The ER-luminal domain of the HCMV glycoprotein US6 inhibits peptide translocation by TAP

Translational mini-review series on infectious disease: congenital cytomegalovirus infection: 50 years on

Cytomegalovirus (CMV) is the leading cause of congenital viral infection, with an incidence of 0.5-3% of live births worldwide. Clinical evidence has shown hearing and vision loss, mental retardation and sometimes death in affected newborns. Primary maternal CMV infection during gestation poses a 40% risk of intrauterine transmission in contrast to recurrent infection. European laboratories have made significant progress in the last decade in solving diagnostic problems linked to infection in pregnancy. With the advances in CMV serology, such as detection of anti-CMV IgM by enzyme immunoassays (EIA), confirmed by Western blot, together with seroconversion and anti-CMV IgG avidity evaluation in pregnant mothers, can help to identify recent infection. Preventative measures such as screening for CMV in the routine serological work-up of pregnant women have been introduced in countries such as Spain and Italy. The development of specific T cell-mediated immune responses in mothers, fetus and neonates is now emerging with regard to antigen-specific CD4 and CD8 T cells, differentiation status, proliferative and cytokine responses. A protective vaccine against CMV is a major public health priority and the study of vaccines in animal model systems has identified potential strategies for interrupting transmission and preventing disease in newborns. Congenital CMV infection has a variable outcome and therefore novel diagnostic methods are required to identify those at risk and therapeutic interventions are needed to improve the long-term prognosis of those infected. CMV was first isolated in 1957. We are now 50 years on, so procrastination is not an option.

A CD8+ T cell immune evasion protein specific to Epstein-Barr virus and its close relatives in Old World primates

γ1-Herpesviruses such as Epstein-Barr virus (EBV) have a unique ability to amplify virus loads in vivo through latent growth-transforming infection. Whether they, like α- and β-herpesviruses, have been driven to actively evade immune detection of replicative (lytic) infection remains a moot point. We were prompted to readdress this question by recent work (Pudney, V.A., A.M. Leese, A.B. Rickinson, and A.D. Hislop. 2005. J. Exp. Med. 201:349–360; Ressing, M.E., S.E. Keating, D. van Leeuwen, D. Koppers-Lalic, I.Y. Pappworth, E.J.H.J. Wiertz, and M. Rowe. 2005. J. Immunol. 174:6829–6838) showing that, as EBV-infected cells move through the lytic cycle, their susceptibility to EBV-specific CD8⁺ T cell recognition falls dramatically, concomitant with a reductions in transporter associated with antigen processing (TAP) function and surface human histocompatibility leukocyte antigen (HLA) class I expression. Screening of genes that are unique to EBV and closely related γ1-herpesviruses of Old World primates identified an early EBV lytic cycle gene, BNLF2a, which efficiently blocks antigen-specific CD8⁺ T cell recognition through HLA-A–, HLA-B–, and HLA-C–restricting alleles when expressed in target cells in vitro. The small (60–amino acid) BNLF2a protein mediated its effects through interacting with the TAP complex and inhibiting both its peptide- and ATP-binding functions. Furthermore, this targeting of the major histocompatibility complex class I pathway appears to be conserved among the BNLF2a homologues of Old World primate γ1-herpesviruses. Thus, even the acquisition of latent cycle genes endowing unique growth-transforming ability has not liberated these agents from evolutionary pressure to evade CD8⁺ T cell control over virus replicative foci.

A short isoform of human cytomegalovirus US3 functions as a dominant negative inhibitor of the full-length form

Human cytomegalovirus encodes four unique short (US) region proteins, each of which is independently sufficient for causing the down-regulation of major histocompatibility complex (MHC) class I molecules on the cell surface. This down-regulation enables infected cells to evade recognition by cytotoxic T lymphocytes (CTLs) but makes them vulnerable to lysis by natural killer (NK) cells, which lyse those cells that lack MHC class I molecules. The 22-kDa US3 glycoprotein is able to down-regulate the surface expression of MHC class I molecules by dual mechanisms: direct endoplasmic reticulum retention by physical association and/or tapasin inhibition. The alternative splicing of the US3 gene generates two additional products, including 17-kDa and 3.5-kDa truncated isoforms; however, the functional significance of these isoforms during viral infection is unknown. Here, we describe a novel mode of self-regulation of US3 function that uses the endogenously produced truncated isoform. The truncated isoform itself neither binds to MHC class I molecules nor prevents the full-length US3 from interacting with MHC class I molecules. Instead, the truncated isoform associates with tapasin and competes with full-length US3 for binding to tapasin; thus, it suppresses the action of US3 that causes the disruption of the function of tapasin. Our results indicate that the truncated isoform of the US3 locus acts as a dominant negative regulator of full-length US3 activity. These data reflect the manner in which the virus has developed temporal survival strategies during viral infection against immune surveillance involving both CTLs and NK cells.

A prospective assessment of cytomegalovirus immune evasion gene transcription profiles in transplant patients with cytomegalovirus infection

BACKGROUND

The cytomegalovirus (CMV) immune evasion genes US3, US6, and US11 may disrupt the host immune response via downregulation of major histocompatibility complex molecules. Transplant recipients with CMV infection were prospectively assessed for immune evasion gene expression.

METHODS

Seventy solid organ transplant patients with CMV infection who were given antiviral therapy were enrolled. Quantitative mRNA levels of US3, US6, and US11 were assessed using real-time polymerase chain reaction assays from peripheral blood mononuclear cells at regular time-points after starting therapy.

RESULTS

High immune evasion mRNA levels were detectable at start-of-therapy (median US3-4.5 log10 copies; US6- 3.7 log10 copies, and US11-3.3 log10 copies/10 cells). With therapy, immune evasion mRNA levels declined exponentially. For example, median calculated US3 half-life was 1.59 days (range 0.74-12.5 days). By day7, US3 mRNA was detectable in 55.7%, US6 in 38.6%, and US11 in 41.4% of patients. Early phase kinetics correlated with outcomes. When adjusted for baseline DNA level, there was a trend to higher mRNA levels in patients who relapsed. Also, detectable mRNA at day 14 after start of therapy was associated with virologic relapse after initial treatment (P<or=0.001 for US3, US6, and US11). For example, if US3 mRNA was still detectable at day 14, then risk of relapse was 84.2% vs. 29.4% if US3 mRNA not detectable at day 14 (P<0.001). This correlation was independent of the DNA viral load.

CONCLUSION

CMV immune evasion gene expression is detectable at high levels in patients with CMV infection and declines exponentially with therapy. Expression levels can be independently correlated with outcomes.

Processing and MHC class I presentation of human cytomegalovirus pp65-derived peptides persist despite gpUS2–11-mediated immune evasion

Immune control of human cytomegalovirus (HCMV) infection can be mediated by CD8+cytolytic T lymphocytes (CTL). Adoptive transfer of antiviral CTL confers protection against HCMV reactivation and disease. The tegument protein pp65 and the immediate-early 1 protein (IE1) are recognized to be major CTL targets, even though during productive infection the viral immunoevasion proteins gpUS2–11 act to suppress major histocompatibility complex (MHC) class I-restricted antigen presentation. Thus it was not clear how infected cells could be labelled with antigenic peptides in the face of immunoevasion. We show here that the immunodominant peptide pp65NLVwas presented by MHC class I in cells infected with a gpUS2–11-competent virus. Presentation of pp65NLVwas still detectable at 96 h post-infection, although at low levels. Partial suppression of pp65NLVpresentation was dependent on the ability of the infecting strain to express gpUS2–11. MHC class I-restricted antigen presentation in HCMV-infected cells (encoding gpUS2–11) exhibited specificity for pp65-derived peptides, as infected fibroblasts did not present the IE1-derived nonapeptide IE1TMY. Remarkably, infected cells could restore pp65NLVpeptide presentation after acid removal of MHC class I despite gpUS2–11 expression. This recovery was shown to be dependent on proteasome functionality. In contrast to IE1, pp65 peptides are loaded on MHC class I molecules to be transported to the cell surface at early and late times after infection in the face of gpUS2–11-mediated immunoevasion. pp65 is therefore the first example of an HCMV protein only incompletely subjected to gpUS2–11-mediated immunoevasion.

Human herpesvirus-6A and -6B encode viral immunoevasins that downregulate class I MHC molecules

Like all other members of the herpesvirus family, the closely related human herpesviruses-6 and -7 (HHV-6,7) persist in their host throughout life. In so doing, without exception, every member of the herpesvirus family has evolved mechanisms to avoid detection by the immune system. In particular, human cytomegalovirus (HCMV), mouse cytomegalovirus (MCMV), human herpesvirus-8 (HHV-8), and herpes simplex virus (HSV) all encode multiple proteins that interfere with proper MHC class I antigen presentation. The mechanisms employed by these viruses to effect removal of MHC class I from the cell surface vary. The U21 open reading frame from HHV-7 diverts class I MHC molecules to an endolysosomal compartment using an as-yet unknown mechanism. The two variants of HHV-6, HHV-6A and -6B, both possess a U21 open reading frame which contain only approximately 30% amino acid identity to the U21 sequence from HHV-7. Here we describe the characterization of the U21 gene products from HHV-6A and HHV-6B. Like HHV-7 U21, both of the HHV-6 U21 molecules bind to and divert class I MHC molecules to an endolysosomal compartment, effectively removing them from the cell surface, and providing a possible means of escape from immune detection.

Modulation of the antigenic peptide transporter TAP by recombinant antibodies binding to the last five residues of TAP1

The transporter associated with antigen processing (TAP) plays a pivotal role in the major histocompatibility complex (MHC) class I mediated immune response against infected or malignantly transformed cells. It belongs to the ATP-binding cassette (ABC) superfamily and consists of TAP1 (ABCB2) and TAP2 (ABCB3), each of which possesses a transmembrane and a nucleotide-binding domain (NBD). Here we describe the generation of recombinant Fv and Fab antibody fragments to human TAP from a hybridoma cell line expressing the TAP1-specific monoclonal antibody mAb148.3. The epitope of the antibody was mapped to the very last five C-terminal amino acid residues of TAP1 on solid-supported peptide arrays. The recombinant antibody fragments were heterologously expressed in Escherichia coli and purified to homogeneity from periplasmic extracts by affinity chromatography. The monoclonal and recombinant antibodies bind with nanomolar affinity to the last five C-terminal amino acid residues of TAP1 as demonstrated by ELISA and surface plasmon resonance. Strikingly, the recombinant antibody fragments confer thermal stability to the heterodimeric TAP complex. At the same time TAP is arrested in a peptide transport incompetent conformation, although ATP and peptide binding to TAP are not affected. Based on our results we suggest that the C terminus of TAP1 modulates TAP function presumably as part of the dimer interface of the NBDs.

The varicellovirus-encoded TAP inhibitor UL49.5 regulates the presentation of CTL epitopes by Qa-1b1

Impairment of MHC class I Ag processing is a commonly observed mechanism that allows viruses and tumors to escape immune destruction by CTL. The peptide transporter TAP that is responsible for the delivery of MHC class I-binding peptides into the endoplasmic reticulum is a pivotal target of viral-immune evasion molecules, and expression of this transporter is frequently lost in advanced cancers. We recently described a novel population of CTL that intriguingly exhibits reactivity against such tumor-immune escape variants and that recognizes self-peptides emerging at the cell surface due to defects in the processing machinery. Investigations of this new type of CTL epitopes are hampered by the lack of an efficient inhibitor for peptide transport in mouse cells. In this article, we demonstrate that the varicellovirus protein UL49.5, in contrast to ICP47 and US6, strongly impairs the activity of the mouse transporter and mediates degradation of mouse TAP1 and TAP2. Inhibition of TAP was witnessed by a strong reduction of surface MHC class I display and a decrease in recognition of conventional tumor-specific CTL. Analysis of CTL reactivity through the nonclassical molecule Qa-1(b) revealed that the presentation of the predominant leader peptide was inhibited. Interestingly, expression of UL49.5 in processing competent tumor cells induced the presentation of the new category of peptides. Our data show that the varicellovirus UL49.5 protein is a universal TAP inhibitor that can be exploited for preclinical studies on CTL-based immune intervention.

Redox Regulation Facilitates Optimal Peptide Selection by MHC Class I during Antigen Processing

Activated CD8(+) T cells discriminate infected and tumor cells from normal self by recognizing MHC class I-bound peptides on the surface of antigen-presenting cells. The mechanism by which MHC class I molecules select optimal peptides against a background of prevailing suboptimal peptides and in a considerably proteolytic ER environment remained unknown. Here, we identify protein disulfide isomerase (PDI), an enzyme critical to the formation of correct disulfide bonds in proteins, as a component of the peptide-loading complex. We show that PDI stabilizes a peptide-receptive site by regulating the oxidation state of the disulfide bond in the MHC peptide-binding groove, a function that is essential for selecting optimal peptides. Furthermore, we demonstrate that human cytomegalovirus US3 protein inhibits CD8(+) T cell recognition by mediating PDI degradation, verifying the functional relevance of PDI-catalyzed peptide editing in controlling intracellular pathogens. These results establish a link between thiol-based redox regulation and antigen processing.

Bovine herpesvirus 1 UL49.5 protein inhibits the transporter associated with antigen processing despite complex formation with glycoprotein M

Bovine herpesvirus 1 (BHV-1) interferes with peptide translocation by the transporter associated with antigen processing (TAP). Recently, the UL49.5 gene product of BHV-1 was identified as the protein responsible for the observed inhibition of TAP. In BHV-1-infected cells and virions, the UL49.5 protein forms a complex with glycoprotein M (gM). Hence, it was investigated whether UL49.5 can combine the interactions with gM and the TAP complex. In cell lines constitutively expressing both UL49.5 and gM, UL49.5 appears to be required for functional processing of gM. Immunofluorescence-confocal laser scanning microscopy demonstrated that both proteins are interdependent for their redistribution from the endoplasmic reticulum to the trans-Golgi network. Remarkably, expression of cloned gM results in the abrogation of the UL49.5-mediated inhibition of TAP and prevents the degradation of the transporter. However, in BHV-1-infected cells, differences in UL49.5 and gM expression kinetics were seen to create a window of opportunity at the early stages of infection, during which time the UL49.5 protein can act on TAP without gM interference. Moreover, in later periods, non-gM-associated UL49.5 can be detected in addition to the UL49.5/gM complex. Thus, it has been deduced that different functions of UL49.5, editing of gM processing and inhibition of TAP, can be combined during BHV-1 infection.

Applications of major histocompatibility complex class I molecules expressed as single chains

Generation of CD8 T-cell responses to pathogens and tumors requires optimal expression of class I major histocompatibility complex/peptide complexes, which, in turn, is dependent on host cellular processing events and subject to interference by pathogens. To create a stable structure that is more immunogenic and resistant to immune evasion pathways, we have engineered class I molecules as single-chain trimers (SCTs), with flexible linkers connecting peptide, beta2m, and heavy chain. Herein we extend our earlier studies with SCTs to the K(b) ligand derived from vesicular stomatitis virus (VSV) to characterize further SCTs as probes of immune function as well as their potential in immunotherapy. The VSVp-beta2m-K(b) SCTs were remarkably stable at the cell surface, and immunization with DNA encoding SCTs elicited complex-specific antibody. In addition, SCTs were detected by cytotoxic T-lymphocytes specific for the native molecule, and the covalently bound peptide was highly resistant to displacement by exogenous peptide. SCTs can also prime CD8 T-cells in vivo that recognize the native molecule. Furthermore, SCTs were resistant to downregulation by the immune evasion protein mK3 of gamma herpesvirus 68. Moreover, owing to their preassembled nature, SCTs should be resistant to other immune evasion proteins that restrict peptide supply. Thus, SCTs possess therapeutic potential both for prophylactic treatment and for the treatment of ongoing infection.

Cytomegalovirus interleukin-10 expression in infected cells does not impair MHC class I restricted peptide presentation on bystanding antigen-presenting cells

Human cytomegalovirus (HCMV) has evolved strategies to counteract its surveillance by the immune system. Mitigation of antiviral immune responses is considered critical for establishment of viral latency and for spread. Recently, a gene encoding an interleukin-10 homologue (cmvIL-10) has been discovered in the HCMV genome. Using recombinant cmvIL-10, several mostly immunosuppressive functions of the molecule have been described. However, the role of cmvIL-10 in the context of viral infection was not addressed. To be able to analyze this issue, we generated cmvIL- 10-negative viral mutants. Using these mutants, we tested whether the expression of cmvIL-10 by infected cells would render bystander antigen-presenting cells less efficient in their capacity to present antigenic peptides in the context of MHC class I. To test this, CTL clones specific for the viral nonapeptides P65(495-503) and IE1(297-305) were used as tools. Culture supernatant from fibroblasts infected with cmv-IL10-negative viruses was supplemented with increasing concentrations of recombinant cmvIL-10. Treatment of human THP-1 cells with these conditioned media did not impair their capacity to present HCMV-derived nonapeptides in the context of MHC-class I, even when high concentrations of cmvIL-10 were used. To investigate whether close cell contact was important, fibroblasts were infected with either wild-type HCMV or cmvIL-10 null mutants and were cocultured with nonpermissive lymphoblastoid cell lines, serving as target cells. No correlation was found between the ability of HCMV strains to express the cmvIL-10 gene and the capacity of neighboring LCL to present peptides in the context of MHC class I. Consequently, we propose that cmvIL- 10 expressed in the context of HCMV infection has no direct impact on MHC class I-restricted antigen presentation of noninfected bystander cells.

The role of BiP in endoplasmic reticulum-associated degradation of major histocompatibility complex class I heavy chain induced by cytomegalovirus proteins

Human cytomegalovirus (HCMV1) US11 and US2 proteins cause rapid degradation of major histocompatibility complex (MHC) molecules, apparently by ligating cellular endoplasmic reticulum (ER)-associated degradation machinery. Here, we show that US11 and US2 bind the ER chaperone BiP. Four related HCMV proteins, US3, US7, US9, and US10, which do not promote degradation of MHC proteins, did not bind BiP. Silencing BiP reduced US11- and US2-mediated degradation of MHC class I heavy chain (HC) without altering the synthesis or translocation of HC into the ER or the stability of HC in the absence of US11 or US2. Induction of the unfolded protein response (UPR) did not affect US11-mediated HC degradation and could not explain the stabilization of HC when BiP was silenced. Unlike in yeast, BiP did not act by maintaining substrates in a retrotranslocation-competent form. Our studies go beyond previous observations in mammalian cells correlating BiP release with degradation, demonstrating that BiP is functionally required for US2- and US11-mediated HC degradation. Further, US2 and US11 bound BiP even when HC was absent and degradation of US2 depended on HC. These data were consistent with a model in which US2 and US11 bridge HC onto BiP promoting interactions with other ER-associated degradation proteins.

A structural determinant of human cytomegalovirus US2 dictates the down-regulation of class I major histocompatibility molecules

Human cytomegalovirus down-regulates cell surface class I major histocompatibility (MHC) molecules, thus allowing the virus to proliferate while avoiding detection by CD8+ T lymphocytes. The unique short gene product US2 is a 199-amino acid type I endoplasmic reticulum glycoprotein that modulates surface expression of class I MHC products by targeting class I heavy chains for dislocation from the endoplasmic reticulum to the cytosol, where they undergo proteasomal degradation. Although the mechanism by which this viral protein targets class I heavy chains for destruction remains unclear, the putative US2 cytoplasmic tail comprised of only 14 residues is known to play a functional role. To determine the specific residues critical for mediating class I degradation, a mutagenesis analysis of the cytoplasmic tail of US2 was performed. Using truncation mutants, the removal of only 4 residues (mutant US2(195)) from the US2 carboxyl terminus completely abolishes class I destruction. Furthermore, site-directed mutagenesis of the US2 cytoplasmic tail revealed that the most critical residues for class I-induced destruction, cysteine 187, serine 190, tryptophan 193, and phenylalanine 196, occurs every third residue. This experimental data supports a model that the US2 cytoplasmic tail is in a 3(10) helical configuration. Such a secondary structure would predict that one side of the 3(10) helical cytoplasmic tail would interact with the extraction apparatus to facilitate the dislocation and subsequent destruction of class I heavy chains.

Radiation modulates the peptide repertoire, enhances MHC class I expression, and induces successful antitumor immunotherapy

Radiotherapy is one of the most successful cancer therapies. Here the effect of irradiation on antigen presentation by MHC class I molecules was studied. Cell surface expression of MHC class I molecules was increased for many days in a radiation dose-dependent manner as a consequence of three responses. Initially, enhanced degradation of existing proteins occurred which resulted in an increased intracellular peptide pool. Subsequently, enhanced translation due to activation of the mammalian target of rapamycin pathway resulted in increased peptide production, antigen presentation, as well as cytotoxic T lymphocyte recognition of irradiated cells. In addition, novel proteins were made in response to gamma-irradiation, resulting in new peptides presented by MHC class I molecules, which were recognized by cytotoxic T cells. We show that immunotherapy is successful in eradicating a murine colon adenocarcinoma only when preceded by radiotherapy of the tumor tissue. Our findings indicate that directed radiotherapy can improve the efficacy of tumor immunotherapy.

Cross-reactive CTL recognizing two HLA-A*02-restricted epitopes within the BK virus and JC virus VP1 polypeptides are frequent in immunocompetent individuals

Two HLA-A*02-restricted epitopes have been identified within the VP1 polypeptide of a human polyomavirus, BK virus, which is associated with polyomavirus-associated nephropathy in kidney transplant patients. Immunization of transgenic mice with recombinant modified vaccinia Ankara expressing BKV VP1 (rMVA-BKV VP1) elicited functional CTL populations recognizing the sequences LLMWEAVTV (amino acids residues 108-116, BKV VP1p108) and AITEVECFL (residues 44-52, BKV VP1p44) and cross-reactive to the previously described JC virus VP1 homologs. Flow-based analyses of PBMC from a panel of thirty healthy HLA-A*02 human volunteers indicated that the majority of these subjects harbored functional CTL populations recognizing the BKV epitopes and cross-reactive with the JCV homologs. CTL recognizing the JCV VP1p100 and JCV VP1p36 epitopes have previously been associated with prolonged survival in progressive multifocal leukoencephalopathy patients. These findings suggest that infection with BKV or JCV could potentially induce cross-protective T-cell immunity against diseases associated with these viruses.

Human cytomegalovirus-encoded US2 and US11 target unassembled MHC class I heavy chains for degradation

Surface MHC class I molecules serve important immune functions as ligands for both T and NK cell receptors for the elimination of infected and malignant cells. In order to reach the cell surface, MHC class I molecules have to fold properly and form trimers consisting of a heavy chain (HC), a beta2-microglobulin light chain and an 8-10-mer peptide. A panel of ER chaperones facilitates the folding and assembly process. Incorrectly assembled or folded MHC class I HCs are detected by the ER quality-control system and transported to the cytosol for degradation by proteasomes. In human cytomegalovirus-infected cells, two viral proteins are synthesized, US2 and US11, which target MHC class I HCs for proteasomal degradation. It is unknown at which stage of MHC class I folding and complex formation US2 and US11 come into play. In addition, it is unclear if the disposal takes place via the same pathway through which proteins are removed that fail to pass ER quality control. In this study, we show with a beta2m-deficient cell line that US2 and US11 both target unassembled HCs for degradation. This suggests that US2 and US11 both act at an early stage of MHC class I complex formation. In addition, our data indicate that US11-mediated degradation involves mechanisms that are similar to those normally used to remove terminally misfolded HCs.

Does cytomegalovirus play a causative role in the development of various inflammatory diseases and cancer?

Human cytomegalovirus (HCMV) is a herpes virus that infects and is carried by 70-100% of the world's population. During its evolution, this virus has developed mechanisms that allow it to survive in an immunocompetent host. For many years, HCMV was not considered to be a major human pathogen, as it appeared to cause only rare cases of HCMV inclusion disease in neonates. However, HCMV is poorly adapted for survival in the immunosuppressed host and has emerged as an important human pathogen in AIDS patients and in patients undergoing immunosuppressive therapy following organ or bone marrow transplantation. HCMV-mediated disease in such patients has highlighted the possible role of this virus in the development of other diseases, in particular inflammatory diseases such as vascular diseases, autoimmune diseases and, more recently, with certain forms of cancers. Current research is focused on determining whether HCMV plays a causative role in these diseases or is merely an epiphenomenon of inflammation. Inflammation plays a central role in the pathogenesis of HCMV. This virus has developed a number of mechanisms that enable it to hide from the cells of the immune system and, at the same time, reactivation of a latent infection requires immune activation. Numerous products of the HCMV genome are devoted to control central functions of the innate and adaptive immune responses. By influencing the regulation of various cellular processes including the cell cycle, apoptosis and migration as well as tumour invasiveness and angiogenesis, HCMV may participate in disease development. Thus, the various drugs now available for treatment of HCMV disease (e.g. ganciclovir, acyclovir and foscarnet), may also prove to be useful in the treatment of other, more widespread diseases.

Physical and Functional Interactions of the Cytomegalovirus US6 Glycoprotein with the Transporter Associated with Antigen Processing

The endoplasmic reticulum-resident human cytomegalovirus glycoprotein US6 (gpUS6) inhibits peptide translocation by the transporter associated with antigen processing (TAP) to prevent loading of major histocompatibility complex class I molecules and antigen presentation to CD8+ T cells. TAP is formed by two subunits, TAP1 and TAP2, each containing one multispanning transmembrane domain (TMD) and a cytosolic nucleotide binding domain. Here we reported that the blockade of TAP by gpUS6 is species-restricted, i.e. gpUS6 inhibits human TAP but not rat TAP. Co-expression of human and rat subunits of TAP demonstrates independent binding of gpUS6 to human TAP1 and TAP2, whereas gpUS6 does not bind to rat TAP subunits. gpUS6 associates with preformed TAP1/2 heterodimers but not with unassembled TAP subunits. To locate domains of TAP required for gpUS6 binding and function, we took advantage of reciprocal human/rat intrachain TAP chimeras. Each TAP subunit forms two contact sites within its TMD interacting with gpUS6. The dominant gpUS6-binding site on TAP2 maps to an N-terminal loop, whereas inhibition of peptide transport is mediated by a C-terminal loop of the TMD. For TAP1, two gpUS6 binding domains are formed by loops of the C-terminal TMD. The domain required for TAP inactivation is built by a distal loop of the C-terminal TMD, indicating a topology of TAP1 comprising 10 endoplasmic reticulum transmembrane segments. By forming multimeric complexes, gpUS6 reaches the distant target domains to arrest peptide transport. The data revealed a nonanalogous multipolar bridging of the TAP TMDs by gpUS6.

Structural basis for a major histocompatibility complex class Ib-restricted T cell response

In contrast to antigen-specific immunity orchestrated by major histocompatibility complex (MHC) class Ia molecules, the ancestrally related nonclassical MHC class Ib molecules generally mediate innate immune responses. Here we have demonstrated the structural basis by which the MHC class Ib molecule HLA-E mediates an adaptive MHC-restricted cytotoxic T lymphocyte response to human cytomegalovirus. Highly constrained by host genetics, the response showed notable fine specificity for position 8 of the viral peptide, which is the sole discriminator of self versus nonself. Despite the evolutionary divergence of MHC class Ia and class Ib molecules, the structure of the T cell receptor-MHC class Ib complex was very similar to that of conventional T cell receptor-MHC class Ia complexes. These results emphasize the evolutionary 'ambiguity' of HLA-E, which not only interacts with innate immune receptors but also has the functional capacity to mediate virus-specific cytotoxic T lymphocyte responses during adaptive immunity.

Dissection of the interaction of the human cytomegalovirus-derived US2 protein with major histocompatibility complex class I molecules: prominent role of a single arginine residue in human leukocyte antigen-A2

Human cytomegalovirus encodes several proteins that interfere with expression of major histocompatibility complex (MHC) class I molecules on the surface of infected cells. The unique short protein 2 (US2) binds to many MHC class I allomorphs in the endoplasmic reticulum, preventing cell surface expression of the class I molecule in question. The molecular interactions underlying US2 binding to MHC class I molecules and its allele specificity have not been fully clarified. In the present study, we first compared the sequences and the structures of US2 retained versus non-retained human leukocyte antigen (HLA) class I allomorphs to identify MHC residues of potential importance for US2 binding. On the basis of this analysis, 18 individual HLA-A2 mutants were generated and the ability of full-length US2 to bind wild-type and mutated HLA-A2 complexes was assessed. We demonstrate that Arg181 plays a critical role in US2-mediated inhibition of HLA-A2 cell surface expression. The structural comparison of all known crystal structures of HLA-A2 either alone, or in complex with T cell receptor or the CD8 co-receptor, indicates that binding of US2 to HLA-A2 results in a unique, large conformational change of the side chain of Arg181. However, although the presence of Arg181 seems to be a prerequisite for US2 binding to HLA-A2, it is not sufficient for binding to all MHC class I alleles.

Human cytomegalovirus protein pp71 disrupts major histocompatibility complex class I cell surface expression

The human cytomegalovirus tegument protein pp71 is the product of the UL82 gene. Roles for pp71 in stimulating gene transcription, increasing infectivity of viral DNA, and the degradation of retinoblastoma family proteins have been described. Here we report a novel function for pp71 in limiting accumulation of cell surface major histocompatibility complex (MHC) class I complexes. MHC molecules were analyzed in glioblastoma cells exposed to a replication-defective adenovirus expressing UL82 (Adpp71) or after transient transfection of the UL82 gene. Accumulation of cell surface MHC class I levels diminished in a specific and dose-dependent manner after exposure to Adpp71 but not after exposure to an adenovirus expressing beta-galactosidase (Adbeta gal). UL82 expression did not interfere with accumulation of either MHC class I heavy-chain transcript or protein, nor did UL82 expression correlate with markers of apoptosis. Rather, UL82 expression correlated with an increased proportion of MHC class I molecules exhibiting sensitivity to endoglycosidase H treatment. Finally, we show that, in cells infected with recombinant virus strain missing all of the unique short region MHC class I evasion genes, disruption of UL82 expression by short, interfering RNAs led to increased accumulation of cell surface MHC class I complexes. These findings support a novel role for HCMV pp71 in disruption of the MHC class I antigen presentation pathway.

Antigen presentation and the ubiquitin-proteasome system in host-pathogen interactions

Relatively small genomes and high replication rates allow viruses and bacteria to accumulate mutations. This continuously presents the host immune system with new challenges. On the other side of the trenches, an increasingly well-adjusted host immune response, shaped by coevolutionary history, makes a pathogen's life a rather complicated endeavor. It is, therefore, no surprise that pathogens either escape detection or modulate the host immune response, often by redirecting normal cellular pathways to their advantage. For the purpose of this chapter, we focus mainly on the manipulation of the class I and class II major histocompatibility complex (MHC) antigen presentation pathways and the ubiquitin (Ub)-proteasome system by both viral and bacterial pathogens. First, we describe the general features of antigen presentation pathways and the Ub-proteasome system and then address how they are manipulated by pathogens. We discuss the many human cytomegalovirus (HCMV)-encoded immunomodulatory genes that interfere with antigen presentation (immunoevasins) and focus on the HCMV immunoevasins US2 and US11, which induce the degradation of class I MHC heavy chains by the proteasome by catalyzing their export from the endoplasmic reticulum (ER)-membrane into the cytosol, a process termed ER dislocation. US2- and US11-mediated subversion of ER dislocation ensures proteasomal degradation of class I MHC molecules and presumably allows HCMV to avoid recognition by cytotoxic T cells, whilst providing insight into general aspects of ER-associated degradation (ERAD) which is used by eukaryotic cells to purge their ER of defective proteins. We discuss the similarities and differences between the distinct pathways co-opted by US2 and US11 for dislocation and degradation of human class I MHC molecules and also a putatively distinct pathway utilized by the murine herpes virus (MHV)-68 mK3 immunoevasin for ER dislocation of murine class I MHC. We speculate on the implications of the three pathogen-exploited dislocation pathways to cellular ER quality control. Moreover, we discuss the ubiquitin (Ub)-proteasome system and its position at the core of antigen presentation as proteolysis and intracellular trafficking rely heavily on Ub-dependent processes. We add a few examples of manipulation of the Ub-proteasome system by pathogens in the context of the immune system and such diverse aspects of the host-pathogen relationship as virus budding, bacterial chromosome integration, and programmed cell death, to name a few. Finally, we speculate on newly found pathogen-encoded deubiquitinating enzymes (DUBs) and their putative roles in modulation of host-pathogen interactions.

Subtle sequence variation among MHC class I locus products greatly influences sensitivity to HCMV US2- and US11-mediated degradation

Human cytomegalovirus (HCMV) interferes with cellular immune responses by modulating surface expression of MHC class I molecules. Here, we focused on HCMV-encoded unique short (US) 2 and US11, which bind newly synthesized MHC class I heavy chains (HCs) and support their dislocation into the cytosol for subsequent degradation by proteasomes. Not all MHC class I locus products are equally sensitive to this down-modulation. The aim of this study was to identify which domains, and ultimately which residues, are responsible for the resistance or sensitivity of MHC class I molecules to US2- and US11-mediated down-regulation. We show that, besides endoplasmic reticulum-lumenal regions, the C-terminus of class I molecules represents an important determinant for allele specificity in US11-mediated degradation. HLA-E becomes sensitive to US11-mediated down-regulation when its cytoplasmic tail is extended. Interestingly, this only requires two additional residues, lysine and valine, at its C-terminus. For US2, the MHC class I allele specificity is largely determined by a small region at the junction of the alpha2/alpha3 domain of the HC. It is quite remarkable that minor changes, in only four residues, can completely revert the sensitivity of naturally US2-resistant HLA-E molecules. With this study we provide better insights into the features underlying the selectivity in MHC class I down-regulation by US2 and US11.

Molecular mechanisms of HLA class I antigen abnormalities following viral infection and transformation

In humans as in other animal species, CD8+ cytotoxic T lymphocytes (CTLs) play an important if not the major role in controlling virus-infected and malignant cell growth. The interactions between CD8+ T cells and target cells are mediated by human leukocyte antigen (HLA) class I antigens loaded with viral and tumor antigen-derived peptides along with costimulatory receptor/ligand stimuli. Thus, to escape from CD8+ T-cell recognition and destruction, viruses and tumor cells have developed strategies to inhibit the expression and/or function of HLA class I antigens. In contrast, cells with downregulated MHC class I surface expression can be recognized by NK cells, although NK cell-mediated lysis could be abrogated by the expression of inhibiting NK cell receptors. This review discusses the molecular mechanisms utilized by viruses to inhibit the formation, transport and/or expression of HLA class I antigen/peptide complexes on the cell surface. The knowledge about viral interference with MHC class I antigen presentation is not only crucial to understand the pathogenesis of viral diseases, but contributes also to the design of novel strategies to counteract the escape mechanisms utilized by viruses. These investigations may eventually lead to the development of effective immunotherapies to control viral infections and virus-associated malignant diseases.

Modulation of the antigen transport machinery TAP by friends and enemies

The transporter associated with antigen processing (TAP) is a key factor of the major histocompatibility complex (MHC) class I antigen presentation pathway. This ABC transporter translocates peptides derived mainly from proteasomal degradation from the cytosol into the ER lumen for loading onto MHC class I molecules. Manifold mechanisms have evolved to regulate TAP activity. During infection, TAP expression is upregulated by interferon-gamma. Furthermore, the assembly and stability of the transport complex is promoted by various auxiliary factors. However, tumors and viruses have developed sophisticated strategies to escape the immune surveillance by suppressing TAP function. The activity of TAP can be impaired on the transcriptional or translational level, by enhanced degradation or by inhibition of peptide translocation. In this review, we briefly summarize existing data concerning the regulation of the TAP complex.

Rat Cytomegalovirus Induces a Temporal Downregulation of Major Histocompatibility Complex Class I Cell Surface Expression

Herpesviruses are known to influence expression of major histocompatibility complex (MHC) class I molecules on the surface of infected cells using a variety of mechanisms. Downregulation of MHC class I expression prohibits detection and elimination of infected cells by cytotoxic T lymphocytes. To investigate the effect of rat cytomegalovirus (RCMV) infection on MHC class I expression, we infected immortalized and primary rat fibroblasts with RCMV and monitored surface expression of MHC class I molecules at various time-points postinfection. These experiments revealed a downregulation of MHC class I surface expression by RCMV, a phenomenon that has also been reported for human and murine CMV. However, in contrast to the other cytomegaloviruses, RCMV causes only a temporal downregulation of MHC class I, with a maximal decrease at 12 h postinfection. Unlike murine and human CMV, RCMV does not induce proteolytic degradation of MHC class I molecules. In RCMV-infected cells, the MHC class I molecules are stable, but their exit from the ER is delayed.

The poxviral scrapin MV-LAP requires a myxoma viral infection context to efficiently downregulate MHC-I molecules

Downregulation of MHC class I molecules is a strategy developed by some viruses to escape cellular immune responses. Myxoma virus (MV), a poxvirus causing rabbit myxomatosis, encodes MV-LAP that is known to increase MHC-I endocytosis and degradation through a C(4)HC(3) motif critical for an E3 ubiquitin ligase activity. Here, we performed a functional mapping of MV-LAP and showed that not only the C(4)HC(3) motif is necessary for a marked downregulation of MHC-I but also a conserved region in the C-terminal part of the protein. We also showed that the putative transmembrane domains are responsible for a specific subcellular localization of the protein: they retain MV-LAP in the ER in transfected cells and in the endolysosomal compartments in infected cells. We observed that a specific MV infection context is necessary for a fully efficient downregulation of MHC-I. Our data suggest that the functionality of viral LAP factors, inherited by herpes- and poxviruses from mammalian cells, is more complex than anticipated.

MHC class I alleles and their exploration of the antigen-processing machinery

At the cell surface, major histocompatibility complex (MHC) class I molecules present fragments of intracellular antigens to the immune system. This is the end result of a cascade of events initiated by multiple steps of proteolysis. Only a small part of the fragments escapes degradation by interacting with the peptide transporter associated with antigen presentation and is translocated into the endoplasmic reticulum lumen for binding to MHC class I molecules. Subsequently, these newly formed complexes can be transported to the plasma membrane for presentation. Every step in this process confers specificity and determines the ultimate result: presentation of only few fragments from a given antigen. Here, we introduce the players in the antigen processing and presentation cascade and describe their specificity and allelic variation. We highlight MHC class I alleles, which are not only different in sequence but also use different aspects of the antigen presentation pathway to their advantage: peptide acquaintance.

Viral strategies for evading antiviral cellular immune responses of the host

The host invariably responds to infecting viruses by activating its innate immune system and mounting virus-specific humoral and cellular immune responses. These responses are aimed at controlling viral replication and eliminating the infecting virus from the host. However, viruses have evolved numerous strategies to counter and evade host's antiviral responses. Providing specific examples from the published literature, we discuss in this review article various strategies that viruses have developed to evade antiviral cellular responses of the host. Unraveling these viral strategies allows a better understanding of the host-pathogen interactions and their coevolution. This knowledge is important for identifying novel molecular targets for developing antiviral reagents. Finally, it may also help devise new knowledge-based strategies for developing antiviral vaccines.

Mechanisms of MHC class I-restricted antigen processing and cross-presentation

In this review, we discuss recent data from our laboratory that address two aspects of major histocompatibility complex (MHC) class I-restricted antigen processing. First, we consider the nature of the peptide-loading complex, which is the assembly of proteins in the endoplasmic reticulum (ER) into which newly synthesized MHC class I-beta(2) microglobulin (beta(2)m) heterodimers are incorporated, and the mechanisms involved in MHC class I assembly and peptide loading that are facilitated by the peptide-loading complex. Second, we discuss mechanisms of cross-presentation, the phenomenon whereby extracellular and luminal protein antigens can be processed by antigen-presenting cells, particularly dendritic cells, and presented by MHC class I molecules to CD8(+) T cells. The focus of the discussion is mainly on the human MHC class I system.

Dendritic cells in viral pathogenesis: protective or defective?

Dendritic cells (DC) are potent antigen-presenting cells that are critical in the initiation of immune responses to control and/or eliminate viral infections. Recent studies have investigated the effects of virus infection on the biology of DC. This review summarizes these changes, focusing on both the DC parameters affected and the viral factors involved. In addition, the central role of DC biology in the pathogenesis of several viral families, including herpesviruses, paramyxoviruses and retroviruses, is explored. The field of pathogen recognition by DC is addressed, focusing on its role in protecting the host from viral infection, as well as the ability of viruses to exploit such host receptor ligation and signalling to their replicative advantage. The hypothesis is proposed that virus and host have evolved a symbiotic relationship to ensure both viral transmission and host survival.

Viral interference with MHC class I antigen presentation pathway: the battle continues

CD8+ cytotoxic T lymphocytes (CTLs) play a critical role in the defense against viral infections. In general, CD8+ CTLs recognize antigenic peptides in the context of the major histocompatibility complex (MHC) class I molecule. The MHC class I molecules are expressed on almost all the nucleated cells in the body. The trimolecular complex consisting of the class I heavy chain, beta2-microglobulin and the peptide are generated by the MHC class I antigen presentation pathway. This pathway is designed to sample the intracellular milieu and present the information to the CTLs trafficking the area. This rigorous sampling of intracellular environment enables the CTLs to quickly identify and eliminate the cells that synthesize non-self proteins as a result of a viral infection. Many viruses, including several viruses of veterinary importance, have evolved astounding strategies to interfere with the MHC class I antigen presentation pathway, as a means of evading the CTL response of the host. This review focuses on the diverse mechanisms of viral evasion of the MHC class I antigen presentation pathway with particular emphasis on viruses of veterinary importance.

Viral Interference with Antigen Presentation to CD8+T Cells: Lessons from Cytomegalovirus

Cytomegaloviruses (CMV), in common with other Herpesviruses, establish lifelong persistence in their hosts. These highly host-specific viruses each encode viral genes that interfere with antigen presentation to CD8+ T cells, although the molecular mechanisms by which this end is achieved differ for human and murine CMVs. In each case, there has been a presumption that these genes are necessary for virus persistence in the host. However, recent data in the murine model casts doubt on that presumption. Here, we review the molecular mechanisms of interference with the class I pathway, and the published data regarding functional significance, with a focus on the murine model.

Impaired transporter associated with antigen processing-dependent peptide transport during productive EBV infection

Human herpesviruses, including EBV, persist for life in infected individuals. During the lytic replicative cycle that is required for the production of infectious virus and transmission to another host, many viral Ags are expressed. Especially at this stage, immune evasion strategies are likely to be advantageous to avoid elimination of virus-producing cells. However, little is known about immune escape during productive EBV infection because no fully permissive infection model is available. In this study, we have developed a novel strategy to isolate populations of cells in an EBV lytic cycle based on the expression of a reporter gene under the control of an EBV early lytic cycle promoter. Thus, induction of the viral lytic cycle in transfected EBV(+) B lymphoma cells resulted in concomitant reporter expression, allowing us, for the first time, to isolate highly purified cell populations in lytic cycle for biochemical and functional studies. Compared with latently infected B cells, cells supporting EBV lytic cycle displayed down-regulation of surface HLA class I, class II, and CD20, whereas expression levels of other surface markers remained unaffected. Moreover, during lytic cycle peptide transport into the endoplasmic reticulum, was reduced to <30% of levels found in latent infection. Because steady-state levels of TAP proteins were unaffected, these results point toward EBV-induced interference with TAP function as a specific mechanism contributing to the reduced levels of cell surface HLA class I. Our data implicate that EBV lytic cycle genes encode functions to evade T cell recognition, thereby creating a window for the generation of viral progeny.

Viral evasion of the MHC class I antigen-processing machinery

In their adaptation to the immune system in vertebrates, viruses have been forced to evolve elaborate strategies for evading the host's immune response. To ensure life-long persistence in the host, herpes viruses, adenoviruses and retroviruses have exploited multiple cellular pathways for their purpose, including the class I antigen-processing machinery. Attractive and prominent targets for viral attacks are the proteasome complex, the transporter associated with antigen processing, and MHC class I molecules. This review briefly outlines the different mechanisms of viral interference with the antigen-presentation pathway.

Functional dissection of HCMV US11 in mediating the degradation of MHC class I molecules

The human cytomegalovirus (HCMV) gene product US11 dislocates MHC I heavy chains from the endoplasmic reticulum (ER) and targets them for proteasomal degradation in the cytosol. To identify the structural and functional domains of US11 that mediate MHC class I molecule degradation, we constructed truncated mutants and chimeric proteins, and analyzed these to determine their intracellular localization and their ability to degrade MHC class I molecules. We found that only the luminal domain of US11 was essential to confer ER localization to the protein but that the ability to degrade MHC class I molecules required both the transmembrane domain and the luminal domain of US11. By analyzing a series of point mutants of the transmembrane domain, we were also able to identify Gln(192) and Gly(196) as being crucial for the functioning of US11, suggesting that these residues may play a critical role in interacting with the components of the protein degradation machinery.

Rhesus cytomegalovirus contains functional homologues of US2, US3, US6, and US11

Human cytomegalovirus (HCMV) is a paradigm for mechanisms subverting antigen presentation by major histocompatibility complex (MHC) molecules. Due to its limited host range, HCMV cannot be studied in animals. Thus, the in vivo importance of inhibiting antigen presentation for the establishment and maintenance of infection with HCMV is unknown. Rhesus cytomegalovirus (RhCMV) is an emerging animal model that shares many of the features of HCMV infection. The recent completion of the genomic sequence of RhCMV revealed a significant degree of homology to HCMV. Strikingly, RhCMV contains several genes with low homology to the HCMV US6 gene family of inhibitors of the MHC I antigen presentation pathway. Here, we examine whether the RhCMV US6 homologues (open reading frames Rh182, -184, -185, -186, -187, and -189) interfere with the MHC I antigen-processing pathway. We demonstrate that Rh182 and Rh189 function similarly to HCMV US2 and US11, respectively, mediating the proteasomal degradation of newly synthesized MHC I. The US3 homologue, Rh184, delayed MHC I maturation. Unlike US3, MHC I molecules eventually escaped retention by Rh184, so that steady-state surface levels of MHC I remained unchanged. Rh185 acted similarly to US6 and inhibited peptide transport by TAP and, consequently, peptide loading of MHC I molecules. Thus, despite relatively low sequence conservation, US6 family-related genes in RhCMV are functionally closely related to the conserved structural features of HCMV immunomodulators. The conservation of these mechanisms implies their importance for immune evasion in vivo, a question that can now be addressed experimentally.

Lamp-2a facilitates MHC class II presentation of cytoplasmic antigens

Extracellular antigens are internalized and processed before binding MHC class II molecules within endosomal and lysosomal compartments of professional antigen presenting cells (APC) for subsequent presentation to T cells. Yet select cytoplasmic peptides derived from autoantigens also intersect and bind class II molecules via an unknown mechanism. In human B lymphoblasts, inhibition of the peptide transporter associated with antigen processing (TAP) failed to alter class II-restricted cytoplasmic epitope presentation. By contrast, decreased display of cytoplasmic epitopes via class II molecules was observed in cells with diminished expression of the lysosome-associated membrane protein-2 (Lamp-2). Overexpression of Lamp-2 isoform A (Lamp-2a), an established component of chaperone-mediated autophagy, enhanced cytoplasmic autoantigen presentation. Manipulating APC expression of heat shock cognate protein 70 (hsc70), a cofactor for Lamp-2a, also altered cytoplasmic class II peptide presentation. These results demonstrate a novel role for the lysosomal Lamp-2a-hsc70 complex in promoting immunological recognition and antigen presentation.

Varicelloviruses avoid T cell recognition by UL49.5-mediated inactivation of the transporter associated with antigen processing

Detection and elimination of virus-infected cells by cytotoxic T lymphocytes depends on recognition of virus-derived peptides presented by MHC class I molecules. A critical step in this process is the translocation of peptides from the cytoplasm into the endoplasmic reticulum by the transporter associated with antigen processing (TAP). Here, we identified the bovine herpesvirus 1-encoded UL49.5 protein as a potent inhibitor of TAP. The expression of UL49.5 results in down-regulation of MHC class I molecules at the cell surface and inhibits detection and lysis of the cells by cytotoxic T lymphocytes. UL49.5 homologs encoded by two other varicelloviruses, pseudorabies-virus and equine herpesvirus 1, also block TAP. Homologs of UL49.5 are widely present in herpesviruses, acting as interaction partners for glycoprotein M, but in several varicelloviruses UL49.5 has uniquely evolved additional functions that mediate its participation in TAP inhibition. Inactivation of TAP by UL49.5 involves two events: inhibition of peptide transport through a conformational arrest of the transporter and degradation of TAP by proteasomes. UL49.5 is degraded along with TAP via a reaction that requires the cytoplasmic tail of UL49.5. Thus, UL49.5 represents a unique immune evasion protein that inactivates TAP through a unique two-tiered process.

MHC expression in a human adult stem cell line and its down-regulation by hCMV US gene transfection

Due to their unique capacity to self-renew and for multiple differentiation, stem cells are considered promising candidates for cell replacement therapy in many devastating diseases. However, studies on immune rejection, which is a major problem facing successful stem cell therapy, are rare. In this study, we examined MHC expression in the M13SV1 cell line, which has previously been shown to have stem cell properties and to be non-tumorigenic, in order to determine whether human adult stem cells might be rejected after transplantation. Our results show low expression levels of MHC class I molecules on the surface of these cells. An induction of MHC class I expression was observed when the cells were treated with IFN-gamma. Maximal induction of MHC class protein expression was observed at 48 h after treatment with concentrations above 5 ng/ml of IFN-gamma. Elevated MHC class I levels were sustained for 72 h after withdrawing IFN-gamma. Therefore, we introduced human cytomegalovirus (hCMV) US genes, which are known to be able to reduce MHC class I expression on the cell surface after infection, into M13SV1 cells. Cells transfected with the hCMV US2, US3, US6 or US11 genes exhibited a reduction (40-60%) of MHC class I expression compared with mock-transfected cells. These results suggest that human adult stem cells are capable of expressing high levels of MHC class I proteins, and thus may be rejected on transplantation unless they are modified. In addition, viral stealth mechanisms can be exploited for stem cell transplantation.

Viral modulation of NK cell immunity

Natural killer (NK) cells have been implicated in innate immune responses against viruses such as herpesviruses, which cause persistent infections in the host. In response to the selective pressure that is exerted by NK cells, many viruses have evolved strategies either to evade detection by NK cells or to modulate the activity of NK cells. Here, we review the unique relationship that exists between NK cells and viruses, with a focus on herpesviruses.

Viral immune evasion molecules attack the ER peptide-loading complex and exploit ER-associated degradation pathways

The CD8+ cytotoxic-T-cell response is a potent mechanism that controls intracellular pathogens, including many viruses. To facilitate transmission, viruses often counter this response by inhibiting the cell surface display of virus-derived peptides on MHC class I molecules. More specifically, recent studies have demonstrated that viruses have evolved remarkable mechanisms to inhibit MHC class I expression by interfering with the function of the MHC class I assembly machinery (the peptide-loading complex) in the endoplasmic reticulum and/or by exploiting endoplasmic-reticulum-associated degradation pathways. These viral molecules are proving invaluable as research tools to illuminate the novel features of physiological pathways that are central to normal cell biology. Furthermore, the detailed characterization of such pathways has yielded significant new insights into host-pathogen interplay.

Receptor modulation in viral replication: HIV, HSV, HHV-8 and HPV: same goal, different techniques to interfere with MHC-I antigen presentation

Evasion of host immunity is a common objective of viruses that cause chronic infections. Viruses involved in sexually transmitted infections constitute no exception to this phenomenon. HIV, HPV, HSV, and HHV-8 subvert the class I major histocompatibility complex (MHC-I) antigen presentation pathway, thereby evading the cellular immune response. Although the goal of these viruses is the same and efficient MHC-I downregulation in infected cells is achieved, their techniques vary considerably. Whether viral inhibition occurs at the transcriptional level, during assembly of MHC-I complexes in the endoplasmic reticulum, during its journey to the cell surface, or after reaching the cell surface, each one of these viruses ingeniously achieves MHC-I downregulation and avoids the cellular immune response. Unraveling the mechanisms of interference with MHC-I antigen presentation employed by these viruses is not only crucial to understand their pathogenesis, but also reveals novel mechanisms of regulation of cellular receptors. When employed as modulators of cellular trafficking pathways, viruses become tools to dissect fundamental cell processes. In return, the precise dissection of these processes may offer new weapons against the ruses viruses employ to propagate and establish chronic infections.

Viral Subversion of the Immune System

The continuous interactions between host and viruses during their co-evolution have shaped not only the immune system but also the countermeasures used by viruses. Studies in the last decade have described the diverse arrays of pathways and molecular targets that are used by viruses to elude immune detection or destruction, or both. These include targeting of pathways for major histocompatibility complex class I and class II antigen presentation, natural killer cell recognition, apoptosis, cytokine signalling, and complement activation. This paper provides an overview of the viral immune-evasion mechanisms described to date. It highlights the contribution of this field to our understanding of the immune system, and the importance of understanding this aspect of the biology of viral infection to develop efficacious and safe vaccines.

Down-regulation of MHC class I expression in human neuronal stem cells using viral stealth mechanism

Due to their unique capacity for self-renewal in addition to their ability to differentiate into cells of all neuronal lineages, neuronal stem cells (NSCs) are promising candidates for cell replacement therapy in neuronal injury and neurodegenerative diseases. However, there are few studies on immune rejection, which is one of the main problems facing successful stem cell therapy. In order to determine if human NSC might be rejected after transplantation the MHC expression level was examined in the HB1.F3 cell line, which has previously been shown to exhibit NSC properties. The results showed low expression levels of the MHC class I molecules on the surfaces of these cells. A dramatic increase in the MHC class I expression level was observed when the cells were treated with IFN-gamma, TNF-alpha, and IL-1beta, alone or in combination. The maximum induction of MHC class I protein expression was observed at above 20ng/ml IFN-gamma 48h after the treatment. The apparent additive effects of TNF-alpha and IL-1beta in combination on the maximum induction of MHC class I expression exerted by IFN-gamma treatment were not observed. The MHC class I levels elevated by IFN-gamma were sustained for 72h after withdrawing the IFN-gamma. Therefore, this study introduced human cytomegalovirus (hCMV) US genes, which are known to be able to reduce the MHC class I expression level on the cell surface after infection, into HB1.F3 cells. The cells transfected with the hCMV US2, US3, US6 or US11 genes showed 20-50% reduction in the MHC class I expression level compared with the mock-transfected cells. These results suggest that NSC expresses high levels of the MHC class I proteins, and unless they are modified, might be rejected upon transplantation. In addition, the various viral stealth mechanisms can be exploited for stem cell transplantation.

Access of soluble antigens to the endoplasmic reticulum can explain cross-presentation by dendritic cells

In dendritic cells (DCs), peptides derived from internalized particulate substrates are efficiently cross-presented by major histocompatibility complex (MHC) class I molecules. Exogenous soluble antigens are also presented by DCs but with substantially lower efficiency. Here we show that particulate and soluble antigens use different transport pathways. Particulate antigens have been shown to access peripheral endoplasmic reticulum (ER)-like phagosomes that are competent for cross-presentation, whereas we show here that soluble proteins that escape proteolysis enter the lumen of the ER. From there, they may be translocated into the cytosol by the pathway established for ER-associated degradation and their derived peptides may be transported back into the ER for binding by MHC class I molecules. MHC class I presentation involving the constitutive retrograde transport of soluble proteins to the ER by DCs may facilitate DC tolerance to components of their extracellular environment.

Virus subversion of protective immunity

The major histocompatibility (MHC) class I antigen presentation pathway plays a pivotal role in immunity to viruses. MHC class I molecules are expressed on the cell surface of all nucleated cells and present peptides derived from intracellular proteins to cytotoxic T lymphocytes (CTLs), which then eliminate virally infected cells. However, many viruses have evolved proteins to inhibit the MHC class I pathway, thus enabling virally infected cells to escape CTL lysis. In this review, we summarize recent findings about viral inhibition of the MHC class I pathway.

Effects of Chlamydia trachomatis infection on the expression of natural killer (NK) cell ligands and susceptibility to NK cell lysis

Natural killer (NK) cells are an important component of the immediate immune response to infections, including infection by intracellular bacteria. We have investigated recognition of Chlamydia trachomatis (CT) by NK cells and show that these cells are activated to produce interferon (IFN)-gamma when peripheral blood mononuclear cells (PBMC) are stimulated with CT organisms. Furthermore, infection of epithelial cell lines with CT renders them susceptible to lysis by human NK cells. Susceptibility was observed 18-24 h following infection and required protein synthesis by the infecting chlamydiae, but not by the host cell; heat or UV inactivated chlamydiae did not induce susceptibility to NK cell lysis. CT infection was also shown to decrease the expression of classical and non-classical major histocompatibility complex (MHC) molecules on infected cells, thus allowing recognition by NK cells when combined with an activating signal. A candidate activating signal is MICA/B, which was shown to be expressed constitutively on epithelial cells.

TAP genes and immunity

The transporter associated with antigen processing (TAP) is a member of the ATP-binding cassette transporter family that specializes in delivering cytosolic peptides to class I molecules in the endoplasmic reticulum. The TAP is a major target of genetic alteration in tumours and disruption by viral inhibitors. In some species, TAP genes have co-evolved with MHC class I molecules to deliver peptides that are customised for particular alleles. In humans, MHC class I polymorphism determines the level of tapasin-mediated association with TAP and subsequent peptide optimisation within the peptide-loading complex (PLC). MHC class I molecules that still load peptides without complexing to the TAP might be more resistant to viral interference of the PLC and less sensitive to competition for TAP by other class I allotypes.