Immunodominant minor histocompatibility antigens expressed by mouse leukemic cells can serve as effective targets for T cell immunotherapy

The Genomic Landscape of Antigenic Targets for T Cell-Based Leukemia Immunotherapy

Intensive fundamental and clinical research in cancer immunotherapy has led to the emergence and evolution of two parallel universes with surprisingly little interactions: the realm of hematologic malignancies and that of solid tumors. Treatment of hematologic cancers using allogeneic hematopoietic cell transplantation (AHCT) serendipitously led to the discovery that T cells specific for minor histocompatibility antigens (MiHAs) could cure hematopoietic cancers. Besides, studies based on treatment of solid tumor with ex vivo-expanded tumor infiltrating lymphocytes or immune checkpoint therapy demonstrated that anti-tumor responses could be achieved by targeting tumor-specific antigens (TSAs). It is our contention that much insight can be gained by sharing the tremendous amount of data generated in the two-abovementioned universes. Our perspective article has two specific goals. First, to discuss the value of methods currently used for MiHA and TSA discovery and to explain the key role of mass spectrometry analyses in this process. Second, to demonstrate the importance of broadening the scope of TSA discovery efforts beyond classic annotated protein-coding genomic sequences.

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.

Next-generation leukemia immunotherapy

Allogeneic hematopoietic cell transplantation led to the discovery of the allogeneic GVL effect, which remains the most convincing evidence that immune cells can cure cancer in humans. However, despite its great paradigmatic and clinical relevance, induction of GVL by conventional allogeneic hematopoietic cell transplantation remains a quite rudimentary form of leukemia immunotherapy. It is toxic and its efficacy is far from optimal. It is therefore sobering that since the discovery of the GVL effect 3 decades ago, the way GVL is induced and manipulated has practically not changed. Preclinical and clinical studies suggest that injection of T cells primed against a single Ag present on neoplastic cells could enhance the GVL effect without causing any GVHD. We therefore contend that Ag-targeted adoptive T-cell immunotherapy represents the future of leukemia immunotherapy, and we discuss the specific strategies that ought to be evaluated to reach this goal. Differences between these strategies hinge on 2 key elements: the nature of the target Ag and the type of Ag receptor expressed on T cells.

Nonengraftment haploidentical cellular immunotherapy for refractory malignancies: tumor responses without chimerism

Allogeneic bone marrow transplantation relies on immunosuppression, which controls graft-versus-host disease (GVHD) and allows engraftment at the expense of diminished graft versus-tumor (GVT) activity. Advances in hematologic transplantation have prompted the development of effective, less-toxic regimens that attempt to balance GVH and GVT immunoreactions. We analyzed the safety and efficacy of haploidentical transplantation in a Phase I/II nonimmunosuppressive, nonmyeloablative setting. A total of 41 patients with relapsed refractory cancer received 100 cGy of total body irradiation (TBI), along with an infusion of 1 x 10(6) to 2 x 10(8) CD3+ cells/kg; 29 patients received the highest dose. A postinfusional cellular graft rejection syndrome resembling engraftment syndrome was noted at the 2 highest CD3+ infusion cohorts. There were 26 patients with hematologic malignancies with 14 responses, 9 of which were major. Two of 6 patients with lymphoma remained free of disease at 76 months and 82 months, respectively; there were 5 durable complete responses and 4 partial responses in 13 patients with acute myelogenous leukemia (AML). All responses occurred outside of donor chimerism. TBI at 100 cGy followed by HLA-haploidentical immunotherapy is a biologically active therapy for patients with refractory AML and lymphoma. Possible mechanisms contributing to its effectiveness include initial GVT kill, breaking of host tolerance to tumor through cross-reactive alloreactive responses, persistent nondetectable microchimerism, or some combination of these.

The MHC class I peptide repertoire is molded by the transcriptome

Under steady-state conditions, major histocompatibility complex (MHC) I molecules are associated with self-peptides that are collectively referred to as the MHC class I peptide (MIP) repertoire. Very little is known about the genesis and molecular composition of the MIP repertoire. We developed a novel high-throughput mass spectrometry approach that yields an accurate definition of the nature and relative abundance of unlabeled peptides presented by MHC I molecules. We identified 189 and 196 MHC I-associated peptides from normal and neoplastic mouse thymocytes, respectively. By integrating our peptidomic data with global profiling of the transcriptome, we reached two conclusions. The MIP repertoire of primary mouse thymocytes is biased toward peptides derived from highly abundant transcripts and is enriched in peptides derived from cyclins/cyclin-dependent kinases and helicases. Furthermore, we found that approximately 25% of MHC I-associated peptides were differentially expressed on normal versus neoplastic thymocytes. Approximately half of those peptides are derived from molecules directly implicated in neoplastic transformation (e.g., components of the PI3K-AKT-mTOR pathway). In most cases, overexpression of MHC I peptides on cancer cells entailed posttranscriptional mechanisms. Our results show that high-throughput analysis and sequencing of MHC I-associated peptides yields unique insights into the genesis of the MIP repertoire in normal and neoplastic cells.

Early lymphocyte recovery after autologous stem cell transplantation predicts superior survival in mantle-cell lymphoma

Autologous stem cell transplantation (ASCT) is an effective treatment strategy for mantle-cell lymphoma (MCL) demonstrating significantly prolonged progression-free survival (PFS) when compared to interferon-alpha maintenance therapy of patients in first remission. The study of absolute lymphocyte count at day 15 (ALC-15) after ASCT as a prognostic factor in non-Hodgkin lymphoma (NHL) included different lymphoma subtypes. The relationship of ALC-15 after ASCT in MCL has not been specifically addressed. We evaluated the impact of ALC-15 recovery on survival of MCL patients undergoing ASCT. We studied 42 consecutive MCL patients who underwent ASCT at the Mayo Clinic in Rochester from 1993 to 2005. ALC-15 threshold was set at 500 cells/microl. The median follow-up after ASCT was 25 months (range, 2-106 months). The median overall survival (OS) and PFS times were significantly better for the 24 patients who achieved an ALC-15 >or=500 cells/microl compared with 18 patients with ALC-15 <500 cells/microl (not reached vs 30 months, P<0.01 and not reached vs 16 months, P<0.0006, respectively). Multivariate analysis demonstrated ALC-15 to be an independent prognostic factor for OS and PFS. The ALC-15 >or=500 cells/microl is associated with a significantly improved clinical outcome following ASCT in MCL.

Host MHC class II+ antigen-presenting cells and CD4 cells are required for CD8-mediated graft-versus-leukemia responses following delayed donor leukocyte infusions

Following bone marrow transplantation, delayed donor leukocyte infusions (DLIs) can induce graft-versus-leukemia (GVL) effects without graft-versus-host disease (GVHD). These antitumor responses are maximized by the presence of host hematopoietic antigen-presenting cells (APCs) at the time of DLI. Using a tumor-protection model, we demonstrate here that GVL activity following administration of DLIs to established mixed chimeras is dependent primarily on reactivity to allogeneic MHC antigens rather than minor histocompatibility or tumor-associated antigens. CD8(+) T-cell-dependent GVL responses against an MHC class II-negative tumor following delayed DLI require CD4(+) T-cell help and are reduced significantly when host APCs lack MHC class II expression. CD4(+) T cells primed by host APCs were required for maximal expansion of graft-versus-host reactive CD8(+) T cells but not their synthesis of IFN-gamma. In contrast, the GVL requirement for CD4(+) T-cell help was bypassed almost completely when DLI was administered to freshly irradiated recipients, indicating that the host environment is a major factor influencing the cellular mechanisms of GVL.

T-cell responses directed against multiple HLA-A*0201-restricted epitopes derived from Wilms' tumor 1 protein in patients with leukemia and healthy donors: identification, quantification, and characterization

PURPOSE

Antigens derived from the Wilms' tumor (WT1) protein, which is overexpressed in leukemias, are attractive targets for immunotherapy. Four HLA-A*0201-restricted WT1-derived epitopes have been identified: WT37, WT126, WT187, and WT235. We determined the natural immunogenecity of these antigens in patients with hematologic malignancies and healthy donor.

EXPERIMENTAL DESIGN

To detect very low frequencies of WT1-specific CD8+ T cells, we used quantitative reverse transcription-PCR to measure IFN-gamma mRNA production by WT1 peptide-pulsed CD8+ T cells from 12 healthy donors, 8 patients with chronic myelogenous leukemia, 6 patients with acute myelogenous leukemia, and 8 patients with acute lymphoblastic leukemia.

RESULTS

Responses were detected in 5 of 8 chronic myelogenous leukemia patients, 4 of 6 patients with acute myelogenous leukemia, and 7 of 12 healthy donors. No responses were detected in patients with acute lymphoblastic leukemia. The magnitude and extent of these CD8+ T-cell responses was greater in patients with myeloid leukemias than in healthy donors. Clonotypic analysis of WT1-specific CD8+ T cells directly ex vivo in one case showed that this naturally occurring population was oligoclonal. Using fluorescent peptide-MHC class I tetramers incorporating mutations in the alpha3 domain (D227K/T228A) that abrogate binding to the CD8 coreceptor, we were able to confirm the presence of high-avidity T-cell clones within the antigen-specific repertoire.

CONCLUSION

The natural occurrence of high-avidity WT1-specific CD8+ T cells in the periphery could facilitate vaccination strategies to expand immune responses against myeloid leukemias.

Immunodominance and Immunodomination: Critical Factors in Developing Effective CD8+ T‐Cell–Based Cancer Vaccines

The focusing of cellular immunity toward one, or just a few, antigenic determinant, even during immune responses to complex microorganisms or antigens, is known as immunodominance. Although described in many systems, the mechanisms of determinant immunodominance are only just beginning to be appreciated, especially in relation to the interplay between T cells of differing specificities and the interactions between T cells and the antigen-presenting cells (APCs). The outcome of these cellular interactions can lead to a form of immune suppression of one specificity by another-described as "immunodomination". The specific and detailed mechanisms involved in this process are now partly defined. A full understanding of all the factors that control immunodominance and influence immunodomination will help us to develop better viral and cancer vaccines.

Organ-Specific T Cell Receptor Repertoire in Target Organs of Murine Graft-Versus-Host After Transplantation Across Minor Histocompatibility Antigen Barriers

BACKGROUND

Minor histocompatibility antigens (miHags) are recognized by alloreactive cytotoxic donor T lymphocytes and trigger potent immune reactions such as graft-versus-host disease (GvHD) after major histocompatibility complex-matched transplantation. Our study focuses on tissue-specific T-cell responses to miHag-encoded peptides in GvHD target organs during the first 30 days in a murine transplant model.

METHODS

Complementarity determining region (CDR)3-size spectratyping was used to study T cell receptor (TCR) repertoires in recipient skin, liver, ileum, colon, spleen, and heart.

RESULTS

GvHD occurred as early as day 14 and was proven by histology in skin, liver, ileum, and colon. The heart was histologically not affected by GvHD but showed endomyocardial "quilty lesions." Two distinct patterns of TCR diversities could be identified. In skin, a restricted V beta usage in combination with all J beta segments contrasted with a complete V beta repertoire in intestinal organs combined with a restricted J beta usage. Interestingly, TCR repertoire in the heart was almost identical with intestinal CDR3-size patterns. Persisting clones were found in skin from day 9 to 30. In intestine and heart, identical sequences were obtained from several organs on day 14 and 21, but no persistence of CDR3 sequences could be observed.

CONCLUSIONS

These results suggest that in the skin a limited number of persisting T cell clones maintains GvHD, whereas in the intestine, temporary expansions of different clones may fuel the process of GvHD. Strategies that eliminate tissue-specific T cells on the basis of their activational status rather than their V beta expression but at the same time preserve a broad, overall TCR repertoire will help to increase the efficacy and safety of allogeneic stem cell transplantation.

A tumour vaccine of fixed tumour fragments in a controlled-release vehicle with cytokines for therapy of hepatoma in mice

BACKGROUND

Cytokines can be strong potentiators for a tumour vaccine, but they have very short life in vivo when administered as a solution.

AIMS

To evaluate the slow release of interleukin 2 from a cytokine-vehicle in vitro and in vivo and to evaluate the anti-tumour activity of a new tumour vaccine in vivo.

METHODS

The tumour vaccine was composed of formalin-fixed Hepa 1-6 hepatoma tissue fragments, tuberculin and a lipid based vehicle containing granulocyte-macrophage colony-stimulating factor and interleukin 2. The quantity of interleukin 2 release from the cytokine-vehicle in vitro and in vivo was determined by a proliferation assay with CTLL-2 cell line. Hepa 1-6 hepatoma model system with C57BL/6J mice was used to examine protective and therapeutic anti-tumour effect of the vaccine.

RESULTS

Release of interleukin 2 from the cytokine-vehicle lasted 5 days in vitro and 3 days in vivo. The vaccine protected 67% of mice from a Hepa 1-6 cell challenge and had a therapeutic effect by prolonging the life span of mice bearing established Hepa 1-6 tumours of 5 mm in diameter. Of the treated mice, 20% became completely tumour-free.

CONCLUSIONS

Formalin-fixed tumour fragments and cytokines in controlled-release vehicle are useful in the rational design of tumour vaccines.

T-cell clones can be rendered specific for CD19: toward the selective augmentation of the graft-versus-B-lineage leukemia effect

Relapse of B-lineage acute lymphoblastic leukemia (B-ALL) after allogeneic hematopoietic stem cell transplantation (HSCT) commonly results from the failure of a graft-versus-leukemia (GVL) effect to eradicate minimal residual disease. Augmenting the GVL effect by the adoptive transfer of donor-derived B-ALL-specific T-cell clones is a conceptually attractive strategy to decrease relapse rates without exacerbating graft-versus-host disease (GVHD). Toward this end, we investigated whether a genetic engineering approach could render CD8(+) cytotoxic T lymphocytes (CTLs) specific for tumor cells that express the B-cell lineage cell surface molecule CD19. This was accomplished by the genetic modification of CTLs to express a chimeric immunoreceptor composed of a CD19-specific single-chain immunoglobulin extracellular targeting domain fused to a CD3-zeta intracellular signaling domain. CD19-redirected CTL clones display potent CD19-specific lytic activity and chimeric immunoreceptor-regulated cytokine production and proliferation. Because B-ALL cells can evade T-cell/natural killer- cell recognition by down-regulation of cell surface accessory molecules that participate in the formation of a functional immunologic synapse, we compared the CD19-specific effector function of genetically modified CD8(+) CTLs toward CD19(+) cells with disparate levels of intercellular adhesion molecule 1 (ICAM-1), leukocyte function-associated antigen 1 (LFA-1), and LFA-3. We observed that recognition of B-lineage tumor lines by CD19-specific CTLs was not impaired by low levels of ICAM-1, LFA-1, and LFA-3 cell surface expression, a functional attribute that is likely a consequence of our high-affinity CD19-specific chimeric immunoreceptor. Furthermore, the CD19-specific CTLs could lyse primary B-ALL blasts. These preclinical observations form the basis for implementing clinical trials using donor-derived CD19-specific T-cell clones to treat or prevent relapse of B-ALL after allogeneic HSCT.

Adoptive cellular therapy: a therapeutic reality?

The past decade has seen numerous lines of evidence emerging that suggest human malignancies may be sensitive to the effects of cellular immunotherapy. An increasing understanding of the nature of the effector cells and their target antigens is now leading to more focused efforts to harness these responses for therapeutic benefit. However, clinical application has proven more challenging than initially envisaged. Advances in the setting of allogeneic stem cell transplantation now allow attempts to augment both immunological recovery and anti-tumour activity. Some of the most attractive targets here are allospecific rather than truly tumour-specific. Application outside of this setting is based on attempts to delineate further tumour-specific or, increasingly, tumour-selective targets. This review summarizes these developments and highlights some of the issues that remain to be resolved.

T-cell therapy of leukemia

BACKGROUND

The demonstration that immune-mediated elimination of leukemia contributes to the success of allogeneic hematopoietic stem cell transplantation (HSCT) has renewed interest in the development of immune-based therapies that might be used to augment the antileukemic effect of HSCT or in patients who are not receiving HSCT.

METHODS

The authors reviewed studies that have analyzed the mechanisms that may be operative in T-cell recognition of leukemia after allogeneic HSCT, identified candidate target antigens for immunotherapy of leukemia in transplant and nontransplant patients, and evaluated expression of candidate antigens on leukemic progenitors.

RESULTS

A large number of potential targets for T-cell therapy or vaccination have now been identified in human leukemia. Studies to evaluate novel immune-based therapies are now being initiated.

CONCLUSIONS

The rapid pace of progress in cellular and molecular immunology has identified new opportunities for developing T-cell therapy or vaccination for leukemia. Obstacles must be addressed before these approaches can be applied broadly, but the promising results of preclinical studies suggest continued efforts in this area will result in the establishment of immunotherapy as a useful modality in clinical practice.

Adoptive transfer of minor histocompatibility antigen-specific T lymphocytes eradicates leukemia cells without causing graft-versus-host disease

Adoptive transfer of T cells reactive to minor histocompatibility antigens has the unmatched ability to eradicate malignant hematopoietic cells. Unfortunately, its use is hampered by the associated graft-versus-host disease. The critical issue of a possible dissociation of the antileukemic effect and graft-versus-host disease by targeting specific minor histocompatibility antigens remains unresolved because of the unknown nature and number of minor histocompatibility antigens necessary or sufficient to elicit anti-leukemic activity and graft-versus-host disease. We found that injection of T lymphocytes primed against a single major histocompatibility complex class I-restricted immunodominant minor histocompatibility antigen (B6dom1) caused no graft-versus-host disease but produced a curative anti-leukemic response. Avoidance of graft-versus-host disease required that no other host-reactive T cells be co-injected with T cells primed with B6dom1. Here we show that effective and non-toxic immunotherapy of hematologic malignancies can be achieved by targeting a single immunodominant minor histocompatibility antigen.

The In Vivo Fate of APCs Displaying Minor H Antigen and/or MHC Differences Is Regulated by CTLs Specific for Immunodominant Class I-Associated Epitopes

The goal of this work was to evaluate the fate of APCs following interactions with T cells in unprimed mice with a normal T cell repertoire. We elaborated a model in which male adherent peritoneal mononuclear cells were injected into the foreleg footpads of naive female recipients mismatched for either minor or major histocompatibility Ags. At various times after injection, APC numbers in the draining (axillary and brachial) lymph nodes were assessed using a Ube1y gene-specific PCR assay. Our experimental model was designed so that the number of APCs expressing the priming epitope was similar to what is observed under real life conditions. Thus, early after injection, the frequency of afferent lymph-derived APCs expressing the priming epitope was in the range of 101–102/106 lymph node cells. We found that APCs presenting some, but not all, nonself epitopes were killed rapidly after entrance into the lymph nodes. Rapid elimination of APCs occurred following interactions with MHC class I-restricted, but not class II-restricted, T cells and was observed when APCs presented an immunodominant (B6dom1/H7a), but not a nondominant (HY), epitope. Killing of APCs was mediated partly, but not exclusively, by perforin-dependent process. We propose that killing of APCs by CTLs specific for immunodominant MHC class I-restricted epitopes may be instrumental in regulating the intensity, duration, and diversity of T cell responses.

Biochemical and Immunogenetic Analysis of an Immunodominant Peptide (B6dom1) Encoded by the Classical H7 Minor Histocompatibility Locus

Of the many minor histocompatibility (H) Ags that have been detected in mice, the ability to induce graft vs host disease (GVHD) after bone marrow transplantation is restricted to a limited number of immunodominant Ags. One such murine Ag, B6dom1, is presented by the H2-Db MHC class I molecule. We present biochemical evidence that the natural B6dom1 peptide is indistinguishable from AAPDNRETF, and we show that this peptide can be isolated from a wide array of tissues, with highest levels from the lymphoid organs and lung. Moreover, we employ a novel, somatic cell selection technique involving CTL-mediated immunoselection coupled with classical genetics, to show that B6dom1 is encoded by the H7 minor H locus originally discovered ∼40 years ago. These studies provide a molecular genetic framework for understanding B6dom1, and exemplify the fact that mouse minor H loci that encode immunodominant CTL epitopes can correspond to classical H loci originally identified by their ability to confer strong resistance to tumor transplantation. Additionally, these studies demonstrate the utility of somatic cell selection approaches toward resolving H Ag immunogenetics.

Shaping the Repertoire of Cytotoxic T-Lymphocyte Responses: Explanation for the Immunodominance Effect Whereby Cytotoxic T Lymphocytes Specific for Immunodominant Antigens Prevent Recognition of Nondominant Antigens

The immunodominance effect, whereby the presence of immunodominant epitopes prevents recognition of nondominant determinants presented on the same antigen-presenting cell (APC) considerably restricts the repertoire of cytotoxic T lymphocyte (CTL) responses. To elucidate the molecular basis of the immunodominance effect, we compared the interactions of a dominant (B6dom1) and a nondominant epitope (H-Y) with their restricting class I molecule (H2-Db), and their ability to trigger cognate CTLs. We found that B6dom1/Db complexes behaved as optimal T-cell receptor (TCR) ligands and triggered a more rapid in vivo expansion of cognate CTLs than H-Y/Db complexes. The superiority of the dominant epitope was explained by its high cell surface density (1,012 copies/cell for B6dom1v 10 copies/cell for H-Y) and its optimal affinity for cognate TCRs. Based on these results, we conclude that dominant class I–associated epitopes are those that have optimal ability to trigger TCR signals in CTLs. We propose that the rapid expansion of CTLs specific for dominant antigens should enable them to compete more successfully than other CTLs for occupancy of the APC surface.

Shaping the Repertoire of Cytotoxic T-Lymphocyte Responses: Explanation for the Immunodominance Effect Whereby Cytotoxic T Lymphocytes Specific for Immunodominant Antigens Prevent Recognition of Nondominant Antigens

The immunodominance effect, whereby the presence of immunodominant epitopes prevents recognition of nondominant determinants presented on the same antigen-presenting cell (APC) considerably restricts the repertoire of cytotoxic T lymphocyte (CTL) responses. To elucidate the molecular basis of the immunodominance effect, we compared the interactions of a dominant (B6dom1) and a nondominant epitope (H-Y) with their restricting class I molecule (H2-Db), and their ability to trigger cognate CTLs. We found that B6dom1/Db complexes behaved as optimal T-cell receptor (TCR) ligands and triggered a more rapid in vivo expansion of cognate CTLs than H-Y/Db complexes. The superiority of the dominant epitope was explained by its high cell surface density (1,012 copies/cell for B6dom1v 10 copies/cell for H-Y) and its optimal affinity for cognate TCRs. Based on these results, we conclude that dominant class I–associated epitopes are those that have optimal ability to trigger TCR signals in CTLs. We propose that the rapid expansion of CTLs specific for dominant antigens should enable them to compete more successfully than other CTLs for occupancy of the APC surface.

Immunodominance in the CTL Response Against Minor Histocompatibility Antigens: Interference Between Responding T Cells, Rather than with Presentation of Epitopes

We have investigated mechanisms involved in immunodominance of the CTL response of C57BL/6 (B6) mice against cells of BALB.B origin. This transplantation barrier consists of at least 40 minor histocompatibility (H) Ags. Insufficient presentation of nondominant epitopes in the presence of dominant epitopes was investigated as a possible mechanism for immunodominance. Ag presentation was assessed by recognition of dendritic cells of BALB.B origin, MLC restimulatory capacity, and quantification of cell surface presentation by peptide elution from intact cells. Cells from BALB.B mice, which fail to elicit CTL against nondominant epitopes, presented nondominant epitopes to a similar extent as cells from minor H congenic mice; the latter do elicit CTL against nondominant minor H Ags. Nevertheless, presentation of nondominant and dominant epitopes by the same APC appeared to be an important factor for immunodominance to occur, since simultaneous immunization with the epitopes on separate cells elicited CTL against both types of epitopes. This suggested that immunodominance is determined in the interaction between different responding T cells and the APC. Support for this was obtained in an in vitro model in which the CTL response against a nondominant epitope was inhibited by the concomitant response against a dominant epitope. This study suggests that immunodominance in the CTL response against certain minor H Ags results from interference between T cell responses and not from insufficient presentation of peptide epitopes. The study also provides an in vitro model for further investigations of the immunodominance phenomenon.

On the mechanisms of immunodominance in cytotoxic T lymphocyte responses to minor histocompatibility antigens

Although there are numerous minor histocompatibility antigens (MiHA), T cell responses leading to graft-versus-host (GVH) and graft-versus-tumor effects involve only a small number of immunodominant MiHA. The goal of the present study was to analyze at the cellular and molecular levels the mechanisms responsible for MiHA immunodominance. Cytotoxic T lymphocytes (CTL) generated in eight combinations of H2b strains of mice were tested against syngeneic targets sensitized with HPLC-fractionated peptides eluted from immunizing cells. The number of dominant MiHA was found to range from as little as two up to ten depending on the strain combination used. The nature of dominant MiHA was influenced by both the antigen profile of the antigen-presenting cells (APC) and the repertoire of responding CTL. When C57BL/6 dominant MiHA (B6dom) and H-Y were presented on separate APC, they showed similar immunogenicity. In contrast, when they were presented on the same APC, B6dom MiHA totally dominated H-Y. B6dom MiHA did not suppress anti-H-Y responses by acting as T cell receptor antagonists for anti-H-Y CTL, nor were anti-B6dom CTL precursors more abundant than anti-H-Y CTL precursors. Dominance resulted from competition for the APC surface between anti-B6dom and anti-H-Y CTL; the crucial difference between the dominant and the dominated MiHA appears to depend on the differential avidity of their respective CTL for APC. The only B6dom epitope thus far identified is the nonapeptide AAPDNRETF presented by H2-D(b). We found that compared with other known D(b)-binding peptides, AAPDNRETF is expressed at very high levels on the cell surface, binds to the D(b) molecule with very high affinity, and dissociates very slowly from its presenting class I molecule. These data indicate that one cannot predict which MiHA will be dominant or dominated based simply on their respective immunogenicity when presented on separate APC. Indeed, the avidity of T cell/APC interactions appears to determine which antigen(s) will trigger T cell responses when numerous epitopes are presented by the same APC.

Identification of an immunodominant mouse minor histocompatibility antigen (MiHA). T cell response to a single dominant MiHA causes graft-versus-host disease

T cell responses to non-MHC antigens are targeted to a restricted number of immunodominant minor histocompatibility antigens whose identity remains elusive. Here we report isolation and sequencing of a novel immunodominant minor histocompatibility antigen presented by H-2Db on the surface of C57BL/6 mouse cells. This nonapeptide (AAPDNRETF) shows strong biologic activity in cytotoxic T lymphocyte sensitization assays at concentrations as low as 10 pM. C3H.SW mice primed with AAPDNRETF in incomplete Freund's adjuvant generated a potent anti-C57BL/6 T cell-mediated cytotoxic activity, and T lymphocytes from AAPDNRETF-primed mice caused graft-versus-host disease when transplanted in irradiated C57BL/6 recipients. These results (a) provide molecular characterization of a mouse dominant minor histocompatibility antigen, (b) identify this peptide as a potential target of graft-versus-host disease and, (c) more importantly, demonstrate that a single dominant minor antigen can cause graft-versus-host disease. These findings open new avenues for the prevention of graft-versus-host disease and should further our understanding of the mechanisms of immunodominance in T cell responses to minor histocompatibility antigens.