key: cord-0924838-4mchpe0c
authors: Sheu, Ting-Ting; Chiang, Bor-Luen
title: Lymphopenia, Lymphopenia-Induced Proliferation, and Autoimmunity
date: 2021-04-16
journal: Int J Mol Sci
DOI: 10.3390/ijms22084152
sha: cb1cab50ced327ee26254ac99e59d1d4d87d025a
doc_id: 924838
cord_uid: 4mchpe0c

Immune homeostasis is a tightly regulated system that is critical for defense against invasion by foreign pathogens and protection from self-reactivity for the survival of an individual. How the defects in this system might result in autoimmunity is discussed in this review. Reduced lymphocyte number, termed lymphopenia, can mediate lymphopenia-induced proliferation (LIP) to maintain peripheral lymphocyte numbers. LIP not only occurs in normal physiological conditions but also correlates with autoimmunity. Of note, lymphopenia is also a typical marker of immune aging, consistent with the fact that not only the autoimmunity increases in the elderly, but also autoimmune diseases (ADs) show characteristics of immune aging. Here, we discuss the types and rates of LIP in normal and autoimmune conditions, as well as the coronavirus disease 2019 in the context of LIP. Importantly, although the causative role of LIP has been demonstrated in the development of type 1 diabetes and rheumatoid arthritis, a two-hit model has suggested that the factors other than lymphopenia are required to mediate the loss of control over homeostasis to result in ADs. Interestingly, these factors may be, if not totally, related to the function/number of regulatory T cells which are key modulators to protect from self-reactivity. In this review, we summarize the important roles of lymphopenia/LIP and the Treg cells in various autoimmune conditions, thereby highlighting them as key therapeutic targets for autoimmunity treatments.

A healthy and competent immune system must not only defend against infection but also protect against self-reactivity. To achieve this, it is critical that immune homeostasis maintains the T cell pool at a relatively constant number with a variety of specificities and tolerance to self-antigens [1, 2] .

In steady-state, large numbers of T cells are released from the thymus; these divide daily in the periphery and replace those that die by apoptosis [1] . Among T cell subsets, the most rapidly dividing population is of CD4 + regulatory T (Treg) cells, which suppress pathogenic, self-reactive cells that escape deletion during negative selection in the thymus [1] . Antigen-presenting dendritic cells (DCs) play the critical roles in the development, function, and homeostasis of Treg cells [3] . All these mechanisms contribute to immune homeostasis, in which self-tolerance is maintained. While the number of lymphocytes is low or reduced (lymphopenia) due to various triggers, a process termed lymphopeniainduced proliferation (LIP) could occur to respond to "fill-the-space" signals in the niche of secondary lymphoid organs [4] [5] [6] . However, this process might be associated with ADs such as type 1 diabetes (T1D) [7] . This article highlights the importance of T cell lymphopenia in the development of autoimmune diseases, as well as overviews LIP conditions to discuss the roles of loss of homeostatic control, focusing on Treg cells and negative costimulatory receptors during this LIP process.

Immune homeostasis is a tightly regulated system that is critical for defense against invasion by foreign pathogens and protects from self-reactivity for the survival of an individual. T lymphocytes play the critical roles in this regard and are the main focus of this review. Thymocytes are developed and produced from the thymus once they become mature T cells. In the periphery, the control of survival, proliferation, and death of T cells are also crucial to maintain T cell numbers. When the T cell number is decreased due to recent infection, treatment with certain cytotoxic medications, or other triggers, a recovery process of the T cell pool occurs through LIP.

Homeostasis of the T cell population could be mediated by two distinct modes of LIP: (i) slow homeostatic proliferation (HP) driven by low-affinity self-peptide/major histocompatibility complex (MHC) ligands and the cytokines interleukin (IL)-7 and IL-15 [4, 5, 8, 9] and (ii) rapid spontaneous proliferation (SP) caused by high-affinity interactions with selfpeptides or commensal bacterium-derived peptides presented by MHC molecules [10] [11] [12] as well as the cytokine IL-6 [13] . Of note, the complexity of T cell receptor (TCR) repertoire also regulate SP. Min et al. demonstrated that the presence of memory CD4 + T cells of broad diversity inhibited the proliferation and differentiation of transferred naïve CD4+ T cells; however, a memory population of similar number with limited diversity were not able to do so [14] . On the other hand, HP could be mediated by competition for cytokines, such as IL-7, by nonspecific T cells [15] .

LIP might occur in both naïve and antigen-experienced memory T cells [16] [17] [18] , and it has been shown that naïve T cells can acquire the characteristics of memory T cells during LIP [19] [20] [21] [22] . Interestingly, Goldrath et al. demonstrated that when naïve CD8 + T cells transferred into sublethally irradiated (lymphopenic) syngeneic mice, they undergo slow HP and have a surface phenotype of memory CD8 + T cells (CD44 hi Ly6C hi CD122/IL-2Rβ hi ) and are transiently able to produce interferon-gamma (IFN-γ) at early time points [21] . Even more interestingly, this HP-associated phenotypic conversion is reversible. After 20-30 days, the transferred cells begin to downregulate their expression of all three activation/memory markers. By 40-50 days after transfer, the majority of cells no longer express higher levels of these three surface markers. Similarly, after 12-31 days of transfer, the cells exhibit significant ability to produce IFN-γ whereas the ability to function as effectors in vitro diminishes by day 45 [21] . Of note, not only the naïve cells undergoing SP rapidly acquire the memory phenotypes, such as high CD44 expression and IFN-γ secretion [15, 23] , but also the SP of naïve cells produces cells that upregulate CD44 and IFN-γ expression to a greater degree than cells undergoing HP, thereby generating a majority of damaging autoreactive T cells [13, 15, 23] . It has been shown that if naïve T cells are transferred into syngeneic hosts with T-cell lymphopenia induced by irradiation, the majority of cells undergo HP, whereas only a small proportion of cells undergo SP [4, 11] . In contrast, if transferred into lymphopenic RAG2 −/− mice, both HP and SP comparably occur within recipient mice [15] . The LIP of cells with the memory phenotype could be resulted from the SP of naïve or preexisting memory cells [4, 10, 11] as well as from HP regulated by IL-7/IL-15 because of a reduction in the number of naïve cells.

Interestingly, in addition to cytokines and the affinity between TCR and the selfpeptide/self-MHC ligand, the rate of LIP of T cells depends on T cell subtype and age. In the case of naïve cells, the LIP might be faster for CD8 + T cells [13] than CD4 + T cells [15] as shown in the lymphopenic models generated by deletion of the Rag2 gene. It has been shown that peripheral T cells from neonates proliferate more strongly compared to adult T cells, whereas it is not matched by increased expression of the activation marker CD44 [24] . In contrast, to prevent self-reactivity mediated by T lymphocytes, multiple tolerance mechanisms have been identified. Central tolerance is mediated by the removal of newly produced, strongly autoreactive lymphocytes (negative selection). Peripheral tolerance includes anergy (functional unresponsiveness), suppression by Treg cells, and deletion due to activation-induced cell death (AICD). It has been established that Treg cells with CD4 + CD25 + Foxp3 + phenotype are crucial for the maintenance of tolerance and originate from naturally occurring thymus-derived Treg cells (nTreg/tTreg) cells [25] and peripherally-induced Treg (iTreg/pTreg) cells generated from naïve cells upon antigen stimulation [26, 27] . Mature self-antigen-reactive tTreg cells leave the thymus and enter peripheral tissues, where they can suppress the activation of other self-reactive T cells [28, 29] . In addition, these two types of Treg cells mediate immune regulation through the production of IL-10, TGF-beta, and IL-35 or combinations of these proteins [30, 31] . Other types of regulatory T cells originated from the periphery [1] comprise TGF-β-producing type 3 helper (Th3) cells [32] and IL-10-producing type 1 regulatory T (Tr1) cells [33] . Tr1 and Th3 cells are induced by DCs that have an activation status distinct from DCs that promote the differentiation of Th1 or Th2 cells.

Moreover, new subsets of Treg cells have been identified. The Foxp3 + Bcl-6 + CXCR5 + follicular regulatory T (Tfr) cells regulate the function of Tfh cells and the germinal center response [34] [35] [36] . IL-35-dependent regulatory cells demonstrate potent suppressive potential in several mouse disease models [31] . Other lymphocyte populations also contain regulatory cells. CD8 + Treg cells secrete either TGF-β or IL-10 [37, 38] . Furthermore, antigen-activated CD8 + γδ T cells can prevent insulin-dependent diabetes in mice [39] , and IL-10-and TGF-β-producing regulatory γδ T cells can suppress the antitumor activity of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells [40] . In addition, natural killer T (NKT) cells can secrete regulatory cytokines, including IL-10 [41] . Additionally, regulatory B (Breg) cells support immunological tolerance through the production of IL-10, IL-35, and TGF-β [42] .

Lymphopenia can occur in normal physiological conditions or be associated with abnormal pathogenic status such as autoimmune diseases. It has been shown that the number of T cells is very low in the peripheral immune system of neonatal mice. In CD4 + T cells, adult percentages are not reached until postnatal day 7 in the lymph nodes and day 15 in the spleen [43] . This lymphopenic state can drive LIP in neonates [44] . With aging, the thymic output in the elderly is reduced, leading to a decreased percentage of peripheral naïve T cells, which can lead to LIP [45] . Furthermore, in animals, it has been demonstrated that CD4 + and CD8 + T-cell lymphopenia exists in the spleens of aged Balb/c mice [46] . In addition, we demonstrated that in Balb/c mice, age was inversely correlated with the number of total and naïve CD4 + T cells, and advanced aging resulted in LIP, given that the percentage of proliferating cells is inversely correlated with the percentage of naïve cells [47] . In fact, T cell lymphopenia, especially a decline of naïve T cells, is a typical marker of immune aging (immunosenescence) in both humans and mice [46] [47] [48] . With aging, the recent thymic emigrants (RTEs) reduce, resulting from thymic involution. Newly generated T cells contain T-cell receptor excision circles (TRECs), which are small circles of DNA generated in T cells during their development in the thymus. TRECs are unable to replicate, and are diluted due to cell division, thereby are considered as a marker of newly generated T cell output. It is demonstrated that the percentage of TREC + cells declines by more than 95% from 20 to 60 years of age [49, 50] . Other than T cell lymphopenia, several typical markers of immune aging are identified, including the loss of CD28 expression, decreased IL-2 production and IL-2 receptor expression, and fewer naïve T cells containing TRECs [48] (Figure 1 ). It is well established that the loss of CD28 expression in CD4 + and CD8 + T cells in the elderly [51] [52] [53] . CD28 is a critical costimulatory signal for complementing TCR derived signals and promoting T cell proliferation and IL-2 production. The loss of CD28 expression is typically seen in T cells that have gone through multiple rounds of replication, and of note, clonal CD28 − T cells have been demonstrated to be autoreactive and resistant to apoptosis [54] . In addition, not only autoimmunity is increased in the elderly [55] , but also lymphopenia is associated with various autoimmune conditions/diseases (Table 1 ). It has been demonstrated that lymphopenia occurs in primary immunodeficiency (PID) disorders, such as Omenn syndrome (OS) and Wiskott-Aldrich syndrome (WAS) [56] [57] [58] [59] , as well as common ADs; the onset of common ADs is secondary to a loss of tolerance to self-antigens. Interestingly, the associations between PIDs and ADs have been demonstrated. In fact, PIDs can be classified into four groups based on the frequency of association with ADs [60] . ADs are classified as organ-specific [61] [62] [63] [64] [65] [66] [67] [68] or systemic [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] disorders (Table 1 ). Autoimmune conditions/diseases could be varied in the age of onset (e.g., early-onset or late-onset rheumatoid arthritis), single-gene traits [81] , human leukocyte antigen (HLA) type [81] [82] [83] [84] [85] associated, or the trigger by viral infections [86, 87] (Table 1 ). This phenomenon indicates that a common mechanism presented as lymphopenia may exist for the induction of autoimmunity. Of note, markers of immune aging have been documented in the ADs in humans [69, [88] [89] [90] [91] [92] [93] [94] [95] [96] (Table 1) , such as reduced percentage of TREC + cells and/or lymphopenia, LIP in T cell subsets, and loss of CD28 + cells/accumulation of CD28 null cells in T cell subsets (Table 1) . 

Lymphopenia has been found in T-cell deficiency PIDs, such as Omenn syndrome (OS) [57, 58] and Wiskott-Aldrich syndrome (WAS) [59] . PIDs are genetic disorders that affect different parts of the immune system [103] . Patients with PIDs are with higher susceptibility to infections. In addition, they could be predisposed to immune dysregulation including malignancies [104] [105] [106] , allergies [107] , inflammation, and autoimmune diseases [108] [109] [110] . Interestingly, in a French national study, Fischer et al. demonstrate that, when analyzing age at onset, the probability of autoimmune or inflammatory manifestations occurred earlier in life up to around 20 years old is highest in patients with T-cell PIDs than in patients with innate and B-cell PIDs [111] . PIDs are monogenic disorders; therefore, their specific association with autoimmunity indicates the importance of a given gene in autoimmunity/tolerance. OS is characterized by the infiltration of autologous and activated T cells into target organs such as the skin and gut [112] . This disorder could be affected by mutations in recombination-activating genes 1 and 2 (RAG1 and RAG2), deoxyribonucleic acid (DNA) cross-link repair 1C (DCLRE1C), or DNA ligase 4 (LIG4) gene; these genes encode RAG1, RAG2, artemis, and DNA ligase 4, respectively. Of note, all these genes are involved in TCR rearrangement in the thymus, thereby contributing the deficiency of T cells. In addition, the TCR repertoire is severely restricted [113, 114] . It has been found that a reduced number of Treg cells may play a critical role in inducing autoimmunity in OS given that the frequency of Treg cells is dramatically reduced in in the thymus and spleen of Rag2 R229Q/R229Q mice [115] . Reportedly, 70% of patients with WAS suffer from at least one of the following autoimmune disorders: hemolytic anemia, neutropenia, arthritis, skin vasculitis, glomerulonephritis, or inflammatory bowel disease [116] . This disorder is caused by mutations of a WAS protein (WASP). WASP is critical in cellular signaling to the actin cytoskeleton [117] to mediate the functions of multiple cell types in immune systems [118] . The immunopathogenic mechanisms in WAS might be linked to Treg function based on the following facts. First, investigations in WAS patients demonstrated that reduced suppressive function of Treg cells contribute to WAS autoimmunity [119] . Second, it has been shown that WASP is required for Treg cell-dependent suppression in vitro and in vivo [119, 120] . Third, reduced production of IL-2, an important factor for the growth of CD4 + CD25 + FOXP3 + Treg cells, has been demonstrated in WAS patients [121] .

In contrast to PID disorders, common ADs are of multifactorial origin with complex genetics involved [122] [123] [124] [125] [126] [127] [128] . The onset of these common ADs is secondary to the loss of self-tolerance and is directed against antigens present only in a particular organ (organspecific) or in many organs and tissues (systemic) of the body, resulting in widespread tissue damage in the host [129] (Table 1) .

Of note, T cell lymphopenia is a distinct characteristic in the ADs associated with T-cell PID disorders such as WAS [130, 131] ; however, despite its association with a variety of ADs as shown in Table 1 , T cell lymphopenia may not be present in patients who are suffering from ADs [132, 133] due to enduring LIP and/or the concomitant occurrence of other lymphoproliferative conditions such as lymphomas [134, 135] . It has been shown that this concomitant occurrence of lymphoproliferative conditions might be linked to treatment with methotrexate (MTX), a disease-modifying antirheumatic drug (DMARD) [136] [137] [138] . Viral infections, including HIV [139, 140] , EBV [140, 141] , and HTLV-1 [142, 143] , may play critical roles in the development of both ADs and lymphoproliferative conditions. The possible roles of viral infections in LIP, autoimmunity, and lymphoproliferative conditions are discussed in Section 6.3 with focus on EBV since it has been shown to be associated with many ADs (Table 1) .

Additionally, marked lymphopenia is associated with severe cases of the coronavirus disease 2019 (COVID-19) pandemic and has been used to predict disease severity [144] [145] [146] in COVID-19 patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It has been demonstrated that the lymphocyte number in COVID-19 patients gradually decreased as the disease progressed. In addition, older patients with lower lymphocyte and platelet counts were at higher risk of severe disease and increased duration of hospitalization [147] . This may be because SARS-CoV-2 invades progenitor/stem cells via angiotensin-converting enzyme 2 (ACE2) with its spike protein [148] . Lymphopenia occurs especially in T cells and NK cells [149] . The numbers of both CD4 + T cells and CD8 + T cells in patients with COVID-19 were lower than those in healthy donors, and the decrease in Th cells was more pronounced in severe cases [149] . Of note, several autoimmune disorders have been demonstrated in COVID-19 patients infected with SARS-CoV-2. These include cutaneous rashes and vasculitis, autoimmune cytopenia, antiphospholipid syndrome, central or peripheral neuropathy, myositis, and myocarditis [150] .

In animal models, it has also been shown that lymphopenia may exist in PID disorders (e.g., OS and WAS) [97] [98] [99] , organ-specific [7,100,101] (e.g., T1D), or systemic [102] (e.g., rheumatoid arthritis [RA]) ADs (Table 1) . It has been shown that lymphopenia is associated with two animal models for OS. The memory mutant (MM) mouse exhibits a spontaneous mutation in Rag1 (R972Q), which was also found in an OS patient [97] . The Rag2 R229Q/R229Q mouse exhibits a hypomorphic R229Q mutation in Rag2 found in some OS patients [115] . Rag2 R229Q/R229Q mice have dramatically reduced frequency of Treg cells in the thymus and spleen, suggesting a reduced number of Treg cells may play a role in inducing autoimmunity in OS [115] . In addition, WASP-deficient mice are lymphopenic [98, 99] , and most of these mice develop colitis [98] . Investigations in WASPdeficient mice demonstrated that reduced suppressive function of Treg cells contribute to WAS autoimmunity [119] . Furthermore, lymphopenia is also found in animal models of nonobese diabetic (NOD) mice and K/BxN mice, which are animal models of T1D and RA, respectively [7, 102] .

Interestingly, LIP not only occurs in normal physiological conditions as mentioned above but also correlates with autoimmunity in both humans and animal models (Tables 1 and 2 ). This might be because the lymphopenic condition favors the production of self-reactive T cells through LIP and promotes autoimmunity based on the following findings. First, self-peptide/self-MHC can drive LIP [4, 24] . Second, LIP is regulated by clonal competition [12, 151] . It has been shown that T cells with higher TCR affinity can better compete for factors for cell survival and LIP [152] . Third, T cells undergoing LIP can acquire effector/memory function and thus contribute to the induction of autoimmunity [13] . The LIP-associated autoimmunity in animal models and patients is listed in Table 2 . The induction of lymphopenia can be mediated by physiological condition [24, 44] , genetic alteration [4, [13] [14] [15] 97, 102, 153] , irradiation [4, 154, 155] , cytokine imbalance [7] , immunosuppressive cytostatic drugs [156, 157] , impaired thymic function [49] , or viral infections [147, 149, 150, 158] . The mechanisms of LIP involving cytokines and TCR affinity are mainly demonstrated by murine models (Table 2 ). Increased CD4 T cell proliferation through enhanced gp130-mediated STAT3 signaling results in increased memory/activated cells 3.

LIP of CD4 + T cells is increased due to elevated production of IL-7 by non-hematopoietic cells as a result of IL-6 family cytokine-gp130-STAT3 signaling [154] Wiede, 2014 Irradiation Sub-lethal irradiation (600-650 Gy), PTPN2-deficient naïve CD8 + T cells 1. PTPN2-deficient naïve CD8 + T cells undergo rapid LIP when transferred into irradiated lymphopenic mice and acquire the characteristics of antigen-experienced effector T cells 2.

The increased LIP response is TCR-dependent but not IL-7-dependent 3.

The LIP results in an altered TCR repertoire and the development of autoimmunity (increased hepatic lymphocytic infiltrates accompanying liver damage) [155] IRIS occurs during HARRT therapy [158] a The rate of HP and/or SP was evaluated by the author of this study. Abbreviations: LIP, lymphopenia-induced proliferation; TCR, T cell receptor; MHC, major histocompatibility complex; CD44, cluster of differentiation 44; CD4, cluster of differentiation 4; HP, homeostatic proliferation; SP, spontaneous proliferation; IL-7, Interleukin-7; Rag, recombination activating gene; IL-6, Interleukin-6; IL-17, Interleukin-17; CD8, cluster of differentiation 8; ADAP, adhesion and degranulation-promoting adapter protein; cGy, centigray; Gy, gray; STAT3, signal transducer and activator of transcription 3; PTPN2, protein tyrosine phosphatase N2; NOD, non-obese diabetic; RIP-GP, rat insulin promoter-glycoprotein; FK506, tacrolimus; GAD, glutamate acid decarboxylase; IRIS, immune reconstitution inflammatory syndrome; HARRT, highly active antiretroviral therapy.

Genetic alterations mediated by transgenes or mutations may mediate lymphopenic conditions. A Rag-mutant mouse model of human OS exhibits lymphopenia, LIP, and activated CD4 + cells [97] . Furthermore, when purified CD8 + T cells were transferred into lymphopenic Rag-deficient mice, both slow HP and rapid SP were found as well as colitis. This rapid SP of naive CD8 + T cells caused colitis through the induction of pathogenic cytokine-producing Th17 cells and memory-type cells characterized by CD44 expression [13] . In addition, the LIP of transferred T cells in Rag1-mutant mice is enhanced in the absence of peripheral autoimmune regulator (AIRE) and induces colitis because of the decreased proliferation of Foxp3 + CD4 + Treg cells [159] . Moreover, the importance of LIP in induction of autoimmunity has been demonstrated in an animal model of retinal autoimmune disease (experimental autoimmune uveoretinitis, EAU) [160] . In this study, a CD4 + β-galactosidase (βgal)-specific TCR transgenic mouse line (βgalTCR) and transgenic mice that are Rag −/− and/or expressed βgal in retinal photoreceptor cells (arrβgal mice) were used. In the recipient mice, the lymphopenic condition was ensured by Rag mutation (Rag −/− ). McPherson et al. showed that higher incidence and severity of EAU could be induced by transferring naïve, CD25-depleted βgal TCR T cells into lymphopenic arrβgal × Rag −/− double transgenic recipient mice, compared to that induced by activated CD25-depleted βgal TCR T cells or in arrβgal transgenic recipient mice. It was also shown that LIP occurred and resulted in EAU in this experimental condition. These results suggest that lymphopenia/LIP plays a promoting role whereas regulatory T cells in the transferred T cells and in the recipient mice play inhibitory roles in the development of EAU [160] .

Irradiation could also result in a lymphopenic state. It has been shown that when transferred into lymphopenic mice induced by irradiation, protein tyrosine phosphatase N2 (PTPN2)-deficient CD8 + T cells undergo rapid LIP and acquire the phenotypes of antigen-experienced effector T cells. PTPN2 is an important regulator of TCR signaling through dephosphorylation and inactivation of the Src family protein tyrosine kinases Lck and Fyn. The enhanced LIP, due to increased TCR-dependent but not IL-7-dependent responses, results in a skewed TCR repertoire and the development of autoimmunity, as indicated by increased hepatic lymphocytic infiltrates accompanying liver damage and production of antinuclear antibodies in sera [155] . Furthermore, it has been demonstrated that the LIP of naïve T cells in lymphopenic Rag1 −/− mice or irradiated mice is further enhanced by the deletion of Tgfbr2, the gene of TGF-βRII. It was found that signaling from TGF-β directly to naïve T cells inhibited their rapid proliferation, acquisition of effector functions, and autoimmunity in Rag1 −/− recipient mice [161, 162] .

Induction of lymphopenia by the drug cyclophosphamide (CTX) may regulate LIP, as demonstrated in an autoimmune diabetes model using the rat insulin promoter (RIP)glycoprotein (GP)-transgenic mouse. In RIP-GP mice, the expression of the lymphocytic choriomeningitis virus (LCMV) glycoprotein in pancreatic β-cells is driven under the control of the RIP. After the adoptive transfer of activated CD4 + T cells specific for the I-A b -restricted LCMV GP61-80 epitope (Smarta), RIP-GP mice with a polyclonal CD8 TCR repertoire developed diabetes with a 20% onset rate. However, the onset rate of diabetes reached 100% when lymphodepletion was performed through CTX administration before the transfer of activated Smarta CD4 + T cells [156] . Therefore, the lymphopenic state in RIP-GP mice may contribute to the LIP of autoreactive CD8 + T cells.

In humans, increased LIP of T cells has been demonstrated as a mechanism for the production of oligoclonal T cell repertoires in OS and WAS patients [58, 163] . In OS, the TCR repertoire is severely restricted [113, 114] and is characterized with large oligoclonal expansions of leaky TCR specificities. In addition, the pathology of OS has been linked to the presence of autologous T cells, which are mainly activated memory (CD25 + HLA-DR + CD45RO + ) cells [58, 114, 164] . This phenomenon is consistent with the concept that lymphopenia promotes the proliferation of cells with higher self-affinity, thereby promoting the accumulation of autoreactive T cells with memory phenotype and skewing the TCR repertoire toward autoimmunity. Furthermore, it has been shown that a decline in the percentage of TREC + T cells in RA patients, which occurred as a consequence of thymic involution and compensatory lymphopenia-induced proliferation in the periphery. LIP has been associated with clinical RA [74] . This may be because increased LIP could lead to TCR repertoire contraction and an increase in autoreactive T cell pool due to competition for self-peptide/self-MHC and cytokines, thereby predisposing the development of autoimmune diseases such as RA [74] . During this process of compensatory proliferation of post-thymic T cells, T cells with premature aging characteristics, including loss of cell surface receptor CD28, shortened telomeres, and production of high levels of IFN-γ, were accumulated. These premature aging T cells contributed autoreactivity and tissue damage. Premature telomere shortening in RA patients occurred in both CD4 + and CD8 + T cells [49] . In addition, it has been demonstrated that in primary Sjögren's syndrome (pSS), lymphopenia mainly occurred in naïve CD4 + T cells. These naïve CD4 + T cells exhibited characteristics of aging such as shortened telomeres and reduced levels of TRECs, suggesting these naïve CD4 + T cells have gone through extensive post-thymic proliferation [95] . In patients of Hashimoto's thyroiditis (HT), lymphopenia was noted especially in CD8 + T populations. Memory (CD45RO + ) T cells were increased in both CD4 + and CD8 + T cells [90] . It has been shown that naïve (CD45RA + ) CD4 + T cells showed lower TRECs numbers compared to healthy controls [90] . Although relative telomere length (RTL) was not significantly different between healthy controls and HT, a trend to shorter RTL was observed in naïve CD4 + T cells of HT patients. In addition, increased percentage of CD28 null cells was observed in T cells. Therefore, reduction of TRECs may be mostly caused by peripheral compensatory proliferation of naïve CD4 + T cells in HT. Nevertheless, whether other mechanisms by which reduction of TRECs existing remains to be examined, such as switching from naïve to memory cells, apoptosis of naive T cells or distribution to secondary lymphoid organs or the thyroid gland. Of note, the loss of CD28 expression was associated with cytomegalovirus (CMV)-seropositivity in HT patients [90] .

It has been demonstrated that after one year of islet transplantation for T1D therapy, many patients were no longer insulin-independent and developed the autoimmune disease over time. These patients demonstrated both T cell and B cell lymphopenia due to the immunosuppressive drugs administered as part of the therapeutic protocol [157] . It is known that the immunosuppressive drugs used in this study, tacrolimus (FK506) and rapamycin, inhibit T cell signaling and thereby block the production of IL-2, a growth factor for T cells. This lymphopenia-induced T cell proliferation was associated with the production of more glutamate acid decarboxylase (GAD)-specific autoreactive T cells (β-cell reactive) and pancreatic islet destruction [157] .

In addition, immune reconstitution inflammatory syndrome (IRIS) associated with HIV infection is the most well-defined lymphopenia-associated autoimmune disorder in humans. This clinical condition demonstrates the potent relationship between lymphopenia and organ-specific autoimmunity. Highly active antiretroviral therapy (HAART) leads to rapid increases in CD4 + T cell numbers in HIV-infected patients. The first increase in CD4 + T cell numbers could be observed days to weeks after the initiation of HAART. During the subsequent weeks and months, the efficiency of LIP is improved and patients show substantially increased CD4 + T cell numbers. IRIS occurs early during the rapid rise in CD4 + T cell numbers via LIP in HIV-infected patients with a history of viral infection or infection-associated inflammation that is well-controlled at the start of HAART. During the rapid rise in CD4 + T cell numbers, inflammation in the previous infection site recurs and the patients develop a recurrence of previous symptoms, most often without evidence of reinfection. Therefore, inflammation appears to be critical because of increased serum IL-6 levels and polymorphisms of IL-6, TNF-α, and IL-12 [165] , and/or high-level inflammation in the tissue when immune reconstitution is initiated, predisposing to IRIS.

Moreover, as mentioned in Section 3, marked lymphopenia is associated with severe cases of COVID-19, and it was found that the proliferation of non-naïve CD4 + and nonnaïve CD8 + T cells was significantly higher in COVID-19 patients than in healthy donors or recovered donors [166] . In the case of CD4 + T cells, the increased proliferation is mainly found in the effector memory population and circulating Tfh cells [166] . Therefore, the rapid reduction in lymphocytes may be critical in the pathogenesis through the accumulation of effector memory cells and contribute to severe COVID-19. In addition to lymphopenia, other markers involving immune aging have been demonstrated in severe and extremely severe COVID-19 patients including decreased CD28 expression and increased CD45RO (memory phenotype) expression on CD4 + and CD8 + T cells [167] .

LIP is not only associated with autoimmunity but has also been shown to play a causative role in the development of autoimmunity in animal models, including organspecific and systemic autoimmune diseases. It is known that adhesion and degranulationpromoting adapter protein (ADAP) regulates positive selection, and ADAP deficiency results in a decreased thymic output. Another model for T1D, ADAP-deficient mice bred to the BDC2.5 TCR (pancreatic islet antigen-specific) transgenic mouse also exhibits lymphopenia, increased LIP, and diabetes. The transfer of either leukocytes or purified T cells into these lymphopenic mice resulted in reduced LIP and decreased diabetes incidence [153] .

In addition, it has been demonstrated that the LIP of T cells occurs and plays a causative role in the development of T1D in NOD mice [7] . NOD mice exhibit CD4 + T-cell lymphopenia. The lymphopenic environment in NOD mice promotes the LIP of transferred pancreatic β cell-specific TCR transgenic NOD T cells. This increased T cell proliferation correlates with lymphocyte infiltration into the pancreatic islets. NOD mice with the greatest fraction of T cells proliferating in the lymphoid organs exhibited the greatest fraction of islets with insulitis. Immunization with complete Freund's adjuvant (CFA), which is composed of immune-activating mycobacterial cell wall components, results in increased T cell numbers, decreased LIP of pathogenic β cell-specific T cells, and protection from T1D development. It was found that the production of cytokine IL-21 and a subsequent IL-21 receptor (IL-21R) expression on the surface of T cells increased in NOD mice compared to their congenic C57BL/6.Idd3.NOD mice. In contrast, it has been shown that NOD mice with IL-21R deficiency are protected from disease and demonstrate increased lymphocyte numbers and decreased HP. These data suggest that IL-21 promotes lymphopenia, HP, and autoimmunity [168] . Another T1D animal model, ADAP-deficient BDC2.5 TCR transgenic mice (pancreatic islet antigen-specific), also demonstrated that LIP causes autoimmunity as mentioned above [153] . Interestingly, by using the same model, NOD mice, we have previously demonstrated the presence of premature CD4 + T cell aging and its contribution to LIP of memory cells [47] .

K/BxN mice (a cross between KRN TCR transgenic mice on a C57BL/6 background (K/B) and NOD mice) are a murine model of spontaneous RA. These mice are lymphopenic, but the resulting LIP of T cells can be prevented by the restoration of cell numbers [102] . During the preclinical phase of the disease, K/BxN mice exhibit CD4 + T-cell lymphopenia, which is followed by a compensatory proliferation of these cells during the early clinical phase. The majority of CD4 + T cells acquired a memory phenotype (CD44 high CD62L low CD25 − ), which is a hallmark of lymphopenia-induced proliferating cells. K/BxN mice transferred with syngeneic T cells did not develop arthritis. This protective effect was associated with decreased proliferation of recipient-derived CD4 + T cells [102] . Furthermore, it has been shown that under lymphopenic conditions induced by irradiation, mice with a mutation in the gp130 IL-6 receptor subunit (F759) in nonhematopoietic cells developed an RA-like joint disease. These mice exhibited increased CD4 + T cell proliferation, and the disease is dependent on CD4 + T cells, given that mice lacking CD4 + T cells do not develop the disease. The F759 mutation resulted in greater proliferation caused by IL-7 through enhanced gp130-mediated STAT3 signaling, thus leading to increased memory/activated cells and disease. In contrast, providing CD4 + T cells prevents LIP and suppresses disease [154] .

Although autoimmunity is strongly associated with LIP as mentioned above, lymphopenia or LIP alone is not sufficient to cause autoimmune diseases. For example, humans with a lymphopenic state are found in diverse clinical conditions, such as congenital and acquired immunodeficiency syndromes, following autologous and allogeneic stem cell transplantation, antibody-depleting therapy to prevent or treat graft rejection, and cytotoxic chemotherapy for cancer [169] . These lymphopenic humans almost universally exhibit LIP, but most of them do not develop clinical evidence of autoimmunity.

In addition, neonatal mice are nearly devoid of peripheral T cells [24] . This lymphopenic state drives LIP without causing the development of autoimmunity [170] . In fact, only a minority of thymic emigrants undergo LIP in neonates. This is probably due to the presence of T cells with a Treg phenotype that is already found on day 3 after birth [171] .

The LIP in neonatal mice leads to autoimmune diseases only if they are AIRE deficient. Interestingly, it has been shown that, during the perinatal period, AIRE promoted the generation of a distinct population of FOXP3 + CD4 + Treg cells. These Treg cells stably existed in adult mice and played a role in maintaining self-tolerance to protect against the autoimmunity typical of AIRE knockout mice [172] . The AIRE expression in the perinatal age window is important since it is necessary and sufficient to avoid autoimmunity characteristic of AIRE-deficient mice [173] . PID patients with mutations in AIRE suffer from the autoimmune disease APECED (autoimmune polyendocrinopathy candidiasis ectodermal dystrophy) [174] . In addition, it was found that independent of the type of TCR stimulation, up to 70% of neonatal CD4 + Foxp3 − T cells became CD4 + Foxp3 + Treg cells, whereas less than 10% of adult CD4 + Foxp3 − T cells became CD4 + Foxp3 + Treg cells under the same conditions [175] .

A two-hit model of autoimmunity states that in the presence of a trigger, as exemplified by lymphopenia, the loss of control over homeostasis may result in the onset of autoimmunity [158, 161, 162] . This two-hit model of autoimmunity is supported by several lines of evidence. For example, it has been shown that, in an animal colitis model, the transfer of T cells with naive phenotype (CD45RB hi ) into lymphopenic severe combined immune deficiency (SCID) mice induces autoimmune colitis, whereas the transfer of T cells with memory phenotype (CD45RB lo ) or mixed populations of CD45RB hi plus CD45RB lo T cells does not [176] . It has been shown that the deficiency in CD4 + CD25 + regulatory cells is important because SCID mice transferred with CD4 + CD25 − T cells also developed colitis, and this autoimmunity could be prevented by providing CD4 + CD25 + cells within 10 days of CD4 + CD25 − T cell transfer [177] .

In addition, it has been shown that neonatal mice with thymectomy treatment developed autoimmune gastritis at high frequencies because of T cell reactivity developed recognizing the H/K ATPase self-antigen [178] . Similar to the models of colitis mentioned above, neonatal thymectomy in mice also leads to overall T cell lymphopenia with a preferential depletion of CD4 + CD25 + Treg cells. This is because mice are lymphopenic at birth and undergo LIP in RTEs with high rates, which serves to effectively seed the peripheral T cell compartment [44] . It has been demonstrated that RTEs are the preferential precursors of Treg cells differentiated in the periphery [179] .

Furthermore, in neonate physiological lymphopenia, induction of loss of homeostatic control by deletion of the Tgfbr2 gene at an early stage results in the onset of autoimmunity and death before five weeks of age [180] . Histological analysis demonstrated severe infiltration of leukocytes into multiple tissues/organs including the stomach, lung, liver, pancreatic islets, and thyroid gland [180] . However, if deletion of the Tgfbr2 gene is controlled to occur at a later stage, then there will be no development of autoimmunity in neonate mice even in the presence of lymphopenia [162] .

In addition, as mentioned previously, IRIS occurs early during the rapid rise in CD4 + T cell numbers that occur via LIP in human immunodeficiency virus (HIV)-infected patients. Nevertheless, if T cell numbers are very low, autoimmune disease is not observed, suggesting lymphopenia alone is not able to cause autoimmunity.

Interestingly, during the LIP-associated autoimmunity mentioned in the previous Section 5, the mechanisms regarding the loss of control over homeostasis are all linked to Treg cells. The deficiency in CD4 + CD25 + regulatory cells is important for lymphopenic SCID mice to develop colitis after transfer with naive CD45RB hi T cells [177] . TGF-βRII deficiency plays important roles in the development of autoimmunity in neonate mice [180] . Interestingly, the TGF-β/TGF-βRII signaling pathway has also been shown to be critical for both the development and function of Treg cells [181] . IL-21 plays a role in lymphopenia induction in NOD mice, and Attridge et al. have shown that IL-21 can counteract Treg suppression through inhibition of T cell IL-2 production [182] . It is well known that IL-2 induces and enhances the growth of Treg cells (CD4 + CD25 + FOXP3 + ), which selectively inhibits the SP form of LIP [23] . Furthermore, increased level of IL-6 is observed in IRIS. It is known that IL-6 inhibits Treg cell function [183] and Treg cell expansion [184] . IL-6 also induces Smad7, thereby inhibiting TGF-β activity [185] (Figure 2 ). Figure 2 . The induction, types and regulation mechanisms of LIP. Several triggers have been shown to induce lymphopenia including infection, irradiation, drugs, genetic defects, and thymus atrophy. LIP could occur via two pathways: slow homeostatic proliferation (HP) and rapid spontaneous proliferation (SP). HP is mediated by cytokines (IL-7 and/or IL-15) and interaction with self-peptide/MHC with low affinity. On the other hand, SP is regulated by cytokine IL-6 and interaction with self-peptide/MHC with high affinity. Treg cells might inhibit SP through LAG-3. Treg cells could be regulated by the presence of CD28, IL-2 production, and the TGF-β/TGFRII pathway; IL-21 could counteract Tregs function by inhibiting IL-2 production. IL-6 inhibits Treg cell function and expansion. PD-1 inhibits the LIP of T cells through its effect on peptide/MHC-II. BTLA negatively regulates the HP. Perp inhibits the persistence of T cells undergoing LIP through its role in activation-induced cell death; it is not known whether it exhibits a general role in both forms of LIP or in a certain form of LIP ( * ). The roles of PIDs in the pathways of LIP are indicated in red. Arrows and T-bars indicate positive and negative effects, respectively.

Several mechanisms have been demonstrated to negatively regulate the LIP of T cells. Programmed cell death protein 1 (PD-1) plays a role in inhibiting the LIP of T cells, and this effect is dependent on peptide/MHC-II but not IL-7 Ralpha signaling [186] . In addition, an MHC class II-binding CD4 homolog, lymphocyte activation gene 3 (LAG-3), regulates the LIP of T cells by both Treg-dependent and independent mechanisms [187] . B and T lymphocyte attenuator (BTLA) negatively regulates the HP of CD8 + T cells [188] . Perp (p53 apoptosis effector related to PMP-22) inhibits the persistence of CD4 + effector/memory T cells undergoing LIP through its role in AICD [189] (Figure 2 ).

Infections may mediate LIP by several ways. First, lymphopenia is frequently associated with many viral infections, such as influenza virus [190, 191] , measles virus [192] , rubella virus [193] , parvovirus [194] , HIV [195] , and SARS-CoV-2 [149] . Second, activation of autoreactive T cells can be mediated by infections with bacteria or viruses through the following mechanisms [196] : (i) molecular mimicry, (ii) bystander activation after tissue damage caused by proinflammatory cytokines (such as TNF-α) which are released from antigen-presenting cells (APCs) after stimulation of Toll-like receptors (TLRs) and other pattern-recognition receptors (PRRs) [141] , and (iii) epitope spreading to other autoreactive B cells after more tissue destruction by activated autoreactive T cells and inflammatory cytokines [196] . Third, proinflammatory cytokine IL-6 can further regulate autoimmunity by inducing LIP and autoantibody production, thereby establishing a vicious cycle (Figure 3 ). The enduring LIP process due to the presence of infections could lead to accumulation of cells with the following characteristics: loss of diversity of the TCR repertoire, increase in number of exhausted CD28 − T cells, and functional changes in CD4 + T cell subsets. These cells are with autoimmune potential yet are waning of the cellular immunity.

Taking EBV infection in RA as an example (Figure 3 ), molecular mimicry occurs between EBV and self-proteins such as: Epstein-Barr nuclear antigen 1 (EBNA1) and human cytokeratin; a sequence in the gp110 EBV glycoprotein and sequence QKRAA located in HLA-DRB1*0401, a susceptibility factor for RA [197] . In addition, during its lytic cycle, EBV can produce a human IL-10 homolog (vIL-10). Human IL-10 stimulates antiinflammatory responses whereas vIL-10 is less efficient in this regard. Moreover, vIL-10 inhibits the increase of suppressors of inflammation induced by hIL-10 [198] . Furthermore, vIL-10 interferes with the activity of CD4 + Th cells [199] , prevents an attack by nature killer cells [199] , promotes proliferation/differentiation of B cells [200] , and stimulates antibody production [200] . EBV is associated with cancers and lymphoproliferative conditions originated from both epithelial cells and hematopoietic cells such as Hodgkin's lymphoma caused by Reed-Sternberg cells (RSCs) [201] .

It is reasoned that EBV infection could enhance LIP of host autoreactive cells, thereby dampening the ability of cellular immunity to clear virus-infected host cells. This concept is supported by several studies. First, it has been demonstrated that EBV infection occurred in most Burkitt's lymphoma (BL) cases in solid organ transplant recipients; the onset rate of BL was decreased in those patients who were EBV-seropositive at baseline compared to EBV-seronegative [202] . EBV is also associated with post-transplant lymphoproliferative disorder (PTLD). Second, it was found that decreased T-cell response to EBV gp110 glycoprotein, expressed in lytic cycle in RA, was correlated with inflammation and systemic involvement [197] . Third, very interestingly, it has been demonstrated the contribution of cytokines, TNF-α, IL-6, and IL-10, to the association of ADs with specific types of lymphoproliferative conditions based on evaluating the effect of single nucleotide polymorphism (SNP) of cytokines in the progression of lymphoma in concurrence with ADs [134] .

In summary, from the perspective of viruses, turning the host's immune system to continuously attack hosts themselves through LIP of host autoreactive cells and thus making it eventually exhausted to avoid being attacked, is an excellent strategy for their own long-term survival and eventual expansion at the expense of host health. Infections could lead to lymphopenia and production of autoreactive T cells (brown circle) and B cells (orange circle) through a series of mechanisms. Under lymphopenic condition, IL-6 may drive LIP of autoreactive T cells. The LIP of autoreactive T cells and the production of autoantibodies increase autoimmunity that may further develop into autoimmune diseases. Cytokine TNF-α released from activated APCs could cause tissue damage. Activation of APCs by EBV infection results in the production of EBV-specific effector T cells such as cytotoxic T cells (green circle). EBV could infect both epithelial cells and B cells, although B cells are the primary targets. For illustration purpose only, it is shown that EBV virions produced from epithelial cells (blue rectangle) during the lytic phase of its life cycle could cause further damage of epithelial cells. EBV could also enter into a latent phase, during which the virus could remain silent in memory B cells (yellow circle). EBV has evolved various strategies to modulate the host immune response, such as production of viral IL-10 (vIL-10), which contribute to development of EBV + B cell lymphomas, such as RSC positive Hodgkin's lymphoma. Arrows and T-bars indicate positive and negative effects, respectively.

A healthy and competent immune system needs the maintenance of immune homeostasis with tolerogenic responses to self-reactive cells. If LIP endures, as opposed to returning to this steady-state homeostasis, it could lead to the development of ADs. The loss of homeostatic control might be linked to Treg cells and several coinhibitory receptors that can negatively regulate the LIP of T cells. Indeed, many single-gene traits linked to autoimmune diseases (e.g., AIRE and CTLA-4) play important roles in the production or function of Treg cells. Importantly, a variety of ADs are associated with lymphopenia, although susceptibility to individual autoimmune diseases might be linked to certain HLA genes and/or infection with viruses. Therefore, it is reasoned that the exploration of the mechanisms by which LIP is regulated may provide information for the development of both preventive and therapeutic strategies to conquer ADs. 

The dynamic co-evolution of memory and regulatory CD4+ T cells in the periphery

TGF-beta puts the brakes on homeostatic proliferation

Circulating precursors of human CD1c+ and CD141+ dendritic cells

The peptide ligands mediating positive selection in the thymus control T cell survival and homeostatic proliferation in the periphery

Low-affinity ligands for the TCR drive proliferation of mature CD8+ T cells in lymphopenic hosts

Naive T cell homeostasis: From awareness of space to a sense of place

Homeostatic expansion of T cells during immune insufficiency generates autoimmunity

Interleukin-7 mediates the homeostasis of naive and memory CD8 T cells in vivo

IL-7 is critical for homeostatic proliferation and survival of naive T cells

Self-recognition is crucial for maintaining the peripheral CD4+ T-cell pool in a nonlymphopenic environment

Recent immune status determines the source of antigens that drive homeostatic T cell expansion

Competition for self ligands restrains homeostatic proliferation of naive CD4 T cells

IL-6-dependent spontaneous proliferation is required for the induction of colitogenic IL-17-producing CD8+ T cells

Spontaneous proliferation, a response of naive CD4 T cells determined by the diversity of the memory cell repertoire

Spontaneous and homeostatic proliferation of CD4 T cells are regulated by different mechanisms

Population dynamics of T lymphocytes. Renewal rate and expansion in the peripheral lymphoid organs

Lymphocyte homeostasis

Cytokines and T cell homeostasis

T cell homeostasis: Keeping useful T cells alive and live T cells useful

The generation of protective memory-like CD8+ T cells during homeostatic proliferation requires CD4+ T cells

Naive T cells transiently acquire a memory-like phenotype during homeostasis-driven proliferation

Homeostasis-stimulated proliferation drives naive T cells to differentiate directly into memory T cells

Regulatory CD4+CD25+Foxp3+ T cells selectively inhibit the spontaneous form of lymphopenia-induced proliferation of naive T cells

Naive T cells proliferate strongly in neonatal mice in response to self-peptide/self-MHC complexes

Thymic regulatory T cells arise via two distinct developmental programs

Induced CD4+Foxp3+ regulatory T cells in immune tolerance

Induction of FoxP3 and acquisition of T regulatory activity by stimulated human CD4+CD25-T cells

Cutting edge: Self-peptides drive the peripheral expansion of CD4+CD25+ regulatory T cells

Natural versus adaptive regulatory T cells

Interleukin-35: Expanding Its Job Profile

IL-35-mediated induction of a potent regulatory T cell population

IL-4 is a differentiation factor for transforming growth factor-beta secreting Th3 cells and oral administration of IL-4 enhances oral tolerance in experimental allergic encephalomyelitis

A CD4+ T-cell subset inhibits antigen-specific T-cell responses and prevents colitis

T follicular regulatory cells in mice and men

Regulatory T cells can migrate to follicles upon T cell activation and suppress GC-Th cells and GC-Th cell-driven B cell responses

CD8(+) T cells from Theiler's virus-resistant BALB/cByJ mice downregulate pathogenic virus-specific CD4(+) T cells

IL-10 producing CD8(+) CD122(+) PD-1(+) regulatory T cells are expanded by dendritic cells silenced for Allograft Inflammatory Factor-1

Aerosol insulin induces regulatory CD8 gamma delta T cells that prevent murine insulin-dependent diabetes

Depletion of IL-10-and TGF-beta-producing regulatory gamma delta T cells by administering a daunomycin-conjugated specific monoclonal antibody in early tumor lesions augments the activity of CTLs and NK cells

NK T cell-derived IL-10 is essential for the differentiation of antigen-specific T regulatory cells in systemic tolerance

Regulatory B cells: Origin, phenotype, and function

T cell immunity in neonates

Neonates support lymphopenia-induced proliferation

The influence of age on T cell generation and TCR diversity

Exercise training increases the naive to memory T cell ratio in old mice

Premature CD4+ T cell aging and its contribution to lymphopenia-induced proliferation of memory cells in autoimmune-prone non-obese diabetic mice

Aging of the immune system: A risk factor for autoimmunity?

T cell homeostasis in patients with rheumatoid arthritis

Changes in thymic function with age and during the treatment of HIV infection

Functional subsets within clonally expanded CD8(+) memory T cells in elderly humans

Biomarkers of immunosenescence within an evolutionary perspective: The challenge of heterogeneity and the role of antigenic load

Functional properties of CD4+ CD28-T cells in the aging immune system

Resistance to apoptosis and elevated expression of Bcl-2 in clonally expanded CD4+CD28-T cells from rheumatoid arthritis patients

Rheumatologic and autoimmune manifestations of primary immunodeficiency disorders

Genetically determined lymphopenia and autoimmune manifestations

Oligoclonal expansion of T lymphocytes with multiple second-site mutations leads to Omenn syndrome in a patient with RAG1-deficient severe combined immunodeficiency

Early deficit of lymphocytes in Wiskott-Aldrich syndrome: Possible role of WASP in human lymphocyte maturation

Tolerance and autoimmunity: Lessons at the bedside of primary immunodeficiencies

Abnormal lymphocyte subsets in Kuwaiti patients with type-1 insulin-dependent diabetes mellitus and their first-degree relatives

Multiple sclerosis and age at infection with common viruses

Graves' disease following immune reconstitution or immunomodulatory treatment: Should we manage it any differently?

Myasthenia gravis and lymphoma. A clinical and immunological association

Double seronegative myasthenia gravis with antiphospholipid syndrome: A case report

Circulating lymphocyte subpopulations in Hashimoto thyroiditis

Primary biliary cirrhosis-phenotypic characterization of immunocompetent cells in peripheral blood and liver tissue

Cellular immune deficiency with autoimmune hemolytic anemia in purine nucleoside phosphorylase deficiency

Premature aging of the immune system in children with juvenile idiopathic arthritis

Remission of juvenile rheumatoid arthritis after infection with parvovirus B19

Prognostic parameters for flare in systemic lupus erythematosus

Alarcon-Segovia, D. Lymphopenia in systemic lupus erythematosus. Clinical, diagnostic, and prognostic significance

Lymphopenia in Sjogren's syndrome with rheumatoid arthritis: Relationship to lymphocytotoxic antibodies, cryoglobulinemia, and impaired mitogen responsiveness

Aging, autoimmunity and arthritis: T-cell senescence and contraction of T-cell repertoire diversitycatalysts of autoimmunity and chronic inflammation

Peripheral blood lymphocyte subset counts in patients with dermatomyositis: Clinical correlations and changes following therapy

T-lymphocyte immunophenotyping in polymyositis and dermatomyositis

CD4+ T-lymphocytopenia without HIV infection: Increased prevalence among patients with primary Sjögren's syndrome

Lymphocyte markers in patients with progressive systemic sclerosis

Thymus-dependent (T) lymphocyte deficiency in progressive systemic sclerosis

Prognostic significance of severe preoperative lymphopenia in patients with Crohn's disease

Janeway's Immunobiology

Risk factors and disease mechanisms in myositis

Relationshipp of HLA-Dw3 and HLA-B8 to Sjogren's syndrome

HLA and autoimmunity in scleroderma (systemic sclerosis)

Linkage of Crohn's disease to the major histocompatibility complex region is detected by multiple non-parametric analyses

The Immune System Recovery Plan: A Doctor's 4-Step Program to Treat Autoimmune Disease

Expression of CD4(+) CD28(null) T lymphocytes in children and adolescents with type 1 diabetes mellitus: Relation to microvascular complications, aortic elastic properties, and carotid intima media thickness

Premature immunosenescence in rheumatoid arthritis and multiple sclerosis patients

Lower CD28+ T cell proportions were associated with CMV-seropositivity in patients with Hashimoto's thyroiditis

Characterization of biliary intra-epithelial lymphocytes at different anatomical levels of intrahepatic bile ducts under normal and pathological conditions: Numbers of CD4+CD28-intra-epithelial lymphocytes are increased in primary biliary cirrhosis

First report of expansion of CD4(+)/CD28 null T-helper lymphocytes in adult patients with idiopathic autoimmune hemolytic anemia

Preferential elimination of CD28+ T cells in systemic lupus erythematosus (SLE) and the relation with activation-induced apoptosis

autoimmunity, and rheumatoid arthritis

Lymphopenia in primary Sjogren's syndrome is associated with premature aging of naive CD4+ T cells

Evolving insights into the cellular and molecular pathogenesis of fibrosis in systemic sclerosis

Murine models of autoimmune hemolytic anemia

Homeostatically proliferating CD4 T cells are involved in the pathogenesis of an Omenn syndrome murine model

Wiskott-Aldrich syndrome protein-deficient mice reveal a role for WASP in T but not B cell activation

Antigen receptor-induced activation and cytoskeletal rearrangement are impaired in Wiskott-Aldrich syndrome protein-deficient lymphocytes

Distinct roles of T-cell lymphopenia and the microbial flora for gastrointestinal and CNS autoimmunity

Spontaneous thyroiditis in thymectomized and irradiated Wistar rats

Prevention of spontaneous arthritis by inhibiting homeostatic expansion of autoreactive CD4+ T cells in the K/BxN mouse model

Autoimmunity and primary immunodeficiency: Two sides of the same coin?

Primary immunodeficiencies in the Netherlands: National patient data demonstrate the increased risk of malignancy

Stoppa-Lyonnet, D. Incidence, presentation, and prognosis of malignancies in ataxia-telangiectasia: A report from the French national registry of primary immune deficiencies

Malignancies in the setting of primary immunodeficiency: Implications for hematologists/oncologists

Food allergy in patients with primary immunodeficiency diseases: Prevalence within the US Immunodeficiency Network (USIDNET)

Autoimmune Disease in Primary Immunodeficiency: At the Crossroads of Anti-Infective Immunity and Self-Tolerance

The crossroads of autoimmunity and immunodeficiency: Lessons from polygenic traits and monogenic defects

Primary immunodeficiency and autoimmunity

Autoimmune and inflammatory manifestations occur frequently in patients with primary immunodeficiencies

Intrathymic restriction and peripheral expansion of the T-cell repertoire in Omenn syndrome

Omenn's syndrome occurring in patients without mutations in recombination activating genes

V(D)J recombination defects in lymphocytes due to RAG mutations: Severe immunodeficiency with a spectrum of clinical presentations

A hypomorphic R229Q Rag2 mouse mutant recapitulates human Omenn syndrome

Autoimmunity in Wiskott-Aldrich syndrome: Risk factors, clinical features, and outcome in a single-center cohort of 55 patients

WASP (Wiskott-Aldrich syndrome protein) gene mutations and phenotype

Wiskott-Aldrich Syndrome at the nexus of autoimmune and primary immunodeficiency diseases

Impaired in vitro regulatory T cell function associated with Wiskott-Aldrich syndrome

WASP regulates suppressor activity of human and murine CD4(+)CD25(+)FOXP3(+) natural regulatory T cells

Autoimmunity in Wiskott-Aldrich syndrome

Type 1 Diabetes Genetics Consortium. Type 1 diabetes: Evidence for susceptibility loci from four genome-wide linkage scans in 1435 multiplex families

Autoimmunity in systemic lupus erythematosus: Integrating genes and biology

Genetics of autoimmune diseases in humans and in animal models

Genetics of human systemic lupus erythematosus: The emerging picture

Update on human systemic lupus erythematosus genetics

The genetics of complex autoimmune diseases: Non-MHC susceptibility genes

Type 1 diabetes genes and pathways shared by humans and NOD mice

Human autoimmune diseases: A comprehensive update

Autoimmune Features of Wiskott-Aldrich Syndrome: A Case Report

Evaluation of Classic Wiskott Aldrich Syndrome with Mild Symptoms in Two Cousins: A Case Report. Iran

A combination of cellular biomarkers predicts failure to respond to rituximab in rheumatoid arthritis: A 24-week observational study

Lymphocytes as Biomarkers of Therapeutic Response in Rheumatic Autoimmune Diseases, is it a Realistic Goal?

Lymphoma in the setting of autoimmune diseases: A review of association and mechanisms

The Role of Autoimmune Diseases in the Prognosis of Lymphoma

Methotrexate-associated Lymphoproliferative Disorders in Patients with Rheumatoid Arthritis: Clinicopathologic Features and Prognostic Factors

Clinicopathological Analysis of "Other Iatrogenic Immunodeficiency-Associated Lymphoproliferative Disorders" Reveals a Favorable Outcome Independent of the Effectiveness of Methotrexate Discontinuation in Autoimmune Disease Patients

Lymphoproliferative disorders in patients with rheumatoid arthritis in the era of widespread use of methotrexate: A review of the literature and current perspective

Incidence of autoimmune diseases in a nationwide HIV/AIDS patient cohort in Taiwan

Hodgkin's lymphoma and its association with EBV and HIV infection

Epstein-Barr virus in systemic autoimmune diseases

HTLV-1, Immune Response and Autoimmunity

HTLV-1-Mediated Epigenetic Pathway to Adult T-Cell Leukemia-Lymphoma

Lymphopenia is associated with severe coronavirus disease 2019 (COVID-19) infections: A systemic review and meta-analysis

Clinical features of patients infected with 2019 novel coronavirus in

Lymphopenia predicts disease severity of COVID-19: A descriptive and predictive study

Lymphopenic community acquired pneumonia as signature of severe COVID-19 infection

SARS-CoV-2 Entry Receptor ACE2 is Expressed on Very Small CD45(-) Precursors of Hematopoietic and Endothelial Cells and in Response to Virus Spike Protein Activates the Nlrp3 Inflammasome

Dysregulation of Immune Response in Patients with Coronavirus

Autoimmunity as the comet tail of COVID-19 pandemic

Cutting edge: Homeostatic proliferation of peripheral T lymphocytes is regulated by clonal competition

A role for TCR affinity in regulating naive T cell homeostasis

Defective positive selection results in T cell lymphopenia and increased autoimmune diabetes in ADAP-deficient BDC2.5-C57BL/6 mice

Autoimmune arthritis associated with mutated interleukin (IL)-6 receptor gp130 is driven by STAT3/IL-7-dependent homeostatic proliferation of CD4+ T cells

PTPN2 attenuates T-cell lymphopenia-induced proliferation

CD4 T cells, lymphopenia, and IL-7 in a multistep pathway to autoimmunity. Proc. Natl. Acad. Sci

Islet transplantation in patients with autoimmune diabetes induces homeostatic cytokines that expand autoreactive memory T cells

Autoimmunity during lymphopenia: A two-hit model

Lymphopeniainduced proliferation in the absence of functional Autoimmune regulator (Aire) induces colitis in mice

Lymphopenia-induced proliferation is a potent activator for CD4+ T cell-mediated autoimmune disease in the retina

TGF-beta signaling to T cells inhibits autoimmunity during lymphopenia-driven proliferation

A two-hit model of autoimmunity: Lymphopenia and unresponsiveness to TGF-beta signaling

Analysis of T-cell repertoire diversity in Wiskott-Aldrich syndrome

Omenn syndrome: A lack of tolerance on the background of deficient lymphocyte development and maturation

Adverse events of desirable gain in immunocompetence: The Immune Restoration Inflammatory Syndromes

Deep immune profiling of COVID-19 patients reveals distinct immunotypes with therapeutic implications

The laboratory tests and host immunity of COVID-19 patients with different severity of illness

IL-21 limits peripheral lymphocyte numbers through T cell homeostatic mechanisms

Reconstitution of the lymphocyte compartment after lymphocyte depletion: A key issue in clinical immunology

Control of peripheral T-lymphocyte tolerance in neonates and adults

Regulatory potential and control of Foxp3 expression in newborn CD4+ T cells

Immune tolerance. Regulatory T cells generated early in life play a distinct role in maintaining self-tolerance

Neonatal tolerance revisited: A perinatal window for Aire control of autoimmunity

A decade of AIRE

Default" generation of neonatal regulatory T cells

Phenotypically distinct subsets of CD4+ T cells induce or protect from chronic intestinal inflammation in C. B-17 scid mice

Immunologic self-tolerance maintained by activated T cells expressing IL-2 receptor alpha-chains (CD25)

An autoimmune disease with multiple molecular targets abrogated by the transgenic expression of a single autoantigen in the thymus

Recent thymic emigrants are the preferential precursors of regulatory T cells differentiated in the periphery

Transforming growth factor-beta controls development, homeostasis, and tolerance of T cells by regulatory T cell-dependent and -independent mechanisms

Yin-Yang' functions of transforming growth factor-beta and T regulatory cells in immune regulation

IL-21 inhibits T cell IL-2 production and impairs Treg homeostasis

Toll pathway-dependent blockade of CD4+CD25+ T cell-mediated suppression by dendritic cells

IL-6 produced by dendritic cells from lupus-prone mice inhibits CD4+CD25+ T cell regulatory functions

Cutting edge: Trans-signaling via the soluble IL-6R abrogates the induction of FoxP3 in naive CD4+CD25 T cells

PD-1 Controls Tonic Signaling and Lymphopenia-Induced Proliferation of T Lymphocytes. Front

Negative regulation of T cell homeostasis by lymphocyte activation gene-3 (CD223)

B and T lymphocyte attenuator regulates CD8+ T cell-intrinsic homeostasis and memory cell generation

The p53 effector Perp mediates the persistence of CD4(+) effector memory T-cell undergoing lymphopenia-induced proliferation

Depletion of lymphocytes and diminished cytokine production in mice infected with a highly virulent influenza A (H5N1) virus isolated from humans

Influenza A (H5N1) in Hong Kong: An overview

T helper cell defect related to severity in measles. Scand

Abnormalities of in vitro lymphocyte responses during rubella virus infections

Experimental parvoviral infection in humans

Sharing CD4+ T Cell Loss: When COVID-19 and HIV Collide on Immune System. Front

99th Dahlem conference on infection, inflammation and chronic inflammatory disorders: Triggering of autoimmune diseases by infections

Epstein-Barr virus in autoimmune diseases

Epstein Barr Virus Interleukin 10 Suppresses Anti-inflammatory Phenotype in Human Monocytes

The EBV immunoevasins vIL-10 and BNLF2a protect newly infected B cells from immune recognition and elimination

Virus-encoded homologs of cellular interleukin-10 and their control of host immune function

Epstein Barr Virus Associated Lymphomas and Epithelia Cancers in Humans

Burkitt lymphoma risk in U.S. solid organ transplant recipients