Mechanisms of complement C3 induced operational tolerance

Immunosuppressive drugs are needed to prevent rejection after organ transplantation. These drugs non-specifically suppress the whole immune system and predispose the recipients to infection and malignancy. In animal models several protocols have shown their potency to induce immunologic tolerance without maintenance immunosuppression. However, the same strategies have yielded limited success in human studies hitherto. An improved understanding of the mechanisms by which these tolerogenic protocols exert their effect will aid the development of effective clinical protocols. Dr Wong`s laboratory has established a male to female mouse skin transplant model (male antigen HY mismatch) using donor lymphocyte infusion (DLI) to study the role of C3 in tolerance and rejection. They demonstrated that C3 is critical for DLI-induced long-term graft acceptance. However, the exact mechanism for this novel and important finding is still unclear. Since both C3a and C5a receptors (C3aR, C5aR) on antigen presenting cells (APC) have been shown to have profound effects on their ability to present antigen, we propose that tolerance induction in the HY mismatched model requires a tolerogenic subset of APC whose function is dependent on C3aR or C5aR signalling. To test this hypothesis we will analyze if C3aR and/or C5aR deficient mice will show defects in tolerance induction in the HY mismatch model. Since CD4 + T cells exhibit C3b receptors that stimulate the development of inhibitory T cell function, we propose that direct action of complement on recipient T cells favours tolerance induction in this HY-mismatched model. Thus we will investigate if CD55 deficient mice in which excessive activation of complement on the T cell surface leads to anti-donor T cell hyper-reactivity will exhibit reduced threshold for the induction of tolerance in this model. CR2 is the main receptor for C3d, the end metabolite of C3b. Mice deficient in CR2 display reduced antibody generation due to impaired antigen retention in lymph nodes and reduced B cell stimulation. Opsonisation of HY antigen by complement might lead to tagging with C3d, and this may be necessary for efficient antigen handling leading to tolerance induction in the DLI model. To test this hypothesis we will analyze if CR2 deficient mice exhibit defective tolerance induction due to a defect in HY antigen uptake and antigen presentation to T cells. This project shall clarify the mechanisms by which the complement system exerts its function in tolerance induction protocols, thereby, paving the way for its introduction to the clinic.