Dendritic cells (DC) are rare, uniquely well-equipped professional antigen-presenting cells (ARC) that are regarded as both initiators and regulators of immune responses. Multiple DC subsets have been identified in mice and humans. In mice, three major DC subsets have been characterized: Myeloid (MDC), Lymphoid-related (LDC) and Plasmacytoid (PDC). Although donor-derived DC are regarded as critical instigators of acute organ allograft rejection, the contribution of recipient DC towards development of chronic rejection has not been clearly defined or extensively studied. We hypothesize that different subsets of recipient DC have unique roles and levels of contribution to heart allograft rejection. Here we propose a novel approach to analyze the role of recipient DC in the outcome of heart transplantation: we will restore defined subsets of DC to recipient mice with profound deficiencies in DC subsets as the result of targeted gene disruption. Specifically, C57BL/6 (B6, H2b) mice that are profoundly deficient in fms-like tyrosine kinase 3 ligand (Flt3L-/-), a molecule crucial for the development and regulation of DC, will be used in these studies as heart graft recipients. Donor BALB/c (H2d) heart allografts will be transplanted into recipient Flt3L-/- B6 mice and the outcome determined by histological, immunohistological, and molecular biologic examination of the grafts at various time points post transplant. Additionally, analysis of anti-donor T cell-mediated and humoral immunity will be performed. To eliminate the possibility of host T cell activation and hence graft rejection due to alloantigen presentation by donor ARC, we will concurrently, use costimulatory molecule- deficient B7-1/B7-2-/- (BALB/c, H2d) mice as heart allograft donors. These B7 molecules normally are expressed by ARC within the graft, and provide costimulatory signals to recipient B6 T cells by engaging CD28, via the direct pathway of immune stimulation. In this proposal, Aim 1 will characterize the role of different subsets of recipient DC in allo-specific T cell activation. In Aim 2. we will determine outcome of experimental heart allografts in the absence of recipient DC; this will reveal additional aspects of the role of different subsets of recipient DC in heart graft rejection. In Aim 3. we will test the in vivo anti-donor T and B cell responses in heart graft recipient mice that lack DC or that are reconstituted with specific subsets of DC. Transplantation is now firmly established as the therapy of choice for these end-stage diseases. Graft failure as the result of chronic rejection is a major problem that exacerbates the organ shortage problem. These studies are expected to affect patients' health positively as the results may elucidate DC-targeted anti- rejection therapies. We believe the data from the proposal will lead to better understanding of the pathogenesis and therapy of graft chronic rejection. [unreadable] [unreadable]