An unstable beta 2-microglobulin: major histocompatibility complex class I heavy chain intermediate dissociates from calnexin and then is stabilized by binding peptide

Chikungunya virus infection disrupts MHC-I antigen presentation via nonstructural protein 2

Infection by chikungunya virus (CHIKV), a mosquito-borne alphavirus, causes severe polyarthralgia and polymyalgia, which can last in some people for months to years. Chronic CHIKV disease signs and symptoms are associated with the persistence of viral nucleic acid and antigen in tissues. Like humans and nonhuman primates, CHIKV infection in mice results in the development of robust adaptive antiviral immune responses. Despite this, joint tissue fibroblasts survive CHIKV infection and can support persistent viral replication, suggesting that they escape immune surveillance. Here, using a recombinant CHIKV strain encoding the fluorescent protein VENUS with an embedded CD8+ T cell epitope, SIINFEKL, we observed a marked loss of both MHC class I (MHC-I) surface expression and antigen presentation by CHIKV-infected joint tissue fibroblasts. Both in vivo and ex vivo infected joint tissue fibroblasts displayed reduced cell surface levels of H2-Kb and H2-Db MHC-I proteins while maintaining similar levels of other cell surface proteins. Mutations within the methyl transferase-like domain of the CHIKV nonstructural protein 2 (nsP2) increased MHC-I cell surface expression and antigen presentation efficiency by CHIKV-infected cells. Moreover, expression of WT nsP2 alone, but not nsP2 with mutations in the methyltransferase-like domain, resulted in decreased MHC-I antigen presentation efficiency. MHC-I surface expression and antigen presentation was rescued by replacing VENUS-SIINFEKL with SIINFEKL tethered to β2-microglobulin in the CHIKV genome, which bypasses the requirement for peptide processing and TAP-mediated peptide transport into the endoplasmic reticulum. Collectively, this work suggests that CHIKV escapes the surveillance of antiviral CD8+ T cells, in part, by nsP2-mediated disruption of MHC-I antigen presentation.

Chikungunya virus infection disrupts MHC-I antigen presentation via nonstructural protein 2

Infection by chikungunya virus (CHIKV), a mosquito-borne alphavirus, causes severe polyarthralgia and polymyalgia, which can last in some people for months to years. Chronic CHIKV disease signs and symptoms are associated with the persistence of viral nucleic acid and antigen in tissues. Like humans and nonhuman primates, CHIKV infection in mice results in the development of robust adaptive antiviral immune responses. Despite this, joint tissue fibroblasts survive CHIKV infection and can support persistent viral replication, suggesting that they escape immune surveillance. Here, using a recombinant CHIKV strain encoding a chimeric protein of VENUS fused to a CD8+ T cell epitope, SIINFEKL, we observed a marked loss of both MHC class I (MHC-I) surface expression and antigen presentation by CHIKV-infected joint tissue fibroblasts. Both in vivo and ex vivo infected joint tissue fibroblasts displayed reduced cell surface levels of H2-Kb and H2-Db MHC proteins while maintaining similar levels of other cell surface proteins. Mutations within the methyl transferase-like domain of the CHIKV nonstructural protein 2 (nsP2) increased MHC-I cell surface expression and antigen presentation efficiency by CHIKV-infected cells. Moreover, expression of WT nsP2 alone, but not nsP2 with mutations in the methyltransferase-like domain, resulted in decreased MHC-I antigen presentation efficiency. MHC-I surface expression and antigen presentation could be rescued by replacing VENUS-SIINFEKL with SIINFEKL tethered to β2-microglobulin in the CHIKV genome, which bypasses the need for peptide processing and TAP-mediated peptide transport into the endoplasmic reticulum. Collectively, this work suggests that CHIKV escapes the surveillance of antiviral CD8+ T cells, in part, by nsP2-mediated disruption of MHC-I antigen presentation.

CD1-mediated immune responses in mucosal tissues: molecular mechanisms underlying lipid antigen presentation system

The cluster of differentiation 1 (CD1) molecule differs from major histocompatibility complex class I and II because it presents glycolipid/lipid antigens. Moreover, the CD1-restricted T cells that recognize these self and foreign antigens participate in both innate and adaptive immune responses. CD1s are constitutively expressed by professional and nonprofessional antigen-presenting cells in mucosal tissues, namely, the skin, lung, and intestine. This suggests that CD1-reactive T cells are involved in the immune responses of these tissues. Indeed, evidence suggests that these cells play important roles in diverse diseases, such as inflammation, autoimmune disease, and infection. Recent studies elucidating the molecular mechanisms by which CD1 presents lipid antigens suggest that defects in these mechanisms could contribute to the activities of CD1-reactive T cells. Thus, improving our understanding of these mechanisms could lead to new and effective therapeutic approaches to CD1-associated diseases. In this review, we discuss the CD1-mediated antigen presentation system and its roles in mucosal tissue immunity.

Engineering Cell Lines for Specific Human Leukocyte Antigen Presentation

Epitope-specific immunotherapies have enabled the targeted treatment of a variety of diseases, ranging from cancer, infection, and autoimmune disorders. For CD8+ T cell-based therapies, the precise identification of immunogenic peptides presented by human leukocyte antigen (HLA) class I is essential which can be achieved by immunopeptidomics. Here, using lentivirus-mediated transduction and cell sorting approaches, we present a method to engineer a cell line that does not express its native HLA but instead expresses an HLA of interest (in this instance HLA-A*02:01). This technique can be used to elucidate the immunopeptidome of cell lines expressing different HLAs.

Crystal structures of N-myristoylated lipopeptide-bound HLA class I complexes indicate reorganization of B-pocket architecture upon ligand binding

Rhesus monkeys have evolved major histocompatibility complex (MHC)-encoded class I allomorphs such as Mamu-B*098 that are capable of binding N-myristoylated short lipopeptides rather than conventional long peptides; however, it remains unknown whether such antigen-binding molecules exist in other species, including humans. We herein demonstrate that human leukocyte antigen (HLA)-A*24:02 and HLA-C*14:02 proteins, which are known to bind conventional long peptides, also have the potential to bind N-myristoylated short lipopeptides. These HLA class I molecules shared a serine at position 9 (Ser9) with Mamu-B*098, in contrast to most MHC class I molecules that harbor a larger amino acid residue, such as tyrosine, at this position. High resolution X-ray crystallographic analyses of lipopeptide-bound HLA-A*24:02 and HLA-C*14:02 complexes indicated that Ser9 was at the bottom of the B pocket with its small hydroxymethyl side chain directed away from the B-pocket cavity, thereby contributing to the formation of a deep hydrophobic cavity suitable for accommodating the long-chain fatty acid moiety of lipopeptide ligands. Upon peptide binding, however, we found the hydrogen-bond network involving Ser9 was reorganized, and the remodeled B pocket was able to capture the second amino acid residue (P2) of peptide ligands. Apart from the B pocket, virtually no marked alterations were observed for the A and F pockets upon peptide and lipopeptide binding. Thus, we concluded that the structural flexibility of the large B pocket of HLA-A*2402 and HLA-C*1402 primarily accounted for their previously unrecognized capacity to bind such chemically distinct ligands as conventional peptides and N-myristoylated lipopeptides.

Homotypic and heterotypic in cis associations of MHC class I molecules at the cell surface

Through the presentation of peptide antigens to cytotoxic T lymphocytes, major histocompatibility complex (MHC) class I molecules mediate the adaptive immune response against tumors and viruses. Additional non-immunological functions include the heterotypic association of class I molecules with cell surface receptors, regulating their activities by unknown mechanisms. Also, homotypic associations resulting in class I dimers and oligomers - of unknown function - have been related to pathological outcomes. In this review, we provide an overview of the current knowledge about the occurrence, biochemical nature, and dynamics of homotypic and heterotypic associations of class I molecules at the cell surface with special focus on the molecular species that take part in the complexes and on the evidence that supports novel biological roles for class I molecules. We show that both heterotypic and homotypic class I associations reported in the literature describe not one but several kinds of oligomers with distinctive stoichiometry and biochemical properties.

•

Major histocompatibility complex class I molecules form homotypic and heterotypic associations at the cell surface.

•

Associations show distinctive stoichiometry and biochemical properties.

•

Associations might regulate immunological and non-immunological processes.

•

Heterotypic association with cell surface receptors might regulate receptor's activity.

•

Homotypic associations have been related to pathological outcomes.

Destabilizing single chain major histocompatibility complex class I protein for repurposed enterokinase proteolysis

The lack of a high throughput assay for screening stabilizing peptides prior to building a library of peptide-major histocompatibility complex class I (pMHC-I) molecules has motivated the continual use of in silico tools without biophysical characterization. Here, based on de novo protein fragmentation, the EASY MHC-I (EZ MHC-I) assay favors peptide antigen screening to an unheralded hands-on time of seconds per peptide due to the empty single chain MHC-I protein instability. Unlike tedious traditional labeling- and antibody-based MHC-I assays, repurposed enterokinase directly fragments the unstable single MHC-I chain protein unless rescued by a stabilizing peptide under luminal condition. Herein, the principle behind EZ MHC-I assay not only characterizes the overlooked stability as a known better indicator of immunogenicity than classical affinity but also the novel use of enterokinase from the duodenum to target destabilized MHC-I protein not bearing the standard Asp-Asp-Asp-Asp-Lys motif, which may protend to other protein instability-based assays.

Crystal structures of lysophospholipid-bound MHC class I molecules

Newly synthesized major histocompatibility complex (MHC) class I proteins are stabilized in the endoplasmic reticulum (ER) by binding 8-10-mer-long self-peptide antigens that are provided by transporter associated with antigen processing (TAP). These MHC class I:peptide complexes then exit the ER and reach the plasma membrane, serving to sustain the steady-state MHC class I expression on the cell surface. A novel subset of MHC class I molecules that preferentially bind lipid-containing ligands rather than conventional peptides was recently identified. The primate classical MHC class I allomorphs, Mamu-B*098 and Mamu-B*05104, are capable of binding the N-myristoylated 5-mer (C14-Gly-Gly-Ala-Ile-Ser) or 4-mer (C14-Gly-Gly-Ala-Ile) lipopeptides derived from the N-myristoylated SIV Nef protein, respectively, and of activating lipopeptide antigen-specific cytotoxic T lymphocytes. We herein demonstrate that Mamu-B*098 samples lysophosphatidylethanolamine and lysophosphatidylcholine containing up to a C20 fatty acid in the ER. The X-ray crystal structures of Mamu-B*098 and Mamu-B*05104 complexed with lysophospholipids at high resolution revealed that the B and D pockets in the antigen-binding grooves of these MHC class I molecules accommodate these lipids through a monoacylglycerol moiety. Consistent with the capacity to bind cellular lipid ligands, these two MHC class I molecules did not require TAP function for cell-surface expression. Collectively, these results indicate that peptide- and lipopeptide-presenting MHC class I subsets use distinct sources of endogenous ligands.

Multifunctional Chaperone and Quality Control Complexes in Adaptive Immunity

The fundamental process of adaptive immunity relies on the differentiation of self from nonself. Nucleated cells are continuously monitored by effector cells of the immune system, which police the peptide status presented via cell surface molecules. Recent integrative structural approaches have provided insights toward our understanding of how sophisticated cellular machineries shape such hierarchical immune surveillance. Biophysical and structural achievements were invaluable for defining the interconnection of many key factors during antigen processing and presentation, and helped to solve several conundrums that persisted for many years. In this review, we illuminate the numerous quality control machineries involved in different steps during the maturation of major histocompatibility complex class I (MHC I) proteins, from their synthesis in the endoplasmic reticulum to folding and trafficking via the secretory pathway, optimization of antigenic cargo, final release to the cell surface, and engagement with their cognate receptors on cytotoxic T lymphocytes.

Targeted Delivery of the HLA-B∗27-Binding Peptide into the Endoplasmic Reticulum Suppresses the IL-23/IL-17 Axis of Immune Cells in Spondylarthritis

Ankylosing spondylitis (AS) is highly associated with the expression of human leukocyte antigen-B27 (HLA-B^∗27). HLA-B^∗27 heavy chain (B27-HC) has an intrinsic propensity to fold slowly, leading to the accumulation of the misfolded B27-HC in the endoplasmic reticulum (ER) and formation of the HLA-B^∗27 HC homodimer, (B27-HC)₂, by a disulfide linkage at Cys-67. (B27-HC)₂ displayed on the cell surface can act as a ligand of the killer-cell Ig-like receptor (KIR3DL2). (B27-HC)₂ binds to KIR3DL2 of NK and Th17 cells and activates both cells, resulting in the activation of the IL-23/IL-17 axis to launch the inflammatory reaction in AS patients. However, activation of the IL-23/IL-17 axis originally derived from the HLA-B^∗27 misfolding in the ER needs to be characterized. In this study, we delivered two HLA-B^∗27-binding peptides, KRGILTLKY and SRYWAIRTR, into the ER by using a tat-derived peptide (GRKKRRQRRR)-His₆-ubiquitin (THU) vehicle. Both peptides are derived from the human actin and nucleoprotein of influenza virus, respectively. Our results demonstrated that targeted delivery of both HLA-B^∗27-binding peptides into the ER can promote the HLA-B^∗27 folding, decrease the levels of (B27-HC)₂, and suppress the activation of the IL-23/IL-17 axis in response to lipopolysaccharide. Our findings can provide a new therapeutic strategy in AS.

Transport and quality control of MHC class I molecules in the early secretory pathway

Folding and peptide binding of major histocompatibility complex (MHC) class I molecules have been thoroughly researched, but the mechanistic connection between these biochemical events and the progress of class I through the early secretory pathway is much less well understood. This review focuses on the question how the partially assembled forms of class I (which lack high-affinity peptide and/or the light chain beta-2 microglobulin) are retained inside the cell. Such investigations offer researchers exciting chances to understand the connections between class I structure, conformational dynamics, peptide binding kinetics and thermodynamics, intracellular transport, and antigen presentation.

Targeted delivery of an antigenic peptide to the endoplasmic reticulum: application for development of a peptide therapy for ankylosing spondylitis

The development of suitable methods to deliver peptides specifically to the endoplasmic reticulum (ER) can provide some potential therapeutic applications of such peptides. Ankylosing spondylitis (AS) is strongly associated with the expression of human leukocytic antigen-B27 (HLA-B27). HLA-B27 heavy chain (HC) has a propensity to fold slowly resulting in the accumulation of misfolded HLA-B27 HC in the ER, triggering the unfolded protein response, and forming a homodimer, (B27-HC)₂. Natural killer cells and T-helper 17 cells are then activated, contributing to the major pathogenic potentials of AS. The HLA-B27 HC is thus an important target, and delivery of an HLA-B27-binding peptide to the ER capable of promoting HLA-B27 HC folding is a potential mechanism for AS therapy. Here, we demonstrate that a His₆-ubiquitin-tagged Tat-derived peptide (THU) can deliver an HLA-B27-binding peptide to the ER promoting HLA-B27 HC folding. The THU-HLA-B27-binding peptide fusion protein crossed the cell membrane to the cytosol through the Tat-derived peptide. The HLA-B27-binding peptide was specifically cleaved from THU by cytosolic ubiquitin C-terminal hydrolases and subsequently transported into the ER by the transporter associated with antigen processing. This approach has potential application in the development of peptide therapy for AS.

Intracellular localization and association of MHC class I with porcine invariant chain

The objective was to investigate the intracellular localization and association of pig major histocompatibility complex (MHC) class I subunits with invariant chain (Ii). Pig MHC class I subunit cDNAs were cloned by RT-PCR and eukaryotic expression plasmids of α and β2m were constructed with fusions to red or enhanced green fluorescent protein (pDsRed2-N1-α, pEGFP-N1-α, pDsRed2-N1-β2m, and pEGFP-N1-β2m). A pig Ii mutant with a deleted CLIP region (DCLIP-Ii) was constructed by overlap extension PCR. Wild-type Ii and mutant Ii were cloned into pEGFP-C1 (pEGFP-C1-Ii, pEGFP-C1-DCLIP-Ii). The recombinant plasmids of MHC I subunits and pEGFP-C1-Ii (pEGFP-C1-ΔCLIP-Ii) were transiently cotransfected into COS-7 cells with Lipofectamine 2000. Immunofluorescence microscopy was performed to detect expression and intracellular localization of Ii and MHC I subunits, and immunoprecipitation was used to analyze their association. Our results indicated that pig Ii associates with integrated MHC I subunits to form oligomers, but cannot associate with single MHC I subunits. Furthermore, deletion of the Ii CLIP sequence blocks association with integrated MHC I subunits. Thus, pig Ii cannot associate with a single MHC I molecule, the α or β2m chain, but Ii and the integrated MHC I molecule can form complexes that colocalize in the endomembranes of COS-7 cells. The Ii of CLIP plays a key role in assembly of Ii and MHC I.

Class I HLA folding and antigen presentation in beta 2-microglobulin-defective Daudi cells

To present virus and tumor Ags, HLA class I molecules undergo a complex multistep assembly involving discrete but transient folding intermediates. The most extensive folding abnormalities occur in cells lacking the class I L chain subunit, called beta(2)-microglobulin (beta(2)m). Herein, this issue was investigated taking advantage of eight conformational murine mAbs (including the prototypic W6/32 mAb) to mapped H chain epitopes of class I molecules, four human mAbs to class I alloantigens, as well as radioimmunoprecipitation, in vitro assembly, pulse-chase, flow cytometry, and peptide-pulse/ELISPOT experiments. We show that endogenous (HLA-A1, -A66, and -B58) as well as transfected (HLA-A2) heavy chains in beta(2)m-defective Burkitt lymphoma Daudi cells are capable of being expressed on the cell surface, although at low levels, and exclusively as immature glycoforms. In addition, HLA-A2 is: 1) partially folded at crucial interfaces with beta(2)m, peptide Ag, and CD8; 2) receptive to exogenous peptide; and 3) capable of presenting exogenous peptide epitopes (from virus and tumor Ags) to cytotoxic T lymphocytes (bulk populations as well as clones) educated in a beta(2)m-positive environment. These experiments demonstrate a precursor-product relationship between novel HLA class I folding intermediates, and define a stepwise mechanism whereby distinct interfaces of the class I H chain undergo successive, ligand-induced folding adjustments in vitro as well as in vivo. Due to this unprecedented class I plasticity, Daudi is the first human cell line in which folding and function of class I HLA molecules are observed in the absence of beta(2)m. These findings bear potential implications for tumor immunotherapy.

The surprising diversity of lipid antigens for CD1-restricted T cells

CD1 proteins have been conserved throughout mammalian evolution and function to present lipid antigens to T cells. Crystal structures of CD1-lipid complexes show that CD1 antigen-binding grooves are composed of four pockets and two antigen entry portals. This structural information now provides a detailed understanding of how CD1-binding grooves capture a surprisingly diverse array of lipid ligands. CD1-expressing APCs are able to acquire lipid antigens from their own pool of lipids and from exogenous sources, including microbial pathogens, bystander cells, or even the systemic circulation. CD1 proteins bind to certain antigens using high stringency loading reactions within endosomes that involve low pH, glycosidases, and lipid transfer proteins. Other antigens can directly load onto CD1 proteins using low stringency mechanisms that are independent of cellular factors. New evidence from in vivo systems shows that CD1-restricted T cells influence outcomes in infectious, autoimmune, and allergic diseases. These studies lead to a broader view of the natural function of alphabeta T cells, which involves recognition of both cellular proteins and lipids.

Total HLA class I loss in a sarcomatoid renal carcinoma cell line caused by the coexistence of distinct mutations in the two encoding beta2-microglobulin genes

In renal cell carcinoma (RCC), HLA class I downregulation has been found in about 40% of the lesions examined. Since only scanty information is available about the molecular basis of these defects, we have investigated the mechanism(s) underlying HLA class I antigen downregulation or loss in six RCC cell lines. Five of them express HLA class I antigens although at various levels; on the other hand, HLA class I antigens are not detectable on the remaining cell line, the RCC52 cell line, belonging to a sarcomatoid subtype, even following incubation with IFN-gamma. beta(2)-microglobulin (beta(2) m) was not detected in RCC52 cells. Surprisingly, RCC52 cells harbor two mutations in the beta ( 2 ) m genes in exon 1: a single G deletion (delG) in codon 6, which introduces a premature stop at codon 7, and a CT dinucleotide deletion (delCT), which leads to a premature stop at codon 55. Analysis of eight clonal sublines isolated from the RCC52 cell line showed that the two beta ( 2 ) m gene mutations are carried separately by RCC52 cell subpopulations. The delG/delCT double mutations were detected in two sublines with a fibroblast-like morphology, while the delCT mutation was detected in the remaining six sublines with an epithelial cell morphology. Furthermore, loss of heterozygosity (LOH) of the beta ( 2 ) m gene at STR D15S-209 was found only in the epithelioid subpopulation, indicating loss of one copy of chromosome 15. Immunostaining results of the tumor lesion from which the cell line RCC52 was originated were consistent with the phenotyping/molecular findings of the cultured cells. This is the first example of the coexistence of distinct beta ( 2 ) m defects in two different tumor subpopulations of a RCC, where loss of one copy of chromosome 15 occurs in one of the subpopulations with total HLA class I antigen loss.

Pathways of CD1 and lipid antigen delivery, trafficking, processing, loading, and presentation

Specific T cell responses to a variety of self and microbial lipids depend on proper assembly and intracellular trafficking of CD 1 molecules that intersect with and load processed lipid antigens. These pathways involve unique membrane trafficking and chaperones that are distinct from those utilized for major histocompatibility complex (MHC)-mediated presentation of peptide antigens, and thus define unique lipid antigen presentation pathways. Furthermore, recent studies have identified components of lipid metabolism that participate in lipid delivery, uptake, processing and loading onto CD1 molecules. Defects in these pathways result in impaired T cell development and function, underscoring their critical role in the lipid-specific T cell immune responses.

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.

Immunosurveillance is active in colorectal cancer as downregulation but not complete loss of MHC class I expression correlates with a poor prognosis

Many colorectal tumors lose or downregulate cell surface expression of MHC class I molecules conferring resistance to T-cell-mediated attack. It has been suggested that this phenomenon is due to in vivo immune-tumor interactions. However, evidence of the impact of MHC class I loss on outcomes from colorectal cancer is scarce. In our study of more than 450 colorectal cancers in tissue microarray format, we have shown that both high levels of MHC class I expression and absent MHC class I expression are associated with similar disease-specific survival times, possibly due to natural killer cell-mediated clearance of MHC class I-negative tumor cells. However, tumors with low level expression of MHC class I were found to confer a significantly poorer prognosis, retaining independent significance on multivariate analysis. The existence of these poor prognosis tumors, which may avoid both NK- and T-cell-mediated immune surveillance, has important implications for the design of immunotherapeutic strategies in colorectal cancer.

Impaired Assembly Results in the Accumulation of Multiple HLA-C Heavy Chain Folding Intermediates

Class I MHC H chains assemble with beta2-microglobulin (beta2m) and are loaded with peptide Ags through multiple folding steps. When free of beta2m, human H chains react with Abs to linear epitopes, such as L31. Immunodepletion and coimmunoprecipitation experiments, performed in this study, detected a preferential association of L31-reactive, beta2m-free H chains with calnexin in beta2m-defective cells, and with calreticulin and TAP in beta2m-expressing cells. In beta2m-defective cells, the accumulation of calnexin-bound H chains stoichiometrically exceeded their overall accumulation, a finding that supports both chaperoning preferences and distinct sorting abilities for different class I folds. No peptide species, in a mass range compatible with that of the classical class I ligands, could be detected by mass spectrometry of acidic eluates from L31-reactive HLA-Cw1 H chains. In vitro assembly experiments in TAP-defective T2 cells, and in cells expressing an intact Ag-processing machinery, demonstrated that L31 H chains are not only free of, but also unreceptive to, peptides. L31 and HC10, which bind nearly adjacent linear epitopes of the alpha1 domain alpha helix, reciprocally immunodepleted free HLA-C H chains, indicating the existence of a local un-/mis-folding involving the N-terminal end of the alpha1 domain alpha helix and peptide-anchoring residues of the class I H chain. Thus, unlike certain murine free H chains, L31-reactive H chains are not the immediate precursors of conformed class I molecules. A model inferring their precursor-product relationships with other known class I intermediates is presented.

Tapasin and other chaperones: models of the MHC class I loading complex

MHC (major histocompatibility complex) class I molecules bind intracellular virus-derived peptides in the endoplasmic reticulum (ER) and present them at the cell surface to cytotoxic T lymphocytes. Peptide-free class I molecules at the cell surface, however, could lead to aberrant T cell killing. Therefore, cells ensure that class I molecules bind high-affinity ligand peptides in the ER, and restrict the export of empty class I molecules to the Golgi apparatus. For both of these safeguard mechanisms, the MHC class I loading complex (which consists of the peptide transporter TAP, the chaperones tapasin and calreticulin, and the protein disulfide isomerase ERp57) plays a central role. This article reviews the actions of accessory proteins in the biogenesis of class I molecules, specifically the functions of the loading complex in high-affinity peptide binding and localization of class I molecules, and the known connections between these two regulatory mechanisms. It introduces new models for the mode of action of tapasin, the role of the class I loading complex in peptide editing, and the intracellular localization of class I molecules.

Mechanisms of Vδ1 γδ T Cell Activation by Microbial Components

There are two major subsets of gammadelta T cell in humans. Vgamma2Vdelta2 T cells predominate in the circulation and significantly expand in vivo during a variety of infectious diseases. Ags identified for the Vdelta2 T cells are nonpeptide phosphate, amine, and aminobisphosphonate compounds. In contrast, Vdelta1-encoded TCRs account for the vast majority of gammadelta T cells in tissues such as intestine and spleen. Some of these T cells recognize CD1c and MHC class I-related chain (MICA/B) molecules [correction]. These T cells are cytotoxic and use both perforin- and Fas-mediated cytotoxicity. A fundamental question is how these gammadelta T cells are activated during microbial exposure to carry out effector functions. In this study, we provide evidence for a mechanism by which Vdelta1 gammadelta T cells are activated by inflammatory cytokines in the context of the Vdelta1 TCR. Dendritic cells are necessary as accessory cells for microbial Ag-mediated Vdelta1 gammadelta T cell activation. Cytokine (IL-12), adhesion (LFA3/CD2, LFA1/ICAM1) and costimulatory (MHC class I-related chain (MICA/B) molecules/NK-activating receptor G2D) molecules play a significant role along with Vdelta1 TCR in this activation.

Quality control of MHC class I maturation

Assembly of MHC class I molecules in the ER is regulated by the so-called loading complex (LC). This multiprotein complex is of definite importance for class I maturation, but its exact organization and order of assembly are not known. Evidence implies that the quality of peptides loaded onto class I molecules is controlled at multiple stages during MHC class I assembly. We recently found that tapasin, an important component of the LC, interacts with COPI-coated vesicles. Biochemical studies suggested that the tapa-sin-COPI interaction regulates the retrograde transport of immature MHC class I molecules from the Golgi network back to the ER. Also other findings now propose that in addition to the peptide-loading control, the quality control of MHC class I antigen presentation includes the restriction of export of suboptimally loaded MHC class I molecules to the cell surface. In this review, we use recent studies of tapasin to examine the efficiency of TAP, the LC constitution, ER quality control of class I assembly, and peptide optimization. The concepts of MHC class I recycling and ER retention are also discussed.

Differential downregulation of endoplasmic reticulum-residing chaperones calnexin and calreticulin in human metastatic melanoma

Characterization of the molecular basis of tumor recognition by T cells has shown that major histocompatibility complex (MHC) class I molecules play a crucial role in presenting antigenic peptide epitopes to cytotoxic T lymphocytes. MHC class Ia downregulation has been repeatedly described on melanoma cells and is thought to be involved in the failure of the immune system to control tumor progression. Proper assembly of MHC class I molecules is dependent on several cofactors, e.g. the chaperones calnexin and calreticulin residing in the endoplasmic reticulum. Alterations in the expression of these chaperones may have important implications for MHC class I assembly, peptide loading, and presentation on the tumor cell surface and thus may contribute to the immune escape phenotype of tumor cells. In the present study, we compared melanoma lesions representing different stages of tumor progression with regard to the expression of calnexin and calreticulin in tumor cells by means of immunohistochemistry. Metastatic melanoma lesions exhibited significant downregulation of calnexin as compared to primary melanoma lesions. In contrast, chaperone calreticulin was expressed in melanoma cells of primary as well as of metastatic lesions. Our data suggest that chaperone-downregulation, particularly calnexin-downregulation, may contribute to the metastatic phenotype of melanoma cells in vivo. Consistently, conserved chaperone expression in metastatic melanoma lesions may be a useful criterion for selection of patients for treatment with T cell-based immunotherapies.

Intracellular pathways of CD1 antigen presentation

Each of the human CD1 proteins takes a different route through secretory and endocytic compartments before finally arriving at the cell surface, where these proteins present glycolipid antigens to T cells. Recent studies have shown that adaptor-protein complexes and CD1-associated chaperones control not only CD1 trafficking, but also the development and activation of CD1-restricted T cells. This indicates that CD1 proteins, similar to MHC class I and II molecules, selectively acquire certain antigens in distinct cellular subcompartments. Here, we summarize evidence supporting the hypothesis that CD1 proteins use separate, but parallel, pathways to survey endosomal compartments differentially for lipid antigens.

Accessory proteins and the assembly of human class I MHC molecules: a molecular and structural perspective

The cell-surface presentation of antigenic peptides by class I major histocompatibility complex (MHC) molecules to CD8+ T-cell receptors is part of an immune surveillance mechanism aimed at detecting foreign antigens. This process is initiated in the endoplasmic reticulum (ER) with the folding and assembly of class I MHC molecules which are then transported to the cell surface via the secretory pathway. In recent years, several accessory proteins have been identified as key components of the class I maturation process in the ER. These proteins include the lectin chaperones calnexin (CNX) and calreticulin (CRT), the thiol-dependent oxidoreductase ERp57, the transporter associated with antigen processing (TAP), and the protein tapasin. This review presents the most recent advances made in characterizing the biochemical and structural properties of these proteins, and discusses how this knowledge advances our current understanding of the molecular events underlying the folding and assembly of human class I MHC molecules in the ER.

Calnexin, calreticulin, and ERp57 cooperate in disulfide bond formation in human CD1d heavy chain

Members of the CD1 family of membrane glycoproteins can present antigenic lipids to T lymphocytes. Like major histocompatibility complex class I molecules, they form a heterodimeric complex of a heavy chain and beta(2)-microglobulin (beta(2)m) in the endoplasmic reticulum (ER). Binding of lipid antigens, however, takes place in endosomal compartments, similar to class II molecules, and on the plasma membrane. Unlike major histocompatibility complex class I or CD1b molecules, which need beta(2)m to exit the ER, CD1d can be expressed on the cell surface as either a free heavy chain or associated with beta(2)m. These differences led us to investigate early events of CD1d biosynthesis and maturation and the role of ER chaperones in its assembly. Here we show that CD1d associates in the ER with both calnexin and calreticulin and with the thiol oxidoreductase ERp57 in a manner dependent on glucose trimming of its N-linked glycans. Complete disulfide bond formation in the CD1d heavy chain was substantially impaired if the chaperone interactions were blocked by the glucosidase inhibitors castanospermine or N-butyldeoxynojirimycin. The formation of at least one of the disulfide bonds in the CD1d heavy chain is coupled to its glucose trimming-dependent association with ERp57, calnexin, and calreticulin.

Tapasin-the keystone of the loading complex optimizing peptide binding by MHC class I molecules in the endoplasmic reticulum

MHC class I molecules are loaded with peptides that mostly originate from the degradation of cytosolic protein antigens and that are translocated across the endoplasmic reticulum (ER) membrane by the transporter associated with antigen processing (TAP). The ER-resident molecule tapasin (Tpn) is uniquely dedicated to tether class I molecules jointly with the chaperone calreticulin (Crt) and the oxidoreductase ERp57 to TAP. As learned from the study of a Tpn-deficient cell line and from mice harboring a disrupted Tpn gene, the transient association of class I molecules with Tpn and TAP is critically important for the stabilization of class I molecules and the optimization of the peptide cargo presented to cytotoxic T cells. The different functions of molecular domains of Tpn and the highly coordinated formation of the TAP-associated peptide loading complex will also be discussed in this review.

Failure of trafficking and antigen presentation by CD1 in AP-3-deficient cells

Endocytosed microbial antigens are primarily delivered to lysosomal compartments where antigen binding to MHC and CD1 molecules occurs in an acidic and proteolytically active environment. Signal-dependent delivery to lysosomes has been suggested for these antigen-presenting molecules, but molecular interactions with vesicular coat proteins and adaptors that direct their lysosomal sorting are poorly understood. Here CD1b but not other CD1 isoforms bound the AP-3 adaptor protein complex. In AP-3-deficient cells derived from patients with Hermansky-Pudlak syndrome type 2 (HPS-2), CD1b failed to efficiently gain access to lysosomes, resulting in a profound defect in antigen presentation. Since MHC class II traffics normally in AP-3-deficient cells, defects in CD1b antigen presentation may account for recurrent bacterial infections in HPS-2 patients.

Calreticulin recognizes misfolded HLA-A2 heavy chains

Our studies investigated functional interactions between calreticulin, an endoplasmic reticulum chaperone, and major histocompatibility complex (MHC) class I molecules. Using in vitro thermal aggregation assays, we established that calreticulin can inhibit heat-induced aggregation of soluble, peptide-deficient HLA-A2 purified from supernatants of insect cells. The presence of HLA-A2-specific peptides also inhibits heat-induced aggregation. Inhibition of heat-induced aggregation of peptide-deficient HLA-A2 by calreticulin correlates with a rescue of the HLA-A2 heavy chain from precipitation, by forming high-molecular-weight complexes with calreticulin. Complex formation between HLA-A2 heavy chains and calreticulin occurs at 50 degrees C but not 37 degrees C, suggesting polypeptide-based interactions between the HLA-A2 heavy chain and calreticulin. Once complexes are formed, the addition of peptide is not sufficient to trigger efficient assembly of heavy chain/beta2m/peptide complexes. Using a fluorescent peptide-based binding assay, we show that calreticulin does not enhance peptide binding by HLA-A2 at 37 degrees C. We also show that calreticulin itself is converted to oligomeric species on exposure to 37 degrees C or higher temperatures, and that oligomeric forms of calreticulin are active in inhibiting thermal aggregation of peptide-deficient HLA-A2. Taken together, these results suggest that calreticulin functions in the recognition of misfolded MHC class I heavy chains in the endoplasmic reticulum. However, in the absence of other endoplasmic reticulum components, calreticulin by itself does not enhance the assembly of misfolded MHC class I heavy chains with beta2m and peptides.

The cell biology of MHC class I antigen presentation

MHC class I antigen presentation refers to the co-ordinated activities of many intracellular pathways that promote the cell surface appearance of MHC class I/beta2m heterodimers loaded with a spectrum of self or foreign peptides. These MHC class I peptide complexes form ligands for CD8 positive T cells and NK cells. MHC class I heterodimers are loaded within the endoplasmic reticulum (ER) with peptides derived from intracellular proteins. Alternatively, MHC class I molecules may be loaded with peptides derived from extracellular proteins in a process called MHC class I cross presentation. This pathway is less well defined but can overlap those pathways operating in classical MHC class I presentation and has recently been reviewed elsewhere (1). This review will address the current concepts regarding the intracellular assembly of MHC class I molecules with their peptide cargo within the ER and their subsequent progress to the cell surface.

[CD1: A new paradigm for antigen presentation]

Molecules of the major histocompatibility complex (MHC) bind protein-derived peptide antigens and present them to T cells. This has been a central dogma in modern immunology, and our appreciation of a variety of cell-mediated immune responses has been based only on this paradigm. However, we now know that T cell recognition also involves non-peptide antigens. Studies over the past several years have established a new paradigm that non-MHC-encoded molecules of the CD1 family mediate presentation of lipid antigens to T cells, and unraveled their significant role in microbial immunity, tumor immunology, and autoimmunity. Identification of a novel pathway for T cell activation mediated by CD1 molecules opens a possibility for new therapeutic strategies, including development of lipid-based vaccines.

Extraction of human Langerhans cells: a method for isolation of epidermis-resident dendritic cells

Langerhans cells (LCs) are immature dendritic cells in the epidermis that play a central role in T-lymphocyte mediated skin immunity. Upon activation with antigenic stimuli, they differentiate drastically into mature dendritic cells while migrating from the epidermis to regional lymph nodes. Thus, in order to study biological details of immature LCs, it is crucial to isolate epidermis-resident, immature LCs without dermal dendritic cell contamination. Methods for extracting LCs from human skin as well as in vitro derivation of LC-like cells from hematopoietic progenitor cells have been described previously, but the cell preparations can potentially contain a significant number of dendritic cells that are not identical to epidermal LCs. Here, we describe a technique by which purely epidermis-resident LCs are extracted from human skin. Following digestion of human skin with dispase, the epidermis was separated mechanically without any attached dermal component. The trypsinized epidermal cells were then fractionated by centrifugation with a discontinuous density gradient composed of bovine albumin and sodium metrizoate. The LC-enriched preparation thus obtained contained 80% to >90% CD1a+, E-cadherin+ cells that expressed Birbeck granules and the Lag protein. Consistent with their being at an immature stage, the freshly isolated LCs lacked the expression of CD83, a marker for mature dendritic cells. The purified LCs were able to activate allogeneic T cells, indicating that the cells retained T-cell stimulation ability even after extraction. Thus, the present work offers an opportunity for precise in vitro studies of epidermal LCs.

Distinct differences in association of MHC class I with endoplasmic reticulum proteins in wild-type, and beta 2-microglobulin- and TAP-deficient cell lines

In this study we have compared the interaction of human MHC class I molecules with IgG heavy chain (HC) binding protein (BiP), calnexin, calreticulin, tapasin and TAP in beta(2)-microglobulin (beta(2)m)- or TAP-deficient cells, as well as in wild-type B-LCL cells. Distinct differences between the association of HC and these endoplasmic reticulum (ER) proteins were found in the three cell lines. In the absence of beta(2)m (Daudi cells), HC associated with both BiP and calnexin. A prominent portion of HC was complexed simultaneously to both chaperones, as indicated by co-precipitation with either anti-calnexin or anti-class I antisera. In the presence of beta(2)m, but absence of TAP (T2 cells), HC could be co-precipitated with calnexin, whereas no detectable interaction with BiP could be demonstrated. This suggests that calnexin interacts with HC at a later stage than BiP. In B-LCL cells, HC-beta(2)m associated with calreticulin and tapasin, whereas no interaction with calnexin and BiP was observed. In the absence of beta(2)m, HC were rapidly degraded in the ER, while the ER retained HC were stabilized in the presence of beta(2)m, even in the absence of TAP. The dissociation of class I molecules from TAP in B-LCL cells correlated with the kinetics of appearance of class I molecules on the cell surface, suggesting that TAP retains peptide-free class I molecules in the ER. Taken together, our results suggest the model that BiP and calnexin sequentially control the folding of MHC class I, before MHC class I molecules associate with the loading complex.

Blocked transport of soluble K(b) molecules containing connecting peptide segment involved in calnexin association

The molecular event governing the assembly of the MHC class I heavy chain-beta(2)-microglobulin-peptide complex is still not fully understood. In order to characterize the transport properties of MHC class I molecules, several truncated H-2K(b) genes were constructed and expressed in COS7 cells. Surprisingly, the expressed soluble molecule containing connecting peptide (CP) segment (sK(b)(CP)) did not secrete as efficiently as the one without CP (sK(b)(CYT)). When the sK(b)(CP) gene was transfected into a calnexin-deficient cell line CEM.NK(R), the amount of soluble K(b) molecules in the supernatant was comparable with sK(b)(CYT)-transfected CEM.NK(R). To further demonstrate the different transport of sK(b)(CP) and sK(b)(CYT) within living cells, we attached green fluorescent protein (GFP) to the C-termini of both molecules and, as a comparison, to the full-length transmembrane counterpart (mK(b)-GFP). While the mK(b)-GFP-transfected cells showed the green fluorescence in the reticular network and the nuclear envelope, sK(b)(CP)-GFP showed obviously lump fluorescence of high intensity within cells. However, the distribution of sK(b)(CYT)-GFP was fairly uniform. Furthermore, GFP-tagged molecules allow us to analyze their interaction with other proteins in a direct, simple and quantitative method, designated immunofluorescence precipitation. The results showed that 60% of sK(b)(CP)-GFP molecules were associated with calnexin, while <10% with tapasin. Taken together with the results from sK(b)(CYT)-GFP and mK(b)-GFP, it is reasonable to deduce that the CP segment is involved in the association of class I molecules with calnexin and the transmembrane region might play a dynamic role in the dissociation from calnexin. The suggested kinetic association of class I molecules with calnexin is likely to contribute to the different maturation rate between several class I alleles.

Pathways for lipid antigen presentation by CD1 molecules: nowhere for intracellular pathogens to hide

A crucial feature of peptide antigen presentation by major histocompatibility complex (MHC) class I and II molecules is their differential ability to sample cytosolic and extracellular antigens. Intracellular viral infections and bacteria that are taken up in phagosomes, but then escape from the endocytic compartment efficiently, enter the class I pathway via the cytosol. In contrast, phagosome-resident bacteria yield protein antigens that are sampled deep in the endocytic compartment and presented in a vacuolar acidification-dependent pathway mediated by MHC class II molecules. Despite this potential for antigen sampling, microbes have evolved a variety of evasive mechanisms that affect peptide transport in the MHC class I pathway or blockade of endosomal acidification and inhibition of phagosome-lysosome fusion that may compromise the MHC class II pathway of antigen presentation. Thus, besides MHC class I and II, a third lineage of antigen-presenting molecules that bind lipid and glycolipid antigens rather than peptides exists and is mediated by the family of CD1 proteins. CD1 isoforms (CD1a, b, c, and d) differentially sample both recycling endosomes of the early endocytic system and late endosomes and lysosomes to which lipid antigens are differentially delivered. These CD1 pathways include vacuolar acidification-independent pathways for lipid antigen presentation. These features of presenting lipid antigens, independently monitoring various antigen-containing intracellular compartments and avoiding certain evasive techniques employed by microbes, enable CD1 molecules to provide distinct opportunities to function in host defense against the microbial world.

Misfolding of HLA-B27 as a Result of Its B Pocket Suggests a Novel Mechanism for Its Role in Susceptibility to Spondyloarthropathies

The MHC class I protein HLA-B27 is strongly associated with susceptibility to spondyloarthropathies and can cause arthritis when expressed in rats and mice, implying a direct role in disease pathogenesis. A prominent hypothesis to explain this role suggests that the unique peptide binding specificity of HLA-B27 confers an ability to present arthritogenic peptides. The B pocket, a region of the peptide binding groove that is an important determinant of allele-specific peptide binding, is thought to be critical for arthritogenicity. However, this hypothesis remains unproven. We show that in addition to its role in peptide selection, the B pocket causes a portion of the pool of assembling HLA-B27 heavy chains in the endoplasmic reticulum to misfold, resulting in their degradation in the cytosol. The misfolding phenotype is corrected by replacing the HLA-B27 B pocket with one from HLA-A2. Our results suggest an alternative to the arthritogenic peptide hypothesis. Misfolding and its consequences, rather than allele-specific peptide presentation, may underlie the strong link between the HLA-B27 B pocket and susceptibility to spondyloarthropathies.

Separate pathways for antigen presentation by CD1 molecules

The ability to sample relevant intracellular compartments is necessary for effective antigen presentation. To detect peptide antigens, MHC class I and II molecules differentially sample cytosolic and endosomal compartments. CD1 constitutes another lineage of lipid antigen-presenting molecules. We show that CD1b traffics deeply into late endosomal compartments, while CD1a is excluded from these compartments and instead traffics independently in the recycling pathway of the early endocytic system. Further, CD1b but not CD1a antigen presentation is dependent upon vesicular acidification. Since lipids and various bacteria are known to traffic differentially, either penetrating deeply into the endocytic system or following the route of recycling endosomes, these findings elucidate efficient monitoring of distinct components of the endocytic compartment by CD1 lipid antigen-presenting molecules.

Calreticulin is expressed on the cell surface of activated human peripheral blood T lymphocytes in association with major histocompatibility complex class I molecules

Calreticulin is an endoplasmic reticulum resident molecule known to be involved in the folding and assembly of major histocompatibility complex (MHC) class I molecules. In the present study, expression of calreticulin was analyzed in human peripheral blood T lymphocytes. Pulse-chase experiments in [35S]methionine-labeled T cell blasts showed that calreticulin was associated with several proteins in the endoplasmic reticulum and suggested that it was expressed at the cell surface. Indeed, the 60-kDa calreticulin was labeled by cell surface biotinylation and precipitated from the surface of activated T cells together with a protein with an apparent molecular mass of 46 kDa. Cell surface expression of calreticulin by activated T lymphocytes was further confirmed by immunofluorescence and flow cytometry, studies that showed that both CD8+ and CD4+ T cells expressed calreticulin in the plasma membrane. Low amounts of cell surface calreticulin were detected in resting T lymphocytes. By sequential immunoprecipitation using the conformation independent monoclonal antibody HC-10, we provided evidence that the cell surface 46-kDa protein co-precipitated with calreticulin is unfolded MHC I. These results show for the first time that after T cell activation, significant amounts of calreticulin are expressed on the T cell surface, where they are found in physical association with a pool of beta2-free MHC class I molecules.

Interaction of Murine MHC Class I Molecules with Tapasin and TAP Enhances Peptide Loading and Involves the Heavy Chain α3 Domain

In human cells the association of MHC class I molecules with TAP is thought to be mediated by a third protein termed tapasin. We now show that tapasin is present in murine TAP-class I complexes as well. Furthermore, we demonstrate that a mutant H-2Dd molecule that does not interact with TAP due to a Glu to Lys mutation at residue 222 of the H chain (Dd(E222K)) also fails to bind to tapasin. This finding supports the view that tapasin bridges the association between class I and TAP and implicates residue 222 as a site of contact with tapasin. The inability of Dd(E222K) to interact with tapasin and TAP results in impaired peptide loading within the endoplasmic reticulum. However, significant acquisition of peptides can still be detected as assessed by the decay kinetics of cell surface Dd(E222K) molecules and by the finding that prolonged viral infection accumulates sufficient target structures to stimulate T cells at 50% the level observed with wild-type Dd. Thus, although interaction with tapasin and TAP enhances peptide loading, it is not essential. Finally, a cohort of Dd(E222K) molecules decays more rapidly on the cell surface compared with wild-type Dd molecules but much more slowly than peptide-deficient molecules. This suggests that some of the peptides obtained in the absence of an interaction with tapasin and TAP are suboptimal, suggesting a peptide-editing function for tapasin/TAP in addition to their role in enhancing peptide loading.

Calreticulin and Calnexin Interact with Different Protein and Glycan Determinants During the Assembly of MHC Class I

Before peptide binding, a variety of endoplasmic reticulum (ER) proteins are associated with class I including calnexin, TAP, calreticulin, and tapasin. Although the selective functions of any one of these ER proteins have been difficult to define, individually or in combination they perform two general chaperone functions for class I. They promote assembly of the class I heterotrimeric molecule (heavy (H) chain, β2m, and peptide) and they retain incompletely assembled complexes in the ER. In this study, we present evidence that calreticulin clearly differs from calnexin in how it associates with class I. Regarding the structural basis of the association, the oligosaccharide moiety in the α1 domain and the amino acid residue at position 227 in the α3 domain were both found to be critical for the interaction of class I with calreticulin. Interestingly, calreticulin displayed sensitivity to class I peptide binding even in TAP-deficient human or mouse cells. Thus, calreticulin is clearly more specific than calnexin in the structures and conformation of the class I molecule with which it can interact.

Structural characterization of a soluble and partially folded class I major histocompatibility heavy chain/beta 2m heterodimer

Class I major histocompatibility (MHC) heavy chain (HC) must fold and assemble with beta 2 microglobulin (beta 2m) prior to binding peptides in the endoplasmic reticulum (ER). Each of these events is mediated by association with chaperones and other proteins and is an essential requirement for the maturation and normal cell surface expression of stable class I MHC-peptide complexes. Here we describe the biochemical and structural characterization of a soluble HC (B*0702)/beta 2m heterodimer, apparently free of peptide. Results suggest that the peptide binding domains (alpha 1 and alpha 2) of this folding intermediate are unstable and possess many of the properties ascribed to the molten globule state. The partially folded state of the HC/beta 2m heterodimer is consistent with the suggestion that it is stabilized by chaperones and other proteins in the ER. This soluble intermediate may be useful for studying protein-assisted folding and peptide binding of class I MHC molecules.

HLA-B27-restricted antigen presentation in the absence of tapasin reveals polymorphism in mechanisms of HLA class I peptide loading

Tapasin is a resident ER protein believed to be critical for antigen presentation by HLA class I molecules. We demonstrate that allelic variation in MHC class I molecules influences their dependence on tapasin for peptide loading and antigen presentation. HLA-B*2705 molecules achieve high levels of surface expression and present specific viral peptides in the absence of tapasin. In contrast, HLA-B*4402 molecules are highly dependent upon human tapasin for these functions, while HLA-B8 molecules are intermediate in this regard. Significantly, HLA-B*2705 like HLA-B*4402, requires tapasin to associate efficiently with TAP (transporters associated with antigen processing). The unusual ability of HLA-B*2705 to form peptide complexes without associating with TAP or tapasin confers flexibility in the repertoire of peptides presented by this molecule. We speculate that these properties might contribute to the role of HLA-B27 in conferring susceptibility to inflammatory spondyloarthropathies.

CD1--a new paradigm for antigen presentation and T cell activation

Despite identification of the CD1 family of molecules in the late 1970s, the function of CD1 was undetermined for more than a decade. Recent evidence has established that CD1 molecules comprise a novel lineage of antigen-presenting molecules, distinct from major histocompatibility complex (MHC) class I and class II molecules. Unlike the MHC molecules, which bind short peptides in their antigen-binding groove for presentation to either CD4+ or CD8+ T cells bearing alpha beta T cell receptors, the CD1 molecules appear to accommodate lipid and glycolipid antigens in their hydrophobic cavity for presentation to a wide variety of T cells, including double-negative alpha beta and gamma delta T cells and CD8+ alpha beta T cells. By using a unique cytoplasmic signal, some CD1 molecules traffic to endosomal compartments for sampling mycobacteria-derived lipid antigens, and subsequently lipid antigen-loaded CD1 molecules are expressed on the cell surface to activate specific T cells. These CD1-restricted T cells kill mycobacteria-infected cells and secrete interferon-gamma, indicating a potential role of CD1-mediated T cell responses in clearing mycobacterial infection. The identification of an MHC-independent antigen presentation pathway for nonpeptide antigens provides new insights into immunoregulation and host defense.

Fast association rates suggest a conformational change in the MHC class I molecule H-2Db upon peptide binding

Major histocompatibility complex (MHC) class I molecules bind peptides in the endoplasmic reticulum (ER). For this binding reaction, when performed in vitro, widely differing association rates have been reported. We have expressed empty soluble H-2Db class I molecules in Chinese hamster ovary (CHO) cells and generated complete sets of association, dissociation, and equilibrium constants of unmodified peptides using tritium-labeled peptides and stopped-flow fluorescence spectroscopy. We find that (i) the transition midpoint of temperature denaturation (Tm) of the protein is shifted from 30.5 to 56 degrees C upon the binding of a high-affinity peptide. (ii) With the peptide SV-324-332 (sequence FAPGNYPAL) at 4 degrees C, the dissociation rate constant of 1.02 x 10(-5) s-1 and an equilibrium constant of 8.5 x 10(7) M-1 predict an association rate constant of 870 M-1 s-1 for a simple one-step model of binding. (iii) In contrast, binding of this peptide proceeds much faster, with 1.4 x 10(6) M-1 s-1. These "mismatch kinetics" suggest that peptide binding occurs in several steps, most likely via a conformational rearrangement of the peptide binding groove. The structure of the peptide-class I complex at the time-point of peptide recognition may therefore be different from the equilibrium crystal structures. (iv) Association of modified peptides, in the presence of detergent, or above the Tm of the empty molecule is considerably slower. This might explain why fast on-rates have not been observed in previous studies.