Major histocompatibility complex conformational epitopes are peptide specific

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

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

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

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Associations show distinctive stoichiometry and biochemical properties.

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Associations might regulate immunological and non-immunological processes.

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

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Homotypic associations have been related to pathological outcomes.

TCR-like antibodies in cancer immunotherapy

Cancer immunotherapy has been regarded as the most significant scientific breakthrough of 2013, and antibody therapy is at the core of this breakthrough. Despite significant success achieved in recent years, it is still difficult to target intracellular antigens of tumor cells with traditional antibodies, and novel therapeutic strategies are needed. T cell receptor (TCR)-like antibodies comprise a novel family of antibodies that can recognize peptide/MHC complexes on tumor cell surfaces. TCR-like antibodies can execute specific and significant anti-tumor immunity through several distinct molecular mechanisms, and the success of this type of antibody therapy in melanoma, leukemia, and breast, colon, and prostate tumor models has excited researchers in the immunotherapy field. Here, we summarize the generation strategy, function, and molecular mechanisms of TCR-like antibodies described in publications, focusing on the most significant discoveries.

The source of MHC class I presented peptides and its implications

The source of peptides that enter the major histocompatibility class I (MHCI) pathway has been intensively debated over the last two decades. The initial assumption that peptides are derived from degradation of full length proteins was challenged by a model in which alternative translation products are a source of peptides. This model has been tested and supported by scientific data. We now need new hypotheses on the physiological implications of different sources of peptides for the MHCI pathway. The aim of this overview is to give an up-to-date account of the source of antigenic peptide material for the MHCI pathway and to incorporate the more recent observations of alternative mRNA translation products into existing models of the direct and cross-presentation pathways.

Gene Expression Driven by a Strong Viral Promoter in MVA Increases Vaccination Efficiency by Enhancing Antibody Responses and Unmasking CD8⁺ T Cell Epitopes

Viral vectors are promising tools for vaccination strategies and immunotherapies. However, CD8⁺ T cell responses against pathogen-derived epitopes are usually limited to dominant epitopes and antibody responses to recombinant encoded antigens (Ags) are mostly weak. We have previously demonstrated that the timing of viral Ag expression in infected professional Ag-presenting cells strongly shapes the epitope immunodominance hierarchy. T cells recognizing determinants derived from late viral proteins have a clear disadvantage to proliferate during secondary responses. In this work we evaluate the effect of overexpressing the recombinant Ag using the modified vaccinia virus early/late promoter H5 (mPH5). Although the Ag-expression from the natural promoter 7.5 (P7.5) and the mPH5 seemed similar, detailed analysis showed that mPH5 not only induces higher expression levels than P7.5 during early phase of infection, but also Ag turnover is enhanced. The strong overexpression during the early phase leads to broader CD8 T cell responses, while preserving the priming efficiency of stable Ags. Moreover, the increase in Ag-secretion favors the induction of strong antibody responses. Our findings provide the rationale to develop new strategies for fine-tuning the responses elicited by recombinant modified vaccinia virus Ankara by using selected promoters to improve the performance of this viral vector.

Endogenous-peptide-dependent alloreactivity: new scientific insights and clinical implications

T-cell alloreactivity is generated via immune responsiveness directed against allogeneic (allo) human leucocyte antigen (HLA) molecules. Whilst the alloresponse is of extraordinary potency and frequency, it has often been assumed to be less peptide-specific than conventional T-cell reactivity. Recently, several human studies have shown that both alloreactive CD8(+) and CD4(+) T cells exhibit exquisite allo-HLA and endogenous peptide specificity that has also underpinned tissue-specific allorecognition. In this review, we summarize former and recent scientific evidence in support of endogenous peptide (self-peptide)-dependence of T-cell alloreactivity. The clinical implications of these findings will be discussed in the context of both solid organ transplantation and haematopoietic stem cell transplantation (HSCT). Insights into the understanding of the molecular basis of T-cell allorecognition will probably translate into improved allograft survival outcomes, lower frequencies of graft vs host disease and could potentially be exploited for selective graft vs leukaemia effect to improve clinical outcomes following HSCT.

A role for UDP-glucose glycoprotein glucosyltransferase in expression and quality control of MHC class I molecules

UDP-glucose:glycoprotein glucosyltransferase 1 (UGT1) serves as a folding sensor in the calnexin/calreticulin glycoprotein quality control cycle. UGT1 recognizes disordered or hydrophobic patches near asparagine-linked nonglucosylated glycans in partially misfolded glycoproteins and reglucosylates them, returning folding intermediates to the cycle. In this study, we examine the contribution of the UGT1-regulated quality control mechanism to MHC I antigen presentation. Using UGT1-deficient mouse embryonic fibroblasts reconstituted or not with UGT1, we show that, although formation of the peptide loading complex is unaffected by the absence of UGT1, the surface level of MHC class I molecules is reduced, MHC class I maturation and assembly are delayed, and peptide selection is impaired. Most strikingly, we show using purified soluble components that UGT1 preferentially recognizes and reglucosylates MHC class I molecules associated with a suboptimal peptide. Our data suggest that, in addition to the extensively studied tapasin-mediated quality control mechanism, UGT1 adds a new level of control in the MHC class I antigen presentation pathway.

Modified vaccinia virus Ankara exerts potent immune modulatory activities in a murine model

Background

Modified vaccinia virus Ankara (MVA), a highly attenuated strain of vaccinia virus, has been used as vaccine delivery vector in preclinical and clinical studies against infectious diseases and malignancies. Here, we investigated whether an MVA which does not encode any antigen (Ag) could be exploited as adjuvant per se.

Methodology/Principal Findings

We showed that dendritic cells infected in vitro with non-recombinant (nr) MVA expressed maturation and activation markers and were able to efficiently present exogenously pulsed Ag to T cells. In contrast to the dominant T helper (Th) 1 biased responses elicited against Ags produced by recombinant MVA vectors, the use of nrMVA as adjuvant for the co-administered soluble Ags resulted in a long lasting mixed Th1/Th2 responses.

Conclusions/Significance

These findings open new ways to potentiate and modulate the immune responses to vaccine Ags depending on whether they are co-administered with MVA or encoded by recombinant viruses.

Mechanism of cellular rejection in transplantation

The explosion of new discoveries in the field of immunology has provided new insights into mechanisms that promote an immune response directed against a transplanted organ. Central to the allograft response are T lymphocytes. This review summarizes the current literature on allorecognition, costimulation, memory T cells, T cell migration, and their role in both acute and chronic graft destruction. An in depth understanding of the cellular mechanisms that result in both acute and chronic allograft rejection will provide new strategies and targeted therapeutics capable of inducing long-lasting, allograft-specific tolerance.

Peptide induction of surface expression of class I MHC

This unit describes a method for comparing the relative binding of different peptides to the same MHC class I (MHC-I) molecule using live cells. Live cells expressing suboptimally loaded MHC-I proteins are incubated with medium containing diluted amounts of synthetic peptides to be tested for binding to class I. After overnight incubation with peptide, surface class I expression is monitored by flow cytometry using an allele-specific MAb. Relative binding affinity of peptide reliably correlates with the amount of surface induction of the class I molecule to which it specifically binds. The mechanistic basis of this assay is that surface MHC-I molecules become conformationally unstable shortly after peptide dissociation. However, the binding of an exogenous peptide can stabilize the surface class I molecule, prevent conformational instability, and thus increase class I surface expression in an allele-specific manner.

Conformation of MHC class II I-Ag7 is sensitive to the P9 anchor amino acid in bound peptide

Type I diabetes is a chronic autoimmune disease resulting in the destruction of insulin-producing beta cells in the pancreas. In humans, disease incidence is linked to expression of specific MHC class II alleles and in mice type I diabetes is associated with the class II allele I-A(g7). I-A(g7) contains a polymorphism that is shared by human class II alleles associated with the disease, at position 57 in the beta chain, in which aspartic acid is changed to a serine. The P9 pocket in the peptide-binding groove is in part shaped by beta57, and therefore the structure of this pocket is modified in I-A(g7). Using mAbs, we have previously determined that alternative conformations of I-A(g7) form in response to peptide binding. In this study, we have extended these findings by examining how peptides induce I-A(g7) molecules to adopt different conformations. By mutating the amino acid in the P9 position of either class II-associated invariant chain peptide (CLIP) or glutamic acid decarboxylase (GAD) 65 (207-220), we have determined that the chemical nature of the P9 anchor amino acid, either acidic or small hydrophobic, affects the overall conformation of the I-A(g7) class II molecule. T cell hybridomas specific for GAD 65 (207-220) in the context of I-A(g7) were also examined for recognition of I-A(g7) bound to GAD 65 (207-220), in which Glu(217) in the P9 position was changed to alanine. We found that although some TCRs were able to recognize both peptides in the context of I-A(g7), and thus both class II conformations, approximately one-third of the T cells tested were not able to recognize the alternate class II conformation formed with the mutated peptide. These results indicate that the I-A(g7) conformations may affect functional activation of T cells, and thus may play a role in autoimmunity.

Dendritic cells cross-dressed with peptide MHC class I complexes prime CD8+ T cells

The activation of naive CD8+ T cells has been attributed to two mechanisms: cross-priming and direct priming. Cross-priming and direct priming differ in the source of Ag and in the cell that presents the Ag to the responding CD8+ T cells. In cross-priming, exogenous Ag is acquired by professional APCs, such as dendritic cells (DC), which process the Ag into peptides that are subsequently presented. In direct priming, the APCs, which may or may not be DC, synthesize and process the Ag and present it themselves to CD8+ T cells. In this study, we demonstrate that naive CD8+ T cells are activated by a third mechanism, called cross-dressing. In cross-dressing, DC directly acquire MHC class I-peptide complexes from dead, but not live, donor cells by a cell contact-mediated mechanism, and present the intact complexes to naive CD8+ T cells. Such DC are cross-dressed because they are wearing peptide-MHC complexes generated by other cells. CD8+ T cells activated by cross-dressing are restricted to the MHC class I genotype of the donor cells and are specific for peptides generated by the donor cells. In vivo studies demonstrate that optimal priming of CD8+ T cells requires both cross-priming and cross-dressing. Thus, cross-dressing may be an important mechanism by which DC prime naive CD8+ T cells and may explain how CD8+ T cells are primed to Ags that are inefficiently cross-presented.

Influence of dominant HIV-1 epitopes on HLA-A3/peptide complex formation

The binding of peptides to MHC class I molecules induces MHC/peptide complexes that have specific conformational features. Little is known about the molecular and structural bases required for an optimal MHC/peptide association able to induce a dominant T cell response. We sought to characterize the interaction between purified HLA-A3 molecules and four well known CD8 epitopes from HIV-1 proteins. To define the characteristics of HLA-peptide complex formation and to identify potential structural changes, we used biochemical assays that detect well formed complexes. We tested the amplitude, stability, and kinetic parameters of the interaction between HLA-A3, peptides, and anti-HLA mAbs. Our results show that the four epitopes Nef73-82, Pol325-333, Env37-46, and Gag20-28 bind strongly to HLA-A3 molecules and form very stable complexes that are detected with differential patterns of mAb reactivity. The most striking result is the nonrecognition of the HLA-A3/Gag20-28 complex by the A11.1M mAb specific to HLA-A3/-A11 alleles. To explain this observation, from the data published on HLA-A11 crystallographic structure, we propose molecular models of the HLA-A3 molecule complexed with Nef73-82, Pol325-333, and Gag20-28 epitopes. In the HLA-A3/Gag20-28 complex, we suggest that Arg at position P1 of the peptide may push the alpha2 helix residue Trp-167 of HLA-A3 and affect mAb recognition. Such observations may have great implications for T cell antigen receptor recognition and the immunogenicity of HLA/peptide complexes.

The Crystal Structure of H-2Db Complexed with a Partial Peptide Epitope Suggests a Major Histocompatibility Complex Class I Assembly Intermediate

In the absence of bound peptide ligands, major histocompatibility complex (MHC) class I molecules are unstable. In an attempt to determine the minimum requirement for peptide-dependent MHC class I stabilization, we have used short synthetic peptides derived from the Sendai virus nucleoprotein epitope (residues 324-332, 1FAPGNYPAL9) to promote its folding in vitro of H-2D(b). We found that H-2D(b) can be stabilized by the pentapeptide 5NYPAL9, which is equivalent to the C-terminal portion of the optimal nonapeptide and includes both the P5 and P9 anchor residues. We have crystallized the complex of the H-2D(b) molecule with the pentamer and determined the structure to show how a quasi-stable MHC class I molecule can be formed by occupancy of a single binding pocket in the peptide-binding groove.

T-cell receptor-like antibodies: novel reagents for clinical cancer immunology and immunotherapy

Major histocompatibility complex class I molecules play a central role in the immune response against a variety of cells that have undergone malignant transformation by shaping the T-cell repertoire and presenting peptide antigens from endogeneous antigens to CD8+ cytotoxic T-cells. Diseased tumor or virus-infected cells are present on class I major histocompatibility complex molecule peptides that are derived from tumor-associated antigens or viral-derived proteins. Due to their unique specificity, such major histocompatibility complex-peptide complexes are a desirable target for novel approaches in immunotherapy. Targeted delivery of toxins or other cytotoxic drugs to cells which express specific major histocompatibility complex-peptide complexes that are involved in the immune response against cancer or viral infections would allow for a specific immunotherapeutic treatment of these diseases. It has recently been demonstrated that antibodies with the antigen-specific, major histocompatibility complex-restricted specificity of T-cells can be generated by taking advantage of the selection power of phage display technology. In addition to their tumor targeting capabilities, antibodies that mimic the fine specificity of T-cell receptors can serve as valuable research reagents that enable study of human class I peptide-major histocompatibility complex ligand presentation, as well as T-cell receptor peptide-major histocompatibility complex interactions. T-cell receptor-like antibody molecules may prove to be useful tools for studying major histocompatibility complex class I antigen presentation in health and disease as well as for therapeutic purposes in cancer, infectious diseases and autoimmune disorders.

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.

Peptide selection by class I molecules of the major histocompatibility complex

Class I molecules of the major histocompatibility complex (MHC) bind peptides derived from cytoplasmic proteins. Comparison of over 100 such peptides reveals the importance of the carboxy-terminal residue in selective binding. Recent evidence implicates the proteases and transporters of the processing pathway in providing peptides with the correct residues at the carboxyl terminus.

The possibility of B-cell-dependent T-cell development

The development of T cells is thought to be independent of B cells. However, defects in cell-mediated immunity in individuals with B-cell deficiency suggest the contrary. To test whether B cells affect T-lymphocyte development, we constructed mice with a monoclonal T-cell compartment (MT) and monoclonal B- and T-cell compartments (MBTs). In these mice, the T cells expressed a DO 11.10 transgenic (DO-T) cell receptor restricted to major histocompatibility complex (MHC) class IId. While CD4+ DO-T lymphocytes are rare in transgenic H-2b MT mice, we found that in H-2b MBT mice under the influence of B cells, DO-T lymphocytes mature into large numbers of CD4+ peripheral T cells. H-2b MBT mice have more CD4+ thymocytes than H-2b MT mice. These data are consistent with the view that B cells play some role in thymocyte development.

Kb, Kd, and Ld molecules share common tapasin dependencies as determined using a novel epitope tag

The endoplasmic reticulum protein tapasin is considered to be a class I-dedicated chaperone because it facilitates peptide loading by proposed mechanisms such as peptide editing, endoplasmic reticulum retention of nonpeptide-bound molecules, and/or localizing class I near the peptide source. Nonetheless, the primary functions of tapasin remain controversial as do the relative dependencies of different class I molecules on tapasin for optimal peptide loading and surface expression. Tapasin dependencies have been addressed in previous studies by transfecting different class I alleles into tapasin-deficient LCL721.220 cells and then monitoring surface expression and Ag presentation to T cells. Indeed, by these criteria, class I alleles have disparate tapasin-dependencies. In this study, we report a novel and more direct method of comparing tapasin dependency by monitoring the ratio of folded vs open forms of the different mouse class I heavy chains, L(d), K(d), and K(b). Furthermore, we determine the amount of de novo heavy chain synthesis required to attain comparable expression in the presence vs absence of tapasin. Our findings show that tapasin dramatically improves peptide loading of all three of these mouse molecules.

The MHC class II molecule I-Ag7 exists in alternate conformations that are peptide dependent

Insulin-dependent diabetes mellitus is an autoimmune disease that is genetically linked to the HLA class II molecule DQ in humans and to MHC I-Ag7 in nonobese diabetic mice. The I-Ag7 beta-chain is unique and contains multiple polymorphisms, at least one of which is shared with DQ alleles linked to insulin-dependent diabetes mellitus. This polymorphism occurs at position 57 in the beta-chain, in which aspartic acid is mutated to a serine, a change that results in the loss of an interchain salt bridge between alphaArg76 and betaAsp57 at the periphery of the peptide binding groove. Using mAbs we have identified alternative conformations of I-Ag7 class II molecules. By using an invariant chain construct with various peptides engineered into the class II-associated invariant chain peptide (CLIP) region we have found that formation of these conformations is dependent on the peptide occupying the binding groove. Blocking studies with these Abs indicate that these conformations are present at the cell surface and are capable of interactions with TCRs that result in T cell activation.

The alloreactive and self-restricted CD4+ T cell response directed against a single MHC class II/peptide combination

The cellular basis for allograft rejection derives from the strong T cell response to cells bearing foreign MHC. While it was originally assumed that alloreactive T cells focus their recognition on the polymorphic residues that differ between syngeneic and allogeneic MHC molecules, studies with MHC class I-restricted CTL have shown that MHC-bound peptides play a critical role in allorecognition. It has been suggested that alloreactive T cells depend more strongly on interactions with the MHC molecule than with the associated peptide, but there is little evidence to support this idea. Here we have studied the alloreactive and self-restricted response directed against the class II H2-Ab molecule bound with a single peptide, Ep, derived from the H2-Ealpha chain. This MHC class II-peptide combination was a poor target and stimulator of alloreactive CD4+ T cell responses, indicating that MHC-bound peptides are as important for alloreactive CD4+ T cells as they are for alloreactive CTL. We also generated alloreactive T cells with exquisite specificity for the Ab/Ep complex, and compared their reactivity with self-restricted T cells specific for the same Ab/Ep complex. Our results showed that peptide-specific alloreactive T cells, as compared with self-restricted T cells, were more sensitive to peptide stimulation, but equally sensitive to amino acid substitutions in the peptide. These findings indicate that alloreactive and self-restricted T cells interact similarly with their MHC/peptide ligand.

Evaluation of T-cell responses to peptides and lipopeptides with MHC class I binding motifs derived from the amino acid sequence of the 19-kDa lipoprotein of Mycobacterium tuberculosis

Cytotoxic T-lymphocyte (CTL) epitopes on the 19-kDa lipoprotein from Mycobacterium tuberculosis were identified by the use of lipopeptides and their cytokine profile studied. Selection of candidate CTL epitopes was based on synthetic peptides derived from the amino acid sequence of the 19-kDa lipoprotein showing major histocompatibility complex class I (MHC-I) binding motifs (H-2D(b) and H-2L(d)). Their ability to up-regulate and stabilize MHC-I molecules on the mouse lymphoma cell line RMA-S was studied. Similar studies were performed with peptides, in which the anchor amino acid of the H-2D(b) MHC-I motif was replaced by alanine. Three out of five peptides with H-2D(b) or H-2L(d) binding motifs and their corresponding lipopeptides as well, up-regulated and stabilized the H-2D(b) molecules on RMA-S cells. Replacement of the anchor amino acid residues of the H-2D(b) MHC-I motif by alanine revealed that the anchor amino acid asparagine at position 5, contributed more to binding of peptide to H-2D(b) molecules than leucine at position 11. The closely related lipopeptides LP19c and LP19d, in combination with incomplete Freund's adjuvant (IFA), induced CTL responses in C57BL/6 (H-2(b)) mice. These CTLs could recognize the naturally processed antigen, i.e. the 19-kDa antigen protein produced and processed by the EX-19 cell line. The capacity of the various lipopeptides to induce CTL correlated well with the ability of the (lipo)peptide to up-regulate and to stabilize H-2D(b) molecules. Lipopeptide LP19c primed spleen cells showed a T helper type one profile after in vitro stimulation with P19c and P19d 19 kDa peptides. The approach to characterize presumptive 19-kDa CTL epitopes might lead to selection of promising CTL epitopes, which can be applied in the development of subunit tuberculosis vaccines.

Evidence of selective processing of immunodominant epitopes in virally infected cells

Recent advances in clarifying the molecular mechanisms involved in Ag processing and presentation have relied heavily on the use of somatic cell mutants deficient in proteasome subunits, TAP transporter, and cell surface expression of MHC class I molecules. Of particular interest currently are those mutants that lack specific protease activity involved in the generation of antigenic peptides. It is theoretically possible that deficiencies of this nature could selectively prevent the cleavage of certain peptide bonds and thus generate only a subset of antigenic peptides. Gro29/Kb cell line is derived from the wild-type murine Ltk- cell line. This cell line is one example of a mutant that lacks specific protease activities. This deficiency manifests itself in an inability to generate a subset of immunodominant peptide epitopes derived from vesicular stomatitis virus and herpes simplex virus. This in turn leads to a general inability to present these viral epitopes to cytotoxic T lymphocytes (CTL). These studies describe a unique Ag processing deficiency and provide new insight into the role of proteasome-independent proteases in MHC class I-restricted peptide generation.

A novel HLA-B*51 allele (B*5116) identified by nucleotide sequencing

We report here an additional HLA-B*51 variant designated HLA-B*5116. Detected by an abnormal serological reactivity pattern, this variant was identified as a B*51 allele by polymerase chain reaction using sequence-specific primers (PCR-SSP) and characterized by nucleotide sequencing. The new variant sequence match closely with the classical HLA-B*5101 excepted two adjacent nucleotide substitutions at positions 216 and 217 of the third exon and the subsequent Leucine to Glutamic acid change at codon 163 of the alpha2 domain (CTG-->GAG). This new variant was not detected in three different ethnic groups (French, Algerian and Lebanese) suggesting a very rare frequency.

Ly-49CB6 NK Inhibitory Receptor Recognizes Peptide-Receptive H-2Kb 1

NK-mediated cytotoxicity involves two families of receptors: activating receptors that trigger lysis of the target cells being recognized and inhibitory receptors specific primarily for MHC I on the target cell surface that can override the activating signal. MHC I molecules on the cell surface can be classified into molecules made stable by the binding of peptide with high affinity or unstable molecules potentially capable of binding high affinity peptide (hence, peptide receptive) and being converted into stable molecules. It has been previously shown that the Ly-49A inhibitory receptor recognizes stable Dd molecules. We show in this study that the inhibitory receptor Ly-49CB6 recognizes peptide-receptive Kb molecules, but does not recognize Kb molecules once they have bound high affinity peptide.

Mapping the Major Interaction Between Binding Protein and Ig Light Chains to Sites Within the Variable Domain

Newly synthesized Ig chains are known to interact in vivo with the binding protein (BiP), a major peptide-binding chaperone in the endoplasmic reticulum. The predominant interactions between the light chain and BiP are observed early in the folding pathway, when the light chain is either completely reduced, or has only one disulfide bond. In this study, we describe the in vitro reconstitution of BiP binding to the variable domain of light chains (VL). Binding of deliberately unfolded VL was dramatically more avid than that of folded VL, mimicking the interaction in vivo. Furthermore, VL binding was inhibited by addition of ATP, was competed with excess unlabeled VL, and was demonstrated with several different VL proteins. Using this assay, peptides derived from the VL sequence were tested experimentally for their ability to bind BiP. Four peptides from both β sheets of VL were shown to bind BiP specifically, two with significantly higher affinity. As few as these two peptide sites, one from each β sheet of VL, are sufficient to explain the association of BiP with the entire light chain. These results suggest how BiP directs the folding of Ig in vivo and how it may be used in shaping the B cell repertoire.

Recognition of an MHC Class I-Restricted Antigenic Peptide Can Be Modulated by para-Substitution of Its Buried Tyrosine Residues in a TCR-Specific Manner

Conformational dependence of TCR contact residues of the H-2Kb molecule on the two buried tyrosine side chains of the vesicular stomatitis virus (VSV)-8 peptide was investigated by systematic substitutions of the tyrosines with phenylalanine, p-fluorophenylalanine (pFF), or p-bromophenylalanine (pBrF). The results of peptide competition CTL assays revealed that all of the peptide variants, except for the pBrF analogues, had near-native binding to the H-2Kb molecule. Epitope-mapped anti-H-2Kb mAbs detected conformational differences among H-2Kb molecules stabilized with these VSV-8 variants on RMA-S cells. Selective recognition of the VSV-8 analogues was displayed by a panel of three H-2Kb-restricted, anti-VSV-8 TCRs. Thus, these substitutions result in an antigenically significant conformational change of the MHC molecular surface structure at both C and D pockets, and the effect of this change on cognate T cell recognition is dependent on the TCR structure. Our results confirm that the structure of buried peptide side chains can determine the surface conformation of the MHC molecule and demonstrate that even a very subtle structural nuance of the buried side chain can be incorporated into the surface conformation of the MHC molecule. The ability of buried residues to modulate this molecular surface augments the number of residues on the MHC-peptide complex that can be recognized as “foreign” by the CD8+ T cell repertoire and allows for a higher level of antigenic discrimination. This may be an important mechanism to expand the total number of TCR specificities that can respond to a single peptide determinant.

Soluble, high-affinity dimers of T-cell receptors and class II major histocompatibility complexes: biochemical probes for analysis and modulation of immune responses

T cell receptors (TCR) and major histocompatibility complex (MHC) molecules are integral membrane proteins that have central roles in cell-mediated immune recognition. Therefore, soluble analogs of these molecules would be useful for analyzing and possibly modulating antigen-specific immune responses. However, due to the intrinsic low-affinity and inherent solubility problems, it has been difficult to produce soluble high-affinity analogs of TCR and class II MHC molecules. This report describes a general approach which solves this intrinsic low-affinity by constructing soluble divalent analogs using IgG as a molecular scaffold. The divalent nature of the complexes increases the avidity of the chimeric molecules for cognate ligands. The generality of this approach was studied by making soluble divalent analogs of two different classes of proteins, a TCR (2C TCR2Ig) and a class II MHC (MCCI-Ek2Ig) molecule. Direct flow cytometry assays demonstrate that the divalent 2C TCR2Ig chimera retained the specificity of the native 2C TCR, while displaying increased avidity for cognate peptide/MHC ligands, resulting in a high-affinity probe capable of detecting interactions that heretofore have only been detected using surface plasmon resonance. TCR2IgG was also used in immunofluorescence studies to show ER localization of intracellular peptide-MHC complexes after peptide feeding. MCCI-Ek2Ig chimeras were able to both stain and activate an MCC-specific T cell hybridoma. Construction and expression of these two diverse heterodimers demonstrate the generality of this approach. Furthermore, the increased avidity of these soluble divalent proteins makes these chimeric molecules potentially useful in clinical settings for probing and modulating in vivo cellular responses.

Peptide dependency of alloreactive CD4+ T cell responses

Alloreactivity, the capacity of a large number of T lymphocytes to react with foreign MHC molecules, represents the cellular basis for the rejection of tissue grafts. Although it was originally assumed that the TCR of alloreactive T cells focus their recognition on the polymorphic residues that differ between the MHC molecules of responder and stimulator cells, studies in the MHC class I system have clearly demonstrated that MHC-bound peptides can influence this interaction. It remains unclear, however, whether peptides play an equally important role for the recognition of MHC class II molecules by alloreactive CD4+ T cells. Another issue that remains unresolved is the overall frequency of peptide-dependent versus peptide-independent alloreactive T cells. We have addressed these questions with antigen-presenting cells (APC) from H2-M mutant mice that predominantly express a single MHC class II-peptide complex, H2-Ab bound by a peptide (CLIP) derived from the class II-associated invariant chain. APC from these mice were used as targets and stimulators for alloreactive CD4+ T cells. Results demonstrated that the vast majority of CD4+ alloreactive T cells recognize MHC class II molecules in a peptide-dependent fashion.

A Region of Conformational Variability Outside the Peptide-Binding Site of a Class I MHC Molecule

Peptide binding is known to influence the conformation of the surface of class I molecules as detected with mAbs and TCR. A new conformationally sensitive epitope on the mouse class I molecule Kb is defined by mAb AF6-88.5. The recognized structure is affected by amino acid substitutions in any of the three external domains of the class I heavy chain and, in addition, is influenced by the substitution of human for mouse β2-microglobulin. Interestingly, the epitope for this Ab is not affected by mutations within the peptide-binding cleft or by the nature of the peptide bound. These findings indicate that the effect of a change in one domain of class I can radiate to other parts of the molecule. Furthermore, the existence of conformationally sensitive structures outside of the peptide-binding site suggests the possibility that class I molecules may change their structure in response to binding by receptors and ligands such as the TCR and the coligand CD8. Such structural changes may represent signals that can influence cellular activation events.

Peptide ligand structure influences the exchange of beta2-microglobulin by cell surface Kb

We have studied the interactions which occur between the peptide ligand and beta2-microglobulin (beta2m) components of the class I MHC complex by analysing the process of beta2m exchange. We have previously shown that the rate of beta2m exchange on a cell-surface class I MHC complex varies with the peptide ligand to which it is bound. It remains unclear, however, whether the ability of peptide ligand to alter beta2m/heavy-chain association is related to peptide affinity, peptide structure, or both. In this article, we examine the effects of variations in peptide ligand structure on the rate of beta2m exchange by cell surface Kb complexes. Using a panel of alanine substituted variants of the MCMV peptide (YPHFMPTNL), we show that single amino acid changes in peptide sequence can have dramatic effects on the rates of beta2m exchange. The observed changes in beta2m exchange rates are directly due to modification of the peptide ligand structure as they do not reflect changes in peptide affinity. These findings suggest that peptide ligand structure can induce conformational changes in the Kb heavy chain which alter the rates of cell surface beta2m exchange, and provide further evidence for peptide-dependent fluidity of the class I heavy chain.

Anti-MUC1 Antibodies React Directly with MUC1 Peptides Presented by Class I H2 and HLA Molecules

Peptides bound in the groove of MHC class I molecules and detected by CTLs are not normally accessible to Ab. We now report that MUC1 peptides that are bound within the groove of MHC class I molecules (H2 and HLA) and that can be detected by CTLs can also be detected by anti-MUC1 Abs. mAbs to the middle and C-terminal regions of the class I-associated peptides but not to the N terminus were able to react with MUC1 peptides bound to H2Kb and HLA-A*0201, and only to the mid-region for H2Db, by flow cytometry and also to block CTL activity. Molecular modeling showed that the N terminus is buried (and not accessible), whereas the midpeptide residues form a loop and the C terminus is free, making these two regions accessible to Ab. The findings demonstrate for the first time that peptides associated with class I molecules can be detected by anti-peptide Abs.

Identification of new cytotoxic T-cell epitopes on the 38-kilodalton lipoglycoprotein of Mycobacterium tuberculosis by using lipopeptides

Induction of cytotoxic T lymphocytes (CTLs) by vaccination has been shown to protect against bacterial, viral, and tumoral challenge. The aim of this study was to identify CTL epitopes on the 38-kDa lipoglycoprotein from Mycobacterium tuberculosis. The identification of these CTL epitopes was based on synthesizing peptides designed from the 38-kDa lipoglycoprotein, with known major histocompatibility complex class I (MHC-I) binding motifs (H-2Db), and studying their ability to up-regulate and stabilize MHC-I molecules on the mouse lymphoma cell line RMA-S. To improve the capacity of the identified peptides to induce CTL responses in mice, palmitic acid with a cysteine-serine-serine spacer amino acid sequence was attached to the amino terminus of the peptide. Two of five peptides with H-2Db binding motifs and their corresponding lipopeptides up-regulated and stabilized the H-2Db molecules on RMA-S cells. Both lipopeptides, in combination with incomplete Freund's adjuvant, induced CTL responses in C57BL/6 (H-2(b)) mice. Moreover, the lipopeptide induced stronger CTL responses than the peptide. The capacity of the various lipopeptides to induce CTL displayed a good relationship with the ability of the (lipo)peptide to up-regulate and to stabilize H-2Db molecules. The capacity of the peptides and lipopeptides to up-regulate and stabilize MHC-I expression can therefore be used to predict their potential to function as a CTL epitope. The newly identified CTL epitopes and their lipid derivatives provide us with important information for future M. tuberculosis vaccine design.

Clarification of HLA-B serologically ambiguous types by automated DNA sequencing

Assignment of HLA-B types can be hampered by ambiguous reactivity of the typing sera resulting in inaccurate HLA-B assignments. In this study, 19 Korean samples exhibiting ambiguous serologic reactivities were characterized by DNA sequencing. Alleles identified from 7 samples were previously undetected in this population (B*1517, B*4101, B*4701, B*5001, and B*5106) and from 9 samples were common alleles in this population (B*4002, B*4003, B*4006, B*1501, B*1401, B*67012, and B*5401). Three samples were putative HLA-B homozygotes. Three major factors causing serologic ambiguity were identified: weak or false negative reactivity of typing sera (52.4%); cross or false positive reactivity of the sera (38.1%); and absence of information on the reaction patterns due to the lack of appropriate sera in the typing kit (e.g. B*4101 encoded molecule) or to the presence of recently characterized molecules (e.g. B*5106 encoded molecule) (9.5%). Overall, sequencing was helpful in clarifying ambiguous serologic reaction patterns improving the HLA typing for the Korean population.

Presentation of antigenic peptides by products of the major histocompatibility complex

Molecules encoded by the major histocompatibility complex (MHC) are polymorphic integral membrane proteins adapted to the presentation of peptide fragments of foreign antigens to antigen-specific T-cells. The diversity of infectious agents to which an immune response must be mounted poses a unique problem for receptor-ligand interactions; how can proteins whose polymorphism is necessarily limited bind an array of peptides almost infinite in its complexity? Both MHC class I and class II determinants have achieved this goal by harnessing a limited number of peptide side chains to anchor the epitope in place while exploiting conserved features of peptide structure, independent of their primary sequence. While class I molecules interact predominantly with the N- and C-termini of peptides, class II determinants form an extensive hydrogen bonding network along the length of the peptide backbone. Such a strategy ensures high-affinity binding, while selectively exposing the unique features of each ligand for recognition by the T-cell receptor.

A basis for alloreactivity: MHC helical residues broaden peptide recognition by the TCR

The high frequency of alloreactive T cells is a major hindrance for transplantation; however, the molecular basis for alloreactivity remains elusive. We examined the I-Ep alloreactivity of a well-characterized Hb(64-76)/I-Ek-specific murine T cell. Using a combinatorial peptide library approach, we identified a highly stimulatory alloepitope mimic and observed that the recognition of the central TCR contact residues (P3 and P5) was much more flexible than that seen with Hb(64-76)/I-Ek, but still specific. Therefore, alloreactive T cells can recognize a self-peptide/MHC surface; however, the allogeneic MHC molecule changes the recognition requirements for the central region of the peptide, allowing a more diverse repertoire of ligands to be recognized.

Human monoclonal antibody with T-cell-like specificity recognizes MHC class I self-peptide presented by HLA-DR1 on activated cells

Alloreactive T cells recognize peptides presented in the binding groove of major histocompatibility complex molecules (MHCs), whereas B cells mainly recognize the MHCs independent of bound peptides. Here, we demonstrate that the human B-cell repertoire comprises B cells which can be stimulated during pregnancy to produce antibodies reacting with MHCs in a way similar to T cells. The human monoclonal antibody UL-5A1 recognizes DR1(DRA/DRB1*0101) molecules on lymphoblastoid cell lines only if they co-express HLA-A2 or if they have been loaded with HLA-A2-derived peptides. The effect of the HLA-A2 peptide 105-117 on UL-5A1 reactivity was specific, time and dose-dependent. Reactivity increased when naturally processed peptides were removed from DR1 molecules before the HLA-A2 peptide 105-117 was loaded. UL-5A1 reacted specifically with cells that had been activated. The results imply a role of activation of cells in peptide processing and/or loading.

Analysis of the expression of peptide-major histocompatibility complexes using high affinity soluble divalent T cell receptors

Understanding the regulation of cell surface expression of specific peptide-major histocompatibility complex (MHC) complexes is hindered by the lack of direct quantitative analyses of specific peptide-MHC complexes. We have developed a direct quantitative biochemical approach by engineering soluble divalent T cell receptor analogues (TCR-Ig) that have high affinity for their cognate peptide-MHC ligands. The generality of this approach was demonstrated by specific staining of peptide-pulsed cells with two different TCR-Ig complexes: one specific for the murine alloantigen 2C, and one specific for a viral peptide from human T lymphocyte virus-1 presented by human histocompatibility leukocyte antigens-A2. Further, using 2C TCR- Ig, a more detailed analysis of the interaction with cognate peptide-MHC complexes revealed several interesting findings. Soluble divalent 2C TCR-Ig detected significant changes in the level of specific antigenic-peptide MHC cell surface expression in cells treated with gamma-interferon (gamma-IFN). Interestingly, the effects of gamma-IFN on expression of specific peptide-MHC complexes recognized by 2C TCR-Ig were distinct from its effects on total H-2 Ld expression; thus, lower doses of gamma-IFN were required to increase expression of cell surface class I MHC complexes than were required for upregulation of expression of specific peptide-MHC complexes. Analysis of the binding of 2C TCR-Ig for specific peptide-MHC ligands unexpectedly revealed that the affinity of the 2C TCR-Ig for the naturally occurring alloreactive, putatively, negatively selecting, complex, dEV-8-H-2 Kbm3, is very low, weaker than 71 microM. The affinity of the 2C TCR for the other naturally occurring, negatively selecting, alloreactive complex, p2Ca-H-2 Ld, is approximately 1000-fold higher. Thus, negatively selecting peptide-MHC complexes do not necessarily have intrinsically high affinity for cognate TCR. These results, uniquely revealed by this analysis, indicate the importance of using high affinity biologically relevant cognates, such as soluble divalent TCR, in furthering our understanding of immune responses.

Affinity biotinylation: nonradioactive method for specific selection and labeling of cellular proteins

Biotinylation has become a popular alternative to radioiodination for labeling cell surface proteins, whereas labeling of the total cellular protein pool is usually achieved metabolically with [35S]methionine and [35S]cysteine. In this paper we describe a new technique in which total cellular lysate proteins that have been affinity bound to a solid phase are labeled efficiently with biotin. This labeling technique is preferable to direct biotinylation of cell lysate since the unreacted biotin can be readily removed from the sample by washing. The affinity step permits preselection of the molecules to be labeled, thereby decreasing the potential for nonspecific binding during subsequent immunoprecipitation. We applied this affinity biotinylation method to a human cellular lysate in order to preselect the total glycoprotein pool for subsequent immunoprecipitation of HLA class I. Following immunoprecipitation, SDS-PAGE, and Western blot, the biotinylated protein could be readily revealed by enhanced chemiluminescence. The results were comparable to those obtained by radiometabolic labeling and Western blot using a monoclonal antibody probe. Overall, the affinity biotinylation method is faster and more practical than conventional radiolabeling, without any loss in sensitivity.

HLA-B alleles associated with the B15 serologically defined antigens

Cells expressing HLA molecules in the B15 family were identified by serologic typing in routine testing of volunteer donors of various ethnic backgrounds for a bone marrow registry. DNA sequencing was used to identify HLA-B15 alleles associated with each serologic type and to examine the diversity within the B15 antigen family. Alleles which appeared predominantly in each B15 serologic cluster included: B15 with no defined serologic subdivision (B*1501), B62 (B*1501), B63 (B*1516, B*1517), B75 (B*1502, B*1521), and B76/77 (B*1513). Other B*15 alleles were also found associated with the serotypes and some of these alleles (e.g., B*1501 and B*1516) were found in two or more serologic clusters illustrating the complexity of this family. The B15 unsplit and B75 groups were the most complex exhibiting 16 and 7 alleles, respectively, within each serotype. Five new B*15 alleles (B*1530, B*1531, B*1533, B*1534, B*1535) and 5 other new HLA-B alleles (B*38022, B*3910, B*4010, B*51012, and B*5108) were also identified.

A recombinant single-chain human class II MHC molecule (HLA-DR1) as a covalently linked heterotrimer of alpha chain, beta chain, and antigenic peptide, with immunogenicity in vitro and reduced affinity for bacterial superantigens

Major histocompatibility complex (MHC) class II molecules bind to numerous peptides and display these on the cell surface for T cell recognition. In a given immune response, receptors on T cells recognize antigenic peptides that are a minor population of MHC class II-bound peptides. To control which peptides are presented to T cells, it may be desirable to use recombinant MHC molecules with covalently bound antigenic peptides. To study T cell responses to such homogeneous peptide-MHC complexes, we engineered an HLA-DR1 cDNA coding for influenza hemagglutinin, influenza matrix, or HIV p24 gag peptides covalently attached via a peptide spacer to the N terminus of the DR1 beta chain. Co-transfection with DR alpha cDNA into mouse L cells resulted in surface expression of HLA-DR1 molecules that reacted with monoclonal antibodies (mAb) specific for correctly folded HLA-DR epitopes. This suggested that the spacer and peptide did not alter expression or folding of the molecule. We then engineered an additional peptide spacer between the C terminus of a truncated beta chain (without transmembrane or cytoplasmic domains) and the N terminus of full-length DR alpha chain. Transfection of this cDNA into mouse L cells resulted in surface expression of the entire covalently linked heterotrimer of peptide, beta chain, and alpha chain with the expected molecular mass of approximately 66 kDa. These single-chain HLA-DR1 molecules reacted with mAb specific for correctly folded HLA-DR epitopes, and identified one mAb with [MHC + peptide] specificity. Affinity-purified soluble secreted single-chain molecules with truncated alpha chain moved in electrophoresis as compact class II MHC dimers. Cell surface two-chain or single-chain HLA-DR1 molecules with a covalent HA peptide stimulated HLA-DR1-restricted HA-specific T cells. They were immunogenic in vitro for peripheral blood mononuclear cells. The two-chain and single-chain HLA-DR1 molecules with covalent HA peptide had reduced binding for the bacterial superantigens staphylococcal enterotoxin A and B and almost no binding for toxic shock syndrome toxin-1. The unique properties of these engineered HLA-DR1 molecules may facilitate our understanding of the complex nature of antigen recognition and aid in the development of novel vaccines with reduced superantigen binding.

Alloreactive cytotoxic T lymphocytes focus on specific major histocompatibility complex-bound peptides

Alloreactive T cells are often specific for individual peptides that are bound to allogeneic major histocompatibility complex (MHC) molecules. Other alloreactive T cells are reported to be peptide-independent or to recognize MHC conformational changes that are induced by multiple peptides. We tested 12 anti-HLA-B7 alloreactive cytotoxic T lymphocyte (CTL) clones that bind a restricted region of HLA-B7, including three CTL clones that were generated in a protocol designed to stimulate peptide-independent T cells. All 12 CTLs recognized multiple point mutations in the HLA-B7 peptide-binding groove. Eleven of the 12 CTLs recognized specific peptides that eluted in one or two fractions on high-performance liquid chromatography (HPLC). None of the CTLs promiscuously recognized 16 HLA-B7-binding synthetic peptides, although one CTL recognized minor by-products in one synthetic peptide preparation. CTL clone KID-9 cross-reacted with allogeneic HLA-B7 and HLA-B27 molecules and recognized a distinct peptide bound to each MHC molecule. CTL clone KD-11 recognized peptides that eluted in two HPLC fractions and recognized HLA-B7-transfected peptide antigen processing defective T2 cells. These results indicate that CTL allorecognition is peptide-specific whether the allogeneic MHC molecules are expressed on normal cells or antigen processing-deficient cells.

On the calculation of binding free energies using continuum methods: application to MHC class I protein-peptide interactions

This paper describes a methodology to calculate the binding free energy (delta G) of a protein-ligand complex using a continuum model of the solvent. A formal thermodynamic cycle is used to decompose the binding free energy into electrostatic and non-electrostatic contributions. In this cycle, the reactants are discharged in water, associated as purely nonpolar entities, and the final complex is then recharged. The total electrostatic free energies of the protein, the ligand, and the complex in water are calculated with the finite difference Poisson-Boltzmann (FDPB) method. The nonpolar (hydrophobic) binding free energy is calculated using a free energy-surface area relationship, with a single alkane/water surface tension coefficient (gamma aw). The loss in backbone and side-chain configurational entropy upon binding is estimated and added to the electrostatic and the nonpolar components of delta G. The methodology is applied to the binding of the murine MHC class I protein H-2Kb with three distinct peptides, and to the human MHC class I protein HLA-A2 in complex with five different peptides. Despite significant differences in the amino acid sequences of the different peptides, the experimental binding free energy differences (delta delta Gexp) are quite small (< 0.3 and < 2.7 kcal/mol for the H-2Kb and HLA-A2 complexes, respectively). For each protein, the calculations are successful in reproducing a fairly small range of values for delta delta Gcalc (< 4.4 and < 5.2 kcal/mol, respectively) although the relative peptide binding affinities of H-2Kb and HLA-A2 are not reproduced. For all protein-peptide complexes that were treated, it was found that electrostatic interactions oppose binding whereas nonpolar interactions drive complex formation. The two types of interactions appear to be correlated in that larger nonpolar contributions to binding are generally opposed by increased electrostatic contributions favoring dissociation. The factors that drive the binding of peptides to MHC proteins are discussed in light of our results.

Localization, quantitation, and in situ detection of specific peptide-MHC class I complexes using a monoclonal antibody

CD8+ T lymphocytes recognize antigens as short peptides bound to MHC class I molecules. Available methods cannot determine the number and distribution of these ligands on individual cells or detect antigen-presenting cells in tissues. Here we describe a method for eliciting and identifying monoclonal antibodies specific for a particular peptide-MHC class I combination. One such antibody can identify antigen complexes with a limit of detection approaching that of T cells. We used this antibody to determine the number of peptide-class I complexes generated upon viral infection, to identify antigen-presenting cells in cell mixtures, to determine the site of peptide-MHC class I interaction inside cells, and to visualize cells bearing specific peptide-MHC class I complexes after in vivo infection. Similar antibodies may prove useful for diagnostic or therapeutic purposes in cancer, infectious diseases, and autoimmune disorders.

Serologic ambiguity and allelic frequency of the HLA-B40 family in the Korean population

The most frequently identified HLA-B type in Koreans is HLA-B40 (13.4%). Due to the lack of mono-specific alloantisera and cross reactivity of sera used as typing reagents, discrimination between the serologic splits of B40, B60 and B61, has been a problem in tissue typing laboratories. In this study, an efficient PCR-SSP typing system was established to distinguish B60 and B61 and to assess the difficulty in serologic assignment for these types. The SSP system was also used to elucidate the frequency of B40 alleles (B*4001-B*4008) encoding B40 molecules in the Korean population. Eighty eight unrelated individuals identified serologically as B40 positive were selected from 358 consecutive volunteers from the unrelated bone marrow registry. Seven sets of PCR that amplify exons 2 and 3 of the HLA-B gene using 10 sequence specific primers (SSP) were used for discrimination between B60 and B61, and for B40 allelic typing. A clear discrimination of B60 and B61 was possible in all samples including 48 serologically ambiguous samples (B60-14/48; B61-34/48) and 5 potentially B40 homozygous samples (B60/ B61 heterozygotes-4/5; B60 homozygote-1/5). Therefore, the use of a focused SSP approach enhances serologic definition of HLA types in routine clinical testing. In allelic typing, all B60 samples (26) appeared to be B*4001, but B61 samples revealed more heterogeneity (B*4002-36/58, B*4003-4/ 58, B*4006-18/58). In addition, B*4003 seemed to be closely associated with the A24-Cw3-DRB1*02 haplotype (3/4). The characterization of allele frequency as well as haplotypic association will be helpful in determination of the optimal size of the volunteer marrow donor pool in the Korean population.

Peptide-independent recognition by alloreactive cytotoxic T lymphocytes (CTL)

We have isolated several H-2K(b)-alloreactive cytotoxic T cell clones and analyzed their reactivity for several forms of H-2K(b). These cytotoxic T lymphocytes (CTL) were elicited by priming with a skin graft followed by in vitro stimulation using stimulator cells that express an H-2K(b) molecule unable to bind CD8. In contrast to most alloreactive T cells, these CTL were able to recognize H-2K(b) on the surface of the antigen processing defective cell lines RMA-S and T2. Furthermore, this reactivity was not increased by the addition of an extract containing peptides from C57BL/6 (H-2(b)) spleen cells, nor was the reactivity decreased by treating the target cells with acid to remove peptides bound to MHC molecules. The CTL were also capable of recognizing targets expressing the mutant H-2K(bm8) molecule. These findings suggested that the clones recognized determinants on H-2K(b) that were independent of peptide. Further evidence for this hypothesis was provided by experiments in which H-2K(b) produced in Drosophila melanogaster cells and immobilized on the surface of a tissue culture plate was able to stimulate hybridomas derived from these alloreactive T cells. Precursor frequency analysis demonstrated that skin graft priming, whether with skin expressing the wild-type or the mutant H-2K(b) molecule, is a strong stimulus to elicit peptide-independent CTL. Moreover, reconstitution experiments demonstrated that the peptide-independent CTL clones were capable of mediating rapid and complete rejection of H-2-incompatible skin grafts. These findings provide evidence that not all allorecognition is peptide dependent.