| The immediate challenge of transplant immunology is to induce donor-specific immune tolerance to improve graft outcome and to eliminate the need for dependency on immunosuppressive therapy. Cell-based therapy employing dendritic cells (DC) for induction of transplant tolerance has been researched intensely due to their critical roles in controlling and regulating immunity. As the most potent antigen (Ag)-presenting cells (APC), DC are well-equipped to capture, process and present Ags, and their unique migratory behavior in vivo is central to the direct and indirect allorecognition pathways of transplantation. This dissertation examined two approaches to exploiting the inherent plasticity of DC with respect to their migration and function to regulate immune responses and enhance allograft survival. First, FTY720 is a novel immunomodulator pro-drug known to cause blood lymphopenia. Its active metabolite, FTY720-phosphate (FTY720-P) is a receptor agonist of sphingosine 1-phosphate (S1P) that regulates cell growth, differentiation, and migration. Our hypothesis was that FTY720 would modulate DC trafficking and function, an effect that could contribute to FTY720-induced immunosuppression. Herein, we show that FTY720 retained DC in the circulation, and concomitantly reduced their number in lymph nodes and spleen, by down-regulating surface intercellular adhesion molecule and homing receptor expression on DC, likely via the S1P 1 signaling pathway. Second, we investigated the immunoregulatory capacity of alternatively-activated (AA) DC and their therapeutic efficacy in a fully MHC-mismatched cardiac allograft model. We hypothesized that AADC, generated in an immunosuppressive milieu and then 'activated' via Toll-like receptor (TLR) ligation, could exhibit tolerogenic properties and offer potential as therapeutic agents to promote long-term vascularized organ allograft survival. We demonstrated that the anti-inflammatory cytokines, IL-10 and TGF-beta, impacted significantly on DC maturation in response to the TLR4 ligand LPS and impaired their ability to induce T cell proliferation. AADC prolonged allograft survival by induction of a tolerogenic cytokine environment and expansion/proliferation of CD4+ regulatory T (Treg) cells, an effect that was markedly potentiated by blockade of the B7-CD28 costimulation pathway. These novel data provide new insights into manipulation of DC migration and function for regulation of alloimmune reactivity and promotion of transplant tolerance by control of S1PR, cytokine, and TLR signaling. |
