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Published online before print October 4, 2005
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Laboratory of Immunovirology, Ste-Justine Hospital Research Center, Department of Microbiolgy and Immunology, University of Montreal, Quebec, Canada
1 Correspondence: Laboratory of Immunovirology, Ste-Justine Hospital Research Center, 3175 Cote Ste Catherine, Montreal, Quebec, H3T 1C5, Canada. E-mail: ali.ahmad{at}recherche-ste-justine.qc.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONANTIVIRAL IMMUNE RESPONSES OF...VIRAL IMMUNE EVASION STRATEGIESCONCLUSIONSREFERENCES |
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Key Words: antigen presentation • CTL • NK cells • MHC antigens • viral infections
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONANTIVIRAL IMMUNE RESPONSES OF...VIRAL IMMUNE EVASION STRATEGIESCONCLUSIONSREFERENCES |
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ANTIVIRAL IMMUNE RESPONSES OF THE HOST |
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TOPABSTRACTINTRODUCTIONANTIVIRAL IMMUNE RESPONSES OF...VIRAL IMMUNE EVASION STRATEGIESCONCLUSIONSREFERENCES |
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(IFN-). The killing of virus-infected cells is an important danger signal to initiate immune response. The NK cell-secreted IFN- plays an important role in inducing an effective antiviral immune response. An important event is the induction of type I (
and ß) IFN. Although almost all cell types can produce these IFN, a specialized cell type, precursor plasmacytoid DC, produces 1000-fold more of these cytokines and is called the natural IFN-producing cell [3
, 4
]. IFNs also increase expression of major histocompatibility complex (MHC) class I and II antigens and of costimulatory molecules on the surface of so-called antigen-presenting cells (APC). The professional APC include DC, macrophages, and B cells. They present virus-derived antigenic peptides to naïve CD8+ T cells and CD4+ T cells in association with MHC class I and class II antigens, respectively (Fig. 1
). This antigen presentation is a critical step in the induction of virus-specific immunity by the adaptive immune system. Activation of the innate immune system plays an instructive role (adjuvant effect) for the induction of virus-specific, adaptive immune responses. In general, exogenous viral particles and viral antigens are phagocytosed and/or endocytosed by APC. They are then degraded in lysosomes, and immunogenic peptides are presented in association with MHC class II antigens to naïve CD4+ T cells [5
]. The virus-specific CD4+ T cells provide essential help for the induction of antiviral CTL, antibodies, and memory T cells. The T helper cells (Th) are further divided into two types: TH-1 and TH-2 [6
]. The two types of the Th cells differ in the expression of their cytokine profiles. The TH-1 and TH-2 cells produce and differentiate in response to IFN- and interleukin (IL)-4, respectively. The role of IFN- in the differentiation of TH-1 cells, however, is indirect, i.e., by inducing the production of IL-12 from macrophages and DC. In addition to IFN-, the TH-1 cells produce IL-2 and TNF-ß. They promote the production of immunoglobulin G2a (IgG2a) in mice and IgG1 and IgG3 in humans and activate macrophages and CD8+ CTL. These responses are essential for clearing intracellular pathogens. The TH-2 cells produce IL-4, IL-5, IL-9, and IL-13 and promote the production of IgG1 and IgE in mice and IgG4 and IgE in humans. They inhibit macrophage activation and promote differentiation and growth of mast cells and eosinophils. These TH-2 cell-induced allergic inflammatory responses are important in clearing extracellular parasites. Efficient induction of virus-specific type 1 CD4+ helper responses is believed important for inducing effective antiviral immune responses in the host. Studies from several viruses have demonstrated an essential role of virus-specific CTL in controlling viral replication [7
, 8
]. For the generation of CTL, APC present antigenic peptides derived from the endogenously expressed viral proteins in association with classical MHC class I molecules to naïve CD8+ T cells. These CD8+ T cells expand and differentiate into virus-specific effector CTL. The virus-specific CD4+ Th cells also play an important role in the generation of CTL and virus-specific memory T cells. The CTL kill virus-infected cells by recognizing their cognate virus-derived peptides in association with MHC class I molecules. They kill them by exocytosing several cytotoxic molecules, e.g., perforin, granzymes, and granulysin, in the immune synapse formed between CTL and the target cell. Fas/FasL and TRAIL/DR interactions may also play a role in this killing. The generation of virus-specific memory T cells is important for an efficient virus-specific anamnestic response, a criterion desired for antiviral vaccines.
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NK cells may also kill virus-infected cells without help from antibodies [10
]. These cells, however, usually do not recognize any viral antigen or viral peptide per se. Their effector function is controlled by a complex system of inhibitory and activating NK cell receptors and coreceptors. They kill target cells unless inhibited by the engagement of inhibitory receptors by their cognate ligands on the target cells. The most important inhibitory receptors include killer cell Ig-like receptors (KIR), NKG2/CD94A, and Ig-like transcripts (ILT), which bind to classical and nonclassical MHC class I antigens; also called human leukocyte antigen (HLA)-A, -B, -C, -E, and -G (Table 1
). It is noteworthy that most of the KIR recognize HLA-C and inhibit NK cells. A down-regulation of the MHC antigens on the surface of virus-infected cells usually makes them susceptible to NK cell-mediated killing [10
, 11
]. Despite their different mechanisms of recognition of virus-infected cells, NK cells and CTL represent the most important cytolytic cells leading to the elimination of tumor and virus-infected cells from the host. Furthermore, both cell types secrete cytokines such as IFN- and TNF-, which interfere with viral replication without causing cell death [12
].
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VIRAL IMMUNE EVASION STRATEGIES |
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TOPABSTRACTINTRODUCTIONANTIVIRAL IMMUNE RESPONSES OF...VIRAL IMMUNE EVASION STRATEGIESCONCLUSIONSREFERENCES |
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Interference with antigen presentation via MHC class I and induction of antiviral immune responses
As APC present virus-derived antigenic peptides in association with MHC class I antigens to prime antiviral CTL, viruses interfere with this antiviral response by down-regulating the expression of MHC class I molecule on the surface of APC. The virus-specific CTL recognize virus-derived antigenic peptides in association with MHC class I antigens. A decreased expression of these antigens on the surface of the virus-infected cells prevents their recognition and killing by the CTL. As shown in Figure 2
and summarized in Table 2
, viruses use many different strategies for this purpose. They may do so by the following:
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Inhibiting proteasome-mediated degradation and generation of peptides
The expression of MHC class I antigens on the cell surface requires the availability of peptides in the ER. The peptides are produced in the cytosol via proteasomal degradation of viral proteins (Fig. 2)
. Many viruses have developed the strategy of inhibiting this degradation and limiting the pool of available peptides. For example, the EBV encodes a nuclear protein, EBNA-1, which is essential for replication of the viral episome in dividing virus-infected/transformed cells. The protein contains a glycine-alanine-rich (GAr) domain, which inhibits its degradation by the 26S proteasome, thus reducing the pool of EBNA-1-derived peptides that could be presented with MHC class I antigens on the cell surface [23
]. Furthermore, the GAr motif also inhibits translation of the EBNA-1 mRNA in cis, and this effect can be distinguished from its effect on proteasomal degradation. By limiting its production at the translational level, EBNA-1 effectively decreases synthesis of defective ribosomal products (DRIPs). It is noteworthy that DRIPs undergo enhanced degradation and are the major source of peptides. Thus, EBV translates a functional level of EBNA-1 needed for its replication without antigenic presentation by the MHC class I, which could lead to the generation of CTL against this viral protein as well as recognition of the infected cells by virus-specific effector CTL [23
].
Blocking TAP functions
Many viruses can inhibit the loading of antigenic peptides onto MHC class I molecules by blocking functions of TAP [24
], which translocates peptides generated in the cytosol by proteasomal degradation into ER for loading onto nascent MHC class I molecules. As stated earlier, without peptides, MHC class I molecules cannot fold properly and be expressed on the cell surface. TAP exists as a heterodimeric complex comprising TAP-1 and TAP-2 and is an essential component of the PLC. The other components of the complex include Tapasin, MHC class I chain, calreticulin, Erp57, and ß2m (Fig. 2)
. Tapasin forms a bridge between TAP and MHC class I and edits quality of the MHC-bound peptides. Calrecticulin monitors proper glycosylation pattern of the nascent MHC molecules specifically recognizing N-linked glycans, and Erp-57 is a thiol oxido-reductase, which isomerizes intrachain S–S bonds. Many viruses encode proteins that can interfere with TAP functions and hence, with the translocation of peptides into the ER. The bovine herpesvirus-1-encoded protein UL49.5 is a potent inhibitor of TAP. It inhibits TAP by inducing a conformational arrest of the transporter as well as by targeting TAP to proteasomal degradation [25
]. It is noteworthy that UL49.5 homologues are found in two other varicelloviruses: pseudorabies virus and equine herpesvirus-1. The adenovirus early transcription unit-3 (E3)-19K and the HSV-1 protein infected cell peptide 47 (ICP47) can also inhibit peptide translocation into ER by blocking functions of TAP leading to a decrease in cell surface expression of MHC class I antigens [26
, 27
]. The ICP47 binds to the cytosolic side of TAP and blocks its function, whereas E3-19K binds TAP and MHC and acts as a competitive inhibitor of tapasin. The HCMV-encoded protein US6 can transiently associate itself with the TAP complex [28
]. This association inhibits the peptide translocation toward the ER and prevents maturation and presentation of MHC class I at the cell surface [29
30
31
32
]. It has also been demonstrated that the US6 binds to the luminal side of TAP and allosterically inhibits its ATPase activity [33
, 34
]. The disruption of TAP function, however, does not affect expression of HLA-E, a nonclassical MHC class I molecule, which binds peptides derived from MHC class I signal sequences and confers protection from NK cell-mediated lysis [35
].
Degradation of PLC and MHC class I antigens
Many viruses can interfere with antigen presentation via MHC class I by degrading the PLC. The HCMV unique short region genes encode at least four proteins US2, US3, US6, and US11. Each of them can independently down-regulate the expression of MHC class I antigens on the surface of the virus-infected cells. The US2 and US11 induce a rapid degradation of the nascent HLA class I molecules during their synthesis [36
, 37
]. The US2 binds to the MHC class I molecules during their glycosylation, leading to their retrograde transport to the cytoplasm and the degradation of the whole complex [38
]. It is interesting that none of these proteins degrades the HCMV-encoded MHC homologue UL18. The latter protein forms heterotrimeric complexes with ß2m and endogenous peptides, providing protection from NK cell-mediated lysis and inhibiting macrophage activation via its interaction with an inhibitory receptor ILT-2, expressed on NK cells and macrophages [39
40
41
42
]. The homologue may also sequester ß2m and inhibit MHC class I expression on the cell surface. Crystallographic studies have shown that US2 associates with HLA-A2 at the junction of the peptide-binding region and the 3 domain, a binding surface that allows US2 to bind the MHC molecule independently of the peptide sequence and to exert its down-regulatory effects [43
].
Poxviruses and -herpesviruses share the K3 family of viral immune evasion proteins (immunoevasins), which possess an amino-terminal plant homeodomain/leukemia-associated protein domain or more specifically, a really interesting new gene with conserved cysteins and histidine residues (RING-CH) domain, followed by two transmembrane domains. The K3 family proteins have ubiquitin (Ub) ligase activity [44
, 45
]. They inhibit the surface expression of glycoproteins, such as MHC class I heavy chains, B7.2, ICAM-1, or CD95, by targeting them to Ub-directed proteasomal degradation. The human homologues of these immunoevasins are the membrane-associated RING-CH (MARCH) proteins, which have functional similarity with K3 proteins. This suggests that these viral immune evasion proteins have been derived from the cellular MARCH proteins. The MARCH proteins regulate endocytosis of cell surface receptors via ubiquitinylation [46
]. The KSHV proteins K3 and K5 [also called modulator of immune recognition (MIR)-1 and -2, respectively] as well as the MHV-68 protein MK3 prevent the surface expression of MHC class I molecules [47
48
49
]. MIR-1 and MIR-2 also down-regulate the surface expression of CD-1 [50
], a family of antigen-presenting molecules, which are distantly related to MHC class I molecules and present lipid and glycolipid antigens to T and NK-T cells. The protein MK3 resides in the ER membrane, where it binds to and ubiquitinylates the cytoplasmic tails of newly synthesized MHC class I heavy chains while bound to peptides in the PLC, leading to their proteasome-dependent degradation [51
]. It can also degrade Tapasin and TAP in a RING finger-dependent manner [52
]. Studies about a model for the interaction of MK3 with MHC-I and the PLC have shown that MK3 interacts with TAP-1 and -2 via their C-terminal domains and with class I molecules via their N-terminal domains [53
]. It is interesting that the K5-mediated down-regulation of MHC class I molecules does not render the virus-infected cells susceptible to NK cell-mediated lysis, as it also down-regulates the expression of ICAM-1 and B7.2 on the infected cells. These molecules act as ligands for NK cell-mediated cytotoxicity. De novo expression of B7.2 and ICAM-1 in the K5-expressing cells restores their sensitivity to NK cells [54
]. Furthermore, unlike K3, which down-regulates all MHC allotypes, K5 only degrades HLA-A and -B but not HLA-E, and the effect on HLA-C is weak [54
]. The myxoma virus (a poxvirus) ER resident protein M153R down-regulates MHC class I and has been shown to have Ub-ligase activity in vitro [55
].
Viruses have evolved strategies to affect intracellular trafficking of MHC class I antigens and cause its retention inside the ER. The HCMV US3 protein associates itself with the MHC class I heavy chain/ß2m complex and causes its retention in the ER without interfering with the maturation [56 , 57 ] and the movement of the complex through the Golgi apparatus [58 , 59 ]. MCMV has been shown to encode three genes, m152, m6, and m4, which are involved in the interference with MHC-I expression and/or recognition. The m152 blocks the export of MHC-I from a pre-Golgi apparatus, whereas m6 directs it to lysosomal degradation (Fig. 2) . The MCMV m4 encodes a glycoprotein, gp34, which is expressed on the cell surface in a complex with MHC class I. It does not inhibit the surface expression of the class I but inhibits its recognition by H-2Kb-restricted CTL. Thus, m4 acts as a viral CTL evasion protein without affecting expression of MHC-1. It is relevant to mention here that the m152/gp40-mediated inhibition of H-2Db is complete, but that of H-2Kb is partial. Therefore, MCMV needed m4 as an additional strategy to inhibit Kb recognition by CTL clones [60 ]. Indeed, m152 appears sufficient to abolish Db-restricted presentation in the virus-infected primary macrophages, but m4, m6, and m152 are required to escape the recognition of virus-infected cells by Kb-restricted CTL [61 ].
The adenovirus E3-19K protein can also block cell surface expression of MHC class I by specifically preventing their terminal glycosylation, correct folding, and export from ER [62 ]. The human herpesvirus 7 (HHV7) protein U21 associates with the MHC class I inside the ER and directs its traffic toward the lysosomes for degradation [63 ].
Differential down-regulation of MHC class I antigens
A global indiscriminate down-regulation of MHC class I molecules on the surface of virus-infected cells may prevent their recognition from virus-specific CTL. However, this strategy also renders the infected cells susceptible to NK cell-mediated killing. As stated earlier, MHC class I molecules, particularly HLA-C, act as ligands for inhibitory NK cell receptors, e.g., KIR. A loss or a decreased expression of these HLA alleles on the surface of virus-infected cells results in a loss of inhibition of NK cells. To evade killing by NK cells and virus-specific CTL, many viruses have evolved strategies to differentially down-regulate MHC class I molecules. More specifically, they down-regulate expression of HLA-A and -B, which mainly present viral epitopes to CTL, but not the expression of HLA-C and HLA-E, which act as ligands for inhibitory NK cell receptors. HIV-1 uses this strategy via Nef protein, which binds hypophosphorylated cytoplasmic tails in early forms of the MHC class I antigens in the ER and redirects them from the trans-Golgi network (TGN) to endosomal degradation [64
]. Indeed, studies have shown that all Nef domains (the N-terminal helix, polyproline, acidic, and oligomerization domains) are involved in this association [65
]. Nef interacts selectively with the intracellular tyrosine motifs of different HLA-A and HLA-B allotypes [66
]. However, the HLA-C and HLA-E do not have these tyrosine motifs and are not targeted by Nef [67
], which interacts with the µ subunit of the cellular adaptor protein (AP) complex and recruits it to the MHC cytoplasmic tails. This interaction with AP causes endocytosis and retrograde trafficking of the MHC molecules from the cell surface. They accumulate in clathrin-coated vesicles and are targeted to degradation. However, the Nef mutants, which do not interact with AP, can also down-regulate MHC expression [64
, 65
]. Piguet et al. [68
] have shown that Nef-mediated down-regulation of the MHC antigens involves interaction between the acidic domain of Nef and phosphofurin acidic cluster sorting (PACS)-1, a molecule that localizes the cellular protein furin to the trans Golgi network (TGN). According to their model, Nef acts as a connector between the cytoplasmic tails of the MHC antigens and PACS-1-dependent protein-sorting pathway. In T cells, however, Nef mediates down-regulation of the MHC molecules via disrupting its secretory pathway from the TGN to the cell surface, whereas in non-T cells, these effects of Nef on the transport of the MHC molecules to the cell surface are less pronounced [64
, 69
]. Overall, Nef inhibits expression of HLA-A and -B alleles on the cell surface and protects the infected cells from CTL-mediated lysis [66
, 69
70
71
]. Indeed, the effects of Nef on MHC surface expression have been shown to be important for the progression of the HIV infection toward AIDS [72
, 73
].
As mentioned above, the KSHV proteins K3 and K5 have the capacity to internalize the MHC class I antigens by Ub-directed degradation from the cell surface. The two proteins differ in their specificity for different MHC alleles. The K5 down-regulates HLA-A and -B efficiently but not HLA-C and -E. The K3, conversely, down-regulates all MHC class I allotypes. The K5 also down-regulates the surface expression of ICAM-1 and B7.2 in the virus-infected cells. This differential down-regulation of the MHC molecules as well as of ICAM-1 and B7.2 confers resistance to NK cell-mediated lysis to the virus-infected, MHC-deficient cells [54 , 74 ].
It seems that many viruses encode proteins to down-regulate the expression of MHC class I molecules from the surface of the infected cells (Fig. 2 ; Table 2 ). They do so primarily to evade host’s antiviral CTL responses. However, certain viruses may in fact increase the expression of these molecules on the surface of the infected cells, at least in the early phase of the infection, when NK cells are activated. For example, flaviviruses stimulate TAP activity by up to 50% [75 ]. More specifically, the Hepatitis C virus (HCV) core protein was shown to activate TAP functions via p53 induction [76 ]. This enhances the TAP-dependent peptide import into the ER lumen and increases the surface expression of MHC class I antigens. The virus-infected cells consequently become more resistant to NK cell-mediated killing. Activated NK cells seem to be important in limiting viral replication, at least in the early phases of the infection before the generation of virus-specific CTL and antibodies.
Down-regulating the expression of MHC class II on the surface of virus-infected cells
The expression of MHC class II molecules on the surface of professional APC is essential for presentation of foreign antigenic peptides to CD4+ T lymphocytes. This presentation results in the generation of antigen-specific CD4+ Th cells. The professional APC-like macrophages, DC and B cells take up exogenous viral proteins by phagocytosis or endocytosis. These cells generate antigenic peptides by protease action in endosomal compartments that are presented by MHC class II molecules, encoded by three different loci (HLA-DP, -DQ, and -DR). The heterodimeric /ß chain constituting the MHC class II is strongly associated with the invariant chain (Ii) in the ER in a nonameric complex and represents an immature MHC-II form. The MHC-II /ß/Ii nonameric complexes are targeted to the MIIC, which are late endosome/lysosome-like compartments. During this transport, proteases present in the endosomes partially cleave the invariant chain, via a series of defined cleavage intermediates, to generate class II-associated Ii peptide, which occupies the peptide-binding groove of the MHC class II until it is exchanged by an antigenic peptide in the MIIC. This process of peptide loading is catalyzed by HLA-DM and -DO (in B cells) inside the MIIC [77
78
79
80
]. This exchange leads to the constitution of a stable heterotrimeric MCH class II peptide complex, mature MHC class II, which can now reach the cell surface. By inhibiting the MHC class II antigenic presentation at different levels, viruses interfere with the generation of virus-specific CD4+ T cells and hence, with the induction of an effective antiviral cellular immune response.
Viruses encode proteins that may interfere with expression of MHC class II antigens (by down-regulating their transcription and/or by disrupting their normal traffic within the cells); loading of peptides onto these antigens; and their presentation to naïve CD4+ T cells by disrupting the interaction between MHC class II antigens and TCR (Fig. 3 , Table 2 ). This is a relatively less-studied aspect of viral immune evasion. However, in recent years, many viral proteins have been shown to interfere with antigen presentation via MHC class II pathway.
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The adenovirus E1A protein can efficiently inhibit IFN--induced up-regulation of HLA class II genes by inhibiting interaction between the cyclic AMP response element-binding protein (CREB)-binding protein (CBP) and the CIITA (ref. [81
], reviewed in ref. [82
]). The IFN-mediated effects on the MHC II expression are important for the induction of an effective antiviral immune response.
Another way to subvert the antigen presentation is to alter the intracellular trafficking of the class II antigens. In HSV-infected cells, the viral glycoprotein B competes with the Ii chain for binding with HLA-DR molecules. In addition, it also associates with HLA-DM. It disturbs intracellular trafficking of MHC class II and prevents them from reaching the cell surface [83
, 84
]. The HIV Nef also impairs the membrane expression of mature (peptide-loaded) MHC class II molecules and promotes the surface expression of their immature (peptide-lacking) forms. The Nef expression induces a marked accumulation of multivesicular bodies (MVB) containing Nef, MHC class II, and high amounts of Ii [85
, 86
]. It is interesting that HIV-1 recruits MVB machinery for budding in macrophages. The HCMV US2 and US3 proteins are also involved in the subversion of antigen presentation to CD4+ T cells via MHC class II. The two proteins collaborate to achieve this end [87
]. US2 causes rapid retrotranslocation of class II proteins DR- and DM- from the ER, followed by their proteasome-mediated degradation [88
]. US3 binds to the newly synthesized MHC class II /ß complexes in the ER and reduces their association with Ii. This complex moves normally to the Golgi apparatus but is not sorted efficiently to the MIIC, leading to a reduction of the peptide-loaded, mature MHC class II complexes on the cell surface and of their recognition by CD4+ T cells [89
].
Prevention of the MHC class II-TCR interaction
The EBV lytic cycle protein gp42 is a type II transmembrane glycoprotein, which binds HLA-DR. This binding is essential for viral entry into DR-positive B cells. The viral protein also associates with MHC class II molecules at various stages of their maturation, e.g., immature -ß-li heterotrimers and mature -ß-peptide complexes, and inhibits antigen presentation to CD4+ T cells. It is interesting that a soluble form of gp42 is generated by proteolytic cleavage in the ER of the virus-infected cells. The protein is secreted and inhibits HLA class II-restricted antigen presentation to T cells by physically hindering the MHC class II-TCR interactions. The transmembrane and soluble forms of the protein are expressed in the EBV genome-positive Burkitt’s lymphoma cells during lytic infection of the virus [90
, 91
]. Another example in this case is the envelope protein of HIV-1, gp120, which binds CD4 and interferes with CD4-MHC class II interactions [92
].
By down-regulating the expression of costimulating molecules
A variety of costimulatory molecules is expressed on the surface of professional APC and other host cells. These molecules interact with their cognate ligands on immune cells. This interaction plays an essential role in the presentation of viral antigens to T cells and B cells and for the induction of an effective antiviral cellular immunity. Costimulation is also important for the efferent or effector phase of the immune response. For example, stimulation of CD4+ T cells via antigen alone (MHC class II molecules loaded with the receptor-specific peptides) would not proliferate and produce IFN- unless costimulated via B7.1 and CD28 interactions. Instead, they would rather become anergic or undergo apoptosis. Similarly, IL-2-activated NK cells would undergo apoptosis if stimulated only via CD16. Many viruses inhibit host’s antiviral immune responses at the inductive and effector phases by down-regulating the expression of costimulatory molecules on host cells. For example, the KSHV-K5 down-regulates surface expression of the costimulatory molecules ICAM-1 and B7.2 on the surface of virus-infected cells [93
, 94
]. The Myxomavirus homologue of the K5, M153R, is also a Ub ligase. It targets MHC class I antigens and CD4 and internalizes and redirects them to proteasomal degradation. The M153R-mediated degradation is dependent on the presence of lysine residues in the cytoplasmic tails of the target proteins [55
, 95
]. The adenovirus oncoprotein E1A decreases the expression of another adhesion molecule lymphocyte function-associated antigen-3 on the surface of Ad5- and Ad12-transformed cells [96
]. It is noteworthy that Nef, Vpu, and Gp160 of HIV-1 reduce surface expression of CD4 and CD28 on the virus-infected cells. Therefore, HIV-infected cells cannot provide proper costimulation when they interact with virus-specific T cells [71
, 85
].
The induction of a virus-specific CTL response to HCMV and MCMV represents the main and most efficient effector function for the control of these pathogens [97
98
99
100
]. The HCMV main tegument protein pp65 and the immediate early protein-1 (IE1) are the major targets for the antiviral CTL raised against HCMV-infected cells [97
]. The pp65 has kinase activity. It phosphorylates and inhibits presentation of IE proteins, such as pp72, to CD8+ T cells via MHC class I antigens [101
]. The pp72 is an essential viral transcription factor. It is interesting that the HCMV-specific CTL response is dominated by the pp65. This protein was recently shown to act as a ligand for NKp30, an activating NK cell receptor. The binding of the protein to the receptor causes dissociation of the receptor-associated signaling component, the 
chain [102
], which acts as a signaling component for several other activating receptors found on the surface of NK and T cells. Thus, the protein may cause a general immunosuppression in the infected host.
Evading host’s NK cell responses
NK cells are a population of bone marrow-derived, low-density, large granular lymphocytes. They constitute 10–15% of the lymphocytes in blood [103
]. They can kill certain virus-infected and tumor cells without prior activation and sensitization. Apart from killing virus-infected cells, NK cells play an important role in immune regulation by secreting immunologically important cytokines and chemokines, e.g., including IFN-, TNF-, macrophage-inflammatory protein-1 (MIP-1), and MIP-1ß. The NK cell-secreted IFN and TNF- may also control viral replication by noncytolytic mechanisms. NK cells and mature DC reciprocally activate each other. As stated earlier, NK cells are usually activated in early phases of a viral infection. Activated NK cells are important in killing virus-infected cells, especially before the generation of virus-specific CTL and antibodies. Unlike T and B cells, which express well-defined, clonally distributed antigen receptors, NK cells activity is controlled by a diverse array of activating and inhibitory receptors and coreceptors, which bind different ligands present on the surface of a target cell and send activating and inhibitory signals to the NK cell. The balance between the inhibitory and activating signals determines whether the NK cells would kill the target cell or be inhibited from killing it. In recent years, a great deal has been learned about these receptors and their ligands [11
, 104
]. The known NK cell receptors and coreceptors as well as their ligands are given in Table 1 . The most important ligands, which bind inhibitory receptors on NK cells and inhibit their activity, are MHC class I antigens, especially HLA-C and -E. Most body cells and tumor cells usually express ligands for some activating NK cell receptors. NK cells would kill these cells by default unless they are inhibited by the engagement of their inhibitory receptors. The presence of MHC class I antigens on the surface of a cell usually makes it resistant to NK cell-mediated killing.
As NK cells could play an important role in controlling virus replication, viruses have evolved many strategies to evade host’s NK cell responses (see Table 3 ).
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The viruses may also evade NK cell responses by increasing the expression of HLA-E. For its expression on the cell surface, this nonclassical HLA molecule needs peptides derived from the signal sequences of HLA-G and many HLA-A, -B, and -C allotypes. The HCMV protein UL-40 acts as a source of the peptides that can bind HLA-E. Thus, by supplying a source of HLA-E-specific peptides, UL-40 stabilizes the expression of HLA-E on the surface of HCMV-infected cells [105 106 107 108 ]. HLA-E inhibits NK cell activation by interacting with the inhibitory receptor CD94/NKG2A. As mentioned earlier, HCMV encodes several proteins to reduce the expression of MHC class I on the surface of the virus-infected cells. As HLA-E needs peptides derived from the signal sequences of other MHC allotypes, the decreased expression of MHC class I would also have decreased the expression of HLA-E on the surface. By encoding UL-40, HCVM compensates for the loss of peptide pool for HLA-E. More recently, an immunodominant CTL epitope derived from the HIV protein p24 was also shown to bind HLA-E and increase the expression of this MHC antigen on the cell surface [109 ]. Thus, HIV may also evade NK cell-mediated lysis by stabilizing and increasing the expression of HLA-E on the surface of the virus-infected cells.
The HCMV also encodes a MHC homologue UL-18, which forms heterodimers with ß2m and can bind endogenous peptides. In addition to decreasing the expression of other MHC molecules by sequestering the ß2m, UL-18 interacts with an inhibitory receptor ILT-2 and inhibits NK cell activation [110 , 111 ]. It is interesting that ILT-2 is also expressed on macrophages, B cells, and DC. The virus-encoded protein may therefore also inhibit activation of these cell types.
When under stress or infected with a virus, the cells express certain stress-inducible proteins MIC-A, MIC-B, and ULBP1–4. These de novo expressed proteins interact with the NK cell receptor NKG2D and trigger NK cell activity. It is interesting that the NK cell activation mediated by NKG2D is not inhibited via inhibitory KIR. Moreover, NKG2D is also expressed on the surface of activated macrophages and certain T cells and is involved in the activation and costimulation of these cells. The HCMV-encoded protein UL16 binds to MIC-B and ULBP1 and -2 and decreases their cell surface expression [112 113 114 ]. This inhibits killing of the virus-infected cells via NKG2D. The ULBPs are glycosylphosphatidylinositol-linked glycoproteins distantly related to the MHC class I family. It is interesting that these proteins were first identified by their ability to bind to the HCMV protein UL16 [115 ]. The UL16 binds the NKG2D ligands intracellularly and redirects their intracellular trafficking for lysosomal degradation via a tyrosine-based sorting signal present in its cytoplasmic tail [115 , 116 ].
It has been demonstrated recently that the HCMV UL141 gene product blocks the surface expression of CD155, which is known as a ligand for the activating NK cell receptors DNAM-1 (CD226) and TACTILE (CD96) [117
]. UL141 is not the only HCVM protein that interferes with the interaction of an activating NK cell receptor with its ligand. The most immunodominant viral protein pp65 can also bind and inhibit the function of another activating NK cell receptor NKp30. The protein causes dissociation of the receptor from the signal-transducing partner, the chain [102
].
Like HCMV, the MCMV has also developed several strategies to evade host’s NK cell responses (Table 3) . Its m155 gene product can subvert the NK cell cytotoxicity by down-regulating H60, which is a stress-inducible protein that acts as a specific, high-affinity ligand for NKG2D [118 ]. The virus encodes two other proteins m152 and m145, which can interfere with the interaction of NKG2D with its ligands. The m152 binds H60 and Rae-1, whereas the m145 can bind and sequester MULT-1 intracellularly [119 , 120 ]. Like H60, Rae-1 and MULT-1 act as ligands for NKG2D for mouse NK cells. These examples clearly show that HCMV and MCVM have developed strategies to inhibit NKG2D-mediated NK cell killing of the virus-infected cells. These strategies may also inhibit macrophages and prevent costimulation of T cells via this activating receptor.
A great deal has been learned about the role of the MCMV protein m157 in determining susceptibility of the virus-infected cells to NK cell-mediated lysis [121 122 123 ]. The protein binds two NK cell receptors Ly49H and Ly49I. The Ly49H is an activating NK cell receptor, whereas Ly49I is an inhibitory one. The MCVM-resistant mouse strains express Ly49H, and MCVM-susceptible strains express Ly49I on their NK cells. The interaction of m157 with Ly49-positive NK cells leads to their activation, proliferation, and release of various cytokines and chemokines. The passage of the m157-positive MCMV in resistant Ly49-positive mice leads to mutations in m157 protein and escape from the NK cell-mediated control of the viral replication. Wild-type MCVM also shows mutations in this viral gene [123 ]. This is a classical example of a virus undergoing mutations under pressure from NK cell-exerted control.
The HCV-encoded major envelope protein E2 interacts with CD81. The latter molecule is a tetraspanin and is expressed as a complex with a variety of receptors on the surface of different cell types including T, B, and NK cells. The effects of CD81 cross-linking with specific antibodies may vary depending on the cell type. This cross-linking inhibits NK cells. Similarly, the binding of E2 to CD81 inhibits NK cell-mediated cytotoxicity and cytokine release (ref. [124 ], reviewed in ref. [125 ]). Furthermore, HCV encodes a serine protease complex, which is essential for cleaving HCV-encoded polyproteins into biologically active proteins. The protease was shown to inhibit activation (phosphorylation) of IFN regulatory factor-3 (IRF-3), probably by cleaving and inactivating an upstream kinase [126 127 128 ]. The activation is an essential step in the induction of type I IFN as well as in the IFN-mediated antiviral effects. As stated above, one of the effects of these IFN is to activate NK cells; HCV can evade NK cell activation by preventing IRF-3 activation. Other viruses also use similar strategies to inhibit NK cell activation. For example, the Ebola and Rabies virus-encodes P proteins and the respiratory syncytial virus (RSV)-encoded NS1 and NS2 proteins inhibit IRF-3 phosphorylation (reviewed in ref. [129 ]).
The ubiquitous human pathogen EBV has evolved a unique strategy to inhibit host’s NK cell responses. The viral protein EBNA-3A supplies peptides, which can bind certain HLA-A allotypes [130 ]. These HLA-peptide complexes are recognized specifically by the inhibitory NK cell receptors KIR3DL2. This recognition inhibits NK cells from killing EBV-infected/transformed host cells. It is interesting that a variety of peptides derived from different other human viruses, which bound these HLA allotypes, was not recognized by these NK cell receptors. It is not yet clear why humans have evolved these KIR receptors, which are used by EBV to evade their NK cell-mediated innate immunity against this virus.
Moreover, certain viruses encode proteins, which are MHC class I homologues and can inhibit NK cell activation. The HCMV encodes MHC class I -chain homologue UL18, which can complex with ß2m and bind endogenous peptides. It is resistant to down-regulation by the viral proteins US2, -3, -6, and -11 [41
, 131
132
133
]. Similarly, the MCMV encodes a MHC class I homologue, m144, which confers protection from NK cell effector functions, even when classical MHC class I antigens are down-regulated from the surface of the virus-infected cells. In vivo studies have shown that m144-expressing MHC class I-deficient lymphoma cells can inhibit activation and accumulation of NK at the site of immune challenge [134
].
Finally, viruses may induce de novo expression of certain MHC antigens and inhibit NK cell functions. For example, HIV induces HLA-G and HLA-E on the surface of HIV-infected cells [109 ]. Both molecules act as ligands for certain inhibitory NK cell receptors.
Evasion from CTL by antigenic variation
This is an important strategy evolved by RNA viruses, which have small genomes and cannot afford to encode many different immune-evasion proteins. Because of poor editing functions of the virus-encoded polymerases and a high rate of virus replication, several point mutations occur at random in structural and nonstructural viral protein genes. This leads to the existence of countless closely related, distinct viruses or "quasi-species" in the infected host, where its antiviral immune response exerts a selective pressure on these quasi-species. The virus-specific CTL are unable to recognize the virus-infected cells if the mutations happened to occur in the amino acid sequence of the epitopes recognized by the CTL. Under pressure from virus-specific CTL, the viruses carrying these mutations (escape mutants) accumulate in the infected individuals. In a similar manner, viruses may mutate to evade virus-specific CD4+ T cells and virus-neutralizing antibodies. When the infected host develops immune responses to the escape mutants, new escape mutants emerge, which can evade host’s antiviral immune responses. Furthermore, viruses may also undergo antigenic variation by recombination between diverse viral strains. By mutating its antigenic determinants, the virus always stays one step ahead of the immune response. This cat and mouse game continues between the virus and the host’s immune responses until host’s ability to mount an immune response is exhausted. All viral epitopes can undergo mutations unless the mutation is in a highly conserved region and compromises a key function of the protein. The mutant viruses may infect another host, and the mutations may persist if the new host does not restrict and present the mutated epitope. The mutated epitopes, at least in vitro, may act as altered peptide ligands and anergize or cause apoptosis of the virus-specific T cell clones. The escape mutants for HIV-1, HCV, and many other viruses have been studied extensively (reviewed in refs. [135
, 136
]). In the case of HIV-1, it has been documented that human populations are selectively accumulating viruses with mutated epitopes, which are presented by the most prevalent HLA allotypes. Nevertheless, the persistence of many epitope-encoding HIV sequences has been documented in the infected individuals having strong epitope-specific CTL responses, suggesting a complex relationship between immune evasion and antigenic variation Large DNA viruses, which cause chronic infections, such as EBV and HCMV, have also been documented to use this strategy to evade CTL responses [137
138
139
]. The RNA viruses with segmented genomes, such as influenza viruses, also undergo antigenic variation (antigenic shift) by a reassortment of genome segments between different viruses. Newly emerged recombinant viruses can evade the immunity, which is prevalent in the host. Such recombinant influenza viruses have caused great havoc in the human history. The influenza virus that caused the 1918 pandemic resulted from such reassortment events occurring between human and nonhuman influenza viruses [140
]. The antigenic variability of viruses is a great hurdle in developing effective antiviral vaccines.
Immune evasion through latency
The state of a reversible, nonproductive viral infection in the host cells is called latency. Viruses may evade immune responses of the host by becoming "latent" and invisible to the immune system. During latency, viruses may infect nonpermissive or semipermissive cells of the host and express only a minimum number of viral genes, which are just necessary to maintain the virus in the cells. The ubiquitous human pathogen EBV represents a classic example of viral latency [141
]. The virus only expresses one protein EBNA-1 and two nonpolyadenylated, short RNA molecules (EBV-encoded small RNA or EBER-1 and -2) in certain latently infected host cells. The virus becomes active and replicates only when the cell becomes activated. The newly produced virions then infect another lot of host cells. Some viruses may persist in immune-privileged tissues of the host, e.g., brain, retina, and kidney. For example, HSV-1 infects and replicates in epithelial cells but persists as latent infection with little gene expression in sensory neurons of Trigeminal ganglia, which do not express MHC antigens [142
]. The virus expresses only one gene, the latency-associated transcript gene, which inhibits viral replication. Upon proper stimuli, such as immunosuppression, trauma, or exposure to sun or ultraviolet radiation, the virus may activate itself and descend down axons of the neurons and infect epithelial cells. Similarly, Herpes zoster virus becomes latent in dorsal root ganglions of the spinal cord. Another herpesvirus, HCMV, persists for long periods of time in kidney, retina, and bone marrow. HIV-1 is known to persist as a latent transcriptionally inactive provirus in the host cell’s genome in long-lived, resting CD4+ memory T cells [143
]. These cells may lack virus-needed transcription factors. The virus may also persist in the brain, which is protected by blood brain barrier from infiltration of lymphocytes. These cells and tissues serve as reservoirs of the virus, which are resistant to chemotherapy and represent a real challenge for a complete elimination of the virus from the infected host.
Targeting immune cells
Many viruses have developed the strategy of infecting immune cells, which play a key role in orchestrating antiviral immune responses. For example, HIV-1 infects CD4+ T cells. The depletion of these cells is a hallmark of HIV-induced AIDS. It has been shown that HIV-specific CD4+ T cells are more susceptible to HIV infection than HCMV-specific CD4+ T cells, as the former cells preferentially migrate to the sites of HIV infection [144
]. CD4+ T cells play an important role in the generation of virus-specific CTL and antibodies. The lack of help from CD4+ T cells is probably one of the reasons for incomplete differentiation of HIV-specific CTL in HIV-infected individuals [145
, 146
]. Consequently, these CTL are compromised in their cytotoxic abilities and are unable to clear the infection [147
]. Many viruses, e.g., the reovirus and measles virus, infect DC and induce the expression of TRAIL and FasL on their surface [148
]. Such DC cannot present antigens and prime T cells for the generation of virus-specific CTL. Instead, they kill interacting T, B, and NK cells via Fas/FasL and TRAIL/DR interactions (Fig. 1)
. The virus-infected DC may in fact induce immunosuppression instead of an antiviral immune response. The human pathogen HSV-1 infects and induces apoptosis in immature DC by decreasing the expression of cellular Fas-associated death domain-like IL-1ß-converting enzyme (FLICE)-inhibitory proteins (cFLIP) at the mRNA level. The virus also increases the expression of TNF- and TRAIL in these cells. These ligands induce apoptosis in the virus-infected DC [149
]. The HIV protein Nef was shown to bind CXC chemokine receptor 4 and induce apoptosis of CD4+ T cells [150
]. Another HIV protein Vpr inhibits DC maturation and impairs their ability to activate virus-specific CTL and memory T cell [151
].
Interference with apoptosis of the virus-infected host cells
Apoptosis or programmed cell death is a physiological process, whereby the cell causes its own death through a regulated and controlled process of degradation of its protein and DNA contents by its own enzymes [152
]. It is a relatively silent and noninflammatory process. The cells may undergo apoptosis through an extrinsic or intrinsic pathway. The extrinsic pathway is activated when external factors such as TNF-, FasL, or TRAIL bind to their specific receptors, so-called death receptors or DR, a family of TNFR-related proteins expressed on the cell surface. The oligomerized DR recruit the adapter Fas-associated death domain (FADD) via their death domains (DD). The death effector domain (DED) of FADD interacts with the DED of procaspase 8 or 10 (also called FLICE). This results in the proteolytic cleavage and activation of these caspases. The intrinsic pathway is activated upon the release of cytochrome c, direct inhibitors of apoptosis proteases (IAP)-binding protein (DIABLO), and other proapoptotic factors from mitochondria. The cytochrome c forms a complex, the death-inducing signaling complex (DISC), with apoptosis protease-activating factor-1 and procaspase-9, resulting in the activation of the latter. DIABLO binds and inhibits cellular IAP and allows activated caspases to mediate their effects. Cells may undergo apoptosis through this pathway when subjected to irreparable DNA damage, viral infections, or physical and chemical insults. There is an active cross-talk between the two pathways. The activation of one may lead to activation of the other pathway. A critical step in this cross-talk is the cleavage of the proapoptotic protein Bid by caspase-8, which is activated by the extrinsic pathway. The cleaved Bid promotes cytochrome c release and activation of the intrinsic pathway of apoptosis. Both pathways lead to a series of caspase and DNase activation events, causing a controlled degradation of cellular proteins and DNA. It is noteworthy that NK and CTL use apoptosis as the principal mechanism for killing virus-infected cells. They do so by releasing certain cytotoxic molecules, such as TNF-, perforin, and granzymes, as well as by engaging DR on the surface of the virus-infected cells. It is noteworthy that the granzyme B, which is released by CTL and NK cells and is endocytosed by the target cells, can activate several caspases directly.
Viruses encode various proteins to modulate apoptosis to their own advantage (Table 4 ). They inhibit premature apoptosis of the virus-infected cells (before replication of the virus has occurred). After completion of the viral replication, viruses may promote apoptosis to disseminate progeny virus without causing inflammatory responses. Viral antiapoptotic strategies also help the virus evade CTL and NK cell-mediated killing of the virus-infected cells.
|
Many viruses can escape the apoptosis mediated via the extrinsic pathway by encoding viral FLIP (vFLIP), which mimick FLICE, contain DED, and associate themselves with DR but lack the caspase activity [153
, 159
]. The mechanism of action of vFLIP is shown in Figure 4
. Many -herpesviruses, including the HHV8, herpesvirus saimiri, equine herpesvirus 2, bovine herpesvirus 4, and moluscum contagiosum virus, encode vFLIP [160
, 161
], which disrupt recruitment of procaspase-8 to the DISC. Two forms (short and long) of the cellular ortholog of the vFLIP have also been identified (see below). They compete with the adaptor FLICE and regulate apoptosis [162
]. The HCMV UL36 gene product, the vICA, also associates with procaspase 8 and blocks its activation (Fig. 4) , but none has sequence identity with other vFLIP, suggesting that this viral protein represents a new class of cell-death suppressors [163
]. The vFLIP can also inhibit apoptosis by increasing the expression of nuclear factor (NF)-
B through their interactions with different adaptor proteins, including TNFR-associated factor-2, NF-B-inducing kinase, and inhibitor of IB kinase-2 [164
]. The cellular ortholog of vFLIP has been cloned, and it generates two protein forms as a result of alternate splicing: a short, 
26 kD, and a long, 55 kD, form. Both forms can delay or inhibit apoptosis by recruitment to the DISC [159
].
|
, and FasL [169
170
171
172
173
]. Eight cellular counterparts of vIAP have been identified. They can inhibit the effector (caspase-3, -6, and -7) and initiator caspases (caspase-9) and modulate apoptosis in cells (reviewed in refs. [152
, 153
]).
The HIV protein Nef protects the virus-infected cells from apoptosis by interfering with an essential signaling molecule, the ASK1, which is a serine/threonine kinase involved in the formation of a key signaling intermediate in the FasL- and TNF--induced death pathway [174
]. This protects HIV-infected cells from apoptosis as a result of the cisligation of Fas by FasL, as the virus increases the expression of Fas and FasL on the surface of the infected cells.
Some viruses can evade host’s cellular immune response by regulating the expression of DR ligands to their own advantage. The measles virus induces the expression of TRAIL in infected human monocyte-derived DC (Fig. 1) . These DC become cytotoxic and induce immunosuppression by killing interacting T cells instead of priming and activating them [148 ]. The HCMV-infected DC also express TRAIL and FasL and delete T cells [175 , 176 ]. Moreover, HSV-1 infects activated human CTL and increases their susceptibility to apoptosis by FasL. Consequently, the antiviral CTL kill each other by fratricide [177 ]. These strategies enable the infecting virus not only to counter and evade host’s antiviral immune response but also to induce immunosuppression in the infected host.
Adenoviruses protect virus-infected cells from apoptosis by inhibiting the expression of DR on the cell surface. The E3 region of all adenoviruses encodes three integral membrane viral proteins: E3-10.4K, E3-14.5K, and E3-6.7K. They are expressed as heteromeric complexes, receptor internalization and degradation (RID) complexes, which reduce the membrane expression of Fas and receptors for TRAIL and epithelial growth factor [178
179
180
181
]. The loss of these receptors leads to protection of the virus-infected cells from the cytototoxic activity exerted by CTL and NK cells [182
]. The RID complexes, however, do not target the transferrin receptor or MHC class I antigens [179
]. The complexes redirect intracellular trafficking of the DR to late endosomes for degradation. The SIV protein Nef increases the expression of FasL on the surface of the virus-infected cells, which can evade antiviral CTL by causing their apoptosis via Fas/FasL interactions [183
]. This mechanism has also been used by HIV protein Nef, which increases the expression of FasL and TNF- in DC. Exogenous Nef also triggers apoptosis of CD8+ T cells by activating caspase-8. Collectively, these effects abrogate the ability of DC to prime and activate alloreactive CD8+ T cells. The cells rather become anergic and show decreased proliferation, cytoxocity, and IFN- production [184
]. The viral protein Tat induces expression of TRAIL in primary human macrophages [185
]. HIV also directly promotes apoptosis of immune cells to evade host’s antiviral immune responses [186
]. The Tat protein acts as a proapoptotic protein by up-regulating the sensitivity of CD4+ T cells to Fas-mediated apoptosis, mainly by increasing the activity and expression of caspase-8 [187
, 188
]. Another HIV protein Vpu also enhances the susceptibility of CD4+ T cells to the Fas-induced apoptosis [189
]. By these mechanisms, HIV-1 manipulates the apoptotic machinery to its advantage in infected and uninfected cells. It promotes unresponsiveness and death of neighboring, uninfected immune cells but protects the virus-infected cells from apoptosis.
Targeting cytokines and chemokines of the host
The cytokines and chemokines are host cell-secreted polypeptides, which bind to their specific cell surface-expressed receptors and modulate activation, proliferation, and migration of various cell types involved in the induction of immune response and inflammation in vial infections. By communicating between different cells, they coordinate and orchestrate different components of the innate and adaptive immune responses (reviewed in ref. [190
]). The host responds to viral infections by stimulating production of a variety of cytokines and chemokines. It is not surprising that viruses have developed several strategies to counter these responses. These strategies include encoding inhibitors, decoy receptors, or modified viral versions of these soluble mediators (summarized in Table 5
; ref. [191
]).
|
and TNF- from monocytes. They also inhibit the mitogen-stimulated proliferation of peripheral blood mononuclear cells (PBMC) and decrease expression of MHC class I and class II antigens and costimulatory molecules ICAM-1, CD80, and CD86 but increase the expression of HLA-G on human PBMC [194
, 195
]. The two viruses seem to have usurped the human IL-10 gene by different mechanisms [195
]. They have modified the gene, retaining its immunosuppressive and anti-inflammatory properties, but not the immunostimulatory ones. The encoding of an IL-10 homologue is not restricted to herpesviruses; a poxvirus-encoded protein Y134R was also recently shown to have IL-10-like activities [196
]. Concerning chemokines, the herpesviruses such as KSHV, HHV6, and HCMV encode proteins, which bear sequence homology to human chemokines MIP-1 [191 ]. Certain virus-encoded chemokines may evade immune responses by acting as antagonists (e.g., vMIP-2 of KSHV), and others may facilitate virus spread by acting as agonists (e.g., U83 protein of HHV6). Furthermore, certain virus-encoded chemokine-like proteins may skew the immune response by chemoattracting TH-2 type CD4+ T cells (e.g., vMIP-1, -2, and -3 of KSHV). Some viruses may encode proteins, which have no sequence homology to any known chemokine but still may have chemoattractant properties. The HIV Tat and the RSV protein G are such proteins. The RSV uses protein G to gain entry in cells via the fractalkine receptor [197 ].
The poxviruses and herpesviruses encode proteins, which are similar in sequence to the extracellular ligand-binding domains of certain cytokine or chemokine receptors but lack their intracytoplasmic tails. Functionally, they act as decoy receptors and neutralize the bound cytokines and chemokines of the host, as they can bind the cytokine or the chemokine but cannot transmit signals. A good example is the cowpox, which encodes at least four different TNFR [191
, 198
]. Similarly, viruses have targeted other cytokine receptors (e.g., including IL-1ßR, IFN-R, CD30). Viruses also modulate the chemokine system of the host by encoding certain chemokine receptor homologues. The examples include the ORF74, US28, and US27 proteins of KSHV, HCMV, and HHV6, respectively. The virus-encoded chemokine receptors are expressed on the surface of the infected cells, and their role in immune evasion is not yet fully understood (reviewed in ref. [191
]).
In addition to encoding decoy receptors, viruses also encode homologues of the cellular proteins that can bind and inhibit a cytokine. The certain poxviruses such as cowpox virus, ectromelia virus, and vaccinia virus encode a soluble protein vIL-18BP, which like its cellular homologue, binds and neutralizes the biological activity of IL-18 [199
]. It is noteworthy that in concert with IL-12, IL-18 strongly stimulates antiviral cellular immunity. Similarly, vaccinia virus encodes a vIFN-/ßBP, which binds to the cell surface after secretion and prevents IFN from binding to its receptors. Similar to the virus-encoded cytokine-binding proteins, viruses also encode proteins that can bind chemokines and neutralize them. The myxoma virus encodes M-T7 [or virus chemokine-binding protein 1 (vCKBP-1)], which can bind and inactivate several chemokines. However, the poxvirus-encoded chemokines do not bind to M-T7. The vCKBP-2 binds to C–C chemokines and is encoded by myxoma and vaccinia viruses. The third vCKBP-3 (also called M-3) is encoded by the MHV-68 and inactivates almost all chemokines. Expectedly, the M-3 mutant MHV was found to be less pathogenic in mice [200
].
The cowpox virus CrmA inhibits caspase-1, also called IL-1-converting enzyme, which is needed to cleave precursor, immature IL-1ß and IL-18 into biologically active, mature cytokines [201 ].
Mimicking FcR
Viruses may also use other strategies to evade host’s cellular immune responses. The MCMV, HCMV, and HSV-1 encode at least one protein, which mimics a FcR [202
, 203
]. The FcR homologues are thought to prevent macrophage activation from immune complexes and protect virus-infected cells from NK cell killing via ADCC. They also inhibit clearance of antibody-coated pathogens from the circulation.
Deregulating immune responses via superantigens (SA)
SA are molecular structures, which bind MHC class II to a site distinct from the antigen-binding groove on APC and to particular variable regions of the ß-chain of the TCR. Each SA binds to a specific subset of Vß elements. SA are powerful T cell mitogens and induce uncontrolled activation of their cognate Vß-bearing T cells. A good deal has been learned about bacterial SA, more than 40 of which have been identified (reviewed in ref. [204
]). A SA-encoding human endogenous retrovirus (HERV)-K18 has been identified in humans. The provirus is located on human chromosome 1 in the first intron of CD48 in reverse orientation and has three alleles. The truncated envelope protein of the virus acts as a SA, which can bind Vß-7- and Vß13-containing TCR (ref. [205
]). Several studies suggest that viruses, such as EBV, HIV, HCMV, and rabies virus, encode SA. These conclusions were drawn, as the individuals suffering from these viral infections exhibited unusual expansions of certain Vß-bearing T cell subsets (reviewed in ref. [206
]). However, the exact identification of the SA encoded by these viruses has remained elusive. It is interesting that Sutkowski et al. [207
] have demonstrated that EBV does not encode any SA per se; it rather activates the SA-encoding HERV-K18 at the transcriptional level. These results explain the expansion of Vß13-positive T cell subsets in EBV-infected individuals. Apart from EBV, IFN- has also been demo
