Specific targeting of the EBV lytic phase protein BNLF2a to the transporter associated with antigen processing results in impairment of HLA class I-restricted antigen presentation

Epstein-Barr virus BGLF4 kinase downregulates NF-κB transactivation through phosphorylation of coactivator UXT

Epstein-Barr virus (EBV) BGLF4 is a member of the conserved herpesvirus kinases that regulate multiple cellular and viral substrates and play an important role in the viral lytic cycles. BGLF4 has been found to phosphorylate several cellular and viral transcription factors, modulate their activities, and regulate downstream events. In this study, we identify an NF-κB coactivator, UXT, as a substrate of BGLF4. BGLF4 downregulates not only NF-κB transactivation in reporter assays in response to tumor necrosis factor alpha (TNF-α) and poly(I·C) stimulation, but also NF-κB-regulated cellular gene expression. Furthermore, BGLF4 attenuates NF-κB-mediated repression of the EBV lytic transactivators, Zta and Rta. In EBV-positive NA cells, knockdown of BGLF4 during lytic progression elevates NF-κB activity and downregulates the activity of the EBV oriLyt BHLF1 promoter, which is the first promoter activated upon lytic switch. We show that BGLF4 phosphorylates UXT at the Thr3 residue. This modification interferes with the interaction between UXT and NF-κB. The data also indicate that BGLF4 reduces the interaction between UXT and NF-κB and attenuates NF-κB enhanceosome activity. Upon infection with short hairpin RNA (shRNA) lentivirus to knock down UXT, a spontaneous lytic cycle was observed in NA cells, suggesting UXT is required for maintenance of EBV latency. Overexpression of wild-type, but not phosphorylation-deficient, UXT enhances the expression of lytic proteins both in control and UXT knockdown cells. Taking the data together, transcription involving UXT may also be important for EBV lytic protein expression, whereas BGLF4-mediated phosphorylation of UXT at Thr3 plays a critical role in promoting the lytic cycle.

ABC transporters and immunity: mechanism of self-defense

The transporter associated with antigen processing (TAP) is a prototype of an asymmetric ATP-binding cassette (ABC) transporter, which uses ATP binding and hydrolysis to translocate peptides from the cytosol to the lumen of the endoplasmic reticulum (ER). Here, we review molecular details of peptide binding and ATP binding and hydrolysis as well as the resulting allosteric cross-talk between the nucleotide-binding domains and the transmembrane domains that drive translocation of the solute across the ER membrane. We also discuss the general molecular architecture of ABC transporters and demonstrate the importance of structural and functional studies for a better understanding of the role of the noncanonical site of asymmetric ABC transporters. Several aspects of peptide binding and specificity illustrate details of peptide translocation by TAP. Furthermore, this ABC transporter forms the central part of the major histocompatibility complex class I (MHC I) peptide-loading machinery. Hence, TAP is confronted with a number of viral factors, which prevent antigen translocation and MHC I loading in virally infected cells. We review how these viral factors have been used as molecular tools to decipher mechanistic aspects of solute translocation and discuss how they can help in the structural analysis of TAP.

The "Bridge" in the Epstein-Barr virus alkaline exonuclease protein BGLF5 contributes to shutoff activity during productive infection

Replication of the human herpesvirus Epstein-Barr virus drastically impairs cellular protein synthesis. This shutoff phenotype results from mRNA degradation upon expression of the early lytic-phase protein BGLF5. Interestingly, BGLF5 is the viral DNase, or alkaline exonuclease, homologues of which are present throughout the herpesvirus family. During productive infection, this DNase is essential for processing and packaging of the viral genome. In contrast to this widely conserved DNase activity, shutoff is only mediated by the alkaline exonucleases of the subfamily of gammaherpesviruses. Here, we show that BGLF5 can degrade mRNAs of both cellular and viral origin, irrespective of polyadenylation. Furthermore, shutoff by BGLF5 induces nuclear relocalization of the cytosolic poly(A) binding protein. Guided by the recently resolved BGLF5 structure, mutants were generated and analyzed for functional consequences on DNase and shutoff activities. On the one hand, a point mutation destroying DNase activity also blocks RNase function, implying that both activities share a catalytic site. On the other hand, other mutations are more selective, having a more pronounced effect on either DNA degradation or shutoff. The latter results are indicative of an oligonucleotide-binding site that is partially shared by DNA and RNA. For this, the flexible "bridge" that crosses the active-site canyon of BGLF5 appears to contribute to the interaction with RNA substrates. These findings extend our understanding of the molecular basis for the shutoff function of BGLF5 that is conserved in gammaherpesviruses but not in alpha- and betaherpesviruses.

The EBV immunoevasins vIL-10 and BNLF2a protect newly infected B cells from immune recognition and elimination

Lifelong persistence of Epstein-Barr virus (EBV) in infected hosts is mainly owed to the virus' pronounced abilities to evade immune responses of its human host. Active immune evasion mechanisms reduce the immunogenicity of infected cells and are known to be of major importance during lytic infection. The EBV genes BCRF1 and BNLF2a encode the viral homologue of IL-10 (vIL-10) and an inhibitor of the transporter associated with antigen processing (TAP), respectively. Both are known immunoevasins in EBV's lytic phase. Here we describe that BCRF1 and BNLF2a are functionally expressed instantly upon infection of primary B cells. Using EBV mutants deficient in BCRF1 and BNLF2a, we show that both factors contribute to evading EBV-specific immune responses during the earliest phase of infection. vIL-10 impairs NK cell mediated killing of infected B cells, interferes with CD4+ T-cell activity, and modulates cytokine responses, while BNLF2a reduces antigen presentation and recognition of newly infected cells by EBV-specific CD8+ T cells. Together, both factors significantly diminish the immunogenicity of EBV-infected cells during the initial, pre-latent phase of infection and may improve the establishment of a latent EBV infection in vivo.

The TAP translocation machinery in adaptive immunity and viral escape mechanisms

The adaptive immune system plays an essential role in protecting vertebrates against a broad range of pathogens and cancer. The MHC class I-dependent pathway of antigen presentation represents a sophisticated cellular machinery to recognize and eliminate infected or malignantly transformed cells, taking advantage of the proteasomal turnover of the cell's proteome. TAP (transporter associated with antigen processing) 1/2 (ABCB2/3, where ABC is ATP-binding cassette) is the principal component in the recognition, translocation, chaperoning, editing and final loading of antigenic peptides on to MHC I complexes in the ER (endoplasmic reticulum) lumen. These different tasks are co-ordinated within a dynamic macromolecular peptide-loading complex consisting of TAP1/2 and various auxiliary factors, such as the adapter protein tapasin, the oxidoreductase ERp57, the lectin chaperone calreticulin, and the final peptide acceptor the MHC I heavy chain associated with β2-microglobulin. In this chapter, we summarize the structural organization and molecular mechanism of the antigen-translocation machinery as well as various modes of regulation by viral factors and in genetic diseases and tumour development.

Different expression levels of the TAP peptide transporter lead to recognition of different antigenic peptides by tumor-specific CTL

Decreased antigenicity of cancer cells is a major problem in tumor immunology. This is often acquired by an expression defect in the TAP. However, it has been reported that certain murine Ags appear on the target cell surface upon impairment of TAP expression. In this study, we identified a human CTL epitope belonging to this Ag category. This epitope is derived from preprocalcitonin (ppCT) signal peptide and is generated within the endoplasmic reticulum by signal peptidase and signal peptide peptidase. Lung cancer cells bearing this antigenic peptide displayed low levels of TAP, but restoration of their expression by IFN-γ treatment or TAP1 and TAP2 gene transfer abrogated ppCT Ag presentation. In contrast, TAP upregulation in the same tumor cells increased their recognition by proteasome/TAP-dependent peptide-specific CTLs. Thus, to our knowledge, ppCT(16-25) is the first human tumor epitope whose surface expression requires loss or downregulation of TAP. Lung tumors frequently display low levels of TAP molecules and might thus be ignored by the immune system. Our results suggest that emerging signal peptidase-generated peptides represent alternative T cell targets, which permit CTLs to destroy TAP-impaired tumors and thus overcome tumor escape from CD8(+) T cell immunity.

Epstein-Barr virus isolates retain their capacity to evade T cell immunity through BNLF2a despite extensive sequence variation

The Epstein-Barr virus (EBV)-encoded immune evasion protein BNLF2a inhibits the transporter associated with antigen processing (TAP), thereby downregulating HLA class I expression at the cell surface. As a consequence, recognition of EBV-infected cells by cytotoxic T cells is impaired. Here, we show that sequence polymorphism of the BNLF2a protein is observed with natural EBV isolates, with evidence for positive selection. Despite these mutations, the BNLF2a variants efficiently reduce cell surface HLA class I levels. This conservation of BNLF2a function during evolution of EBV implies an important role for the viral TAP inhibitor in preventing T cell recognition during viral infection.

Epstein-Barr viral BNLF2a protein hijacks the tail-anchored protein insertion machinery to block antigen processing by the transport complex TAP

Virus-infected cells are eliminated by cytotoxic T lymphocytes, which recognize viral epitopes displayed on major histocompatibility complex class I molecules at the cell surface. Herpesviruses have evolved sophisticated strategies to escape this immune surveillance. During the lytic phase of EBV infection, the viral factor BNLF2a interferes with antigen processing by preventing peptide loading of major histocompatibility complex class I molecules. Here we reveal details of the inhibition mechanism of this EBV protein. We demonstrate that BNLF2a acts as a tail-anchored protein, exploiting the mammalian Asna-1/WRB (Get3/Get1) machinery for posttranslational insertion into the endoplasmic reticulum membrane, where it subsequently blocks antigen translocation by the transporter associated with antigen processing (TAP). BNLF2a binds directly to the core TAP complex arresting the ATP-binding cassette transporter in a transport-incompetent conformation. The inhibition mechanism of EBV BNLF2a is distinct and mutually exclusive of other viral TAP inhibitors.

Persistent infection drives the development of CD8+ T cells specific for late lytic infection antigens in lymphocryptovirus-infected macaques and Epstein-Barr virus-infected humans

We examined the CD8(+) T cell repertoire against lytic infection antigens in rhesus macaques persistently infected with the Epstein-Barr virus (EBV)-related lymphocryptovirus (rhLCV). CD8(+) T cells specific for late (L) antigens were detected at rates comparable to those for early antigens and were associated with increasing duration of infection. L antigen-specific CD8(+) T cells were also readily detected in adult, EBV-positive humans. Thus, viral major histocompatibility complex class I (MHCI) immune evasion genes expressed during lytic LCV infection do not prevent L-specific CD8(+) T cell development over time during persistent infection.

Structural and functional analysis of the TAP-inhibiting UL49.5 proteins of varicelloviruses

Viral infections are counteracted by virus-specific cytotoxic T cells that recognize the infected cell via MHC class I (MHC I) molecules presenting virus-derived peptides. The loading of the peptides onto MHC I molecules occurs in the endoplasmic reticulum (ER) and is facilitated by the peptide loading complex. A key player in this complex is the transporter associated with antigen processing (TAP), which translocates the viral peptides from the cytosol into the ER. Herpesviruses have developed many strategies to evade cytotoxic T cells. Several members of the genus Varicellovirus encode a UL49.5 protein that prevents peptide transport through TAP. These include bovine herpesvirus (BoHV) 1, BoHV-5, bubaline herpesvirus 1, cervid herpesvirus 1, pseudorabies virus, felid herpesvirus 1, and equine herpesvirus 1 and 4. BoHV-1 UL49.5 inhibits TAP by preventing conformational changes essential for peptide transport and by inducing degradation of the TAP complex. UL49.5 consists of an ER luminal N-terminal domain, a transmembrane domain and a cytosolic C-terminal tail domain. In this study, the following features of UL49.5 were deciphered: (1) chimeric constructs of BoHV-1 and VZV UL49.5 attribute the lack of TAP inhibition by VZV UL49.5 to its ER-luminal domain, (2) the ER-luminal and TM domains of UL49.5 are required for efficient interaction with and inhibition of TAP, (3) the C-terminal RXRX sequence is essential for TAP degradation by BoHV-1 UL49.5, and (4) in addition to the RXRX sequence, the cytoplasmic tail of BoHV-1 UL49.5 carries a motif that is required for efficient TAP inhibition by the protein. A model is presented depicting how the different domains of UL49.5 may block the translocation of peptides by TAP and target TAP for proteasomal degradation.

Epstein-Barr virus Zta-induced immunomodulators from nasopharyngeal carcinoma cells upregulate interleukin-10 production from monocytes

During lytic infection with Epstein-Barr virus (EBV), several viral lytic proteins function to evade immune recognition or to actively suppress immune cells. An EBV lytic transactivator, Zta, induces an immunosuppressive cytokine interleukin 10 (IL-10) in B cells, but whether it regulates IL-10 in the context of epithelial cells is unclear. In this study, we tested nasopharyngeal carcinoma (NPC) cell lines and found that Zta did not induce IL-10 in these epithelial cells. Interestingly, the conditioned medium of Zta-expressing NPC cells enhanced IL-10 production from monocytes. We further revealed that the IL-10-inducing effect involved at least two immunomodulators that were upregulated by Zta and secreted from NPC cells: granulocyte-macrophage colony-stimulating factor (GM-CSF) and prostaglandin E(2) (PGE(2)). Zta was recruited to and activated the GM-CSF promoter, thus upregulating GM-CSF expression. Zta also activated the promoter of cyclooxygenase-2 (COX-2), and Zta-induced COX-2 increased downstream PGE(2) production. Cotreatment with GM-CSF and PGE(2) synergistically induced IL-10 production from monocytes. The IL-10-inducing effect of the Zta-conditioned medium was reduced when GM-CSF or the COX-2/PGE(2) pathway was blocked. The conditioned medium of NPC cells with EBV lytic infection showed a similar increase of GM-CSF and PGE(2) levels as well as the IL-10-inducing effect on monocytes, and knockdown of Zta abolished all the effects. Therefore, through Zta-induced immunomodulators, EBV lytic infection in NPC cells can direct bystander monocytes to produce IL-10, which may be a novel way of EBV to promote local immunosuppression.

Association of HLA polymorphisms with post-transplant lymphoproliferative disorder in solid-organ transplant recipients

The association between HLA polymorphisms and PTLD was investigated in a case-control study, comparing 110 predominantly adult solid-organ transplant recipients who developed PTLD to 5601 who did not. Donor and recipient HLA were analyzed. We detected a significant association between recipient HLA-A26 and the development of PTLD (OR 2.74; p = 0.0007). In Caucasian recipients, both recipient and donor HLA-A26 were independently associated with development of PTLD (recipient A26 OR 2.99; p = 0.0004, donor A26 OR 2.81; p = 0.002). Analysis of HLA-A and -B haplotypes revealed that recipient HLA-A26, B38 haplotype was strongly correlated with a higher incidence of EBV-positive PTLD (OR 3.99; p = 0.001). The common ancestral haplotype HLA-A1, B8, DR3, when carried by the donor, was protective against PTLD (OR 0.41; p = 0.05). Several other HLA specificities demonstrated associations with clinical and pathological characteristics as well as survival. These findings demonstrate the importance of HLA polymorphisms in modulating the risk for PTLD, and may be useful in risk stratification and development of monitoring and prophylaxis strategies.

EBV protein BNLF2a exploits host tail-anchored protein integration machinery to inhibit TAP

EBV, the prototypic human γ(1)-herpesvirus, persists for life in infected individuals, despite the presence of vigorous antiviral immunity. CTLs play an important role in the protection against viral infections, which they detect through recognition of virus-encoded peptides presented in the context of HLA class I molecules at the cell surface. The viral peptides are generated in the cytosol and are transported into the endoplasmic reticulum (ER) by TAP. The EBV-encoded lytic-phase protein BNLF2a acts as a powerful inhibitor of TAP. Consequently, loading of antigenic peptides onto HLA class I molecules is hampered, and recognition of BNLF2a-expressing cells by cytotoxic T cells is avoided. In this study, we characterize BNLF2a as a tail-anchored (TA) protein and elucidate its mode of action. Its hydrophilic N-terminal domain is located in the cytosol, whereas its hydrophobic C-terminal domain is inserted into membranes posttranslationally. TAP has no role in membrane insertion of BNLF2a. Instead, Asna1 (also named TRC40), a cellular protein involved in posttranslational membrane insertion of TA proteins, is responsible for integration of BNLF2a into the ER membrane. Asna1 is thereby required for efficient BNLF2a-mediated HLA class I downregulation. To optimally accomplish immune evasion, BNLF2a is composed of two specialized domains: its C-terminal tail anchor ensures membrane integration and ER retention, whereas its cytosolic N terminus accomplishes inhibition of TAP function. These results illustrate how EBV exploits a cellular pathway for TA protein biogenesis to achieve immune evasion, and they highlight the exquisite adaptation of this virus to its host.

Inhibition of mouse TAP by immune evasion molecules encoded by non-murine herpesviruses

Herpesviruses escape elimination by cytotoxic T lymphocytes through specific interference with the antigen-presenting function of MHC class I (MHC I) molecules. The transporter associated with antigen processing (TAP) forms a bottleneck in the MHC I antigen presentation pathway. The fact that multiple viruses, especially herpesviruses, encode molecules blocking TAP function is a case in point. The action of these viral immuno evasins is usually potent and very specific, making these proteins valuable tools for studying the cell biology of antigen presentation, including alternative antigen processing pathways. Yet, no dedicated TAP inhibitor has been described for any of the mouse herpesviruses. To permit the use of immuno evasins derived from non-mouse herpesviruses in mouse models, we assessed the cross-species activity of four TAP inhibitors and one tapasin inhibitor in the context of three different mouse haplotypes, H-2(b), H-2(d), and H-2(k). Two of the four TAP inhibitors, the bovine herpesvirus 1-encoded UL49.5 and the human cytomegalovirus (HCMV)-encoded US6 protein, potently inhibited mouse TAP. ICP47 and BNLF2a, encoded by herpes simplexvirus 1 and Epstein-Barr virus, respectively, failed to inhibit TAP in all mouse cells tested. Previous work, however, demonstrated that US6 did not cross the mouse species barrier. We now show that substitution of the cysteine residue at position 108 was responsible for this lack of activity. The HCMV-encoded tapasin inhibitor US3 efficiently downregulated H-2(d) molecules on 3T3 cells, but not in other cell lines tested. Finally, we show that synthetic peptides comprising the functional domain of US6 can be exploited as a versatile TAP inhibitor. In conclusion, a complete overview is presented of the applicability of herpesvirus-encoded TAP and tapasin inhibitors in mouse cells of different genetic background.

Immune defence against EBV and EBV-associated disease

Epstein-Barr virus (EBV), a B-lymphotropic herpesvirus widespread in the human population and normally contained as an asymptomatic infection by T cell surveillance, nevertheless causes infectious mononucleosis and is strongly linked to several types of human cancer. Here we describe new findings on the range of cellular immune responses induced by EBV infection, on viral strategies to evade those responses and on the links between HLA gene loci and EBV-induced disease. The success of adoptive T cell therapy for EBV-driven post-transplant lymphoproliferative disease is stimulating efforts to target other EBV-associated tumours by immunotherapeutic means, and has reawakened interest in the ultimate intervention strategy, a prophylactic EBV vaccine.

Exploitation of herpesvirus immune evasion strategies to modify the immunogenicity of human mesenchymal stem cell transplants

BACKGROUND

Mesenchymal stem cells (MSCs) are multipotent cells residing in the connective tissue of many organs and holding great potential for tissue repair. In culture, human MSCs (hMSCs) are capable of extensive proliferation without showing chromosomal aberrations. Large numbers of hMSCs can thus be acquired from small samples of easily obtainable tissues like fat and bone marrow. MSCs can contribute to regeneration indirectly by secretion of cytokines or directly by differentiation into specialized cell types. The latter mechanism requires their long-term acceptance by the recipient. Although MSCs do not elicit immune responses in vitro, animal studies have revealed that allogeneic and xenogeneic MSCs are rejected.

METHODOLOGY/PRINCIPAL FINDINGS

We aim to overcome MSC immune rejection through permanent down-regulation of major histocompatibility complex (MHC) class I proteins on the surface of these MHC class II-negative cells through the use of viral immune evasion proteins. Transduction of hMSCs with a retroviral vector encoding the human cytomegalovirus US11 protein resulted in strong inhibition of MHC class I surface expression. When transplanted into immunocompetent mice, persistence of the US11-expressing and HLA-ABC-negative hMSCs at levels resembling those found in immunodeficient (i.e., NOD/SCID) mice could be attained provided that recipients' natural killer (NK) cells were depleted prior to cell transplantation.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate the potential utility of herpesviral immunoevasins to prevent rejection of xenogeneic MSCs. The observation that down-regulation of MHC class I surface expression renders hMSCs vulnerable to NK cell recognition and cytolysis implies that multiple viral immune evasion proteins are likely required to make hMSCs non-immunogenic and thereby universally transplantable.

Innate immune modulation in EBV infection

Epstein-Barr Virus (EBV) belongs to the gammaherpesvirus family, members of which are oncogenic. Compared with other closely related herpesviruses, EBV has developed much more elaborate and sophisticated strategies for subverting host immune system, which may account for its high prevalence in immune competent hosts. Thus, study of EBV-specific immune dysregulation is important for understanding EBV latency and oncogenesis, and will identify potential molecular targets for immunotherapeutic interventions. Here I summarize the recent findings of individual EBV products in regulating host immune responses, with emphasis on the innate immune modulation.

The capacity of UL49.5 proteins to inhibit TAP is widely distributed among members of the genus Varicellovirus

The lifelong infection by varicelloviruses is characterized by a fine balance between the host immune response and immune evasion strategies used by these viruses. Virus-derived peptides are presented to cytotoxic T lymphocytes by major histocompatibility complex (MHC) class I molecules. The transporter associated with antigen processing (TAP) transports the peptides from the cytosol into the endoplasmic reticulum, where the loading of MHC-I molecules occurs. The varicelloviruses bovine herpesvirus 1 (BoHV-1), pseudorabies virus, and equid herpesviruses 1 and 4 have been found to encode a UL49.5 protein that inhibits TAP-mediated peptide transport. To investigate to what extent UL49.5-mediated TAP inhibition is conserved within the family of Alphaherpesvirinae, the homologs of another five varicelloviruses, one mardivirus, and one iltovirus were studied. The UL49.5 proteins of BoHV-5, bubaline herpesvirus 1, cervid herpesvirus 1, and felid herpesvirus 1 were identified as potent TAP inhibitors. The varicella-zoster virus and simian varicellovirus UL49.5 proteins fail to block TAP; this is not due to the absence of viral cofactors that might assist in this process, since cells infected with these viruses did not show reduced TAP function either. The UL49.5 homologs of the mardivirus Marek's disease virus 1 and the iltovirus infectious laryngotracheitis virus did not block TAP, suggesting that the capacity to inhibit TAP via UL49.5 has been acquired by varicelloviruses only. A phylogenetic analysis of viruses that inhibit TAP through their UL49.5 proteins reveals an interesting hereditary pattern, pointing toward the presence of this capacity in defined clades within the genus Varicellovirus.

Viral evasion of T cell immunity: ancient mechanisms offering new applications

Upon infecting a host, viruses are confronted by a coordinated and multi-faceted immune response. Indeed, evolutionary combat between virus and host has contributed signally to the host's development of a formidable innate and adaptive immune defense arsenal, and to the virus' acquisition of effective means to evade it. Cytotoxic T lymphocytes play a key role in the elimination of virus-infected cells, which they detect through recognition of virus-derived peptides displayed at the cell surface in the context of MHC class I molecules. This highly sensitive recognition system is a prime target for immune evasion strategies deployed by many viruses, particularly large DNA viruses such as herpesviruses and poxviruses. Elucidation of the mode of action of the immune evasion proteins encoded by these viruses has not only provided new insights into viral pathogenesis, but has also led to the discovery of hitherto unknown cell biological and immunological phenomena. Moreover, viral immune evasion proteins constitute extremely useful tools to block defined stages of the MHC class I presentation pathway, not only for research purposes, but also for clinical applications.

Cloning of the Epstein-Barr virus-related rhesus lymphocryptovirus as a bacterial artificial chromosome: a loss-of-function mutation of the rhBARF1 immune evasion gene

Rhesus macaques are naturally infected with a gammaherpesvirus which is in the same lymphocryptovirus (LCV) genus as and closely related to Epstein-Barr virus (EBV). The rhesus macaque LCV (rhLCV) contains a repertoire of genes identical to that of EBV, and experimental rhLCV infection of naive rhesus macaques accurately models acute and persistent EBV infection of humans. We cloned the LCL8664 rhLCV strain as a bacterial artificial chromosome to create recombinant rhLCV for investigation in this animal model system. A recombinant rhLCV (clone 16 rhLCV) carrying a mutation in the putative immune evasion gene rhBARF1 was created along with a rescued wild-type (rWT) rhLCV in which the rhBARF1 open reading frame (ORF) was repaired. The rWT rhLCV molecular clone demonstrated viral replication and B-cell immortalization properties comparable to those of the naturally derived LCL8664 rhLCV. Qualitatively, clone 16 rhLCV carrying a mutated rhBARF1 was competent for viral replication and B-cell immortalization, but quantitative assays showed that clone 16 rhLCV immortalized B cells less efficiently than LCL8664 and rWT rhLCV. Functional studies showed that rhBARF1 could block CSF-1 cytokine signaling as well as EBV BARF1, whereas the truncated rhBARF1 from clone 16 rhLCV was a loss-of-function mutant. These recombinant rhLCV can be used in the rhesus macaque animal model system to better understand how a putative viral immune evasion gene contributes to the pathogenesis of acute and persistent EBV infection. The development of a genetic system for making recombinant rhLCV constitutes a major advance in the study of EBV pathogenesis in the rhesus macaque animal model.

CD8+ T cell responses against TAP-inhibited cells are readily detected in the human population

Target cell recognition by CTLs depends on the presentation of peptides by HLA class I molecules. Tumors and herpes viruses have adopted strategies to greatly hamper this peptide presentation at the important bottleneck, the peptide transporter TAP. Previously, we described the existence of a CD8(+) CTL subpopulation that selectively recognizes such TAP-deficient cells in mouse models. In this study, we show that the human counterpart of this CTL subset is readily detectable in healthy subjects. Autologous PBMC cultures were initiated with dendritic cells rendered TAP-impaired by gene transfer of the viral evasion molecule UL49.5. Strikingly, specific reactivity to B-LCLs expressing one of the other viral TAP-inhibitors (US6, ICP47, or BNLF2a) was already observed after three rounds of stimulation. These short-term T cell cultures and isolated CD8(+) CTL clones derived thereof did not recognize the normal B-LCL, indicating that the cognate peptide-epitopes emerge at the cell surface upon an inhibition in the MHC class I processing pathway. A diverse set of TCRs was used by the clones, and the cellular reactivity was TCR-dependent and HLA class I-restricted, implying the involvement of a broad antigenic peptide repertoire. Our data indicate that the human CD8(+) T cell pool comprises a diverse reactivity to target cells with impairments in the intracellular processing pathway, and these might be exploited for cancers that are associated with such defects and for infections with immune-evading herpes viruses.

ABC proteins in antigen translocation and viral inhibition

How ABC transporters work is a key issue because of their important roles in multidrug resistance of pathogenic bacteria, reduced efficacy of antitumor drugs, cholesterol metabolism, cell homeostasis and immune response. In the past few years, significant progress has been made in crystallization and structure determination of (mostly) bacterial ABC transporters, as well as in functional studies on ABC systems involved in human pathology. In this review, we use the transporter associated with antigen processing (TAP) to illustrate what is known regarding the mechanism of substrate transport. We also discuss the chemical basis of substrate recognition by TAP and the allosteric cross-talk between the binding of substrate, the release of chemical energy by ATP hydrolysis and cross-membrane translocation. Finally, we detail the role of TAP in a large macromolecular assembly, which optimally loads MHC class I molecules, and the interference with this machinery by TAP-targeted viral factors. Because of structural and probable mechanistic similarities, the understanding of the detailed structure and mechanism of TAP will be applicable to all ABC systems, including those of medical relevance.

ABC proteins in antigen translocation and viral inhibition

How ABC transporters work is a key issue because of their important roles in multidrug resistance of pathogenic bacteria, reduced efficacy of antitumor drugs, cholesterol metabolism, cell homeostasis and immune response. In the past few years, significant progress has been made in crystallization and structure determination of (mostly) bacterial ABC transporters, as well as in functional studies on ABC systems involved in human pathology. In this review, we use the transporter associated with antigen processing (TAP) to illustrate what is known regarding the mechanism of substrate transport. We also discuss the chemical basis of substrate recognition by TAP and the allosteric cross-talk between the binding of substrate, the release of chemical energy by ATP hydrolysis and cross-membrane translocation. Finally, we detail the role of TAP in a large macromolecular assembly, which optimally loads MHC class I molecules, and the interference with this machinery by TAP-targeted viral factors. Because of structural and probable mechanistic similarities, the understanding of the detailed structure and mechanism of TAP will be applicable to all ABC systems, including those of medical relevance.

Cowpox virus inhibits the transporter associated with antigen processing to evade T cell recognition

Cowpox virus encodes an extensive array of putative immunomodulatory proteins, likely contributing to its wide host range, which includes zoonotic infections in humans. Unlike Vaccinia virus, cowpox virus prevents stimulation of CD8(+) T cells, a block that correlated with retention of MHC class I in the endoplasmic reticulum by the cowpox virus protein CPXV203. However, deletion of CPXV203 did not restore MHC class I transport or T cell stimulation. Here, we demonstrate the contribution of an additional viral protein, CPXV12, which interferes with MHC class I/peptide complex formation by inhibiting peptide translocation by the transporter associated with antigen processing (TAP). Importantly, human and mouse MHC class I transport and T cell stimulation was restored upon deletion of both CPXV12 and CPXV203, suggesting that these unrelated proteins independently mediate T cell evasion in multiple hosts. CPXV12 is a truncated version of a putative NK cell ligand, indicating that poxviral gene fragments can encode new, unexpected functions.

Immune responses to Epstein-Barr virus: molecular interactions in the virus evasion of CD8+ T cell immunity

Persistent viruses have mechanisms for modulating the host immune responses that are essential for achieving a lifelong virus–host balance while minimizing the viral pathogenicity. Here we review some of the immune-modulating mechanisms evolved by the ubiquitous but potentially oncogenic Epstein–Barr virus, with particular emphasis on the molecular mechanisms of genes interfering with HLA class I antigen presentation.

TAP-inhibitors from old world primate 1-herpesviruses and their use: WO2009008713

Cytotoxic T lymphocytes (CTLs) recognize peptide antigens in the context of major histocompatibility complex (MHC) class I molecules. CTL-mediated immunity is important for defense against cancers and virus infections, and thus viruses and tumors have evolved CTL-evasion mechanisms. The transporter associated with antigen processing (TAP) is a key factor for MHC class I assembly, and TAP is a frequent target of immune evasion by viruses and tumors. WO2009008713 discloses potential therapeutic uses of TAP inhibitors encoded by Epstein-Barr virus and its lymphocryptovirus (LCV) homologs. In particular, WO2009008713 proposes the use of LCV TAP inhibitors and their combinations with other viral TAP inhibitors to elicit novel categories of CTLs that destroy target cells in which the function of TAP or other components of the MHC class I assembly pathway is inhibited. Alternatively, WO2009008713 proposes the use of LCV TAP inhibitors and their combinations with other viral TAP inhibitors to more efficiently generate tumor-specific T-cell epitopes for immunotherapy. The methods described in WO2009008713 offer the promise of new strategies of cancer immunotherapy. However, significant optimization of therapy conditions and characterizations of eligible tumor types will be needed to further develop therapeutic use of TAP inhibitors.

MHC class I antigen presentation: learning from viral evasion strategies

The cell surface display of peptides by MHC class I molecules to lymphocytes provides the host with an important surveillance mechanism to protect against invading pathogens. However, in turn, viruses have evolved elegant strategies to inhibit various stages of the MHC class I antigen presentation pathway and prevent the display of viral peptides. This Review highlights how the elucidation of mechanisms of viral immune evasion is important for advancing our understanding of virus-host interactions and can further our knowledge of the MHC class I presentation pathway as well as other cellular pathways.

Peptide trafficking and translocation across membranes in cellular signaling and self-defense strategies

Cells are metastable per se and a fine-tuned balance of de novo protein synthesis and degradation shapes their proteome. The primary function of peptides is to supply amino acids for de novo protein synthesis or as an energy source during starvation. Peptides are intrinsically short-lived and steadily trimmed by an armada of intra and extracellular peptidases. However, peptides acquired additional, more sophisticated tasks already early in evolution. Here, we summarize current knowledge on intracellular peptide trafficking and translocation mediated by ATP-binding cassette (ABC) transport machineries with a focus on the functions of protein degradation products as important signaling molecules in self-defense mechanisms.