Abstract
During homeostatic expansion, peripheral T cells proliferate in response to lymphopenia, in the absence of cognate antigen or costimulatory signals. Host CD8 reconstitution following hematopoietic cell transplantation (HCT) involves the de novo-generation of T cells in addition to the homeostatic expansion of mature donor T cells present in the graft and donor lymphocyte infusion as well as host T cells that survive conditioning. Although it is well appreciated that CD8 homeostatic expansion contributes to host CD8 reconstitution following HCT, the factors governing the extent of the contribution by donor CD8 homeostatic expansion to host reconstitution have not been precisely and systematically examined. The ex vivo generation of memory CD8 T cells specific for an epitope of the immunodominant minor histocompatibility antigen H60 was demonstrated. By adapting a previously described culture system designed to generate large numbers of transgenic memory CD8 T cells, memory CD8 cells (TM) specific for a physiological antigen were elicited from a heterogeneous population of CD8 T cells. The ex vivo-generated antigen-specific memory CD8 cells were then expanded under conditions of lymphopenia in ablatively conditioned syngeneic transplant recipients and found to persist greater than 2 months post-transplant. These findings support the notion that the transplantation of small numbers ex vivo-generated memory CD8 cells, specific for a physiologically relevant antigen, can help restore host immune function following HCT. Transplant conditions were found to modulate the contribution of homeostatically expanded donor CD8 TM to the host CD8 compartment. Varying the conditioning intensity, timing of infusion, and infusion dose affected the kinetics of expansion as well as the homeostatic set-point. In my HCT model, delayed infusions of 3 weeks and transplantation of small numbers of donor CD8 T cells resulted in significant contributions to host CD8 T cell compartment. These findings could be applied clinically to enhance the effectiveness of clinical immunotherapy in restoring host immune function post-transplant. Moreover, transplantation of varying doses of donor CD8 TM demonstrated there was maximal donor contribution to host CD8 reconstitution. Finally, the homeostatic expansion, persistence, and function of transgenic memory and naïve CD8 populations were investigated following transplantation into ablatively conditioned syngeneic recipients. Both donor CD8 populations underwent 2 weeks of expansion post-transplant. The naïve CD8 population reproducibly achieved higher homeostatic numbers than the memory CD8 population. By one month post-HCT, the transplanted naïve CD8 populations also exhibited a memory CD8 phenotype. Moreover, the CD8 TN→TM population mediated an effective response to a primary challenge, comparable to the antigen-experienced memory CD8 population. Thus, regardless of the state of differentiation of the donor CD8 population at the time of transplant, donor CD8 homeostatic expansion contributes a memory CD8 cells population, able to mediating effective immune responses, to host reconstitution.