The nature of the MHC class I peptide loading complex

Lipid protein interactions: the assembly of CD1d1 with cellular phospholipids occurs in the endoplasmic reticulum

CD1d1 is a member of a family of lipid Ag-presenting molecules. The cellular ligands associated with CD1d1 were isolated and characterized by biochemical means as an approach to elucidate the mechanism by which CD1 molecules assemble in vivo. Natural ligands of mouse CD1d1 included cellular phosphatidylinositol and phosphatidylinositol-glycans that are synthesized in the endoplasmic reticulum. Further biochemical data revealed that the two CD1d1 mutants, one defective in recycling from-and-to the plasma membrane and the other in efficiently negotiating the secretory pathway, associated with phosphatidylinositol. Thus phosphatidylinositol associated with CD1d1 in the early secretory pathway. Phosphatidylinositol also associated with CD1d1 in Pig-A-deficient cells that are defective in the first glycosylation step of glycosylphosphatidylinositol biosynthesis. Moreover, cellular phosphatidylinositol-glycans are not Valpha14Jalpha15 natural T cell Ags. Therefore, we predict that cellular lipids occlude the hydrophobic Ag-binding groove of CD1 during assembly until they are exchanged for a glycolipid Ag(s) within the recycling compartment for display on the plasma membrane. In this manner, cellular lipids might play a chaperone-like role in the assembly of CD1d1 in vivo, akin to the function of invariant chain in MHC class II assembly.

Down-regulation of MHC class I by bovine papillomavirus E5 oncoproteins

The papillomavirus E5 protein is localized in the endoplasmic reticulum (ER) and Golgi apparatus (GA) of the host cell. Transformed bovine fibroblasts expressing bovine papillomavirus (BPV) E5 are highly vacuolated and have a much enlarged, distorted and fragmented GA. Major histocompatibility complex class I (MHC I) is processed and transported to the cell surface through the GA. Given the cellular localization of E5 in the GA and the morphologically abnormal GA, we investigated the expression of MHC I in cells transformed by E5 from BPV-1 and BPV-4. Two cell lines were used: bovine cells that also express E6, E7 and activated ras, and NIH3T3 cells that express only E5. In addition, PalF cells acutely infected with a recombinant retrovirus expressing E5 were also examined. In contrast to non-transformed normal cells, or transformed cells expressing other papillomavirus proteins, cells expressing E5 do not express MHC I on their surface, but retain it intracellularly, independently of the presence of other viral or cellular oncogenes, or of whether the cells are long-term transformants or acutely infected. We conclude that expression of E5 prevents expression of MHC I to the cell surface and causes its retention within the cell. In addition, lower amounts of total MHC I heavy chain and of heavy chain RNA are detected in E5-transformed cells than in control cells. As surface expression of another glycosylated membrane protein, the transferrin receptor, is not affected, it appears that E5 targets MHC I with at least a degree of specificity. In papillomavirus lesions this effect would have important implications for antigen presentation by, and immunosurveillance of, virally infected cells.

Accessory proteins that control the assembly of MHC molecules with peptides

The stable assembly of Major Histocompatibility Complex (MHC) molecules with peptides is controlled by a number of cofactors, including proteins with general housekeeping functions and proteins with dedicated functions in MHC assembly. Recent work in my laboratory has focused on two chaperones, tapasin (tpn) and DM, that play critical roles in the loading of peptides onto MHC class I and MHC class II molecules, respectively. Tapasin is a transmembrane protein that tethers empty class I molecules in the endoplasmic reticulum to the transporter associated with antigen processing. DM is a peptide exchange factor that binds with empty and peptide-loaded class II molecules in endosomal and lysosomal compartments. Although a number of different functions for tapasin and DM have been proposed, emerging evidence suggests that both of these chaperones retain unstable MHC molecules in peptide-loading compartments until they bind with high-affinity peptides. These cofactors therefore promote the surface expression of long-lived MHC-peptide complexes.

Disulfide bond isomerization and the assembly of MHC class I-peptide complexes

The presence of a disulfide bond inside the peptide binding groove of MHC class I molecules and of the thiol oxidoreductase ERp57 in the class I loading complex suggests that disulfide bond isomerization may play a role in peptide loading. Here we show that ERp57 and tapasin are disulfide linked inside the loading complex. Mutagenesis of cysteine 95 in tapasin not only abolishes formation of the ERp57-tapasin bond but also prevents complete oxidation of the class I heavy chain in the loading complex. The resulting MHC class I-beta2m heterodimers are poorly loaded with high-affinity peptides in the ER but nevertheless escape to the cell surface where they are unstable. These findings suggest a role for disulfide bond isomerization in tapasin-mediated peptide loading.

The transporter associated with antigen processing: function and implications in human diseases

The adaptive immune systems have evolved to protect the organism against pathogens encountering the host. Extracellular occurring viruses or bacteria are mainly bound by antibodies from the humoral branch of the immune response, whereas infected or malignant cells are identified and eliminated by the cellular immune system. To enable the recognition, proteins are cleaved into peptides in the cytosol and are presented on the cell surface by class I molecules of the major histocompatibility complex (MHC). The transport of the antigenic peptides into the lumen of the endoplasmic reticulum (ER) and loading onto the MHC class I molecules is an essential process for the presentation to cytotoxic T lymphocytes. The delivery of these peptides is performed by the transporter associated with antigen processing (TAP). TAP is a heterodimer of TAP1 and TAP2, each subunit containing transmembrane domains and an ATP-binding motif. Sequence homology analysis revealed that TAP belongs to the superfamily of ATP-binding cassette transporters. Loss of TAP function leads to a loss of cell surface expression of MHC class I molecules. This may be a strategy for tumors and virus-infected cells to escape immune surveillance. Structure and function of the TAP complex as well as the implications of loss or downregulation of TAP is the topic of this review.

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.

The quantity of naturally processed peptides stably bound by HLA-A*0201 is significantly reduced in the absence of tapasin

Tapasin plays a critical role in promoting peptide binding by major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum. In its absence, cell surface expression of most allotypes is significantly reduced. Two exceptions are HLA-A*0201 and HLA-B*2705. In this study, the repertoire of peptides bound endogenously by these allotypes in the absence of tapasin was examined and stability of the HLA class I/peptide complexes assessed. Similar quantities of peptides were recovered from B*2705 complexes expressed in the absence and presence of tapasin and the composition of the peptide pools were not radically different. However, the stability of B*2705 molecules expressed at the surface of tapasin-deficient cells was found to be reduced which suggests there are subtle changes to the peptide repertoire. The impact of the absence of tapasin was more dramatic for A*0201. Although equivalent levels of cell surface A*0201 are expressed in the presence and absence of tapasin, very little A*0201 glycoprotein was recovered from tapasin-deficient cells suggesting the complexes readily dissociate. Consistent with reduced stability, A*0201 complexes were found to be rapidly lost from the surface of tapasin-deficient cells. Analysis of the small quantity of endogenously bound peptides recovered from A*0201 expressed in the absence of tapasin revealed a complex mixture typical of A*0201 molecules expressed in normal cells. Therefore these molecules are unable to exploit the alternative supply of TAP-independent A*0201-binding peptides present in the endoplasmic reticulum. Loading of A*0201 with peptides from both TAP-dependent and TAP-independent sources is significantly compromised without tapasin.

Stability of surface H-2K(b), H-2D(b), and peptide-receptive H-2K(b) on splenocytes

We have used flow cytometry to study the stability and peptide-binding capability of MHC class I (MHC-I) on the surface of normal C57BL/6 mouse T lymphoblasts. The MHC-I molecules on each cell are nearly evenly divided into two populations with mean half-life values of approximately 1 and 20 h. Our observations suggest that members of the later contain peptide bound with medium to high affinity. Cell surface MHC-I molecules capable of binding exogenous peptide (thus, "peptide-receptive") belong almost entirely to the less stable population. Before exogenous peptide can bind, MHC-I must undergo a change, probably loss of a very low affinity peptide. For MHC-I-K(b), we found that the maximum rate for binding of exogenous peptide corresponds to a t(1/2) value of 12 min. To maintain the 50:50 steady-state distribution of long- vs short-lived MHC-I molecules on the cell surface, approximately 20 short-lived molecules must be exported to the cell surface for each long-lived molecule.

The murine cytomegalovirus immune evasion protein m4/gp34 forms biochemically distinct complexes with class I MHC at the cell surface and in a pre-Golgi compartment

We have recently demonstrated that the murine CMV (MCMV) gene m4 is an immune evasion gene that protects MCMV-infected targets from some virus-specific CTL clones. m4 encodes m4/gp34, a 34-kDa glycoprotein that binds to major histocompatibility complex class I in the endoplasmic reticulum and forms a detergent-stable complex that is exported to the surface of the cell. To investigate how m4/gp34 promotes CTL evasion, we analyzed the assembly and export of m4/gp34-K(b) complexes. We found that 50-70% of K(b) exported over the course of MCMV infection was m4/gp34 associated. Because these complexes are present at the cell surface, it is possible that m4 mediates CTL evasion by interfering with contact between class I and receptors on the T cell. In addition, we found that K(b) retained by the MCMV immune evasion gene m152 formed a novel type of complex with Endo H-sensitive m4/gp34; these complexes are distinguished from the exported complexes by being stable in 1% digitonin and unstable in 1% Nonidet P-40. Because this association occurs in a pre-Golgi compartment, m4/gp34 might also interfere with Ag presentation by affecting some aspect of class I assembly, such as peptide loading. Although m4/gp34 requires beta(2)-microglobulin to bind class I, there was no significant binding of m4/gp34 to beta(2)-microglobulin in the absence of class I H chain, demonstrating that m4/gp34 forms Nonidet P-40-stable complexes specifically with folded conformations of class I. We conclude that m4/gp34 promotes immune evasion by a novel mechanism involving altered assembly and/or T cell recognition of class I molecules.

Toxoplasma gondii and MHC-restricted antigen presentation: on degradation, transport and modulation

Resistance against Toxoplasma gondii, an obligate intracellular protozoan parasite surrounded by a parasitophorous vacuolar membrane, is mediated by the cellular arm of the immune system, namely CD8+ and CD4+ T cells. Thus, priming and activation of these cells by presentation of antigenic peptides in the context of major histocompatibility complex class I and class II molecules have to take place. This is despite the fact that the vacuolar membrane avoids fusion with the endocytic compartment and acts like a molecular sieve, restricting passive diffusion of larger molecules. This raises several cell biological and immunological questions which will be discussed in this review in the context of our current knowledge about major histocompatibility complex-restricted antigen presentation in other systems: (1) By which pathways are parasite-derived antigens presented to T cells? (2) Has the parasite evolved mechanisms to interfere with major histocompatibility complex-restricted antigen presentation in order to avoid immune recognition? (3) To what extent and by which mechanism is antigenic material, originating from the parasite, able to pass through the vacuolar membrane into the cytosol of the infected cell and is it then accessible to the antigen presentation machinery of the infected cell? (4) What are the actual antigen-presenting cells which prime specific T cells in lymphoid organs? An understanding of these mechanisms will not only provide new insights into the pathogenesis of Toxoplasma gondii and possibly other intravacuolar parasites, but will also improve vaccination strategies.

Efficient assembly of recombinant major histocompatibility complex class I molecules with preformed disulfide bonds

The expression of major histocompatibility class I (MHC-I) crucially depends upon the binding of appropriate peptides. MHC-I from natural sources are therefore always preoccupied with peptides complicating their purification and analysis. Here, we present an efficient solution to this problem. Recombinant MHC-I heavy chains were produced in Escherichia coli and subsequently purified under denaturing conditions. In contrast to common practice, the molecules were not reduced during the purification. The oxidized MHC-I heavy chain isoforms were highly active with respect to peptide binding. This suggests that de novo folding of denatured MHC-I molecules proceed efficiently if directed by preformed disulfide bond(s). Importantly, these molecules express serological epitopes and stain specific T cells; and they bind peptides specifically. Several denatured MHC-I heavy chains were analyzed and shown to be of a quality, which allowed quantitative analysis of peptide binding. The analysis of the specificity of the several hundred human MHC haplotypes, should benefit considerably from the availability of pre-oxidized recombinant MHC-I.

Structure of the ABC ATPase domain of human TAP1, the transporter associated with antigen processing

The transporter associated with antigen processing (TAP) is an ABC transporter formed of two subunits, TAP1 and TAP2, each of which has an N-terminal membrane-spanning domain and a C-terminal ABC ATPase domain. We report the structure of the C-terminal ABC ATPase domain of TAP1 (cTAP1) bound to ADP. cTAP1 forms an L-shaped molecule with two domains, a RecA-like domain and a small alpha-helical domain. The diphosphate group of ADP interacts with the P-loop as expected. Residues thought to be involved in gamma-phosphate binding and hydrolysis show flexibility in the ADP-bound state as evidenced by their high B-factors. Comparisons of cTAP1 with other ABC ATPases from the ABC transporter family as well as ABC ATPases involved in DNA maintenance and repair reveal key regions and residues specific to each family. Three ATPase subfamilies are identified which have distinct adenosine recognition motifs, as well as distinct subdomains that may be specific to the different functions of each subfamily. Differences between TAP1 and TAP2 in the nucleotide-binding site may be related to the observed asymmetry during peptide transport.

Efficient MHC class I-independent amino-terminal trimming of epitope precursor peptides in the endoplasmic reticulum

MHC class I ligands are produced mainly by proteasomal proteolysis, in conjunction with an unknown extent of trimming by peptidases. Trimming of precursor peptides in the endoplasmic reticulum, a process postulated to be class I dependent, may substantially enhance the efficiency of antigen presentation. However, monitoring of luminal peptide processing has not so far been possible. Here we show that several precursor peptides with amino-terminal extensions are rapidly converted to HLA-A2 ligands by one or several highly efficient metallo-peptidases found on the outer surface of, but also within, microsomes. Surprisingly, luminal trimming is fully active in HLA class I- or TAP-deficient microsomes and precedes peptide association with HLA class I molecules. Trimmed peptides are rapidly depleted from, and become undetectable in, microsomes lacking the restricting class I molecules.

Designing peptide vaccines for cellular cross-presentation

Synthetic peptides are safe and relatively cheap vaccine components. However, the efficiency of peptide vaccines is limited by peptide interaction with non-professional antigen-presenting cells, which may hamper induction of productive T-cell responses. This paper argues that peptide vaccines should be modified for exclusive uptake by cells with the capacity to prime T-cell responses. Moreover, design of peptide vaccines should take intracellular antigen processing into account and exploit cellular mechanisms of proteolysis, transport and HLA class I assembly of antigenic peptides to enhance efficiency of T-cell priming and stimulation.

Identification of novel HLA-B27 ligands derived from polymorphic regions of its own or other class I molecules based on direct generation by 20 S proteasome

HLA-B27 is strongly associated with ankylosing spondylitis. Natural HLA-B27 ligands derived from polymorphic regions of its own or other class I HLA molecules might be involved in autoimmunity or provide diversity among HLA-B27-bound peptide repertoires from individuals. In particular, an 11-mer spanning HLA-B27 residues 169-179 is a natural HLA-B27 ligand with homology to proteins from Gram-negative bacteria. Proteasomal digestion of synthetic substrates demonstrated direct generation of the B27-(169-179) ligand. Cleavage after residue 181 generated a B27-(169-181) 13-mer that was subsequently found as a natural ligand of B*2705 and B*2704. Its binding to HLA-B27 subtypes in vivo correlated better than B27-(169-179) with association to spondyloarthropathy. Proteasomal cleavage generated also a peptide spanning B*2705 residues 150-158. This region is polymorphic among HLA-B27 subtypes and class I HLA antigens. The peptide was a natural B*2704 ligand. Since this subtype differs from B*2705 at residue 152, it was concluded that the ligand arose from HLA-B*3503, synthesized in the cells used as a source for B*2704-bound peptides. Thus, polymorphic HLA-B27 ligands derived from HLA-B27 or other class I molecules are directly produced by the 20 S proteasome in vitro, and this can be used for identification of such ligands in the constitutive HLA-B27-bound peptide pool.

Clustering of peptide-loaded MHC class I molecules for endoplasmic reticulum export imaged by fluorescence resonance energy transfer

Fluorescence resonance energy transfer between cyan fluorescent protein- and yellow fluorescent protein-tagged MHC class I molecules reports on their spatial organization during assembly and export from the endoplasmic reticulum (ER). A fraction of MHC class I molecules is clustered in the ER at steady state. Contrary to expectations from biochemical models, this fraction is not bound to the TAP. Instead, it appears that MHC class I molecules cluster after peptide loading. This clustering points toward a novel step involved in the selective export of peptide-loaded MHC class I molecules from the ER. Consistent with this model, we detected clusters of wild-type HLA-A2 molecules and of mutant A2-T134K molecules that cannot bind TAP, but HLA-A2 did not detectably cluster with A2-T134K at steady state. Lactacystin treatment disrupted the HLA-A2 clusters, but had no effect on the A2-T134K clusters. However, when cells were fed peptides with high affinity for HLA-A2, mixed clusters containing both HLA-A2 and A2-T134K were detected.

The truncated cytoplasmic tail of HLA-G serves a quality-control function in post-ER compartments

In contrast to the current model of MHC class I trafficking, which predicts that once a MHC class I molecule leaves the ER, it moves to the cell surface by bulk flow, we show that HLA-G that is loaded with suboptimal peptides is retrieved from post-ER compartments to the ER. Loading of HLA-G with high-affinity peptides abrogates this retrieval due to the lack of binding affinity to coatomer. Moreover, the loss of the endocytosis motif in the truncated cytoplasmic tail results in the prolonged half-life of HLA-G on the cell surface. Our findings reveal that surface expression of HLA-G can be further regulated in post-ER compartments and that the truncated cytoplasmic tail plays a critical role in such quality-control mechanisms.

MHC class II allosteric site drugs: new immunotherapeutics for malignant, infectious and autoimmune diseases

The discovery of the interactions of the 'Ii-Key' segment of the Ii protein with the major histocmpatibility complex (MHC) Class II allosteric site, which is adjacent to the antigenic peptide-binding site, creates therapeutic opportunities by regulating the antigenic peptide binding to MHC class II molecules. The binding of Ii-Key to the MHC class II allosteric site loosens the hold of the MHC Class II 'clamshell' on antigenic peptides and leads to highly efficient antigenic peptide charging to or releasing from the MHC class II antigenic peptide-binding groove. Ii-Key peptide-induced spilling of bound antigenic peptide, or replacement with inert blockers, leads to 'inert immunosuppression'. Highly efficient replacement of ambient with vaccine peptides by Ii-Key permits 'active immunosuppression' for antigen-specific control of autoimmune diseases in the absence of cytokines or adjuvants. On the other hand, active immunization against cancer or infectious disease can result from epitope replacement mediated by Ii-Key and accompanied by cytokines or other adjuvants. Finally, linking the Ii-Key peptide through a simple polymethylene bridge to an antigenic sequence vastly increases the potency of MHC Class II peptide vaccines. In summary, the discovery of the MHC class II allosteric site allows one to increase the efficiency of MHC class II-related, antigenic epitope-specific therapy for malignant, infectious, and autoimmune diseases. The focus of this review is on the mechanism and potential clinical use of such novel allosteric site-directed, Ii-key drugs.

MHC class I molecules can direct proteolytic cleavage of antigenic precursors in the endoplasmic reticulum

The large set of peptides presented by MHC (major histocompatibility complex) class I molecules are generated by proteolysis of diverse precursors in the cytoplasm and possibly in the endoplasmic reticulum (ER). To define the potential peptide trimming events in the ER, we analyzed proteolytic products generated in isolated microsomes. The residues flanking the N terminus of the final antigenic peptide were rapidly removed within the microsomes but only in the presence of appropriate MHC molecules. Remarkably, the precursor peptide was bound to the MHC molecules in a distinct conformation and required an aminopeptidase activity to generate the optimal peptide. The MHC molecules are therefore not only the final repositories of antigenic peptides, but they can also direct their excision from longer precursors.

Distinct functions of the ATP binding cassettes of transporters associated with antigen processing: a mutational analysis of Walker A and B sequences

The transporters associated with antigen processing (TAP1/TAP2) provide peptides to MHC class I molecules in the endoplasmic reticulum. Like other ATP-binding cassette proteins, TAP uses ATP hydrolysis to power transport. We have studied peptide binding to as well as translocation by TAP proteins with mutations in the Walker A and B sequences that are known to mediate ATP binding and hydrolysis. We show that a mutation in the TAP1 Walker B sequence reported to abrogate class I expression by a lung tumor does not affect ATP binding affinity, suggesting a defect restricted to ATP hydrolysis. This mutation reduces peptide transport by only 50%, suggesting that TAP function can be highly limiting for antigen presentation in non-lymphoid cells. Single substitutions in Walker A sequences (TAP1K544A, TAP2K509A), or their complete replacements, abrogate nucleotide binding to each subunit. Although all of these mutations abrogate peptide transport, they reveal distinct roles for nucleotide binding to the two transporter subunits in TAP folding and in regulation of peptide substrate affinity, respectively. Alteration of the TAP1 Walker A motif can have strong effects on TAP1 and thereby TAP complex folding. However, TAP1 Walker A mutations compatible with correct folding do not affect peptide binding. In contrast, abrogation of the TAP2 nucleotide binding capacity has little or no effect on TAP folding but eliminates peptide binding to TAP at 37 degrees C in the presence of nucleotides. Thus, nucleotide binding to TAP2 but not to TAP1 is a prerequisite for peptide binding to TAP. Based on these results, we propose a model in which nucleotide and peptide release from TAP are coupled and followed by ATP binding to TAP2, which induces high peptide affinity and initiates the transport cycle.

Use of chimeric proteins to investigate the role of transporter associated with antigen processing (TAP) structural domains in peptide binding and translocation

The transporter associated with antigen processing (TAP) comprises two subunits, TAP1 and TAP2, each containing a hydrophobic membrane-spanning region (MSR) and a nucleotide binding domain (NBD). The TAP1/TAP2 complex is required for peptide translocation across the endoplasmic reticulum membrane. To understand the role of each structural unit of the TAP1/TAP2 complex, we generated two chimeras containing TAP1 MSR and TAP2 NBD (T1MT2C) or TAP2 MSR and TAP1 NBD (T2MT1C). We show that TAP1/T2MT1C, TAP2/T1MT2C, and T1MT2C/T2MT1C complexes bind peptide with an affinity comparable to wild-type complexes. By contrast, TAP1/T1MT2C and TAP2/T2MT1C complexes, although observed, are impaired for peptide binding. Thus, the MSRs of both TAP1 and TAP2 are required for binding peptide. However, neither NBD contains unique determinants required for peptide binding. The NBD-switched complexes, T1MT2C/T2MT1C, TAP1/T2MT1C, and TAP2/T1MT2C, all translocate peptides, but with progressively reduced efficiencies relative to the TAP1/TAP2 complex. These results indicate that both nucleotide binding sites are catalytically active and support an alternating catalytic sites model for the TAP transport cycle, similar to that proposed for P-glycoprotein. The enhanced translocation efficiency of TAP1/T2MT1C relative to TAP2/T1MT2C complexes correlates with enhanced binding of the TAP1 NBD-containing constructs to ATP-agarose beads. Preferential ATP interaction with TAP1, if occurring in vivo, might polarize the transport cycle such that ATP binding to TAP1 initiates the cycle. However, our observations that TAP complexes containing two identical TAP NBDs can mediate translocation indicate that distinct properties of the nucleotide binding site per se are not essential for the TAP catalytic cycle.

Distinct functions and cooperative interaction of the subunits of the transporter associated with antigen processing (TAP)

The ATP-binding cassette (ABC) transporter TAP translocates peptides from the cytosol to awaiting MHC class I molecules in the endoplasmic reticulum. TAP is made up of the TAP1 and TAP2 polypeptides, which each possess a nucleotide binding domain (NBD). However, the role of ATP in peptide binding and translocation is poorly understood. We present biochemical and functional evidence that the NBDs of TAP1 and TAP2 are non-equivalent. Photolabeling experiments with 8-azido-ATP demonstrate a cooperative interaction between the two NBDs that can be stimulated by peptide. The substitution of key lysine residues in the Walker A motifs of TAP1 and TAP2 suggests that TAP1-mediated ATP hydrolysis is not essential for peptide translocation but that TAP2-mediated ATP hydrolysis is critical, not only for translocation, but for peptide binding.

Antigen presentation subverted: Structure of the human cytomegalovirus protein US2 bound to the class I molecule HLA-A2

Many persistent viruses have evolved the ability to subvert MHC class I antigen presentation. Indeed, human cytomegalovirus (HCMV) encodes at least four proteins that down-regulate cell-surface expression of class I. The HCMV unique short (US)2 glycoprotein binds newly synthesized class I molecules within the endoplasmic reticulum (ER) and subsequently targets them for proteasomal degradation. We report the crystal structure of US2 bound to the HLA-A2/Tax peptide complex. US2 associates with HLA-A2 at the junction of the peptide-binding region and the alpha3 domain, a novel binding surface on class I that allows US2 to bind independently of peptide sequence. Mutation of class I heavy chains confirms the importance of this binding site in vivo. Available data on class I-ER chaperone interactions indicate that chaperones would not impede US2 binding. Unexpectedly, the US2 ER-luminal domain forms an Ig-like fold. A US2 structure-based sequence alignment reveals that seven HCMV proteins, at least three of which function in immune evasion, share the same fold as US2. The structure allows design of further experiments to determine how US2 targets class I molecules for degradation.

Human cytomegalovirus US2 endoplasmic reticulum-lumenal domain dictates association with major histocompatibility complex class I in a locus-specific manner

The human cytomegalovirus-encoded US2 glycoprotein targets endoplasmic reticulum-resident major histocompatibility complex (MHC) class I heavy chains for rapid degradation by the proteasome. We demonstrate that the endoplasmic reticulum-lumenal domain of US2 allows tight interaction with class I molecules encoded by the HLA-A locus. Recombinant soluble US2 binds properly folded, peptide-containing recombinant HLA-A2 molecules in a peptide sequence-independent manner, consistent with US2's ability to broadly downregulate class I molecules. The physicochemical properties of the US2/MHC class I complex suggest a 1:1 stoichiometry. These results demonstrate that US2 does not require additional cellular proteins to specifically interact with soluble class I molecules. Binding of US2 does not significantly alter the conformation of class I molecules, as a soluble T-cell receptor can simultaneously recognize class I molecules associated with US2. The lumenal domain of US2 can differentiate between the products of distinct class I loci, as US2 binds several HLA-A locus products while being unable to bind recombinant HLA-B7, HLA-B27, HLA-Cw4, or HLA-E. We did not observe interaction between soluble US2 and either recombinant HLA-DR1 or recombinant HLA-DM. The substrate specificity of US2 may help explain the presence in human cytomegalovirus of multiple strategies for downregulation of MHC class I molecules.

Mapping and expression pattern analysis of key components of the major histocompatibility complex class I antigen processing and presentation pathway in a representative human renal cell carcinoma cell line

Renal cell carcinoma (RCC) represent approximately 5% of all cancer deaths. At the time of presentation, over 50% of the patients have already developed locally advanced or metastatic disease with five-year survival rates of less than 20%. Although relative resistant to conventional regimens, RCC are partially susceptible to T cell-based immunotherapy. To further develop this treatment modality, two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) was applied for both the mapping of the key components of the major histocompatibility complex (MHC) class I antigen processing and presentation machinery (APM) and the characterization of the constitutive and cytokine-regulated protein expression profiles in a representative human RCC cell line. The latter aspect is based on the fact, that the expression level of some of the APM components can be altered in response to interferon (IFN)-gamma treatment. Total cell lysates from untreated and IFN-gamma-treated tumor cells were separated on 2-D PAGE gels using broad range immobilized pH gradient (IPG) strips. Serial Western blot analyses using sets of APM-specific antibodies were performed to target the relevant protein spots. Protein verification was mostly accomplished via peptide mass finger-printing using matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). To date, the majority of the APM-related components have been identified and mapped. In addition, the different protein expression profiles of untreated and IFN-gamma-treated RCC cells are under investigation.

Kinetics and thermodynamics of beta 2-microglobulin binding to the alpha 3 domain of major histocompatibility complex class I heavy chain

The major histocompatibility complex (MHC) class I molecule plays a crucial role in cytotoxic lymphocyte function. Functional class I MHC exists as a heterotrimer consisting of the MHC class I heavy chain, an antigenic peptide fragment, and beta2-microglobulin (beta2m). beta2m has been previously shown to play an important role in the folding of the MHC heavy chain without continued beta2m association with the MHC complex. Therefore, beta2m is both a structural component of the MHC complex and a chaperone-like molecule for MHC folding. In this study we provide data supporting a model in which the chaperone-like role of beta2m is dependent on initial binding to only one of the two beta2m interfaces with class 1 heavy chain. beta2-Microglobulin binding to an isolated alpha3 domain of the class I MHC heavy chain accurately models the biochemistry and thermodynamics of beta2m-driven refolding. Our results explain a 1000-fold discrepancy between beta2m binding and refolding of MHC1. The biochemical study of the individual domains of complex molecules is an important strategy for understanding their dynamic structure and multiple functions.

Glycoprotein folding in the endoplasmic reticulum

Our understanding of eukaryotic protein folding in the endoplasmic reticulum has increased enormously over the last 5 years. In this review, we summarize some of the major research themes that have captivated researchers in this field during the last years of the 20th century. We follow the path of a typical protein as it emerges from the ribosome and enters the reticular environment. While many of these events are shared between different polypeptide chains, we highlight some of the numerous differences between proteins, between cell types, and between the chaperones utilized by different ER glycoproteins. Finally, we consider the likely advances in this field as the new century unfolds and we address the prospect of a unified understanding of how protein folding, degradation, and translation are coordinated within a cell.

Tapasin: an ER chaperone that controls MHC class I assembly with peptide

The stable assembly of MHC class I molecules with peptides in the endoplasmic reticulum (ER) involves several accessory molecules. One of these accessory molecules is tapasin, a transmembrane protein that tethers empty class I molecules to the peptide transporter associated with antigen processing (TAP). Here, evidence is presented that tapasin retains class I molecules in the ER until they acquire high-affinity peptides.

Glycosylation and the immune system

Almost all of the key molecules involved in the innate and adaptive immune response are glycoproteins. In the cellular immune system, specific glycoforms are involved in the folding, quality control, and assembly of peptide-loaded major histocompatibility complex (MHC) antigens and the T cell receptor complex. Although some glycopeptide antigens are presented by the MHC, the generation of peptide antigens from glycoproteins may require enzymatic removal of sugars before the protein can be cleaved. Oligosaccharides attached to glycoproteins in the junction between T cells and antigen-presenting cells help to orient binding faces, provide protease protection, and restrict nonspecific lateral protein-protein interactions. In the humoral immune system, all of the immunoglobulins and most of the complement components are glycosylated. Although a major function for sugars is to contribute to the stability of the proteins to which they are attached, specific glycoforms are involved in recognition events. For example, in rheumatoid arthritis, an autoimmune disease, agalactosylated glycoforms of aggregated immunoglobulin G may induce association with the mannose-binding lectin and contribute to the pathology.

TAPI polymorphisms in several human ethnic groups: characteristics, evolution, and genotyping strategies

Genetic variations in the locus encoding the transporter associated with antigen processing, subunit 1 (TAP1), were systematically studied using samples from Caucasians, Africans, Brazilians, and compared with data from chimpanzees. PCR-amplified genomic sequences corresponding to the 11 exons were analyzed by single-strand conformation polymorphism (SSCP) and sequencing. Six nonsynonymous and 2 synonymous single nucleotide polymorphisms (SNPs) were found to be common in one ethnic group or another, and they involved codons 254 (Gly-GGC/Gly-GGT) in exon 3, 333 (Ile-ATC/Val-GTC) in exon 4, 370 (Ala-GCT/Val-GTT) in exon 5, 458 (Val-GTG/Leu-TTG) in exon 6, 518 (Val-GTC/Ile-ATC) in exon 7, 637 (Asp-GAC/Gly-GGC), 648 (Arg-CGA/Gln-CAA) and 661 (Pro-CCG/Pro-CCA) in exon 10. At each SNP site the sequence listed first was predominant in all ethnic groups. Several SNPs segregated on the same chromosome regardless of populations and species. Together, the SNPs produced 5 major human TAP1 alleles, 4 of which matched the officially recognized alleles *0101, *02011, *0301, and *0401; the 5th allele differed from each of those by at least 4 SNPs. Overall, TAP1*0101 was the predominant allele in all ethnic groups, with frequencies ranging from 0.667 in Zambians to 0.808 in US Caucasians. The TAP1*0401 frequency showed the greatest difference between Africans (0.221-0.254) and Caucasians (0.033), with Brazilians (0.058) fitting in the middle. Consistent with earlier work based on Caucasians and gorillas, *0101 appeared to be the newest human TAP1 allele, suggesting a dramatic spread of *0101 into all human populations examined. Characterization of TAP1 polymorphisms allowed the design of a PCR-based genotyping scheme that targeted 7 SNP sites and required 2 separate genotyping techniques.

A role for calnexin in the assembly of the MHC class I loading complex in the endoplasmic reticulum

Heterodimers of MHC class I glycoprotein and beta(2)-microglobulin (beta(2)m) bind short peptides in the endoplasmic reticulum (ER). Before peptide binding these molecules form part of a multisubunit loading complex that also contains the two subunits of the TAP, the transmembrane glycoprotein tapasin, the soluble chaperone calreticulin, and the thiol oxidoreductase ERp57. We have investigated the assembly of the loading complex and provide evidence that after TAP and tapasin associate with each other, the transmembrane chaperone calnexin and ERp57 bind to the TAP-tapasin complex to generate an intermediate. These interactions are independent of the N:-linked glycan of tapasin, but require its transmembrane and/or cytoplasmic domain. This intermediate complex binds MHC class I-beta(2)m dimers, an event accompanied by the loss of calnexin and the acquisition of calreticulin, generating the MHC class I loading complex. Peptide binding then induces the dissociation of MHC class I-beta(2)m dimers, which can be transported to the cell surface.

Non-conservative substitutions distinguish previously uncharacterized HLA-A molecules

The extent of class I HLA polymorphism is not yet realized, and to provide a glimpse of the HLA-A polymorphism which remains undetected, we have analyzed approximately 3,700 National Marrow Donor Program (NMDP) Donor/Recipient Pair Retrospective Study Samples with HLA-A DNA sequence-based typing (SBT). Seventeen new HLA-A alleles were detected, with a total of 19 nucleotide substitutions distinguishing these new alleles from their closest HLA-A relatives. Nearly all of the new alleles differ by single nucleotide substitutions; a majority of these substitutions can be explained by gene conversion events but 6 alleles likely originated by point mutation. Fifteen of the 19 nucleotide substitutions translate into amino acid differences in the molecule. Structurally, the inferred amino acid alterations were non-conservative in terms of chemical property, and most substitutions were positioned in 1 or more of the specificity pockets which determine peptide binding. Although these new alleles were identified in a primarily Caucasian sample population, 9 of the 17 new HLA-A alleles were found in samples of non-Caucasoid origin. A new allele detection rate of 1 in approximately 200 individuals in our data set would, therefore, be higher in a non-Caucasoid sample population. In summary, the single nucleotide substitutions that distinguish undetected HLA-A alleles translate into functionally distinct HLA-A molecules. Further studies of the role of HLA-A in transplantation, in disease association, and in evolution must therefore accommodate the discovery of new alleles differing by single nucleotides.

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.

Phosphorylated peptides are naturally processed and presented by major histocompatibility complex class I molecules in vivo

Posttranslational modification of peptide antigens has been shown to alter the ability of T cells to recognize major histocompatibility complex (MHC) class I-restricted peptides. However, the existence and origin of naturally processed phosphorylated peptides presented by MHC class I molecules have not been explored. By using mass spectrometry, significant numbers of naturally processed phosphorylated peptides were detected in association with several human MHC class I molecules. In addition, CD8(+) T cells could be generated that specifically recognized a phosphorylated epitope. Thus, phosphorylated peptides are part of the repertoire of antigens available for recognition by T cells in vivo.

C-terminal epitope tagging facilitates comparative ligand mapping from MHC class I positive cells

Purification of specific class I molecules prior to peptide ligand characterization is complicated by the presence of multiple class I proteins in most cell lines. Immortalized B, T, and tumor cell lines typically express endogenous HLA-A, -B, and -C; and most individuals from which the cell lines are derived are heterozygous at these loci. Antibodies specific for a particular HLA molecule may be used for purification, but allele-specific antibodies can be biased by ligands occupying the peptide-binding groove. Through the use of C-terminal tagging, we have developed a method of soluble HLA production such that downstream purification does not skew the peptide analysis of the examined molecule. Comparison of peptides eluted from HLA class I molecules with and without C-terminal tags demonstrates that addition of a tag does not abrogate the peptide binding specificity of the original molecule. Both pooled Edman sequencing and mass spectrometric sequencing identified no substantial differences in peptides bound by untailed, 6-HIS-tailed, and FLAG-tailed class I molecules, demonstrating that the peptide specificity of a given molecule is not distorted by either tag. This production methodology bypasses problems with isolation of specific molecules and permits ligand mapping and epitope discovery in a variety of pathogen-infected and tumor cell lines.

Selective export of MHC class I molecules from the ER after their dissociation from TAP

It has been assumed that upon dissociation from TAP, MHC class I molecules exit the ER by nonselective bulk flow. We now show that exit must occur by association with cargo receptors. Inconsistent with exit by bulk flow, loading of MHC class I molecules with high-affinity peptides triggers dissociation from TAP but has no effect on rates of ER-to-Golgi transport. Moreover, peptide-loaded MHC class I molecules accumulate at ER exit sites from which TAP molecules are excluded. Consistent with receptor-mediated exit, ER-to-Golgi transport of MHC class I molecules is independent of their cytoplasmic tails, which themselves lack ER export motifs. In addition, we show that MHC class I molecules associate with the putative cargo receptor BAP31.

Immunological principles of adverse drug reactions: the initiation and propagation of immune responses elicited by drug treatment

Adverse drug reactions account for between 2 to 5% of all hospital admissions and can prevent the administration of an otherwise effective therapeutic agent. Hypersensitivity or immune-mediated reactions, although less common, tend to be proportionately more serious. There is convincing evidence to implicate the immune system in the pathogenesis of hypersensitivity reactions. Our understanding of the way in which the immune system recognises drugs is based on the hapten hypothesis; the onset of hypersensitivity involves drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and T cell proliferation. Central to this hypothesis is the critical role of drug metabolism, with the balance between metabolic bioactivation and detoxification being one important component of individual susceptibility. The purpose of this review is to classify drug hypersensitivity reactions in terms of their clinical presentation, and also to consider recent advances in our understanding of the chemical, biochemical and, in particular, cellular immunological mechanisms of hypersensitivity. The following topics are reviewed: (i) drug disposition and cellular metabolism; (ii) mechanisms of antigen processing and presentation; (iii) the role of cytokines and co-stimulatory molecules in the induction and maintenance of a polarised immune response; and (iv) the application of the hapten hypothesis, danger hypothesis and serial triggering model to drug hypersensitivity. A greater understanding of the mechanism(s) of hypersensitivity may identify novel therapeutic strategies and help to combat one of the more severe forms of adverse reactions to drugs.

Impaired assembly yet normal trafficking of MHC class I molecules in Tapasin mutant mice

Loading of peptides onto major histocompatibility complex class I molecules involves a multifactorial complex that includes tapasin (TPN), a membrane protein that tethers empty class I glycoproteins to the transporter associated with antigen processing. To evaluate the in vivo role of TPN, we have generated Tpn mutant mice. In these animals, most class I molecules exit the endoplasmic reticulum (ER) in the absence of stably bound peptides. Consequently, mutant animals have defects in class I cell surface expression, antigen presentation, CD8+ T cell development, and immune responses. These findings reveal a critical role of TPN for ER retention of empty class I molecules. Tpn mutant animals should prove useful for studies on alternative antigen-processing pathways that involve post-ER peptide loading.

Tapasin-mediated retention and optimization of peptide ligands during the assembly of class I molecules

The murine class I H-2Kb molecule achieves high level surface expression in tapasin-deficient 721.220 human cells. Compared with their behavior in wild-type cells, Kb molecules expressed on 721.220 cells are more receptive to exogenous peptide, undergo more rapid surface decay, and fail to form macromolecular peptide loading complexes. As a result, they are rapidly transported to the cell surface, reflecting a failure of endoplasmic reticulum retention mechanisms in the absence of loading complex formation. Despite the failure of Kb molecules to colocalize to the TAP and their rapid egress to the cell surface, Kb is still capable of presenting TAP-dependent peptides in the absence of tapasin. Furthermore, pool sequencing of peptides eluted from these molecules revealed strict conservation of their canonical H-2Kb-binding motif. There was a reduction in the total recovery of peptides associated with Kb molecules purified from the surface of tapasin-deficient cells. Comparison of the peptides bound to Kb in the presence and absence of tapasin revealed considerable overlap in peptide repertoire. These results indicate that in the absence of an interaction with tapasin, Kb molecules fail to assemble with calreticulin and TAP, yet they are still capable of acquiring a diverse array of peptides. However, a significant proportion of these peptides appear to be suboptimal, resulting in reduced cell surface stability of Kb complexes. Taken together, the findings indicate that tapasin plays an essential role in the formation of the class I loading complex, which retains class I heterodimers in the endoplasmic reticulum until optimal ligand selection is completed.