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

The role of tapasin in MHC class I protein trafficking in embryos and T cells

Preimplantation mouse embryos express both classical (class Ia) and nonclassical (class Ib) MHC class I proteins, and yet are not rejected by the maternal immune system. Although the function of the embryonic MHC class Ia proteins is unknown, one MHC class Ib protein, Qa-2, the product of the preimplantation embryo development (Ped) gene, actually enhances reproductive success. Similar in structure to MHC class Ia proteins, Qa-2 protein is a trimer of the alpha (heavy) chain, beta(2) microglobulin and a bound peptide. Studies on the folding, assembly and trafficking of MHC class Ia molecules to the cell surface have revealed this process to be dependent on multiple protein chaperone molecules, but information on the role of chaperone molecules in Qa-2 expression is incomplete. Here, we report the detection of mRNA for four chaperone molecules (TAP1, TAP2, calnexin and tapasin) in preimplantation embryos. We then focused on the role of the MHC-dedicated chaperone, tapasin, on Qa-2 protein expression. First, we demonstrated that tapasin protein is expressed by preimplantation embryos. Then, we used tapasin knockout mice to evaluate the role of tapasin in Qa-2 protein expression on both T cells and preimplantation embryos. We report here that optimal cell surface expression of Qa-2 is dependent on tapasin in both T cells and preimplantation embryos. Identification of the molecules involved in regulation of MHC class I protein expression in early embryos is an important first step in gaining insight into mechanisms of escape of embryos from destruction by the maternal immune system.

Comparative modeling of marsupial MHC class I molecules identifies structural polymorphisms affecting functional motifs

Major histocompatibility complex (MHC) class I molecules are transmembrane glycoproteins that present antigenic peptides to CD8+ T cells and are subsequently important for the initiation of an immune response. In this study novel MHC class I sequences from the tammar wallaby (Macropus eugenii) have been characterized. Analysis and comparative modeling of these and existing marsupial molecules reveals potential functional polymorphisms within peptide-binding grooves, MHC assembly motifs and the T cell receptor recognition interface. In addition, we show that a previously identified marsupial-specific insertion is within a region, which is known as a putative NK cell receptor (Ly49A) binding site in the mouse, suggesting that this site may be functionally active in marsupials. Further, the analysis highlighted differences in structural and sequence based grouping of marsupial MHC class I molecules.

Comparative molecular dynamics analysis of tapasin-dependent and -independent MHC class I alleles

MHC class I molecules load antigenic peptides in the endoplasmic reticulum and present them at the cell surface. Efficiency of peptide loading depends on the class I allele and can involve interaction with tapasin and other proteins of the loading complex. Allele HLA-B*4402 (Asp at position 116) depends on tapasin for efficient peptide loading, whereas HLA-B*4405 (identical to B*4402 except for Tyr116) can efficiently load peptides in the absence of tapasin. Both alleles adopt very similar structures in the presence of the same peptide. Comparative unrestrained molecular dynamics simulations on the alpha(1)/alpha(2) peptide binding domains performed in the presence of bound peptides resulted in structures in close agreement with experiments for both alleles. In the absence of peptides, allele-specific conformational changes occurred in the first segment of the alpha(2)-helix that flanks the peptide C-terminal binding region (F-pocket) and contacts residue 116. This segment is also close to the proposed tapasin contact region. For B*4402, a shift toward an altered F-pocket structure deviating significantly from the bound form was observed. Subsequent free energy simulations on induced F-pocket opening in B*4402 confirmed a conformation that deviated significantly from the bound structure. For B*4405, a free energy minimum close to the bound structure was found. The simulations suggest that B*4405 has a greater tendency to adopt a peptide receptive conformation in the absence of peptide, allowing tapasin-independent peptide loading. A possible role of tapasin could be the stabilization of a peptide-receptive class I conformation for HLA-B*4402 and other tapasin-dependent alleles.

More than one reason to rethink the use of peptides in vaccine design

The use of peptides as therapeutics is experiencing renewed enthusiasm owing to advances in delivery, stability and design. Moreover, there is a growing emphasis on the use of peptides in vaccine design as insights into tissue-specific processing of the immunogenic epitopes of proteins and the discovery of unusually long cytotoxic T-lymphocyte epitopes broaden the range of targets and give clues to enhancing peptide immunogenicity. Peptides can also be synthesized with known post-translational modifications and/or deliberately introduced protease-resistant peptide bonds to regulate their processing independent of tissue-specific proteolysis and to stabilize these compounds in vivo. We discuss the potential of peptide-based vaccines for the treatment of chronic viral diseases and cancer, and review recent developments in the field of peptide-based vaccines.

Major Histocompatibility Complex Class I-ERp57-Tapasin Interactions within the Peptide-loading Complex

The endoplasmic reticulum-located multimolecular peptide-loading complex functions to load optimal peptides onto major histocompatibility complex (MHC) class I molecules for presentation to CD8(+) T lymphocytes. Two oxidoreductases, ERp57 and protein-disulfide isomerase, are known to be components of the peptide-loading complex. Within the peptide-loading complex ERp57 is normally found disulfide-linked to tapasin, through one of its two thioredoxin-like redox motifs. We describe here a novel trimeric complex that disulfide links together MHC class I heavy chain, ERp57 and tapasin, and that is found in association with the transporter associated with antigen processing peptide transporter. The trimeric complex normally represents a small subset of the total ERp57-tapasin pool but can be significantly increased by altering intracellular oxidizing conditions. Direct mutation of a conserved structural cysteine residue implicates an interaction between ERp57 and the MHC class I peptide-binding groove. Taken together, our studies demonstrate for the first time that ERp57 directly interacts with MHC class I molecules within the peptide-loading complex and suggest that ERp57 and protein-disulfide isomerase act in concert to regulate the redox status of MHC class I during antigen presentation.

The Double Role of the Endoplasmic Reticulum Chaperone Tapasin in Peptide Optimization of HLA Class I Molecules

During the assembly of the HLA class I molecules with peptides in the peptide-loading complex, a series of transient interactions are made with ER-resident chaperones. These interactions culminate in the trafficking of the HLA class I molecules to the cell surface and presentation of peptides to CD8(+) T lymphocytes. Within the peptide-loading complex, the glycoprotein tapasin exhibits a relevant function. This immunoglobulin (Ig) superfamily member in the endoplasmic reticulum membrane tethers empty HLA class I molecules to the transporter associated with antigen-processing (TAP) proteins. This review will address the current concepts regarding the double role that tapasin plays in the peptide optimization and surface expression of the HLA class I molecules.

Analysis of interactions in a tapasin/class I complex provides a mechanism for peptide selection

We examined interactions in a soluble tapasin (TPN)/HLA-B*0801 complex to gain mechanistic insights into the functions of TPN. Results show that TPN acts as a chaperone by increasing the ratio of active-to-inactive peptide-deficient HLA-B*0801 molecules in solution. TPN causes peptides to associate and dissociate faster owing to its effect on widening the binding groove of HLA-B*0801 molecules. Our data indicate that a TPN-assisted mechanism of peptide selection relies on disruption of conserved hydrogen bonds at the C-terminal end of the groove. Peptide sequence-dependent interactions along the entire length of the groove also play a role in this mechanism. We suggest that TPN influences presentation of antigenic peptides according to a mechanistically complicated process in which bound candidate peptides that are unable to conformationally disengage TPN from class I molecules are excluded from the repertoire. Overall, these studies unify our understanding of the functions of TPN.

Direct peptide-regulatable interactions between MHC class I molecules and tapasin

Tapasin (Tpn) has been implicated in multiple steps of the MHC class I assembly pathway, but the mechanisms of function remain incompletely understood. Using purified proteins, we could demonstrate direct binding of Tpn to peptide-deficient forms of MHC class I molecules at physiological temperatures. Tpn also bound to M10.5, a pheromone receptor-associated MHC molecule that has an open and empty groove and that shares significant sequence identity with class I sequences. Two types of MHC class I-Tpn complexes were detectable in vitro depending on the input proteins; those depleted in beta(2)m, and those containing beta(2)m. Both were competent for subsequent assembly with peptides, but the latter complexes assembled more rapidly. Thus, the assembly rate of Tpn-associated class I was determined by the conditions under which Tpn-MHC class I complexes were induced. Peptide loading of class I inhibited Tpn-class I-binding interactions, and peptide-depletion enhanced binding. In combination with beta(2)m, certain peptides induced efficient dissociation of preformed Tpn-class I complexes. Together, these studies demonstrate direct Tpn-MHC class I interactions and preferential binding of empty MHC class I by Tpn, and that the Tpn-class I interaction is regulated by both beta(2)m and peptide. In cells, Tpn is likely to be a direct mediator of peptide-regulated binding and release of MHC class I from the TAP complex.

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.

HLA-B44 polymorphisms at position 116 of the heavy chain influence TAP complex binding via an effect on peptide occupancy

A single residue polymorphism distinguishes HLA-B*4402(D116) from HLA-B*4405(Y116), which was suggested to allow HLA-B*4405 to acquire peptides without binding to tapasin-TAP complexes. We show that HLA-B*4405 is not inherently unable to associate with tapasin-TAP complexes. Under conditions of peptide deficiency, both allotypes bound efficiently to TAP and tapasin, and furthermore, random nonamer peptides conferred higher thermostability to HLA-B*4405 than to HLA-B*4402. Correspondingly, under conditions of peptide sufficiency, more rapid peptide-loading, dissociation from TAP complexes, and endoplasmic reticulum exit were observed for HLA-B*4405, whereas HLA-B*4402 showed greater endoplasmic reticulum retention and enhanced tapasin-TAP binding. Together, these studies suggest that position 116 HLA polymorphisms influence peptide occupancy, which in turn determines binding to tapasin and TAP. Relative to HLA-B*4405, inefficient peptide loading of HLA-B*4402 is likely to underlie its stronger tapasin dependence for cell surface expression and thermostability, and its enhanced susceptibility to pathogen interference strategies.

Mechanisms of Disease: the immunopathogenesis of spondyloarthropathies

Research into the pathogenesis of the spondyloarthropathies has examined the role of HLA-B27 and other genes in susceptibility to these diseases. Novel characteristics of HLA-B27 have been discovered, which have allowed hypotheses for an influence of HLA-B27 on disease to be developed that do not reflect its ability to present arthritogenic peptides to CD8(+) T cells. Although a role for CD8(+) T cells has not been excluded, they are not required in the HLA-B27 transgenic rat model, and do not dominate at sites of disease in humans. Studies have also focused on the consequences of the (rather inefficient) intracellular folding of the HLA-B27 heavy chain, the ability of cells to deal with intracellular infection, and their expression of unusual forms of HLA-B27 on cell surfaces (including free heavy chains and dimers). Unusual surface forms of HLA-B27 interact with a different set of receptors from those that recognize conventional class I MHC molecules and thus can be implicated in driving inflammatory responses. Additional candidate susceptibility genes are being identified, either using gene-targeting technology in mice, or genomic screening approaches in humans. In several cases, as with HLA-B27, the evidence suggests that these genes influence the response of the host to bacteria, including pathogens and commensal organisms of the skin and gastrointestinal tract. The concept that spondyloarthropathies are the result of interactions between susceptibility genes, bacteria and the immune system remains a useful model for the pathogenesis of these diseases.

The amino acid at position 97 is involved in folding and surface expression of HLA-B27

HLA-B27 confers susceptibility to ankylosing spondylitis (AS) but the mechanism linking this association remains unknown. Other properties unrelated to its natural role of antigen presenting function may be important in disease pathogenesis. We determined here the impact of N97D substitution on the folding and expression of HLA-B*2704 transfected in the 721.221 cell line. The mutation at position 97 abolishes the surface expression of non-conformational (HC10) and conformational (ME1) forms. The expression of ME1 forms was found to be absent in B*2704 N97D by immunoprecipitation and flow cytometry of fixed and permeabilized cell experiments with the conformation-sensitive ME1 antibody. However, immunoblotting cell lysates with HC10 revealed the presence of unfolded heavy chain (HC) and HC-dimer forms. The impact of the N97D mutation in the exit from the endoplasmic reticulum (ER) was analysed by western blot after endoglycosidase-H treatment, and it was found that B*2704 N97D was retained and accumulated as unfolded molecules. We tested for mutant association with transporter associated with antigen processing (TAP), calnexin (CNX), calreticulin (CLR) and beta2 microglobulin (beta2m). The wild-type B*2704 and N97D mutants were associated with TAP, CNX and CLR, although HC10 forms of mutant N97D interact more weakly with TAP. Only folded molecules of HLA-B*2704 were associated with beta2m. Surprisingly, the peptide-binding assay demonstrated the ability of unfolded N97D molecules to bind high-affinity peptides. It has been suggested that AS may arise because of aberrant folding of HLA-B27 molecules within the ER. Future work must therefore aim to clarify the functional connection between the unfolded protein response pathway in response to the accumulation of HLA-B27 in the ER. This mutant could be useful as a model for the misfolding of HLA-B27.

Impaired assembly of the major histocompatibility complex class I peptide-loading complex in mice deficient in the oxidoreductase ERp57

The thiol-oxidoreductase ERp57 is an integral component of the peptide-loading complex of the major histocompatibility complex (MHC) class I pathway, but its function is unknown. To investigate its function in antigen presentation, we generated ERp57-deficient mice. Death in utero caused by ubiquitous ERp57 deletion was prevented by specific deletion in the B cell compartment. We demonstrate that ERp57 was central for recruitment of MHC class I molecules into the loading complex. In ERp57-deficient cells, we found short-lived interaction of MHC class I molecules with the loading complex. Thus, in the steady state, very few MHC class I molecules were present in the loading complex. Surface H-2K(b)-peptide expression and stability were reduced, and presentation of a model antigen was decreased. Our results indicate that ERp57 does not influence the redox state of MHC class I molecules but is an essential structural component required for stable assembly of the peptide-loading complex.

Recognition of open conformers of classical MHC by chaperones and monoclonal antibodies

There is considerable evidence that the conformation and stability of class I and class II major histocompatibility complex (MHC) proteins is dependent upon high-affinity peptide ligation, but structural data for an empty MHC protein unfortunately is lacking. However, several monoclonal antibodies (mAbs) that specifically detect open MHC conformers have been characterized, and they provide insights into the changes associated with peptide loading and unloading. Here, the structural changes make the argument that certain of these open conformer-specific mAbs recognize analogous MHC segments as the molecular chaperones tapasin and DM. MHC residues located in regions flanking the peptide-terminal anchoring pockets have been implicated in both chaperone and monoclonal antibody binding. Indeed, we propose these regions serve as peptide-binding hinges that are uniquely accessible in open MHC.

The optimization of peptide cargo bound to MHC class I molecules by the peptide-loading complex

Major histocompatibility complex (MHC) class I complexes present peptides from both self and foreign intracellular proteins on the surface of most nucleated cells. The assembled heterotrimeric complexes consist of a polymorphic glycosylated heavy chain, non-polymorphic beta(2) microglobulin, and a peptide of typically nine amino acids in length. Assembly of the class I complexes occurs in the endoplasmic reticulum and is assisted by a number of chaperone molecules. A multimolecular unit termed the peptide-loading complex (PLC) is integral to this process. The PLC contains a peptide transporter (transporter associated with antigen processing), a thiooxido-reductase (ERp57), a glycoprotein chaperone (calreticulin), and tapasin, a class I-specific chaperone. We suggest that class I assembly involves a process of optimization where the peptide cargo of the complex is edited by the PLC. Furthermore, this selective peptide loading is biased toward peptides that have a longer off-rate from the assembled complex. We suggest that tapasin is the key chaperone that directs this action of the PLC with secondary contributions from calreticulin and possibly ERp57. We provide a framework model for how this may operate at the molecular level and draw parallels with the proposed mechanism of action of human leukocyte antigen-DM for MHC class II complex optimization.

Accessory molecules in the assembly of major histocompatibility complex class I/peptide complexes: how essential are they for CD8+ T-cell immune responses?

Assembly of major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum is a highly coordinated process that results in abundant class I/peptide complexes at the cell surface for recognition by CD8(+) T cells and natural killer cells. During the assembly process, a number of chaperones and accessory molecules, such as transporter associated with antigen processing, tapasin, ER60, and calreticulin, assist newly synthesized class I molecules to facilitate loading of antigenic peptides and to optimize the repertoire of surface class I/peptide complexes. This review focuses on the relative importance of these accessory molecules for CD8(+) T-cell responses in vivo and discusses reasons that may help explain why some CD8(+) T-cell responses develop normally in mice deficient in components of class I assembly, despite impaired antigen presentation.

Evidence for MR1 antigen presentation to mucosal-associated invariant T cells

The novel class Ib molecule MR1 is highly conserved in mammals, particularly in its alpha1/alpha2 domains. Recent studies demonstrated that MR1 expression is required for development and expansion of a small population of T cells expressing an invariant T cell receptor (TCR) alpha chain called mucosal-associated invariant T (MAIT) cells. Despite these intriguing properties it has been difficult to determine whether MR1 expression and MAIT cell recognition is ligand-dependent. To address these outstanding questions, monoclonal antibodies were produced in MR1 knock-out mice immunized with recombinant MR1 protein, and a series of MR1 mutations were generated at sites previously shown to disrupt the ability of class Ia molecules to bind peptide or TCR. Here we show that 1) MR1 molecules are detected by monoclonal antibodies in either an open or folded conformation that correlates precisely with peptide-induced conformational changes in class Ia molecules, 2) only the folded MR1 conformer activated 2/2 MAIT hybridoma cells tested, 3) the pattern of MAIT cell activation by the MR1 mutants implies the MR1/TCR orientation is strikingly similar to published major histocompatibility complex/alphabetaTCR engagements, 4) all the MR1 mutations tested and found to severely reduce surface expression of folded molecules were located in the putative ligand binding groove, and 5) certain groove mutants of MR1 that are highly expressed on the cell surface disrupt MAIT cell activation. These combined data strongly support the conclusion that MR1 has an antigen presentation function.

Biochemical and structural impact of natural polymorphism in the HLA-A3 superfamily

Class I alleles of the HLA-A3 superfamily (-A*0301, -A*1101, -A*3101, -A*3301, and -Aw*6801) share largely overlapping peptide repertoires. Cross-reactive T cell responses between HLA-A3-like molecule/peptide complexes have been demonstrated in vitro and during natural diseases. In spite of this immune relatedness, HLA-A3-like molecules exhibit noticeable differences in their antigen-selecting and -presenting properties. Identifying molecular and structural features responsible for these differences is important for understanding how natural polymorphism leads to functional divergence within the HLA-A3 superfamily. Towards this goal, we used an approach that combines thermal stability data on recombinant, soluble HLA-A3-like molecules complexed with a nonamer and decamer HIV-1 peptide, together with a detailed structural analysis of these HLA-A3-like molecule/peptide complexes based on crystal and molecular model structures. Our studies revealed the importance of residues 9 and 67 for modulating peptide selection within the B pocket; of residue 97 for modulating peptide selection within the F pocket interdependently with the presence (or absence) of a middle, secondary anchor residue; and of residues 70, 73, 97, 152, and 156 for modulating peptide presentation in the central region of the groove that leads to altered antigenic surfaces. Overall, our detailed assessment of the biochemical and structural impact of natural polymorphism within the HLA-A3 superfamily has permitted to understand how HLA-A3-like molecules differ at the level of their primary and secondary anchor pockets causing fine differences in their peptide-selecting and -presenting properties. A better understanding of the molecular immunological properties of HLA-A3-like molecules is significantly important for the rationale design of broad peptide-based vaccines.

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.

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.

Spondylarthritis-associated and non–spondylarthritis-associated B27 subtypes differ in their dependence upon tapasin for surface expression and their incorporation into the peptide loading complex

OBJECTIVE

B27 subtypes associated with susceptibility to ankylosing spondylitis (AS), and those reported not to be associated with AS, are found to differ in the amino acids that are known in other HLA class I molecules to alter the requirements for tapasin and incorporation into the peptide loading complex. The purpose of this study was to examine the behavior of B*2704 and B*2705 in comparison with B*2706 and B*2709 during early events in HLA class I antigen expression, and determine if their behavior correlates with disease association.

METHODS

Cell lines with nonfunctional tapasin were transiently transfected with different B27 subtypes and their site-directed mutants, and surface expression analyzed by flow cytometry. The association with the peptide loading complex was determined by immunoprecipitation of heterodimeric transporter-associated peptide and analysis of coprecipitated B27.

RESULTS

Amino acids at positions 114, 116, and 152 in the different B27 subtypes were shown to perform key roles in defining a requirement for interaction with tapasin. Not all disease-associated alleles were expressed optimally in the absence of tapasin; furthermore, dependence on tapasin for cell surface expression did not correlate with disease association. Although B*2706, which is not associated with disease, exhibited a number of properties different from those of the disease-associated subtypes, these properties were not displayed by the non-disease-associated allele B*2709.

CONCLUSION

These results indicate that the ability to exhibit optimal cell surface expression in the absence of tapasin is not a prerequisite for susceptibility to AS.

Generation of a functional, soluble tapasin protein from an alternatively spliced mRNA

The loading of newly synthesised MHC class I molecules (MHCI) with peptides requires the involvement of several endoplasmic reticulum (ER)-resident cofactors including calnexin, calreticulin, transporter associated with antigen processing, ERp57 and tapasin. In the absence of tapasin, MHC I complexes are loaded with suboptimal peptides and their recognition by cytotoxic T cells raised to high-affinity, immunodominant peptide epitopes is impaired. Here, we describe the cloning and functional assessment of an alternative spliced form of tapasin. From the EST database, we obtained a partially spliced tapasin cDNA that retained introns 4-6. When transfected into the tapasin-deficient cell line 0.220, the cDNA produced an alternatively spliced tapasin transcript that contained intron 5 (74 bp). This introduced a new stop codon that terminated translation immediately before the putative transmembrane domain and led to a tapasin molecule containing the lumenal domain plus 8 extra novel amino acids at its C-terminus. This molecule promoted peptide loading of HLA-B5 in 0.220 cell line, and restored normal HLA-B5 surface expression. However, the peptides loaded onto HLA-B5 were suboptimal compared to those loaded onto HLA-B5 in the presence of wild-type tapasin.

TAP genes and immunity

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

The MHC-encoded class I molecule, H-2Kk, demonstrates distinct requirements of assembly factors for cell surface expression: roles of TAP, Tapasin and β2-microglobulin

Major histocompatibility complex encoded class I (MHC-I) molecules display peptides derived from endogenous proteins for perusal by CD8+ T lymphocytes. H6, a mouse hepatoma cell line, expresses low levels of surface H-2Dd but not H-2Kk. Surface H-2Dd molecules are unstable and their levels, but not H-2Kk, are induced at 22 degrees C. Immunoprecipitation experiments revealed that H-2Kk, H-2Dd and beta2-microglobulin (beta2m) are expressed intracellularly; however no conformed MHC-I are present. Transcriptional profiling of factors required for MHC-I assembly demonstrated greatly reduced levels of the Transporter associated with antigen processing (Tap)2 subunit. The role of key assembly molecules in the MHC-I pathway was investigated by ectopic expression studies. Overexpression of beta2m enhanced surface H-2Dd, but not H-2Kk, levels whereas overexpression of TAP2 rescued surface H-2Kk, but not H-2Dd, levels. Interestingly, Tapasin plays a dual role: first, in quality control by reducing the induced surface expression of TAP2-mediated H-2Kk and beta2m-mediated H-2Dd levels. Secondly, Tapasin overexpression increases Tap2 transcripts and cooperates with TAPl or human beta2m to enhance surface H-2Kk expression; this synergy is TAP-dependent as demonstrated by infected cell protein 47 (ICP47) inhibition studies. Unlike the well studied H-2 MHC-I alleles, H-2Kb, H-2Db, H-2Kd and H-2Dd, a functional TAP is "essential" for H-2Kk cell surface expression.

Natural HLA class I polymorphism controls the pathway of antigen presentation and susceptibility to viral evasion

HLA class I polymorphism creates diversity in epitope specificity and T cell repertoire. We show that HLA polymorphism also controls the choice of Ag presentation pathway. A single amino acid polymorphism that distinguishes HLA-B*4402 (Asp116) from B*4405 (Tyr116) permits B*4405 to constitutively acquire peptides without any detectable incorporation into the transporter associated with Ag presentation (TAP)-associated peptide loading complex even under conditions of extreme peptide starvation. This mode of peptide capture is less susceptible to viral interference than the conventional loading pathway used by HLA-B*4402 that involves assembly of class I molecules within the peptide loading complex. Thus, B*4402 and B*4405 are at opposite extremes of a natural spectrum in HLA class I dependence on the PLC for Ag presentation. These findings unveil a new layer of MHC polymorphism that affects the generic pathway of Ag loading, revealing an unsuspected evolutionary trade-off in selection for optimal HLA class I loading versus effective pathogen evasion.

HLA-B27: a registry of constitutive peptide ligands

The very strong association of human leukocyte antigen (HLA)-B27 with spondyloarthritis might be related to its peptide-presenting properties. The natural polymorphism of this molecule influences both peptide specificity and disease susceptibility. In this study, we present a comprehensive compilation of known natural ligands of HLA-B27 arising from endogenous proteins of human cells, together with a statistical assessment of residue usage among constitutive peptide repertoires of multiple HLA-B27 subtypes. This analysis provides evidence that every peptide position, including "non-anchor" ones, may be subjected to selection on the basis of its contribution to HLA-B27 binding and also allows a quantization of residue preferences at known anchor positions. The present registry is intended as a basis on which to build up reliable criteria to assess the effect of HLA-B27 polymorphism on peptide presentation, for T-cell epitope predictions, and for molecular mimicry studies.

Competition for Access to the Rat Major Histocompatibility Complex Class I Peptide-loading Complex Reveals Optimization of Peptide Cargo in the Absence of Transporter Associated with Antigen Processing (TAP) Association

Major histocompatibility complex (MHC) class I molecules load peptides in the endoplasmic reticulum in a process during which the peptide cargo is normally optimized in favor of stable MHC-peptide interactions. A dynamic multimolecular assembly termed the peptide-loading complex (PLC) participates in this process and is composed of MHC class I molecules, calreticulin, ERp57, and tapasin bound to the transporter associated with antigen processing (TAP) peptide transporter. We have exploited the observation that the rat MHC class I allele RT1-Aa, when expressed in the rat C58 thymoma cell line, effectively competes and prevents the endogenous RT1-Au molecule from associating with TAP. However, stable RT1-Au molecules are assembled efficiently in competition with RT1-Aa, demonstrating that cargo optimization can occur in the absence of TAP association. Defined mutants of RT1-Aa, which do not allow formation of the PLC, fail to become thermostable in C58 cells. Wild-type RT1-Aa, which does allow PLC formation, also fails to become thermostable in this cell line, which carries the rat TAPB transporter that supplies peptides incompatible for RT1-Aa binding. Full optimization of RT1-Aa requires the presence of the TAP2A allele, which is capable of supplying suitable peptides. Thus, formation of the PLC alone is not sufficient for optimization of the MHC class I peptide cargo.

Allelic variation in class I K gene as candidate for a second component of MHC-linked susceptibility to type 1 diabetes in non-obese diabetic mice

AIMS/HYPOTHESIS

Recent studies have revealed that MHC-linked susceptibility to Type 1 diabetes is determined by multiple components. In the non-obese diabetic (NOD) mouse, a second component (Idd16) has been mapped to a region adjacent to, but distinct from Idd1 in the class II region. In this study, we investigated the class I K gene as a candidate gene for Idd16.

METHODS

We determined the genomic sequences of the class I K gene as well as the reactivity of K molecules with monoclonal antibodies in the NOD mouse, the Cataract Shionogi (CTS) mouse, and the NOD.CTS-H-2 congenic strain, which possesses a resistance allele to Type 1 diabetes at the Idd16 on the NOD genetic background genes.

RESULTS

While the K sequence of the NOD mouse was identical to that of Kd type, ten nucleotide substitutions were identified in the CTS mouse compared with the NOD mouse. Of these, three were in exon 4, giving two amino acid substitutions, which were identical to those seen in KK type. These characteristics were retained in the NOD.CTS-H-2 congenic strain, which had a lower incidence and delayed onset of Type 1 diabetes owing to a resistance allele at Idd16. Lymphocytes from NOD.CTS-H2 congenic mice reacted with anti-Kd and anti-Kk monoclonal antibodies, reflecting the unique sequence of the K gene. The nucleotide sequence of the K gene in the non-obese non-diabetic (NON) mouse was also unique, consisting of a combination of Kk- and Kb-like sequences.

CONCLUSIONS/INTERPRETATION

These data suggest that H2-K is unique in CTS and NON mice, and that allelic variation of the class I K gene may be responsible for Idd16.

Human cytomegalovirus inhibits tapasin-dependent peptide loading and optimization of the MHC class I peptide cargo for immune evasion

The immune evasion protein US3 of human cytomegalovirus binds to and arrests MHC class I molecules in the endoplasmic reticulum (ER). However, substantial amounts of class I molecules still escape US3-mediated ER retention, suggesting that not all class I alleles are affected equally by US3. Here, we identify tapasin inhibition as the mechanism of MHC retention by US3. US3 directly binds tapasin and inhibits tapasin-dependent peptide loading, thereby preventing the optimization of the peptide repertoire presented by class I molecules. Due to the allelic specificity of tapasin toward class I molecules, US3 affects only class I alleles that are dependent on tapasin for peptide loading and surface expression. Accordingly, tapasin-independent class I alleles selectively escape to the cell surface.

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.