Minors held by majors: the H13 minor histocompatibility locus defined as a peptide/MHC class I complex

Genetics of HLA Peptide Presentation and Impact on Outcomes in HLA-Matched Allogeneic Hematopoietic Cell Transplantation

Minor histocompatibility antigens (mHAs), recipient-derived peptide epitopes presented on the cell surface, are known to mediate graft-versus-host disease (GVHD); however, there are no current methods to associate mHA features with GVHD risk. This deficiency is due in part to the lack of technological means to accurately predict, let alone confirm, the tremendous number of potential mHAs in each individual transplant. Previous studies have shown that different HLA molecules present varying fractions of candidate peptide epitopes; however, the genetic "distance" between HLA-matched donors and recipients is relatively constrained. From these 2 observations, it is possible that the HLA type for a donor-recipient pair (DRP) would provide a surrogate measurement of the number of predicted mHAs, which could be related to GVHD risk. Because different HLA molecules present variable numbers of peptide antigens, a predicted cumulative peptide-binding efficiency can be calculated for individual DRP based on the pair's HLA type. The purpose of this study was to test whether cumulative peptide-binding efficiency is associated with the risk of acute GVHD (aGVHD) or relapse. In this retrospective Center for International Blood and Marrow Transplant Research study, a total of 3242 HLA-matched DRPs were analyzed for predicted cumulative peptide-binding efficiency using their HLA types and were divided into tertiles based on their scores. Univariable and multivariable analyses was performed to test for associations between cumulative peptide-binding efficiency for DRPs, divided into the HLA-matched related donor (MRD) and HLA-matched unrelated donor (MUD) cohorts, and the primary outcomes of aGVHD and relapse. Secondary outcomes investigated included overall survival, disease-free survival, and transplantation-related mortality. Using a computationally generated peptidome as a test dataset, the tested series of HLA class I displayed peptide-binding frequencies ranging from 0.1% to 3.8% of the full peptidome, and HLA class II molecules had peptide-binding frequencies of 12% to 77% across the HLA-DRB1 allotypes. By increasing binding efficiency tertile, the cumulative incidence of aGVHD at 6 months for MUD patients was 41%, 41%, and 45% for HLA class I (P = .336) and 44%, 41%, and 42% for HLA class II (P = .452). The cumulative incidences of relapse at 3 years for MUD transplant recipients were 36%, 38%, and 38% for HLA class I (P = .533) and 37%, 37%, and 38% for HLA class II (P = .896). The findings were similar for MRD transplant recipients. Multivariable analysis did not identify any impact of peptide-binding efficiency on aGVHD or relapse in MUD or MRD transplant recipients. Whereas GVHD is mediated by minor antigen mismatches in the context of HLA-matched allo-HCT, peptide-binding efficiency, which was used as a surrogate measurement for predicted number of binding antigens, did not provide additional clinical information for GVHD risk assessment. The negative result may be due to the limitations of this surrogate marker, or it is possible that GVHD is driven by a subset of immunogenic mHAs. Further research should be directed at direct mHA epitope and immunogenicity prediction.

H60: A Unique Murine Hematopoietic Cell-Restricted Minor Histocompatibility Antigen for Graft-versus-Leukemia Effect

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an important treatment for many types of hematological malignancies. Matching of donor and recipient for the major histocompatibility complex (MHC) improves the HSCT reconstitution, but donor-derived T cells reactive to non-MHC encoded minor histocompatibility antigens (MiHAs) can induce graft-versus-host disease (GVHD) while also being needed for graft-versus-leukemia (GVL) effects. MiHAs are allelically variant self-peptides presented conventionally on MHC molecules, but are alloantigenic in transplantation settings. Immunodominant MiHAs are most strongly associated with GVHD and GVL. There is need for mouse paradigms to understand these contradictory effects. H60 is a highly immunodominant mouse MiHA with hematopoietic cell-restricted expression. Immunodominance of H60 is tightly associated with its allelic nature (presence vs. absence of the transcripts), and the qualitative (TCR diversity) and quantitative (frequency) traits of the reactive T cells. The identity as a hematopoietic cell-restricted antigen (HRA) of H60 assists the appearance of the immunodominace in allo-HSCT circumstances, and generation of GVL effects without induction of serious GVHD after adoptive T cell transfer. Also it allows the low avidity T cells to escape thymic negative selection and exert GVL effect in the periphery, which is a previously unevaluated finding related to HRAs. In this review, we describe the molecular features and immunobiology in detail through which H60 selectively exerts its potent GVL effect. We further describe how lessons learned can be extrapolated to human allo-HCST.

Major and minor histocompatibility antigens to NIMA: Prediction of a tolerogenic NIMA effect

The immunologic effects of developmental exposure to non-inherited maternal antigens (NIMA) are heterogeneous, either tolerogenic or immunogenic. The role of minor histocompatibility antigens (MiHA) in NIMA effects is unknown. We have recently reported that the NIMA effect can be classified into two distinct reactivities, low and high responder, to NIMA in utero and during nursing depending on the degree of maternal microchimerism (MMc) and Foxp3 expression of peripheral blood CD4(+)CD25(+) cells after graft-versus-host disease (GVHD) induction. These reactivities were predictable before transplantation, using an MLR-ELISPOT (mixed lymphocyte reaction; enzyme-linked immunospot) assay by comparing the number of IFNγ-producing cells stimulated with NIMA. Moreover, this assay was also applicable in both major and minor NIMA-mismatched setting. These observations are clinically relevant and suggest that it is possible to predict the immunological tolerance to NIMA.

Presentation of Cryptic Peptides by MHC Class I Is Enhanced by Inflammatory Stimuli

Cytolytic T cells eliminate infected or cancer cells by recognizing peptides presented by MHC class I molecules on the cell surface. The antigenic peptides are derived primarily from newly synthesized proteins including those produced by cryptic translation mechanisms. Previous studies have shown that cryptic translation can be initiated by distinct mechanisms at non-AUG codons in addition to conventional translation initiated at the canonical AUG start codon. In this study, we show that presentation of endogenously translated cryptic peptides is enhanced by TLR signaling pathways involved in pathogen recognition as well as by infection with different viruses. This enhancement of cryptic peptides was caused by proinflammatory cytokines, secreted in response to microbial infection. Furthermore, blocking these cytokines abrogated the enhancement of cryptic peptide presentation in response to infection. Thus, presentation of cryptic peptides is selectively enhanced during inflammation and infection, which could allow the immune system to detect intracellular pathogens that might otherwise escape detection because of inhibition of conventional host translation mechanisms.

Rejection of leukemic cells requires antigen-specific T cells with high functional avidity

In a context where injection of antigen (Ag)-specific T cells probably represents the future of leukemia immunotherapy, identification of optimal target Ags is crucial. We therefore sought to discover a reliable marker for selection of the most potent Ags. To this end, (1) we immunized mice against 8 individual Ags: 4 minor histocompatibility Ags (miHAs) and 4 leukemia-associated Ags (LAAs) that were overexpressed on leukemic relative to normal thymocytes; (2) we assessed their ability to reject EL4 leukemic cells; and (3) we correlated the properties of our Ags (and their cognate T cells) with their ability to induce protective antileukemic responses. Overall, individual miHAs instigated more potent antileukemic responses than LAAs. Three features had no influence on the ability of primed T cells to reject leukemic cells: (1) MHC-peptide affinity; (2) the stability of MHC-peptide complexes; and (3) epitope density at the surface of leukemic cells, as assessed using mass spectrometry. The cardinal feature of successful Ags is that they were recognized by high-avidity CD8 T cells that proliferated extensively in vivo. Our work suggests that in vitro evaluation of functional avidity represents the best criterion for selection of Ags, which should be prioritized in clinical trials of leukemia immunotherapy.

Bim mediates the elimination of functionally unfit Th1 responders from the memory pool

Selective clonal deletion in the CD4⁺ T cell compartment during the transition from effector to memory is accompanied by enhanced expression of the pro-apoptotic Bcl-2 family member Bim. Here, we show that Bim deficiency enables the survival of poorly functional Th1 responders that are normally eliminated during contraction. However, rescued bim^−/− CD4⁺ “memory” T cells continued to demonstrate deficient effector functions, poor sensitivity to antigen and an inability to respond to secondary challenge. Our results demonstrate that Bim activity plays a key role in shaping the CD4⁺ memory T cell repertoire, ensuring the emergence of highly functional CD4⁺ memory T cells and the elimination of Th1 effector cells with sub-optimal function. We propose that Bim is a key mediator of T cell death in the absence of appropriate TCR-driven activation and differentiation.

Tolerogenic effect of non-inherited maternal antigens in hematopoietic stem cell transplantation

Major histocompatibility complex antigens that provoke severe transplant reactions are referred to as the human leukocyte antigen (HLA) in human and as the H-2 in mice. Even if the donor and recipient are HLA-identical siblings, graft-versus-host reactions have been linked to differences in the minor histocompatibility antigen. As the chance of finding an HLA-identical sibling donor is only 25%, attention has been focused on using alternative donors. An HLA-mismatched donor with non-inherited maternal antigens (NIMA) is less immunogenic than that with non-inherited paternal antigens, because the contact between the immune systems of the mother and child during pregnancy affects the immune response of the child against NIMA. However, the immunologic effects of developmental exposure to NIMA are heterogeneous, and can be either tolerogenic or immunogenic. We recently have devised a novel method for predicting the tolerogenic effect of NIMA. In this review, we overview the evidence for the existence of the NIMA tolerogenic effect, the possible cellular and molecular basis of the phenomenon, and its utilization in hematopoietic stem cell transplantation. We suggest a future direction for the safe clinical use of this phenomenon, fetomaternal tolerance, in the transplantation field.

Prediction of reactivity to noninherited maternal antigen in MHC-mismatched, minor histocompatibility antigen-matched stem cell transplantation in a mouse model

The immunologic effects of developmental exposure to noninherited maternal Ags (NIMAs) are quite variable. Both tolerizing influence and inducing alloreaction have been observed on clinical transplantation. The role of minor histocompatibility Ags (MiHAs) in NIMA effects is unknown. MiHA is either matched or mismatched in NIMA-mismatched transplantation because a donor of the transplantation is usually limited to a family member. To exclude the participation of MiHA in a NIMA effect for MHC (H-2) is clinically relevant because mismatched MiHA may induce severe alloreaction. The aim of this study is to understand the mechanism of NIMA effects in MHC-mismatched, MiHA-matched hematopoietic stem cell transplantation. Although all offsprings are exposed to the maternal Ags, the NIMA effect for the H-2 Ag was not evident. However, they exhibit two distinct reactivities, low and high responder, to NIMA in utero and during nursing depending on the degree of maternal microchimerism. Low responders survived longer with less graft-versus-host disease. These reactivities were correlated with Foxp3 expression of peripheral blood CD4(+)CD25(+) cells after graft-versus-host disease induction and the number of IFN-γ-producing cells stimulated with NIMA pretransplantation. These observations are clinically relevant and suggest that it is possible to predict the immunological tolerance to NIMA.

Minor histocompatibility antigens: presentation principles, recognition logic and the potential for a healing hand

PURPOSE OF REVIEW

There is ample evidence indicating a pathologic role for minor histocompatibility antigens in inciting graft-versus-host disease in major histocompatibility complex (MHC)-matched bone marrow transplantation and rejection of solid organ allografts. Here we review the current knowledge of the genetic and biochemical bases for the cause of minor histoincompatibility and the structural basis for the recognition of the resulting alloantigens by the T-cell receptor.

RECENT FINDINGS

Recent evidence indicates that we as independently conceived individuals are genetically unique, thus, offering a mechanism for minor histoincompatibility between MHC-identical donor-recipient pairs. Furthermore, advances in delineating the mechanisms underlying antigen cross-presentation by MHC class I molecules and a critical role for autophagy in presenting cytoplasmic antigens by MHC class II molecules have been made. These new insights coupled with the X-ray crystallographic solution of several peptide/MHC-T-cell receptor structures have revealed mechanisms of histoincompatibility.

SUMMARY

On the basis of these new insights, ways to test for allograft compatibility and concoction of immunotherapies are discussed.

Major histocompatibility complex class I-mediated inhibition of neurite outgrowth from peripheral nerves

Studies of mice deficient in classical major histocompatability complex class I (MHCI) revealed that MHCI plays an important role in neurodevelopment in the central nervous system. We previously studied the effects of recombinant MHCI molecules on wildtype retina explants and observed that MHCI can inhibit retina neurite outgrowth, with self-MHCI molecules having greater inhibitory effect than non-self MHCI molecules. Here, we examined classical MHCI's effects on axon outgrowth from neurons of the peripheral nervous system (PNS). We used the embryonic dorsal root ganglia (DRG) explant model since their neurons express MHCI and because DRG explants have been widely used to assess the effects of molecules on axonal outgrowth from PNS neurons. We observed that picomolar levels of a recombinant self-MHCI molecule, but not non-self MHCI molecules, inhibited axon outgrowth from DRG explants. This differential sensitivity to self- vs. non-self MHCI suggests that early in development, self-MHCI may "educate" PNS neurons to express appropriate MHCI receptors, as occurs during natural killer cell development. Furthermore, we observed that a MHCI tetramer stained embryonic DRG neurons, indicating the expression of classical MHCI receptors. These results suggest that MHCI and MHCI receptors play roles during early stages of PNS development and may provide new targets of therapeutic strategies to promote neuronal outgrowth after PNS injury.

Differential suppression of tumor-specific CD8+ T cells by regulatory T cells

In the CT26 BALB/c murine model of colorectal carcinoma, depletion of regulatory T cells (Tregs) prior to tumor inoculation results in protective immunity to both CT26 and other BALB/c-derived tumors of diverse histological origin. In this paper, we show that cross-protection can be conferred by adoptively transferred CD8(+) CTLs. Other schedules for inducing immunity to CT26 have been described, but they do not lead to cross-protection. We show that Treg ablation facilitates the development of new CTL specificities that are normally cryptic, and have mapped the root epitope of one of these responses. This work has allowed us to demonstrate how the specificity of CTL responses to tumor Ags can be controlled via differential suppression of CTL specificities by Tregs, and how this can result in very different physiological outcomes.

Sensitization to minor antigens is a significant barrier in bone marrow transplantation and is prevented by CD154:CD40 blockade

Sensitization to major histocompatibility complex (MHC) alloantigens is critical in transplantation rejection. The mechanism of sensitization to minor histocompatibility antigens (Mi-HAg) has not been thoroughly explored. We used a mouse model of allosensitization to Mi-HAg to study the Mi-HAg sensitization barrier in bone marrow transplantation (BMT). AKR mice were sensitized with MHC congenic Mi-HAg disparate B10.BR skin grafts. Adaptive humoral (B-cells) and cellular (T cells) responses to Mi-HAg are elicited. In subsequent BMT, only 20% of sensitized mice engrafted, while 100% of unsensitized mice did. In vivo cytotoxicity assays showed that Mi-HAg sensitized AKR mice eliminated CFSE labeled donor splenocytes significantly more rapidly than naïve AKR mice but less rapidly than MHC-sensitized recipients. Sera from Mi-HAg sensitized mice also reacted with cells from other mouse strains, suggesting that Mi-HAg peptides were broadly shared between mouse strains. The production of anti-donor-Mi-HAg antibodies was totally prevented in mice treated with anti-CD154 during skin grafting, suggesting a critical role for the CD154:CD40 pathway in B-cell reactivity to Mi-HAg. Moreover, anti-CD154 treatment promoted BM engraftment to 100% in recipients previously sensitized to donor Mi-HAg. Taken together, Mi-HAg sensitization poses a significant barrier in BMT and can be overcome with CD154:CD40 costimulatory blockade.

Endoplasmic reticulum aminopeptidase associated with antigen processing defines the composition and structure of MHC class I peptide repertoire in normal and virus-infected cells

The MHC class I (MHC-I) molecules ferry a cargo of peptides to the cell surface as potential ligands for CD8(+) cytotoxic T cells. For nearly 20 years, the cargo has been described as a collection of short 8-9 mer peptides, whose length and sequences were believed to be primarily determined by the peptide-binding groove of MHC-I molecules. Yet the mechanisms for producing peptides of such optimal length and composition have remained unclear. In this study, using mass spectrometry, we determined the amino acid sequences of a large number of naturally processed peptides in mice lacking the endoplasmic reticulum aminopeptidase associated with Ag processing (ERAAP). We find that ERAAP-deficiency changed the oeuvre and caused a marked increase in the length of peptides normally presented by MHC-I. Furthermore, we observed similar changes in the length of viral peptides recognized by CD8(+) T cells in mouse CMV-infected ERAAP-deficient mice. In these mice, a distinct CD8(+) T cell population was elicited with specificity for an N-terminally extended epitope. Thus, the characteristic length, as well as the composition of MHC-I peptide cargo, is determined not only by the MHC-I peptide-binding groove but also by ERAAP proteolysis in the endoplasmic reticulum.

Neurons preferentially respond to self-MHC class I allele products regardless of peptide presented

Studies of mice lacking MHC class I (MHC I)-associated proteins have demonstrated a role for MHC I in neurodevelopment. A central question arising from these observations is whether neuronal recognition of MHC I has specificity for the MHC I allele product and the peptide presented. Using a well-established embryonic retina explant system, we observed that picomolar levels of a recombinant self-MHC I molecule inhibited neurite outgrowth. We then assessed the neurobiological activity of a panel of recombinant soluble MHC Is, consisting of different MHC I heavy chains with a defined self- or nonself-peptide presented, on cultured embryonic retinas from mice with different MHC I haplotypes. We observed that self-MHC I allele products had greater inhibitory neuroactivity than nonself-MHC I molecules, regardless of the nature of the peptide presented, a pattern akin to MHC I recognition by some innate immune system receptors. However, self-MHC I molecules had no effect on retinas from MHC I-deficient mice. These observations suggest that neuronal recognition of MHC I may be coordinated with the inherited MHC I alleles, as occurs in the innate immune system. Consistent with this notion, we show that MHC I and MHC I receptors are coexpressed by precursor cells at the earliest stages of retina development, which could enable such coordination.

The immunological impact of genetic drift in the B10.BR congenic inbred mouse strain

The MHC-matched, minor histocompatibility Ag (miHA)-mismatched B10.BR-->CBA strain combination has been used to elucidate the immunobiology of graft-vs-host disease (GVHD) following allogeneic bone marrow transplantation. Studies conducted in the 1980s had established that B10.BR CD8+ T cells were capable of mediating GVHD in the absence of CD4+ T cells, and that CD4+ T cells were unable to induce lethal disease. In more recent studies with this GVHD model, we detected etiological discrepancies with the previously published results, which suggested that genetic drift might have occurred within the B10.BR strain. In particular, there was increased allorecognition of CBA miHA by B10.BR CD4+ T cells, as determined by both TCR Vbeta spectratype analysis and the induction of lethal GVHD in CBA recipients. Additionally, alloreactivity was observed between the genetically drifted mice (B10.BR/Jdrif) and mice rederived from frozen embryos of the original strain (B10.BR/Jrep) using Vbeta spectratype analysis and IFN-gamma ELISPOT assays, suggesting that new miHA differences had arisen between the mice. Furthermore, T cell-depleted B10.BR/Jdrif bone marrow cells were unable to provide long-term survival following either allogeneic or syngeneic bone marrow transplantation. Gene expression analysis revealed several genes involved in hematopoiesis that were overexpressed in the lineage-negative fraction of B10.BR/Jdrif bone marrow, as compared with B10.BR/Jrep mice. Taken together, these results suggest that genetic drift in the B10.BR strain has significantly impacted the immune alloreactive response in the GVHD model by causing altered expression of miHA and diminished capacity for survival following transplantation into lethally irradiated recipients.

Endoplasmic reticulum aminopeptidase associated with antigen processing regulates quality of processed peptides presented by MHC class I molecules

Effective immune surveillance by CD8 T cells depends on the presentation of diverse peptides by MHC class I (pMHC I) molecules on the cell surface. The pMHC I repertoire is shaped in the endoplasmic reticulum (ER) by the ER aminopeptidase associated with Ag processing (ERAAP). The ERAAP activity is required for producing peptides of appropriate length for generating optimal pMHC I. Paradoxically, ERAAP also inhibits generation of certain peptides such as the SVL9 (SSVVGVWYL) peptide encoded by the H13(a) histocompatibility gene and presented by D(b) MHC by an unknown mechanism. In this study, we show that the presentation of the SVL9-D(b) complex is inhibited when other peptides compete for binding D(b). Conversely, improving the binding of SVL9 peptide to D(b) suppresses the inhibition. Interestingly, the inhibitory effect of competitor peptides is observed only when ERAAP is expressed in the same cells. Thus, ERAAP, in concert with MHC I molecules, regulates the quality of processed peptides presented on the cell surface.

Recognition of allo-peptide is governed by novel anchor imposition and limited variations in TCR contact residues

Immune specificity of a T cell is determined by the TCR contact residues exposed on the antigenic peptide/MHC complex. Naturally processed, biallelic epitopes from H7 minor histocompatibility (mH) antigen vary in position 7 (p7) from aspartic acid (D) to a glutamic acid (E), which differ by an additional methylene (-CH(2)) in the side chain. Here, we show that this variation generates a strong anti-H7a or anti-H7b cytotoxic T cell responses. Further, the H7 allelic peptides use p6 asparagine as their central anchor residue and amino acid variations in either the canonical p5 or the predicted p6 anchor positions in the antigenic epitope were detrimental for TCR recognition. In addition, introduction of any other amino acids, except asparagine, in the polymorphic p7 significantly abolished the ability of anti-H7b TCR recognition. This demonstrates that only an asparagine with an amine group as a side chain instead of a charged oxygen radical could effectively stimulate the anti-H7b specific T cells. Our findings provide evidence that mH antigen-specific TCRs are highly stringent in recognizing their cognate epitopes.

T cell allorecognition and MHC restriction--A case of Jekyll and Hyde?

A great paradox in cellular immunology is how T cell allorecognition exists at high frequencies (up to 10%) despite the stringent requirements of discriminating 'self' from 'non-self' imposed by MHC restriction. Thus, in tissue transplantation, a substantial proportion of the recipient's T cells will have the ability to recognize the graft and instigate an immune response against the transplanted tissue, ultimately resulting in graft rejection--a manifestation of T cell alloreactivity. Transplantation of human organs and lymphoid cells as treatment for otherwise life-threatening diseases has become a more routine medical procedure making this problem of great importance. Immunologists have gained important insights into the mechanisms of T cell alloreactivity from cytotoxic T cell assays, affinity-avidity studies, and crystal structures of peptide-MHC (pMHC) molecules and T cell receptors (TCRs) both alone and in complex. Despite the clinical significance of alloreactivity, the crystal structure of an alloreactive human TCR in complex with both cognate pMHC and an allogeneic pMHC complex has yet to be determined. This review highlights some of the important findings from studies characterizing the way in which alloreactive T cell receptors and pMHC molecules interact in an attempt to resolve this great irony of the cellular immune response.

Distributions of single nucleotide polymorphisms in differential chromosome segments of congenic resistant strains that define minor histocompatibility antigens

Minor histocompatibility antigens (MiHAs) stimulate the rejection of allografts when donors and recipients are matched at the major histocompatibility complex (MHC). The majority of identified autosomal MiHAs were generated by non-synonymous (NS) substitutions that alter MHC class I-binding peptides. The mosaic distribution of single nucleotide polymorphisms (SNPs) that distinguish inbred mouse strains led us to hypothesize that MiHA genes defined by congenic strains on C57BL/6 and C57BL/10 backgrounds map to chromosomal regions with relatively high numbers of NS SNPs that distinguish C57 strains from other common inbred strains. To test this hypothesis, we mapped the ends of differential chromosome segments of congenic strains, which define 12 MiHAs, relative to microsatellites and SNPs. The lengths of differential segments ranged from 9.7 to 105.9 Mbp in congenic strains where no attempts were made to select recombinants within these segments. There was no apparent correlation between differential segment length and number of backcrosses, suggesting that factors other than the number of opportunities for recombination affected the differential segment lengths in these congenics. These differential segments included higher numbers of NS SNPs that distinguish C57BL/6J from A/J, DBA/2J, and 129S1/J than would be predicted if these SNPs were uniformly distributed along the chromosomes. The most extreme case was the H8 congenic that included 74% of the SNPs on chromosome 14 within its 9.7-11.1 Mbp differential segment. These results point toward a direct relationship between the level of genomic divergence, as indicated by numbers of NS SNPs, and numbers of MiHAs that collectively determine the magnitude of allograft rejection.

Mechanisms of minor histocompatibility antigen immunogenicity: the role of infinitesimal versus structurally profound polymorphisms

Minor histocompatibility antigens (mHAgs) are a diverse collection of major histocompatibility complex (MHC)-bound peptides that play a critical role in the induction of detrimental graft-versus-host disease (GVHD) or the development of beneficial graft-versustumor (GVT) effects after allogeneic hematopoietic stem cell transplantation. mHAgs are a consequence of allelic polymorphism that translates to disparity in MHC-presented peptide epitopes between transplant donor and recipient. This donor/recipient allelic disparity can range from infinitesimal amino side chain differences between MHC-presented peptides, to profound structural polymorphisms in genes and proteins that can nullify transcription or translation of one allelic variant and result in the complete abrogation of its presentation by MHC.

Effective graft depletion of MiHAg T-cell specificities and consequences for graft-versus-host disease

Minor histocompatibility antigen (MiHAg) differences between donor and recipient in MHC-matched allogeneic hematopoietic stem cell transplantation (allo-HSCT) often result in graft-versus-host disease (GVHD). While MiHAg-specific T-cell responses can in theory be directed against a large number of polymorphic differences between donor and recipient, in practice, T-cell responses against only a small set of MiHAgs appear to dominate the immune response, and it has been suggested that immunodominance may predict an important contribution to the development of GVHD. Here, we addressed the feasibility of graft engineering by ex vivo removal of T cells with 1 or more defined antigen specificities in a well-characterized experimental HSCT model (B6 → BALB.B). We demonstrate that immunodominant H60- and H4-specific CD8+ T-cell responses can be effectively suppressed through MHC class I tetramer–mediated purging of the naive CD8+ T cell repertoire. Importantly, the development of GVHD occurs unimpeded upon suppression of the immunodominant MiHAg-specific T-cell response. These data indicate that antigen-specific graft engineering is feasible, but that parameters other than immunodominance may be required to select T-cell specificities that are targeted for removal.

The contributions of mass spectrometry to understanding of immune recognition by T lymphocytes

Over the last 15 years, the ability of mass spectrometry to analyze complex peptide mixtures and identify individual species has provided unprecedented insights into the repertoire of peptide antigens displayed by MHC molecules and recognized by T lymphocytes. These include: understanding the peptide binding specificity of MHC molecules; understanding of roles of different intracellular components of the antigen processing pathways in determining the peptide display; and identification of a large number of individual peptide antigens associated with infectious diseases, cancer, and transplant rejection that have provided the basis for new immunologically based therapies. This review will summarize the impact that the application of mass spectrometry has had on these advances, with particular attention to the contributions of Professor Donald Hunt and members of his laboratory, and point out the opportunities for future work.

A screen for retrotransposed imprinted genes reveals an association between X chromosome homology and maternal germ-line methylation

Imprinted genes undergo epigenetic modifications during gametogenesis, which lead to transcriptional silencing of either the maternally or the paternally derived allele in the subsequent generation. Previous work has suggested an association between imprinting and the products of retrotransposition, but the nature of this link is not well defined. In the mouse, three imprinted genes have been described that originated by retrotransposition and overlap CpG islands which undergo methylation during oogenesis. Nap1l5, U2af1-rs1, and Inpp5f_v2 are likely to encode proteins and share two additional genetic properties: they are located within introns of host transcripts and are derived from parental genes on the X chromosome. Using these sequence features alone, we identified Mcts2, a novel candidate imprinted retrogene on mouse Chromosome 2. Mcts2 has been validated as imprinted by demonstrating that it is paternally expressed and undergoes promoter methylation during oogenesis. The orthologous human retrogenes NAP1L5, INPP5F_V2, and MCTS2 are also shown to be paternally expressed, thus delineating novel imprinted loci on human Chromosomes 4, 10, and 20. The striking correlation between imprinting and X chromosome provenance suggests that retrotransposed elements with homology to the X chromosome can be selectively targeted for methylation during mammalian oogenesis.

The conventional view is that DNA carries all of our heritable information and our genes control development into adulthood. The discovery of epigenetics, a term coined to describe effects that are not coded for by DNA sequence, but can nonetheless affect our development and well-being, has added another layer of complexity to our understanding of genetics. One class of genes under epigenetic control are imprinted genes. Mammals inherit two copies of every gene, one from mother and one from father, and in most cases, both are active. However, for a small number of imprinted genes in mammals, only one is active, either the maternal or the paternal copy. Epigenetics amounts to a control system for switching genes on and off appropriately. We focus on a group of little-studied imprinted genes that share features that give clues to their evolutionary origins. These so-called “retrogenes” are protein-coding sequences of DNA that have undergone duplication and jumped into novel locations in the genome. Because of this, it is possible to determine where, and roughly when, many of the imprinted retrogenes originated. This provides an opportunity to study the molecular events that have generated imprinted genes during mammalian evolution.

Pooling and PCR as a method to combat low frequency gene targeting in mouse embryonic stem cells

The introduction of germ line modifications by gene targeting in mouse embryonic stem (ES) cells has proven a fundamental technology to relate genes to mammalian biology. Critical aspects required for successful gene targeting have traditionally been experimental enhancements that increase the frequency or detection of homologous recombination within ES cells; however, the utilization of such methods may still result in the failed isolation of a positively targeted ES cell clone. In this study, we discuss the current enhancement methods and describe an ES cell pooling strategy that maximizes the ability to detect properly targeted ES cells regardless of an inherent low targeting efficiency. The sensitivity required to detect correctly targeted events out of a pool of ES cell clones is provided by polymerase chain reaction (PCR), and only those pools containing positives need to be expanded and screened to find individually targeted clones. This method made it possible to identify targeted clones from a screen of approximately 2,300 ES cell colonies by performing only 123 PCR reactions. This technically streamlined approach bypasses the need to troubleshoot and re-engineer an existing targeting construct that is functionally suitable despite its low targeting frequency.

Structural analysis of H2-Db class I molecules containing two different allelic forms of the type 1 diabetes susceptibility factor beta-2 microglobulin: Implications for the mechanism underlying variations in antigen presentation

Beta-2 microglobulin (beta2m) is a member of the immunoglobulin-like domain superfamily that is an essential structural subunit of the MHC class I (MHC-I) molecule. beta2m was previously identified as a susceptibility factor for the development of type 1 diabetes (T1D) in NOD mice, whereby transgenic expression of the beta2ma variant, but not the beta2mb variant, restored diabetes susceptibility to normally resistant NOD.beta2mnull mice. Here we report the crystal structures and thermodynamic stabilities of the NOD MHC-I molecule H2-Db containing these two variants. Our results reveal subtle differences in the structures of the beta2m variants, namely in minor loop shifts and in variations in the hydrogen bonding networks at the interfaces between the components of the ternary complex. We also demonstrate that the thermodynamic stabilities of the beta2m variants in isolation differ. However, the conformation of the peptide in the MHC cleft is unchanged in beta2m allelic Db complexes, as are the TCR recognition surfaces. Thus, despite modest structural differences between allelic complexes, the evidence indicates that Db peptide presentation of the representative peptide is unchanged in the context of either beta2m allelic variant. These data suggest that other mechanisms, such as differential association of MHC-I in multiprotein complexes, are likely responsible for the effect of beta2m on T1D development.

Molecular basis for the high affinity interaction between the thymic leukemia antigen and the CD8alphaalpha molecule

The mouse thymic leukemia (TL) Ag is a nonclassical MHC class I molecule that binds with higher affinity to CD8alphaalpha than CD8alphabeta. The interaction of CD8alphaalpha with TL is important for lymphocyte regulation in the intestine. Therefore, we studied the molecular basis for TL Ag binding to CD8alphaalpha. The stronger affinity of the TL Ag for CD8alphaalpha is largely mediated by three amino acids on exposed loops of the conserved alpha3 domain. Mutant classical class I molecules substituted with TL Ag amino acids at these positions mimic the ability to interact with CD8alphaalpha and modulate lymphocyte function. These data indicate that small changes in the alpha3 domain of class I molecules potentially can have profound physiologic consequences.

CD8 T-cell ability to exert immunodomination correlates with T-cell receptor: Epitope association rate

When presented alone, H7 a and HY antigens elicit CD8 T-cell responses of similar amplitude, but H7 a totally abrogates the response to HY when both antigens are presented on the same antigen-presenting cell. We found that H7a- and HY-specific T-cell precursors had similar frequencies in nonimmune mice and expressed similar levels of CD5. The H7a -specific CD8 T-cell repertoire harvested at the time of primary response showed highly restricted T-cell receptor (TCR) diversity. Furthermore, T cells specific for H7a and HY expressed equivalent levels of CD8 and TCR and displayed similar tetramer decay rates. The key difference was that anti-H7a T cells exhibited a much more rapid TCR:epitope on-rate than anti-HY T cells. Coupled with evidence that primed CD8 T cells limit the duration of antigen presentation by killing or inactivating antigen-presenting cells, our data support a novel and simple model for immunodomination: the main feature of T cells that exert immunodomination is that, compared with other T cells, they are functionally primed after a shorter duration of antigen presentation.

Induction of obliterative airway disease in murine tracheal allografts by CD8+ CTLs recognizing a single minor histocompatibility antigen

The role of minor histocompatibility Ag (mHAg)-specific CD8+ CTLs in the pathogenesis of chronic lung allograft rejection (bronchiolitis obliterans syndrome) remains to be elucidated. Thus, the goal of this study was to define the role of a single mHAg mismatch at the polymorphic H13 allele in the development of obliterative airway disease (OAD) after murine heterotopic tracheal transplantation. The H13a and H13b alleles encode for the SSVVGVWYL (SVL9) and SSVIGVWYL (SIL9) mHAgs, respectively, presented in the context of the H2Db MHC class I molecule. Toward this, C56BL/10SnJ (H13a) tracheal allografts were transplanted into congenic B10.CE-H13b Aw(30NX)/Sn (H13b) recipients. The allografts were harvested at 30, 60, and 90 days after transplantation, and OAD lesions (epithelial damage, cellular infiltration, and luminal fibrosis) were confirmed histologically. Selected groups of mice were immunized (s.c.) or tolerized (i.v.) with the SVL9 peptide before transplantation. This single mHAg mismatch induced the development of OAD within 90 days. SVL9 immunization significantly accelerated the kinetics of the OAD lesions. In contrast, SVL9 tolerization completely abrogated the development of OAD. This was correlated with a complete abrogation of H13a-specific CD8+ CTL responses with a significant reduction in the frequency of IFN-gamma-producing CTLs and the activation of TGF-beta-producing CD8+ T cells. In conclusion, a single mHAg mismatch can induce the development of OAD. These data also suggest that mHAg-reactive CD8+ CTLs may play an important role in the pathogenesis of chronic lung allograft rejection in humans.

Unanticipated antigens: translation initiation at CUG with leucine

Major histocompatibility class I molecules display tens of thousands of peptides on the cell surface for immune surveillance by T cells. The peptide repertoire represents virtually all cellular translation products, and can thus reveal a foreign presence inside the cell. These peptides are derived from not only conventional but also cryptic translational reading frames, including some without conventional AUG codons. To define the mechanism that generates these cryptic peptides, we used T cells as probes to analyze the peptides generated in transfected cells. We found that when CUG acts as an alternate initiation codon, it can be decoded as leucine rather than the expected methionine residue. The leucine start does not depend on an internal ribosome entry site–like mRNA structure, and its efficiency is enhanced by the Kozak nucleotide context. Furthermore, ribosomes scan 5′ to 3′ specifically for the CUG initiation codon in a eukaryotic translation initiation factor 2–independent manner. Because eukaryotic translation initiation factor 2 is frequently targeted to inhibit protein synthesis, this novel translation mechanism allows stressed cells to display antigenic peptides. This initiation mechanism could also be used at non-AUG initiation codons often found in viral transcripts as well as in a growing list of cellular genes.

Proteins have been identified for which a unique translational machinery makes use of unconventional start codons

Rapid identification of preformed alloreactive T cells for use in a clinical setting

BACKGROUND

In clinical practice, HLA matching is generally applied to minimize the incidence of graft rejection after transplantation. Recently, graft rejection has been directly associated with the presence of preformed alloreactive T cells before transplantation. Despite this knowledge, assays to rapidly quantify preformed alloreactivity are not available for use in a clinical setting. In this study, such an assay was developed and evaluated in a large cohort to correlate alloreactive T-cell reactivity with HLA matching.

METHODS

Stimulator peripheral blood mononuclear cells were prestained with CD45-fluorescein isothiocyanate antibody and mixed with responder peripheral blood mononuclear cells. Activation-induced cytokine secretion was blocked using brefeldin A. After 6 hr, functionally active alloreactive responder CD4 and CD8 T cells were quantified among fluorescein isothiocyanate-negative cells by their expression of interferon-gamma on flow cytometry.

RESULTS

Directly alloreactive CD4 and CD8 T cells among both stimulators and responders were easily distinguished after 6 hr of stimulation without being affected by bystander activation. Among 128 paired combinations, 23.4% of individuals had alloreactive CD8 T cells, 15.7% had alloreactive CD4 T cells, and 12.5% had alloreactivity in both T-cell subpopulations. Alloreactive T cells decreased from circulation within a few days after transplantation. In line with well-known clinical observations that associate HLA matching with graft outcome, the number of HLA-A and -B mismatches correlated with alloreactive CD8 T-cell frequencies in the whole study population, whereas it did not predict alloreactivity on an individual basis.

CONCLUSION

Alloreactive T cells may rapidly be quantified after 6 hr of stimulation. Thus, the flow cytometric approach may be applied in a clinical setting to facilitate the individualization of immunosuppressive therapy and studies on the identification of patients who are at increased risk to develop graft rejection.

The impact of self-tolerance on the polyclonal CD8+ T cell repertoire

TCRs possess considerable cross-reactivity toward structurally related Ags. Because the signaling threshold for negative selection is lower than that required for activation of mature T cells, the question arises as to which extent thymic deletion of self-specific T cells affects T cell responsiveness toward foreign peptides. In this study we show, in three different mouse models systems, that the polyclonal CD8(+) T cell repertoire has a marked ability to react against the majority of Ags related to self despite self-tolerance, even in cases where self and foreign differ only marginally at a single TCR-contact residue. Thus, while individual T cells are markedly cross-reactive, the ability to distinguish between closely related Ags is introduced at the polyclonal T cell level.

Duration of Alloantigen Presentation and Avidity of T Cell Antigen Recognition Correlate with Immunodominance of CTL Response to Minor Histocompatibility Antigens

CD8 T lymphocytes (CTL) responsive to immunodominant minor histocompatibility (minor H) Ags are thought to play a disproportionate role in allograft rejection in MHC-identical solid and bone marrow transplant settings. Although many studies have addressed the mechanisms underlying immunodominance in models of infectious diseases, cancer immunotherapy, and allograft immunity, key issues regarding the molecular basis of immunodominance remain poorly understood. In this study, we exploit the minor H Ag system to understand the relationship of the various biochemical parameters of Ag presentation and recognition to immunodominance. We show that the duration of individual minor H Ag presentation and the avidity of T cell Ag recognition influence the magnitude and, hence, the immunodominance of the CTL response to minor H Ags. These properties of CTL Ag presentation and recognition that contribute to immunodominance have implications not only for tissue transplantation, but also for autoimmunity and tumor vaccine design.

Chronic rejection of murine cardiac allografts discordant at the H13 minor histocompatibility antigen correlates with the generation of the H13-specific CD8+ cytotoxic T cells

BACKGROUND

Minor histocompatibility antigen (mHag) discordances have been shown to play a critical role in graft-versus-host disease after bone marrow transplantation. However, the role of mHag in rejection of solid-organ allografts remains unknown. Therefore, the goal of this study was to define the role of a single mHag discordance derived from the polymorphic H13 allele in the development of cardiac allograft rejection in mice. The H13a and H13b alleles encode for the SSVVGVWYL (SVL9) and SSVIGVWYL (SIL9) mHag bound to the H2Db molecule, respectively.

METHODS

C56BL/10SnJ (H13a) cardiac allografts were transplanted into congenic B10.CE-H13b Aw(30NX)/Sn (H13b) mice. Allograft function was monitored daily and rejection was defined by cessation of heart beat. Rejection was confirmed histologically. The phenotypic and functional characteristics of the graft-infiltrating cells were analyzed by in situ and in vitro staining with H13a-specific tetramers and by chromium-51 (51Cr)-release assay, respectively.

RESULTS

Sixty-five percent of H13-incompatible allografts were rejected in 37.0+/-14.5 days. Sixty-eight percent of the H13a allografts transplanted into H13a-sensitized mice were rejected earlier, in 27.6+/-15.9 days. Rejected allografts showed histopathologic signs of chronic rejection with diffuse mononuclear cell infiltration, concentric intimal hyperplasia, and fibrosis. Both CD8+ (87%) and CD4+ (13%) T cells were observed in rejected allografts. In addition, 60% of the graft-infiltrating CD8+ T cells recognized a H2Db/SVL9 tetramer. Graft-infiltrating CD8+ T cells showed a significant H2Db-restricted, SVL9-specific cytotoxic activity.

CONCLUSIONS

A single mHag discordance, as demonstrated with H13 disparity, results in the pathogenesis of chronic rejection of major histocompatibility complex-matched vascularized solid-organ allograft.

The H4b minor histocompatibility antigen is caused by a combination of genetically determined and posttranslational modifications

Minor histocompatibility (H) Ag disparities result in graft-vs-host disease and chronic solid allograft rejection in MHC-identical donor-recipient combinations. Minor H Ags are self protein-derived peptides presented by MHC class I molecules. Most arise as a consequence of allelic variation in the bound peptide (p) that results in TCR recognizing the p/MHC as foreign. We used a combinational peptide screening approach to identify the immune dominant H2K(b)-restricted epitope defining the mouse H4(b) minor H Ag. H4(b) is a consequence of a P3 threonine to isoleucine change in the MHC-bound peptide derived from epithelial membrane protein-3. This allelic variation also leads to phosphorylation of the H4(b) but not the H4(a) epitope. Further, ex vivo CD8(+) T lymphocytes bind phosphorylated Ag tetramers with high efficiency. Although we document the above process in the minor H Ag system, posttranslational modifications made possible by subtle amino acid changes could also contribute to immunogenicity and immune dominance in tumor immunotherapeutic settings.

Analysis of immunodominance among minor histocompatibility antigens in allogeneic hematopoietic stem cell transplantation

In major histocompatibility complex (MHC)-matched allogeneic hematopoietic stem cell transplantation (HSCT), donor responses are directed against multiple host minor histocompatibility antigens (mHAgs), producing graft-versus-host disease (GVHD) and graft-versus-tumor (GVT) effects. We studied MHC-matched, mHAg-mismatched C3H.SW>C57BL/6 HSCT in which three mHAg are molecularly defined (B6dom1, H3, H13) to determine if there is a hierarchy of immunodominance among the mHAgs and to learn the contribution of each to GVHD. We found that B6dom1 was the immunodominant mHAg. B6dom1 did not block responses to the subdominant mHAgs H3 and H13. The mechanism of immunodominance was not mHAg avidity or affinity for class I. B6dom1 elicited a broader variety of Vbeta clonotypes than either H3 or H13. Severe GVHD could occur in the absence of a strong B6dom1 response. Alloreactivity to isolated B6dom1, H3 or H13 differences did not produce severe GVHD. We concluded that immunodominance is explained by both mHAg density on host cells and the repertoire of donor T cells capable of responding to the mHAgs. Clinically significant GVHD requires donor responses to multiple mHAgs. Modulation of responses to a single immunodominant mHAg is insufficient for the prevention of GVHD, while immunotherapies directed against isolated mHAgs may not provoke severe GVHD.

Differences in MHC-class I presented minor histocompatibility antigens extracted from normal and graft-versus-host disease (GVHD) mice

Graft-versus-host disease (GVHD) may develop after allogeneic bone marrow transplantation (BMT) between donors and recipients incompatible for minor histocompatibility antigens (mHAg). Here, we examined the possible relationship between tissue-specific distribution of dominant mHAg peptides and specific organ destruction caused by GVHD. In the B6 anti-Balb/b (H-2b) strain combination, a GVHD developed against Balb/b mHAgs. Despite the high number of incompatible mHAgs between these two strains, both cytotoxic T lymphocyte (CTL) response and GVHD could be attributed to a limited number of dominant mHAgs. We studied CTL-defined expression of dominant mHAgs in normal tissues and their GVHD-related modifications. mHAg peptides were prepared by acid elution and reversed-phase high pressure liquid chromatography fractionation from the spleen, liver, gut and skin as GVHD target tissues and from the heart and kidney as control tissues. Peptidic fractions extracted from normal and GVHD tissues were incubated with RMA-S targets and analysed using bulk B6 anti-Balb/b CTL. In each tissue several fractions were recognized with a given pattern of mHAg expression. GVHD induced qualitative and quantitative changes in antigenic peptide expression. Modifications in mHAg presentation during GVHD concerned preferentially GVHD target organs as opposed to non-GVHD target organs. In addition, when immunizing tissues were derived from GVHD mice instead of normal mice, the profile of CTL recognition was different. In conclusion, these data indicate that broad differences could exist in peptide presentation between various normal and GVHD-target organs.

Second class minors: molecular identification of the autosomal H46 histocompatibility locus as a peptide presented by major histocompatibility complex class II molecules

CD4 T cells regulate immune responses that cause chronic graft rejection and graft versus host disease but their target antigens remain virtually unknown. We developed a new method to identify CD4 T cell-stimulating antigens. LacZ-inducible CD4 T cells were used as a probe to detect their cognate peptide/MHC II ligand generated in dendritic cells fed with Escherichia coli expressing a library of target cell genes. The murine H46 locus on chromosome 7 was thus found to encode the interleukin 4-induced IL4i1 gene. The IL4i1 precursor contains the HAFVEAIPELQGHV peptide which is presented by A(b) major histocompatibility complex class II molecule via an endogenous pathway in professional antigen presenting cells. Both allelic peptides bind A(b) and a single alanine to methionine substitution at p2 defines nonself. These results reveal novel features of H loci that regulate CD4 T cell responses as well as provide a general strategy for identifying elusive antigens that elicit CD4 T cell responses to tumors or self-tissues in autoimmunity.

Real-time T-cell profiling identifies H60 as a major minor histocompatibility antigen in murine graft-versus-host disease

Although CD8 T cells are thought to be a principal effector population of graft-versus-host disease (GVHD), their dynamics and specificity remain a mystery. Using a mouse model in which donor and recipient were incompatible at many minor histocompatibility antigens (minor H Ags), the CD8 T-cell response was tracked temporally and spatially through the course of GVHD. Donor CD8 T cells in the circulation, spleen, lung, and liver demonstrated virtually identical kinetics: rapid expansion and then decline prior to morbidity. Remarkably, up to one fourth of the CD8 T cells were directed against a single minor antigen, H60. Extreme H60 immunodominance occurred regardless of sampling time, site, and genetic background. This study is the first to analyze the T cells participating in GVHD in "real-time," demonstrates the exceptional degree to which immunodominance of H60 can occur, and suggests that such superdominant minor H Ags could be risk factors for GVHD.

The immunogenomics of minor histocompatibility antigens

Minor histocompatibility (H) antigens are a diverse assemblage of major histocompatibility complex (MHC)-bound peptides with the unifying property of acting as alloantigens that induce allogeneic tissue rejection. They are a consequence of any form of accumulated genetic variation that translates to differential MHC-presented peptide epitopes, the most common form of which is simple sequence polymorphisms. The universe of potential minor H antigens is large when transplantation is performed between genetically unrelated, MHC-matched individuals, especially considering the remarkable discriminative sensitivity of T cells. However, the phenomenon of immunodominance greatly simplifies immune responses that ensue. One mouse minor H antigen, H60, stands out in that the preponderance of the CD8 T cell response elicited in a complex alloantigenic setting is directed against this single minor H antigen epitope. Its immunodominance is because mice lacking H60 develop an unusually robust T cell repertoire dedicated to this single minor H antigen. The now well-characterized mouse minor H antigen system should provide a vehicle to assess the degree to which immunodominant alloantigens contribute to transplant rejection.

Immunodominance of H60 is caused by an abnormally high precursor T cell pool directed against its unique minor histocompatibility antigen peptide

The H60 minor histocompatibility (H) antigen peptide is derived from a glycoprotein that serves as a ligand for the stimulatory NKG2D receptor. We show that this peptide is remarkably immunodominant in that it competes effectively with MHC alloantigens, is efficiently crosspresented by host antigen-presenting cells (APCs), and readily elicits naive CD8 T cell responses in vitro. H60 immunodominance is neither a consequence of NKG2D engagement nor competition among minor H antigens on APCs. Instead, H60 immunodominance is a consequence of an abnormally high naive precursor frequency of H60 peptide reactive CD8 T cells. Understanding why the H60 peptide is so immunogenic has important implications in tissue transplantation and vaccine design.

Insights into antigen processing gained by direct analysis of the naturally processed class I MHC associated peptide repertoire

MHC class I molecules are responsible for the presentation of antigenic peptides to CD8+ T lymphocytes. Based on their relatively promiscuous binding of peptides, these molecules display information derived from a large fraction of proteins that are made inside the cell. This review describes our characterization of the peptides comprising this repertoire, with particular attention given to their complexity and quantities, their post-translational modification, and the pathways leading to their expression.

Identification of a novel HLA-B60-restricted T cell epitope of the minor histocompatibility antigen HA-1 locus

The polymorphic minor histocompatibility Ag HA-1 locus encodes two peptides, HA-1(H) and HA-1(R), with a single amino acid difference. Whereas the immunogenicity of the HA-1(R) allele has not yet been shown, the nonameric HA-1(H) peptide induces HLA-A2-restricted cytotoxic T cells in vivo and in vitro. It is not known whether the mHag HA-1(H) or HA-1(R) associates with other HLA class I molecules. Therefore, the polymorphic regions of both HA-1 alleles were analyzed to identify HLA class I binding peptides that are properly processed by proteasomal degradation. Peptide binding analyses were performed for all nonameric HA-1(H/R) peptides for binding to nine HLA class I molecules with >10% prevalence in the Caucasian population and for seven nonameric/decameric HA-1(H/R) peptides predicted to bind to HLA-A3, -B14, and -B60. Only the nonameric KECVL(H)/(R)DDL and decameric KECVL(H)/(R)DDLL peptides showed strong and stable binding to HLA-B60. In vitro digestion of 29-aa-long HA-1 peptides by purified 20S proteasomes revealed proper cleavage at the COOH termini of both HLA-B60 binding HA-1(H) and HA-1(R) peptides. In subsequent analyses, dendritic cells pulsed with the nonameric HA-1(R) peptide did not induce CTLs that recognize the natural HLA-B60/HA-1(R) ligand. In contrast, dendritic cells pulsed with the nonameric HA-1(H) peptide induced IFN-gamma-secreting T cells specific for the natural HLA-B60/HA-1(H) ligand in three HLA-B60(+) HA-1(RR) individuals, demonstrating the immunogenicity of the HLA-B60/HA-1(H) ligand. In conclusion, this study shows a novel HLA-B60-restricted T cell epitope of the minor histocompatibility Ag HA-1 locus.

Viral and bacterial infections induce expression of multiple NK cell receptors in responding CD8(+) T cells

NK cells express several families of receptors that play central roles in target cell recognition. These NK cell receptors are also expressed by certain memory phenotype CD8(+) T cells, and in some cases are up-regulated in T cells responding to viral infection. To determine how the profile of NK receptor expression changes in murine CD8(+) T cells as they respond to intracellular pathogens, we used class I tetramer reagents to directly examine Ag-specific T cells during lymphocytic choriomeningitis virus and Listeria monocytogenes infections. We found that the majority of pathogen-specific CD8(+) T cells initiated expression of the inhibitory CD94/NKG2A heterodimer, the KLRG1 receptor, and a novel murine NK cell marker (10D7); conversely, very few Ag-specific T cells expressed Ly49 family members. The up-regulation of these receptors was independent of IL-15 and persisted long after clearance of the pathogen. The expression of CD94/NKG2A was rapidly initiated in naive CD8(+) T cells responding to peptide Ags in vitro and on many of the naive T cells that proliferate when transferred into lymphopenic (Rag-1(-/-)) hosts. Thus, CD94/NKG2A expression is a common consequence of CD8(+) T cell activation. Binding of the CD94/NKG2A receptor by its ligand (Qa-1(b)) did not significantly inhibit CD8(+) T cell effector functions. However, expression of CD94 and NKG2A transgenes partially inhibited early events of T cell activation. These subtle effects suggest that CD94/NKG2A-mediated inhibition of T cells may be limited to particular circumstances or may synergize with other receptors that are similarly up-regulated.

Minor H antigens: genes and peptides

In this review, we describe the evidence from which the existence of non-MHC histocompatibility (H) antigens was deduced, the clinical setting of bone marrow transplantation in which they are important targets for T cell responses, and the current understanding of their molecular identity. We list the peptide epitopes, their MHC restriction molecules and the genes encoding them, of the human and murine minor H antigens now identified at the molecular level. Identification of the peptide epitopes allows T cell responses to these antigens following transplantation of MHC-matched, minor H-mismatched tissues to be enumerated using tetramers and elispot assays. This will facilitate analysis of correlations with HVG, GVH and GVL reactions in vivo. The potential to use minor H peptides to modulate in vivo responses to minor H antigens is discussed. Factors controlling immunodominance of T cell responses to one or a few of many potential minor H antigens remain to be elucidated but are important for making predictions of in vivo HVG, GVH and GVL responses and tailoring therapy after HLA-matched BMT and DLI.

Producing nature's gene-chips: the generation of peptides for display by MHC class I molecules

Gene-chips contain thousands of nucleotide sequences that allow simultaneous analysis of the complex mixture of RNAs transcribed in cells. Like these gene-chips, major histocompatibility complex (MHC) class I molecules display a large array of peptides on the cell surface for probing by the CD8(+) T cell repertoire. The peptide mixture represents fragments of most, if not all, intracellular proteins. The antigen processing machinery accomplishes the daunting task of sampling these proteins and cleaving them into the precise set of peptides displayed by MHC I molecules. It has long been believed that antigenic peptides arose as by-products of normal protein turnover. Recent evidence, however, suggests that the primary source of peptides is newly synthesized proteins that arise from conventional as well as cryptic translational reading frames. It is increasingly clear that for many peptides the C-terminus is generated in the cytoplasm, and N-terminal trimming occurs in the endoplasmic reticulum in an MHC I-dependent manner. Nature's gene-chips are thus both parsimonious and elegant.