| Dendritic cells (DC) are uniquely well-equipped, professional antigen-presenting cells (APC), with the ability to initiate and regulate immune responses. In transplantation, DC of both donor and host origin contribute to graft rejection by inducing T cell activation and proliferation, via the direct and indirect pathways of allorecognition, respectively. Evidence has also accumulated, however, that DC, particularly in an immature state, can promote tolerance induction and prolong organ allograft survival. Rapamycin is a potent immunosuppressant pro-drug that is well-recognized for its inhibitory effects on T cell proliferation. Despite extensive research on rapamycin's impact on lymphocytes, little is known to date regarding its effects on DC. The central hypothesis in these studies was that, rapamycin interferes with the DC maturation and enhances their tolerogenic potential. We first analyzed the influence of rapamycin, in pharmacologically-relevant concentrations, on the maturation, functional activation and T cell stimulatory potential of murine myeloid DC. Herein we show that rapamycin targets DC antigen (Ag)-uptake and IL-4-mediated maturation both in vitro (in bone marrow-derived DC), and in vivo (in freshly-isolated DC, following in vivo administration of rapamycin). Exposure to rapamycin impairs inflammatory cytokine production and effective T cell stimulation by DC. Furthermore, rapamycin-treated DC induce Ag-specific T cell anergy. Next, we determined that presentation of alloAgs to T cells by rapamycin-pretreated DC of host origin, under in vivo (pre-transplant) steady-state conditions, could induce hyporesponsiveness to subsequent challenge and prolong organ (heart) graft survival. A single infusion of these cells, seven days prior to transplant, led to a significant improvement in transplant outcome in an Ag-specific manner. Furthermore, repeated infusion resulted in marked prolongation of graft survival. These studies demonstrate, for the first time, that the immunosuppressive action of rapamycin can be ascribed, in part, to its inhibitory effects on DC and that rapamycin can potentiate the tolerogenic properties of DC. They also reveal the potential of rapamycin-treated DC as therapeutic vectors of transplant tolerance. |
