doi:10.1006/smim.2001.0294, available online at http://www.idealibrary.com onseminars in IMMUNOLOGY, Vol. 13, 2001: pp. 41–49

Immune evasion by human cytomegalovirus: lessons inimmunology and cell biology

Wil A. M. Loenen a, C. A. Bruggeman a and E. J. H. J. Wiertz b,∗

The human cytomegalovirus (HCMV) has dedicated a sig-nificant part of its genome to genes encoding molecules thatmodulate the host immune response. Many of these geneshave homologues in the host genome. Others, however, areunique in the sense that no obvious primary sequence iden-tity is found in the available databases. The HCMV geneproducts interfere with the activation of MHC class I andclass II restricted T cells and NK cells, modify the function ofcytokines and their receptors, interact with complement fac-tors and modulate signal transduction and transcriptionfactor activity, in addition to interference with many othercellular functions. Investigation of these evasion strategieshas not only improved our understanding of HCMV patho-genesis, but has also provided unexpected, novel insights intobasic cell biological and immunological processes.

Key words: cytomegalovirus / immune evasion / MHCclass I homologue / US2 family

c© 2001 Academic Press

Introduction

Human cytomegalovirus (HCMV) causes persistentinfections with little or no harm to healthy individu-als. To immunocompromised patients, such as heartor kidney transplant recipients and AIDS patients,however, the virus forms a serious threat. In theseindividuals, the delicate balance, developed betweenthe virus and its host in the course of evolution, is

From the aDepartment of Medical Microbiology, University ofMaastricht, PO BOX 5800, 6202 AZ Maastricht, the Netherlands andbDepartment of Medical Microbiology, Leiden University Medical Center,PO BOX 9600, 2300 RC Leiden, the Netherlands. *Correspondingauthor. E-mail: wiertz@lumc.nl

c©2000 Academic Press1044–5323/01/010041+ 09/$35.00/0

disrupted. In addition, HCMV infection of seroneg-ative pregnant women is not without danger. In fact,HCMV is the most common viral cause of congenitaldefects. Research of the past few years has providednew insights into the fascinating interplay of hostimmune surveillance and viral evasion strategies. Thisknowledge is instrumental in the design of new strate-gies to treat and prevent HCMV-associated diseases.

HCMV, as well as its counterparts in other mam-malians, such as the murine, rat and rhesus CMV,have acquired homologs of host genes with a rolein the normal immune response. These includemajor histocompatibility complex (MHC) class Imolecules, intracellular and extracellular receptors,as well as cytokines and chemokines. Recently, severallarge blocks of related open reading frames withno known cellular homologues have been shownto affect the expression of MHC class I moleculesat the surface of infected cells. Examples capableof inducing such down regulation are the HCMV-encoded US genes. In the absence of MHC class Imolecules, cytotoxic T cells (CTLs) will no longerbe able to detect the infected cells. In addition, theCMV genome contains relatives of these genes, towhich no function has been assigned and which haveprobably arisen from duplication events. Togetherwith the host homologues, these genes cover as muchas 10–20% of the total genome of cytomegalovirusesand represent a potentially powerful army, actingagainst the host immune system during lytic andlatent infection.

The products of the unique short region genesUS2, US3, US6 and US11 are examples of im-munoregulatory molecules that do not have homo-logues in the host. Although they belong to onefamily, they affect MHC class I molecules in com-pletely different ways (discussed below). In addition,gpUS2 has recently been shown to interfere withthe function of MHC II molecules. The non-classicalMHC Ib molecule HLA-G, which occurs on placental

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Table 1. HCMV genes with cellular homologues that affect immune responses

Gene Cellular homologue Potential role

UL18 MHC I Binds LIR-1Inhibitory effect on cells carrying the ligand CD94/NKG2subsets NK and T cells, including memory and γ δ cellsthe majority of antigen-presenting DC and B cellsmonocytes

UL33 CCR1 Chemokine receptor, present in viral envelopRole in inflammatory response,

UL78 CCR1 Chemokine receptor with similarity to the fMLP receptorRole in inflammatory response, leukocyte migration

UL111A IL-10 Inhibition of IL-12 and IFN-gammaInhibition of DC, promotion of B cell growth

UL144 TNFR supergene family Most members bind TRAFs, upregulate NF-kB and kinases andstimulate differentiation of many subsets of lymphocytes;others are part of the apoptotic pathway involving TRADD.Interference with second signal in the Ag-specific immuneresponse

UL146 &UL147

IL-8 ChemokinesRole in inflammatory response, leukocyte migration

US27 CCR1 Chemokine receptor-like protein with no known ligandRole in inflammatory response, leukocyte migration

US28 CCR1 Chemokine receptor, binds RANTES, MCP-1, MIP-1α, MIP-1βRole in inflammatory response, leukocyte migration

Un FcR Unknown host or viral geneElimination of virus-specific antibodies

tissues, is not spared either and is attacked by gpUS3and gpUS6. Excitement has arisen by the findingthat HCMV affects antigen presentation by anotherMHC Ib molecule, HLA-E, via a protein unrelated tothe US2 family, namely gpUL40. This may well turnout to be a highly effective evasion strategy, as theligand for HLA-E is the natural killer cell receptorCD94/NKG2. In addition to NK cells, this receptoris expressed on antigen presenting cells (APCs) andsubsets of T cells, including γ δ cells. The moleculehas also been reported on memory T cells, a T cellsubset that is potentially important during latencyand possibly also during reactivation of HCMV.

Immune modulation by HCMV genes withhomology to host genes

So far, nine genes have been identified in the HCMVgenome, that share homology with host proteinsand manipulate the host immune response (listedin Table 1).

UL18

More than a decade has passed since the discoveryof the MHC I homologue UL18.1 Only recentlyit was shown that UL18 serves as a decoy for NKcells.2 The eventual identification of its ligand, LIR-1,allowed a more detailed investigation of its function3.The interaction of UL18 with LIR-1 downregu-lates NK-mediated toxicity. LIR-1 is also expressed onnearly all monocytes, dendritic cells, B cells and a sub-set of T cells, in which cells it also blocks cytotoxic re-sponses. It can interact with a large number of MHCIa and Ib alleles.3, 4 An important clue for its potentialrole in immune evasion has come from elegant workby the laboratory of Dr P. Bjorkman. Using BIAcoresensor chips coated with various truncated, solubleforms of LIR-1, the authors show LIR-1 to bind UL18with a 1000-fold higher affinity than host MHC class Imolecules, with a Kd in the nM range.5 This allowseven very small amounts of UL18 at the cell surface(as indeed found) to be sufficient for efficient en-

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gagement of LIR-1, which may result in inhibition ofthe NK cell attack. Binding probably occurs betweenthe N-terminal Ig-domain of LIR-1 and the α3 domainof UL18, c.q. MHC class I molecules, in a fashionanalogous to the interactions between CD4 and MHCclass II molecules. LIR-1 binds UL18 with and with-out peptide with similar affinity, which is particularlyinteresting in the context of the MHC class I homo-logues of mouse and rat CMV, m144 and r144 respec-tively. These homologues lack part of the α2 domainand thus part of the peptide binding pocket.6, 7

G-protein coupled receptor homologues

Four HCMV orfs, UL33, UL78, US27 and US28,encode homologues of G protein-coupled chemokinereceptors.8 Chemokine (or chemotactic cytokine)receptors play an important role in inflammation.They direct leukocyte migration to sites of virusinfection, which is crucial for effective clearance ofthe lytic infection. Positional homologues of UL33and UL78 have also been identified in rat andmurine CMV.8–11

IL-10

A CMV homologue of host IL-10 has been identifiedin studies with rhesus monkeys.12 The IL10 gene,which hitherto escaped attention due to the presenceof introns, has a positional homologue in HCMV,UL111A. IL10 is a pleiotropic cytokine that candownregulate MHC class II expression, among otherimmunoregulatory functions.

IL-8

The sequence of HCMV published by Chee et al.13

concerns the commonly used laboratory strainAD169. Clinical isolates of HCMV exist that containadditional orfs, two of which, UL146 and UL147,encode chemokines14. Murine and rat CMV do nothave positional homologues of these genes, butexpress different chemokines from the (spliced)m129/m131 and (probably unspliced) r131genes,respectively7, 11. These genes are likely to be ofphysiological importance and may control traffickingof leukocytes to sites of infection.

UL144

UL144 encodes a homologue of the tumor necrosisfactor receptor (TNFR) supergene family,15 whosemembers are of great importance for regulation of

the immune response.?, 16 Transfection studies infibroblasts indicate that, unlike the host TNFR-likereceptors, this protein is not expressed on the cellmembrane. The protein contains a large numberof N-linked glycans of unknown function, whereasno ligand has been identified so far. The authorsreported that sera of a group of 25 HIV patientsshowed a correlation between positivity for HCMVand the presence of UL144-specific antibodies, whichsuggests a potential clinical relevance of this receptorin vivo.

FcR

One or more HCMV genes induce FcR activity oninfected cells, which may in some as yet unknown wayallow escape from antibody-mediated destruction.18

Whether this FcR itself is virus- or host-encodedremains to be established, though the murine m138orf encodes a FcR that is conserved in rat CMV7, 11.

CTL evasion by gpUS2, gpUS3, gpUS6 andgpUS11: interference with cell surfaceexpression of MHC class I molecules

MHC class I–peptide complexes are assembled soonafter synthesis and translocation of the nascentpolypeptide chains into the endoplasmatic reticulum(ER).19 Several chaperones stabilize the folding in-termediates, as only the trimeric complex composedof heavy chain (HC), β2-microglobulin (β2m) andantigenic peptide is stable. The peptide componentis derived from proteins that have been targeted fordestruction by the proteasome. Such proteins aredegraded into short peptides of 8–12 amino acidresidues. The transporter associated with antigenprocessing, TAP, transports the resulting peptidesinto the ER. If viral proteins are present in thecytoplasm, these will be degraded in a similar fash-ion. At the cell surface, complexes of peptides andMHC class I molecules are recognized by the T cellreceptor for antigen.

HCMV is capable of blocking this antigen presenta-tion route using at least four different gene products,gpUS2, gpUS3, gpUS6 and gpUS11. These glycopro-teins are related (Figure 1). The US genes have no po-sitional homologues in the rodent CMVs. The mouseCMV genome, however, encodes at least three genes,m04, m06 and m152, that affect intracellular traffick-ing MHC class I molecules. Interestingly, the mech-

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gpUS2 --MNNLWKAWVGLWTSMGPLIRLPDGITKAGEDALRPWKSTAKH--PWFQIEDNRCYIDN gpUS3 ------MKPVLVLAILAVLFLRLADSVPRPLDVVVSEIRS-AH-----FRVEENQCWFHM gpUS6 ---MDLLIRLGFLLMCALPTPGERSSRDPKTLLSLSPRQQ-ACV--PRTKSHRPVCYNDT gpUS11 MNLVMLILALWAPVAGSMPELSLTLFDEPPPLVETEPLPPLSDVSEYRVEYSEARCVLRS gpUS2 GK-LFARGSIVGNMSRFVFDPKADYGG-VGENLYVHADDVEFVP-GESLKWNVRNLDVMP gpUS3 GM-LYFKGRMSGNFTEKHFVNVGIVSQSYMDRLQVSGEQYHHDERGAYFEWNIGGHPVTH gpUS6 G--DCTDADDSWKQLGEDFAHQCLQAAKKRPKTHKSRPNDRNLEGRLTCQRVRRLLPCDL gpUS11 GGRLEALWTLRGNLSVPTPTPRVYYQTLEGYADRVPTPVEDVSESLVAKRYWLRDYRVPQ gpUS2 IFETLALRLVLQG--DVIWLRCVPELRVDYT-----SSAYMWNMQYGMVRKSYTHVAWTI gpUS3 TVDMVDITLSTRWGDPKKYAACVPQVRMDYS-----SQTINWYLQRSMRDDNWGLLFRTL gpUS6 DIHPSHRLLTLMN-------NCVCDGAVWNA-----FRLIERHGFFAVTLYLCCGITLLV gpUS11 RTKLVLFYFSPCHQCQTYYVECEPRCLVPWVPLWSSLEDIERLLFEDRRLMAYYALTIKS gpUS2 VFYSINITLLVLFIVYVTVDCNLSMMWMRFFVC-- gpUS3 LVYLFSLVVLVLLTVGVSARLRFI----------- gpUS6 VILALLCSITYESTGRGIRRCGS------------ gpUS11 AQYTLMMVAVIQVFWGLYVKGWLHRHFPWMFSDQW

Figure 1. Alignment of amino acid sequences of HCMV-encoded gpUS2, gpUS3, gpUS6 and gpUS11. The sequences werealigned using the ClustalW alignment program.

Table 2. HCMV genes that affect antigen presentation by MHC class Ia, class Ib and class II

Gene Expression Target Mechanism

UL83 Pre-IE MHC I Pp65 blocks presentation of IE1 peptideUS3 IE MHC I Intracellular retention of MHC I in ERUS2 E MHC I

MHC IIDislocation of MHC I complex to cytoplasm and degradationDegradation HLA-DR-α and DM-α

US11 E MHC I Dislocation of MHC I to cytoplasm and degradationUS6 E/L MHC I Inhibition TAP-dependent peptide translocationUL40 E MHC Ib TAP-independent gpUL40 peptide presented by HLA-E allows

potential interaction with CD94/NKG2 on APC, T and NK cellsUn MHC II Unknown gene blocks IFNγ signalling via CIITA and JAK/STATUn (s) MHC I

MHC IIBlock surface expression of metalloproteases CD10 and CD13Involved in peptide trimming for presentation by class I and II

anisms involved are distinct from those employed bythe HCMV US gene products (see elsewhere in thisissue). Genes related to m152 appear to be present inthe rat CMV genome, though not all at similar posi-tions11.

GpUS2

gpUS2 is an early gene product that induces rapiddegradation of newly synthesized MHC class Imolecules, reducing the half life from more than6 h to less than 2 min20 (Table 2). In the ER, gpUS2appears to bind MHC class I molecules after orconcomitantly with core glycosylation. This resultsin redox-senstive21 retrograde transport to the cyto-plasm. After deglycosylation by an N-glycanase, bothMHC class I molecules and gpUS2 are degraded.An interesting aspect of this novel transport process

was the discovery that this involved the Sec61 porecomplex, which was initially believed to unidirec-tionally translocate nascent polypeptides into theER.20 It was subsequently shown that this route isin fact a constitutive cellular pathway that serves todegrade improperly folded or assembled proteincomplexes.20, 22 The cytoplasmic domain of MHCclass I molecules is essential for dislocation to the cy-tosol, whereas it is not required for binding gpUS2.23

Thus, substrate recognition and destruction of MHCclass I molecules are separable events.

GpUS3

In contrast to US2, US6 and US11, US3 encodes animmediate early gene product. GpUS3 physically asso-ciates with MHC class I heavy-chain/β2m complexesand retains these molecules in the ER. GpUS3 does

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Table 3. MHC allele specificity of gpUS2, gpUS3, gpUS6 and gpUS11

MHC Ia MHC Ib MHC II

Gene HLA-A HLA-B HLA-C HLA-E HLA-G HLA-DRUS2 + + − − − +

US3 + + + − + ?US6 + + + − + ?US11 + + − − − ?

not interfere with peptide loading.24 In addition toits interaction with MHC class I molecules, gpUS3 hasbeen reported to retain the MHC class Ib moleculeHLA-G (see References 25–27, Table 2).

GpUS6

GpUS6 is expressed at later times after infectionthan gpUS2 and gpUS3, and remains expressedthroughout the viral life cycle. GpUS6 interfereswith peptide loading of MHC class I molecules inthe ER in yet another way (see References 28–30, Table 2) GpUS6 associates transiently with thecomplex of TAP, MHC class I heavy chains andβ2m, that also contains the chaperones tapasin andcalreticulin. This interaction requires the ER-luminaldomain of gpUS6. The presence of gpUS6 resultsin inhibition of peptide translocation across the ERmembrane and effectively prevents assembly of thetrimeric MHC class I complex. Like gpUS3, gpUS6has been reported to affect HLA-G.25 This block,however, appears to be ineffectual for HLA-E, assurface expression of this MHC class Ib moleculeis facilitated in a TAP-independent way by the viralgpUL40 protein, as will be discussed below.

GpUS11

Like gpUS2, gpUS11 is an early protein thatcauses dislocation of MHC class I molecules tothe cytoplasm, thereby downregulating the surfaceexpression of classical MHC class I molecules, butnot HLA-G (see References 20, 27, 31, Table 2).Recently, it was shown that US11-mediated down-regulation of HLA-C effectively triggers lysis byNK cells in vitro.32 Binding of US11 does notrequire the cytoplasmic domain of MHC class Imolecules, though destruction of MHC class Imolecules does require this region to be present.23

In contrast to gpUS2, gpUS11 is not degradedtogether with the MHC class I molecules, whichalso points towards different mechanisms of degra-

dation on gpUS2 and gpUS11.31 Perhaps the twoproteins bind to different domains of the MHCclass I molecules. This possibility is supported bythe fact that gpUS2 and gpUS11 clearly differ incross-species recognition of murine MHC class I al-leles.33 Recent data show that the MHC class I heavychains become ubiquitinated while still attached tothe ER membrane. This has led to a new modelfor the sequence of events in quality control ofmisfolded proteins.34

More evasion: interference with cell surfaceexpression of MHC class II molecules

HCMV does not limit its subversive maneuvers to theMHC class I pathway of antigen presentation. Severalexciting reports have appeared, which indicate thatthe function of non-classical MHC Ib and MHCclass II molecules is also compromized by HCMV.Unlike MHC class I molecules, which are present onvirtually all cells in the body, MHC class II moleculesare mainly restricted to professional APCs: B cells,dendritic cells and cells of the monocytic lineage.These cells provide signals to both CTLs (via MHCclass I molecules) and T helper (Th) cells (via MHCclass II molecules). The interaction between APCsand Th cells occurs via a dual signal involving MHCclass I molecules and CD40, a member of the TNFRgene family (reviewed in References 35, 36). Thisinteraction would precede the ability of the APCto co-stimulate the CTL. It can thus be reasonedthat downregulation of MHC class II molecules willeffectively prevent a full CTL response. The virus in-terferes with MHC class II expression in several ways,including degradation of MHC class II glycoproteinsvia gpUS2, interference with the CIITA/JAK/STATpathway, inhibition of surface expression of the met-alloproteases CD10 and CD13, and finally perhapsalso via the newly found IL-10 homologue UL111A(discussed above).

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GpUS2

Using an adenovirus vector carrying an inducibleconstruct of the US2 gene, the gpUS2 has recentlybeen shown to affect not only MHC class I, butalso MHC class II expression. Infection leads torapid degradation of the HLA-DRα chain in aproteasome-dependent manner.37 In contrast tothe effect of gpUS2 on MHC class I molecules, nodegradation intermediates could be identified inthe cytoplasm, suggesting that the protein may bedirectly transported from the ER into the lumen ofthe proteasome. GpUS2 binds free α chains as wellas the complex of α, β and invariant chain (Ii). Theformation of functional MHC class II complexes isalso affected by gpUS2 indirectly, i.e. via degradationof HLA-DMA molecules. Together with HLA-DMB,DMA forms a heterodimer, which stabilizes MHCclass II molecules in the MHC class II compartmentand catalyzes the peptide loading process. Otherproteins with sequence homology to MHC class I andclass II molecules, such as HLA-DRβ and HLA-DMB,fail to be degraded by gpUS2. Based on these obser-vations and the lack of degradation of HLA-C andHLA-G by gpUS2, the authors conclude that gpUS2probably binds to the membrane–distal α1 domain.

CIITA

The class II transactivator (CIITA) protein tightlycontrols the promoter of the MHC class II region.CIITA itself is regulated by at least four differentpromoters allowing constitutive expression in den-dritic cells and B cells, and inducible expressionin other cells. The latter induction is IFN-γ depen-dent, involves the JAK/STAT pathway and can beblocked by HCMV.38 This block can be reversed byco-transfection with a CIITA gene controlled by theIFNγ -independent SRα promoter, which restoresMHC class II expression. The HCMV-induced inhibi-tion of this route may involve proteasome-dependentdegradation of JAK1, preventing activation by IFNγof the JAK/STAT pathway and thus of CIITA39 (seealso Sudmali, this volume).

Inhibition of CD10 and CD13 metalloproteases

CD10 (CALLA) and CD13 (aminopeptidase N) aremembers of the ADAM family of metalloproteases,which are involved in peptide trimming in both theMHC class I and class II antigen presentation path-ways.40, 41 Surface expression of CD10 and CD13 is af-fected by probably more than one as yet unidentified

HCMV gene. The block of CD10 expression appearsto occur primarily at the RNA level, whereas inhibi-tion of CD13 lies at the protein level and is possiblycaused by ER retention.41

A new evasion strategy: interference with thenon-classical MHC molecule HLA-E

Lysis of target cells by NK cells is controlled by theengagement of natural killer cell receptors withMHC class I molecules. NK cell receptors can bedivided into two groups, depending on the presenceof immunoreceptor tyrosine-based activation orinhibition motifs, ITAM or ITIM, in their cytoplasmicdomain. Upon interaction with their ligands theseregions regulate intracellular kinase/phosphataseactivity and thus influence the activation state of thecell. Three groups of receptor molecules, belongingto entirely different protein families, have beenidentified: the immunoglobulin-like killer inhibitoryreceptors (KIR), C-lectin-type ly49 receptors and theCD94/NKG2 heterodimeric receptors.4 The story iscomplicated as these receptors are also present onother cells such as dendritic cells, B cells, monocytesand subsets of T cells, including memory and γ δ cells.In addition, MHC class Ib molecules may serve asligands for NK cells and these molecules arealso being discovered on an expanding number ofcell types.26, 42

The CD94/NKG2 complex interacts with HLA-E,which has a very restricted peptide binding capacityand was initially shown to bind only peptides derivedfrom the signal sequences of MHC class I molecules.4

Recently,43 have show that surface expression ofHLA-E is not blocked by HCMV. It was discoveredthat HLA-E binds a peptide derived from the HCMVearly protein gpUL40, which in fact enhances surfaceexpression of HLA-E (Table 2). This peptide, whichhas a sequence related to MHC I signal peptides, canbe translocated into the ER in a TAP-independentmanner, thereby circumventing the gpUS6-inducedblock of the TAP transporter. TAP-independent pep-tide presentation has also been reported for HLA-G,which interacts with members of the KIR familyof natural killer receptors [which family includesLIR-1, the receptor to which the HCMV MHC class Ihomologue UL18 binds (see above)]. The data onthe allele specificity of the US gene products aresummarized in Table 3. Surprisingly, no publicationshave appeared yet on immune evasion by the US7,

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HCMV immune evasion strategies

US8 and US9 gene products, despite their highhomology to US11.

The answer of the host immune system to viralevasion strategies

Though viral immune evasion genes buy sufficienttime to establish HCMV latency, the body doesgenerate virus-specific antibodies and memory Band T cells. In recent years, HCMV-specific memoryCTLs have been characterized in more detail. TheCTLs are oligoclonal and primarily directed againstpeptides derived from the tegument phosphoproteinpp65, though other peptides can also be recognized.Clonal expansions that may comprise up to 3% of allPBMC, have now been pinpointed by two laboratoriesto specific T cell subsets, based on the expression ofCD57, CD27 and CD28. The first group identifiedoligoclonal CD28−CD57+CD8+HCMV-specific CTLprecursors in HCMV carriers by TCRβ-based RT-PCRanalysis.44 These results are in accordance with olderclinical data describing expansion of CD57+ CD8+

T cells in allograft recipients suffering from primaryHCMV infection.45, 46 The other laboratory used aread-out system for HCMV-reactive T cells that isbased on HCMVpp65 peptide-induced IFNγ andTNFα production, and assigned the CTLs to theCD57+ CD8+ CD27−/CD27+/− T cell subsets.47

These populations parallel the antigen-specificmemory and effector populations described earlier,the latter of which is marked by loss of both CD27and CD28.48 As CD27 is a hallmark for memoryB cells,49, 50 it strengthens the notion that differentialCD27 expression could serve as a useful tool to linkHCMV-specific memory cells of both B and T lineagewithin one individual.

The future

The past decade has seen major breakthroughs invirology, immunology and cell biology. New insightshave been gained in these fields by studies on viralstrategies to evade the host immune system. Virusesinterfere with, or perhaps take over control of pro-tein transport and processing in the cell in orderto generate productive progeny. Many questions,however, remain: how does the virus deal with thecontinuous flow of proteins recycling from thecell surface to the Golgi, as only some 10% of thistraffic consists of newly-synthesized molecules?51

What is the function of the molecular chaperonesin the presence of viral gene products that causedegradation or intracellular retention of cellularproteins? Why has HCMV acquired so many genes,each targeting the immune system in a differentway? Have they been duplicated in order to dealwith polymorphism of MHC molecules? Or do theyperhaps affect other immune receptors and ligands?And finally, we would like to learn more about HCMVitself: what is the function of its immune evasiongenes in vivo during lytic and latent phases of theviral life cycle?

References

1. Beck S, Barrell BG (1988) Human cytomegalovirus encodesa glycoprotein homologous to MHC class-I antigens. Nature331:269–272

2. Reyburn HT, Mandelboim O, Vales-Gomez M, Davis DM, Paz-many L, Strominger JL (1997) The class I MHC homologue ofhuman cytomegalovirus inhibits attack by natural killer cells.Nature 386:514–517

3. Cosman D, Fanger N, Borges L, Kubin M, Chin W, PetersonL, Hsu M (1999) Human CMV, MHC class I and inhibitorysignalling receptors: more questions than answers. ImmunolRev 168:177–185

4. Lopez-Botet M, Bellon T (1999) Natural killer cell activationand inhibition by receptors for MHC class I. Curr OpinImmunol 11:301–307

5. Chapman TL, Heikeman AP, Bjorkman PJ (1999) Theinhibitory receptor LIR-1 uses a common binding interactionto recognize MHC class I molecules and the viral homologUL18. Immunity 11:603–613

6. Beisser PS, Kloover JS, Grauls GE, Blok MJ, Bruggeman CA,Vink C (2000) The r144 major histocompatibility complexclass I-like gene of rat cytomegalovirus is dispensable for bothacute and long-term infection in the immunocompromisedhost. J Virol 74:1045–1050

7. Farrell HE, Degli-Esposti MA, Davis-Poynter NJ (1999) Cy-tomegalovirus evasion of natural killer cell responses. Im-munol Rev 168:187–197

8. Lalani AS, Barrett JW, McFadden G (2000) Modulatingchemokines: more lessons from viruses. Immunol Today21:100–106

9. Beisser PS, Grauls G, Bruggeman CA, Vink C (1999) Dele-tion of the R78 G protein-coupled receptor gene from ratcytomegalovirus results in an attenuated, syncytium-inducingmutant strain. J Virol 73:7218–7230

10. Beisser PS, Vink C, Van Dam JG, Grauls G, Vanherle SJV,Bruggeman CA (1998) The R33 G protein-coupled receptorgene of rat cytomegalovirus plays an essential role in thepathogenesis of viral infection. J Virol 72:2352–2363

11. Vink C, Beisser PS, Bruggeman CA (1999) Molecular mimicryby cytomegaloviruses. Function of cytomegalovirus-encodedhomologues of G protein-coupled receptors, MHC class Iheavy chains and chemokines. Intervirology 42:342–349

12. Lockridge KM, Zhou SS, Kravitz RH, Johnson JL, Sawai ET,Blewett EL, Barry PA (2000) Primate cytomegaloviruses en-code and express an IL-10-like protein. Virology 268:272–280

13. Chee MS et al. (1990) Analysis of the protein-coding content

47

W. A. M. Loenen et al.

of the sequence of human cytomegalovirus strain AD169.Curr Top Microbiol Immunol 154:125–169

14. Penfold ME, Dairaghi DJ, Duke GM, Saederup N, MocarskiES, Kemble GW, Schall TJ (2000) Cytomegalovirus encodes apotent alpha chemokine. Proc Natl Acad Sci 96:9839–9844

15. Benedict CA, Butrovich KD, Lurain NS, Corbeil J, Rooney I,Schneider P, Tschopp J, Ware CF (1999) Cutting edge: a novelviral TNF receptor superfamily member in virulent strains ofhuman cytomegalovirus. J Immunol 162:6967–6970

16. Gravestein LA, Borst J (1998) Tumor necrosis factor receptorfamily members in the immune system. Semin Immunol10:423–434

17. Loenen WAM (1998) CD27 and (TNFR) relatives in theimmune system: their role in health and disease. SeminImmunol 10:417–422

18. Furukawa T, Hornberger E, Sakuma S, Plotkin SA (1975)Demonstration of immunoglobulin G receptors inducedby human cytomegalovirus. J Clin Microbiol 2:332–336

19. Rock KL, Goldberg AL (1999) Degradation of cell proteinsand the generation of MHC class I-presented peptides. AnnuRev Immunol 17:739–779

20. Wiertz EJHJ, Tortorella D, Bogyo M, Yu J, Mothes W, Jones TR,Rapoport TA, Ploegh HL (1996a) Sec61-mediated transfer ofa membrane protein from the endoplasmic reticulum to theproteasome for destruction. Nature 384:432–443

21. Tortorella D, Story CM, Huppa JB, Wiertz EJ, Jones TR, BacikI, Bennink JR, Yewdell JW, Ploegh HL (1998) Dislocation oftype I membrane proteins from the ER to the cytosol is sensi-tive to changes in redox potential. J Cell Biol 142:365–376

22. Hughes EA, Hammond C, Cresswell P (1997) Misfolded ma-jor histocompatibility complex class I heavy chains are translo-cated into the cytoplasm and degraded by the proteasome.Proc Natl Acad Sci USA 94:1896–1901

23. Story CM, Furman MH, Ploegh HL (1999) The cytosolic tail ofclass I MHC heavy chain is required for its dislocation by thehuman cytomegalovirus US2 and US11 gene products. ProcNatl Acad Sci 96:8516–8521

24. Jones TR, Wiertz EJHJ, Sun L, Fish KN, Nelson JA, PloeghHL (1996) Human cytomegalovirus US3 impairs transportand maturation of major histocompatibility complex class Iheavy chains. Proc Natl Acad Sci USA 93:11327–11333

25. Jun Y, Kim E, Jin M, Sung HC, Han H, Geraghty DE, AhnK (2000) Human cytomegalovirus gene products US3 andUS6 down-regulate trophoblast class I MHC molecules. JImmunol 164:805–811

26. Le Bouteiller P, Blaschitz A (1999) The functionality of HLA-G is emerging. Immunol Rev 167:233–244

27. Schust DJ, Tortorella D, Seebach J, Phan C, Ploegh HL (1998)Trophoblast class I major histocompatibility complex (MHC)products are resistant to rapid.degradation imposed by thehuman cytomegalovirus (HCMV) gene products US2 andUS11. J Exp Med 188:497–503

28. Ahn K, Gruhler A, Galocha B, Jones TR, Wiertz EJHJ, PloeghH, Peterson PA, Yang Y, Früh K (1997) The ER-luminaldomain of the HCMV glycoprotein US6 inhibits peptidetranslocation by TAP. Immunity 6:613–621

29. Hengel H, Flohr T, Hammerling GJ, Koszinowski UH, Mom-burg F (1996) Human cytomegalovirus inhibits peptidetranslocation into the endoplasmic reticulum for MHC class Iassembly. J Gen Virol 77:2287–2296

30. Hengel H, Koopmann J, Flohr T, Muranyi W, Goulmy E,Hämmerling GJ, Koszinowski UH, Momburg F (1997) Aviral ER-resident glycoprotein inactivates the MHC-encodedpeptide transporter. Immunity 6:623–632

31. Wiertz EJHJ, Jones TR, Sun L, Bogyo M, Geuze HJ, Ploegh

HL (1996b) The human cytomegalovirus US11 gene productdislocates MHC class I heavy chains from the endoplasmicreticulum to the cytosol. Cell 84:769–779

32. Huard B, Fruh K (2000) A role for MHC class I down-regulation in NK cell lysis of herpes virus-infected cells. EurJ Immunol 30:509–515

33. Machold RP, Wiertz EJHJ, Jones TR, Ploegh HL (1997) TheHCMV gene products US11 and US2 differ in their abilityto attack allelic forms of murine major histocompatibilitycomplex (MHC) class I heavy chains. J Exp Med 185:363–366

34. Shamu CE, Story CM, Rapoport TA, Ploegh HL (1999) Thepathway of US11-dependent degradation of MHC class I heavychains involves a ubiquitin-conjugated intermediate. J CellBiol 147:45–58

35. Toes REM, Schoenberger SP, Van der Voort EIH, OffringaR, Melief CJM (1998) CD40-CD40ligand interactions andtheir role in cytotoxic T lymphocyte priming and anti-timorimmunity. Semin Immunol 10:443–448

36. Vogel LA, Noelle RJ (1998) CD40 and its crucial role as amember of the TNFR family. Semin Immunol 10:435–442

37. Tomazin R, Boname J, Hegde NR, Lewinsohn DM, AltschulerY, Jones TR, Cresswell P, Nelson JA, Riddell SR, JohnsonDC (1999) Cytomegalovirus US2 destroys two components ofthe MHC class II pathway, preventing recognition by CD4+T cells. Nat Med 5:1039–1043

38. Le Roy E, Muhlethaler-Mottet A, Davrinche C, Mach B, Davi-gnon JL (1999) Escape of human cytomegalovirus from HLA-DR-restricted CD4(+) T-cell response is mediated by repres-sion of gamma interferon-induced class II transactivator ex-pression. J Virol 73:6582–6589

39. Miller DM, Rahill BM, Boss JM, Lairmore MD, Durbin JE,Waldman WJ, Sedmak DD (1998) Human cytomegalovirusinhibits major histocompatibility complex class II expressionby disruption of the Jak/Stat pathway. J Exp Med 187:675–683

40. Barclay AN, Brown MH, Law SKA, McKnight AJ, TomlinsonMG, Van der Merwe PA (1997) The Leucocyte AntigenFactsBook. Academic Press Inc

41. Phillips AJ, Tomasec P, Wang EC, Wilkinson GW, BorysiewiczLK (1998) Human cytomegalovirus infection downregulatesexpression of the cellular aminopeptidases CD10 and CD13.Virology 250:350–358

42. Braud VM, Allan DS, McMichael AJ (1999) Functions ofnonclassical MHC and non-MHC-encoded class I molecules.Curr Opin Immunol 11:100–108

43. Tomasec P, Braud VM, Rickards C, Powell MB, McSharryBP, Gadola S, Cerundolo V, Borysiewicz LK, McMichaelAJ, Wilkinson GW (2000) Surface expression of HLA-E,an inhibitor of natural killer cells, enhanced by humancytomegalovirus gpUL40. Science 287:1031–1033

44. Weekes MP, Wills MR, Mynard K, Hicks R, Sissons JG,Carmichael AJ (1999) Large clonal expansions of hu-man virus-specific memory cytotoxic T lymphocytes withinthe CD57+CD28− CD8+ T-cell population. Immunology98:443–449

45. Hazzan M, Labalette M, Noel C, Lelievre G, Dessaint JP (1997)Recall response to cytomegalovirus in allograft recipients:mobilization of CD57+, CD28+ cells before expansion ofCD57+, CD28− cells within the CD8+ T lymphocyte compart-ment. Transplantation 63:693–698

46. Labalette M, Salez F, Pruvot FR, Noel C, Dessaint JP (1994)CD8 lymphocytosis in primary cytomegalovirus (CMV) in-fection of allograft recipients: expansion of an uncom-mon CD8+ CD57− subset and its progressive replacement byCD8+ CD57+ T cells. Clin Exp Immunol 95:465–471

47. Kern F, Khatamzas E, Surel I, Frommel C, Reinke P, Wal-

48

HCMV immune evasion strategies

drop SL, Picker LJ, Volk HD (1999) Distribution of humanCMV-specific memory T cells among the CD8pos. subsets de-finedby CD57, CD27, and CD45 isoforms. Eur J Immunol29:2908–2915

48. Hamann D, Baars PA, Rep MH, Hooibrink B, Kerkhof-GardeSR, Klein MR, van Lier RA (1997) Phenotypic and functionalseparation of memory and effector human CD8+ T cells. JExp Med 186:1407–1418

49. Lens SM, Tesselaar K, van Oers MH, van Lier RA (1998) Con-trol of lymphocyte function through CD27-CD70 interactions.Semin Immunol 10:491–499

50. Loenen WAM (2000) CD27, the receptor for CD70. Cy-tokine Reference DOI: 10.1006/rwcy.2000.16010, AcademicPress Ltd

51. Kelly (1999) Deconstructing membrane traffic. Trends CellBiol 9:M29–33

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