key: cord-0072352-u5ivt3pd
authors: Pansy, Katrin; Uhl, Barbara; Krstic, Jelena; Szmyra, Marta; Fechter, Karoline; Santiso, Ana; Thüminger, Lea; Greinix, Hildegard; Kargl, Julia; Prochazka, Katharina; Feichtinger, Julia; Deutsch, Alexander JA.
title: Immune Regulatory Processes of the Tumor Microenvironment under Malignant Conditions
date: 2021-12-10
journal: Int J Mol Sci
DOI: 10.3390/ijms222413311
sha: 15ee67f51e4736c04e7b2a063bc3b0b3d61a2169
doc_id: 72352
cord_uid: u5ivt3pd

The tumor microenvironment (TME) is a critical regulator of tumor growth, progression, and metastasis. Since immune cells represent a large fraction of the TME, they play a key role in mediating pro- and anti-tumor immune responses. Immune escape, which suppresses anti-tumor immunity, enables tumor cells to maintain their proliferation and growth. Numerous mechanisms, which have been intensively studied in recent years, are involved in this process and based on these findings, novel immunotherapies have been successfully developed. Here, we review the composition of the TME and the mechanisms by which immune evasive processes are regulated. In detail, we describe membrane-bound and soluble factors, their regulation, and their impact on immune cell activation in the TME. Furthermore, we give an overview of the tumor/antigen presentation and how it is influenced under malignant conditions. Finally, we summarize novel TME-targeting agents, which are already in clinical trials for different tumor entities.

The tumor microenvironment (TME) consists of immune cells, non-immune stromal cells, and extracellular matrix proteins. All of these components function as critical regulators of tumor growth, progression, and metastasis as well as anti-tumor immune responses. Recently, treatment options for various tumor types have changed tremendously with the development of immunotherapy. This novel therapy differs from conventional chemotherapeutic agents in that it reactivates the immune responses to malignant cells, rather than directly affecting tumor cell growth, pointing out the importance of the TME, especially of immune cells, in tumors. Immune evasion is a mechanism that prevents the immune cell-mediated tumor cell lysis and it has become an important research field in various tumor entities in the last few years. This complex mechanism is mediated by multiple interacting molecular processes, including: (1) suppression of the lymphocyte activation and effector functions mediated by direct interaction of tumor and immune cells and/or the active recruitment of cells possessing immunosuppressive properties and (2) antigen presentation ( Figure 1 ). Here, we review how the TME and its interaction with tumor cells and/or the immune system affect immune evasion, which regulators and pathways are involved, especially focusing on immune cell activation and antigen presentation.

The TME consists of more than 30 distinct tumor-infiltrating non-malignant types of cells, as well as the extracellular matrix (ECM), and its composition differs among tumor types [1] [2] [3] [4] [5] [6] . The cellular component comprises immune cells of the innate immune system (neutrophils, eosinophils, macrophages, dendritic cells (DCs), mast cells, and natural killer (NK) cells, as described in detail in Table 1 ) and the adaptive immune system (T and B cells, as described in detail in Tables 2 and 3) , fibroblasts, endothelial cells and various tissue-associated cells. These cells interact with each other and with cancer cells via complex communication networks through secreted cytokines, chemokines, growth factors, and proteins of the ECM (fibrous structural network, glycoproteins, growth factors, and proteoglycans). The TME cells and the ECM can have a tumor-promoting function but can also play a key role in the regulation of anti-tumor immune responses.

The immune cells of the TME can possess both tumor-promoting as well as anti-tumor functions in the TME (summarized in Tables 1-3 ). The key player in immune cellmediated tumor rejection is the subgroup of cytotoxic T cells (CD8+ T cells, Table 3 ). Here, we review how the TME and its interaction with tumor cells and/or the immune system affect immune evasion, which regulators and pathways are involved, especially focusing on immune cell activation and antigen presentation.

The TME consists of more than 30 distinct tumor-infiltrating non-malignant types of cells, as well as the extracellular matrix (ECM), and its composition differs among tumor types [1] [2] [3] [4] [5] [6] . The cellular component comprises immune cells of the innate immune system (neutrophils, eosinophils, macrophages, dendritic cells (DCs), mast cells, and natural killer (NK) cells, as described in detail in Table 1 ) and the adaptive immune system (T and B cells, as described in detail in Tables 2 and 3) , fibroblasts, endothelial cells, and various tissue-associated cells. These cells interact with each other and with cancer cells via complex communication networks through secreted cytokines, chemokines, growth factors, and proteins of the ECM (fibrous structural network, glycoproteins, growth factors, and proteoglycans). The TME cells and the ECM can have a tumor-promoting function but can also play a key role in the regulation of anti-tumor immune responses. Macrophages derive from monocytes. They play an important role in host defense against pathogens, stimulation of the adaptive immune system mainly by their function as antigen-presenting cells (APCs), and tissue remodeling. By Th1 cytokines (IFN-γ and/or lipopolysaccharide (LPS)) macrophages differentiate into M1 phenotype and produce pro-inflammatory cytokines.

In contrast, macrophages activated by Th2 cytokines (IL-4, IL-10, IL-2) possess the M2 phenotype producing anti-inflammatory factors. Malignant conditions: Tumor-associated macrophages (TAM) can contribute to tumor cell proliferation, invasion, and metastasis, as well as angiogenesis and suppression of T cell-mediated anti-tumor immune responses. TAMs can adopt their differentiation stage in a wide range between M1 and M2 expressing both markers. [1, 6] Neutrophils

Neutrophils are essential effector cells of the innate immune system. They are the first responders in infection, injury, and defense against pathogens. Malignant conditions: Tumor-associated neutrophils (TANs) can exhibit anti-tumor properties as N1 TANs-mediating cytotoxicity-or pro-tumoral effects as N2 TANs-secreting angiogenesis and invasion promoting factors. and TNF-α. [6] Myeloid-derived suppressor cells (MDSCs)

MDSCs compromise a heterogeneous immature immune cell population derived from the myeloid compartment. This cell population plays an essential role in the negative regulation of immune responses. Malignant conditions: MDSCs can be induced by GM-CSF, VEGF, and IL-6, which are mainly produced by tumor cells. They can modulate the inflammatory microenvironment via depletion of amino acids and/or via expression of immune inhibitory ligands to inhibit T cell effector function [6] DCs HLA-DR + lineage − IFNs DCs are the central coordinator of immune response and play a central role in immunity. Their main functions are endocytosis, antigen presentation, and IFN production. Malignant conditions: DCs can play a key role in inducing and maintaining anti-tumor immunity, but in the TME their antigen-presenting function may be inefficient. DCs can differentiate into immunosuppressive regulatory DCs, which limit the T cell activity.

[2] [3] Table 2 . Cells of the adaptive immune system in TME: T cell subpopulations.

T cells CD3+ various cytokines T cells express the TCR complex, which consists of two variable regions-the αand the β-chains (αβTCR)-in the vast majority of human T cells. The smaller T cell subset-γδ-T cells-just express γand δ-chains.

Reactive conditions: T cells can recognize foreign or "non-self" material presented as peptides bound to MHC class I or II molecules at the cell surface. Therefore, these cells play an essential role in immune response (bacterial and viral infection via MHC-mediated antigen presentation and tissue/cell graft rejection caused by MHC mismatches). [ 1, 6] The immune cells of the TME can possess both tumor-promoting as well as anti-tumor functions in the TME (summarized in Tables 1-3). The key player in immune cell-mediated tumor rejection is the subgroup of cytotoxic T cells (CD8+ T cells, Table 3 ).

A subpopulation of cancer-associated fibroblasts (CAFs) possesses the ability to modulate anti-tumor immune responses. This function is mainly mediated by the secretion of anti-inflammatory cytokines, prostaglandins (e.g., PGE-2), chemokines, and ECM proteins [4] [5] [6] . CAFs can increase the number of Tregs by the secretion of chemokines and cytokines. Furthermore, CAFs can directly inhibit the activity of effector T cells and cytotoxic T cells (CD8+ T cells) by the secretion of anti-inflammatory cytokines and PGE-2, by the expression of immune inhibitory ligands, such as PD-L1 and PD-L2 (described in detail in Sections 3.1.3 and 3.1.4) and/or by the production of metabolic reprogramming factors. The immune inhibitory effects of CAFs can also be promoted by their production of dense collagen networks, which represent a physical barrier that inhibits the presence of activated T cells within the tumor. ECM remodeling mediated by CAFs can also acts on the macrophage differentiation into the M2 anti-inflammatory phenotype, thereby playing a key role in their immune inhibitory functions.

Endothelial cells in the TME play a significant role in the pathogenesis of different types of tumors. They are crucial for angiogenesis and are therefore needed for delivering nutrients to tumor cells, promoting tumor cell survival, and aiding in metastasis [8] . Moreover, a study indicated that pericytes of the TME vessels can induce anergy of the CD4+ T cells through a regulator of G-protein signaling-5 (RGS-5)-and IL-6-dependent pathways [9] .

T cells play a key role in immune cell-mediated tumor rejection. An antigen-presenting cell (APC) is needed for T cell activation. The APC bears antigenic peptides, which are non-covalently bound to a major histocompatibility complex (MHC) class I or class II molecule. Under malignant conditions, the antigenic peptides are tumor-specific antigens, e.g., neoantigens arising from somatic mutations and/or cancer-associated antigens. The TCR only recognizes the tumor-specific antigen when it is bound to the MHC molecule. In T cells, the TCR associates with the CD3 membrane complex, whose cytosolic region is responsible for propagating the intracellular signal upon TCR ligation. Each TCR is associated with either a CD4 or CD8 co-receptor, which both bind to MHC (class I for CD8 and class II for CD4) to further stabilize the T cell and APC interaction [10] . Besides the TCR-MHC interaction, other relevant molecules (co-stimulatory and/or adhesion) are needed for T cell activation. The primary co-stimulatory pathway is the binding of CD28 expressed on T cells and CD80 and/or CD86 expressed on APCs [11] . However, additional membrane-bound and/or membrane-soluble signals are necessary for a complete T cell activation. These signals are achieved by the TNF receptor superfamily (CD27, OX-40, 4-1BB, and CD30) when interacting with their appropriate ligand on APCs (CD70, OX-40L, 4-1BBL, and CD30L). The culmination of the TCR and co-stimulatory signals cause the production/secretion of IL-2, autocrine and paracrine factors that stimulate T cell proliferation [12] [13] [14] . T cells also express a set of inhibitory receptors, called immune checkpoints, which are implicated in the fine-tuning of inflammatory processes. These inhibitory receptors can limit the co-stimulatory signal as well as the co-stimulatory molecule ligation and are needed for T cell homeostasis [15] . Under malignant conditions, T cell activation and effector function are inhibited by soluble factors, e.g., cytokines, PGE and toll-like receptor (TLR) ligands, by inhibitory receptors/immune checkpoints, and/or by cells possessing regulatory functions that result in T cell dysfunction, e.g., Tregs, M2 macrophages and myeloid-derived suppressor cells.

T cell anergy is described as the induced hyporesponsive state of T cells caused by incomplete activation and/or low IL-2 production, resulting in little to no T cell proliferation. Human tumor cells and tumor-associated APCs often express high levels of PD-L1, PD-L2, ICOS-L, and B7-H3, which causes high co-inhibitory signals that are accompanied by a low CD28 co-stimulation of T cells, which is itself caused by low to absent CD80 and/or CD86 signals [16] [17] [18] [19] . Consequently, the factors that negatively regulate TCR-coupled pathways, together with the epigenetic mechanisms that cause transcriptional silencing, are activated in T cells under these circumstances [20] . The T cell anergy process is mediated by the combination of the nuclear factor of activated T cells (NFAT) homodimer formation, members of the E3 ubiquitin family, which are also known as casitas B-lineage lymphoma (CBL), epigenetic factors such as Ikaros family zinc finger protein 1 (IKAROS), and the regulatory protein SIR2 homologue (Sirt1) [21] [22] [23] .

Exhausted T cells are described as effector T cells with decreased cytokine expression and effector cell function, as well as being re-activation resistant. This dysfunction occurs when T cells are chronically stimulated and/or activated in chronic inflammation and in cancer [24] . T cell exhaustion seems to be a progressive process in which T cells experience repeated activation. As a result, T cells acquire multiple inhibitory surface molecules in persisting disease conditions such as cancer [25, 26] . In cancer, the expression of the programmed cell death protein 1 (PD-1), T cell immunoglobulin domain and mucin domaincontaining protein-3 (TIM-3), lymphocyte activation gene 3 (LAG-3), T cell immunoglobulin and ITIM domains (TIGIT), and B and T lymphocyte attenuator (BTLA) are found on exhausted T cells [25, 27, 28] . This results in reduced IL-2, IFN-γ, and TNF-α secretion as well as cell cycle arrest, which defines immune cell dysfunction [26] . Notably, it is also possible that exhausted T cells may express multiple inhibitory surface molecules [28] [29] [30] [31] . They have a unique expression signature with alterations in the TCR-and cytokine-signaling pathways, as well as the signaling pathways that are implicated in migration and metabolism, and in the expression of transcription factors (B lymphocyte-induced maturation protein-1 (Blimp-1), basic leucine zipper transcription factor (BATF), NFAT, T-box expressed in T cells (T-bet), and T-box brain protein 2 (Eomes)) [24, [32] [33] [34] [35] [36] .

T Cell Senescence Senescent T cells are characterized by telomere shortening, loss of co-stimulatory molecule CD28, and cell-cycle arrest [37, 38] . Telomere shortening is a by-product of cell division and causes senescence [39] . Senescent T cells develop defective T cell effector abilities with decreased expression of IFN-γ, granzyme B, and perforin [40] [41] [42] [43] and negative regulatory function [44, 45] . Compared to T cell exhaustion and anergy, T cell senescence is irreversible [40, 46, 47] . T cell senescence can be induced by tumor cells via direct interaction, causing the activation of p53, p21, and p16, or by glucose competition and/or hypoxic TME [48] .

In addition to low CD28 expression, T cell senescence is associated with low expression of CD27 and high expression of TIM-2, CD57, and killer cell lectin-like receptor G1 (KLRG-1), as well as p26 and p21, which are implicated in cell cycle regulation [26, [49] [50] [51] [52] [53] . Moreover, the levels of other immune inhibitory surface molecules, e.g., TIGIT and CTLA-4, are increased on senescent T cells [48] . T cells with this surface molecule-expression pattern were found in melanoma [52] and lymphoma [48] . A recent study reported that IL-7 and its receptor also play a key role in immunosenescence [48] .

Conditions in the TME Immune checkpoints and their ligands (the most important members are depicted in Figure 2 ) can be subdivided into the B7-CD28 families, TIM family, immunoglobulin (Ig) superfamily, nectin and nectin-like binding receptors, and butyrophilins, as well as intracellular immune checkpoints. Blocking various regions of these immune checkpoint axes by therapeutic antibodies, which is referred to as immune checkpoint blockade (ICB), represents a novel and promising strategy that has proven its worth in clinical practice for different types of cancer, at least for CTLA-4 and PD-1 blockades [54] .

Within this review, we focus on different types of immune checkpoints/inhibitory co-stimulatory molecules, their regulation, and their key function in the suppression of anti-tumor immune responses.

Overview of immune checkpoint molecules/co-inhibitory receptors causing immunosuppressive conditions in the TME. Different immune checkpoint molecules are expressed on T cells and are shown with their ligands expressed on APCs/tumor cells and/or T cells triggering a co-inhibitory signal to suppress effector T cell responses. Immunosuppressive mechanisms are described in detail below. Created with BioRender.com.

Within this review, we focus on different types of immune checkpoints/inhibitory costimulatory molecules, their regulation, and their key function in the suppression of antitumor immune responses.

The B7-CD28 family comprises six CD28 receptor family members and ten ligand members consisting of co-stimulatory or co-inhibitory molecules, which play an essential role in T cell tolerance and homeostasis ( Figure 2 ) [55] . The B7 family co-inhibitory pathway includes CD80/CD86-CTLA-4, PD-L1/PD-L2-PD-1, HVEM-BTLA, as well as VISTAaxes, all of which are implicated in the mediation of the immune evasion of malignant cells [56] .

One of the most studied inhibitory receptors is CTLA-4, which has structural and biochemical similarities to the co-stimulatory receptor CD28. CTLA-4 is the inhibitory counterpart of CD28, which shares the same ligands CD80 and CD86 [57] . CTLA-4 competes with CD28 for CD80/CD86 binding with a much higher affinity than CD28 in order to reduce T cell responses via cell-intrinsic and -extrinsic pathways [57, 58] . In tumorigenesis, CTLA-4 binding to CD80/CD86 dampens T cell activation by blocking downstream PI3K/AKT and NF-κB pathways [56, 59, 60] .

Another well-characterized member of the B7-CD28 family is PD-1. Its expression is caused by persistent exposure to antigens and/or to inflammatory signals during chronic infection [33, 61, 62] . PD-L1, the corresponding ligand, is broadly expressed on numerous immune cells such as (activated) T cells, B cells, DCs, and many cancer cells under malignant conditions to suppress T cell responses. The PD-L2 expression as the second PD-1 ligand is restricted to APCs [63] [64] [65] . The expression of PD-L1 and, to a lesser degree, of PD-L2, is induced in response to pro-inflammatory cytokines such as TNF-α and type I and type II IFNs [66] [67] [68] . Moreover, PD-L2 expression is upregulated through IL-4, granulocyte-macrophage colony-stimulating factor (GM-CSF) as well as interferons [69, 70] . The binding of PD-L1 or PD-L2 to PD-1 weakens T cell activation and lowers the activation 

The B7-CD28 family comprises six CD28 receptor family members and ten ligand members consisting of co-stimulatory or co-inhibitory molecules, which play an essential role in T cell tolerance and homeostasis ( Figure 2 ) [55] . The B7 family co-inhibitory pathway includes CD80/CD86-CTLA-4, PD-L1/PD-L2-PD-1, HVEM-BTLA, as well as VISTA-axes, all of which are implicated in the mediation of the immune evasion of malignant cells [56] .

One of the most studied inhibitory receptors is CTLA-4, which has structural and biochemical similarities to the co-stimulatory receptor CD28. CTLA-4 is the inhibitory counterpart of CD28, which shares the same ligands CD80 and CD86 [57] . CTLA-4 competes with CD28 for CD80/CD86 binding with a much higher affinity than CD28 in order to reduce T cell responses via cell-intrinsic and -extrinsic pathways [57, 58] . In tumorigenesis, CTLA-4 binding to CD80/CD86 dampens T cell activation by blocking downstream PI3K/AKT and NF-κB pathways [56, 59, 60] .

Another well-characterized member of the B7-CD28 family is PD-1. Its expression is caused by persistent exposure to antigens and/or to inflammatory signals during chronic infection [33, 61, 62] . PD-L1, the corresponding ligand, is broadly expressed on numerous immune cells such as (activated) T cells, B cells, DCs, and many cancer cells under malignant conditions to suppress T cell responses. The PD-L2 expression as the second PD-1 ligand is restricted to APCs [63] [64] [65] . The expression of PD-L1 and, to a lesser degree, of PD-L2, is induced in response to pro-inflammatory cytokines such as TNF-α and type I and type II IFNs [66] [67] [68] . Moreover, PD-L2 expression is upregulated through IL-4, granulocytemacrophage colony-stimulating factor (GM-CSF) as well as interferons [69, 70] . The binding of PD-L1 or PD-L2 to PD-1 weakens T cell activation and lowers the activation of immune cells by reducing the activation of intracellular signaling pathways and downregulation of effector cytokines [63] . In malignant conditions, it has been observed that PD-1-ligands prevent tumor cells from immune cell-mediated lysis [71] .

The inhibitory receptor B and T cell lymphocyte attenuator (BTLA) is another member of the B7-CD28 family [72] . The primary known ligand is the Herpes virus entry mediator (HVEM), which is a TNF receptor (TNFR) family member. HVEM may also act as a ligand for CD160, tumor necrosis factor superfamily member 14 (LIGHT/TNFSF14), another TNFSF ligand, and Lymphotoxin α (LTα). The interaction of BTLA and/or CD160 with HVEM leads to inhibitory signals and causes the suppression of T cells. In contrast, BTLA binding by LIGHT and/or LTα results in co-stimulatory signals to activate T cells [73] . Under malignant conditions, the BTLA/CD160/HVEM axis is associated with an impaired anti-tumor immune response in various tumor entities, as reviewed in detail by Ning et al. [74] .

V-domain immunoglobulin suppressor of T cell activation (VISTA) is a novel, rather uninvestigated B7 family checkpoint protein. Recently, it has been shown that V-Set and immunoglobulin domain-containing protein 3 (VSIG-3) binds to VISTA, resulting in T cell inhibition and suppression through a novel VSIG-3/VISTA pathway [75] . Furthermore, VISTA is implicated in a broad spectrum of immune responses and suppresses T cell activation [76] . As a paradigm, the inhibitory immune checkpoint VISTA can act as a ligand and as a receptor on T cells and APCs to attenuate the immune response by suppressing the T cell-mediated immune response [77, 78] . The specific physiological role of VISTA remains unclear. Observations in malignant settings suggest that VISTA expression suppresses the T cell-mediated response and promotes immune evasion [79] [80] [81] [82] . Additionally, it was reported that VISTA is mainly expressed on immune cells, especially CD8+ T cells, and is associated with immunosuppressive conditions in various tumor types, including melanoma, acute myeloid leukemia (AML), hepatocellular carcinoma (HCC), small-lung-cell carcinoma, gastric cancer, and colorectal cancer, as reviewed in detail by ElTanbouly et al. [83] and Tagliamento et al. [84] .

B7-H3 and B7-H4 are immunosuppressive B7-CD28 family members, which dampen T cell effector function. For both, no corresponding receptor on T cells has been identified so far. Elevated levels of both molecules were observed and correlated with poor clinical outcomes in various tumor entities [85, 86] .

The cell-surface molecule T cell immunoglobulin and mucin domain-containing protein-3 (TIM-3) is a key member of effector T cell function and belongs to the TIM family. The TIM family contains two additional members (TIM-1 and TIM-4) in humans. TIM-1 and TIM-4 function as co-stimulatory molecules for T cell activation, whereas TIM-3 possesses an immune inhibitory function. The major function of TIM-3 is to inhibit the Th1 and cytotoxic T cell responses and the expression of TNF and IFN-γ. Hence, TIM-3 is an important negative regulator of adaptive and innate immunity [87, 88] . So far, four distinct ligands have been identified to bind to TIM-3: galectin-9, phosphatidylserine (PtdSer), high mobility group box 1 (HMGB1), and carcinoembryonic AG-related cell adhesion molecule 1 (CEACAM1) [89] [90] [91] [92] . Galectin-9 expression is upregulated by IFN-γ contributing as part of a negative feedback loop such as PD-L1 and is expressed by immune cells such as T cells, B cells, macrophages, and mast cells, and by non-immune cells such as endothelial cells and fibroblasts [26, 93] . PtdSer is a phospholipid that is exposed on the surfaces of apoptotic cells and interacts as a ligand with all of the TIM family members, but with a remarkably lower affinity [90, 94, 95] . The interaction of PtdSer and TIM-3 is crucial for the clearance of apoptotic cells, but its role in cancer remains unclear [95] . HMGB1, a DNA-binding protein, is secreted by cancer cells and by other cell types and can interact with other receptors alone or in complex with DNA or LPS [91, 96, 97] . HMGB1, but not galectin-9 or PtdSer, binding to TIM-3 is responsible for regulating the nucleic acid-mediated innate immune responses [91] . CEACAM1 acts as a self-ligand on T cells. It is expressed at high levels on activated T cells, but not on naïve T cells, and functions as a negative regulator of T cell responses [92, 98, 99] . CEACAM1 expression has also been identified on macrophages, DCs, monocytes, and tumor cells [100] [101] [102] . The anti-tumor immunity is inhibited by TIM-3 by mediating T cell exhaustion [103] [104] [105] . In cancer, TIM3+ PD-1+ CD8+ TILs are associated with the most dysfunctional or terminal stage of CD8+-T cell exhaustion, whereas single positive TIM3-PD-1+ CD8+ TILs exhibit a weaker exhaustion/dysfunctional stage and double negative TIM3-PD-1-CD8+ TILs possess good effector function [25, 52, 106] .

The co-inhibitory receptor lymphocyte activation gene 3 (LAG-3) is a transmembrane protein that is found on the surface of cells and exerts a high structural homology to CD4 [107] . Continuous antigen exposure in tumors or during chronic viral infections leads to high and persistent co-expression of LAG-3 with other co-inhibitory molecules such as PD-1 and TIM3 on T cells, resulting in T cell dysfunction [108] . Tumor-infiltrating T cells are also typically exposed to tumor-associated antigens and consequently express high levels of inhibitory co-receptors such as LAG-3, leading to a state of exhaustion [109] [110] [111] . To date, several ligands are known to interact with LAG-3, e.g., galactin-3, LSECtin, and MHC class II complex [112] [113] [114] [115] [116] . A recent work identified fibrinogen-like protein 1 (FGL1), which is a liver-secreted protein, as an additional immune inhibitory ligand that is highly expressed in human cancer cells [117] . LAG-3 facilitates inhibitory effects on effector T cells and Tregs. In CD8+ T cells, LAG-3 crosslinks with the CD3/TCR complex and inhibits TCR-induced T cell proliferation, the production of cytokines, and calcium influx [118] .

The signaling lymphocyte activation molecule F4 (SLAMF4, CD244, 2B4) is a member of the SLAM family of proteins and is also part of the immunoglobulin (Ig) family [99, [119] [120] [121] . SLAMF4 binds to the high-affinity ligand SLAMF2 (CD48), as well as to CD2 with low affinity, causing either co-stimulatory or inhibitory signals depending on the presence of intracellular proteins within the immune cells. When SLAM-associated protein (SAP) is present, co-stimulatory signals are induced. In contrast, when Ewing sarcoma-activated transcript 2 (EAT2) is present, immune inhibitory signals are triggered [122] [123] [124] [125] . In human cancers, SLAMF4 is expressed on exhausted CD8+ T cells and it is co-expressed with other inhibitory receptors [126, 127] . SLAMF4 seems to be implicated in immune evasion, but its interplay with other co-stimulatory and/or co-inhibitory signals has not been fully deciphered so far.

T cell immunoglobulin and ITIM domains (TIGIT, or WUCAM, Vstm3) is a receptor belonging to the nectin and nectin-like molecules (Necls) [128] . TIGIT binds to three ligands, namely CD155 (PVR), CD112 (Nectin-2), and CD113 (Nectin-3), that are expressed on APCs and cancer cells [129] [130] [131] . The main ligand of TIGIT is CD155 with the highest affinity, and it binds with lower affinity to CD112 and CD113 [129, 130, 132] . CD155 expression is mainly found on DCs, T cells, B cells, and macrophages, but also in non-hematopoietic tissues [120] . CD112 is expressed in both hematopoietic and non-hematopoietic tissues [133, 134] . Analogous to the CTLA-4/CD28 pathway, CD226 (DNAM-1), which is a co-stimulatory counterpart of TIGIT, and CD96 (Tactile), which is a novel inhibitory receptor, compete with TIGIT to bind to CD112 and CD155 in order to fine-tune the immune responses. However, TIGIT binds to CD155 with much higher affinity, followed by CD96 and then CD226 [128, [135] [136] [137] . The TIGIT/CD96/CD226 pathway is more complex than the CTLA-4/CD28 pathway. CD226 can bind CD112 as a ligand and the CD112R (PVRIG), which is another novel immune checkpoint, competes with CD226 and TIGIT to bind to CD112 [128, 134, 138, 139] . Within the TME, the expression of TIGIT is increased in human and murine TILs and is observed in a broad range of malignancies [140] [141] [142] [143] [144] . In tumor tissue, TIGIT expression on CD8+ T cells and Tregs plays a major role in driving suppression within the TME [133, 140, 142] . It was observed that a TIGIT blockade might restore T cell responses in various malignancies [142] .

Butyrophilin (BTN), butyrophilin-like (BTNL) and the selection and upkeep of intraepithelial T cell-like factors (SKINTL) are a more recently identified class of T cell co-inhibitory/co-stimulatory molecules that share strong structural similarities with the B7 family. They belong to the Ig superfamily that consists of extracellular IgV and/or IgC domains followed by a transmembrane domain and a cytoplasmic B30.2 domain, which was found in most of the members [145] [146] [147] . In humans, the BTN family consists of seven proteins (BTN1A1, BTN2A1, BTN2A2, BTN2A3, BTN3A1, BTN3A2, and BTN3A3), BTNLs of five proteins (BTNL2, BTNL3, BTNL8, BTNL9, and BTNL10), and SKINTL [148] [149] [150] .

Most of the BTN/BTNLs, which have been characterized, inhibit T cell proliferation and cytokine production through currently unidentified receptors. However, for BTN3A1, BTN3A2, and BTNL8, the co-stimulatory function resulting in T cell activation has been described [148, [151] [152] [153] [154] .

The number of studies investigating BTN/BTNLs in malignancies is rather limited. It was observed that BTN3A2 expression was associated with the infiltration of CD4+ and CD8+ T cells and a better prognosis in ovarian cancer, indicating its immune co-stimulatory properties [155] . Furthermore, it was shown that BTN3A1 is highly expressed in human high-grade serous ovarian carcinomas, where it inhibits tumor-reactive αβ TCR activation, demonstrating its co-inhibitory effects [156] .

In addition to surface molecules and soluble factors, intracellular immune checkpoints that modulate T cell function have been identified, namely nuclear receptor subfamily 2 group F member 6 (NR2F6) and poly(rC)-binding protein 1 (PCPB1) [157, 158] .

NR2F6 possesses a feedback function upon TCR ligation, by repressing the transactivation/DNA binding of NFAT and activator protein 1 (AP-1), which are key transcription factors of TCR stimulation. The DNA-binding affinity of NR2F6 is regulated at the posttranslational level by Ser86 phosphorylation mediated by PKC, resulting in diminished DNA binding. It has been demonstrated that NR2F6 mechanistically controls the magnitude and duration of the IL-2, IFN-γ, and TNF-α expression of effector CD4+ and CD8+ T cells, thereby affecting T cell activation and effector outcomes. Data generated from tumor-bearing NR2F-deficient mice that showed reduced tumor growth caused by their hyperactive tumor immunity indicate the key function of NR2F6 as a negative intracellular regulator of the T cell immune response and as a potential target to restore anti-tumor immunity [157] .

PCPB1 is an RNA-binding protein, which is upregulated in activated CD4+ and CD8+ T cells upon TCR stimulation. It prevents the conversion of the effector T cells into Tregs, and thereby stabilizes effector function and subverts immunosuppressive conditions. T cell-specific PCPB1 deletion in mouse tumor models caused Treg differentiation and the induction of multiple checkpoint molecules including PD-1, TIGIT, and VISTA on tumor-infiltrating lymphocytes and reduced anti-tumor immunity [158] .

The transcriptional control of immune checkpoints in immune cells, as well as of inhibitory ligands in cancer cells, have been understudied so far.

It was reported that the regulators of cell survival and proliferation control the expression of inhibitory ligands, namely members of AP-1, MYC, which is a signal transducer and activator of transcription 3 (STAT3), hypoxia-inducible factor 1-alpha (HIF-1a), and nuclearestrogen receptor α (ER-α) [159] [160] [161] [162] [163] . AP-1 is a dimeric transcription factor that comprises four DNA-binding-protein family members (Jun, Fos, musculoaponeurotic fibrosarcoma (Maf), and activating transcription factor (ATF)) [159, 164] . It regulates the inhibitory ligands upon immune cell activation via direct binding of the promotor region, e.g., as observed in Hodgkin's lymphoma by the binding of Jun to the enhancer region of the PD-L1 promoter, resulting in PD-L1 expression [165, 166] . MYC, STAT3, and HIF-1a also possess a binding site in the PD-L1 promoter and seem to induce its expression [160, 161, 167, 168] . ER-α also binds the PD-L1 promoter but represses PD-L1 expression, indicating its function as a negative regulator of inhibitor ligands [162] . These transcription factors are often stimulated when oncogenic-signaling cascades, including mitogen-activated protein kinases (MAPK), JAK/STAT, Wnt, and PI3K/AKT, are activated [164, 169, 170] . Moreover, a high expression of PD-L1 and PD-L2 is frequently observed because of genetic alterations directly affecting the gene locus of PD-L1 and PD-L2 and/or the JAK/STAT activation that is caused by its amplifications [166, [171] [172] [173] .

For immune checkpoints, it was reported that their promoter is bound by transcription factors of the NFAT family (mainly NFAT1 and NFAT2), the AP-1 family, and members of the NR4A nuclear-orphan-receptor family [174] [175] [176] [177] [178] [179] [180] [181] . For NFAT1 and NFAT2, it is known that both are responsible for the expression of immune checkpoints such as PD-1, TIM-3, LAG-3, TIGIT, and CTLA-4, as well as exhaustion-associated genes in the absence of AP-1 transcription factors [176, 182, 183] . For the nuclear receptor subfamily 4A (NR4A), it was observed that two members of this family, namely NR4A2 and NR4A3, are highly expressed in exhausted T cells [182] and those exhaustion-specific accessible regions are enriched for the consensus of binding sites for NR4As in exhausted T cells [184, 185] . Furthermore, it was reported that NR4A1, which is the third and last member of the NR4A family, binds the PD-1 promoter [180, 181] and that this receptor is preferentially recruited to the AP-1 transcription factor, where it represses T cell effector gene expression by inhibiting AP-1 function [180] . Furthermore, CAR T cells that were lacking all three NR4A members displayed the phenotypes and gene expression profiles of CD8+ effector T cells and chromatin accessibility, which was enriched for the binding of motifs for transcription factors involved in the activation of T cells [181] . Recently, another study supported the central role of TOX proteins in CD8+ T cell exhaustion. TOX is a transcription factor family consisting of TOX1, TOX2, TOX3, and TOX4 [186] . Models causing T cell exhaustion demonstrated that TOX protein was upregulated and remained high in exhausted CD8+ T cells. Additionally, the deletion of TOX promotes the production of IFN-γ and TNF in these conditions [187] [188] [189] [190] . Like NR4As, in a CAR T cell model, TOX1 and TOX2 were highly upregulated in exhausted T cells. The deficiency of TOX1 and TOX2 in CAR T cells decreased the expression of inhibitory receptors and increased effector T cell function. Furthermore, TOX proteins cooperate with NR4As causing T cell exhaustion-mediated calcineurin signaling via NFAT in the absence of AP-1. NFAT, NR4As, and TOXs promote the expression of immune checkpoints [191] .

Furthermore, several microRNAs (miRNAs) have been identified to directly bind and downregulate immune checkpoints and their inhibitory ligands, whereas miR-34, miR-200, and miR-570 bind and suppress PD-L1 [192] , miR-138 binds and suppresses PD-1 and PD-L1 [192] , miR-28 binds and suppresses PD-1 as well as BTLA [193] , and miR-155 binds and suppresses the immune checkpoints [194, 195] . Additionally, one long non-coding RNA (lncRNA), namely NKX2-1-AS1, has been identified to negatively regulate the PD-1/PD-L1 axis [196] . Cytokines are small, soluble molecules with pleiotropic effects in normal cells, in host defenses against pathogens, and in the eradication of tumor cells [197] and can be either pro-or anti-inflammatory by nature. They act as major regulators of the innate and adaptive immune systems that allow immune cells to communicate over short distances in a paracrine and autocrine fashion by controlling their proliferation, differentiation, effector functions, and survival [198] . The action of cytokines is usually short-lived and limited; however, dysregulated and chronic cytokine release is a hallmark of auto-immune disease, cancer, and immunosuppression [199] . Cytokines play a key role in tumorigenesis by stimulating intracellular signaling. On the one hand, they cause tumor cell proliferation and/or invasion [200] , and the regulation of immune-activation and/or immunosuppressive processes, on the other hand. The immune regulatory functions of cytokines are listed in Table 4 . Interestingly, the cytokine signaling is mainly mediated by the JAK/STAT-pathway that is known to control immunosurveillance as well as evasion [201] . IFN-γ Enhanced antigen presentation by induction of MHC class I Promotion of Th1 together with inhibition of Th2 response, and activation of macrophages [203] IL-33 Local increasement of CD8+ T cells and NK cells Increased type I immune response (increased expression of IFN-γ, IL-12 and granzyme B) [204] IL-36 Enhanced effector function of CD8+ T cells, NK T cells, and γδ T cells [204] IL-12 Enhanced cellular cytotoxicity Enhanced IFN-γ production and differentiation of naïve T cells towards Th1 cells [205] IL-2 Expansion of CD8+ T cells by IL-2 Acquisition of effector and memory functions of CD8+ T cells [206] IL-18 Activation of CD4+ T cells and/or NK immune responses [204] IL-15 Homeostasis and activation of NK cells Expansion and activation of memory T cells [207, 208] IL-21 Regulation of lymphoid cell, NK cells and myeloid cells [209] IL-1 T cell activation Proliferation of B cells [204] IL-6

Inhibition of Tregs differentiation Stimulation of Tfh differentiation Production of IL-21 Promotion of differentiation of B cells into IgA secreting plasma cells Involvement in CD28-independent T cell activation [210] pro-tumorigenic effects-immunosuppressive function CSF-1 TAM recruitment and differentiation onto an M2 phenotype [211] IFN-γ Upregulation of indoleamine-pyrrole 2,3-dioxygenase (IDO) and HLA-G, PD-L1 and other immunoregulatory molecules by the JAK/STAT pathway Induction of MDSCs [203] IL-18 Regulation of PD-1 by conventional NK cells [204] Type I IFN (IFN-α and-β) 

pro-tumorigenic effects-immunosuppressive function

Increased expression of immune inhibitory molecules mediated through the transcription factors c-Mac and Prdm1

The key factor for the maximal effector cell expression of PD-L1, LAG-3, CTLA-4, and TIGIT [219, 220] IL-33 Increased number of immunosuppressive immune cells and innate lymphoid cells [204] CCL2 Activation of Tregs and inhibition of T cell effector function [221] TGF-β Inhibition of CD8+, CD4+, NK cells proliferation and cytotoxicity Polarization of macrophages and neutrophils towards a suppressive phenotype Together with FOXP3 driver of Tregs differentiation Together with IL-6 and IL-21 driver of Th17 differentiation [222] 3.1.

The first line of defense is mediated by the innate immune system. To establish an effective immune response, these innate immune cells use intracellular or membrane-bound pattern recognition receptors (PRRs) to recognize the pathogen-associated molecular pattern (PAMP) or damage-associated molecular pattern (DAMP) molecules that are released from dying cells [223] . PRRs can be classified based on their protein domain homology as toll-like receptors (TLRs), nucleotide-binding oligomerization domain-like receptors (NLRs), retinoic-acid-inducible gene-I-like receptors (RLRs), C-type lectin receptors (CLRs), or cytosolic DNA sensors (CDSs). They are not only expressed on the cell surface but also widely distributed in the intracellular membrane compartment and the cytoplasm. PRRs are mainly expressed on innate immune cells (macrophages and DCs) and B cells as well as, to a lesser extent, on other immune cells such as mast cells, Tregs, monocytes, and basophils. Additionally, they can also be expressed on tumor cells. Under malignant conditions, ligand binding causes the activation of downstream pathways that directly affect tumor cells and/or promote the production and secretion of cytokines, e.g., IFNs, pro-inflammatory factors, and chemokines, in order to activate the adaptive immune system. Thus, PRRs possess regulatory functions of immunosurveillance and tolerance in tumor immunity [224] [225] [226] [227] .

TLRs, which are the most studied PRRs in tumors, consist of 13 subtypes, of which TLR11, 12, and 13 are not expressed in humans. Some TLRs (TLR1, 2, 4, 5, 6, and 10) are presented on the cell surface, whereas others (TLR3, 7, 8, and 9) are located in the intracellular compartment [224, 225] . Ligand binding (membrane components to surface receptors or nucleic acids to intracellular members) causes NF-κB-and MAPK-pathway activation in either a MyD88-dependent or MyD88-independent manner, which subsequently causes the secretion of pro-inflammatory cytokines and IFNs. TLR signaling is crucial in the activation of adaptive immunity [228] [229] [230] [231] [232] . Numerous negative regulators, which suppress TLR signaling and thereby modulate immune responses, can also be found [233] . In the TME, almost all of the immune cell types express TLRs. Thus, TLR stimulation can modulate immune cell function and activation. It can suppress Tregs and/or MSDCs, increase cytotoxic T cell activity, cause switches from Th1 to Th17 responses, lead to M1 macrophage differentiation and increase antigen presentation, suggesting the usage of TLR agonists to promote anti-tumor immune responses [234] .

NLRs are intracellular PRRs that comprise five family members (NLRA, NRLB, NLRC, NLRP, and NLRX) and are derived from 22 genes in humans [224] [225] [226] . Upon activation, NLRs form protein complexes called inflammasomes, whereas others orchestrate NF-κB and MAPK signaling [235] . It has been demonstrated by a few studies that NLR signaling can prime anti-tumor immune responses that are mediated by DCs, macrophages, and T cell activation [236] .

RLRs are also intracellular PPRs that mainly include three members: RIG-I, MDA5, and LGP2. They can be activated by double-stranded RNA, resulting in activation of the IRF (IFN-regulatory factors, mainly IRF-1, IRF-3, and IRF-7) transcription factors and the NF-κB pathway [224] [225] [226] . In the TME, RLR stimulation enhances anti-tumor immunity through the activation of cytotoxic T cells and NK cells, and the inhibition of Tregs [224] .

CLRs are expressed on the plasma membrane and can recognize polysaccharides that are present on pathogens and self antigens [225, 226] . CLRs are widely expressed on myeloid cells such as macrophages, neutrophils, and DCs. Many CLRs, such as dectin-1, 2, 3, Mincle, and DEC-203, trigger cellular immune responses, which are mainly caused by NF-κB-and MAPK signaling-induced cytokine secretion. In contrast, some CLRs, such as MICL and DCIR, have immune inhibitory effects by controlling the maturation, activation, and proliferation of DCs caused by the inhibition of TLR signaling [225, 237] .

CDSs are expressed in the cytoplasmic compartment and recognize abnormal DNA. CDSs include IF116, cGAS, AIM2, and DAI. Among them, cytosolic double-stranded DNA promotes the AIM2-mediated inflammasome and enhances the production of proinflammatory factors such as IL-1β and IL-18. In contrast, the recognition of doublestranded DNA by cGAS or IF116 causes the activation of STING, leading to the expression of IFN-β and other cytokines such as TNF, IL-1β, and IL-6. Furthermore, it has been demonstrated that cGAS-STING activation in the TAMs and DCs of the TME can enhance anti-tumor immunity [225, 226] .

PGE-2 is one of the most widely produced prostaglandins in the human body. It carries out paracrine and autocrine signaling functions and is involved in multiple physiological processes. However, there is also a large body of evidence implicating PGE-2 and its metabolic enzymes in cancer progression. In tumors, PGE-2 can be produced by the stroma, by TILs, or by cancer cells themselves. High levels of PGE-2 play a role in promoting tumor formation, growth, and metastasis in different types of cancer, including colon cancer, breast cancer, lung cancer, and melanoma [238] [239] [240] . The reported effects of PGE-2 in tumors are very diverse. They range from apoptosis resistance [241] , angiogenesis [242, 243] , and enhanced migration of tumor cells [244, 245] to the suppression of anti-tumor immunity [246] . PGE-2 promotes a TME that contains low numbers of certain infiltrating immune cell types by directly or indirectly affecting different immune cell populations, making it a key disruptor of the anti-tumor immune response [247] . A prominent effect of PGE-2 is the suppression of the conventional type one DC (cDC1s)-mediated immune response. By inhibiting the conventional NK cell (cNK) functions, PGE-2 leads to a lower presence of cDC1 in the TME [248] . cDC1s have been shown to be involved in the recruitment and activation of CD8+ T cells [249, 250] . Moreover, they help maintain the cytotoxic responses of CD8+ effector cells in the TME [251] . The sequential failure to recruit cDC1s and CD8+ T cells into the tumor, due to the presence of PGE-2, leads to disruptions at key steps of the cancer immune cycle, such as the antigen presentation, priming, and activation of T cells, and the trafficking and infiltration of T cells into the tumor. PGE-2 in the TME further leads to the differentiation and activation of immunosuppressive myeloid cell species, such as monocyte and neutrophil immunosuppressive phenotypes [252] [253] [254] [255] [256] , M2 macrophages [257, 258] , and Tregs [259] , thereby further contributing to immune evasion. Zelenay et al. showed in vivo that cyclo-oxygenase (COX) inhibition synergizes with immune checkpoint blockade therapy using anti-PD-1 monoclonal antibodies in a mouse melanoma model [246] . A more recent study could identify a COX-2-associated inflammatory gene signature, which can be used to predict responses to immune checkpoint therapy in multiple cancers in humans [260] . When combined, the effects of PGE-2 result in poorly infiltrated tumors and may therefore lead to immune checkpoint inhibitor therapy resistance.

The heterogeneous and dynamic metabolite profile of the TME affects the proliferation and activity of all cellular players involved. Some general features have been ascribed to TME of solid tumors, e.g., hypoxia, scarce nutrient availability, and increased concentrations of waste products, such as lactate, reactive oxygen species, or cysteine [261] . However, these features are spatially unamenable to each part of a single tumor, and to each tumor type, as the TME is highly heterogeneous and some tumors can also rely on oxidative metabolic pathways or can be well vascularized and well supplied with metabolites [262] [263] [264] .

Rapidly proliferating tumors nevertheless possess a highly anabolic appetite and are voracious consumers of glucose [265, 266] , which enables the ramped-up aerobic glycolysis that is needed for their rapid proliferation [265, 266] . Similarly, T cells preferentially use aerobic glycolysis during their clonal expansion into the CD8+ effector phenotype [267, 268] . Therefore, the metabolic rewiring occurring in non-transformed, proliferating cells can be attributed to both cancer cells and effector T cells [265, 266, 269] . Importantly, a specific metabolic profile defines each differentiation state and lineage subtype of T cells, ranging from quiescent, mostly oxidative metabolism in naïve T cells to highly glycolytic metabolism in terminally differentiated effector T cells [270] . Alternatively, as described in the context of acute function and microenvironment, the metabolism of T cells can be graded from hypermetabolic to hypometabolic state [269] .

Given their coexistence, cancer cells and T cells are in competition for the nutrients available in the TME. In highly glycolytic tumors, cancer cells tend to outcompete T cells in glucose consumption, thereby inhibiting effector T cell differentiation and functionality [267, 271] . Furthermore, other T cell populations, such as CD8+ memory T cells or Treg cells, are less reliant on glucose and are less affected by nutrient competition [267, 271] . Therefore, in these conditions, the expansion of immunosuppressive TILs is favored, thus ensuring immune evasion by differentially regulating effector and immunosuppressive TILs. In addition to metabolic disturbance, glucose limitation can also affect effector T cell function through miRNA-regulated epigenetic modifications [272] or by blocking IFN-γ production [267] . Furthermore, the increased aerobic glycolysis of activated T cells allows the maintenance of the Acetyl-CoA pools that are needed for the epigenetic promotion of IFNG expression [273] , as well as for GAPDH production, which regulates the IFN-γ protein translation [267] .

In addition to glucose, the scarcity of other metabolites such as amino acids can also affect T cell activity. For example, the depletion of glutamine, arginine, and tryptophan has immunosuppressive effects that inhibit T cell function and stimulate immune suppressor cell function (e.g., Tregs and/or MDSCs). Arginine is known to induce proliferation, differentiation, and activity of T cells, thus improving their anti-tumor response by promoting oxidative pathways [274] . Tryptophan is needed for protein synthesis in activated T cells, as well as for cell-cycle progression [275] , while its catabolites in the kynurenine pathway, such as 3-hydroxyanthranilic and quinolinic acids, can induce selective apoptosis in vitro in Th1 but not Th2 cells [276] . The key players in the depletion of arginine and tryptophan in the TME are arginase and IDO1, respectively, which have both been shown to be upregulated in tumor cells. IDO1 catabolizes tryptophan, thereby depleting tryptophan in the TME, and at the same time produces the oncometabolite kyneurine [277] . In addition to IDO1, IDO2 and tryptophan-2,3-dioxygenase (TDO) regulate the rate-limiting enzymatic conversion of tryptophan in the kyneurine pathway and are therefore being investigated as single or co-targets for the inhibition of the facilitation of the immunosuppressive TME through this metabolic pathway [278, 279] . Indeed, a recent publication showed that a novel TDO inhibitor, together with anti-CTLA-4, reduced ectopic colon cancer growth [280] .

The waste products of cancer metabolism are also part of the TME and affect immune cell function. The disturbed vascularization that is characteristic of tumors is the major cause of hypoxia and necrosis. Chronic hypoxia [281] and an acidic environment also support immune evasion by inducing T cell differentiation into immunosuppressive Tregs via different mechanisms [282, 283] . In highly necrotic areas, dying cancer cells release a high number of cations, specifically potassium, leading to functional starvation of the tumor-infiltrating T cells, which is characterized by autophagy induction and oxidative metabolism, in parallel to downregulation of AKT/mTOR signaling [284, 285] . The final outcome is limited T cell effector function and preserved T cell stemness [285] .

The interaction of innate immune cells, such as macrophages and NK cells, with the TME is less well examined. However, distinct metabolic features also characterize the two borderline macrophage phenotypes, whereby M1 macrophages have been shown to have enhanced anabolic metabolism, including anaerobic glycolysis, pentose phosphate pathway and fatty acid synthesis, whereas M2 macrophages rely more on catabolic metabolism and oxidative phosphorylation [261] .

Results of studies investigating tumor metabolism as a therapeutic target are still diverse and not easily translatable because of the metabolism dynamics and tumor heterogeneity. A balance between immune cell infiltrate, tumor size, and vascularity, as well as the metabolic phenotype, should be taken into consideration before assessing the therapeutic strategy and outcome. For example, even though it seems that nutrient deprivation (applied in the form of fasting, ketogenic, fasting-mimicking diets, etc.) will cause immune suppression, in some cases different findings have been observed. Interestingly, fasting alone reduced acute lymphoblastic leukemia development in mice [286] . Furthermore, fastingmimicking diets in combination with chemotherapy enhanced the CD8+ T cell-dependent tumor cytotoxicity of chemotherapy in a mouse model of breast cancer [277] . The involvement of the immune system in response to starvation or caloric restriction mimetics has also been demonstrated through the differential regulation of cytotoxic and regulatory T cell activity in the TME [287] .

The metabolic profile of T cells is especially important in adoptive immunotherapies. For successful therapy, T cells should display persistence and durability after transfer [270] . There is a possibility to boost these characteristics in ex vivo cell cultures, in order to increase the "quality" of cells for therapy (reviewed in [270] ). The metabolic status of the tumor can be predictive of the response to adoptive T cell therapy, as Cascone et al. recently demonstrated that highly glycolytic tumors that were more therapy-resistant [288] .

In the TME, a high level of adenosine triphosphate (ATP) is present. ATP is catabolized by CD73, which is an enzyme that is normally expressed by MSDC, TMAs, Tregs, exhausted T cells, and tumor cells in malignant conditions. Thus, CD73 causes high levels of adenosine, which belongs to the group of immunosuppressive metabolites [289] . The high expression of CD73 has been found in many tumor entities and is associated with poor prognosis [290] . Extracellular levels of adenosine are usually low in normal tissue. They increase dramatically upon injury in order to suppress excess inflammation and allow wound healing [289] . These effects are mediated by the adenosine receptors, A2aR and A2bR, which are expressed on multiple immune cells, including T cells, APCs, neutrophils, and NK cells. In these immune cells, both receptors possess inhibitory effects [291] .

An elegant experiment using single-cell RNA sequencing provided an overarching picture of the metabolic tumor landscape [292] . Interestingly, the high metabolic heterogeneity of tumors between patients has been observed, while the metabolic profiles of immune cells showed less variability, implying once more the high metabolic plasticity and adaptability of cancer cells compared to immune cells [292] . RNA sequencing could also reveal valuable biomarkers, as exemplified in a recent study showing that the expression of two metabolic enzymes could be differentially correlated to the immune infiltrate in the tumor bed, as well as to patient survival [293] .

Antigen presentation via MHC class I and class II molecules is important for activating immune responses [294] [295] [296] . Nucleated cells synthesize proteins, which are processed and presented as immunogenic peptide-MHC class I complexes on their surfaces. These complexes are recognized by cytotoxic CD8+ T cells, with the purpose of detecting and eliminating altered cells (tumor or virus-infected cells). In contrast, APCs, such as DCs, macrophages and B cells, can ingest antigens, which are processed to be presented in conjunction with the MHC class II molecules to CD4+ T cells.

For the antigen presentation to CD8+ T cells, proteins in the tumor or virus-infected cells need to be proteasomally digested in order to generate short oligopeptides, which are transported from the cytosol to the endoplasmic reticulum (ER) by transporters associated with antigen-processing 1 and 2 (TAP1 and TAP2). In the ER, oligopeptides are loaded onto a nascent MHC class I molecule with the assistance of chaperones. Antigen-loaded MHC class I molecules are then delivered to the cell surface for antigen presentation [297, 298] .

In humans, classical MHC class I molecules comprise a polymorphic alpha heavy chain that is encoded by the classical HLA genes -HLA-A, HLA-B, and HLA-C -as well as an invariant light chain that is encoded by the β-2 microglobulin gene (B2M) [295, 296] . In addition to the classical MHC class I molecules, there are also non-classical HLA class I molecules. Their heavy chains are encoded by the HLA-E, HLA-F, and HLA-G genes. MHC class II molecules comprise an alpha and a beta chain and are encoded by genes in three different loci (HLA-DP, HLA-DQ, and HLA-DR) [294] .

Antigen presentation on MHC class I molecules is a key component of immunosurveillance, and cytotoxic CD8+ T cells can recognize altered cells through tumor antigens (such as neoantigens, human endogenous retroviral (HERV) antigens, tumor-associated antigens and cancer testis antigens (CTAs)) [299] [300] [301] [302] . Therefore, tumor cells are pressured to develop strategies to evade destruction by the host's immune system [302] . Altered expression of MHC class I components in order to avoid antigen presentation is common in human malignancies and plays a pivotal role in immune escape [303, 304] . The percentage of cases with a low MHC class I phenotype varies greatly depending on the tumor type [305] . Additionally, tumors can exhibit heterogeneous MHC class I expression and the MHC class I expression may change during cancer progression. Moreover, a low MHC class I phenotype is also often associated with a worse prognosis [304] .

Altered MHC class I expression can result from hard or soft lesions [306] . Structural genetic alterations, such as mutations affecting the class I heavy chain genes or the B2M gene, cause irreversible defects (hard lesions) [295] . In contrast, reversible defects (soft lesions) can derive from alterations in the transcriptional or post-transcriptional regulation of MHC class I antigen presentation pathway components (e.g., genes encoding the class I heavy chain, the invariant light chain and the antigen processing machinery components) [295, 305, 306] . For example, the repression of gene transcription can be caused by hypermethylated promoter regions of MHC class I antigen presentation pathway genes. MHC class I expression is mediated by NF-κB, IFNs, and NLRC5 [307] , and dysregulations in these pathways are also often responsible for MHC class I soft defects [304] . Furthermore, alterations in the regulation of the antigen presentation pathway components may arise from dysregulated miRNAs or lncRNAs [295, 305] .

Even though tumor cells can escape cytotoxic CD8+ T cells by defective MHC class I expression, NK cells have the capability to destroy these cells via 'missing-self' recognition [308, 309] . This is due to the inhibitory ligand function of MHC class I complexes, which, when lowered or absent, can result in NK cell activation, leading to cytotoxicity; however, ligands for activating receptors that are upregulated on tumor cells must also contribute to this process [310] [311] [312] . However, many advanced malignancies exhibit defective MHC class I expression, suggesting that NK-mediated surveillance is circumvented [313] . Hence, tumor cells can employ evading strategies and may also exhibit plasticity to avoid the cytotoxicity of both cytotoxic CD8+ T and NK cells [304, 305] . Among others, such strategies include the expression of the non-classical MHC class I HLA-G, which can lead to NK cell inhibition, and the induction of NK cell dysfunction [305, [311] [312] [313] [314] . Additionally, NK cell infiltration into solid tumors is often limited and they are mainly confined to the stroma compartments at the tumor invasive margin, also in MHC class I defective tumors [303] .

A low MHC class I phenotype represents a major hurdle for effective immunotherapy, as it relies mainly on the cytotoxicity of CD8+ T cells [315] . Indeed, defective MHC class I expression has been associated with a resistance to such regimens [305] . If reversible defects are responsible for the low MHC class I phenotype, then therapeutic drugs could serve to restore MHC class I expression [295] . The clinical benefit of regimens based on NK cells has been shown to be limited so far; however, a number of therapies are currently in development [311, 316] .

HLA-E acts as a ligand of the heterodimeric receptor NKG2A/CD94. This receptor is present on circulating NK cells and on T cells with cytolytic function. The interaction of HLA-E and NKG2A/CD94 inhibits this cytolytic function [317, 318] . Virally infected cells downregulate HLA-E to favor NK and T cell activation and antiviral responses [319] . In contrast, cancers evade the immune system by the overexpression of HLA-E as well as by recruiting TILs that have strong expression of NKG2A/CD94 [318, 320] . High levels of HLA-E were reported in several tumor types, including gynecologic cancers, breast cancer, non-small cell lung cancer (NSCLC), liver, pancreas, kidney, melanoma, prostate, head and neck, stomach, rectal, and colon cancer [321] . The surface expression of HLA-E is associated with functional antigen-processing molecules and cytotoxic T cell infiltration [321] . The blockade of NKG2A enhances the anti-tumor response by NK and T cells. However, the currently available data suggest that monotherapy may be insufficient to achieve anti-tumor effects [318, 320] .

HLA-F has not been well investigated so far. It was demonstrated that HLA-F can bind to immune inhibitory receptors (ILT2, ILT4, and KIR3DL2), suggesting its potential immunosuppressive function [322] . Under malignant conditions, HLA-F expression seems to be of clinical relevance, as studies demonstrated that it functions as an unfavorable prognostic factor in various cancer types, including NSCLC [323] , esophageal squamous cell [324] , breast and gastric cancer [325, 326] . However, more comprehensive studies are needed in order to fully elucidate its function.

The non-classical MHC class I molecule HLA-G is under non-malignant conditions mainly expressed on extravillious trophoblasts [327, 328] , which invade maternal tissues during pregnancy, and promotes immune tolerance at the fetal-maternal interface [329] [330] [331] . Interestingly, many tumors, including colorectal, renal, breast, and lung tumors, have been shown to express HLA-G, which has often been associated with a worse outcome [201, [332] [333] [334] [335] [336] . In cancer, HLA-G expression can be involved in the evasion from anti-tumor immunity [337] . Indeed, HLA-G has been proposed as a new immune checkpoint in cancer [338] . Similar to other immune checkpoints, a higher expression of pro-inflammatory transcripts appears to be associated with a higher HLA-G expression in cancer, suggesting that HLA-G could be upregulated to counteract the host's immune response [339] . HLA-G exhibits immunomodulatory potential by binding to inhibitory receptors (ILT2 and ILT4) on immune cells, including NK, T, and B cells, as well as macrophages [336] . In addition to direct suppression through binding to inhibitory receptors, further regulatory effects of HLA-G include the induction of immunosuppressive cells such as Tregs as well as the intercellular transfer of HLA-G through mechanisms such as trogocytosis [336, 340, 341] .

Epigenetic mechanisms, including DNA methylation and histone modifications, which are involved in gene expression, are often dysregulated, as indicated by frequent mutation in DNA-methylating, histone-acetylating and -deacetylating, and methylating genes. These mechanisms are critical for the interactions between tumor and immune cells. It has been demonstrated that tumors commonly hijack these epigenetic mechanisms in order to escape anti-tumor immunity [342, 343] .

Epigenetic dysregulation contributes to a reduced antigen-presenting function in tumor cells, which enables tumor cells to become invisible to T cells. There are two possi-bilities to diminish antigen presentation: (1) on the gene expression levels by DNA and/or histone methylation, as well as histone deacetylation, resulting in the low expression of self and tumor antigens (neoantigens and CTAs); (2) on the MHC class I levels (antigenpresenting machinery) by the epigenetic silencing of the genes involved in the antigenpresenting machinery, such as B2M, TAP1, and TAP2 [342, 343] . Using DNA-hypomethylating agents and inhibitors of histone deacetylase and/or methyltransferase can boost tumor antigen presentation and immunogenicity. In addition, it has been observed that the same epigenetic drugs can induce co-immunostimulatory molecules [344] .

It has also been demonstrated that more than 8% of the human genome consists of HERVs arising from retroviruses that infected the human genome millions of years ago [345] . These genomic regions are predominantly silenced by DNA methylation and histone deacetylation and methylations. Treatment with DNA-hypomethylation inhibitors of histone deacetylases and/or methyltransferase restores the expression of HERVs, causing a dsRNA and/or dsDNA sensor, which finally triggers the innate immune response, especially the expression and secretion of cytokines, such as interferons, leading to the enhanced expression of MHC class I molecules on malignant cells [299, 342] .

In the TME, certain subsets of APCs, which hinder immune activation and promote tolerance, are present. These cells are usually immature DCs, expressing low levels of costimulatory factors resulting in an insufficient T cell activation [346] [347] [348] . Additionally, these immature DCs can suppress immune responses by the secretion of anti-inflammatory cytokines such as IL-10 and TGF-β [349] . Furthermore, certain DC subpopulations (CD11c low , CD45RB+) and plasmacytoid DCs can induce Tregs [350, 351] .

Investigations of the DC transcriptional signature revealed that the absence of the transcription factor Rel B, which belongs to the NF-κB family, is associated with increased populations of not only Tregs, but also IL-2-producing memory Th cells [352] . The transcription factor interferon regulatory factor 4 (IRF4) in immunogenic DCs is associated with the IL-10-and IL-33-mediated-differentiation of Th2 (and the priming of Tregs [353] ). Furthermore, another transcription factor, namely the dendritic-cell-specific transcript (DC-SCRIPT), promotes IL-10 expression, which is associated with decreased MAPK and enhanced ERK signaling [354] , and decreased IL-12 expression [355] .

Like tumor cells, DCs can also be involved in the catabolism of the amino acid tryptophan by the expression of IDO (described in detail in Section 3.1.4.4) and thereby promote an immunosuppressive condition in the TME [356, 357] . An alternative catabolic pathway causing an immunosuppressive condition in the TME is mediated by TDO (described in detail in Section 3.1.4.4), which can be expressed by specialized myeloid cells [358, 359] .

Besides the immune checkpoint components, cytokines, metabolites, and genes that are implicated in antigen presentation, numerous, mostly membrane-bound molecules also exist, that possess immune inhibitory function mainly through their impact on immune cells other than T cells.

CEACAM is a family of proteins including CEACAM1, CEACAM5, and CEACAM6. All three members play a key role in immune modulation: CEACAM1 serves as a ligand of TIM3 (described in detail in Section 3.1). CEACAM5 possesses inhibitory effects on NK-mediated tumor cell lysis, and CEACAM6 dampens myeloid and T cell activation [360] [361] [362] .

Another immunosuppressive gene is the leukemia inhibitory factor (LIF), which is a crucial factor in embryogenesis. It normally promotes an immunosuppressive microenvironment to protect the embryo from the mother's immune system. However, it has been shown that it is dysregulated under malignant conditions, favoring immunosuppressive conditions by the inhibition of cytotoxic T cell recruitment to the TME [363] .

CD47 serves as a marker of self-recognition. It binds signal regulatory protein α (SIRPα), which is located on macrophages, in order to prompt an antiphagocytic signal [364, 365] . In malignant cells it is frequently over-expressed, thereby blocking phagocytosis and favoring metastasis. A high expression of CD47 has been considered as a poor prognostic factor [365] .

Semaphorins are a transmembrane protein family that is involved in axonal repair after neuronal injury, cytoskeletal changes, and the migration of endothelial and immune cells [366] . Among this family, SEMA3A, SEMA3B, and SEMA4D (CD100) have all been implicated in the recruitment of TAMs to the TME, and they promote an immunosuppressive microenvironment [367] . SEMA4D binds three types of receptors, Plexin-B1 (PLXNB1), Plexin-B2 (PLXNB2), and CD72, which are all expressed by APCs, endothelial cells, and tumor cells [368] . Upon binding to its receptor, SEMA4D blocks the immune infiltration of active T cells and favors a shift toward Tregs. It also causes macrophage recruitment and differentiation into the M2 phenotype in the TME [368, 369] .

Common lymphatic endothelial and vascular endothelial receptor-1 (CLEVER-1) is a scavenger receptor that is expressed on endothelial cells and tissues of M2 macrophages [370] . This molecule is implicated in cell trafficking and cellular adhesion and has also been linked with immune modulation, which is mediated by M2 macrophages. Elevated CLEVER-1 levels have been associated with a poor prognosis in certain malignancies. The blockade of the pathway induces an M1 macrophage phenotype in the TME and reactivates and recruits CD8+ T cells [371] .

Axl is a tyrosine kinase receptor and a member of the TAM receptor family. It is generally expressed by platelets as well as endothelial, cardiac, hepatic, nervous, and immune cells such as monocytes, macrophages, NKs, and DCs [372] . The binding of Gas6, which is a ligand of Axl, activates phagocytosis and induces an immunosuppressive phenotype in DCs, macrophages, and NK cells [372, 373] . Axl also possesses the potential to decrease antigen presentation, to increase immunosuppressive cytokines, and to indirectly interfere with T cell activation [374] .

Phosphatidylserine (PS) is a phospholipid located in the inner layer of the plasma membrane of eukaryotic cells. Once the cell dies, the PS is exposed to the outer layer of the membrane. PS receptors are expressed by endothelial cells, MDSCs, macrophages and DCs, as well as B, T, and NK cells. PS receptors can directly or indirectly bind to PS. The PS/PSR interaction triggers efferocytosis and activates inhibitory pathways that suppress the inflammatory responses to apoptosis [375] . PS is overexpressed by the tumor, endothelial cells, the tumor vasculature, and the TME. Its levels are further increased by anti-tumor therapies that result in cancer cell death [376] . Targeting this pathway, either by blocking PS or PS receptors, can enhance immune responses against the tumor and potentiate the effects of chemotherapy and radiotherapy [375] .

In past decades, the development of immunotherapies has led to a fundamental improvement in the treatment of solid and hematological malignancies. With the rise of checkpoint inhibitors, hitherto incurable and rapidly evolving NSCLCs or malignant melanoma have changed into chronic diseases with extended prognoses in many cases. Nevertheless, a combination with chemotherapeutic agents is often required in order to obtain sufficient efficacy. In hematological malignancies, toxic chemotherapies with longterm effects such as the development of secondary malignancies have now been replaced by immunotherapies [377] [378] [379] . As shown in Table 5 , numerous candidates have been chosen to target specific molecules of the TME in order to restore immunosurveillance. 

The TME is no longer the under-investigated background of malignant cells and has become the main research field in many tumor entities. Based on these findings, novel concepts of cancer development, cancer progression, and points of action for new therapeutic approaches have been developed. Additionally, the role of the immune cells in the TME has been intensively studied. On the one hand, the tumor-promoting function of TILs has been reported, and on the other hand it was demonstrated that TILs are suppressed in their effector function, resulting in the inhibition of anti-tumor immune responses. These immune-evasive processes might be caused by T cell dysfunction induced by tumor and immune cell interaction (e.g., receptor-mediated and/or by secreted molecules) and/or altered antigen presentation. These two affected mechanisms have been investigated in a limited number of tumor entities so far, indicating the need for further comprehensive studies.

The data on the complex interplay of tumor and immune cells are rather limited and are mainly focused on the CTLA-4-CD80-CD86 and PD-1-PD-L1-PD-L2 axes for most of the cancer types. Data on less common co-inhibitory receptors and on secreted molecules are completely missing for most of the malignancies so far. Therefore, detailed investigations aimed at the comprehensive study of the TME and tumor cell interactions are mandatory in order to gain knowledge for the development of novel immune-based therapies to restore an effective anti-tumor immune response.

Collectively, there is an urgent need for genomic studies using a combination of state-of-the-art sequencing techniques (at the single cell level) and imaging-based methods on clinical tumor samples and robust preclinical models to gain knowledge about (1) how these immune-evasive mechanisms are regulated; (2) how these mechanisms can be therapeutically targeted; and (3) which patients can benefit from such therapeutic interventions. 

Molecular Aspects and Future Perspectives of Cytokine-Based Anti-Cancer Immunotherapy

Dendritic Cells in Cancer Immunology and Immunotherapy

Fibroblast Heterogeneity in Tumor Micro-Environment: Role in Immunosuppression and New Therapies

The Dark Side of Fibroblasts: Cancer-Associated Fibroblasts as Mediators of Immunosuppression in the Tumor Microenvironment

Effect of Tumor Microenvironment on Pathogenesis of the Head and Neck Squamous Cell Carcinoma: A Systematic Review

Eosinophils: The Unsung Heroes in Cancer?

Immunotherapeutic Targeting of Tumor-Associated Blood Vessels

Tumor-Derived Vascular Pericytes Anergize Th Cells

Molecular Coordination of Aβ T-Cell Receptors and Coreceptors CD8 and CD4 in Their Recognition of Peptide-MHC Ligands

CD28-Mediated Signalling Co-Stimulates Murine T Cells and Prevents Induction of Anergy in T-Cell Clones

Signal 3 Determines Tolerance versus Full Activation of Naive CD8 T Cells: Dissociating Proliferation and Development of Effector Function

Danger and OX40 Receptor Signaling Synergize to Enhance Memory T Cell Survival by Inhibiting Peripheral Deletion

Interleukin 12 Synergizes with B7/CD28 Interaction in Inducing Efficient Proliferation and Cytokine Production of Human T Cells

Hallmarks of Response to Immune Checkpoint Blockade

PD-L1/B7H-1 Inhibits the Effector Phase of Tumor Rejection by T Cell Receptor (TCR) Transgenic CD8+ T Cells

Inhibitory B7-Family Molecules in the Tumour Microenvironment

Blockade of B7-H1 Improves Myeloid Dendritic Cell-Mediated Antitumor Immunity

B7-H4 Expression Identifies a Novel Suppressive Macrophage Population in Human Ovarian Carcinoma

New Insights into the Molecular Basis of T Cell Anergy: Anergy Factors, Avoidance Sensors, and Epigenetic Imprinting

Transcriptional Complexes Formed by NFAT Dimers Regulate the Induction of T Cell Tolerance

Ikaros Enforces the Costimulatory Requirement for IL2 Gene Expression and Is Required for Anergy Induction in CD4 + T Lymphocytes

Interleukin 2 Gene Transcription Is Regulated by Ikaros-Induced Changes in Histone Acetylation in Anergic T Cells

Cell Exhaustion

Targeting Tim-3 and PD-1 Pathways to Reverse T Cell Exhaustion and Restore Anti-Tumor Immunity

Tim-3/Galectin-9 Signaling Pathway Mediates T-Cell Dysfunction and Predicts Poor Prognosis in Patients with Hepatitis B Virus-Associated Hepatocellular Carcinoma

Coexpression and Functional Cooperation of CTLA-4 and CD28 on Activated T Lymphocytes

CD8+ T Cells Specific for Tumor Antigens Can Be Rendered Dysfunctional by the Tumor Microenvironment through Upregulation of the Inhibitory Receptors BTLA and PD-1

BTLA Mediates Inhibition of Human Tumor-Specific CD8+ T Cells That Can Be Partially Reversed by Vaccination

Coregulation of CD8+ T Cell Exhaustion by Multiple Inhibitory Receptors during Chronic Viral Infection

Immune Inhibitory Molecules LAG-3 and PD-1 Synergistically Regulate T-Cell Function to Promote Tumoral Immune Escape

Bat3 Promotes T Cell Responses and Autoimmunity by Repressing Tim-3-Mediated Cell Death and Exhaustion

Molecular Signature of CD8+ T Cell Exhaustion during Chronic Viral Infection

A Role for the Transcriptional Repressor Blimp-1 in CD8+ T Cell Exhaustion during Chronic Viral Infection

Progenitor and Terminal Subsets of CD8+ T Cells Cooperate to Contain Chronic Viral Infection

Molecular Signatures of T-Cell Inhibition in HIV-1 Infection

Long-Term Culture of Monoclonal Human T Lymphocytes: Models for Immunosenescence?

Replicative Senescence in the Immune System: Impact of the Hayflick Limit on T-Cell Function in the Elderly

Cell Anergy, Exhaustion, Senescence, and Stemness in the Tumor Microenvironment

Human Immunodeficiency Virus Type 1 Clade B Superinfection: Evidence for Differential Immune Containment of Distinct Clade B Strains

A Biomarker That Identifies Senescent Human Cells in Culture and in Aging Skin in Vivo

Protocols to Detect Senescence-Associated Beta-Galactosidase (SA-Bgal) Activity, a Biomarker of Senescent Cells in Culture and in Vivo

HIV-Specific CD8+ T Cells Produce Antiviral Cytokines but Are Impaired in Cytolytic Function

Are Senescence and Exhaustion Intertwined or Unrelated Processes That Compromise Immunity?

Truly Naïve" to

Immunosenescence: A Key Player in Cancer Development

Expression of CD57 Defines Replicative Senescence and Antigen-Induced Apoptotic Death of CD8+ T Cells

Loss of T Cell Receptor-Induced Bmi-1 in the KLRG1+ Senescent CD8+ T Lymphocyte

Homeostasis and Effector Function of Lymphopenia-Induced "Memory-Like" T Cells in Constitutively T Cell-Depleted Mice

Upregulation of Tim-3 and PD-1 Expression Is Associated with Tumor Antigen-Specific CD8+ T Cell Dysfunction in Melanoma Patients

P53 Isoforms Regulate Aging-and Tumor-Associated Replicative Senescence in T Lymphocytes

Next Generation of Immune Checkpoint Inhibitors and Beyond

Co-Inhibitory Molecules of the B7-CD28 Family in the Control of T-Cell Immunity

Coinhibitory Pathways in the B7-CD28 Ligand-Receptor Family

The Interaction Properties of Costimulatory Molecules Revisited

CTLA-4: New Insights into Its Biological Function and Use in Tumor Immunotherapy

CTLA-4 and PD-1 Receptors Inhibit T-Cell Activation by Distinct Mechanisms

CTLA-4 and PD-1 Control of T-Cell Motility and Migration: Implications for Tumor Immunotherapy

Molecular and Cellular Insights into T Cell Exhaustion

Restoring Function in Exhausted CD8 T Cells during Chronic Viral Infection

The PD-1 Pathway in Tolerance and Autoimmunity

PD-L1 Expression in Human Cancers and Its Association with Clinical Outcomes

B7-H1 (Programmed Death-1 Ligand) on Dendritic Cells Is Involved in the Induction and Maintenance of T Cell Anergy

Mechanisms Regulating PD-L1 Expression on Tumor and Immune Cells

Engagement of the PD-1 Immunoinhibitory Receptor by a Novel B7 Family Member Leads to Negative Regulation of Lymphocyte Activation

PD-1 and Its Ligands in Tolerance and Immunity

PD-L1 and PD-L2 Are Differentially Regulated by Th1 and Th2 Cells

Expression of Programmed Death 1 Ligands by Murine T Cells and APC

Regulation and Function of the PD-L1 Checkpoint

Attenuator Is Highly Expressed on CMV-Specific T Cells during Infection and Regulates Their Function

B and T Lymphocyte Attenuator Regulates T Cell Activation through Interaction with Herpesvirus Entry Mediator

Roles of BTLA in Immunity and Immune Disorders

VSIG-3 as a Ligand of VISTA Inhibits Human T-Cell Function

VISTA, a Novel Mouse Ig Superfamily Ligand That Negatively Regulates T Cell Responses

Coinhibitory Receptor PD-1H Preferentially Suppresses CD4+ T Cell-Mediated Immunity

VISTA Is a Novel Broad-Spectrum Negative Checkpoint Regulator for Cancer Immunotherapy

Expression of the Immune Checkpoint VISTA in Breast Cancer

VISTA Expressed in Tumour Cells Regulates T Cell Function

Spatially Resolved and Quantitative Analysis of Vista/Pd-1h as a Novel Immunotherapy Target in Human Non-Small Cell Lung Cancer

VISTA Expression Is Associated with a Favorable Prognosis in Patients with High-Grade Serous Ovarian Cancer

VISTA: A Novel Immunotherapy Target for Normalizing Innate and Adaptive Immunity

VISTA: A Promising Target for Cancer Immunotherapy? ImmunoTargets Ther

Potential Targeting of B7-H4 for the Treatment of Cancer

B7-H3, a Checkpoint Molecule, as a Target for Cancer Immunotherapy

A Family of Cell Surface Phosphatidylserine Receptors That Regulate Innate and Adaptive Immunity

Tim-3: An Emerging Target in the Cancer Immunotherapy Landscape

Ligand Galectin-9 Negatively Regulates T Helper Type 1 Immunity

/Transmembrane, Ig, and Mucin-3 Allelic Variants Differentially Recognize Phosphatidylserine and Mediate Phagocytosis of Apoptotic Cells

Tumor-Infiltrating DCs Suppress Nucleic Acid-Mediated Innate Immune Responses through Interactions between the Receptor TIM-3 and the Alarmin HMGB1

CEACAM1 Regulates TIM-3-Mediated Tolerance and Exhaustion

Interferon-Gamma Stimulates the Expression of Galectin-9 in Cultured Human Endothelial Cells

TIM-1 and TIM-4 Glycoproteins Bind Phosphatidylserine and Mediate Uptake of Apoptotic Cells

Tim-3 Mediates Phagocytosis of Apoptotic Cells and Cross-Presentation

HMGB1 Mediates Endogenous TLR2 Activation and Brain Tumor Regression

IL-1α, IL-33 and S100 Proteins: Dual-Function Alarmins

Contact-Dependent Control of Immunity

Activating Interactions in Human NK Cell Recognition: The Role of 2B4-CD48

Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 on Murine Dendritic Cells Is a Potent Regulator of T Cell Stimulation

Lymphocyte and Macrophage Expression of Carcinoembryonic Antigen-related Adhesion Molecules That Serve as Receptors for Murine Coronavirus

Induction and Transcriptional Regulation of the Co-Inhibitory Gene Module in T Cells

IL-27 and TCR Stimulation Promote T Cell Expression of Multiple Inhibitory Receptors

An Increased Number of PD-1+ and Tim-3+ CD8+ T Cells Is Involved in Immune Evasion in Gastric Cancer

TIM-3, a Promising Target for Cancer Immunotherapy

LAG-3, a Novel Lymphocyte Activation Gene Closely Related to CD4

Reversing T-Cell Dysfunction and Exhaustion in Cancer

Lymphocytes Infiltrating Various Tumour Types Express the MHC Class II Ligand Lymphocyte Activation Gene-3 (LAG-3): Role of LAG-3/MHC Class II Interactions in Cell-Cell Contacts

Expression of LAG-3 by Tumor-Infiltrating Lymphocytes Is Coincident with the Suppression of Latent Membrane Antigen-Specific CD8+ T-Cell Function in Hodgkin Lymphoma Patients

Progression of Lung Cancer Is Associated with Increased Dysfunction of T Cells Defined by Coexpression of Multiple Inhibitory Receptors

Phenotypic Analysis of the Murine CD4-Related Glycoprotein, CD223 (LAG-3)

Cellular Expression and Tissue Distribution of the Human LAG-3-Encoded Protein, an MHC Class II Ligand

Galectin-3 Shapes Antitumor Immune Responses by Suppressing CD8 T Cells via LAG-3 and Inhibiting Expansion of Plasmacytoid Dendritic Cells

LSECtin Expressed on Melanoma Cells Promotes Tumor Progression by Inhibiting Antitumor T-Cell Responses

Galectin-3: An Open-Ended Story

Fibrinogen-like Protein 1 Is a Major Immune Inhibitory Ligand of LAG-3

CD3/TCR Complex-Associated Lymphocyte Activation Gene-3 Molecules Inhibit CD3/TCR Signaling

Functional Role of Human NK Cell Receptor 2B4 (CD244) Isoforms

2B4 (CD244) and CS1: Novel Members of the CD2 Subset of the Immunoglobulin Superfamily Molecules Expressed on Natural Killer Cells and Other Leukocytes

2B4, a New Member of the Immunoglobulin Gene Superfamily, Is Expressed on Murine Dendritic Epidermal T Cells and Plays a Functional Role in Their Killing of Skin Tumors

The X-Linked Lymphoproliferative-Disease Gene Product SAP Regulates Signals Induced through the Co-Receptor SLAM

Structural Basis for the Interaction of the Free SH2 Domain EAT-2 with SLAM Receptors in Hematopoietic Cells

Negative Regulation of Natural Killer Cell Function by EAT-2, a SAP-Related Adaptor

Molecular Dissection of 2B4 Signaling: Implications for Signal Transduction by SLAM-Related Receptors

Pseudo-Exhaustion of CD8+ T Cells in AML

Exhaustion of Tumor-Specific CD8+ T Cells in Metastases from Melanoma Patients

TIGIT and CD96: New Checkpoint Receptor Targets for Cancer Immunotherapy

The Surface Protein TIGIT Suppresses T Cell Activation by Promoting the Generation of Mature Immunoregulatory Dendritic Cells

The Interaction of TIGIT with PVR and PVRL2 Inhibits Human NK Cell Cytotoxicity

Clinical Significance of CD155 Expression in Human Pancreatic Cancer

Vstm3 Is a Member of the CD28 Family and an Important Modulator of T-Cell Function

The Human Poliovirus Receptor Related 2 Protein Is a New Hematopoietic/Endothelial Homophilic Adhesion Molecule

Identification of CD112R as a Novel Checkpoint for Human T Cells

Coming of Age: CD96 Emerges as Modulator of Immune Responses

The TIGIT/CD226 Axis Regulates Human T Cell Function

Arenas-Hernandez, M. Immune Cells in Term and Preterm Labor

Shibuya, A. Functional Characterization of DNAM-1 (CD226) Interaction with Its Ligands PVR (CD155) and Nectin

The Murine Pan T Cell Marker CD96 Is an Adhesion Receptor for CD155 and Nectin-1

The Immunoreceptor TIGIT Regulates Antitumor and Antiviral CD8+ T Cell Effector Function

T-Cell Immunoglobulin and ITIM Domain (TIGIT) Associates with CD8+ T-Cell Exhaustion and Poor Clinical Outcome in AML Patients

TIGIT and PD-1 Impair Tumor Antigen-Specific CD8+ T Cells in Melanoma Patients

Expression of Lymphocyte-Activating Gene 3 and T-Cell Immunoreceptor with Immunoglobulin and ITIM Domains in Cutaneous Melanoma and Their Correlation with Programmed Cell Death 1 Expression in Tumor-Infiltrating Lymphocytes

CD155T/TIGIT Signaling Regulates CD8+ T-Cell Metabolism and Promotes Tumor Progression in Human Gastric Cancer

Antibodies to the Major Insoluble Milk Fat Globule Membrane-Associated Protein: Specific Location in Apical Regions of Lactating Epithelial Cells

Butyrophilin, a Milk Protein, Modulates the Encephalitogenic T Cell Response to Myelin Oligodendrocyte Glycoprotein in Experimental Autoimmune Encephalomyelitis

Structure and Dimerization of a Soluble Form of B7-1

Butyrophilins: An Emerging Family of Immune Regulators

Cloning, Localization, and Structure of New Members of the Butyrophilin Gene Family in the Juxta-Telomeric Region of the Major Histocompatibility Complex

Butyrophilins: An Important New Element of Resistance. Cent

Like Molecule That Functions to Inhibit T Cell Activation

BTNL2, a Butyrophilin/B7-Like Molecule, Is a Negative Costimulatory Molecule Modulated in Intestinal Inflammation

BTN1A1, the Mammary Gland Butyrophilin, and BTN2A2 Are Both Inhibitors of T Cell Activation

BTN3A2 Expression in Epithelial Ovarian Cancer Is Associated with Higher Tumor Infiltrating T Cells and a Better Prognosis

BTN3A1 Governs Antitumor Responses by Coordinating Aβ and Γδ T Cells

Beyond CTLA-4 and PD-1: Orphan Nuclear Receptor NR2F6 as T Cell Signaling Switch and Emerging Target in Cancer Immunotherapy

RNA Binding Protein Pcbp1 Is an Intracellular Immune Checkpoint for Shaping t Cell Responses in Cancer Immunity

AP-1 Transcription Factors as Regulators of Immune Responses in Cancer

HDAC3 Modulates Cancer Immunity via Increasing PD-L1 Expression in Pancreatic Cancer

Master Regulator of Immune Privilege

ERα Is a Negative Regulator of PD-L1 Gene Transcription in Breast Cancer

The Role of Hypoxia-Inducible Factor 1 in Tumor Immune Evasion

Transcriptional Regulation of Cancer Immune Checkpoints: Emerging Strategies for Immunotherapy

Aberrantly Expressed C-Jun and JunB Are a Hallmark of Hodgkin Lymphoma Cells, Stimulate Proliferation and Synergize with NF-KB

Constitutive AP-1 Activity and EBV Infection Induce PD-L1 in Hodgkin Lymphomas and Posttransplant Lymphoproliferative Disorders: Implications for Targeted Therapy

Oncogenic Kinase NPM/ALK Induces through STAT3 Expression of Immunosuppressive Protein CD274 (PD-L1, B7-H1). Proc. Natl. Acad. Sci

Targeting Hypoxia at the Forefront of Anticancer Immune Responses

The PI3K-AKT-MTOR Pathway and Prostate Cancer: At the Crossroads of AR, MAPK, and WNT Signaling

Integrative Analysis Reveals Selective 9p24.1 Amplification, Increased PD-1 Ligand Expression, and Further Induction via JAK2 in Nodular Sclerosing Hodgkin Lymphoma and Primary Mediastinal Large B-Cell Lymphoma

Genetic Basis of PD-L1 Overexpression in Diffuse Large B-Cell Lymphomas

Genomic Amplification of CD274 (PD-L1) in Small-Cell Lung Cancer

Follicular Regulatory T Cells Control Humoral Autoimmunity via NFAT2-Regulated CXCR5 Expression

Cutting Edge: NFAT Transcription Factors Promote the Generation of Follicular Helper T Cells in Response to Acute Viral Infection

NFATc1 Regulates PD-1 Expression upon T Cell Activation

The CD40L Promoter Contains Nuclear Factor of Activated T Cells-Binding Motifs Which Require AP-1 Binding for Activation of Transcription

NFAT1 and NFAT2 Are Positive Regulators of IL-4 Gene Transcription

Calcium-Dependent Activation of Interleukin-21 Gene Expression in T Cells

Genome-Wide Analysis Identifies NR4A1 as a Key Mediator of T Cell Dysfunction

NR4A Transcription Factors Limit CAR T Cell Function in Solid Tumours

The Transcription Factor NFAT Promotes Exhaustion of Activated CD8+ T Cells. Immunity

Transcriptional Mechanisms Underlying Lymphocyte Tolerance

Dynamic Changes in Chromatin Accessibility Occur in CD8+ T Cells Responding to Viral Infection

Exhaustion-Associated Regulatory Regions in CD8+ Tumor-Infiltrating T Cells

TOX Defines a Conserved Subfamily of HMG-Box Proteins

TOX Transcriptionally and Epigenetically Programs CD8+ T Cell Exhaustion

TOX Reinforces the Phenotype and Longevity of Exhausted T Cells in Chronic Viral Infection

TOX Is a Critical Regulator of Tumour-Specific T Cell Differentiation

Single-Cell RNA-Seq Reveals TOX as a Key Regulator of CD8+ T Cell Persistence in Chronic Infection

TOX and TOX2 Transcription Factors Cooperate with NR4A Transcription Factors to Impose CD8+ T Cell Exhaustion

The Roles of MicroRNAs in Regulating the Expression of PD-1/PD-L1 Immune Checkpoint

MiR-28 Modulates Exhaustive Differentiation of T Cells through Silencing Programmed Cell Death-1 and Regulating Cytokine Secretion

Attenuator Is a Target of MiR-155 during Naive CD4+ T Cell Activation

MiR-155 Contributes to Df1-Induced Asthma by Increasing the Proliferative Response of Th Cells via CTLA-4 Downregulation

NKX2-1-AS1 Negatively Regulates CD274/PD-L1, Cell-Cell Interaction Genes, and Limits Human Lung Carcinoma Cell Migration

Cytokines in the Treatment of

Interferon-γ Is a Master Checkpoint Regulator of Cytokine-Induced Differentiation

IFN-γ from Lymphocytes Induces PD-L1 Expression and Promotes Progression of Ovarian Cancer

Identification of Discrete Tumor-Induced Myeloid-Derived Suppressor Cell Subpopulations with Distinct T Cell Suppressive Activity

Tumor-Specific up-Regulation of the Nonclassical Class I HLA-G Antigen Expression in Renal Carcinoma

TNF-Alpha in Cancer Treatment: Molecular Insights, Antitumor Effects, and Clinical Utility

Roles of IFN-γ in Tumor Progression and Regression: A Review

IL-1 Family Members in Cancer; Two Sides to Every Story

Impact of IL-12 in Cancer. Curr. Cancer Drug Targets

Role of IL-2 in Cancer Immunotherapy

IL-15 Is an Essential Mediator of Peripheral NK-Cell Homeostasis

Interleukin (IL)-2 and IL-15 Have Different Effects on Human Natural Killer Lymphocytes

Interleukin-21 in Cancer Immunotherapy: Friend or Foe? Oncoimmunology

Antitumor Effects of Interleukin-6 (IL-6)/Interleukin-6 Receptor (IL-6R) Signaling Pathway Inhibition in Clear Cell Carcinoma of the Ovary

Biology and Therapeutic Targeting of Tumour-Associated Macrophages

Interferons α and β in Cancer: Therapeutic Opportunities from New Insights

IL-1 and IL-1 Regulatory Pathways in Cancer Progression and Therapy

The IL-8/IL-8R Axis: A Double Agent in Tumor Immune Resistance

The Paradoxical Role of IL-10 in Immunity and Cancer

Master Switch from Tumor-Promoting Inflammation to Antitumor Immunity

IL-4 Blockade Alters the Tumor Microenvironment and Augments the Response to Cancer Immunotherapy in a Mouse Model

Targeting of IL-4 and IL-13 Receptors for Cancer Therapy

Dual Roles of IL-27 in Cancer Biology and Immunotherapy

An IL-27/NFIL3 Signalling Axis Drives Tim-3 and IL-10 Expression and T-Cell Dysfunction

Transforming Growth Factor-β Signaling in Immunity and Cancer. Immunity

Signaling in Innate Immunity and Inflammation

Promising Targets for Cancer Immunotherapy: TLRs, RLRs, and STING-Mediated Innate Immune Pathways

Promising Targets Based on Pattern Recognition Receptors for Cancer Immunotherapy

Pattern Recognition Receptors in Health and Diseases

The Next Generation of Pattern Recognition Receptor Agonists: Improving Response Rates in Cancer Immunotherapy

Innate Immune Recognition

Endogenous Ligands of Toll-like Receptors: Implications for Regulating Inflammatory and Immune Responses

Mammalian Toll-like Receptors: To Immunity and Beyond

Toll-like Receptor Signalling

Inferences, Questions and Possibilities in Toll-like Receptor Signalling

Negative Regulation of Toll-like Receptor-Mediated Immune Responses

The Role of TLRs in Anti-Cancer Immunity and Tumor Rejection

NOD-like Receptors: Master Regulators of Inflammation and Cancer

Nod-like Receptors: Key Molecular Switches in the Conundrum of

Targeting C-Type Lectin Receptors for Cancer Immunity

Prostaglandin E2 as a Potent Therapeutic Target for Treatment of Colon Cancer

COX-2 and PGE2-Dependent Immunomodulation in Breast Cancer

Increased Expression of Cyclooxygenase 2 Occurs Frequently in Human Lung Cancers, Specifically in Adenocarcinomas

COX-2-Dependent Stabilization of Survivin in Non-Small Cell Lung Cancer

EP2, a Receptor for PGE2, Regulates Tumor Angiogenesis through Direct Effects on Endothelial Cell Motility and Survival

Cyclo-Oxygenase 2 Expression Is Associated with Angiogenesis and Lymph Node Metastasis in Human Breast Cancer

Inhibition of Ep3 Attenuates Migration and Promotes Apoptosis of Non-Small Cell Lung Cancer Cells via Suppression of TGF-β/Smad Signaling

Role of β-Arrestin 1 in the Metastatic Progression of Colorectal Cancer

Cyclooxygenase-Dependent Tumor Growth through Evasion of Immunity

Oncology Meets Immunology: The Cancer-Immunity Cycle

Reis e Sousa, C. NK Cells Stimulate Recruitment of CDC1 into the Tumor Microenvironment Promoting Cancer Immune Control

Plasmacytoid Dendritic Cells Cross-Prime Naive CD8 T Cells by Transferring Antigen to Conventional Dendritic Cells through Exosomes

Tumor-Residing Batf3 Dendritic Cells Are Required for Effector T Cell Trafficking and Adoptive T Cell Therapy

Dissecting the Tumor Myeloid Compartment Reveals Rare Activating Antigen-Presenting Cells Critical for T Cell Immunity

Prostaglandin E2 Promotes Tumor Progression by Inducing Myeloid-Derived Suppressor Cells

Positive Feedback between PGE2 and COX2 Redirects the Differentiation of Human Dendritic Cells toward Stable Myeloid-Derived Suppressor Cells

Doxorubicin Resistant Cancer Cells Activate Myeloid-Derived Suppressor Cells by Releasing PGE2

Prostaglandin E2 Leads to the Acquisition of DNMT3A-Dependent Tolerogenic Functions in Human Myeloid-Derived Suppressor Cells

Melanoma-Educated CD14+ Cells Acquire a Myeloid-Derived Suppressor Cell Phenotype through COX-2-Dependent Mechanisms

Altered Expression of 15-Hydroxyprostaglandin Dehydrogenase in Tumor-Infiltrated CD11b Myeloid Cells: A Mechanism for Immune Evasion in Cancer

Prostaglandin E2 2 Signaling and Bacterial Infection Recruit Tumor-Promoting Macrophages to Mouse Gastric Tumors

Tumor Cyclooxygenase-2/Prostaglandin E2-Dependent Promotion of FOXP3 Expression and CD4+CD25+ T Regulatory Cell Activities in Lung Cancer

Antagonistic Inflammatory Phenotypes Dictate Tumor Fate and Response to Immune Checkpoint Blockade

Immunometabolism in Cancer at a Glance. DMM Dis

Metabolic Heterogeneity in Human Lung Tumors

Mechanisms and Implications of Metabolic Heterogeneity in Cancer

Quantification of Microenvironmental Metabolites in Murine Cancers Reveals Determinants of Tumor Nutrient Availability

Metabolic Regulation of Cell Growth and Proliferation

Metabolic Competition in the Tumor Microenvironment Is a Driver of Cancer Progression

The Spectrum of T Cell Metabolism in Health and Disease

Metabolic Regulation of T Cell Longevity and Function in Tumor Immunotherapy

Nutrient Competition: A New Axis of Tumor Immunosuppression

Cancer Mediates Effector T Cell Dysfunction by Targeting MicroRNAs and EZH2 via Glycolysis Restriction

Molecular Characterization and Clinical Relevance of Metabolic Expression Subtypes in Human Cancers

Tryptophan Deprivation Sensitizes Activated T Cells to Apoptosis Prior to Cell Division

MYC Promotes Tryptophan Uptake and Metabolism by the Kynurenine Pathway in Colon Cancer

Role of IDO and TDO in Cancers and Related Diseases and the Therapeutic Implications

Tryptophan Catabolism in Cancer: Beyond IDO and Tryptophan Depletion

Inhibition of Tryptophan-Dioxygenase Activity Increases the Antitumor Efficacy of Immune Checkpoint Inhibitors

Oxygen Sensing by T Cells Establishes an Immunologically Tolerant Metastatic Niche

Lactate in the Regulation of Tumor Microenvironment and Therapeutic Approaches

Cancer Acidity: An Ultimate Frontier of Tumor Immune Escape and a Novel Target of Immunomodulation

Ionic Immune Suppression within the Tumour Microenvironment Limits T Cell Effector Function

Stemness and Dysfunction in Tumors Are Triggered by a Common Mechanism

Fasting Selectively Blocks Development of Acute Lymphoblastic Leukemia via Leptin-Receptor Upregulation

Caloric Restriction Mimetics Enhance Anticancer Immunosurveillance

Increased Tumor Glycolysis Characterizes Immune Resistance to Adoptive T Cell Therapy

Targeting Adenosine for Cancer Immunotherapy

Comprehensive Evaluation of NT5E/CD73 Expression and Its Prognostic Significance in Distinct Types of Cancers

A2aR Antagonists: Next Generation Checkpoint Blockade for Cancer Immunotherapy

Metabolic Landscape of the Tumor Microenvironment at Single Cell Resolution

Metabolic Enzymes Expressed by Cancer Cells Impact the Immune Infiltrate

Present Yourself! By MHC Class I and MHC Class II Molecules

Role of Human Leukocyte Antigen System as A Predictive Biomarker for Checkpoint-Based Immunotherapy in Cancer Patients

Major Histocompatibility Complex (MHC) Class I and MHC Class II Proteins: Conformational Plasticity in Antigen Presentation

Atypical MHC Class II-Expressing Antigen-Presenting Cells: Can Anything Replace a Dendritic Cell?

Biological Consequences of MHC-II Expression by Tumor Cells in Cancer

Alternative Tumour-Specific Antigens

Tumor Neoantigens: Building a Framework for Personalized Cancer Immunotherapy

Cancer Germline Gene Activation: Friend or Foe? Cell Cycle

A Few Good Peptides: MHC Class I-Based Cancer Immunosurveillance and Immunoevasion

Cancer Immune Escape: MHC Expression in Primary Tumours versus Metastases

MHC Class I Downregulation in Cancer: Underlying Mechanisms and Potential Targets for Cancer Immunotherapy

Cancer Immune Evasion through Loss of MHC Class I Antigen Presentation

Hard" and "Soft" Lesions Underlying the HLA Class I Alterations in Cancer Cells: Implications for Immunotherapy

The Regulatory Network behind MHC Class I Expression

Natural Killer Cell Recognition of Missing Self

Search of the 'Missing Self': MHC Molecules and NK Cell Recognition

Human NK Cell Education by Inhibitory Receptors for MHC Class I

Characterizing the Dysfunctional NK Cell: Assessing the Clinical Relevance of Exhaustion

The Molecular Mechanism of Natural Killer Cells Function and Its Importance in Cancer Immunotherapy

Cytokine Therapy Reverses NK Cell Anergy in MHC-Deficient Tumors

IL-21-Mediated Reversal of NK Cell Exhaustion Facilitates Anti-Tumour Immunity in MHC Class I-Deficient Tumours

The Urgent Need to Recover MHC Class I in Cancers for Effective Immunotherapy

NK Cell-Based Cancer Immunotherapy: From Basic Biology to Clinical Development

Anti-NKG2A MAb Is a Checkpoint Inhibitor That Promotes Anti-Tumor Immunity by Unleashing Both T and NK Cells

The NKG2A Immune Checkpoint-A New Direction in Cancer Immunotherapy

Conserved and Variable Natural Killer Cell Receptors: Diverse Approaches to Viral Infections

Adding to the Checkpoint Blockade Armamentarium

The NKG2A-HLA-E Axis as a Novel Checkpoint in the Tumor Microenvironment

Immunomodulation of Classical and Non-Classical HLA Molecules by Ionizing Radiation

HLA-F Expression Is a Prognostic Factor in Patients with Non-Small-Cell Lung Cancer

Alteration of HLA-F and HLA I Antigen Expression in the Tumor Is Associated with Survival in Patients with Esophageal Squamous Cell Carcinoma

Expression in Gastric Cancer-PubMed

Clinical Implication of Human Leukocyte Antigen (HLA)-F Expression in Breast Cancer

Human Leucocyte Antigen (HLA) Expression of Primary Trophoblast Cells and Placental Cell Lines, Determined Using Single Antigen Beads to Characterize Allotype Specificities of Anti-HLA Antibodies

Protection from Natural Killer Cell-Mediated Lysis by HLA-G Expression on Target Cells

Roles of HLA-G in the Maternal-Fetal Immune Microenvironment. Front. Immunol. 2020, 11, 592010

At the Interface of Maternal-Fetal Tolerance

Expression in Classical HLA Class I-Negative Tumors Is of Prognostic Value for Clinical Outcome of Early Breast Cancer Patients

Combined Analysis of HLA Class I, HLA-E and HLA-G Predicts Prognosis in Colon Cancer Patients

Expression of Human Leucocyte Antigen G (HLA-G) Is Associated with Prognosis in Non-Small Cell Lung Cancer

Prognostic and Clinicopathological Value of Human Leukocyte Antigen G in Gastrointestinal Cancers: A Meta-Analysis. Front. Oncol. 2021, 11, 642902

Heterogeneity of HLA-G Expression in Cancers: Facing the Challenges

Human Leukocyte Antigen-G (HLA-G) Expression in Cancers: Roles in Immune Evasion, Metastasis and Target for Therapy

An Immune Checkpoint Molecule

HLA-G: A New Immune Checkpoint in Cancer?

Intercellular transfer of HLA-G: its potential in cancer immunology

Targeting the Epigenetic Regulation of Antitumour Immunity

a Promising Anti-PD1 Clinical Candidate Antibody Derived from a New Chicken Antibody Discovery Platform

Human endogenous retroviruses and their implication for immunotherapeutics of cancer

Tolerogenic Dendritic Cells

Dendritic Cells Induce Peripheral T Cell Unresponsiveness under Steady State Conditions in Vivo

Decay-Accelerating Factor Modulates Induction of T Cell Immunity

Lipopolysaccharide-Enhanced, Toll-like Receptor 4-Dependent T Helper Cell Type 2 Responses to Inhaled Antigen

Characterization of Dendritic Cells That Induce Tolerance and T Regulatory 1 Cell Differentiation in Vivo

Alloantigen-Presenting Plasmacytoid Dendritic Cells Mediate Tolerance to Vascularized Grafts

RelB+ Steady-State Migratory Dendritic Cells Control the Peripheral Pool of the Natural Foxp3+ Regulatory T Cells. Front

Transcription Factor IRF4 Drives Dendritic Cells to Promote Th2 Differentiation

Dendritic Cells Actively Limit Interleukin-10 Production under Inflammatory Conditions via DC-SCRIPT and Dual-Specificity Phosphatase 4

Dendritic Cell-Specific Transcript: Dendritic Cell Marker and Regulator of TLR-Induced Cytokine Production

IDO Expression by Dendritic Cells: Tolerance and Tryptophan Catabolism

Immunosuppressive Enzymes in the Tumor Microenvironment

Cancer Immunotherapy by Targeting IDO1/TDO and Their Downstream Effectors

Towards Understanding the Mechanisms of Actions of Carcinoembryonic Antigen-Related Cell Adhesion Molecule 6 in Cancer Progression

Emerging Role and Targeting of Carcinoembryonic Antigen-Related Cell Adhesion Molecule 6 (CEACAM6) in Human Malignancies

On the Dual Role of Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1 (CEACAM1) in Human Malignancies

LIF Regulates CXCL9 in Tumor-Associated Macrophages and Prevents CD8+ T Cell Tumor-Infiltration Impairing Anti-PD1 Therapy

Cancer Immunotherapy Targeting the CD47/SIRPα Axis

CD47 Is a Novel Potent Immunotherapy Target in Human Malignancies: Current Studies and Future Promises

Role of Semaphorins during Axon Growth and Guidance

Importance of Semaphorins in Cancer Immunity

Antibody Blockade of Semaphorin 4D Promotes Immune Infiltration into Tumor and Enhances Response to Other Immunomodulatory Therapies

Linking Tumor-Associated Macrophages, Inflammation, and Intestinal Tumorigenesis: Role of MCP-1

Stabilin-1, a Homeostatic Scavenger Receptor with Multiple Functions

Immunotherapeutic Blockade of Macrophage Clever-1 Reactivates the CD8 + T-Cell Response against Immunosuppressive Tumors

Biology of the TAM Receptors

AXL Receptor Tyrosine Kinase as a Promising Anti-Cancer Approach: Functions, Molecular Mechanisms and Clinical Applications

Molecular Pathways: Oncologic Pathways and Their Role in T-Cell Exclusion and Immune Evasion-A New Role for the AXL Receptor Tyrosine Kinase

Targeting: Implications for Cancer Immunotherapy

Phosphatidylserine-Targeting Antibody Induces M1 Macrophage Polarization and Promotes Myeloid-Derived Suppressor Cell Differentiation

The Evolving Role of Immune Checkpoint Inhibitors in Cancer Treatment

Immunotherapy for Hematological Malignancies

Combinatorial Approach to Improve Cancer Immunotherapy: Rational Drug Design Strategy to Simultaneously Hit Multiple Targets to Kill Tumor Cells and to Activate the Immune System

Adjuvant Ipilimumab versus Placebo after Complete Resection of High-Risk Stage III Melanoma (EORTC 18071): A Randomised, Double-Blind, Phase 3 Trial

Nivolumab plus Ipilimumab versus Sunitinib in Advanced Renal-Cell Carcinoma

Nivolumab in Patients with Metastatic DNA Mismatch Repair-Deficient or Microsatellite Instability-High Colorectal Cancer (CheckMate 142): An Open-Label, Multicentre, Phase 2 Study

Efficacy and Safety of Nivolumab plus Ipilimumab in Patients with Advanced Hepatocellular Carcinoma Previously Treated with Sorafenib: The CheckMate 040 Randomized Clinical Trial

Nivolumab plus Ipilimumab in Advanced Non-Small-Cell Lung Cancer

First-Line Nivolumab plus Ipilimumab Combined with Two Cycles of Chemotherapy in Patients with Non-Small-Cell Lung Cancer (CheckMate 9LA): An International, Randomised, Open-Label, Phase 3 Trial

First-Line Nivolumab plus Ipilimumab in Unresectable Malignant Pleural Mesothelioma (CheckMate 743): A Multicentre, Randomised, Open-Label, Phase 3 Trial

Durvalumab with or without Tremelimumab for Patients with Metastatic Pancreatic Ductal Adenocarcinoma: A Phase 2 Randomized Clinical Trial

Phase III Randomized Clinical Trial Comparing Tremelimumab with Standard-of-Care Chemotherapy in Patients with Advanced Melanoma

CheckMate 025 Randomized Phase 3 Study: Outcomes by Key Baseline Factors and Prior Therapy for Nivolumab Versus Everolimus in Advanced Renal Cell Carcinoma

Nivolumab for Relapsed/Refractory Classic Hodgkin Lymphoma After Failure of Autologous Hematopoietic Cell Transplantation: Extended Follow-Up of the Multicohort Single-Arm Phase II CheckMate 205 Trial

CheckMate 037): A Randomised, Controlled, Open-Label, Phase 3 Trial

Nivolumab in Previously Untreated Melanoma without BRAF Mutation

Nivolumab Alone and Nivolumab plus Ipilimumab in Recurrent Small-Cell Lung Cancer (CheckMate 032): A Multicentre, Open-Label, Phase 1/2 Trial

Five-Year Outcomes from the Randomized, Phase Iii Trials Checkmate 017 and 057: Nivolumab versus Docetaxel in Previously Treated Non-Small-Cell Lung Cancer

Nivolumab versus Docetaxel in Advanced Nonsquamous Non-Small-Cell Lung Cancer

Results of KEYNOTE-240: Phase 3 Study of Pembrolizumab (Pembro) vs Best Supportive Care (BSC) for Second Line Therapy in Advanced Hepatocellular Carcinoma (HCC)

Pembrolizumab versus Investigator-Choice Chemotherapy for Ipilimumab-Refractory Melanoma (KEYNOTE-002): A Randomised, Controlled, Phase 2 Trial

Efficacy and Safety of Pembrolizumab in Previously Treated Advanced Cervical Cancer: Results from the Phase II KEYNOTE-158

Pembrolizumab plus Axitinib versus Sunitinib for Advanced Renal-Cell Carcinoma

Pembrolizumab Alone or with Chemotherapy versus Cetuximab with Chemotherapy for Recurrent or Metastatic Squamous Cell Carcinoma of the Head and Neck (KEYNOTE-048): A Randomised, Open-Label, Phase 3 Study

Randomized Phase III KEYNOTE-181 Study of Pembrolizumab Versus Chemotherapy in Advanced Esophageal Cancer

Efficacy and Safety of Pembrolizumab for Heavily Pretreated Patients with Advanced, Metastatic Adenocarcinoma or Squamous Cell Carcinoma of the Esophagus: The Phase 2 KEYNOTE-180 Study

Pembrolizumab in Microsatellite-Instability-High Advanced Colorectal Cancer

Pembrolizumab versus Docetaxel for Previously Treated, PD-L1-Positive, Advanced Non-Small-Cell Lung Cancer (KEYNOTE-010): A Randomised Controlled Trial

Pembrolizumab versus Chemotherapy for PD-L1-Positive Non-Small-Cell Lung Cancer

Pembrolizumab versus Chemotherapy for Previously Untreated, PD-L1-Expressing, Locally Advanced or Metastatic Non-Small-Cell Lung Cancer (KEYNOTE-042): A Randomised, Open-Label, Controlled, Phase 3 Trial

Pembrolizumab plus Chemotherapy in Metastatic Non-Small-Cell Lung Cancer

Pembrolizumab plus Chemotherapy for Squamous Non-Small-Cell Lung Cancer

Pembrolizumab in Patients with Extensive-Stage Small-Cell Lung Cancer: Results from the Phase Ib KEYNOTE-028 Study

Association of Tumour Mutational Burden with Outcomes in Patients with Advanced Solid Tumours Treated with Pembrolizumab: Prospective Biomarker Analysis of the Multicohort, Open-Label, Phase 2 KEYNOTE-158 Study

Pembrolizumab in Patients with MSI-H Advanced Endometrial Cancer from the KEYNOTE-158 Study

Clinical Activity and Safety of the Anti-Programmed Death 1 Monoclonal Antibody Dostarlimab for Patients with Recurrent or Advanced Mismatch Repair-Deficient Endometrial Cancer: A Nonrandomized Phase 1 Clinical Trial

Dostarlimab in the Treatment of Recurrent or Primary Advanced Endometrial Cancer

Toripalimab: First Global Approval

Atezolizumab for First-Line Treatment of PD-L1-Selected Patients with NSCLC

Atezolizumab with or without Chemotherapy in Metastatic Urothelial Cancer (IMvigor130): A Multicentre, Randomised, Placebo-Controlled Phase 3 Trial

Atezolizumab versus Docetaxel in Patients with Previously Treated Non-Small-Cell Lung Cancer (OAK): A Phase 3, Open-Label, Multicentre Randomised Controlled Trial

Durvalumab after Chemoradiotherapy in Stage III Non-Small-Cell Lung Cancer

A Randomised Phase II Study Investigating Durvalumab in Addition to an Anthracycline Taxane-Based Neoadjuvant Therapy in Early Triple-Negative Breast Cancer: Clinical Results and Biomarker Analysis of GeparNuevo Study

Durvalumab in Urothelial Cancers

Efficacy and Safety of First-Line Avelumab Treatment in Patients with Stage IV Metastatic Merkel Cell Carcinoma a Preplanned Interim Analysis of a Clinical Trial

Avelumab plus Axitinib versus Sunitinib for Advanced Renal-Cell Carcinoma

Updated Efficacy Results from the JAVELIN Renal 101 Trial: First-Line Avelumab plus Axitinib versus Sunitinib in Patients with Advanced Renal Cell Carcinoma

Avelumab Maintenance Therapy for Advanced or Metastatic Urothelial Carcinoma

Characterization of a Novel Anti-Human Lymphocyte Activation Gene 3 (LAG-3) Antibody for Cancer Immunotherapy

Expression of LAG-3 and Efficacy of Combination Treatment with Anti-LAG-3 and Anti-PD-1 Monoclonal Antibodies in Glioblastoma

Blocking TIM-3 in Treatment-Refractory Advanced Solid Tumors: A Phase Ia/b Study of LY3321367 with or without an Anti-PD-L1

A Phase Ia/b Study of TIM-3/PD-L1 Bispecific Antibody in Patients with Advanced Solid Tumors

Simplify-1: A Phase III Randomized Trial of Momelotinib versus Ruxolitinib in Janus Kinase Inhibitor-Naïve Patients with Myelofibrosis

Phase 1B Study of Momelotinib Combined with Trametinib in Metastatic, Kirsten Rat Sarcoma Viral Oncogene Homolog-Mutated Non-Small-Cell Lung Cancer After Platinum-Based Chemotherapy Treatment Failure

Long-Term Survival in Patients Treated with Ruxolitinib for Myelofibrosis: COMFORT-I and-II Pooled Analyses

Ruxolitinib in GvHD (RIG) Study: A Multicenter, Randomized Phase 2 Trial to Determine the Response Rate of Ruxolitinib and Best Available Treatment (BAT) versus BAT in Steroid-Refractory Acute Graft-versus-Host Disease (AGvHD) (NCT02396628)

Anti-Adult T-Cell Leukemia/Lymphoma Activity of Cerdulatinib, a Dual SYK/JAK Kinase Inhibitor

Efficacy and Safety of the Dual SYK/JAK Inhibitor Cerdulatinib in Patients with Relapsed or Refractory B-Cell Malignancies: Results of a Phase I Study

Phase 2 Study of Gandotinib (LY2784544) in Patients with Myeloproliferative Neoplasms

A Randomized Assessment of Adding the Kinase Inhibitor Lestaurtinib to First-Line Chemotherapy for FLT3-Mutated AML

Phase I Dose Escalation Study of Lestaurtinib in Patients with Myelofibrosis

Pacritinib vs Best Available Therapy, Including Ruxolitinib

Aldesleukin in Advanced Renal Cell Carcinoma

NKTR-214) plus Nivolumab in Patients with Advanced Solid Tumors: Phase I Dose-Escalation Study of Safety, Effi Cacy, and Immune Activation (PIVOT-02)

Galunisertib plus Gemcitabine vs. Gemcitabine for First-Line Treatment of Patients with Unresectable Pancreatic Cancer

A Phase 2 Study of Galunisertib (TGF-B1 Receptor Type i Inhibitor) and Sorafenib in Patients with Advanced Hepatocellular Carcinoma

Phase II Trial of Adjuvant Dendritic Cell Vaccine in Combination with Celecoxib, Interferon-α, and Rintatolimod in Patients Undergoing Cytoreductive Surgery and Hyperthermic Intraperitoneal Chemotherapy for Peritoneal Metastases

Imiquimod and the Treatment of Cutaneous T-Cell Proliferative Diseases: At the Threshold

Imiquimod in Mycosis Fungoides

Investigative Drugs for the Treatment of Cutaneous T-Cell Lymphomas (CTCL): An Update

Topical Resiquimod Can Induce Disease Regression and Enhance T-Cell Effector Functions in Cutaneous T-Cell Lymphoma

Results from an Integrated Safety Analysis of Urelumab, an Agonist Anti-CD137 Monoclonal Antibody

Safety and Activity of Varlilumab, a Novel and First-in-Class Agonist Anti-CD27 Antibody, for Hematologic Malignancies

Safety and Activity of Varlilumab, a Novel and First-in-Class Agonist Anti-CD27 Antibody, in Patients with Advanced Solid Tumors

Daratumumab plus Lenalidomide and Dexamethasone for Untreated Myeloma

Lenalidomide, Bortezomib, and Dexamethasone for Transplant-Eligible Newly Diagnosed Multiple Myeloma: The GRIFFIN Trial

How We Manage Adults with Myelodysplastic Syndrome

AUGMENT: A Phase III Study of Lenalidomide plus Rituximab versus Placebo plus Rituximab in Relapsed or Refractory Indolent Lymphoma

Cereblon Is a Direct Protein Target for Immunomodulatory and Antiproliferative Activities of Lenalidomide and Pomalidomide

Drug Therapy: Multiple Myeloma

Elotuzumab plus Pomalidomide and Dexamethasone for Multiple Myeloma