Tapasin is required for efficient peptide binding to transporter associated with antigen processing

Disturbances in microbial skin recolonization and cutaneous immune response following allogeneic stem cell transfer

The composition of the gut microbiome influences the clinical course after allogeneic hematopoietic stem cell transplantation (HSCT), but little is known about the relevance of skin microorganisms. In a single-center, observational study, we recruited a cohort of 50 patients before undergoing conditioning treatment and took both stool and skin samples up to one year after HSCT. We could confirm intestinal dysbiosis following HSCT and report that the skin microbiome is likewise perturbed in HSCT-recipients. Overall bacterial colonization of the skin was decreased after conditioning. Particularly patients that developed acute skin graft-versus-host disease (aGVHD) presented with an overabundance of Staphylococcus spp. In addition, a loss in alpha diversity was indicative of aGVHD development already before disease onset and correlated with disease severity. Further, co-localization of CD45⁺ leukocytes and staphylococci was observed in the skin of aGVHD patients even before disease development and paralleled with upregulated genes required for antigen-presentation in mononuclear phagocytes. Overall, our data reveal disturbances of the skin microbiome as well as cutaneous immune response in HSCT recipients with changes associated with cutaneous aGVHD.

A urinary proteomic landscape of COVID-19 progression identifies signaling pathways and therapeutic options

Signaling pathway alterations in COVID-19 of living humans as well as therapeutic targets of the host proteins are not clear. We analyzed 317 urine proteomes, including 86 COVID-19, 55 pneumonia and 176 healthy controls, and identified specific RNA virus detector protein DDX58/RIG-I only in COVID-19 samples. Comparison of the COVID-19 urinary proteomes with controls revealed major pathway alterations in immunity, metabolism and protein localization. Biomarkers that may stratify severe symptoms from moderate ones suggested that macrophage induced inflammation and thrombolysis may play a critical role in worsening the disease. Hyper activation of the TCA cycle is evident and a macrophage enriched enzyme CLYBL is up regulated in COVID-19 patients. As CLYBL converts the immune modulatory TCA cycle metabolite itaconate through the citramalyl-CoA intermediate to acetyl-CoA, an increase in CLYBL may lead to the depletion of itaconate, limiting its anti-inflammatory function. These observations suggest that supplementation of itaconate and inhibition of CLYBL are possible therapeutic options for treating COVID-19, opening an avenue of modulating host defense as a means of combating SARS-CoV-2 viruses.

In silico analysis reveals interrelation of enriched pathways and genes in type 1 diabetes

Type 1 diabetes (T1D) is a multifactorial, polygenic complex autoimmune disease damaging pancreatic islet β cells. Numerous genes linked to T1D have been discovered through genetical studies, GWAS and polymorphisms. Most genetical studies focused on independent genes while others overemphasized on SNPs. Here, a collective analysis of documented T1D-associated genes was performed using bioinformatics tools. Enriched biological pathways, functions, enrichment clustering, networks and interactomes were analysed. Besides, meta-analyses of T1D-associated genes and T1D-related genes from SNPs were investigated to find common genes, pathways, enrichment and interrelationships. Notable enriched pathways comprised of cytokine-mediated signalling, cytokine production, interferon gamma production, myeloid leukocyte activation, activation of immune response, lymphocyte activation, adaptive immune response, Th17 cell differentiation etc. Enrichment analysis of T1D-associated genes emphasized the role of immune-linked machineries in metabolism, disease progression and aetiology of type 1 diabetes. Interactome analysis revealed overrepresentation of T1D-associated genes compared with T1D-related genes from SNPs. MCODE components highlighted the significance of pathways linked to vitamin D metabolism, signalling by interleukins, toll-like receptors, chemokines, PD-1, NOTCH, antigen processes etc. About 153 genes from MCODE complexes representing enriched pathways of T1D-associated genes and T1D-related genes from SNPs play a crucial role and may be important for further investigations. The information may be valuable for designing precision medicine-based therapeutics.

TET2 deficiency reprograms the germinal center B cell epigenome and silences genes linked to lymphomagenesis

The TET2 DNA hydroxymethyltransferase is frequently disrupted by somatic mutations in diffuse large B cell lymphomas (DLBCLs), a tumor that originates from germinal center (GC) B cells. Here, we show that TET2 deficiency leads to DNA hypermethylation of regulatory elements in GC B cells, associated with silencing of the respective genes. This hypermethylation affects the binding of transcription factors including those involved in exit from the GC reaction and involves pathways such as B cell receptor, antigen presentation, CD40, and others. Normal GC B cells manifest a typical hypomethylation signature, which is caused by AID, the enzyme that mediates somatic hypermutation. However, AID-induced demethylation is markedly impaired in TET2-deficient GC B cells, suggesting that AID epigenetic effects are partially dependent on TET2. Last, we find that TET2 mutant DLBCLs also manifest the aberrant TET2-deficient GC DNA methylation signature, suggesting that this epigenetic pattern is maintained during and contributes to lymphomagenesis.

Coxsackievirus-Induced Proteomic Alterations in Primary Human Islets Provide Insights for the Etiology of Diabetes

Enteroviral infections have been associated with the development of type 1 diabetes (T1D), a chronic inflammatory disease characterized by autoimmune destruction of insulin-producing pancreatic beta cells. Cultured human islets, including the insulin-producing beta cells, can be infected with coxsackievirus B4 (CVB4) and thus are useful for understanding cellular responses to infection. We performed quantitative mass spectrometry analysis on cultured primary human islets infected with CVB4 to identify molecules and pathways altered upon infection. Corresponding uninfected controls were included in the study for comparative protein expression analyses. Proteins were significantly and differentially regulated in human islets challenged with virus compared with their uninfected counterparts. Complementary analyses of gene transcripts in CVB4-infected primary islets over a time course validated the induction of RNA transcripts for many of the proteins that were increased in the proteomics studies. Notably, infection with CVB4 results in a considerable decrease in insulin. Genes/proteins modulated during CVB4 infection also include those involved in activation of immune responses, including type I interferon pathways linked to T1D pathogenesis and with antiviral, cell repair, and inflammatory properties. Our study applies proteomics analyses to cultured human islets challenged with virus and identifies target proteins that could be useful in T1D interventions.

Tapasin facilitation of natural HLA-A and -B allomorphs is strongly influenced by peptide length, depends on stability, and separates closely related allomorphs

Despite an abundance of peptides inside a cell, only a small fraction is ultimately presented by HLA-I on the cell surface. The presented peptides have HLA-I allomorph-specific motifs and are restricted in length. So far, detailed length studies have been limited to few allomorphs. Peptide-HLA-I (pHLA-I) complexes of different allomorphs are qualitatively and quantitatively influenced by tapasin to different degrees, but again, its effect has only been investigated for a small number of HLA-I allomorphs. Although both peptide length and tapasin dependence are known to be important for HLA-I peptide presentation, the relationship between them has never been studied. In this study, we used random peptide libraries from 7- to 13-mers and studied binding in the presence and absence of a recombinant truncated form of tapasin. The data show that HLA-I allomorphs are differentially affected by tapasin, different lengths of peptides generated different amounts of pHLA-I complexes, and HLA-A allomorphs are generally less restricted than HLA-B allomorphs to peptides of the classical length of 8-10 aa. We also demonstrate that tapasin facilitation varies for different peptide lengths, and that the correlation between high degree of tapasin facilitation and low stability is valid for different random peptide mixes of specific lengths. In conclusion, these data show that tapasin has specificity for the combination of peptide length and HLA-I allomorph, and suggest that tapasin promotes formation of pHLA-I complexes with high on and off rates, an important intermediary step in the HLA-I maturation process.

Label-free quantification and shotgun analysis of complex proteomes by one-dimensional SDS-PAGE/NanoLC-MS: evaluation for the large scale analysis of inflammatory human endothelial cells

To perform differential studies of complex protein mixtures, strategies for reproducible and accurate quantification are needed. Here, we evaluated a quantitative proteomic workflow based on nanoLC-MS/MS analysis on an LTQ-Orbitrap-VELOS mass spectrometer and label-free quantification using the MFPaQ software. In such label-free quantitative studies, a compromise has to be found between two requirements: repeatability of sample processing and MS measurements, allowing an accurate quantification, and high proteomic coverage of the sample, allowing quantification of minor species. The latter is generally achieved through sample fractionation, which may induce experimental bias during the label-free comparison of samples processed, and analyzed independently. In this work, we wanted to evaluate the performances of MS intensity-based label-free quantification when a complex protein sample is fractionated by one-dimensional SDS-PAGE. We first tested the efficiency of the analysis without protein fractionation and could achieve quite good quantitative repeatability in single-run analysis (median coefficient of variation of 5%, 99% proteins with coefficient of variation <48%). We show that sample fractionation by one-dimensional SDS-PAGE is associated with a moderate decrease of quantitative measurement repeatability while largely improving the depth of proteomic coverage. We then applied the method for a large scale proteomic study of the human endothelial cell response to inflammatory cytokines, such as TNFα, interferon γ, and IL1β, which allowed us to finely decipher at the proteomic level the biological pathways involved in endothelial cell response to proinflammatory cytokines.

IL-2-engineered nano-APC effectively activates viral antigen-mediated T cell responses from chronic hepatitis B virus-infected patients

Impaired function of virus-specific T cells resulting from virus persistence is one of the major mechanisms underlying the development of chronic hepatitis B viral infection. Previously, we found that IL-2 can restore the effector function of T cells rendered tolerant by Ag persistence. However, systemic administration of IL-2 induces organ pathology and expansion of T regulatory cells. In this study, we show that nano-APC with engineered HLA alleles and IL-2 deliver peptide-MHC complexes, costimulatory molecules, and IL-2 to Ag-responding T cells, resulting in enhanced expression of CD25 and activation of TCR signaling pathways, while suppressing PD-1 expression on viral-responding CD8 T cells from chronic hepatitis B virus patients. The enhanced activation of CD4 and CD8 T cells induced by IL-2-nano-APC was Ag dependent and IL-2-nano-APC did not affect T regulatory cells. At a size of 500 nm, the nano-APC effectively induce immune synapse formation on Ag-specific T cells and accumulate as free particles in the lymphoid organs. These attributes of IL-2-nano-APC or other bioadjuvant-engineered nano-APC have profound implications for their use as a therapeutic strategy in the treatment of chronic hepatitis B virus infection or other chronic viral diseases.

Effect of a tapasin mutant on the assembly of the mouse MHC class I molecule H2-K(d)

Major histocompatibility complex (MHC) class I heavy chain/beta(2)m heterodimers assemble with antigenic peptides through interactions with peptide-loading complex proteins, including tapasin and ERp57. In human cells, a cysteine residue within tapasin (C95) has been shown to form a covalent bond with ERp57. In this study, we focused on the effect of this tapasin amino-acid residue in mouse cells expressing the MHC class I molecule H2-K(d). We showed that a large disulfide-bonded complex was present in the mouse cells that included ERp57, tapasin, and K(d). Furthermore, in mouse cells, unlike human cells, we found that tapasin mutated at C95 can participate in a non-covalent complex with ERp57. Comparison of our findings to earlier findings with a human molecule (HLA-B(*)4402) also revealed that a tapasin C95 mutation has a stronger effect on the maturation and stability of K(d) than HLA-B(*)4402. Overall, our results characterize the influence of this tapasin cysteine residue on the stable surface expression of a mouse MHC class I molecule and reveal differences in tapasin C95 interactions and effects between mouse and human systems.

Influence of the tapasin C terminus on the assembly of MHC class I allotypes

Several endoplasmic reticulum proteins, including tapasin, play an important role in major histocompatibility complex (MHC) class I assembly. In this study, we assessed the influence of the tapasin cytoplasmic tail on three mouse MHC class I allotypes (H2-K(b), -K(d), and -L(d)) and demonstrated that the expression of truncated mouse tapasin in mouse cells resulted in very low K(b), K(d), and L(d) surface expression. The surface expression of K(d) also could not be rescued by human soluble tapasin, suggesting that the surface expression phenotype of the mouse MHC class I molecules in the presence of soluble tapasin was not due to mouse/human differences in tapasin. Notably, soluble mouse tapasin was able to partially rescue HLA-B8 surface expression on human 721.220 cells. Thus, the cytoplasmic tail of tapasin (either mouse or human) has a stronger impact on the surface expression of murine MHC class I molecules on mouse cells than on the expression of HLA-B8 on human cells. A K408W mutation in the mouse tapasin transmembrane/cytoplasmic domain disrupted K(d) folding and release from tapasin, but not interaction with transporter associated with antigen processing (TAP), indicating that the mechanism whereby the tapasin transmembrane/cytoplasmic domain facilitates MHC class I assembly is not limited to TAP stabilization. Our findings indicate that the C terminus of mouse tapasin plays a vital role in enabling murine MHC class I molecules to be expressed at the surface of mouse cells.

Molecular machinations of the MHC-I peptide loading complex

The acquisition of an optimal peptide ligand by MHC class I molecules is crucial for the generation of immunity to viruses and tumors. This process is orchestrated by a molecular machine known as the peptide loading complex (PLC) that consists of specialized and general ER-resident molecules. These proteins collaborate to ensure the loading of an optimal peptide ligand into the antigen binding cleft of class I molecules. The surprising diversity of peptides bound to MHC class I molecules and recapitulation of class I assembly in vitro have provided new insights into the molecular machinations of peptide loading. Coupled with the extraordinary polymorphism of class I molecules and their differential dependence on various components of the PLC for cell surface expression, a picture of peptide loading at the molecular level has recently emerged and will be discussed herein.

HLA-B*2704, an Allotype Associated with Ankylosing Spondylitis, Is Critically Dependent on Transporter Associated with Antigen Processing and Relatively Independent of Tapasin and Immunoproteasome for Maturation, Surface Expression, and T Cell Recognition: Relationship to B*2705 and B*2706

B*2704 is strongly associated to ankylosing spondylitis in Asian populations. It differs from the main HLA-B27 allotype, B*2705, in three amino acid changes. We analyzed the influence of tapasin, TAP, and immunoproteasome induction on maturation, surface expression, and T cell allorecognition of B*2704 and compared some of these features with B*2705 and B*2706, allotypes not associated to disease. In the tapasin-deficient .220 cell line, this chaperone significantly influenced the extent of folding of B*2704 and B*2705, but not their egress from the endoplasmic reticulum. In contrast, B*2706 showed faster folding and no accumulation in the endoplasmic reticulum in the absence of tapasin. Surface expression of B*2704 was more tapasin dependent than B*2705. However, expression of free H chain decreased in the presence of this chaperone for B*2705 but not B*2704, suggesting that more suboptimal ligands were loaded on B*2705 in the absence of tapasin. Despite its influence on surface expression, tapasin had little effect on allorecognition of B*2704. Both surface expression and T cell recognition of B*2704 were critically dependent on TAP, as established with TAP-deficient and TAP-proficient T2 cells. Both immunoproteasome and surface levels of B*2704 were induced by IFN-gamma, but this had little effect on allorecognition. Thus, except for the differential effects of tapasin on surface expression, the tapasin, TAP, and immunoproteasome dependency of B*2704 for maturation, surface expression, and T cell recognition are similar to B*2705, indicating that basic immunological features are shared by the two major HLA-B27 allotypes associated to ankylosing spondylitis in human populations.

The double lysine motif of tapasin is a retrieval signal for retention of unstable MHC class I molecules in the endoplasmic reticulum

Tapasin (tpn), an essential component of the MHC class I (MHC I) loading complex, has a canonical double lysine motif acting as a retrieval signal, which mediates retrograde transport of escaped endoplasmic reticulum (ER) proteins from the Golgi back to the ER. In this study, we mutated tpn with a substitution of the double lysine motif to double alanine (GFP-tpn-aa). This mutation abolished interaction with the coatomer protein complex I coatomer and resulted in accumulation of GFP-tpn-aa in the Golgi compartment, suggesting that the double lysine is important for the retrograde transport of tpn from late secretory compartments to the ER. In association with the increased Golgi distribution, the amount of MHC I exported from the ER to the surface was increased in 721.220 cells transfected with GFP-tpn-aa. However, the expressed MHC I were less stable and had increased turnover rate. Our results suggest that tpn with intact double lysine retrieval signal regulates retrograde transport of unstable MHC I molecules from the Golgi back to the ER to control the quality of MHC I Ag presentation.

Single Nucleotide Polymorphism Discovery in the Avian Tapasin Gene

Tapasin is a transmembrane glycoprotein located in the endoplasmic reticulum. Its function is to assist the assembly of major histocompatibility complex class I molecules. The chicken Tapasin gene includes 8 exons and is localized inside the major histocompatibility complex between the 2 class IIbeta genes. The aim of the current study was the estimation of single nucleotide polymorphism frequency within the avian Tapasin gene. The Tapasin gene sequence from exon 5 to exon 6 was amplified for the chicken, turkey, and pheasant, and sequences of different lengths were obtained. The sequence analysis based on PolyBayes identified 25 putative single nucleotide polymorphism sites when the 3 species were compared. The coding sequences were further translated and analyzed to identify amino acid substitutions. The results indicated that polymorphisms within this region of the gene was mainly observed in the heterozygous state. The level of conservation of the Tapasin gene sequence among species is likely to be related to the functional importance of the gene.

Rhesus macaque MHC class I molecules present HLA-B-like peptides

SIV-infected Indian rhesus macaques (Macaca mulatta) are an important animal model for humans infected with HIV. Understanding macaque (M. mulatta class I (Mamu)) MHC class I-peptide binding facilitates the comparison of SIV- and HIV-specific cellular immune responses. In this study, we characterized the endogenous peptide-binding properties of three Mamu-A (A*02, A*08, A*11) and three Mamu-B (B*01, B*03, B*12) class I molecules. Motif comparisons revealed that five of the six macaque class I molecules (A*02, A*08, A*11, B*01, and B*03) have peptide-binding motifs similar to those of human class I molecules. Of the 65 macaque endogenous peptide ligands that we sequenced by tandem mass spectroscopy, 5 were previously eluted from HLA class I molecules. Nonamers predominated among the individual ligands, and both the motifs and the individual ligands indicated P2, P9, and various ancillary anchors. Interestingly, peptide binding of the Mamu-A and Mamu-B molecules exhibited cross-species peptide-presentation overlap primarily with HLA-B molecules. Indeed, all of the macaque class I molecules appeared HLA-B-like in peptide presentation. Remarkably, the overlap in macaque- and HLA-peptide presentation occurred despite divergent class I peptide-binding grooves. Macaque and human class I differing by up to 42 aa (13-23%) within the alpha-1 and alpha-2 domains, including substantial divergence within specificity pockets A-F, bound the same endogenous peptide. Therefore, endogenous peptide characterization indicates that macaque class I molecules may be the functional equivalents of HLA-B molecules.

Qualitative and Quantitative Differences in Peptides Bound to HLA-B27 in the Presence of Mouse versus Human Tapasin Define a Role for Tapasin as a Size-Dependent Peptide Editor

Tapasin (Tpn) is a chaperone of the endoplasmic reticulum involved in peptide loading to MHC class I proteins. The influence of mouse Tpn (mTpn) on the HLA-B*2705-bound peptide repertoire was analyzed to characterize the species specificity of this chaperone. B*2705 was expressed on Tpn-deficient human 721.220 cells cotransfected with human (hTpn) or mTpn. The heterodimer to beta(2)-microglobulin-free H chain ratio on the cell surface was reduced with mTpn, suggesting lower B*2705 stability. The B*2705-bound peptide repertoires loaded with hTpn or mTpn shared 94-97% identity, although significant differences in peptide amount were observed in 16-17% of the shared ligands. About 3-6% of peptides were bound only with either hTpn or mTpn. Nonamers differentially bound with mTpn had less suitable anchor residues and bound B*2705 less efficiently in vitro than those loaded only with hTpn or shared nonamers. Decamers showed a different pattern: those found only with mTpn had similarly suitable residues as shared decamers and bound B*2705 with high efficiency. Peptides differentially presented by B*2705 on human or mouse cells showed an analogous pattern of residue suitability, suggesting that the effect of mTpn on B*2705 loading is comparable in both cell types. Thus, mTpn has quantitative and qualitative effects on the B*2705-bound peptide repertoire, impairing presentation of some suitable ligands and allowing others with suboptimal anchor residues and lower affinity to be presented. Our results favor a size-dependent peptide editing role of Tpn for HLA-B*2705 that is species-dependent and suboptimally performed, at least for nonamers, by mTpn.

Involvement of the chaperone tapasin in HLA-B44 allelic losses in colorectal tumors

Tumors can exhibit selective allelic losses of HLA class I antigens as part of altered HLA phenotypes. In colorectal tumors, the HLA class I allele most frequently lost is HLA-B44, although the precise mechanism responsible for this loss has not been described to date. From a total of 95 colorectal cryopreserved tumor samples, we selected (by immunohistochemical staining) 13 tumors with HLA-B44-negative expression. Loss of heterozygosity at 6p21.3 was demonstrated to be the cause of the negative expression in 4 cases. In the remaining 9 cases, structural analyses of microdissected tissue samples of the 3 subtypes of HLA-B44 loss in these tumors (B*4402, B*4403 and B*4405) did not reveal any mutations. However, all 3 subtypes of HLA-B44 presented in this study shared a common characteristic: the presence of an aspartic amino acid residue at position 114 in the HLA class I heavy chain. This residue has been described as determining tapasin dependence for the surface expression of these alleles and therefore for antigen presentation. We studied tapasin transcription by RT-PCR in these tumors and found tapasin downregulation in all 9 tumors samples with the HLA-B44-negative phenotype. In contrast, tapasin was normally transcribed in HLA-B44-positive colorectal tumors samples, as well as in 3 HLA-B44-negative laryngeal carcinomas and 1 bladder tumor. Defective tapasin transcription seems to be an alteration responsible for the absence of HLA-B44 expression in colorectal tumors, thus contributing to the generation of tumor immune escape phenotypes.

A Charged Amino Acid Residue in the Transmembrane/Cytoplasmic Region of Tapasin Influences MHC Class I Assembly and Maturation

Tapasin influences the quantity and quality of MHC/peptide complexes at the cell surface; however, little is understood about the structural features that underlie its effects. Because tapasin, MHC class I, and TAP are transmembrane proteins, the tapasin transmembrane/cytoplasmic region has the potential to affect interactions at the endoplasmic reticulum membrane. In this study, we have assessed the influence of a conserved lysine at position 408, which lies in the tapasin transmembrane/cytoplasmic domain. We found that substitutions at position K408 in tapasin affected the expression of MHC class I molecules at the cell surface, and down-regulated tapasin stabilization of TAP. In addition to affecting TAP interaction with tapasin, the substitution of alanine, but not tryptophan, for the lysine at tapasin position 408 increased the amount of tapasin found in association with the open, peptide-free form of the HLA-B8 H chain. Tapasin K408A was also associated with more folded, beta(2)-microglobulin-assembled HLA-B8 molecules than wild-type tapasin. Consistent with our observation of a large pool of tapasin K408A-associated HLA-B8 molecules, the rate at which HLA-B8 migrated from the endoplasmic reticulum was slower in tapasin K408A-expressing cells than in wild-type tapasin-expressing cells. Thus, the alanine substitution at position 408 in tapasin may interfere with the stable acquisition by MHC class I molecules of peptides that are sufficiently optimal to allow MHC class I release from tapasin.

HLA-E surface expression is independent of the availability of HLA class I signal sequence-derived peptides in human tumor cell lines

Human leukocyte antigen (HLA)-E is a nonclassic HLA class I molecule whose expression at the cell surface of tumor cells might allow them to escape T- and natural killer (NK)-cell immune surveillance. In this study, we analyzed HLA-E expression in a panel of human HLA-typed tumor cell lines of different histotypes by flow cytometry with anti-HLA-E monoclonal antibodies and by reverse transcriptase-polymerase chain reaction. Although specific HLA-E transcripts were detected in all cell lines, except in HELA, surface expression was detected at different intensities on seven (23%) of 30 cell lines with higher frequency and intensity among osteosarcoma cell lines. HLA-E-positive tumor cell lines mainly expressed the HLA-A*02 class I allele. Some tumor cell lines demonstrating HLA class I A* or Cw* alleles, which we expected to allow HLA-E surface expression on the basis of reported data on lymphoid cells, instead were HLA-E negative. All tumor cell lines were either tapasin and TAP-1 positive by flow cytometry, except two osteosarcoma cell lines, a finding that suggests an intact assembly machinery for peptide loading. We conclude that the concomitant presence of the appropriate HLA class I alleles with leader sequence-derived peptides and HLA-E heavy chain may not be sufficient to allow HLA-E surface expression in tumor cell lines as opposed to lymphoid cells.

The processing of antigens delivered as DNA vaccines

The ability of DNA vaccines to provide effective immunological protection against infection and tumors depends on their ability to generate good CD4+ and CD8+ T-cell responses. Priming of these responses is a property of dendritic cells (DCs), and so the efficacy of DNA-encoded vaccines is likely to depend on the way in which the antigens they encode are processed by DCs. This processing could either be via the synthesis of the vaccine-encoded antigen by the DCs themselves or via its uptake by DCs following its synthesis in bystander cells that are unable to prime T cells. These different sources of antigen are likely to engage different antigen-processing pathways, which are the subject of this review. Understanding how to access different processing pathways in DCs may ultimately aid the rational development of plasmid-based vaccines to pathogens and to cancer.

Differential Requirement for Tapasin in the Presentation of Leader- and Insulin-Derived Peptide Antigens to Qa-1b-Restricted CTLs

The loading of MHC class I molecules with peptides involves a variety of accessory proteins, including TAP-associated glycoprotein (tapasin), which tethers empty MHC class I molecules to the TAP peptide transporter. We have evaluated the role of tapasin for the assembly of peptides with the class Ib molecule Qa-1b. In normal cells, Qa-1b is predominantly bound by a peptide, the Qa-1 determinant modifier (Qdm), derived from the signal sequence of class Ia molecules. Our results show that tapasin links Qa-1b to the TAP peptide transporter, and that tapasin facilitates the delivery of Qa-1b molecules to the cell surface. Tapasin was also required for the presentation of endogenous Qdm peptides to Qdm-specific, Qa-1b-restricted CTLs. In sharp contrast, tapasin expression was dispensable for the presentation of an insulin peptide to insulin-specific, Qa-1b-restricted CTL isolated from TCR transgenic mice. However, tapasin deficiency significantly impaired the positive selection of these insulin-specific, Qa-1b-restricted transgenic CD8+ T cells. These findings reveal that tapasin plays a differential role in the loading of Qdm and insulin peptides onto Qa-1b molecules, and that tapasin is dispensable for retention of empty Qa-1b molecules in the endoplasmic reticulum, and are consistent with the proposed peptide-editing function of tapasin.

The ABCs of Immunology: Structure and Function of TAP, the Transporter Associated with Antigen Processing

The transporter associated with antigen processing (TAP) is essential for peptide delivery from the cytosol into the lumen of the endoplasmic reticulum (ER), where these peptides are loaded on major histocompatibility complex (MHC) I molecules. Loaded MHC I leave the ER and display their antigenic cargo on the cell surface to cytotoxic T cells. Subsequently, virus-infected or malignantly transformed cells can be eliminated. Here we discuss the structure, function, and mechanism of TAP as a central part of the peptide-loading complex. Furthermore, aspects of virus and tumor escape strategies are presented.

Identification of HLA a*0201 glioblastoma multiforme cell lines for immunotherapy by PCR-SSP and DNA sequencing

Most tumor specific antigens characterized to date are restricted by HLA a*0201, which is the major HLA subtype in many ethnic groups. Cancer cells that express tumor antigens in association with the HLA a*0201 subtype have been shown to be responsive to various immunotherapies. We therefore sought to identify glioma cell lines that also express this HLA subtype and determine whether they had the molecular properties needed for tumor-peptide presentation. The HLA a*0201 allele was identified with PCR using sequence-specific primers followed by DNA sequencing. With this method, we screened 15 glioma cell lines to determine if they were of the HLA a*0201 genotype. Glioma cell lines that express the HLA a*0201 subtype were further studied for the expression of MHC class I and beta-2-microglobulin (beta2m) molecules by flow cytometry, and peptide presentation molecules TAP-1, TAP-2, and tapasin by RT-PCR. We identified six out of fifteen cell lines that were of the HLA a*0201 subtype. These cell lines are U87, T98, U373, U138, CRL2365 and UMN-4. All these six cell lines exhibited high levels of MHC class I and beta2m molecules. In addition, these cell lines all expressed molecules required for peptide presentation as shown by the presence of peptide presentation-related molecules TAP-1, TAP-2 and tapasin. The identification of glioma cell lines that express the HLA a*0201 subtype along with the necessary molecules for peptide-presentation will enable their use in developing new immunotherapeutic approaches for treating brain tumors. The method used to identify HLA a*0201 glioma cells is rapid and inexpensive, and suitable for screening tumor cells.

The Ig-Like Domain of Tapasin Influences Intermolecular Interactions

Presentation of antigenic peptides to T lymphocytes by MHC class I molecules is regulated by events involving multiple endoplasmic reticulum proteins, including tapasin. By studying the effects of substitutions in the tapasin Ig-like domain, we demonstrated that H-2L(d)/tapasin association can be segregated from reconstitution of folded L(d) surface expression. This finding suggests that peptide acquisition by L(d) is influenced by tapasin functions that are independent of L(d) binding. We also found that the presence of a nine-amino acid region in the Ig-like domain of mouse or human tapasin is required for association with L(d), and certain point substitutions in this sequence abrogate human, but not mouse, tapasin association with L(d). These data are consistent with a higher overall affinity between L(d) and mouse tapasin compared with human tapasin. In addition, we found that other point mutations in the same region of the tapasin Ig-like domain affect MHC class I surface expression and Ag presentation. Finally, we showed that the cysteine residues in the Ig-like domain of tapasin influence tapasin's stability, its interaction with the MHC class I H chain, and its stabilization of TAP. Mutagenesis of these cysteines decreases tapasin's electrophoretic mobility, suggesting that these residues form an intramolecular disulfide bond. Taken together, these results reveal a critical role for the tapasin Ig-like domain in tapasin function.

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.

Evasion of the immune system by adenoviruses

Human adenoviruses (Ads) have the ability to transform primary cells, and certain Ads, the subgenus A adenoviruses such as Ad12, induce tumours in immunocompetent rodents. The oncogenic phenotype of the subgenus A adenoviruses is determined by the viral E1A oncogene. In order to generate tumours, Ad12-transformed cells must evade the cellular immune system of the host. Ad12 E1A gene products mediate transcriptional repression of several genes in the major histocompatibility complex (MHC) involved in antigen processing and presentation, resulting in evasion of cytotoxic T lymphocyte (CTL) killing of transformed cells. In this review, the molecular mechanisms of E1A-mediated transcriptional repression of MHC gene expression are described. In addition, evasion of natural killer (NK) cell killing by Ad-transformed cells is also considered.

Species-specific differences in proteasomal processing and tapasin-mediated loading influence peptide presentation by HLA-B27 in murine cells

Expression of HLA-B27 in murine cells has been used to establish animal models for human spondyloarthritis and for antigen presentation studies, but the effects of xenogeneic HLA-B27 expression on peptide presentation are little known. The issue was addressed in this study. HLA-B27-bound peptide repertoires from human and murine cells overlapped by 75-85%, indicating that many endogenous HLA-B27 ligands are generated and presented in both species. Of 20 differentially presented peptides that were sequenced, only 40% arose from obvious inter-species protein polymorphism, suggesting that differences in antigen processing-loading accounted for many species-specific ligands. Digestion of synthetic substrates with human and murine 20 S proteasomes revealed cleavage differences that accounted for or correlated with differential expression of particular peptides. One HLA-B27 ligand found only in human cells was similarly generated in vitro by human and murine proteasomes. Differential presentation correlated with significantly decreased amounts of this ligand in human tapasin-deficient cells reconstituted with murine tapasin, indicating that species-specific interactions between HLA-B27, tapasin, and/or other proteins in the peptide-loading complex influenced presentation of this peptide. Our results indicate that differences in proteasomal specificity and in interactions involving tapasin determine differential processing and presentation of a significant number of HLA-B27 ligands in human and murine cells.

Dissecting the role of peptides in the immune response: theory, practice and the application to vaccine design

Analytical biochemistry and synthetic peptide based chemistry have helped to reveal the pivotal role that peptides play in determining the specificity, magnitude and quality of both humoral (antibody) and cellular (cytotoxic and helper T cell) immune responses. In addition, peptide based technologies are now at the forefront of vaccine design and medical diagnostics. The chemical technologies used to assemble peptides into immunogenic structures have made great strides over the past decade and assembly of highly pure peptides which can be incorporated into high molecular weight species, multimeric and even branched structures together with non-peptidic material is now routine. These structures have a wide range of applications in designer vaccines and diagnostic reagents. Thus the tools of the peptide chemist are exquisitely placed to answer questions about immune recognition and along the way to provide us with new and improved vaccines and diagnostics.

An essential function of tapasin in quality control of HLA-G molecules

Tapasin plays an important role in the quality control of major histocompatibility complex (MHC) class I assembly, but its precise function in this process remains controversial. Whether tapasin participates in the assembly of HLA-G has not been studied. HLA-G, an MHC class Ib molecule that binds a more restricted set of peptides than class Ia molecules, is a particularly interesting molecule, because during assembly, it recycles between the endoplasmic reticulum (ER) and the cis-Golgi until it is loaded with a high affinity peptide. We have taken advantage of this unusual trafficking property of HLA-G and its requirement for high affinity peptides to demonstrate that a critical function of tapasin is to transform class I molecules into a high affinity, peptide-receptive form. In the absence of tapasin, HLA-G molecules cannot bind high affinity peptides, and an abundant supply of peptides cannot overcome the tapasin requirement for high affinity peptide loading. The addition of tapasin renders HLA-G molecules capable of loading high affinity peptides and of transporting to the surface, suggesting that tapasin is a prerequisite for the binding of high-affinity ligands. Interestingly, the "tapasin-dependent" HLA-G molecules are not empty in the absence of tapasin but are in fact associated with suboptimal peptides and continue to recycle between the ER and the cis-Golgi. Together with the finding that empty HLA-G heterodimers are strictly retained in the ER and degraded, our data suggest that MHC class I molecules bind any available peptides to avoid ER-mediated degradation and that the peptides are in turn replaced by higher affinity peptides with the aid of tapasin.

A single polymorphic residue within the peptide-binding cleft of MHC class I molecules determines spectrum of tapasin dependence

Different HLA class I alleles display a distinctive dependence on tapasin for surface expression and Ag presentation. In this study, we show that the tapasin dependence of HLA class I alleles correlates to the nature of the amino acid residues present at the naturally polymorphic position 114. The tapasin dependence of HLA class I alleles bearing different residues at position 114 decreases in the order of acidity, with high tapasin dependence for acidic amino acids (aspartic acid and glutamic acid), moderate dependence for neutral amino acids (asparagine and glutamine), and low dependence for basic amino acids (histidine and arginine). A glutamic acid to histidine substitution at position 114 allows the otherwise tapasin-dependent HLA-B4402 alleles to load high-affinity peptides independently of tapasin and to have surface expression levels comparable to the levels seen in the presence of tapasin. The opposite substitution, histidine to glutamic acid at position 114, is sufficient to change the HLA-B2705 allele from the tapasin-independent to the tapasin-dependent phenotype. Furthermore, analysis of point mutants at position 114 reveals that tapasin plays a principal role in transforming the peptide-binding groove into a high-affinity, peptide-receptive conformation. The natural polymorphisms in HLA class I H chains that selectively affect tapasin-dependent peptide loading provide insights into the functional interaction of tapasin with MHC class I molecules.

Tapasin interacts with the membrane-spanning domains of both TAP subunits and enhances the structural stability of TAP1 x TAP2 Complexes

The transporter associated with antigen processing (TAP) proteins are involved in transport of peptides from the cytosol into the endoplasmic reticulum. Two subunits, TAP1 and TAP2, are necessary and sufficient for peptide binding and peptide translocation across the endoplasmic reticulum membrane. TAP1 and TAP2 contain an N-terminal hydrophobic membrane-spanning region and a C-terminal nucleotide binding domain. Tapasin is an endoplasmic reticulum resident protein that has been found associated with the TAP subunits and shown to increase expression levels of TAP. Here we investigated TAP-tapasin interactions and their effects on TAP function in insect cells. We show tapasin binding to both TAP1 and TAP2 and to the corresponding nucleotide binding domain-exchanged chimeras as well as to a truncated TAP1.TAP2 complex containing just the membrane-spanning regions of TAP1 and TAP2. However, tapasin interactions with either the truncated TAP construct containing just the nucleotide binding domain are not observed. Tapasin is not required for high affinity peptide binding to TAP1.TAP2 complexes, and in fact, the presence of tapasin slightly reduces the affinity of TAP complexes for peptides. However, at near physiological temperatures, both tapasin and nucleotides stabilize the peptide binding site of TAP1.TAP2 complexes against inactivation, and enhanced thermostability of both TAP subunits is observed in the presence of tapasin. The enhanced structural stability of TAP1.TAP2 complexes in the presence of tapasin might explain the observations that tapasin increases TAP protein expression levels in mammalian cells.

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.

Association of tapasin and COPI provides a mechanism for the retrograde transport of major histocompatibility complex (MHC) class I molecules from the Golgi complex to the endoplasmic reticulum

Tapasin is a subunit of the transporter associated with antigen processing (TAP). It associates with the major histocompatibility complex (MHC) class I. We show that tapasin interacts with beta- and gamma-subunits of COPI coatomer. COPI retrieves membrane proteins from the Golgi network back to the endoplasmic reticulum (ER). The COPI subunit-associated tapasin also interacts with MHC class I molecules suggesting that tapasin acts as the cargo receptor for packing MHC class I molecules as cargo proteins into COPI-coated vesicles. In tapasin mutant cells, neither TAP nor MHC class I are detected in association with the COPI coatomer. Interestingly, tapasin-associated MHC class I molecules are antigenic peptide-receptive and detected in both the ER and the Golgi. Our data suggest that tapasin is required for the COPI vesicle-mediated retrograde transport of immature MHC class I molecules from the Golgi network to the ER.

Optimization of the MHC class I peptide cargo is dependent on tapasin

The loading of MHC class I molecules with their peptide cargo is undertaken by a multimolecular peptide loading complex within the endoplasmic reticulum. We show that MHC class I molecules can optimize their peptide repertoire over time and that this process is dependent on tapasin. Optimization of the peptide repertoire is both quantitatively and qualitatively improved by tapasin. The extent of optimization is maximal when MHC class I molecules are allowed to load within the fully assembled peptide loading complex. Finally, we identify a single natural polymorphism (116D>Y) in HLA-B*4402 that permits tapasin-independent loading of HLA-B*4405 (116Y). In the presence of tapasin, the tapasin-independent allele B*4405 (116Y) acquires a repertoire of peptides that is less optimal than the tapasin-dependent allele B*4402 (116D).

Mutant MHC class I molecules define interactions between components of the peptide-loading complex

Class I histocompatibility molecules, consisting of a heavy chain, beta2-microglobulin and peptide, are assembled in the endoplasmic reticulum (ER) with the assistance of several molecular chaperones and accessory proteins. Peptide binding occurs when assembling class I molecules associate with a loading complex consisting of the transporter associated with antigen processing (TAP) peptide transporter, tapasin, ERp57 and calreticulin (CRT)/calnexin. To assess the physical organization of this complex, we generated a series of mutants in the murine H-2Dd heavy chain and assessed their association with components of the complex. Seven mutations, clustered between amino acids 122 and 136 in the heavy chain alpha2 domain plus one mutation at position 222 in the alpha3 domain, resulted in loss of interaction with tapasin. Association with TAP was always lost simultaneously, supporting the view that tapasin acts as an obligatory bridge between class I molecules and TAP. Compared with previous studies on the HLA-A2 molecule, some differences in points of tapasin interaction were observed. Failure of the H-2Dd mutants to bind tapasin resulted in low cell-surface expression and altered intracellular transport. Most mutants retained a substantial degree of peptide loading, consistent with the view that although tapasin may promote peptide binding to class I, it is not required. A surprising observation was that all mutants lacking tapasin interaction retained normal association with CRT. This contrasts with previous observations on other class I molecules and, combined with differences in tapasin interaction, suggests that the organization of the ER peptide-loading complex can vary depending on the specific class I molecule examined.

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.

A region of tapasin that affects L(d) binding and assembly

Tapasin has been shown to stabilize TAP and to link TAP to the MHC class I H chain. Evidence also has been presented that tapasin influences the loading of peptides onto MHC class I. To explore the relationship between the ability of tapasin to bind to TAP and the MHC class I H chain and the ability of tapasin to facilitate class I assembly, we have created novel tapasin mutants and expressed them in 721.220-L(d) cells. One mutant has a deletion of nine amino acid residues (tapasin Delta334-342), and the other has amino acid substitutions at positions 334 and 335. In this report we describe the ability of these mutants to interact with L(d) and their effects on L(d) surface expression. We found that tapasin Delta334-342 was unable to bind to the L(d) H chain, and yet it facilitated L(d) assembly and expression. Tapasin Delta334-342 was able to bind and stabilize TAP, suggesting that TAP stabilization may be important to the assembly of L(d). Tapasin mutant H334F/H335Y, unlike tapasin Delta334-342, bound to L(d). Expression of tapasin H334F/H335Y in 721.220-L(d) reduced the proportion of cell surface open forms of L(d) and retarded the migration of L(d) from the endoplasmic reticulum. In total, our results indicate that the 334-342 region of tapasin influences L(d) assembly and transport.

Major histocompatibility class I folding, assembly, and degradation: a paradigm for two-stage quality control in the endoplasmic reticulum

Protein folding in living cells is a complex process involving many interdependent factors. The primary site for folding of nascent proteins destined for secretion is the endoplasmic reticulum (ER). Several disease states, including cystic fibrosis, are brought about because of irregularities in protein folding. Under normal cellular conditions, "quality control" mechanisms ensure that only correctly folded proteins are exported from the ER, with incorrectly folded or incompletely assembled proteins being degraded. Quality control mechanisms can be divided into two broad processes: (1) Primary quality control involves general mechanisms that are not specific for individual proteins; these monitor the fidelity of nascent protein folding in the ER and mediate the destruction of incompletely folded proteins. (2) Partially folded or assembled proteins may be subject to secondary quality control mechanisms that are protein- or protein-family-specific. Here we use the folding and assembly of major histocompatibility complex (MHC) class I as an example to illustrate the processes of quality control in the ER. MHC class I, a trimeric complex assembled in the ER of virally infected or malignant cells, presents antigenic peptide to cytotoxic T lymphocytes; this mediates cell killing and thereby prevents the spread of infection or malignancy. The folding and assembly of MHC class I is subjected to both primary and secondary quality control mechanisms that lead either to correct folding, assembly, and secretion or to degradation via a proteasome-associated mechanism.

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.

Functional roles of TAP and tapasin in the assembly of M3-N-formylated peptide complexes

H2-M3 is a MHC class Ib molecule with a high propensity to bind N-formylated peptides. Due to the paucity of endogenous Ag, the majority of M3 is retained in the endoplasmic reticulum (ER). Upon addition of exogenous N-formylated peptides, M3 trafficks rapidly to the cell surface. To understand the mechanism underlying Ag presentation by M3, we examined the role of molecular chaperones in M3 assembly, particularly TAP and tapasin. M3-specific CTLs fail to recognize cells isolated from both TAP-deficient (TAP(o)) and tapasin-deficient mice, suggesting that TAP and tapasin are required for M3-restricted Ag presentation. Impaired M3 expression in TAP(o) mice is due to instability of the intracellular pool of M3. Addition of N-formylated peptides to TAP(o) cells stabilizes M3 in the ER and partially restores surface expression. Surprisingly, significant amounts of M3 are retained in the ER in tapasin-deficient mice, even in the presence of N-formylated peptides. Our results define the role of TAP and tapasin in the assembly of M3-peptide complexes. TAP is essential for stabilization of M3 in the ER, whereas tapasin is critical for loading of N-formylated peptides onto the intracellular pool of M3. However, neither TAP nor tapasin is required for ER retention of empty M3.

Allosteric crosstalk between peptide-binding, transport, and ATP hydrolysis of the ABC transporter TAP

The transporter associated with antigen processing (TAP) is essential for intracellular transport of protein fragments into the endoplasmic reticulum for loading of major histocompatibility complex (MHC) class I molecules. On the cell surface, these peptide-MHC complexes are monitored by cytotoxic T lymphocytes. To study the ATP hydrolysis of TAP, we developed an enrichment and reconstitution procedure, by which we fully restored TAP function in proteoliposomes. A TAP-specific ATPase activity was identified that could be stimulated by peptides and blocked by the herpes simplex virus protein ICP47. Strikingly, the peptide-binding motif of TAP directly correlates with the stimulation of the ATPase activity, demonstrating that the initial peptide-binding step is responsible for TAP selectivity. ATP hydrolysis follows Michaelis-Menten kinetics with a maximal velocity V(max) of 2 micromol/min per mg TAP, corresponding to a turnover number of approximately 5 ATP per second. This turnover rate is sufficient to account for the role of TAP in peptide loading of MHC molecules and the overall process of antigen presentation. Interestingly, sterically restricted peptides that bind but are not transported by TAP do not stimulate ATPase activity. These results point to coordinated dialogue between the peptide-binding site, the nucleotide-binding domain, and the translocation site via conformational changes within the TAP complex.

Thermal stability of MHC class I-beta 2-microglobulin peptide complexes in the endoplasmic reticulum is determined by the peptide occupancy of the transporter associated with antigen processing complex

Once MHC class I heavy chain binds beta(2)-microglobulin (beta(2)m) within the endoplasmic reticulum, an assembly complex comprising the class I heterodimer, TAP, TAPasin, calreticulin, and possibly Erp57 is formed before the binding of high affinity peptide. TAP-dependent delivery of high affinity peptide to in vitro translated K(b)beta(2)m complexes within microsomes (TAP(+)/TAPasin(+)) was studied to determine at which point peptide binding becomes resistant to thermal denaturation. It was determined that the thermal stability of K(b)-beta(2)m-peptide complexes depends on the timing of peptide binding to K(b)beta(2)m relative to TAP binding high affinity peptide. Premature exposure of the TAP complex to high affinity peptide before its association with class I heavy chain results in K(b)beta(2)m-peptide-TAP complexes that lose peptide upon exposure to elevated temperature after solubilization away from microsome-associated proteins. These findings suggest that the order in which class I heavy chain associates with endoplasmic reticulum-resident chaperones and peptide determines the stability of K(b)beta(2)m-peptide complexes.

Quantitative and qualitative influences of tapasin on the class I peptide repertoire

Tapasin is critical for efficient loading and surface expression of most HLA class I molecules. The high level surface expression of HLA-B*2705 on tapasin-deficient 721.220 cells allowed the influence of this chaperone on peptide repertoire to be examined. Comparison of peptides bound to HLA-B*2705 expressed on tapasin-deficient and -proficient cells by mass spectrometry revealed an overall reduction in the recovery of B*2705-bound peptides isolated from tapasin-deficient cells despite similar yields of B27 heavy chain and beta(2)-microglobulin. This indicated that a proportion of suboptimal ligands were associated with B27, and they were lost during the purification process. Notwithstanding this failure to recover these suboptimal peptides, there was substantial overlap in the repertoire and biochemical properties of peptides recovered from B27 complexes derived from tapasin-positive and -negative cells. Although many peptides were preferentially or uniquely isolated from B*2705 in tapasin-positive cells, a number of species were preferentially recovered in the absence of tapasin, and some of these peptide ligands have been sequenced. In general, these ligands did not exhibit exceptional binding affinity, and we invoke an argument based on lumenal availability and affinity to explain their tapasin independence. The differential display of peptides in tapasin-negative and -positive cells was also apparent in the reactivity of peptide-sensitive alloreactive CTL raised against tapasin-positive and -negative targets, demonstrating the functional relevance of the biochemical observation of changes in peptide repertoire in the tapasin-deficient APC. Overall, the data reveal that tapasin quantitatively and qualitatively influences ligand selection by class I molecules.