key: cord-0689284-tfok8ep7 authors: Alcami, Antonio; Koszinowski, Ulrich H. title: Viral mechanisms of immune evasion date: 2000-09-01 journal: Trends Microbiol DOI: 10.1016/s0966-842x(00)01830-8 sha: ec9e8df5e5e837517a79e8423114209104d1f05f doc_id: 689284 cord_uid: tfok8ep7 During the millions of years they have coexisted with their hosts, viruses have learned how to manipulate host immune control mechanisms. Viral gene functions provide an overview of many relevant principles in cell biology and immunology. Our knowledge of viral gene functions must be integrated into virus–host interaction networks to understand viral pathogenesis, and could lead to new anti-viral strategies and the ability to exploit viral functions as tools in medicine. V iruses must be extremely successful predators as they depend on living cells for replication. Almost all living species represent prey for a viral invader. Viruses have coevolved with their hosts and therefore have limited pathogenicity in an immunocompetent natural host. In turn, probably as a result of the constant evolutionary pressure from viral invaders, higher vertebrates have developed a complex immune system. Only in the last decade have we caught a glimpse of what viruses do beyond invading cells for replication. For millions of years viruses have studied cell biology and immunology the hard way, to acquire and defend an ecological niche. It is remarkable that, in the process, individual virus families have targeted many common immunological principles. Viruses that belong to different families are subject to different constraints. Owing to the low fidelity of RNA polymerase, the genome size of RNA viruses is limited. Although this confers the advantage of being able to use mutation to escape immune control, there is little room in the genome to allow immune defenses to be encoded by individual genes. The proteins encoded by RNA viruses are therefore multifunctional. This particular constraint is less rigid for DNA viruses as their genome size allows a larger number of genes to be devoted to host control. In the case of herpesviruses and poxviruses, these genes probably account for Ͼ50% of the total genome. Viruses can exist in two forms: extracellular virion particles and intracellular genomes. Virions are more During the millions of years they have coexisted with their hosts, viruses have learned how to manipulate host immune control mechanisms. Viral gene functions provide an overview of many relevant principles in cell biology and immunology. Our knowledge of viral gene functions must be integrated into virus-host interaction networks to understand viral pathogenesis, and could lead to new antiviral strategies and the ability to exploit viral functions as tools in medicine. resistant to physical stress than genomes but are susceptible to humoral immune control. Virus genomes can be maintained in host cells by limited gene expression and can evade the host immune response. Nevertheless, to exist as a species, virus replication and transfer to a new host are essential. These processes are associated with the production of antigenic proteins that make the virus vulnerable to immune control mechanisms 'warning' the host of the presence of an invader. However, viruses have evolved strategies to evade such immune control mechanisms, and the list of these strategies forms the 'Who's who' of today's immunology. There are two classes of viral immunoregulatory proteins: those encoded by genes with, and those encoded by genes without, sequence homology to cellular genes. Viral homologs of host genes involved in the immune system are mainly found in large DNA viruses (herpesviruses and poxviruses) and their existence suggests that viruses have 'stolen' genes from the host that were subsequently modified for the benefit of the virus. Viral genes without sequence similarity to cellular genes might represent a paradigm for coevolution or could simply be examples of proteins for which the host homologs have not yet been identified. These proteins might possess specific motifs or particular folding properties required for interaction with the host cellular machineries. In this review, and the accompanying poster, we provide an overview of the different mechanisms that viruses use to evade host immune responses. The basic concepts of virus immune evasion will be discussed, with some examples to illustrate particular points; however, space constraints have not allowed a comprehensive review of all immune-evasion strategies. The strategies are listed in the accompanying tables and are discussed in more detail in the references given throughout the text. Antigenic variability was one of the first viral immune-evasion strategies to be identified. Because of the low fidelity of RNA polymerases, viral RNA genomes comprise a collection of RNA species (quasispecies) with random mutations. Therefore, in RNA viruses, the generation and selection of variants with different antigenic properties that can evade recognition by neutralizing antibodies is common. Genetic variability can also generate variant peptide sequences that are either new antigens or that do not bind to major histocompatibility complex (MHC) molecules at all. The complement system is a major non-specific host defense mechanism 1-3 . Viruses encode homologs of complement regulatory proteins that are secreted and block complement activation and neutralization of virus particles (Table 1 ; Box 1). The cowpox virus (CPV) complement inhibitor, termed inflammation Inhibition of soluble vCP/C21L, IMP, SPICE, gC, VV, CPV, VaV, HSV-1, HSV-2 Viral homologs of C4BP, CR1, 1-3 complement factors ORF4, (Table 1) . Antibodies bound to infected cells or virus particles might therefore be bound at the Fc region, thereby inhibiting Fc-dependent immune activation of complement and phagocytes. Fc receptors probably have additional functions in vivo 4 . Interferons (IFNs) were discovered because of their ability to protect cells from viral infection. The key role of both type I (␣ and ␤) and type II (␥) IFNs as one of the first anti-viral defense mechanisms is highlighted by the fact that anti-IFN strategies are present in most viruses ( Table 2) [5] [6] [7] . Viruses block IFNinduced transcriptional responses and the janus kinase (JAK)/signal transducers and activators of transcription (STAT) signal transduction pathways, and also inhibit the activation of IFN effector pathways that induce an anti-viral state in the cell and limit virus replication. This is mainly achieved by inhibiting double-stranded (ds)-RNA-dependent protein kinase (PKR) activation, the phosphorylation of eukaryotic translation initiation factor-2␣ (eIF-2␣) and the RNase L system, which might degrade viral RNA and arrest translation in the host cell. Poxviruses encode soluble versions of receptors for IFN-␣ and -␤ (IFN-␣/␤R) and IFN-␥ (IFN-␥R), which also block the immune functions of IFNs 6 . The VVsecreted IFN-␣/␤R is also localized at the cell surface to protect cells from IFN (Table 3) . Additionally, several viruses inhibit the activity of IFN-␥, a key activator of cellular immunity, by blocking the synthesis or activity of factors required for its production, such as interleukin (IL)-18 or IL-12 (Table 4 ): CPV cytokine response modifier (Crm)A inhibits caspase-1, which processes the mature forms of IL-1␤ and IL-18 (Refs 2,6); various poxviruses encode soluble IL-18-binding proteins (IL-18BPs) [8] [9] [10] ; measles virus (MeV) binds CD46 in macrophages and inhibits IL-12 production 1 ; and herpesviruses and poxviruses express IL-10 homologs that diminish the Th1 response by downregulating the production of IL-12 (Refs 1,11,12). Cytokines play a key role in the initiation and regulation of the innate and adaptive immune responses, and viruses have learned how to block cytokine production, activity and signal transduction (Tables 3,4 ). African swine fever virus (ASFV) replicates in macrophages and encodes an IB homolog that blocks cytokine expression mediated by nuclear factor (NF)-B and the nuclear factor activated T cell (NFAT) transcription factors 13 . Many viruses block signal tranduction by ligands of the tumor necrosis factor (TNF) family, whereas others deliberately induce some cytokine pathways; for example, the Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) recruits components of the TNF receptor (TNFR) and CD40 transduction machinery to mimic cytokine responses that could be beneficial for the virus, such as cell proliferation 14 (Table 4 ). One of the most interesting mechanisms identified in recent years is the mimicry of cytokines (virokines) and cytokine receptors (viroceptors) by large DNA viruses (herpesviruses and poxviruses) 1, 2, 11, 15, 16 (Table 3 ). The functions of these molecules in the animal host are diverse. Soluble viral cytokine receptors might neutralize cytokine activity and cytokine homologs might redirect the immune response for the benefit of the virus. Alternatively, viruses that infect immune cells might use these homologs to induce signaling pathways in the infected cell that promote virus replication. The herpesvirus cytokine homologs vIL-6 and vIL-17 might have immunomodulatory activity but might also increase proliferation of cells that are targets for viral replication 1, 11 . Viral semaphorin homologs have uncovered a role for the semaphorin family -previously known as chemoattractants or chemorepellents involved in axonal guidance during development -in the immune system, and have identified a semaphorin receptor in macrophages that mediates cytokine production 17 . Secreted cytokine receptors or binding proteins are mainly encoded by poxviruses 2,6,11,15,18 . These proteins were originally identified as homologs of host TNFRs, IL-1Rs and IFN-␥Rs. The discovery of four distinct soluble poxvirus TNFRs, and a membrane TNF-binding activity in VV infections, is remarkable and suggests that viral TNFRs might have additional functions [18] [19] [20] (M. Saraiva and A. Alcami, unpublished). Binding and activity assays have identified secreted proteins that bind IFN-␣ and -␤, chemokines (CKs) or granuloctye-macrophage colony-stimulating factor (GM-CSF) and IL-2, and that have no sequence similarity to cellular counterparts 6, 18, 21 . In poxviruses, three distinct secreted IL-18BPs that have recently been identified are homologs of human and mouse secreted IL-18BPs but not of membrane IL-18Rs [8] [9] [10] . Inactivation of poxvirus cytokine receptor genes results in virus attenuation in vivo but, interestingly, deletion of the VV IL-1␤R enhances virus virulence and the onset of fever, suggesting that the purpose of some immuneevasion mechanisms is to reduce the immunopathology caused by viral infection 18 . Herpesviruses and poxviruses modulate the activity of chemoattractant cytokines or CKs that regulate leukocyte trafficking to sites of infection 16, 18, 22 . Virusencoded CKs are either antagonists that block leukocyte recruitment to sites of infection, or agonists that could enhance the recruitment of cells that support viral replication or prevent Th1 anti-viral responses. Murine cytomegalovirus (MCMV) chemokine 1 (MCK-1) activates monocytes in vitro and increases monocyte-associated viremia in vivo 23 . HIV Tat is partially homologous to CKs and is a potent monocyte chemoattractant 24 . Herpesviruses encode many CK receptors (vCKRs) but their function is not clear. constitutively activated and induces cell proliferation, which might favor virus replication. vCKRs encoded by HCMV and HHV-6 reduce the amount of the factor regulated upon activation normal T-cell expressed and secreted (RANTES) in tissue culture and/or its transcription and might inhibit CK activity locally 16, 25 . A role for vCKRs in vivo has been shown for MCMV. vCKs and vCKRs might contribute directly to pathology. The angiogenic properties of HHV-8 macrophage inflammatory protein 1 (vMIP-1) could account for the increased vascularization found in HHV-8-associated tumors, human cytomegalovirus (HCMV) US28 mediates vascular smooth muscle cell migration and perhaps vascular disease 26 , and vCKBP-I is a soluble IFN-␥R encoded by MV, but not VV, which binds the heparin-binding domain of a wide range of CKs and might prevent the correct localization of CKs in vivo by blocking their interaction with proteoglycans. The poxvirus-secreted vCKBP-II, which has a novel protein structure 28 , binds CC CKs with high affinity and blocks their activity. Murine gamma-herpesvirus 68 (MHV-68) has recently been shown to encode a distinct secreted protein (vCKBP-III) that sequesters C, CC, CXC and CX 3 C CKs 29 . Apoptosis, or programmed cell death, can be triggered by a variety of inducers, including ligands of the TNF family, irradiation, cell-cycle inhibitors or infectious agents such as viruses. Apoptosis can be considered an innate cellular response to limit viral propagation, and viruses express proteins that block the death response (Table 5) ; however, apoptosis might also facilitate virus dissemination, and viral pro-apoptotic mechanisms have been described 30 . In addition, cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells kill virus-infected cells by inducing apoptosis via secretion of cytokines such as TNF, the release of perforin and granzymes, or the activation of Fas in the target cell. The cellular proteins implicated in the control of apoptosis are targeted by viral anti-apoptotic mechanisms 1, 5, 30, 31 . Viruses inhibit activation of caspases, encode homologs of the anti-apoptotic protein Bcl-2, block apoptotic signals triggered by activation of TNFR family members by encoding death-effectordomain-containing proteins, and inactivate IFNinduced PKR and the tumor suppressor p53, both of which promote apoptosis. An alternative mechanism is provided by the glutathione peroxidase of Molluscum contagiosum virus (MCV), which provides protection from peroxide-or UV-induced apoptosis, and perhaps from peroxides induced by TNF, macrophages or neutrophils. How to achieve persistence in the face of a vigorous host immune response is a problem that must be solved by viruses that establish life-long infections. Cellular proteins are degraded by the proteasome, the complex major intracellular protease, and the resulting peptides are translocated by transporters associated with antigen processing (TAP) molecules into the endoplasmic reticulum (ER), where they contribute to the assembly of MHC class I molecules 1, 11, [32] [33] [34] . MHC class I molecules indicate the composition of cellular proteins to cells of the immune system. The presentation of foreign peptides activates and attracts cytolytic CD8 ϩ T cells. Interference with antigen processing [e.g. Epstein-Barr nuclear antigen A1 (EBNA1)] or TAP function [e.g. herpes simplex virus (HSV) infected cell protein 47 (ICP47) and HCMV US6 and pp65] prevents peptide generation and transport either specifically or generally (Table 6) . Various mechanisms in different viruses directly target maturation, assembly and export of MHC class I molecules. To date, no cellular homologs have been found for the proteins and functions that target peptide processing, transport and MHC maturation. With few exceptions 35 , the viral proteins bind their target molecule directly. There is only limited functional homology and no sequence homology among the different viral effectors. Nevertheless, the general outcome of these functions is the same: downregulation of MHC class I molecules or of some MHC class I alleles. The study of MHC class I regulation has revealed additional genes in herpesviruses of different species [36] [37] [38] , which might affect many cell types or only those tissues relevant for virus maintenance. Although the downregulation of MHC class I expression prevents CD8 ϩ T-cell recognition, cells that downregulate these molecules are targets for NK cells 1, 11, [32] [33] [34] . NK cells, the first line of cellular defense against viruses, have receptors for certain MHC molecules. Some of these receptors silence the cytolytic machinery of NK cells and act as killer cell inhibitory receptors (KIR). Other receptors, designated leukocyte immunoglobulin-like receptors (LIR), are expressed mainly on monocytes and B cells. Engagement of an NK receptor can also result in NK activation as not all receptors carry an intracellular silencing domain. The HCMV protein UL18 and the MCMV m144 protein, which are homologous to MHC class I, could be associated with NK killing, and UL18 was instrumental in the identification of LIR-1. In addition, the HCMV UL40 protein provides a peptide selectively required for the maturation of the HLA-E molecule, an NK target 39, 40 . However, clinical isolates of HCMV confer a much stronger NK resistance than the laboratory strains sequenced and tested so far, and this resistance is unrelated to MHC class I expression and LIR-1 (Ref. 41) . Clinical isolates carry additional genes, and in vitro propagation has probably led to a loss of certain NK-specific gene functions. Effects on MHC class II expression fall into two classes, namely effects on transcription and posttranslational effects 1, 11, [32] [33] [34] . Adenovirus, MCMV and HCMV affect MHC class II transcription but the target in the signal cascade, although known to be different for these viruses, has not been defined and the viral gene or genes responsible are unknown. At the post-translational level, the HCMV US2 protein, which affects MHC class I, apparently also translocates the DR␣ and the DM␣ chain into the cytosol for degradation by the proteasome. Another target involved in interference with MHC class II function is the shuttling between endosomal peptide loading and surface expression. Human papilloma virus (HPV) and HIV Nef affect vesicle traffic as well as the function of the endocytic machinery. Accordingly, in addition to MHC class II, other proteins that use this pathway, for example the CD4 molecule, are also affected. An understanding of the functions of the viral immunoregulatory genes isolated to date is now emerging. However, we do not yet know whether the list is complete (Table 7) . Additionally, it is unclear when and why a virus deploys one specific function rather than another. Many questions therefore remain unanswered, including which genes are needed during primary infection to 'conquer the territory'; which genes are required to support active replication; and which genes are required to ensure transmission to a new host in the face of a vigorous host immune response? Moreover, why is there such complexity and functional redundancy? Is there a hierarchy in terms of general importance or do some functions operate only in certain tissues? Is complexity and redundancy a viral strategy to enable viruses to infect individuals resistant to some functions? Are the functions of an individual viral gene modulated by its genetic context, and is there any evidence for cooperativity? To date, we only have limited information because the construction of virus mutants and the in vivo testing of the predicted gene function is still in its infancy and, additionally, owing to the species specificity of many viruses, this information can only be gathered from some animal models. The identification of novel immune-evasion strategies and the analysis of their functions in the context of a viral infection should lead to a better understanding of the immune system and the interaction of viruses with their hosts. This will help us to treat virus-induced pathology, to design safer and more immunogenic virus vectors as vaccines or gene delivery systems, and to identify new strategies of immune modulation. Viral subversion of the immune system Vaccinia virus immune evasion Poxviral mimicry of complement and chemokine system components: what's the end game? Virus attenuation after deletion of the cytomegalovirus Fc receptor gene is not due to antibody control Virus interception of cytokineregulated pathways Poxviruses: interfering with interferons Interferons: cell signalling, immune modulation, antiviral responses and virus countermeasures IL-18 binding and inhibition of interferon gamma induction by human poxvirus-encoded proteins Ectromelia, vaccinia and cowpox viruses encode secreted interleukin-18-binding proteins A poxvirus protein that binds to and inactivates IL-18 and inhibits NK cell response One step ahead of the game: viral immunomodulatory molecules Human cytomegalovirus harbors its own unique IL-10 homolog (cmvIL-10) A viral mechanism for inhibition of the cellular phosphatase calcineurin Signal transduction from the Epstein-Barr virus LMP-1 transforming protein Immunomodulation by viruses: the myxoma virus story Modulating chemokines: more lessons from viruses Shared resources between the neural and immune systems: semaphorins join the ranks Poxviruses: capturing cytokines and chemokines A third distinct tumor necrosis factor receptor of orthopoxviruses Vaccinia virus strains Lister, USSR and Evans express soluble and cell-surface tumour necrosis factor receptors Orf virus encodes a novel secreted protein inhibitor of granulocyte-macrophage colony-stimulating factor and interleukin-2 Abduction of chemokine elements by herpesviruses Cytomegalovirus-encoded beta chemokine promotes monocyte-associated viremia in the host HIV-1 Tat protein mimicry of chemokines RANTES binding and down-regulation by a novel human herpesvirus-6 beta chemokine receptor The human cytomegalovirus chemokine receptor US28 mediates vascular smooth muscle cell migration Transgenic expression of the chemokine receptor encoded by human herpesvirus 8 induces an angioproliferative disease resembling Kaposi's sarcoma Structure of a soluble secreted chemokine inhibitor vCCI (p35) from cowpox virus A broad spectrum secreted chemokine binding protein encoded by a herpesvirus Apoptosis: an innate immune response to virus infection Control of apoptosis by poxviruses A comparison of viral immune escape strategies targeting the MHC class I assembly pathway From sabotage to camouflage: viral evasion of cytotoxic T lymphocyte and natural killer cell-mediated immunity Immune evasion by cytomegalovirussurvival strategies of a highly adapted opportunist The luminal part of the murine cytomegalovirus glycoprotein gp40 catalyses the retention of MHC class I molecules Downregulation of major histocompatibility complex class I molecules by Kaposi's sarcoma-associated herpesvirus K3 and K5 proteins Kaposi's sarcoma-associated herpesvirus encodes two proteins that block cell surface display of MHC class I chains by enhancing their endocytosis Inhibition of MHC class I-restricted antigen presentation by gamma2-herpesviruses Surface expression of HLA-E, an inhibitor of natural killer cells, enhanced by human cytomegalovirus gpUL40 The human cytomegalovirus UL40 gene product contains a ligand for HLA-E and prevents NK cellmediated lysis Human cytomegalovirus straindependent changes in NK cell recognition of infected fibroblasts Immune evasion by adenoviruses The equine herpesvirus 2 E1 open reading frame encodes a functional chemokine receptor A highly selective CC chemokine receptor (CCR)8 antagonist encoded by the poxvirus molluscum contagiosum A novel vascular endothelial growth factor encoded by Orf virus, VEGF-E, mediates angiogenesis via signalling through VEGFR-2 (KDR) but not VEGFR-1 (Flt-1) receptor tyrosine kinases M11L: a novel mitochondria-localized protein of myxoma virus that blocks apoptosis of infected leukocytes Ectromelia virus virulence factor p28 acts upstream of caspase-3 in response to UV light-induced apoptosis A cytomegalovirus-encoded mitochondria-localized inhibitor of apoptosis structurally unrelated to Bcl-2 Retrograde protein translocation: ERADication of secretory proteins in health and disease Deletion of a CD2-like gene, 8-DR, from African swine fever virus affects viral infection in domestic swine Letters to Trends in Microbiology Trends in Microbiology welcomes correspondence and discussion. Do you wish to share your views with other microbiologists or comment on recent articles in Trends in Microbiology or other literature sources? Please send letters to tim@current-trends.com, marked for the attention of the Editor The work in the authors' laboratories is funded by the Wellcome Trust and the Deutsche Forschungsgemeinschaft. Antonio Alcami is a Wellcome Trust Senior Research Fellow.