Roles and targets of hematopoietic progenitor cell-reactive T cells in acquired aplastic anemia

Aplastic anemia comprises a variety of hematologic disorders that are characterized by hypocellular bone marrow and peripheral blood pancytopenia. This proposal is focused on acquired/idiopathic aplastic anemia, in the following referred to as “aplastic anemia”, which accounts for approx. 70% of all cases, and is almost exclusively immune-mediated. Aplastic anemia can cause life-threatening complications, and effective treatment options are limited to allogeneic stem cell transplantation and immunosuppressive therapy. While stem cell transplantation offers potential cure and is recommended for subsets of younger patients, immunosuppressive therapy can induce remissions in approx. 70% of patients. The pathophysiologic mechanisms underlying aplastic anemia are incompletely understood. Remissions upon immunosuppressive therapy with anti-thymocyte globulin (ATG) and calcineurin inhibition with cyclosporine A suggest a critical involvement of the immune system, especially T cells. A substantial body of data suggests important roles for memory stem and T helper type 1 (Th1) cells. Furthermore, oligoclonal expansion of effector T cells has been shown to be particularly associated with aplastic anemia. In contrast, regulatory T cells can suppress progenitor cell-directed immune reactions and appear to play a major role in the beneficial effects of ATG-mediated immunosuppression. Despite the undebatable role of T cells in aplastic anemia pathophysiology, their molecular targets and specific contribution to hematopoietic progenitor cell elimination are incompletely understood. We will build on our preliminary experimental evidence that expanded bone marrow T-cell clones recognize hematopoietic progenitor cells and can be cross reactive with epitopes derived from Epstein-Barr virus. We hypothesize that clonally expanded CD8+ T cells recognize antigens presented on hematopoietic progenitor cells leading to immunologically mediated bone marrow aplasia; the critical target epitopes are, among others, related to chronically persistent (viral) antigens. We have established single-cell methodologies that allow identification of clonally expanded hematopoietic progenitor cell-reactive T cells, and determination of their molecular targets. Selected clones will be tracked over time to potentially inform clinical decision making on immunosuppressive therapy in the future. Our research addresses the unmet clinical need of better understanding aplastic anemia pathophysiology to critically guide currently available treatment options and develop novel targeted therapies. We will advance the field by i) providing an in-depth determination of the clonal and phenotypic diversity of bone marrow T cells in aplastic anemia, and ii) establishing a model for the roles of marrow-infiltrating T cells upon perturbation by immunosuppressive therapy, potentially leading to iii) the identification of targets and mechanisms accessible for future therapeutic intervention.