From current immunosuppressive strategies to clinical tolerance of allografts

In order to prevent allograft rejection, most current immunosuppressive drugs nonspecifically target T-cell activation, clonal expansion or differentiation into effector cells. Experimental models have shown that it is possible to exploit the central and peripheral mechanisms that normally maintain immune homeostasis and tolerance to self-antigens, in order to induce tolerance to alloantigens. Central tolerance results from intrathymic deletion of T cells with high avidity for thymically expressed antigens. Peripheral tolerance to nonself-molecules can be achieved by various mechanisms including deletion of activated/effector T cells, anergy induction and active regulation of effector T cells. In this article, we briefly discuss the pathways of allorecognition and their relevance to current immunosuppressive strategies and to the induction of transplantation tolerance (through haematopoietic mixed chimerism, depleting protocols, costimulatory blockade and regulatory T cells). We then review the prospect of clinical applicability of these protocols in solid organ transplantation.