The role of CD154 in organ transplant rejection and acceptance.
CD154 plays a critical role in determining the outcome of a transplanted organ. This simple statement is amply supported by experimental evidence demonstrating that anti-CD154 antibodies are potent inhibitors of allograft rejection in many rigorous transplant models. Unfortunately, despite intensive investigation over the past ten years, the precise mechanisms by which antibodies against CD154 exert their anti-rejection effects have remained less obvious. Though originally classified with reference to B-cell function, CD154-CD40 interactions have also been shown to be important in T cell-antigen-presenting cell interactions. Accordingly, CD154 has been classified as a T-cell co-stimulatory molecule. However, mounting data suggest that treatment with anti-CD154 antibodies does not simply block costimulatory signals, but rather that the antibodies appear to induce signalling in receptor-bearing T cells. Other data suggest that anti-CD154 effects may be mediated by endothelial cells and possibly even platelets. In fact, the current literature suggests that CD154 can either stimulate or attenuate an immune response, depending upon the model system under study. CD154 has secured a fundamental place in transplant biology and general immunology that will no doubt be the source of considerable investigation and therapeutic manipulation in the coming decade.