Immunotherapy of cancer through targeting of minor histocompatibility antigens

Minor histocompatibility antigens are allogeneic targets of T-cell mediated graft-versus-tumour effects following allogeneic stem cell transplantation. Recent research has identified several minor histocompatibility antigens as tumour proteins and has also disclosed their unique properties in both the induction and the effector phase of graft-versus-tumour effects. Targeting tumour-specific minor histocompatibility antigens by adoptive immunotherapy will battle against tumour tolerance and evoke allo-immune responses, thereby enhancing graft-versus-tumour effects against leukaemia and solid tumours. Recently acquired knowledge of the role of donor immunisation status, new techniques in the generation of minor histocompatibility antigen-specific cytotoxic T lymphocytes in vitro, and innovative principles in vaccination will help to design clinical trials that exploit minor histocompatibility antigens in the immunotherapy of cancer.     

Minor histocompatibility antigens: from transplantation problems to therapy of cancer

The idea of transferring healthy marrow for the therapeutic treatment of the various diseases of the blood and of the immune system made a significant contribution to controlling diseases and to advancing modern clinical sciences. The first series of bone marrow transplantations in the 1960s were confronted with severe complications. It became clear that matching for the human leukocyte antigen (HLA) system between donor and recipient significantly improved the clinical results. Nonetheless, an unacceptable percentage of severe complications remained that is mainly attributable to non-HLA histocompatibility systems, i.e., minor histocompatibility antigens. Observations in the 1970s that minor histocompatibility antigens cause serious problems in human bone marrow transplantation laid the basis for their use as curative antigens in stem cell transplantation to date. Thus, the allo-immune T cell activities caused by minor histocompatibililty antigen disparities between HLA-matched donor and recipient can now be applied for the benefit of the transplant patient.     

Hematopoietic stem cell recipients do not develop post-transplantation immune tolerance to antigens present on minimal residual disease.

The immune environment present after allogeneic hematopoietic stem cell transplantation (HSCT) contributes to the control of leukemia. Our laboratory has demonstrated in a murine model that vaccination of recipients after transplantation with recipient tumor vaccines does not exacerbate graft-versus-host disease but does induce meaningful graft-versus-tumor effects. We previously demonstrated that part of the reason for the lack of graft-versus-host disease from post-transplantation vaccination is due to gradual acquisition of tolerance or unresponsiveness to recipient immunodominant minor histocompatibility antigens that are ubiquitously expressed in the recipient. However, our prior studies have not critically addressed the question of whether a similar process of acquisition of unresponsiveness to or tolerance of antigens present on minimal residual disease also occurs. The present study tested the hypothesis that unresponsiveness to antigens present on minimal residual disease present at the time of HSCT would also occur. The answer to this question would have a significant effect on the potential efficacy of post-transplantation tumor vaccines. In a murine model of major histocompatibility complex matched, minor histocompatibility antigen mismatched HSCT (C3.SW female donors and C57BL/6 female recipients), we tested whether transplant recipients would acquire unresponsiveness to antigens present on small numbers of residual leukemia/lymphoma cells. We employed a male C57BL/6 lymphoid malignancy with an immunoglobulin/c-myc oncogene in these studies using as a model of tumor-restricted antigen the well-characterized male (HY) antigen system present only on the tumor but not present as ubiquitous minor antigens in the recipient. After HSCT, recipients did not mount immune responses to the ubiquitously distributed immunodominant recipient strain H7 minor histocompatibility antigen, but did retain the capacity to mount significant T cell responses to HY antigens present on small numbers of HY+ tumor cells present at transplantation. Additional studies using small numbers of nonmalignant recipient male B cells or dendritic cells as models of minimal residual disease also demonstrated that the transplant recipients retained their capacity to mount anti-HY T cell responses. After HSCT, recipients may retain the capacity to mount effective T cell responses to antigens present on minimal residual disease and still acquire relative tolerance to ubiquitously distributed immunodominant minor antigens that are related to graft-versus-host disease.     

Translational mini-review series on vaccines: Peptide vaccines for myeloid leukaemias

The graft-versus-leukaemia (GVL) effect following allogeneic stem cell transplantation is clear evidence that T lymphocytes can control and eliminate myeloid leukaemias. The successful identification of a range of leukaemia specific antigens (LSA) in recent years has stimulated efforts to induce leukaemia specific T cell responses to these antigens with peptide vaccines. An ideal LSA should be restricted in its expression to leukaemia including progenitor cells, intrinsically connected with the leukaemic phenotype, and capable of inducing strong cytotoxic T cell responses to the leukaemia. Peptides from three well-characterized LSA, the breakpoint cluster region-abelson (BCR-ABL) fusion protein of chronic myelogenous leukaemia, proteinase-3 and Wilms tumour 1 protein, serve as the basis for several clinical trials using peptide and adjuvants to treat patients with a variety of myeloid malignancies. Preliminary results from these studies indicate that these peptides induce immune responses which can translate into clinical responses which include complete remissions from leukaemia. These promising early results point the way to optimizing the administration of peptide vaccines and suggest ways of combining vaccination with allogeneic stem cell transplantation to boost GVL effects.     

HapMap SNP Scanner: an online program to mine SNPs responsible for cell phenotype

Minor histocompatibility (H) antigens are targets of graft-vs-host disease and graft-vs-tumor responses after human leukocyte antigen matched allogeneic hematopoietic stem cell transplantation. Recently, we reported a strategy for genetic mapping of linkage disequilibrium blocks that encoded novel minor H antigens using the large dataset from the International HapMap Project combined with conventional immunologic assays to assess recognition of HapMap B-lymphoid cell line by minor H antigen-specific T cells. In this study, we have constructed and provide an online interactive program and demonstrate its utility for searching for single-nucleotide polymorphisms (SNPs) responsible for minor H antigen generation. The website is available as 'HapMap SNP Scanner', and can incorporate T-cell recognition and other data with genotyping datasets from CEU, JPT, CHB, and YRI to provide a list of candidate SNPs that correlate with observed phenotypes. This method should substantially facilitate discovery of novel SNPs responsible for minor H antigens and be applicable for assaying of other specific cell phenotypes (e.g. drug sensitivity) to identify individuals who may benefit from SNP-based customized therapies.     

Minor histocompatibility antigens as determinants for graft‐versus‐host disease after allogeneic haematopoietic stem cell transplantation

Minor histocompatibility antigens (minor H antigens) are genetically polymorphic peptides that have been shown to elicit immune response when mismatched between donor and recipient of haematopoietic stem cell transplantation (HSCT). Depending on the expression profiles, mismatches in these genes may either lead to harmful graft‐versus‐host (GvH) reaction or desired graft‐versus‐leukaemia (GvL) effect. We analysed retrospectively the effect of HLA‐restricted matching 11 established autosomal minor H antigens on the risk of graft‐versus‐host disease and relapse in 311 HLA‐matched sibling HSCT of a single centre. Increased incidence of chronic GvH disease was shown to be associated with mismatches in the HA‐8 and ACC‐1. The mRNA expression profiles in a large set of healthy and malignant tissue samples of minor H antigen genes demonstrated in silico that the expression profiles of HA‐8 and ACC‐1 were surprisingly different: HA‐8 gene was expressed in practically all tissues, whereas ACC‐1 gene had a restricted profile. The results demonstrated that mismatches in minor H antigens HA‐8 and ACC‐1 predisposed to chronic graft‐versus‐host disease (GvHD).     

Nonmyeloablative blood stem cell transplantation as adoptive allogeneic immunotherapy for metastatic renal cell carcinoma

Allogeneic stem cell transplantation has emerged as a potentially curative form of immunotherapy for patients with hematological malignancies that are resistant to conventional chemo/radiotherapy. Donor T cell populations targeting allogeneic minor histocompatibility antigens expressed on the patient's malignant cells are felt to be the driving force of the graft-versus-leukemia reaction, although to date only a handful of these antigens have been fully characterized. Recent data from experimental animal models and limited clinical data in humans suggest that graft-versus-tumor effects, analogous to the graft-versus-leukemia reaction, may be generated against malignancies of epithelial origin. This article reviews the results of a pilot trial demonstrating graft-versus-renal cell carcinoma effects following nonmyeloablative stem cell transplantation, highlighting the potential of allogeneic immunotherapy for treating cancer.     

Clinical tolerance in allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) has been a curative therapeutic option for a wide range of immune hematologic malignant and non-malignant disorders including genetic diseases and inborn errors. Once in the host, allogeneic transplanted cells have not only to ensure myeloid repopulation and immunological reconstitution but also to acquire tolerance to host human leukocyte antigens via central or peripheral mechanisms. Peripheral tolerance after allogeneic HSCT depends on several regulatory mechanisms aimed at blocking alloimmune reactivity while preserving immune responses to pathogens and tumor antigens. Patients transplanted with HSCT represent an ideal model system in humans to identify and characterize the key cellular and molecular players underlying these mechanisms. The knowledge gained from these studies has allowed the development of novel therapeutic strategies aimed at inducing long-term peripheral tolerance, which can be applicable not only in allogeneic HSCT but also in autoimmune diseases and solid-organ transplantation. In the present review, we describe Type 1 regulatory T cells, initially discovered and characterized in chimeric patients transplanted with human leukocyte antigen-mismatched HSCT, and how their presence correlates to tolerance induction and maintenance. Furthermore, we summarize different cell therapy approaches with regulatory T cells, designed to facilitate tolerance induction, minimizing pharmaceutical interventions.     

The immunogenomics of minor histocompatibility antigens

Summary: 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.     

Donor cell origin of multiple myeloma occurring after allogeneic haematopoietic stem cell transplantation in a patient with refractory anaemia with ring sideroblast

Post-transplant lymphoproliferative disorder (PTLD) is a rare but life-threatening complication after solid organ and haematopoietic stem cell transplantation. A 40-year-old woman who was diagnosed as having refractory anaemia with ring sideroblast 6 years ago took an ABO mismatched, unrelated allogeneic haematopoietic stem cell transplantation (HSCT) from a 32-year-old healthy male donor. The bone marrow (BM) study was carried out because of progressing pancytopenia, serum biclonal gammopathy and a distorted ratio of serum level of free κ and λ light chain 138 days after HSCT. The BM examination showed an increased number of plasma cells (12% of total marrow cells) comprising mainly CD45-CD19-CD138+ malignant plasma cells with an immunoglobulin heavy-chain gene rearrangement. Conventional cytogenetics and molecular personal identification studies revealed that all BM cells were totally replaced by donor cells, thus indicating the donor cell origin of PTLD-multiple myeloma. The BM microenvironment of the recipient might be associated with the development of PTLD-multiple myeloma.     

Minor transplantation antigens: their role in shaping the T cell repertoire

Minor transplantation, or histocompatibility (H), antigens are the targets of host-versus-graft (hvg) and graft-versus-host (gvh) reactions that occur when organs or tissues are exchanged between members of the same species who, although genetically not identical, are matched for their major histocompatibility complex (MHC) encoded transplantation antigens. Genes encoding minor H antigens map outside the MHC, on a number of different chromosomes. Whilst gvh and hvg reactions against individual minor H antigens are relatively weak, certainly in comparison with such reactions against MHC antigens, the presence of multiple minor H differences (the situation encountered in man) gives rise to very vigorous reactions that can endanger the survival of graft or host, or both. This is the pathological role of minor H antigens and, indeed, it was this role which was first designated to the MHC antigens, before their physiological role as guidance molecules for T lymphocytes was discovered. Recently, a potential physiological role for minor H antigens has been uncovered by the finding that the presence of certain minor H alleles in mice leads to removal in the thymus (negative selection) of all those T cells expressing a particular T cell receptor (TCR) gene. Such cells therefore never reach the periphery, where they might otherwise give rise to autoimmune reactions. The T cell repertoire is thus moulded by at least some minor H antigens, which may therefore be regarded as non-MHC immune response genes. Furthermore, T cell receptor usage by T cells specific for allogeneic minor H antigens appears not to be representative of T cell receptor usage in the peripheral pool.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cytotoxic T-lymphocyte (CTL) responses against acute or chronic myeloid leukemia

Summary: In addition lo chemotherapy and irradiation, in the context of allogeneic stem cell transplantation (SCT), the donor cell-mediated anti-leukemic effect can lead to sustained complete remissions, also in cases of a large tumor load. This phenomenon appears to be an immunologically mediated response, possibly due to various effector cell populations. Cytotoxic T-lymphocyte (CTL) responses against minor histocompatibility antigens with restricted tissue distribution, in particular restricted to some or all hematopoietic cells, may be highly efficient in inducing and-leukemic responses for adoptive immunotherapy Specific CTL responses against leukemia-associated antigens may be generated using leukemic cells modified to co-express costimulatory molecules identical to professional antigens may be used in the context of allogeneic tells Donor derived T cell recognizing such antigens may be used in the context of allogeneic SCT to induce complete and sustained remissions, also in patients with leukemia refractory to chemotherapy. In these to circumstances, the primary objective of allogeneic SCT may be not to diminish the number of malignant cells by the chemotherapy and irradiation as part of the conditioning regimen, but to allow immunotherapy against leukemic cells using donor lymphocyte populations.     

Transplantation tolerance: from theory to clinic

Tolerance induction and alloreactivity can be applied to the clinic for the transplantation of solid organs and in the treatment of human cancers respectively. Hematopoietic chimerism, the stable coexistence of host and donor blood cells, guarantees that a solid organ from the same donor will be tolerated without a requirement for maintenance immunosuppression, and it also serves as a platform for the adoptive immunotherapy of hematologic malignancies using donor lymphocyte infusions. This review focuses on clinically relevant methods for inducing hematopoietic chimerism and transplantation tolerance, with a special emphasis on reduced intensity transplantation conditioning and high dose, post-transplantation cyclophosphamide to prevent graft rejection and graft-versus-host disease (GVHD). Reduced intensity transplantation regimens permit a transient cooperation between donor and host immune systems to eradicate malignancy without producing GVHD. Their favorable toxicity profile also enables the application of allogeneic stem cell transplantation to treat non-malignant disorders of hematopoiesis and to induce tolerance for solid organ transplantation.     

Bi-directional allelic recognition of the human minor histocompatibility antigen HB-1 by cytotoxic T lymphocytes

Human minor histocompatibility antigens (mHag) are target antigens of the graft-versus-leukemia response observed after allogeneic HLA-identical stem cell transplantation. We previously defined the molecular nature of the B cell lineage-specific mHag HB-1. The CTL epitope was identified as the decamer peptide EEKRGSLHVW presented in the context of HLA-B44. The HB-1 antigen is encoded by a locus of yet unknown function on chromosome 5q32. A single nucleotide polymorphism within this locus results in an amino acid change from histidine (H) to tyrosine (Y) at position P8 within the CTL epitope. Based on genomic information, we have developed a PCR-RFLP assay to perform HB-1 typing at the DNA level. We determined that the allelic frequency for the H and Y variant is 0.79 and 0.21, respectively. From these data, we calculated that the expected recipient disparity between HLA-B44-matched sibling pairs for HB-1H is 2.8%, whereas recipient disparity for HB-1Y is expected to be 12.4%. Therefore, we addressed whether the HB-1Y peptide is reciprocally immunogenic. We revealed that both peptide variants bind equally efficient to HLA-B44 molecules and that the H/Y substitution has no influence on formation of epitope precursor peptides by 20 S proteasome-mediated degradation. More directly, CTL recognizing the naturally presented HB-1Y peptide could be generated from a HB-1H homozygous donor using peptide-pulsed dendritic cells. Using a set of synthetic structurally related peptide variants, we found that the H/Y substitution has a major impact on TCR recognition by CTL specific for either of the HB-1 allelic homologues. HB-1 is the first human mHag described that induces bi-directional allogeneic CTL responses that may contribute to a specific graft-versus-leukemia response following allogeneic stem cell transplantation.     

Adaptive transfer of B10 cells: A novel therapy for chronic rejection after solid organ transplantation

Chronic rejection occurs between almost all MHC-mismatched donors and recipients after transplantation. Immunosuppressive agents have been administrated indiscriminately to manage potential rejection, but complications from lifelong immunosuppressive therapy threaten transplant recipients. Recent studies demonstrated that a number of regulatory B cells (B10 cells) negatively regulate T cell mediated immune responses without inducing systemic immune suppression. Therefore, we propose that adaptive transfer of B10 cells suppresses alloreactive CD8⁺ cytotoxic T cell activation induced by allogeneic solid organ transplantation, reduces T cell mediated rejection and prolongs allograft survival.     

Donor T Cells Administered Over HLA Class II Barriers Mediate Antitumor Immunity without Broad Off-Target Toxicity in a NOD/Scid Mouse Model of Acute Leukemia

Alloreactive (allo)-HLA–directed T cell responses after HLA-mismatched allogeneic hematopoietic stem cell transplantation and donor lymphocyte infusion are typically considered detrimental responses mediating graft-versus-host disease (GVHD). Allo-HLA-reactive T cells with beneficial and selective graft-versus-leukemia (GVL) reactivity, however, can also be identified within an HLA-mismatched context. We investigated whether allo-HLA class II–directed T cells with beneficial GVL reactivity induced in NOD/scid mice engrafted with human chronic myelogenous leukemia in lymphoid blast crisis after treatment with donor lymphocyte infusion – mediated detrimental xenogeneic GVHD as a result of broad off-target cross-reactivity. The results demonstrate that beneficial GVL reactivity and xenogeneic GVHD are mediated by separate T cells. GVL reactivity was mediated by human T cells recognizing allo-HLA class II molecules, whereas xenoreactivity was exerted by human T cells recognizing H-2 molecules. Taken together, our data indicate a limited risk for detrimental off-target effects by allo-HLA class II–directed T cells and thereby provide a basis for the development of strategies for selecting allo-HLA–restricted T cells with selective GVL reactivity for adoptive transfer after HLA-mismatched allogeneic hematopoietic stem cell transplantation.     

Modulation of alloimmune response by commensal gut microbiota and potential new avenues to influence the outcome of allogeneic transplantation by modification of the ‘gut culture’

Host defence response against microbial infections was the foundation for the Science of Immunology. Now, we know the mechanisms of such host defence which include innate immune responses that is generally nonspecific but effective in many cases and lead to more specific responses called adaptive immune response. The gene loci of class I, II and III of the major histocompatibility complex (MHC) play a major role in directing the adaptive immune responses by presenting processed antigens to T and B cells to induce appropriate antigen‐specific cellular and or humoral immune responses. In humans, these are commonly referred to as human leucocyte antigens class I/II‐HLA I/II). The class III region, the gamma region in the MHC complex, is mostly associated with regulation of immune responses along with genes associated with complement activation. The adaptive immune responses are orchestrated by T and B cells that are tuned to respond to antigens that are normally foreign to the body, because these cells are educated to avoid self‐antigens by a process of thymic education and selection of the T cells that are mostly non‐self‐reactive which also helps the B cells in eliciting specific immune responses to non‐self‐antigens. A by‐product of this is the ability of the T and B cells to elicit strong immune responses to foreign HLA/MHC (alloimmune response), which developed into the field of histocompatibility testing for allogeneic transplantation of stem cells and organs. Now, we are beginning to learn that such alloimmune responses can be influenced by the microbiota that symbiotically live in our body especially on the mucosal surfaces and on the skin. This review deals with new and emerging data on how the commensal mucosal and skin microbiota influence the immune homeostasis, and how manipulating the commensal microbiota of the mucosa and skin could influence the survival and long‐term functions of the allografts. Also, alterations of the microbiota by the inevitable immunosuppression prior to and following allogeneic transplantation could contribute towards the outcome of the allografts by alloimmune responses generated due to microbial antigen vs HLA cross‐reactivity.     

Antigen and Lymphopenia-Driven Donor T Cells Are Differentially Diminished by Post-Transplantation Administration of Cyclophosphamide after Hematopoietic Cell Transplantation

Administration of cyclophosphamide after transplantation (post-transplantation cyclophosphamide, PTC) has shown promise in the clinic as a prophylactic agent against graft-versus-host disease (GVHD). An important issue with regard to recipient immune function and reconstitution after PTC is the extent to which, in addition to diminution of antihost allo-reactive donor T cells, the remainder of the nonhost allo-reactive donor T cell pool may be affected. To investigate PTC's effects on nonhost reactive donor CD8 T cells, ova-specific (OT-I) and gp100-specific Pmel-1 T cells were labeled with proliferation dyes and transplanted into syngeneic and allogeneic recipients. Notably, an intermediate dose (66 mg/kg) of PTC, which abrogated GVHD after allogeneic HSCT, did not significantly diminish these peptide-specific donor T cell populations. Analysis of the rate of proliferation after transplantation illustrated that lymphopenic-driven, donor nonhost reactive TCR Tg T cells in syngeneic recipients underwent slow division, resulting in significant sparing of these donor populations. In contrast, after exposure to specific antigens at the time of transplantation, these same T cells were significantly depleted by PTC, demonstrating the global susceptibility of rapidly dividing T cells after an encounter with cognate antigen. In total, our results, employing both syngeneic and allogeneic minor antigen-mismatched T cell replete models of transplantation, demonstrate a concentration of PTC that abrogates GVHD can preserve most cells that are dividing because of the accompanying lymphopenia after exposure. These findings have important implications with regard to immune function and reconstitution in recipients after allogeneic hematopoietic stem cell transplantation.     

Immunogenomics of hematopoietic stem cell transplantation

Recipients of allogeneic hematopoietic stem cell transplantation (HSCT) incur the risk of graft-versus-host disease even when the donor is a sibling who shares the Major Histocompatibility Antigens. Therefore, even the perfect HLA match does not represent the optimal genetic match between donors and recipients in HSCT. In addition to the HLA complex other genetic systems operate and affect the outcome of HSCT. These include minor histocompatibility systems (Martin P. Applicability of matching for minor histocompatibility antigens in human bone marrow transplantation. In: Roopenian DC, Simpson E, editors. Minor histocompatibility antigens: From the laboratory to the clinic. Georgetown: Landis Bioscience; 2000. p. 97-103) (inducing bona fide allogeneic responses) as well as a series of functional polymorphisms in cytokines and chemokines and receptors genes (Transplantation 1997;64:553). Among the items affecting the outcome of HSCT the incidence and severity of infections have an important impact. Polymorphisms of genes controlling both arms of the immune responses to pathogens (innate versus cognate) are strong candidates for susceptibility factors to infection in allogeneic transplantation. These include the MHC alleles (HLA class I, class II, MIC) CD1, Toll and TLR genes MBP, MPO genes, ...). In addition to the NK alloreactivity induced by HLA class I epitopes mismatching (a common situation in HSCT) variations in the genotype of the KIR genes (Tissue Antigens 2001;57:358) may also be encountered between the donor and the recipient leading to potentially harmful or beneficial combinations. An integrated knowledge of the role and hierarchy of the most important genetic factors (MHC and non-MHC) will provide the rationale for a comprehensive matching in HSCT (Curr Opin Hematol 3 (1996) 416). This short review provides a panorama of this strategic issue for further development of HSCT.     

Cerebral Toxoplasmosis After Autologous Peripheral Blood Stem Cell Transplantation

Toxoplasmosis appears to be a rare opportunistic protozoal infection following haematopoietic stem cell transplantation (HSCT). Most cases have been reported in allogeneic HSCT recipients, with only anecdotal reports of infection occurring after autologous transplantation. Reported here is the case of a patient who developed cerebral toxoplasmosis following autologous peripheral blood stem cell transplantation for non-Hodgkin's lymphoma.