Mechanisms of donor-specific transfusion synergy with anti-CD154 for transplant acceptance.

Organ transplantation is a life-saving treatment for patients with end-stage organ failure but requires lifelong immunosuppression that can result in significant complications. Achieving stable and durable donor-specific tolerance, whereby immunosuppression can be stopped without precipitating graft rejection, holds the promise to circumvent these problems. While transient inhibition of the CD40/CD154 costimulatory pathway delays transplant rejection in animal models, and antibodies blocking this interaction are currently in clinical trials, the efficacy of anti-CD154 (αCD154) in mouse models is significantly enhanced by the addition of donor splenocyte transfusion (DST). Indeed, αCD154 + DST, but not αCD154 alone, can successfully induce donor-specific transplantation tolerance to fully mismatched cardiac allografts in mice. Why DST needs to be added to αCD154 is not fully understood. By integrating tracking of graft-specific T cells and of donor cells, we show that systemic, but not subcutaneous, injection of DST enables alloantigen dissemination to secondary lymphoid organs beyond those directly draining the transplant. This wider biodistribution results in a greater number of alloreactive T cells interacting with donor alloantigens in all lymphoid organs, such that more alloreactive T cells can be the target of CD154 blockade. Furthermore, the duration of DST persistence, facilitated by the sharing of MHC alleles between the donor and the recipient, emerges as a critical factor in promoting αCD154-mediated graft acceptance. These results provide insights into rational approaches to improve translation of αCD154 in the clinic.