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provided by Elsevier - Publisher Connector Immunity, Vol. 15, 867–870, December, 2001, Copyright 2001 by Cell Press Infections: Escape, Minireview Resistance, and Counterattack

Xiao-Ning Xu, Gavin R. Screaton, chains from the ER to the cytosol (US2 and 11), or retain and Andrew J. McMichael class I molecules in the ER (US3). In another example, MRC Human Immunology Unit CTL recognition is inhibited by downregulation of MHC Weatherall Institute of Molecular Medicine class I expression on the surface of infected cells by John Radcliffe Hospital human immunodeficiency virus (HIV) protein (Collins Headington, Oxford OX3 9DS et al., 1998). Nef can reroute the MHC class I molecules United Kingdom to clathrin-coated pits for endosomal degradation, which is dependent on a sequence motif in the cyto- plasmic tail of the classical class I molecules (HLA-A and HLA-B locus) (Le Gall et al., 1998). On the other Many establish life-long infections in their nat- hand, HLA-C and HLA-E molecules that are able to bind ural host with few if any clinical manifestations. The to inhibitory receptors of natural killer (NK) cells do not relationship between virus and host is a dynamic pro- contain this motif and are not downregulated by Nef cess in which the virus has evolved the means to coex- (Cohen et al., 1999). Thus, HIV-1 has evolved a strategy ist by reducing its visibility, while the host immune aimed at selective elimination of class I expression that system attempts to suppress and eliminate infection avoids downregulation of those class I molecules which without damage to itself. This short review describes inhibit NK cell killing. a variety of strategies that are employed by viruses to Viral mutation represents another way for a evade host immune responses. These include virus- virus escape from CTL and is probably of central impor- associated escape from recognition, and resis- tance in HIV infection. Because HIV reverse tran- tance to apoptosis and counterattack, with special scriptase is error prone, the high virus turnover continu- reference to two papers published in this issue of Im- ally generates many possible mutations. Under the munity (Mueller et al., 2001; Raftery et al., 2001). pressure of CTL responses, viruses that contain mutated critical amino acids in recognized by CTL are Escape selected, provided the mutation is not harmful to the It is well established that control of virus infection by virus. Many studies have shown that mutational escape the immune response normally requires virus-specific of HIV-1 or SIV occurs during the course of infection in cytotoxic T cells (CTL). CTL are primed by dendritic cells patients or an animal model (reviewed in McMichael and and other professional presenting cells which Rowland-Jones, 2001). process viral proteins that are either endogenously pro- duced or phagocytosed from apoptotic infected cells Resistance (crosspriming). Following clonal expansion, the primed Prolonging the survival of infected cells is clearly in the CTL can act against virus by killing infected cells via interest of infecting viruses, as it will allow full maturation perforin- and/or Fas-dependent pathways before new and dissemination. On the other hand, apoptosis of the virus particles are made. The killing takes less than 4 hr infected cells triggered by either immune attack or as following CTL recognition of viral peptides presented a direct outcome of viral infection is an important com- by MHC class I molecules at the surface of the infected ponent of the host antiviral response. Therefore, it is not cells. In parallel with this direct form of attack, CTL also surprising that many viruses encode viral proteins that release cytokines and chemokines that have antiviral inhibit this process (Figure 1). There are two main path- activity (reviewed in Guidotti and Chisari, 2001). To avoid ways for induction of apoptotic cell death. One involves CTL attack, viruses have developed many tricks to en- death receptors on the cell surface (TNF receptor fam- sure that the infected cells are not targeted by CTL. ily), TNF-R1, Fas, DR4, and DR5 by interaction with their These include interference with the peptide-presenting respective ligands TNF-␣, Fas ligand (FasL), and TRAIL. pathway and viral epitope mutation. The second pathway involves the participation of mito- For example, virus (HSV) establishes chondria and is regulated by members of Bcl-2 family life-long infection with resistance to CTL recognition by including the antiapoptotic molecules, bcl-2 or bcl-XL, inhibition of peptide transport from the cytosol into the or by the proapoptotic effectors, bad, bax,orbak. Both endoplasmic reticulum (ER) where MHC class I/peptide death receptor and mitochondria pathways share a complexes are assembled. The inhibition is mediated common set of caspases that cleave specific cellular by the infected cell protein 47 (ICP47) (York et al., 1994), substrates, leading to DNA fragmentation, the hallmark which prevents peptide loading of MHC class I mole- of apoptosis. Viruses can block one or both pathways. cules by directly inhibiting the function of TAP, the pep- For instance, Kaposi’s sarcoma-associated herpesvi- tide transporter that puts antigenic peptides into the rus (KSHV) encodes a viral inhibitor called vFLIP ER. Similarly, human (HCMV) interferes (ORF71), which disrupts the recruitment of caspase 8 with the MHC class I pathway using genes from the to FADD (Fas-associated death domain) to form the unique short (US) region of HCMV such as US2, 3, 6, death-inducing signaling complex (DISC). If caspase 8 and 11 (reviewed in Tortorella et al., 2000). These gene is not included in the DISC, cell death triggered by death products can independently block TAP-mediated pep- receptors such as Fas is inhibited (Thome et al., 1997). tide translocation (US6), dislocate MHC class I heavy In addition, KSHV encodes a gene, ORF16, that is highly Immunity 868

Figure 1. Evasion of Immune Responses by Viruses in Human Three major mechanisms used by viruses to evade immune responses in human are illustrated. These include antigen presenting pathway (MHC class I molecule/peptide complex), death receptors, and death-inducing ligands. DD, death domain; HIV, human immunodeficiency virus; HCMV, human cytomegalovirus; HSV, ; KSHV, Kaposi’s sarcoma-associated herpesvirus; HTLV-1, human T cell leukemia virus type 1; EBV, Epstein-Barr virus. homologous to endogenous bcl-2 and is able to sup- nalization of Fas from the cell surface and forces its press bax induction apoptotic signaling (Sarid et al., degradation inside lysosomes. This allows the infected 1997). Recently, the HIV-1 nef gene has been found to cells to resist Fas-mediated cell death and promotes inhibit Fas and TNF receptor-mediated apoptosis of HIV- their survival (Tollefson et al., 1998). Perforin-deficient 1-infected cells by binding directly to and blocking the CTL are unable to lyse Adenovirus-infected cells but function of ASK1 (the apoptosis-signal-regulating ki- can kill cells infected with virus that has a deletion of nase 1) (Geleziunas et al., 2001). ASK1 is a member of the E3 proteins. the mitogen-activated protein 3 (MAPK3) family and can activate the c-Jun NH2-terminal kinase pathway, which Counterattack leads to apoptosis. More recently, Nef has been shown There are therefore many examples of viruses that avoid to inactivate the proapoptotic protein bad by phosphor- CTL responses and resist apoptosis, prolonging the life ylation and consequently blocks the mitochondria- of the infected cells in the host. In some cases, despite induced apoptotic pathway (Wolf et al., 2001). Therefore, strong CTL responses in acute infection, the virus can HIV-1 nef can block both apoptotic pathways. Adenovi- lead to generalized immunosuppression, for instance rus uses a different mechanism to protect infected cells HIV-1, , or certain strains of lymphocytic chorio- from apoptosis. The E3-10.4/14.5K protein causes inter- meningitis virus (LCMV). In the case of HIV, this leads Minireview 869

to an impaired CTL response at the late stage of disease. can stimulate DC to upregulate TRAIL expression This is associated with, though not necessarily caused (Fanger et al., 1999; Liu et al., 2001), so an indirect by, widespread apoptosis in T cells (Meyaard et al., pathway is possible. 1992). As the majority of apoptotic cells are not HIV There are now many examples of how viruses subvert infected (Finkel et al., 1995) this apoptosis could include the immune responses (Figure 1). It should be noted HIV-specific CTL. In this issue of Immunity, Mueller et that the idea of counterattack mediated by FasL has al. have compared the susceptibility of HIV- and CMV- yielded some conflicting results in models of tumor and specific CTL to Fas-mediated apoptosis. Interestingly, autoimmune disease. Originally, it was thought that FasL they found that HIV-specific CTL were more prone to might contribute to immune privilege in tissues like testis apoptosis induced by anti-Fas antibody than CMV-spe- or eye and in some tumors by killing attacking CTL cific CTL from the same patient (Mueller et al., 2001). (O’Connell et al., 2001). However, tumors engineered to This was further confirmed by coculture of HIV-specific express FasL were rejected faster than their parent cells CTL with macrophages infected with HIV-1, which in- (Favre-Felix et al., 2000). Similarly, induction of FasL duces surface expression of FasL (Badley et al., 1996). expression on islet cells resulted in severe diabetes in Why are HIV- but not CMV-specific CTL, which express an animal model (Chervonsky et al., 1997). Therefore, comparable levels of Fas on the cell surface, more sensi- FasL expression may stimulate inflammation rather than tive to Fas-mediated killing? Mueller et al. showed HIV- immune privilege, although the balance between FasL- Ϫ Ϫ ϩ specific CTL were mainly CD45RA CD62L CD8 T mediated inflammation and immune privilege could dif- cells, whereas most CMV-specific CTL had a terminally fer in different tissues. It would probably not be to the ϩ Ϫ ϩ differentiated phenotype (CD45RA CD62L CD8 ). To- advantage of a virus such as HIV or HCMV to provoke gether with studies of Champagne et al. (2001) and Ap- a strong inflammatory response through induction of pay et al. (2000), these data suggest that HIV skews FasL and TRAIL expression. maturation of HIV-specific CTL, making them more vul- Whether or not the counterattack hypothesis is impor- nerable to Fas-mediated apoptosis. HIV nef induces tant in virus infections needs to be addressed in vivo, FasL expression on infected cells, and these can there- where the effects of particular death-inducing ligands fore counterattack by killing the CTL through the Fas- could be blocked. 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