Human keratinocyte induction of rapid effector function in antigen-specific memory CD4+ and CD8+ T cells

The ability of human keratinocytes to present antigen to T cells is controversial and, indeed, it has been suggested that keratinocytes may promote T cell hyporesponsiveness. Furthermore, it is unclear whether keratinocytes can process antigen prior to MHC class I and class II presentation. We tested the ability of keratinocytes to induce functional responses in epitope-specific CD4+ and CD8+ memory T cells using peptides, protein and recombinant expression vectors as sources of antigen. Keratinocytes were able to efficiently process and present protein antigen to CD4+ T cells, resulting in cytokine secretion (Th1 and Th2). This interaction was dependent on keratinocyte expression of HLA class II and ICAM-1, which could be induced by IFN-gamma. In addition, keratinocytes could present virally encoded or exogenous peptide to CD8+ T cells, resulting in T cell cytokine production and target cell lysis. Finally, T cell lines grown using keratinocytes as stimulators showed no loss of function. These findings demonstrate that keratinocytes are able to efficiently process and present antigen to CD4+ and CD8+ memory T cells and induce functional responses. The findings have broad implications for the pathogenesis of cutaneous disease and for transcutaneous drug or vaccine delivery.

Early activation markers of human peripheral dendritic cells

Two major populations of dendritic cells (DCs), myeloid and plasmacytoid, can be isolated from human peripheral blood, and are distinguished by differential expression of the cell surface markers CD11c and CD123. These two populations of DCs also are different in their expression of Toll-like receptor (TLRs), which are involved in their activation. To investigate the early events during activation of peripheral DCs, the cells were stimulated in vitro with ligands for TLR-4 (as in lipopolysaccharides [LPS]) or TLR-9 (CpG-containing oligonucleotide [CpG]). The earliest change in protein expression detected after stimulating peripheral DCs with lipopolysaccharide (LPS) or CpG was increased production of the chemokine interleukin (IL)-8. Enhanced production of IL-8 occurred already within 2 hours of stimulation in both myeloid dendritic cells (M-DCs) and plasmacytoid dendritic cells (P-DCs), and preceded expression of the well established activation marker CD40. Although both populations of DCs secreted IL-8 upon activation, the levels of IL-8 produced was several times higher within the M-DCs compared with the P-DCs population. Before activation, both subsets of DCs expressed the IL-8 receptor type B (CD128b); but after stimulation the IL-8 receptor was down-regulated in both populations of DCs. Increased expression of MHC class II molecules is generally regarded as an early activation marker of DCs. However, only the P-DCs showed a significant up-regulation of MHC class II after stimulation. The M-DC population up-regulated MHC class II without any prior activation; thus care should be taken using increased expression of MHC class II molecules as an early activation marker of peripheral M-DCs after activation in vitro. In conclusion, we propose that during activation of human DCs the production of IL-8 and loss of CD128b are the earliest signs of activation preceding both MHC class II, CD40, CD80, and CD86 expression.

The DMalpha and DMbeta chain cooperate in the oxidation and folding of HLA-DM

HLA-DM (DM) is a heterodimeric MHC molecule that catalyzes the peptide loading of classical MHC class II molecules in the endosomal/lysosomal compartments of APCs. Although the function of DM is well-established, little is known about how DMalpha and beta-chains fold, oxidize, and form a complex in the endoplasmic reticulum (ER). In this study, we show that glycosylation promotes, but is not essential for, DMalphabeta ER exit. However, glycosylation of DMalpha N15 is required for oxidation of the alpha-chain. The DMalpha and beta-chains direct each others fate: single DMalpha chains cannot fully oxidize without DMbeta, while DMbeta forms disulfide-linked homodimers without DMalpha. Correct oxidation and subsequent ER egress depend on the unique DMbeta C25 and C35 residues. This suggests that the C25-C35 disulfide bond in the peptide-binding domain overcomes the need for stabilizing peptides required by other MHC molecules.

HLA-DM targets the hydrogen bond between the histidine at position beta81 and peptide to dissociate HLA-DR-peptide complexes

The peptide editor HLA-DM (DM) mediates exchange of peptides bound to major histocompatibility (MHC) class II molecules during antigen processing; however, the mechanism by which DM displaces peptides remains unclear. Here we generated a soluble mutant HLA-DR1 with a histidine-to-asparagine substitution at position 81 of the beta-chain (DR1betaH81N) to perturb an important hydrogen bond between MHC class II and peptide. Peptide-DR1betaH81N complexes dissociated at rates similar to the dissociation rates of DM-induced peptide-wild-type DR1, and DM did not enhance the dissociation of peptide-DR1betaH81N complexes. Reintroduction of an appropriate hydrogen bond (DR1betaH81N betaV85H) restored DM-mediated peptide dissociation. Thus, DR1betaH81N might represent a 'post-DM effect' conformation. We suggest that DM may mediate peptide dissociation by a 'hit-and-run' mechanism that results in conformational changes in MHC class II molecules and disruption of hydrogen bonds between betaHis81 and bound peptide.

Expression and upregulation of cathepsin S and other early molecules required for antigen presentation in activated hepatic stellate cells upon IFN-gamma treatment

Hepatic stellate cells (HSCs) have been shown to be able to activate T-cells and upregulate expression of surface molecules essential for this process, when treated with IFN-gamma. But little is known about the early molecules expressed by activated hepatic stellate cells under the same treatment. In this study, we investigate the effect of IFN-gamma on the transcription and expression of these early molecules in hepatic stellate cells. We show on the molecular level that activated rat hepatic stellate cells express the class II transactivator, the invariant chain (CD74), the MHC class II molecules, as well as cathepsin S, all of which are known to be responsible for the initial steps of successful antigen presentation. The mRNA and the protein expression level of these molecules is upregulated by IFN-gamma. Importantly, IFN-gamma increases cathepsin S activity, suggesting a possible involvement of this protease in CD74 processing. Our data also show that not only can the HSCs take up antigenic proteins, they can also process them. Our comparative study indicates that the rat HSC-T6 cell line displays sufficient similarity to the activated rat HSCs in order to serve as a model for in vitro studies on the molecular mechanisms of inflammatory response.

Detection of aberrant association of DM with MHC class II subunits in the absence of invariant chain

Human HLA-DM or mouse H2-DM plays a vital role for presentation of antigenic sequences by MHC class II peptide receptors. These non-classical MHC class II molecules catalyze the release of the invariant chain (Ii) fragment CLIP from the class II cleft and facilitate acquisition of antigenic peptides by MHC class II peptide receptors. H2-DM- or Ii-deficient mice display an impaired ability of their antigen-presenting cell to present peptides to CD4+ T cells and a molecular link between the immunodeficiencies of these mouse strains may exist. We show that in transfected cells the presence of HLA-DM molecules in endocytic vesicles was strongly reduced when HLA-DM was accompanied by HLA-DR. Exclusion of HLA-DM from endocytic vesicles is explained by mixed association of HLA-DM with HLA-DR subunits and retention of the aggregates in the endoplasmic reticulum. Expression of Ii, however, impairs formation of mixed HLA-DR and HLA-DM complexes and directs matched pairing of HLA-DR and HLA-DM heterodimers. In Ii gene-deficient mice, aberrant association of H2A with H2-DM polypeptides was detected. Low expression of Ii in transgenic mice inhibits interaction of H2A with H2-DM subunits and facilitates formation of H2-DM alphabeta heterodimers. A role of Ii for assembly of H2-DM heterodimers partially explains the immunodeficient phenotype of Ii-/- mice.

BCR-bound antigen is targeted to exosomes in human follicular lymphoma B-cells

BACKGROUND INFORMATION

Exosomes are small membrane vesicles secreted by several cell types during exocytic fusion of multivesicular bodies with the plasma membrane. Exosomes from tumour cells can transfer antigens from cell to cell, a property favouring antigen-specific immune responses in vitro and in vivo, and are thus an interesting putative therapeutic tool in human cancers. Exosomes have been well studied in EBV (Epstein-Barr virus)-transformed human B-cell lines; however, biological stimuli regulating exosome secretion quantitatively and/or qualitatively still remain poorly defined.

RESULTS

We analysed the effect of the BCR stimulation on exosome release in the human follicular lymphoma B-cell line DOHH2. We found that BCR (B-cell receptor) triggering of DOHH2 cells induced the polarization of CD63(+) MHC class II compartments. Moreover, BCR stimulation increased the release of exosome-associated proteins in the extracellular space. Finally, we found that the BCR was expressed at the surface of exosomes, and could target a bound anti-human IgG to these vesicles.

CONCLUSIONS

BCR can modulate the protein content of exosomes upon stimulation, and can target its bound antigen to these vesicles.

Melan-A/MART-1-Specific CD4 T Cells in Melanoma Patients: Identification of New Epitopes and Ex Vivo Visualization of Specific T Cells by MHC Class II Tetramers

Over the past decade, many efforts have been made to identify MHC class II-restricted epitopes from different tumor-associated Ags. Melan-A/MART-1(26-35) parental or Melan-A/MART-1(26-35(A27L)) analog epitopes have been widely used in melanoma immunotherapy to induce and boost CTL responses, but only one Th epitope is currently known (Melan-A51-73, DRB1*0401 restricted). In this study, we describe two novel Melan-A/MART-1-derived sequences recognized by CD4 T cells from melanoma patients. These epitopes can be mimicked by peptides Melan-A27-40 presented by HLA-DRB1*0101 and HLA-DRB1*0102 and Melan-A25-36 presented by HLA-DQB1*0602 and HLA-DRB1*0301. CD4 T cell clones specific for these epitopes recognize Melan-A/MART-1+ tumor cells and Melan-A/MART-1-transduced EBV-B cells and recognition is reduced by inhibitors of the MHC class II presentation pathway. This suggests that the epitopes are naturally processed and presented by EBV-B cells and melanoma cells. Moreover, Melan-A-specific Abs could be detected in the serum of patients with measurable CD4 T cell responses specific for Melan-A/MART-1. Interestingly, even the short Melan-A/MART-1(26-35(A27L)) peptide was recognized by CD4 T cells from HLA-DQ6+ and HLA-DR3+ melanoma patients. Using Melan-A/MART-1(25-36)/DQ6 tetramers, we could detect Ag-specific CD4 T cells directly ex vivo in circulating lymphocytes of a melanoma patient. Together, these results provide the basis for monitoring of naturally occurring and vaccine-induced Melan-A/MART-1-specific CD4 T cell responses, allowing precise and ex vivo characterization of responding T cells.

Tumor cell expression of HLA-DM associates with a Th1 profile and predicts improved survival in breast carcinoma patients

Studies aimed at elucidating the immunological and prognostic significance of HLA-DR expression on breast carcinoma cells have yielded contradictory results. To expand on previous studies, we have investigated the associations of tumor cell expression of HLA-DR and its related co-chaperones, invariant chain (Ii) and HLA-DM, with infiltrating inflammatory cells, in situ cytokine mRNA levels and prognosis and outcome in 112 breast carcinoma patients with a median follow-up of 59 months. While the majority of HLA-DR+ tumors co-express Ii, only a minority express HLA-DM. Tumor cell expression of HLA-DR and co-chaperones positively associated with both infiltrating CD4+ and CD8+ T-cell subsets (P < 0.01). Expression of HLA-DR and Ii associated with decreased estrogen receptor alpha levels and younger age at diagnosis, suggesting a role for hormones in the control of HLA class II expression in breast carcinoma. Patients with DR+Ii+DM- tumors had markedly decreased recurrence-free and disease-specific survival as compared with patients with DR+Ii+DM+ tumors (P < 0.05) and HLA-DR/co-chaperone expression was an independent predictor of survival by multivariate Cox regression analysis, controlling for standard prognostic indicators. Tumors that co-express HLA-DR, Ii and HLA-DM have increased levels of IFN-gamma, IL-2 and IL-12 mRNA, suggesting improved survival of patients with DR+Ii+DM+ tumors may be attributable to Th1-dominated immunity. We conclude that expression of determinants of the immune response by tumor cells may influence breast tumor progression and patient outcome.

15-Deoxy-12,14-prostaglandin J2 inhibits interferon gamma induced MHC class II but not class I expression on ARPE cells through a PPAR gamma independent mechanism

Retinal pigment epithelial (RPE) cells constitute the external part of the blood-retinal-barrier and play a pivotal role in the regulation of retinal immunity. In the present work, we investigated the effects of 15-deoxy-12,14-prostaglandin J2 (15 PGJ2), an endogenous ligand of PPARgamma, on the IFNgamma-induced expression of MHC class II on RPE cells. Indeed, pathological expression of MHC class II molecules at the surface of RPE cells is a common feature of many blinding conditions. We demonstrated that 15 PGJ2 inhibited the IFNgamma-mediated induction of MHC class II on RPE cells without affecting the level of MHC class I and CD54 expression. The other PPARgamma agonist rosiglitazone or troglitazone had no similar effects. Moreover, the inhibitory effect of 15 PGJ2 was not abrogated by co-incubation with PPARgamma antagonists and did not involve the modulation of STAT-1, AKT or ERK1/2 phosphorylation, nor CIITA, IRF1 or IRF2 transcription. In conclusion, 15 PGJ2 inhibits strongly and specifically the IFNgamma-induced MHC class II expression on RPE cells by a PPARgamma independent mechanism. Given the differential role of MHC classes I and II in the development of autoimmune uveitis and the potential toxicity of 15 PGJ2, our data's suggest that the development of novel small molecules targeting similar PPARgamma independent pathways would be useful for the future management of uveitis.

An expectation-maximization algorithm for probabilistic reconstructions of full-length isoforms from splice graphs

Reconstructing full-length transcript isoforms from sequence fragments (such as ESTs) is a major interest and challenge for bioinformatic analysis of pre-mRNA alternative splicing. This problem has been formulated as finding traversals across the splice graph, which is a directed acyclic graph (DAG) representation of gene structure and alternative splicing. In this manuscript we introduce a probabilistic formulation of the isoform reconstruction problem, and provide an expectation-maximization (EM) algorithm for its maximum likelihood solution. Using a series of simulated data and expressed sequences from real human genes, we demonstrate that our EM algorithm can correctly handle various situations of fragmentation and coupling in the input data. Our work establishes a general probabilistic framework for splice graph-based reconstructions of full-length isoforms.

MHC class II proteins and disease: a structural perspective

MHC class II molecules on the surface of antigen-presenting cells display a range of peptides for recognition by the T-cell receptors of CD4+ T helper cells. Therefore, MHC class II molecules are central to effective adaptive immune responses, but conversely, genetic and epidemiological data have implicated these molecules in the pathogenesis of autoimmune diseases. Indeed, the strength of the associations between particular MHC class II alleles and disease render them the main genetic risk factors for autoimmune disorders such as type 1 diabetes. Here, we discuss the insights that the crystal structures of MHC class II molecules provide into the molecular mechanisms by which sequence polymorphisms might contribute to disease susceptibility.

Small molecules that enhance the catalytic efficiency of HLA-DM

HLA-DM (DM) plays a critical role in Ag presentation to CD4 T cells by catalyzing the exchange of peptides bound to MHC class II molecules. Large lateral surfaces involved in the DM:HLA-DR (DR) interaction have been defined, but the mechanism of catalysis is not understood. In this study, we describe four small molecules that accelerate DM-catalyzed peptide exchange. Mechanistic studies demonstrate that these small molecules substantially enhance the catalytic efficiency of DM, indicating that they make the transition state of the DM:DR/peptide complex energetically more favorable. These compounds fall into two functional classes: two compounds are active only in the presence of DM, and binding data for one show a direct interaction with DM. The remaining two compounds have partial activity in the absence of DM, suggesting that they may act at the interface between DM and DR/peptide. A hydrophobic ridge in the DMbeta1 domain was implicated in the catalysis of peptide exchange because the activity of three of these enhancers was substantially reduced by point mutations in this area.

Human Dendritic Cell Expression of HLA-DO Is Subset Specific and Regulated by Maturation

Expression of HLA-DO (DO) in cells that express HLA-DM (DM) results in an altered repertoire of MHC class II/peptide complexes, indicating that DO modulates DM function. Human and murine B cells and thymic epithelial cells express DO, while monocytes/macrophages do not. Monocyte-derived dendritic cells (DC) also have been found to be DO-negative, leading to the assumption that DC do not express DO. In this study, we report that, in fact, certain types of human primary DC express DO. These include Langerhans cells (LC) and some subtypes of circulating blood DC. Specifically, the majority of BDCA-3(+) DC, a small subset of uncertain function, are DO(+), while smaller proportions of CD11c(+), BDCA-1(+) (myeloid) DC, at most a minority of CD123(+)/BDCA-2(+) (plasmacytoid) DC, and no detectable CD16(+) (myeloid) DC, express DO. Immunohistochemistry of human tonsil sections demonstrates that tonsillar interdigitating DC are also DO(+). In a subset of immature LC with higher DO expression, an increased fraction of surface DR molecules carry CLIP peptides, indicating that DO functions as a DM inhibitor in these cells. LC expression of DO is down-regulated by maturation stimuli. DM levels also decrease under these conditions, but the DM:DO ratio generally increases. In the myeloid cell types tested, DO expression correlates with levels of DObeta, but not DOalpha, implying that modulation of DObeta regulates DO dimer abundance in these cells. The range of APC types shown to express DO suggests a broader role for DO in immune function than previously appreciated.

Early cellular events in multiple sclerosis. Intimations of an extrinsic myelinolytic antigen

In a previous immunohistological study of tissues from unusually early cases of MS cluster analysis revealed a progression of demyelination through five distinct stages [Gay F, Drye T, Dick G, et al. The application of multifactorial cluster analysis in the staging of plaques in early multiple sclerosis. Identification and characterization of the primary demyelinating lesion. Brain 1997;120:1461-83]. Tissues from six of the earliest cases in this series contained regions of normal appearing white and grey matter in which well developed inflammatory events, concentrated in perivascular spaces, were found to extend locally into the perivascular parenchyma to envelop ostensibly intact myelin sheaths. The beginnings of myelin sheath lysis and phagocytosis were subsequently detected within these lesions and similar foci were found in subpial and in subependymal tissues. They were characterised by a spreading HLA Class II antigen expression on microglia, and by the presence of co-locating C3 complement-IgG complexes on capillary basement membranes, on microglial cell membranes and within the cytoplasm of large bodied activated astrocytes. Parenchymal lesions contained significantly few CD4+ T cells and showed no evidence of capillary leakage of plasma proteins. Despite the presence of complexed immunoglobulin and complement, opsonization of the myelin sheath could not be demonstrated. These observations point to the presence in early MS of a diffusing, complement-fixing, myelinolytic antigen, processed mainly within the Virchow-Robin spaces and distributed in the cerebrospinal and extracellular fluid compartments of the central nervous system.

HLA Class II molecules on haplotypes associated with type 1 diabetes exhibit similar patterns of binding affinities for coxsackievirus P2C peptides

Enteroviruses such as coxsackievirus B4 (CVB4) are proposed as possible environmental triggers or accelerants of the autoimmune process that leads to type 1 diabetes mellitus. One putative mechanism to account for this association is mimicry between virus components and islet autoantigens. Particular interest has focused on the CVB4 non-structural protein P2C, which we previously showed to be a major target of the effector memory anti-CVB4 CD4 T-cell response, and which harbours a region of sequence similarity with the islet autoantigen, glutamic acid decarboxylase (GAD65). Since several distinct human leucocyte antigen (HLA) Class II molecules are associated with development of type 1 diabetes, we hypothesized that for functional mimicry to be important, any potential region(s) of mimicry in P2C should bind to each of these susceptibility molecules. In the present study therefore we examined the affinity of 20-mer overlapping P2C peptides for soluble HLA-DR4, -DR3, -DQ2 and -DQ8. We identified one discrete region of P2C with high binding affinities for all of these HLA Class II molecules. Moreover, the binding affinity of P2C peptides was significantly correlated between HLA molecules present on the same susceptibility haplotype (e.g. DR4 and DQ8, P =0.0076; DR3 and DQ2 P = 0.002). We conclude that possession of these haplotypes favours restricted presentation of viral epitopes, and speculate that this could promote the potential for mimicry between microbial proteins and islet autoantigens.

MHC class II compartment subtypes: structure and function

Reports from the past couple of years point to an emerging association of the biogenesis, composition and ultrastructural morphology of MHC class II compartments (MIICs) with their functions in antigen processing and loading. Growth factors and cytokines involved in dendritic cell maturation have been shown to regulate MIIC biogenesis, and the MHC-class-II-associated invariant chain chaperone has been reported to regulate endosomal morphology and vacuolation. Differences among ultrastructurally distinct MIICs have begun to be appreciated with regard to variation in antigen loading capacity and to polarization of MHC class II conformational variants among different compartments. Finally, the MIIC ultrastructure organizes the mechanism of MHC class II surface trafficking. Together, these findings begin to shed light on the connection between MIIC protein content, MIIC morphology and MHC class II-related antigen processing.

Achieving stability through editing and chaperoning: regulation of MHC class II peptide binding and expression

In antigen-presenting cells (APCs), loading of major histocompatibility complex class II (MHC II) molecules with peptides is regulated by invariant chain (Ii), which blocks MHC II antigen-binding sites in pre-endosomal compartments. Several molecules then act upon MHC II molecules in endosomes to facilitate peptide loading: Ii-degrading proteases, the peptide exchange factor, human leukocyte antigen-DM (HLA-DM), and its modulator, HLA-DO (DO). Here, we review our findings arguing that DM stabilizes a globally altered conformation of the antigen-binding groove by binding to a lateral surface of the MHC II molecule. Our data imply changes in the interactions between specificity pockets and peptide side chains, complementing data from others that suggest DM affects hydrogen bonds. Selective weakening of peptide/MHC interactions allows DM to alter the peptide repertoire. We also review our studies in cells that highlight the ability of several factors to modulate surface expression of MHC II molecules via post-Golgi mechanisms; these factors include MHC class II-associated Ii peptides (CLIP), DM, and microbial products that modulate MHC II traffic from endosomes to the plasma membrane. In this context, we discuss possible mechanisms by which the association of some MHC II alleles with autoimmune diseases may be linked to their low CLIP affinity.

Right place, right time, right peptide: DO keeps DM focused

Peptide loading of major histocompatibility class II molecules is catalyzed in late endosomal and lysosomal compartments of cells by the catalytic action of human leukocyte antigen (HLA)-DM (H-2M in mice). In B cells, dendritic cells and thymic epithelial cells, the peptide loading of class II molecules is modified by the expression of the non-classical class II molecule, HLA-DO (H-2O in mice). Collectively, studies to date support that DO/H-2O expression inhibits the presentation of antigens acquired by cells via fluid phase endocytosis. However, in B cells, the expression of H-2O promotes the presentation of antigens internalized by the B-cell receptor. In this review, we summarize the literature pertaining to DO assembly, transport, and function, with an emphasis on the function of DO/H-2O.

Allogeneic versus xenogeneic immune reaction to bioengineered skin grafts

There are conflicting reports on the survival and immune reaction to allografts and xenografts of cultured skin substitutes (CSS). In this study, we investigated the allogeneic and xenogeneic responses to CSS of human keratinocytes and genetically engineered CSS expressing keratinocyte growth factor (KGF) that forms a hyperproliferative epidermis. CSS (control and KGF modified) and neonatal human foreskins were evaluated by immunohistochemistry for the expression of MHC class I and II. To study allograft rejection, grafts were transplanted to human peripheral blood mononuclear cell (huPBMC)-reconstituted SCID mice. To study xenograft rejection, grafts were transplanted to immunocompetent mice. Graft survival and immune reaction were assessed visually and microscopically. After transplantation, control CSS formed a normal differentiated epidermis, whereas KGF CSS formed a hyperproliferative epidermis. Control and KGF CSS expressed class I similar to neonatal foreskin, but did not express class II. In the allograft model, rejection of neonatal foreskins was between 5 and 9 days. In contrast, neither control nor KGF CSS was rejected by huPBMC-SCID mice. Histology showed dense mononuclear cell infiltration in human foreskins, with few, if any, mononuclear cells in control or KGF CSS. In contrast to the allogeneic reaction, CSS (control and KGF) were rejected in the xenograft model, but rejection was delayed (9-21 days) compared with neonatal skin (5-8 days). Humanized SCID mice rejected allografts of human neonatal foreskins, but did not reject control CSS or KGF CSS, even though the KGF CSS formed a hyperproliferative epidermis. Rejection of control and KGF CSS by immunocompetent mice in a xenograft model was comparable and their survival was significantly prolonged compared with neonatal skin. These results demonstrate that control CSS and hyperproliferative KGF CSS are less immunogenic than normal human skin and that sustained hyperproliferation of the epidermis does not accelerate rejection.

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.

In silico identification of supertypes for class II MHCs

The development of epitope-based vaccines, which have wide population coverage, is greatly complicated by MHC polymorphism. The grouping of alleles into supertypes, on the basis of common structural and functional features, addresses this problem directly. In the present study we applied a combined bioinformatics approach, based on analysis of both protein sequence and structure, to identify similarities in the peptide binding sites of 2225 human class II MHC molecules, and thus define supertypes and supertype fingerprints. Two chemometric techniques were used: hierarchical clustering using three-dimensional Comparative Similarity Indices Analysis fields and nonhierarchical k-means clustering using sequence-based z-descriptors. An average consensus of 84% was achieved, i.e., 1872 of 2225 class II molecules were classified in the same supertype by both techniques. Twelve class II supertypes were defined: five DRs, three DQs, and four DPs. The HLA class II supertypes and their fingerprints given in parenthesis are DR1 (Trp(9beta)), DR3 (Glu(9beta), Gln(70beta), and Gln/Arg(74beta)), DR4 (Glu(9beta), Gln/Arg(70beta), and Glu/Ala(74beta)), DR5 (Glu(9beta), Asp(70beta)), and DR9 (Lys/Gln(9beta)); DQ1 (Ala/Gly(86beta)), DQ2 (Glu(86beta), Lys(71beta)), and DQ3 (Glu(86beta), Thr/Asp(71beta)); DPw1 (Asp(84beta) and Lys(69beta)), DPw2 (Gly/Val(84beta) and Glu(69beta)), DPw4 (Gly/Val(84beta) and Lys(69beta)), and DPw6 (Asp(84beta) and Glu(69beta)). Apart from the good agreement between known binding motifs and our classification, several new supertypes, and corresponding thematic binding motifs, were also defined.

X Box-Like Sequences in the MHC Class II Region Maintain Regulatory Function

Sequences homologous to the canonical MHC class II (MHC-II) gene X box regulatory elements were identified within the HLA-DR subregion of the human MHC and termed X box-like (XL) sequences. Several XL box sequences were found to bind the MHC class II-specific transcription factors regulatory factor X and CIITA and were transcriptionally active. The histone code associated with the XL boxes and that of the HLA-DRA X box was determined. Using CIITA-positive and -negative B cell lines, CIITA-specific histone modifications were identified and found to be consistent among the active XL boxes. Although a remarkable similarity was observed for most modifications, differences in magnitude between the HLA-DRA promoter for modifications associated with the assembly of the general transcription factors, such as histone H3 lysine 9 acetylation and H3 lysine 4 trimethylation, distinguished the very active HLA-DRA promoter from the XL box regions. In response to IFN-gamma, XL box-containing histones displayed increased acetylation, coincident with CIITA expression and that observed in B cells, suggesting that the end point mechanisms of chromatin remodeling for cell type-specific MHC-II expression were similar. Lastly, an interaction between one XL box and the HLA-DRA promoter was observed in a chromatin-looping assay. Therefore, these data provide evidence that certain XL box sequences contribute to a global increase in chromatin accessibility of the HLA-DR region in B lymphocytes and in response to IFN-gamma and supports the involvement of these XL sequences in the regulation of MHC-II genes.

A point mutation in the groove of HLA-DO allows egress from the endoplasmic reticulum independent of HLA-DM

B lymphocytes express the nonclassical class II molecule HLA-DO, which modulates the peptide loading activity of HLA-DM in the endocytic pathway. Binding to HLA-DM is required for HLA-DO to egress from the endoplasmic reticulum (ER). To gain insights into the mode of action of DO and on the role of DM in ER release, we sought to identify DM-binding residues on DO. Our results show that DOalpha encompasses the binding site for HLA-DM. More specifically, mutation of residue DOalpha41 on an exposed lateral loop of the alpha1 domain affects the binding to DM, ER egress, and activity of DO. Using a series of chimeric DR/DO molecules, we confirmed the role of the alpha chain and established that a second DM-binding region is located C-terminal to the DOalpha80 residue, most probably in the alpha2 domain. Interestingly, after mutation of a buried proline (alpha11) on the floor of the putative peptide-binding groove, HLA-DO remained functional but became independent of HLA-DM for ER egress and intracellular trafficking. Collectively, these results suggest that the binding of HLA-DM to DOalpha allows the complex to egress from the ER by stabilizing intramolecular contacts between the N-terminal antiparallel beta-strands of the DOalphabeta heterodimer.

HLA-DO transduced in human monocyte-derived dendritic cells modulates MHC class II antigen processing

Through the regulation of human leukocyte antigen (HLA)-DM (DM) in B cells, HLA-DO (DO) modulates positively or negatively the presentation of specific peptides. Transduction of DO into human blood monocyte-derived dendritic cells (MoDC) has been proposed as a mean of modifying the peptide repertoire of major histocompatibility complex class II molecules. However, maturation of DC induced by inflammatory stimuli or possibly the adenoviral vector itself triggers acidification of vesicles and shuts down transcription of the class II transactivator gene as well as de novo biosynthesis of class II-related molecules and DM activity. In these conditions, it is unclear that transduced DO could alter the peptide repertoire. Our Western blot and reverse transcriptase-polymerase chain reaction analyses revealed that human DC derived from blood monocytes express small amounts of DOalpha. Transduction of DObeta alone resulted in the accumulation of a small pool of DO in DM(+) CD63(+) vesicles and at the plasma membrane of mature DC. The cell-surface increase in class II-associated invariant chain peptide (CLIP)/class II complexes is in line with an inhibitory role of DO on DM. Cotransduction of DOalpha and DObeta only slightly increased CLIP and DO levels at the cell surface. Together with the fact that a large fraction of transduced DO remains in the endoplasmic reticulum, this suggests that DM is limiting in these conditions. DO expression did not affect a mixed lymphocyte reaction but reduced presentation of the exogenous gp100 antigen to a specific T cell clone. These results show that transduced DO modulates antigen presentation in human mature MoDC, evoking the possible use of this chaperone for immunotherapy.