Minor Ags Research Articles

Natural IgM initiates the immunological cascade against minor antigen-mismatched cells, illustrating the dependency and unique role of APC subtypes

Abstract In organ transplantation a major obstacle is the immunological response against mismatches of major and minor histocompatibility antigen (Ags). Even when MHC Ags are matched between donor and recipient, minor Ags can elicit graft rejection. To date, it is unclear how endogenous antigen-presenting cells (APCs) recognize, coordinate and induce immunological responses against minor Ag-mismatched cells, particularly in the absence of pathogen-associated molecular patterns (PAMPs). We hypothesized that each APC subtype is dependent on each other to complete an immunological response against mismatches of minor Ags. Using a well-established minor Agmismatch model, we demonstrated that the elimination of any one APC subtype (Batf3+ DCs, Irf4+ DCs, Ly6C+ monocytes or B cells) resulted in a diminished or abolished immune response against minor Ag-mismatches. Specifically, we demonstrated that this immunological response began with the recognition of minor Ag-mismatched cells by natural IgM, which resulted in an immune complex formation. This cellular immune complex was then acquired by MHC II presenting APCs, of which one was MHC II expressing Ly6C+ monocytes. Subsequently, Ly6C+ monocyte induced Ag-specific CD4+ T cells licensed Batf3+ DCs via CD40 to present mismatches of minor antigen to Ag-specific CD8+T cells, which were required for the cytotoxic elimination of minor-Ag mismatched cells. Overall, our in vivo findings suggest a sequential, coordinated immunological event is required for the rejection of minor Ag-mismatched cells.

Cutting Edge: Roles for Batf3-Dependent APCs in the Rejection of Minor Histocompatibility Antigen-Mismatched Grafts.

In transplantation, a major obstacle for graft acceptance in MHC-matched individuals is the mismatch of minor histocompatibility Ags. Minor histocompatibility Ags are peptides derived from polymorphic proteins that can be presented by APCs on MHC molecules. The APC subtype uniquely responsible for the rejection of minor Ag-mismatched grafts has not yet been identified. In this study, we examined graft rejection in three mouse models: 1) mismatch of male-specific minor Ags, 2) mismatch of minor Ags distinct from male-specific minor Ags, and 3) skin transplant. This study demonstrates that in the absence of pathogen-associated molecular patterns, Batf3-dependent dendritic cells elicit the rejection of cells and grafts expressing mismatched minor Ags. The implication of our findings in clinical transplantation may be significant, as minor Ag reactivity has been implicated in the pathogenesis of multiple allograft tissues.

Intracellular MHC Class II Controls Regulatory Tolerance to Allogeneic Transplants

MHC class II (MHCII) genes have been implicated in the regulation of T lymphocyte responses. However, the mechanism of MHCII-driven regulation remains unknown. Matching for MHCII between donors and recipients of allografts favors regulatory T cell tolerance to transplants and provides a unique opportunity to study this regulation. In this study, we investigated MHCII regulation using transfer of donor MHCII genes in recipients of cardiac allografts. Transfer of MHCII IA(b) genes in the bone marrow of CBA mice (H-2(k)) prior to the grafting of IA(b+) fully allogeneic C57BL/6 (B6, H-2(b)) heart transplants resulted in donor-specific tolerance associated with long-term survival of B6, but not third-party, allografts without sustained immunosuppression. Strikingly, the majority of accepted heart transplants (>170 d) were devoid of allograft vasculopathy. Further studies indicated that intracellular IA(b) initiated the tolerogenic process, which was mediated by regulatory T cells (Tregs) that polarized antigraft responses to Th2 cytokine producers. This mechanism seems to be unique to MHCII genes, because previous MHC class I gene-based therapies failed to produce Tregs. These results demonstrate the key role of MHCII in the induction of Tregs. They also underscore a potential mechanism of specific inactivation of T cells in this model; when activated by IA(b+) grafts, IA(b)-specific Tregs repress the entire alloresponse to C57BL/6 transplants (including MHC I and minor Ags), thus mediating T cell tolerance.

T-cell therapy targeting minor histocompatibility Ags for the treatment of leukemia and renal-cell carcinoma.

healthsciences The graft-versus-host (GvH) and graft-versus-leukemia (GvL) reactions that occur after MHC-identical allogeneic hematopoietic cell transplantation (HCT) are mediated by T lymphocytes derived from the donor. We have previously shown that CD8 1 cytotoxic T lymphocytes (CTL) specific for minor histocompatibility (H) Ags expressed in recipient hematopoietic cells, including leukemic cells, but not widely expressed in non-hematopoietic tissues, can be isolated from most MHC-identical transplant recipients [1]. These CTL lyse AML and ALL blasts in vitro, and can inhibit the engraftment of human AML stem cells in nonobese diabetic (NOD)/SCID mice [2]. CD8 1 CTL clones specific for tissue-restricted minor H-Ags represent potential therapeutic reagents for adoptive T-cell therapy, designed to selectively enhance the GvL effect without inducing GvHD. A Phase I trial has been initiated at the Fred Hutchinson Cancer Research Center to evaluate the safety and antileukemic efficacy of adoptive T-cell therapy with CD8 1 CTL clones specific for tissue-restricted minor H-Ags, in patients with advanced myelodysplastic syndrome (MDS) and acute leukemia who relapse after HCT from an MHCidentical donor. The CTL clones are generated from transplant recipients prospectively at the time of transplant, and characterized for tissue specificity and Class I MHC restriction. Sequencing of each clone’s uniquely rearranged T-cell receptor b -chain V-D-J region allows the design of PCR assays that facilitate in vivo tracking of adoptively transferred CTL. Upon post-transplant relapse, patients receive cytoreductive chemotherapy, followed by three infusions of minor H-Ag-specific CTL at an escalating dose. If no toxicity is observed, additional CTL infusions are administered at weekly intervals, followed by a 14-day course of low-dose IL-2. In concurrent laboratory studies, cDNA expression cloning is being used to identify the genes encoding the Ags recognized by the CTL clones used in T-cell therapy. Graft-versus-tumor effects are also observed after nonmyeloablative MHC-identical HCT for renal-cell carcinoma (RCC), and studies to investigate the specificity of effector cells involved have recently been initiated [3]. CD8 1 minor H-Ag-specific CTL clones have been isolated from RCC patients after nonmyeloablative HCT. Several CTL clones lyse RCC cell-lines in an MHCrestricted fashion, suggesting that adoptive T-cell therapy targeting minor Ags expressed by RCC cells could be used to enhance the graft-versus-tumor effect after allogeneic HCT for metastatic RCC.

Cutting Edge: Alloimmune Responses Against Major and Minor Histocompatibility Antigens: Distinct Division Kinetics and Requirement for CD28 Costimulation

Comparative study of alloimmune responses against major and minor histocompatibility Ags has been limited by the lack of suitable assays. Here, we use a bioassay that permits tracking of alloreactive CD4+ T cell populations as they proliferate in response to major or minor histocompatibility Ags in vivo. Division of alloreactive CD4+ T cells proceeded more rapidly in response to major histocompatibility Ags than minor Ags, although CD4+ T cells alloreactive to minor Ags had a similar capacity to divide successively up to eight times after stimulation. Allorecognition of minor histocompatibility Ags was highly dependent on CD28 costimulation, with the frequency of CD4+ T cells proliferating in response to minor Ags in the absence of CD28 costimulation reduced up to 20-fold. These findings highlight differences in signaling processes that lead to allorecognition of major and minor histocompatibility Ags and have implications on the design of interventions aimed at abrogating these responses.

T cell suppression in transplantation tolerance through linked recognition.

Allogeneic tissues transplanted to mice treated with CD4- and CD8-specific Abs are often accepted indefinitely due to the induction of immunologic tolerance. When transplantation tolerance was induced to grafts mismatched at multiple minor histocompatibility loci, Ag specificity was inferred because third party grafts, mismatched at the MHC, were rejected normally. However, some "third party" grafts were either accepted, or rejected more slowly. Tolerant mice possess CD4+ cells, which suppress rejection by T cells reacting to the same grafts. Therefore, we hypothesized that tolerated third party grafts might share Ags with the original tolerizing graft, and that these Ags are a target for such suppression. To test this idea, we tolerized mice to a set of minor Ags (B10 minors) and challenged them with third party grafts that carried those minors, as well as an additional strong transplantation Ag, the class I MHC molecule, H-2Kb. This class I molecule acts as a good target for rejection in both naive mice and in mice tolerized to B10 minors. However, when this third party class I molecule is provided "linked" to those B10 minors on an F1 graft, rejection was significantly impaired. The data suggest that suppression within tolerant animals operates locally (perhaps on the same APC) via linked recognition. In addition, our preliminary findings suggest that suppression via linked recognition can also lead to tolerance to the third party Ag.