Long-Term Survival of Skin Allografts Induced by Donor Splenocytes and Anti-CD154 Antibody in Thymectomized Mice Requires CD4

Treatment of C57BL/6 mice with one transfusion of BALB/c spleen cells and anti-CD154 (anti–CD40-ligand) antibody permits BALB/c islet grafts to survive indefinitely and BALB/c skin grafts to survive for z 50 d without further intervention. The protocol induces long-term allograft survival, but the mechanism is unknown. We now report: ( a ) addition of thymectomy to the protocol permitted skin allografts to survive for . 100 d, suggesting that graft rejection in euthymic mice results from thymic export of alloreactive T cells. ( b ) Clonal deletion is not the mechanism of underlying long-term graft survival, as recipient thymectomized mice were immunocompetent and harbor alloreactive T cells. ( c ) Induction of skin allograft acceptance initially depended on the presence of IFNg , CTLA4, and CD4 1 T cells. Addition of anti-CTLA4 or anti–IFNg mAb to the protocol was associated with prompt graft rejection, whereas anti–IL-4 mAb had no effect. The role of IFNg was confirmed using knockout mice. ( d ) Graft survival was associated with the absence of IFNg in the graft. ( e ) Long-term graft maintenance required the continued presence of CD4 1 T cells. The results suggest that, with modification, our short-term protocol may yield a procedure for the induction of long-term graft survival without prolonged immunosuppression. ( J. Clin. Invest. 1998. 101:2446–2455.)