Subversion of the MHC class I antigen-presentation pathway by adenoviruses and herpes simplex viruses

R E V I E W S

Subversion of the MHC class I antigen-presentation pathway by adenoviruses and herpes simplex viruses Hans-Gerhard Burgert morphic transmembrane glycocells play an important Complete elimination of viruses by the protein (heavy chain) is nonrole in recovery from immune system often requires efficient covalently associated with a lysis of infected cells by cytotoxic T cells primary viral infection secreted protein, called 132and protection from recurrent (CTLs). It is therefore not surprising microglobulin (I}2m).The heavy infection 1. Hence, viral escape that persistent viruses have evolved chain consists of three extrafrom T cell surveillance fresophisticated mechanisms to escape CTL cellular domains (ixl, ix2 and quently results in prolonged recognition. One strategy followed by ix3), containing about 90 amino survival of the virus in its host. adenoviruses and herpes simplex viruses acid residues each, a transmemis to subvert antigen presentation by In particular, CD8 + cytotoxic brane domain of about 30 resimajor histocompatibility complex T cells (CTLs) are often critical dues and a hydrophilic cytofor clearing a virus infection. class I molecules. plasmic tail of 30-40 residues. CTLs express on their cell surface a highly specific, clonotypic H-G. Burgert is in the Hans-Spemann-Laboratorium, Crystallographic analyses of Max-Planck-lnstitut fiir Immunbiologie, MHC class I moleculesr have reantigen receptor, the T cell reStiibeweg 51, D-79108 Freiburg, Germany. vealed two membrane-proximal ceptor (TCR). This molecule tel: + 4 9 761 5108487, fax: + 4 9 761 5108221, immunoglobulin-like domains recognizes viral antigenic pepe-maih [email protected] (ix3 and 132m)and two more distides bound to major histotal domains, ixl/ix2, which form compatibility complex (MHC) the peptide-binding pocket. Eight antiparallel [3strands class I antigens on target cells 2. Additional interactions between accessory molecules such as lymphocyte form the bottom of the peptide-binding groove and two long 0~ helices its walls. The remarkable polymorfunction-associated antigen 1 and its ligand on the target cell surface, intercellular adhesion molecule 1, phism concentrated in the ix1/0~2 domains of M H C class I molecules confers unique peptide- and TCRincrease the avidity between CTLs and their targets. Complete activation of CTLs triggers the fusion of binding properties to each MHC class I molecule 7. Assembly of M H C class I molecules takes place in cytolytic granules with the plasma membrane to release the endoplasmic reticulum (Fig. 2a). Newly synthesized the pore-forming protein perforin, as well as proteases, heavy chain associates transiently with a membraneknown as granzymes, all of which are involved in the bound chaperone of the endoplasmic reticulum, called killing process 3. Another mechanism of CTL-induced calnexin s,9. In the human system, binding of [32mto the cell death is mediated by the Fas-ligand-Fas/Apo-1 interaction 4. Activation is accompanied by the secretion heavy chain results in the release of a labile heavy-chainof cytokines, such as tumour necrosis factor ix (TNF-IX), 132m dimer that is stabilized by the binding of short peptides of 8-10 amino acid residues 1°. As most of lymphotoxin, interleukin 2 and interferon 2 (IFN-y), which are either directly cytotoxic or promote T cell pro- the peptides eluted from M H C class I molecules are derived from proteins located in the cytosol or the nuliferation and activation of macrophages. In addition, cleus, it is believed that the peptides are generated by both TNF-IX and IFN-y stimulate transcription of MHC proteasomes (multicatalytic protease complexes located genes, thereby enhancing T cell recognition (Fig. 1). Considering the complexity of T cell activation, sev- in these compartments) 11,12.Their substrate specificity eral possibilities arise as to how a virus might escape is compatible with the hydrophobic and basic amino acids found at the carboxy-terminal ends of MHCthe CTL response s,6. In this review, I focus on the mechabound peptides. Furthermore, the discovery that two nisms used by adenovirus and herpes simplex virus proteasomal subunits, which modulate the proteolytic (HSV) to impair viral antigen presentation and diminactivity of proteasomes, are encoded within the M H C ish CTL-mediated killing. supports the idea that proteasomes participate in antigen processing 12. The MHC class I antigen-presentation pathway To bind to MHC molecules, cytosolic peptides must M H C class I antigens are cell-surface glycoproteins consisting of two polypeptide chains. A highly poly- be translocated into the lumen of the endoplasmic

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Subversionof antigenpresentationby adenovirus

~ D e a t h

The first virus discovered to influence antigen presentation was adenovirus. Human adenovirus infections can become persistent 16 and the early transcription unit 3 (E3) of the virus has been implicated in this process. E3 genes encode proteins that apparently counteract the lytic attack of the host immune system. For example, several E3 proteins (14.7K, 10.4K and 14.5K) protect infected cells from the lytic activity of TNF (Ref. 17), while the most abundant E3 protein, E3/19K, binds MHC class I antigens in the endoplasmic reticulum, thereby inhibiting the transport of newly synthesized MHC molecules to the cell surface ~8. Consequently, allogeneic and antigen-specific T cell recognition in vitro is drastically suppressedlg-2L Consistent with these data, the lungs of cotton rats infected with wild-type adenovirus show a less severe immunopathology than those induced by a mutant virus lacking E3/19K (Ref. 22). Moreover, E3/19K, in combination with the other E3 proteins, prevents allograft rejection in mice for more than 3 months23.Therefore, it is postulated that E3/19Kmediated inhibition of the transport of MHC molecules contributes to the ability of adenovirus to establish persistent infections in humans. With the exception of subgroups A and F, all human adenovirus serotypes (subgroups B-E) express an E3/19K-like protein 17,24. All E3/19K-like proteins analysed so far share the same basic structure (Fig. 3): they are all type I transmembrane glycoproteins of approximately the same size (25-35 kDa, depending on the number of N-linked oligosaccharides). The lumenal portion of approximately 106 (-+4) amino acid residues is N-glycosylated at variable positions and is separated by a transmembrane segment (-23-29 amino acid residues) from a cytoplasmic tail of 12-15 residues 2s,26. Although the amino acid sequence identity is poor 2s,26 (only -25 %), the function of binding to and inhibition of the transport of MHC class I antigens is conserved among the E3/19K-like proteins24. Therefore, the question arises, which structure of the protein is essential for its function? E3/19K-like proteins consist of two modules: an MHC antigen-binding module combined with a structurally separate retrieval signal for the endoplasmic reticulum. The capacity to bind MHC class I molecules resides in the lumenal portion of the protein 25-z8and appears to require a certain tertiary structure rather than a linear stretch of specific amino acids. In the adenovirus 2 protein this conformation seems to depend on two disulphide bonds (Fig. 3) formed between Cysl 1-Cys28 and Cys22-Cys83 (ReL 25). The importance of these cysteines is reflected by their conservation in all known adenovirus E3/19K-like proteins. Other conserved amino acids are found dispersed throughout the lumenal domain but are enriched

O l AM-1 CD8,

Fig. 1. Role of major histocompatibility complex (MHC) molecules and cytotoxic T cells (CTLs) for clearing virus-infected cells. Some of the essential steps in the adenovirus life cycle are outlined: completely assembled virions accumulate in the nucleus; only a small fraction of viruses are released while the cell is still alive. Viral peptides are bound inside the cell by MHC class I molecules and are transported to the cell surface where they are recognized by the T cell receptor (TCR) on the surface of CTLs. Activation of CTLs induces the release of perforin and granzymes, which promote lysis of the infected cell. A second mechanism of CTL-mediated cell death is induction of apoptosis by interaction of the Fas ligand (FasL) on the T cell with the Fas/Apo-1 receptor on the target cell surface. CTLs also secrete tumour necrosis factor (TNF) and lymphotoxin (LT), which might be involved in target cell killing. However, most TNF is produced by macrophages (M
reticulum. This is accomplished by a complex of two integral membrane proteins, termed transporter associated with antigen presentation (TAP), which efficiently transports peptides of 8-15 amino acid residues ~,12,13.The physical association of TAP with MHC molecules9,12suggests that peptides are directly transferred to MHC molecules (Fig. 2a). However, there is also evidence for the involvement of additional molecules in this process, including the cytosolic heat-shock protein 70 (Hsp70), chaperones in the lumen of the endoplasmic reticulum (grp78 and grp94) and the

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Fig. 2. Assembly of major histocompatibility complex (MHC) class I molecules in uninfected and virus-infected cells. (a) Assembly of MHC class I molecules in normal human cells. Nascent heavy chain (HC) associates transiently with calnexin (dark pink box), which retains incompletely folded HC in the endoplasmic reticulum (ER). HCs that fail to bind to calnexin disintegrate. When j%-microglobulin (132m;blue) binds to the complex, calnexin dissociates releasing a labile HC-~2m heterodimer, which can be stabilized by peptide binding. Peptide loading might occur either directly during complex formation with transporter associated with antigen presentation (TAP; yellow) or may be mediated by abundant ER chaperones [BIP = grp78 (green) and/or Gp96=grp94 (rose)] or p100/110 (black). Similar peptide-chaperone intermediates with Hsp70 (light blue) might be formed in the cytosol. The majority of MHC-bound peptides are produced in the cytosol (purple circles), probably by proteasomes, but a proportion is also generated by proteases in the ER, such as the signal peptidase (ER peptides; purple triangles). Formation of the ternary complex results in efficient transport to the cell surface. The question marks indicate points at which it is not clear to what extent each mechanism contributes to the formation of complete MHC complexes. (b) Assembly of MHC class I molecules in adenovirusinfected cells. In the presence of E3/19K (black box), assembly and peptide delivery are apparently not significantly altered. Transport of the MHC-E3/19K complexes proceeds, perhaps with reduced efficiency, until they reach the cis-Golgi network where they are retrieved back into the ER. During this process highermolecular-mass complexes might be formed (not depicted). The thickness of the arrows illustrates the extent of particular processes. (c) Assembly of MHC class I molecules in fibroblasts infected with herpes simplex virus (HSV). During HSV infection 1CP47 (shown in black) inhibits TAP-mediated transport resulting in empty heterodimers, which are unstable. In such a cell, MHC molecules can still be loaded with ER-derived peptides allowing transport to the cell surface. The question mark indicates that it is unclear how efficient this process is.

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Fig. 3. Schematic model of the adenovirus 2 E3/19K protein. The E3/19K structure is drawn as a line. Carbohydrates are depicted as circles. The positions of the cysteines are numbered and the dashes between numbers indicate proposed disulphide bond linkages. Both disulphide bonds are absolutely essential for binding to major histocompatibility complex (MHC) class I molecules. Cysteines 101, 109 and 122 are neither involved in intramolecular cysteine bridges nor do they significantly influence complex formation with MHC molecules. Regions of the molecule responsible for binding to MHC molecules and for retention in the endoplasmic reticulum are indicated.

in a stretch in front of the transmembrane segment. This latter part may be important for correct folding of the more distal portions of the molecule 29. Another important feature is the presence of an endoplasmic reticulum retention signal in the cytoplasmic tail of the adenovirus protein ~° 32. It consists of two lysines positioned either -3 and - 4 or -3 and -5 from the carboxyl terminus (KKXX or KXKXX) 32. This dilysine motif has now been found in several resident endoplasmic reticulum proteins. As these proteins gain early Golgi modifications, it is believed that they reach the cis-Golgi but are then continuously retrieved back to the endoplasmic reticulum >. It is likely that this retrograde transport is mediated by coat proteins that can bind to the dilysine motif 34. Thus, the combination of an MHC-antigen-binding module with an endoplasmic reticulum retrieval signal yields the specific function of the protein. The adenovirus 2 and adenovirus 5 E3/19K molecules, and presumably also the homologous proteins of other adenoviruses, are very promiscuous in that they bind the majority of, if not all, human MHC antigens as well as MHC alleles from other s p e c i e s r 7,18,27,29,35. However, some murine MHC alleles do not bind E3/19K and are not susceptible to its transport inhibition function 21,27.This observation was the basis for the identification of the M H C domains essential for complex formation with E3/19K (Ref. 27). Hybrid M H C molecules containing domains from E3/19K-binding and non-binding M H C alleles revealed the importance of the polymorphic otl and •2 domains that form the peptide-binding pocket 27,>. This is somewhat surprising

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considering the broad specificity of E3/19K and suggests that a rather conserved feature within the 0tl and ot2 domains seems to be involved. Site-directed mutagenesis and antibody-binding studies suggest that the contact site is formed, or at least influenced, by amino acids within the carboxy-terminal part of the o~2 helix and the amino-terminal part of the (,1 helix 36,37. Although the E3/19K binding site is in the vicinity of the peptide-binding pocket, there is no evidence as yet that E3/19K interferes with peptide binding 31. Consistent with this idea, no significant inhibition of complex formation between TAP and M H C molecules has been observed in the presence of E3/19K. Additional data suggest that the interaction between E3/19K and MHC molecules occurs early, probably before or during binding to calnexin (M. Sester, T. Preckel and H-G. Burgert, unpublished). Thus, E3/1 9K binding seems to be an early event in the assembly of MHC class I molecules but one that does not grossly alter their subsequent interactions, except to abolish egress of the completely assembled complex out of the endoplasmic reticulum/cis-Golgi (Fig. 2b). Potentiation of E3 functions by TNF The efficacy of E3/19K in vivo will depend on whether E3/19K-mediated transport inhibition can be overcome by cytokines like TNF and IFN-2 that stimulate transcription of MHC genes and thereby enhance T cell recognition. Likewise, the upregulation of TAP by IFN- 7 should improve this process 13. As TNF is induced in adenovirus-infected tissue in mice -38, studies have focused on the effect of TNF. Surprisingly, TNF is unable to restore MHC class I expression in E3/l 9K-expressing cells but instead leads to a further reduction of M H C antigens on the cell surface 39. This effect is due to an increased synthesis of E3/19K (Ref. 39). The coordinated upregulation of all E3 proteins 4° can be accounted for by the stimulation of the E3 promoter by the cytosolic transcription factor nuclear factor KB (NF-KB; Ref. 41; F. Deryckere and H-G. Burgert, unpublished). Unlike other viral systems where TNF activates viral genes to promote replication, TNF stimulates the production of adenovirus proteins capable of neutralizing the lytic activity of TNF and CTLs. Without showing severe immunopathology in vivo, adenoviruses prolong their survival in the host. This mechanism would appear to assure efficient virus reproduction despite the presence of TNF during the early phase of the immune response (Fig. 4). Alternatively, it may be required for establishing persistent infections in lymphoid tissue or cells in which NF-KB is constitutively active.

An alternative strategy for MHC repression Adenovirus 12 from the highly oncogenic subgroup A, adenovirus 40 and adenovirus 41 (subgroup F) lack an E3/19K-encoding gene42and are unable to retain MHC molecules in the endoplasmic reticulum. Interestingly, in transformed cells the adenovirus 12 EIA protein downregulates MHC expression 43 by interfering predominantly with its transcription 44,4s. The low level of MHC mRNA correlates with increased binding of repressors to the M H C enhancer 4-s and/or decreased

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binding and synthesis of activating transcription factors such as NF-I¢B (Ref. 46). In addition, adenovirus-12mediated suppression of TAP transporters might contribute to reduced M H C expression on the cell surface47. The question remains whether these mechanisms are relevant for acute or persistent infections in the natural host, since human tissue-culture cells infected with adenovirus 12 did not show any downregulation of M H C molecules z4. At present, it is unclear whether a state similar to the transformed one exists during persistence. Interestingly, adenoviruses of subgroups A and F are predominantly associated with gastrointestinal infections. If adenovirus 12 is unable to downregulate MHC expression in its natural host in vivo, this function may not be necessary for the adenovirus 12 infection cycle in the gut environment and therefore may have been lost or never acquired. In support of the latter idea, mouse adenovirus also lacks an E3/19Klike function 48. These observations suggest that the E3/19K function is not absolutely essential for survival of the virus in its natural host per se but it might be beneficial during infection of the respiratory tract and certain other tissues favoured by most adenoviruses.

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It has been known since 1985 that mouse fibroblasts infected with HSV-1 and HSV-2 exhibit a reduced level of M H C class I molecules on their cell surface 49. The effect was greater with HSV-2-infected cells and could not be attributed solely to its host shut-off function. A similar phenomenon has now also been described for HSV-infected human dermal fibroblasts. These cells are resistant to lysis by CD8 ÷CTL clones, although the same clones readily lyse HSV-infected lymphoblast cell lines s°. A reduction in sialylation of human leukocyte antigen (HLA) molecules (sialylation is a carbohydrate modification occurring in the late Golgi stack) indicated that transport of HLA molecules was arrested 51. The viral protein responsible for this effect was identified as the immediate-early protein ICP47 (Ref. 52). Its localization in the cytosol, together with the observed instability of MHC class I molecules in detergent extracts of HSV-infected cells, suggested that it may interfere with peptide generation or delivery into the endoplasmic reticulum. Indeed, two groups demonstrated that ICP47 interferes with TAP-dependent transport of peptides by direct binding to the TAP or the TAPMHC complex (Fig. 2C) s3'54. Like binding of peptide to TAP, the association of ICP47 with TAP requires both TAP subunits 12,-54,-5s.This indicates that ICP47 blocks binding or transport of peptides rather than binding of ATP or M H C antigen to TAP, which occurs with individual TAP subunits. The inhibitory effect of ICP47 on TAP may explain the initial predominance of CD4 ÷ HSV-specific CTLs in herpetic lesions s6,sv. Downregulation of M H C antigens is also used by human (HCMV) and mouse (MCMV) cytomegaloviruses to escape T cell surveillance s8-6°. For a long time, this was a controversial issue 6. The current knowledge suggests that multiple viral gene products are involveds8 and that the mechanisms will differ from those used by adenovirus and HSV (Refs 6,18,53,54).

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Fig. 4. Potentiation of the effects of E3 by tumour necrosis factor (TNF). The scheme outlined is based on the existing in vivo and in vitro data 3~-4°. Macrophages and monocytes first infiltrate the site of infection and produce TNF. TNF is unable to lyse infected cells since they are protected by the E3 proteins 14.7K in the cytosol, 14.5K and 10.4K in the plasma membrane and the EIB/19K protein in the nuclear envelope. Instead, the binding of TNFto the TNF receptor results in activation of the cytosolic transcription factor, nuclear factor KB (NF-KB), which stimulates the transcription of major histocompatibility complex (MHC) genes. The concomitant stimulation of the E3 promoter by NF-~B increases E3 expression, including 19K, which in turn leads to a more efficient retention of MHC molecules and inhibition of antigen presentation. Thus, the second line of defence, the subsequently infiltrating CTLs, might be unable to lyse infected cells. It seems, then, that adenovirus has adapted to the presence of TNF and uses this immune mediator to escape the lyric attack of the immune system more efficiently. Abbreviations used: M O, macrophage; red arrows, stimulation; blue arrows, inhibition; TR, TNF receptor. Concluding remarks

A decade ago, manipulation of M H C class I molecules by the adenovirus E3/19K protein appeared to be a rather exotic property for a viral protein. Most recently, it has become clear that many viruses, particularly persistent viruses, have evolved multiple mechanisms to block the functional expression of M H C molecules (reviewed in Ref. 61). Surely, more remain to be discovered. Although the MHC class I antigen-presentation

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Questions for future research • Which cell type is infected during the persistent state of the adenovirus infection in vivo? • When is the E3/19K function needed during the adenovirus life cycle in vivo? • Is there any correlation between subtype-specific binding of E3/19K to human leukocyte antigen alleles and adenovirus persistence? • What is the role of human CD4 ÷ and CD8 ÷ T cells as well as natural killer cells for adenovirus-specific immunity? • What is the specific function of ICP47 in the in vivo life cycle of herpes simplex virus (HSV)? • Does HSV-2 encode a protein similar to ICP47? • Why does ICP47 not inhibit transporter associated with antigen presentation (TAP) in lymphoid cells?

p a t h w a y seems to be a prime target, the structure of the viral protein involved and the individual mechanism of each virus appear to be shaped by coevolution with their respective hosts. For example, E3/19K molecules of human adenoviruses 2 and 5 inhibit the transport of all H L A molecules examined albeit with differential efficiency, while three out of seven murine M H C alleles are not affected lr-19,-'7,3s. Second, ICP47 only affects h u m a n TAP but not the mouse counterparts s2-s4. Third, the mechanism of M C M V (Refs 6,59) seems to differ from that of H C M V (Ref. 60). Understanding the molecular details of h o w the i m m u n o m o d u l a t o r y proteins of viruses function will give further insights into the cell biology of antigen presentation and processing. As well as providing valuable research tools, there may be some potential here for the development of new therapeutic agents. Acknowledgements I apologizeto friends and colleagues whose work I have not cited due to space limitations. I thank the current and previous members of my laboratory for valuable help through the years, and M. Sester,H. Pahl, H.U. Weltzienand F. Deryckerefor critical reading of the manuscript. Part of this work was supported by the Max-Planck-Societ7 and the Deutsche Forschungsgemeinschaft(SFB388, project B4-Burgert). References 1 DoherD, P.C. et al. (1992) Annu. Rev. lmmunol. 10, 123-151 2 Townsend, A. and Bodmer,H. (1989) Annu. Rev. Immunol. 7, 601-624 3 Berke, G. (1994) Annu. Rev. Immunol. 12, 735-773 4 Nagata, S. and Golstein, P. (1995)Science267, 1449-1456 5 Koup, R.A. (1994)J. Exp. Med. 180, 779-782 6 Rinaldo, C.R., Jr (1994) Am. ]. Pathol. 144, 637-650 7 Bjorkman, P.J. and Parham, P. (1990) Annu. Rev. Biochem. 59, 253-288 8 Germain, R.N. (1994) Cell 76,287-299 9 Williams, D.B. and Watts, T.H. (1995) Curr. Opin. Immunol. 7, 77-84 10 Rammensee, H.G., Falk, K. and R6tzschke, O. (1993) Annu. Rev. Irnmunol. 1l, 213-244 11 Goldberg, A.L. and Rock, K.L. (1992) Nature 357, 375-379 12 Howard, J.C. (1995) Curt. Opin. Immunol. 7, 69-76 13 Momburg, F., Neefjes,J.J. and H~immerling,G.J. (1994) Curt. Opin. Immunol. 6, 32-37 14 Srivastava,P.K. and Udono, H. (1994) Curr. Opin. lmmunol. 6, 728 -732

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15 Feuerbach, D. and Burgert, H-G. (1993) EMBOJ. 12, 3153-3161 16 Horwitz, M.S. (1990)in Virology (2nd edn) (Fields, B.N. et al., eds), pp. 1723-1740, Raven Press 17 Wold, W.S.M., Hermiston, T.W. and Tollefson, A.E. (1995) Curr. Top. Microbiol. Immunol. 199/I, 237-274 18 Burgert, H-G. and Kvist, S. (1985) Cell 41,987-997 19 Burgert, H-G., Maryanski, J.L. and Kvist, S. (1987) Proc. Natl Acad. Sci. USA 84, 1356-1360 20 Andersson, M., McMichael, A. and Peterson, P.A. (1987) J. Immunol. 138, 3960-3966 21 Rawle, F.C. et al. (1991)J. Immunol. 146, 3977-3984 22 Ginsberg,H.S. et al. (1989) Proc. Natl Acad. Sci. USA 86, 3823-3827 23 Efrat, S. et al. (1995)Proc. Natl Acad. Sci. USA 92, 6947-6951 24 P~i~ibo,S., Nilsson, T. and Peterson, P.A. (1986) Proc. Natl Acad. Sci. USA 83, 9665-9669 25 Sester,M. and Burgert, H-G. (1994)J. Virol. 68, 5423-5432 26 Hermiston, T.W. et al. (1993)J. Virol. 67, 5289-5298 27 Burgert, H-G. and Kvist, S. (1987) EMBO J. 6, 2019-2026 28 P/i/ibo, S. etal. (1986) EMBOJ. 5, 1921-1927 29 Flomenberg,P. et al. (1992)J. Virol. 66, 4778-4783 30 Gabathuler, R. and Kvist, S. (1990) J. Cell Biol. 111, 1803-1810 31 Cox, J.H., Bennink,J.R. and Yewdell,J.W. (1991) J. Exp. Med. 174, 1629-1637 32 Jackson, M.R., Nilsson, T. and Peterson, P.A. (1990) EMBO J. 9, 3153-3162 33 Jackson, M.R., Nilsson, T. and Peterson, P.A. (1993)J. Cell Biol. 121,317-333 34 Cosson, P. and Letourneur, F. (1994) Science 263, 1629-1631 35 Beier,D.C. et al. (1994)J. Immunol. 152, 3862-3872 36 Feuerbach, D. et al. (1994}J. Immunol. 153, 1626-1636 37 Flomenberg,P., Gutierrez, E. and Hogan, K.T. (1994) Mol. 1mmunol. 31, 1277-1284 38 Ginsberg, H.S. et al, (1991) Proc. Natl Acad. Sci. USA 88, 1651-1655 39 K6rner, H., Fritzsche, U. and Burgert, H-G. (1992) Proc. Natl

Acad. Sci. USA 89, 11857-11861 40 Deryckere,F. et al. (1995) Immunobiology 193, 186-192 41 Williams,J.L. et al. (1990) EMBO J. 9, 4435-4442 42 Bailey,A. and Maumer, V. (1994) Virology 205,438-452 43 Schrier, P.I. et al. (1983) Nature 305, 771-775 44 Ackrill, A.M. and Blair, G.E. (1988) Oncogene 3,483-487 45 Ge, R. et al. (1992)J. Virol. 66, 6969-6978 46 Schouten, G.J., van der Eb, A.J. and Zantema, A. (1995) EMBOJ. 14, 1498-1507 47 Rotem-Yehudar, R. et al. (1994)J. Exp. Med. 180, 477-488 48 Beard, C.W. and Spindler, K.R. (1995)Virology 208, 457-466 49 Jennings, S.R. et al. (1985)J. Virol. 56, 757-766 50 Kodle, D.M. et al. (1993)J. Clin. Invest. 91,961-968 51 Hill, A.B. et al. (1994)J. lmmunol. 152, 2736-2741 52 York, I.A. et al. (1994) Cell 77, 525-535 53 Frtih, K. et al. (1995} Nature 375, 415-418 54 Hill, A. et al. (1995) Nature 375,411-415 55 Androlewicz,M.J. et al. (1994) Proc. Natl Acad. Sci. USA 91, 12716-12720 56 Cunningham, A.L. and Noble, J.R. (1993)J. Clin. Invest. 83, 490-496 57 Schmid, D.S. and Rouse, B.T. (1992) in Herpes Simplex Virus: Pathogenesis, 1mmunobiology and Control (Rouse, B.T., ed.), pp. 57-74, Springer-Verlag 58 Jones, T.R. et al. (1995)J. Virol. 69, 4830-4841 59 Del Val, M. et aI. (1992)J. Exp. Meal. 176, 729-738 60 Beersma, M.F., Bijlmakers,M.J. and Ploegh, H.L. (1993) J. lmmunol. 151, 4455-4464 61 McFadden, G. and Kane, K. (1994) Adv. Cancer Res. 63, 117-209

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