Bone marrow failure syndrome-acquired aplastic anemia is a condition in which bone marrow does not generate a sufficient number of new cells to replenish blood cells. In most cases, the inability of bone marrow to produce new blood cells results from immune-mediated destruction of hematopoietic stem and progenitor cells, which is characterized by lower counts of red blood cells, white blood cells, and platelets. Patients with this disorder suffer from anemia and life-threatening bleeding and infectious complications. Treatment, therefore, is immunosuppression, typically with antithymocyte globulin in combination with cyclosporine A, and can induce a hematologic response in about two-thirds of aplastic anemia patients. However, relapse occurs in up to 35% of aplastic anemia patients when cyclosporine A is withdrawn at 6 months. Thus, a better understanding of whether and how marrow-destructing T cells persist through treatment could significantly improve aplastic anemia therapy.

The only available experimental animal models to study human aplastic anemia are allogeneic T cell-mediated bone marrow destruction. Taking advantage of these animal models, we have recently discovered that donor T cells deprived of Notch signaling failed to destruct bone marrow in allogeneic recipient mice. Furthermore, inactivation of Notch signaling only in donor CD8+ T cells was sufficient to prevent host tissue injury mediated by allogeneic T cells. We have also identified that host-reactive donor CD8+ effector and memory T cells proliferated and persisted upon chronic exposure to host antigens, causing severe damage to the host tissues, including bone marrow. Most importantly, the immunosuppressive drug rapamycin augmented the generation of long-lived CD8+ T memory cells capable of elaborating effector functions when rapamycin was withdrawn. This would follow that withdrawal of immunosuppression in aplastic anemia patients could result in reactivation of memory T cells to become marrow-destructing effector cells. These observations suggest that immunosuppression may induce marrow-destructing CD8+ effector T cells to become functionally dormant memory T cells that can be reactivated when immunosuppressive drugs are withdrawn.

To improve the efficacy of aplastic anemia therapy, we propose this project to focus on the following three specific aims. First, we will use genetic approaches to define the impact of Notch signaling in the reactivation of effector programs in T memory cells in mouse models of bone marrow failure. Second, we will determine whether and how marrow-destructive memory T cells are generated and maintained during immunosuppression. We will assess whether in vivo administration of cyclosporine A results in restoration / enhancement of regulatory antigen-presenting cells in bone marrow to repress effector programs in memory T cells. Third, we will determine the beneficial effects of memory T cell inhibition in mouse models of bone marrow failure by using pharmacological gamma-secretase inhibitor of Notch signaling and adoptive transfer of marrow-derived regulatory antigen-presenting.

The proposed aims present proof of concept studies to potentially improve therapy of acquired aplastic anemia. Results from these proposed experiments will identify the impact of memory T cells in immune-mediated marrow destruction and the beneficial effects of Notch blockade and regulatory antigen-presenting cells in the treatment of aplastic anemia in mouse models. If blocking memory T cells can prevent the relapse of bone marrow failure, the overall outcomes of bone marrow failure treatment can be potentially improved using an alternative immunomodulation approach. This will lead to the development of novel and clinically relevant approaches to induce the tolerance of pathogenic memory T cells to the host marrow.