New insights into the interactions between T-cell costimulatory blockade and conventional immunosuppressive drugs.

Objective: To determine the precise in vivo interaction between T-cell costimulatory blockade and conventional immunosuppression in transplantation.

Summary Background Data: Blocking B7 or CD154 T-cell costimulatory activation pathways prevents allograft rejection in small and large animal transplant models and is considered a promising strategy for clinical organ transplantation.

Methods: A fully MHC-mismatched vascularized mouse cardiac allograft model was used to test the interactions between anti-CD154 or CTLA4Ig monotherapy and conventional immunosuppressive drugs in promoting long-term graft acceptance. The frequency of alloreactive T cell was measured by ELISPOT. Chronic rejection was examined by histology.

Results: Cyclosporine, tacrolimus, and anti-IL-2R monoclonal antibody therapy abrogated the effect of a single-dose protocol of anti-CD154 therapy. In contrast, rapamycin acted synergistically with anti-CD154 therapy in promoting long-term allograft survival. The addition of calcineurin inhibitors did not abolish this synergistic effect. Intense CD154-CD40 blockade by a multiple-dose schedule of anti-CD154 resulted in long-term graft survival and profound alloreactive T-cell unresponsiveness and overcame the opposite effects of calcineurin inhibitors. CTLA4Ig induced long-term graft survival, and the effect was not affected by the concomitant use of any immunosuppressive drugs.

Conclusions: The widespread view that calcineurin inhibitors abrogate the effects of T-cell costimulatory blockade should be revisited. Sufficient costimulatory blockade and synergy induced by CD154 blockade and rapamycin promote allograft tolerance and prevent chronic rejection.