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Cytomegaloviruses use multiple mechanisms to elude the host immune response
Immunology Letters 57 (1997) 213 – 216
Review
Cytomegaloviruses use multiple mechanisms to elude the host immune response Emmanuel Wiertz a,*, Ann Hill b, Dominic Tortorella c, Hidde Ploegh c a
b
National Institute of Public Health and the En6ironment (RIVM), P.O. Box 1, 3720 BA Biltho6en, The Netherlands Department of Molecular Microbiology and Immunology, Oregon Health Sciences Uni6ersity, 3181 SW Gainess, Portland, OR 97 201, USA c Center for Cancer Research, Massachusetts Institute of Technology, 40 Ames Street, Cambridge, MA 02139, USA
Abstract The study of the effects of cytomegaloviruses on the MHC class I-restricted antigen presentation pathway has yielded an embarrassment of riches. The human cytomegalovirus (HCMV) encodes at least five to six different glycoproteins, each interfering in a different way with elimination of the virus by the host immune system. Most likely, it is the concerted action of these glycoproteins that allows HCMV to escape from elimination by the host immune system during acute and perhaps also persistent infection. Prime targets of these CMV glycoproteins are MHC class I glycoproteins: the very molecules that signal the presence of a virally infected cell to the immune system. Recently, several novel links in the multi-step process of immune evasion by HCMV have been discovered. © 1997 Elsevier Science B.V. Keywords: Cytomegalovirus; Immune system; MHC class I
1. HCMV US2 and US11 destroy newly synthesized MHC class I molecules The US region of the HCMV genome encodes most if not all glycoproteins known to play a role in downregulation of MHC class I expression. Using defined deletion mutants, the early gene product US11 was first identified as a major cause of the long observed instability of MHC class I molecules in HCMV infected cells [1]. More recently, another early gene product, US2, was found to cause class I degradation in a manner indistinguishable from that caused by US11 [3]. Simultaneous expression of two gene products with a similar function is unexpected, especially in view of the absence of significant similarity between US2 and US11. Using cell lines stably transfected with US11 and US2 some of the highly unusual characteristics of what appears to be a novel protein * Corresponding author. Tel.: +31 30 2743665; fax: + 31 30 2744429; e-mail: [email protected] 0165-2478/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 5 - 2 4 7 8 ( 9 7 ) 0 0 0 7 3 - 4
breakdown pathway have been characterized [2,3]. The most compelling observations include: (1) the occurrence of an MHC class I breakdown intermediate in the presence of protease inhibitors specific for the proteasome; (2) involvement of a peptide-N-glycanase in the generation of the class I breakdown intermediate; (3) occurrence of the deglycosylated intermediate (distinguishable by the apparent Asn-to-Asp conversion) as a soluble molecule in the cytosol; (4) involvement in the ‘dislocation’ process of components of the Sec61p complex, the translocation apparatus via which proteins normally enter the ER, and (5) isolation of the breakdown intermediate from proteasomes whose function has been blocked termporarily by protease inhibitors. This unexpected series of events defines a novel breakdown pathway, involving backtransport of glycoproteins from the ER to the cytosol via a dedicated transport channel, inter alia composed of Sec61a and b chains, deglycosylation by a host N-glycanase and, finally, degradation by the proteasome [2,3].
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E. Wiertz et al. / Immunology Letters 57 (1997) 213–216
This pathway is also operational in the absence of viral gene products and appears to be used to dispose of class I heavy chains that fail to fold and assemble properly, for example because antigenic peptides are lacking (as is the case in TAP-deficient cells), or because the light chain, b2m, is absent (in b2m-negative Daudi cells) [4]. Recent reports indicate that retrograde transport, followed by proteasomal degradation, is not limited to MHC molecules but occurs for many other proteins, including the paradigm of ER protein degradation, TCRa (Huppa and Ploegh, submitted) and soluble proteins, in mammalian as well as yeast cells (reviewed in [5]). 2. US3 retains MHC class I molecules in the endoplasmic reticulum Although the US3-encoded glycoprotein shows homology with US2 and US11, the mechanisms by which these molecules influence MHC class I expression are fundamentally different [6,7]. Whereas US2 and US11 induce rapid breakdown of class I heavy chains, stable heavy chain-b2m complexes are formed in US3 expressing transfectants. Instead, US3 retains the class I complexes in the ER as indicated by their EndoH sensitivity. Accordingly, immunofluorescence microscopy reveals perinuclear accumulation of MHC class I molecules while US3 itself is also confined to the ER. When cells are lysed in the presence of digitonin (but not NP40) US3 is found in a physical complex with class I heavy chains and b2m. Association with US3 takes place prior to peptide loading and slows down—but does not prohibit — the acquisition of antigenic peptides [6,7]. A careful analysis of the sequence of expression of US2, US3 and US11 indicates that US3 is present at immediate-early times during infection whereas US11 and US2 are expressed at early and late times [6]. The half life of US3 and US11 is estimated at 3 and 6 h p.i., respectively, indicating that expression of US3 and US11/US2 overlap briefly. It is tempting to propose a model in which US3, being the first non-regulatory immediate-early protein expressed, retains stable MHC class I heterodimers in the ER, thereby rendering them susceptible to destruction by US11/US2 expressed at later time points. However, experiments with US11 or US2 transfected cells suggest that the majority of class I molecules are attacked as free heavy chains: rapid degradation occurs immediately after completion of the polypeptide chain, prior to association with b2m [2,3]. In contrast, US3 appears to interact specifically with heavy chain-b2m complexes [6,7]. Nevertheless, the interplay between US3 and US2/US11 may apply to the minor population of MHC class I molecules that escapes immediate destruction by US11 and US2, and forms a complex with b2m.
ER retention of MHC class I molecules is not unique for HCMV: the adenovirus E3-19k gene product [8,9] possesses similar functions and also murine cytomegalovirus retains class I molecules in the ER [10,11,24]. US3 and E3-19k are not homologous; both are type I membrane proteins and contain high mannose type N-linked glycans.
3. US6 blocks peptide transport by TAP Although ER retention of class I molecules, and also degradation by US2, is mediated via a direct physical interaction with class I heavy chains, this is not a prerequisite for class I downregulation. Other viral gene products have been shown to act indirectly and block functions required for assembly of MHC class I–peptide complexes. Such a mechanism has been described for the HSV 1 and 2-encoded ICP47 [12,13], a small cytosolic protein that blocks the transporter associated with antigen presentation (TAP). Recent data indicate that HCMV also encodes a protein, US6, capable of inhibiting TAP function [14,15]. Most likely, inhibition by US6 involves a novel type of interaction with TAP, since US6 is a glycoprotein and does not demonstrate any homology with the the cytosolic ICP47.
4. UL18 acts as a decoy for NK cells NK cells carry triggering receptors and inhibitory receptors. Activation of the triggering receptor by a target cell will result in its destruction unless the inhibitory receptor detects an MHC class I molecule. Thus, cells that have lost cell surface expression of MHC class I are recognized and destroyed by NK cells. The murine CMV encodes a class I homologue, called m144, which acts as a molecular decoy in vivo [17]. A recombinant MCMV in which the m144 gene has been disrupted demonstrates severely restricted replication compared with wild type MCMV [17]. In vivo depletion studies show that NK cells are responsible for the observed attenuation of the infection. Interestingly, in addition to the class I retaining or destroying functions, HCMV encodes a class I homologue, UL18, which is capable of inhibiting the function of NK cells in vitro [16].
5. Phosphorylation of the HCMV immediate early (I-E) protein by pp65 inhibits T-cell recognition of I-E During the immediate-early phase of viral gene expression, an essential viral transcription factor, 72 kDa immediate-early (I-E) protein, is produced. Since this I-E gene product is expressed abundantly prior to
E. Wiertz et al. / Immunology Letters 57 (1997) 213–216
synthesis of most glycoproteins known to be involved in immune suppression, a CTL response against I-E would be expected. However, very few I-E-specific CTL’s are detected in seropositive individuals. Recent data suggest that a CMV matrix protein, pp65, might be responsible for this apparent immune suppression [18]. Pp65 catalyzes phophorylation of I-E. The modified I-E gene product fails to activate CTL’s in vitro [18].
6. Immune evasion by murine CMV MCMV has its own long literature with regard to immune evasion affecting the class I presentation pathway. Early MCMV gene products (i.e. ones which require immediate early gene products for their transcription) cause peptide loaded MHC class I molecules to be retained in the ER; expression of early genes abolishes CTL recognition of an IE encoded antigen or constitutively expressed cellular antigens [10,11,19,20,24]. The MCMV gene M152 is capable of mediating this effect. M152 encodes a 40kD glycoprotein, which when expressed using a vaccinia vector causes retention of class I molecules and lack of recognition of simultaneously expressed pp99. Interestingly, class I molecules appear retained in the ER-cisGolgi intermediate compartment [24]. As mentioned, MCMV also encodes an MHC class I homologue that inhibits NK activity [17], analogous to the UL18 product of HCMV [16]. Furthermore the importance of NK cells in control of MCMV infection has been easy to document, and a resistance locus to MCMV maps close to the NK locus on mouse chromosome 6 [21]. Recently, a 34kDa glycoprotein encoded by the MCMV gene MO4 has been discovered which is synthesized early in MCMV infection (although only after ER retention becomes apparent) [22]. GpMO4 is largely ER resident, although a small portion binds to class I MHC, and this portion travels with class I to the cell surface where they remain stably associated. The function of GpMO4 (to interfere with CTL recognition at the cell surface? To rescue a portion of class I and so interfere with NK lysis?) has not yet been determined. MCMV thus expresses an array of immune intervention factors which is analogous in complexity to that of HCMV; although in details of molecular function there appear to be many differences.
7. Discussion The redundancy of mechanisms interfering with antigen presentation provokes the question: what is the advantage to the virus? As mentioned, Ahn et al. [6] speculate that the sequential expression of US3 and
215
US11 in virus infection leads to a more efficient interference with class I restricted antigen presentation, with retention preceding degradation. Another possibility is raised by the differential ability of US2 and US11 to cause degradation of mouse class I molecules [23]. There is a clear difference in susceptibility of different class I alleles to degradation; and thus US2 and US11 apparently differ in their allelic preferences. We can extrapolate to suggest that in order to adequately cover the range of human alleles the virus has found it necessary to employ at least two molecules with apparently redundant functions. If true, this suggests a fascinating picture of a sort of cold war standoff, with both virus and the immune system diversifying their arsenals to reach an equilibrium within an evolutionary timeframe. These two types of explanation-that the mechanisms synergise to achieve optimal class I retention, and that the mechanisms are diversified to cope with the polymorphism amongst MHC class I alleles—are not mutually exclusive. Finally, the genes might act differently in the different cell types that the virus infects in vivo, in order to optimize the efficiency of immune control to serve the virus’ overall strategy.
References [1] T.R. Jones, L.K. Hanson, L. Sun, J.S. Slater, R.M. Stenberg, A.E. Campbell, Multiple independent loci within the human cytomegalovirus unique short region down-regulate expression of major histocompatibility complex class I heavy chains, J. Virol. 69 (1995) 4830 – 4841. [2] E.J.H.J. Wiertz, T.R. Jones, L. Sun, M. Bogyo, H.J. Geuze, H.L. Ploegh, The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the ER to the cytosol, Cell 84 (1996) 769 – 779. [3] E.J.H.J. Wiertz, D. Tortorella, M. Bogyo, J. Yu, W. Mothes, T.R. Jones, T.A. Rapoport, H.L. Ploegh, Sec61-mediated transfer of a membrane protein from the endoplasmic reticulum to the proteasome for destruction, Nature 384 (1996) 432–438. [4] E.A. Hughes, C. Hammond, P. Cresswell, Misfolded major histocompatibility complex class I heavy chains are translocated into the cytoplasm and degraded by the proteasome, Proc. Natl. Acad. Sci. USA 94 (1997) 1896 – 1901. [5] J.S. Bonifacino, Reversal of fortune for nascent proteins, Nature 384 (1996) 405 – 406. [6] K. Ahn, A. Angulo, P. Ghazal, P.A. Peterson, Y. Yang, K. Fru¨h, Human cytomegalovirus inhibits antigen presentation by a sequential multistep process, Proc. Natl. Acad. Sci. USA 93 (1996) 10990 – 10995. [7] T.R. Jones, E.J.H.J. Wiertz, L. Sun, K.N. Fish, J.A. Nelson, H.L. Ploegh, Human cytomegalovirus US3 impairs transport and maturation of MHC Class I heavy chains, Proc. Natl. Acad. Sci. USA 93 (1996) 11327 – 11333. [8] M. Andersson, S. Paabo, T. Nilsson, P.A. Peterson, Impaired intracellular transport of class I MHC antigens as a possible means for adenoviruses to evade immune surveillance, Cell 43 (1985) 215 – 222. [9] H.-G. Burgert, S. Kvist, An adenovirus type glycoprotein blocks cell surface expression of human histocompatibility class I antigens, Cell 41 (1985) 987 – 997.
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[10] R. Thale, U. Szepan, H. Hengel, G. Geginat, P. Lucin, U.H. Koszinowski, Identification of the mouse cytomegalovirus genomic region affecting major histocompatibility complex class I molecule transport, J. Virol. 69 (1995) 6098–6105. [11] M. del Val, H. Hengel, H. Ha¨cker, U. Hartlaub, T. Ruppert, P. Lucin, U. Koszinowski, Cytomegalovirus prevents antigen presentation by blocking the transport of peptide-loaded major histocompatibility complex class I molecules into the medialGolgi compartment, J. Exp. Med. 176 (1992) 729–738. [12] A. Hill, P. Jugovic, I. York, G. Russ, J. Bennink, J. Yewdell, H.L. Ploegh, D. Johnson, Herpes simplex virus turns off the TAP to evade host immunity, Nature 375 (1995) 411–415. [13] K. Fru¨h, K. Ahn, H. Djaballah, P. Sempe´, P.M. van Endert, R. Tampe´, P.A. Peterson, Y.A. Yang, A viral inhibitor of peptide transporters for antigen presentation, Nature 375 (1995) 415 – 418. [14] K. Ahn, A. Gruhler, B. Galocha, T.R. Jones, E.J.H.J. Wiertz, H.L. Ploegh, P.A. Peterson, Y. Yang, K. Fru¨h, The ER-luminal domain of the HCMV glycoprotein US6 inhibits peptide translocation by TAP, Immunity, in press. [15] H. Hengel, J.O. Koopman, T. Flohr, W. Muranyi, E. Goulmy, G.J. Ha¨mmerling, U.H. Koszinowski, F. Momburg, A viral ER resident glycoprotein inactivates the MHC encoded peptide transporter, Immunity, in press. [16] H.T. Reyburn, O. Mandelboim, M. Vale´s-Go´mez, D.M. Davis, L. Pazmany, J.L. Strominger, The class I MHC homologue of human cytomegalovirus inhibits attack by natural killer cells, Nature 386 (1997) 514–517. [17] H.E. Farrell, H. Vally, D.M. Lynch, P. Fleming, G.R. Shellam, A.A. Scalzo, N.J. Davis-Poynter, Inhibition of natural killer cells by a cytomegalovirus MHC class I homologue in vivo, Nature
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.
386 (1997) 510 – 514. [18] M.J. Gilbert, S.R. Riddell, B. Plachter, P.D. Greenberg, Cytomegalovirus selectively blocks antigen processing of its immediate-early gene product, Nature 384 (1996) 720 – 722. [19] A.E. Campbell, J.S. Slater, V.J. Cavanaugh, R.M. Stenberg, An early event in murine cytomegalovirus replication inhibits presentation of cellular antigens to cytotoxic T-lymphocytes, J. Virol. 66 (1992) 3011 – 3017. [20] M. Del Val, K. Munch, M.J. Reddehase, U.H. Koszinowski, Presentation of CMV early immediate early antigen to cytolytic T-lymphocytes is selectively prevented by viral genes expressed in the early phase, Cell 58 (1989) 305 – 315. [21] A.A. Scalzo, P.A. Lyons, N.A. Fitzgerald, C.A. Forbes, W.M. Yokoyama, G.R. Shellam, Genetic mapping of Cmv1 in the region of mouse chromosome 6 encoding the NK gene complexassociated loci Ly49 and musNKR-P1, Genomics 27 (1995) 435 – 441. [22] M.F. Kleijnen, J.B. Huppa, P. Lucin, S. Mukherjee, H. Farrel, A.E. Campbell, U.H. Koszinowski, A.B. Hill, H.L. Ploegh, A mouse cytomegalovirus glycoprotein, gp34, forms a complex with folded class I MHC molecules in the ER which is not retained but is transported to the cell surface, EMBO J. 16 (1997) 685 – 694. [23] R. Machold, E.J.H.J. Wiertz, T.R. Jones, H.L. Ploegh, The HCMV gene products US11 and US2 differ in their ability to attack allelic forms of murine MHC class I heavy chains, J. Exp. Med. 185 (1997) 363 – 366. [24] H. Ziegler, R. Tha¨le, P. Lucin, W. Muranyi, T. Flohr, H. Hengel, H. Farrell, W. Rawlinson, U.H. Koszinowski, A mouse cytomegalovirus glycoprotein retains MHC class I complexes in the ERGIC/cis-Golgi compartments, Immunity 6 (1997) 57–66.
Review
Cytomegaloviruses use multiple mechanisms to elude the host immune response Emmanuel Wiertz a,*, Ann Hill b, Dominic Tortorella c, Hidde Ploegh c a
b
National Institute of Public Health and the En6ironment (RIVM), P.O. Box 1, 3720 BA Biltho6en, The Netherlands Department of Molecular Microbiology and Immunology, Oregon Health Sciences Uni6ersity, 3181 SW Gainess, Portland, OR 97 201, USA c Center for Cancer Research, Massachusetts Institute of Technology, 40 Ames Street, Cambridge, MA 02139, USA
Abstract The study of the effects of cytomegaloviruses on the MHC class I-restricted antigen presentation pathway has yielded an embarrassment of riches. The human cytomegalovirus (HCMV) encodes at least five to six different glycoproteins, each interfering in a different way with elimination of the virus by the host immune system. Most likely, it is the concerted action of these glycoproteins that allows HCMV to escape from elimination by the host immune system during acute and perhaps also persistent infection. Prime targets of these CMV glycoproteins are MHC class I glycoproteins: the very molecules that signal the presence of a virally infected cell to the immune system. Recently, several novel links in the multi-step process of immune evasion by HCMV have been discovered. © 1997 Elsevier Science B.V. Keywords: Cytomegalovirus; Immune system; MHC class I
1. HCMV US2 and US11 destroy newly synthesized MHC class I molecules The US region of the HCMV genome encodes most if not all glycoproteins known to play a role in downregulation of MHC class I expression. Using defined deletion mutants, the early gene product US11 was first identified as a major cause of the long observed instability of MHC class I molecules in HCMV infected cells [1]. More recently, another early gene product, US2, was found to cause class I degradation in a manner indistinguishable from that caused by US11 [3]. Simultaneous expression of two gene products with a similar function is unexpected, especially in view of the absence of significant similarity between US2 and US11. Using cell lines stably transfected with US11 and US2 some of the highly unusual characteristics of what appears to be a novel protein * Corresponding author. Tel.: +31 30 2743665; fax: + 31 30 2744429; e-mail: [email protected] 0165-2478/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 5 - 2 4 7 8 ( 9 7 ) 0 0 0 7 3 - 4
breakdown pathway have been characterized [2,3]. The most compelling observations include: (1) the occurrence of an MHC class I breakdown intermediate in the presence of protease inhibitors specific for the proteasome; (2) involvement of a peptide-N-glycanase in the generation of the class I breakdown intermediate; (3) occurrence of the deglycosylated intermediate (distinguishable by the apparent Asn-to-Asp conversion) as a soluble molecule in the cytosol; (4) involvement in the ‘dislocation’ process of components of the Sec61p complex, the translocation apparatus via which proteins normally enter the ER, and (5) isolation of the breakdown intermediate from proteasomes whose function has been blocked termporarily by protease inhibitors. This unexpected series of events defines a novel breakdown pathway, involving backtransport of glycoproteins from the ER to the cytosol via a dedicated transport channel, inter alia composed of Sec61a and b chains, deglycosylation by a host N-glycanase and, finally, degradation by the proteasome [2,3].
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E. Wiertz et al. / Immunology Letters 57 (1997) 213–216
This pathway is also operational in the absence of viral gene products and appears to be used to dispose of class I heavy chains that fail to fold and assemble properly, for example because antigenic peptides are lacking (as is the case in TAP-deficient cells), or because the light chain, b2m, is absent (in b2m-negative Daudi cells) [4]. Recent reports indicate that retrograde transport, followed by proteasomal degradation, is not limited to MHC molecules but occurs for many other proteins, including the paradigm of ER protein degradation, TCRa (Huppa and Ploegh, submitted) and soluble proteins, in mammalian as well as yeast cells (reviewed in [5]). 2. US3 retains MHC class I molecules in the endoplasmic reticulum Although the US3-encoded glycoprotein shows homology with US2 and US11, the mechanisms by which these molecules influence MHC class I expression are fundamentally different [6,7]. Whereas US2 and US11 induce rapid breakdown of class I heavy chains, stable heavy chain-b2m complexes are formed in US3 expressing transfectants. Instead, US3 retains the class I complexes in the ER as indicated by their EndoH sensitivity. Accordingly, immunofluorescence microscopy reveals perinuclear accumulation of MHC class I molecules while US3 itself is also confined to the ER. When cells are lysed in the presence of digitonin (but not NP40) US3 is found in a physical complex with class I heavy chains and b2m. Association with US3 takes place prior to peptide loading and slows down—but does not prohibit — the acquisition of antigenic peptides [6,7]. A careful analysis of the sequence of expression of US2, US3 and US11 indicates that US3 is present at immediate-early times during infection whereas US11 and US2 are expressed at early and late times [6]. The half life of US3 and US11 is estimated at 3 and 6 h p.i., respectively, indicating that expression of US3 and US11/US2 overlap briefly. It is tempting to propose a model in which US3, being the first non-regulatory immediate-early protein expressed, retains stable MHC class I heterodimers in the ER, thereby rendering them susceptible to destruction by US11/US2 expressed at later time points. However, experiments with US11 or US2 transfected cells suggest that the majority of class I molecules are attacked as free heavy chains: rapid degradation occurs immediately after completion of the polypeptide chain, prior to association with b2m [2,3]. In contrast, US3 appears to interact specifically with heavy chain-b2m complexes [6,7]. Nevertheless, the interplay between US3 and US2/US11 may apply to the minor population of MHC class I molecules that escapes immediate destruction by US11 and US2, and forms a complex with b2m.
ER retention of MHC class I molecules is not unique for HCMV: the adenovirus E3-19k gene product [8,9] possesses similar functions and also murine cytomegalovirus retains class I molecules in the ER [10,11,24]. US3 and E3-19k are not homologous; both are type I membrane proteins and contain high mannose type N-linked glycans.
3. US6 blocks peptide transport by TAP Although ER retention of class I molecules, and also degradation by US2, is mediated via a direct physical interaction with class I heavy chains, this is not a prerequisite for class I downregulation. Other viral gene products have been shown to act indirectly and block functions required for assembly of MHC class I–peptide complexes. Such a mechanism has been described for the HSV 1 and 2-encoded ICP47 [12,13], a small cytosolic protein that blocks the transporter associated with antigen presentation (TAP). Recent data indicate that HCMV also encodes a protein, US6, capable of inhibiting TAP function [14,15]. Most likely, inhibition by US6 involves a novel type of interaction with TAP, since US6 is a glycoprotein and does not demonstrate any homology with the the cytosolic ICP47.
4. UL18 acts as a decoy for NK cells NK cells carry triggering receptors and inhibitory receptors. Activation of the triggering receptor by a target cell will result in its destruction unless the inhibitory receptor detects an MHC class I molecule. Thus, cells that have lost cell surface expression of MHC class I are recognized and destroyed by NK cells. The murine CMV encodes a class I homologue, called m144, which acts as a molecular decoy in vivo [17]. A recombinant MCMV in which the m144 gene has been disrupted demonstrates severely restricted replication compared with wild type MCMV [17]. In vivo depletion studies show that NK cells are responsible for the observed attenuation of the infection. Interestingly, in addition to the class I retaining or destroying functions, HCMV encodes a class I homologue, UL18, which is capable of inhibiting the function of NK cells in vitro [16].
5. Phosphorylation of the HCMV immediate early (I-E) protein by pp65 inhibits T-cell recognition of I-E During the immediate-early phase of viral gene expression, an essential viral transcription factor, 72 kDa immediate-early (I-E) protein, is produced. Since this I-E gene product is expressed abundantly prior to
E. Wiertz et al. / Immunology Letters 57 (1997) 213–216
synthesis of most glycoproteins known to be involved in immune suppression, a CTL response against I-E would be expected. However, very few I-E-specific CTL’s are detected in seropositive individuals. Recent data suggest that a CMV matrix protein, pp65, might be responsible for this apparent immune suppression [18]. Pp65 catalyzes phophorylation of I-E. The modified I-E gene product fails to activate CTL’s in vitro [18].
6. Immune evasion by murine CMV MCMV has its own long literature with regard to immune evasion affecting the class I presentation pathway. Early MCMV gene products (i.e. ones which require immediate early gene products for their transcription) cause peptide loaded MHC class I molecules to be retained in the ER; expression of early genes abolishes CTL recognition of an IE encoded antigen or constitutively expressed cellular antigens [10,11,19,20,24]. The MCMV gene M152 is capable of mediating this effect. M152 encodes a 40kD glycoprotein, which when expressed using a vaccinia vector causes retention of class I molecules and lack of recognition of simultaneously expressed pp99. Interestingly, class I molecules appear retained in the ER-cisGolgi intermediate compartment [24]. As mentioned, MCMV also encodes an MHC class I homologue that inhibits NK activity [17], analogous to the UL18 product of HCMV [16]. Furthermore the importance of NK cells in control of MCMV infection has been easy to document, and a resistance locus to MCMV maps close to the NK locus on mouse chromosome 6 [21]. Recently, a 34kDa glycoprotein encoded by the MCMV gene MO4 has been discovered which is synthesized early in MCMV infection (although only after ER retention becomes apparent) [22]. GpMO4 is largely ER resident, although a small portion binds to class I MHC, and this portion travels with class I to the cell surface where they remain stably associated. The function of GpMO4 (to interfere with CTL recognition at the cell surface? To rescue a portion of class I and so interfere with NK lysis?) has not yet been determined. MCMV thus expresses an array of immune intervention factors which is analogous in complexity to that of HCMV; although in details of molecular function there appear to be many differences.
7. Discussion The redundancy of mechanisms interfering with antigen presentation provokes the question: what is the advantage to the virus? As mentioned, Ahn et al. [6] speculate that the sequential expression of US3 and
215
US11 in virus infection leads to a more efficient interference with class I restricted antigen presentation, with retention preceding degradation. Another possibility is raised by the differential ability of US2 and US11 to cause degradation of mouse class I molecules [23]. There is a clear difference in susceptibility of different class I alleles to degradation; and thus US2 and US11 apparently differ in their allelic preferences. We can extrapolate to suggest that in order to adequately cover the range of human alleles the virus has found it necessary to employ at least two molecules with apparently redundant functions. If true, this suggests a fascinating picture of a sort of cold war standoff, with both virus and the immune system diversifying their arsenals to reach an equilibrium within an evolutionary timeframe. These two types of explanation-that the mechanisms synergise to achieve optimal class I retention, and that the mechanisms are diversified to cope with the polymorphism amongst MHC class I alleles—are not mutually exclusive. Finally, the genes might act differently in the different cell types that the virus infects in vivo, in order to optimize the efficiency of immune control to serve the virus’ overall strategy.
References [1] T.R. Jones, L.K. Hanson, L. Sun, J.S. Slater, R.M. Stenberg, A.E. Campbell, Multiple independent loci within the human cytomegalovirus unique short region down-regulate expression of major histocompatibility complex class I heavy chains, J. Virol. 69 (1995) 4830 – 4841. [2] E.J.H.J. Wiertz, T.R. Jones, L. Sun, M. Bogyo, H.J. Geuze, H.L. Ploegh, The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the ER to the cytosol, Cell 84 (1996) 769 – 779. [3] E.J.H.J. Wiertz, D. Tortorella, M. Bogyo, J. Yu, W. Mothes, T.R. Jones, T.A. Rapoport, H.L. Ploegh, Sec61-mediated transfer of a membrane protein from the endoplasmic reticulum to the proteasome for destruction, Nature 384 (1996) 432–438. [4] E.A. Hughes, C. Hammond, P. Cresswell, Misfolded major histocompatibility complex class I heavy chains are translocated into the cytoplasm and degraded by the proteasome, Proc. Natl. Acad. Sci. USA 94 (1997) 1896 – 1901. [5] J.S. Bonifacino, Reversal of fortune for nascent proteins, Nature 384 (1996) 405 – 406. [6] K. Ahn, A. Angulo, P. Ghazal, P.A. Peterson, Y. Yang, K. Fru¨h, Human cytomegalovirus inhibits antigen presentation by a sequential multistep process, Proc. Natl. Acad. Sci. USA 93 (1996) 10990 – 10995. [7] T.R. Jones, E.J.H.J. Wiertz, L. Sun, K.N. Fish, J.A. Nelson, H.L. Ploegh, Human cytomegalovirus US3 impairs transport and maturation of MHC Class I heavy chains, Proc. Natl. Acad. Sci. USA 93 (1996) 11327 – 11333. [8] M. Andersson, S. Paabo, T. Nilsson, P.A. Peterson, Impaired intracellular transport of class I MHC antigens as a possible means for adenoviruses to evade immune surveillance, Cell 43 (1985) 215 – 222. [9] H.-G. Burgert, S. Kvist, An adenovirus type glycoprotein blocks cell surface expression of human histocompatibility class I antigens, Cell 41 (1985) 987 – 997.
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