General T-cell receptor antagonists to immunomodulate HLA-A2-restricted minor histocompatibility antigen HA-1-specific T-cell responses

Dimerization of soluble disulfide trap single-chain major histocompatibility complex class I molecules dependent on peptide binding affinity

Stable presentation of peptide epitope by major histocompatibility complex (MHC) class I molecules is a prerequisite for the efficient expansion of CD8(+) T cells. The construction of single-chain MHC class I molecules in which the peptide, β(2)-microglobulin, and MHC heavy chain are all joined together via flexible linkers increases peptide-MHC stability. We have expressed two T cell epitopes that may be useful in leukemia treatment as single-chain MHC class I molecules, aiming to develop a system for the expansion of antigen-specific CD8(+) T cells in vitro. Disulfide trap versions of these single-chain MHC molecules were also created to improve anchoring of the peptides in the MHC molecule. Unexpectedly, we observed that soluble disulfide trap single-chain molecules expressed in eukaryotic cells were prone to homodimerization, depending on the binding affinity of the peptide epitope. The dimers were remarkably stable and efficiently recognized by conformation-specific antibodies, suggesting that they consisted of largely correctly folded molecules. However, dimerization was not observed when the disulfide trap molecules were expressed as full-length, transmembrane-anchored molecules. Our results further emphasize the importance of peptide binding affinity for the efficient folding of MHC class I molecules.

Secondary anchor polymorphism in the HA-1 minor histocompatibility antigen critically affects MHC stability and TCR recognition

T cell recognition of minor histocompatibility antigens (mHags) underlies allogeneic immune responses that mediate graft-versus-host disease and the graft-versus-leukemia effect following stem cell transplantation. Many mHags derive from single amino acid polymorphisms in MHC-restricted epitopes, but our understanding of the molecular mechanisms governing mHag immunogenicity and recognition is incomplete. Here we examined antigenic presentation and T-cell recognition of HA-1, a prototypic autosomal mHag derived from single nucleotide dimorphism (HA-1(H) versus HA-1(R)) in the HMHA1 gene. The HA-1(H) peptide is restricted by HLA-A2 and is immunogenic in HA-1(R/R) into HA-1(H) transplants, while HA-1(R) has been suggested to be a "null allele" in terms of T cell reactivity. We found that proteasomal cleavage and TAP transport of the 2 peptides is similar and that both variants can bind to MHC. However, the His>Arg change substantially decreases the stability and affinity of HLA-A2 association, consistent with the reduced immunogenicity of the HA-1(R) variant. To understand these findings, we determined the structure of an HLA-A2-HA-1(H) complex to 1.3A resolution. Whereas His-3 is accommodated comfortably in the D pocket, incorporation of the lengthy Arg-3 is predicted to require local conformational changes. Moreover, a soluble TCR generated from HA-1(H)-specific T-cells bound HA-1(H) peptide with moderate affinity but failed to bind HA-1(R), indicating complete discrimination of HA-1 variants at the level of TCR/MHC interaction. Our results define the molecular mechanisms governing immunogenicity of HA-1, and highlight how single amino acid polymorphisms in mHags can critically affect both MHC association and TCR recognition.

The diallelic locus encoding the minor histocompatibility antigen HA-1 is evolutionarily conserved

The polymorphic minor histocompatibility antigen HA-1 induces powerful T-cell alloreactivities with important consequences for graft-vs-tumor activity and development of graft-vs-host disease in patients after human leukocyte antigen-matched stem-cell transplantation (SCT). In view of possible translational animal studies, we analyzed the evolutionary conservation of the diallelic HA-1 locus in four mammalian species. Our results show that rodents do not encode the HA-1(H) allele, neither show polymorphism in this position on the HA-1 gene. Contrariwise, the HA-1(H) allele is present in non-human primate species and dogs. Interestingly, both the HA-1(H) T-cell epitope and its non-immunogenic counterpart HA-1(R) are present in the latter species. Thus, the HA-1 allelic polymorphism is conserved in evolution in primates and dogs.

Induction of dominant transplantation tolerance by an altered peptide ligand of the male antigen Dby

T cell reactivity to minor histocompatibility (mH) antigens is responsible for rejection of HLA-matched allografts, limiting the effectiveness of transplantation for the treatment of end-stage organ failure. The deadbox gene Dby is located on the Y chromosome and encodes an mH antigen that prompts rejection of male tissues by female mice. Establishing a network of regulatory T (T(reg)) cells that is capable of coercing naive cells to adopt a tolerant phenotype offers an attractive strategy for immune intervention in such deleterious immune responses. While various approaches have successfully induced a dominant form of transplantation tolerance, they share the propensity to provoke chronic, incomplete activation of T cells. By identifying the T cell receptor (TCR) contact sites of the dominant epitope of the Dby gene product, we have designed an altered peptide ligand (APL) that delivers incomplete signals to naive T cells from A1 infinity RAG1(-/-) mice that are transgenic for a complementary TCR. Administration of this APL to female transgenic mice polarizes T cells toward a regulatory phenotype, securing a form of dominant tolerance to male skin grafts that is capable of resisting rejection by naive lymphocytes. Our results demonstrate that incomplete signaling through the TCR may establish a network of T(reg) cells that may be harnessed in the service of transplantation tolerance.

The frequency of interferon-gproducing cells reflects alloreactivity against minor histocompatibility antigens

BACKGROUND

In human leukocyte antigen (HLA)-identical living-related renal transplant recipients, no donor-specific alloreactivity can be detected in regular tests (mixed lymphocyte culture, helper T-lymphocyte precursor frequencies, cytotoxic T-lymphocyte precursor frequencies) to identify patients responding to minor histocompatibility antigens (mHag). We questioned whether a more sensitive method like the Elispot-assay could be more appropriate.

METHODS AND RESULTS

Peripheral blood mononuclear cells (PBMC) from 16 HLA-identical living-related kidney transplant patients 3 months (range, 2 weeks to 5 months) after transplantation were tested for the frequency of interferon (IFN)-gamma producing cells by the Elispot-assay. PBMC from the recipient were stimulated with irradiated donor PBMC and corrected for backward stimulation. Donor-specific IFN-gamma producing cells (pc) in the range of 5 to 115 per million PBMC (median, 30 per million PBMC) were found. To evaluate the frequency of spot forming cells in time, PBMC from six patients who received an HLA-identical renal transplant were stimulated with irradiated donor PBMC before, approximately 3 months after, and 12 months after transplantation. Four patients who received an HLA-mismatched living-related kidney served as positive control. In the HLA-identical group, frequencies in the range of 0 to 10 IFN-gamma pc per million PBMC were found before transplantation, 0 to 30 per million PBMC 3 months after transplantation, and 0 to 45 per million PBMC 12 months after transplantation. In the HLA-mismatched group, significantly higher numbers were found: 10 to 480 IFN-gamma pc per million PBMC before, 20 to 360 per million PBMC at 3 months, and 30 to 590 per million PBMC 12 months after transplantation.

CONCLUSION

Under immunosuppressive therapy, IFN-gamma pc specific for donor mHag can be found after HLA-identical living-related renal transplantation.

New Developments in Allotransplant Immunology

After allogeneic stem cell transplantation, the establishment of the donor’s immune system in an antigenically distinct recipient confers a therapeutic graft-versus-malignancy effect, but also causes graft-versus-host disease (GVHD) and protracted immune dysfunction. In the last decade, a molecular-level description of alloimmune interactions and the process of immune recovery leading to tolerance has emerged. Here, new developments in understanding alloresponses, genetic factors that modify them, and strategies to control immune reconstitution are described.

In Section I, Dr. John Barrett and colleagues describe the cellular and molecular basis of the alloresponse and the mechanisms underlying the three major outcomes of engraftment, GVHD and the graft-versus-leukemia (GVL) effect. Increasing knowledge of leukemia-restricted antigens suggests ways to separate GVHD and GVL. Recent findings highlight a central role of hematopoietic-derived antigen-presenting cells in the initiation of GVHD and distinct properties of natural killer (NK) cell alloreactivity in engraftment and GVL that are of therapeutic importance. Finally, a detailed map of cellular immune recovery post-transplant is emerging which highlights the importance of post-thymic lymphocytes in determining outcome in the critical first few months following stem cell transplantation. Factors that modify immune reconstitution include immunosuppression, GVHD, the cytokine milieu and poorly-defined homeostatic mechanisms which encourage irregular T cell expansions driven by immunodominant T cell–antigen interactions.

In Section II, Prof. Anne Dickinson and colleagues describe genetic polymorphisms outside the human leukocyte antigen (HLA) system that determine the nature of immune reconstitution after allogeneic stem cell transplantation (SCT) and thereby affect transplant outcomethrough GVHD, GVL, and transplant-related mortality. Polymorphisms in cytokine gene promotors and other less characterized genes affect the cytokine milieu of the recipient and the immune reactivity of the donor. Some cytokine gene polymorphisms are significantly associated with transplant outcome. Other non-HLA genes strongly affecting alloresponses code for minor histocompatibility antigens (mHA). Differences between donor and recipient mHA cause GVHD or GVL reactions or graft rejection. Both cytokine gene polymorphisms (CGP) and mHA differences resulting on donor-recipient incompatibilities can be jointly assessed in the skin explant assay as a functional way to select the most suitable donor or the best transplant approach for the recipient.

In Section III, Dr. Nelson Chao describes non-pharmaceutical techniques to control immune reconstitution post-transplant. T cells stimulated by host alloantigens can be distinguished from resting T cells by the expression of a variety of activation markers (IL-2 receptor, FAS, CD69, CD71) and by an increased photosensitivity to rhodamine dyes. These differences form the basis for eliminating GVHD-reactive T cells in vitro while conserving GVL and anti-viral immunity. Other attempts to control immune reactions post-transplant include the insertion of suicide genes into the transplanted T cells for effective termination of GVHD reactions, the removal of CD62 ligand expressing cells, and the modulation of T cell reactivity by favoring Th2, Tc2 lymphocyte subset expansion. These technologies could eliminate GVHD while preserving T cell responses to leukemia and reactivating viruses.