BACKGROUND The rate of success in clinical transplantation of islets of Langerhans has dramatically improved with perspectives of wide-scale applicability for patients with type 1 diabetes. One drawback is the need for lifelong immunosuppression, which is associated with significant side effects. Immunomodulatory strategies devoid of side effects and with tolerogenic potential, such as co-stimulatory blockade, would be a great improvement if successful. In this study, the authors have explored the effect of simultaneous blockade of CD40/CD154 and intercellular adhesion molecule (ICAM)/lymphocyte function-associated antigen (LFA)-1 interactions. METHODS Spontaneously diabetic nonobese diabetic (NOD) mice underwent transplantation with allogeneic (C57BL/6) islets and were treated with anti-CD154 monoclonal antibody (mAb) (500 microg, three doses), anti-LFA-1 mAb (100 microg, three doses), or a combination of both in the early peritransplant period. In another set of experiments, LFA-1 engagement was impaired by transplanting islets isolated from ICAM-1-knockout (KO) mice. RESULTS Untreated animals rejected their grafts within 10 days. LFA-1 blockade alone did not result in improved islet graft survival, whereas CD154 blockade alone increased graft survival to 18 days. Simultaneous blockade of both pathways led to significantly improved islet graft survival to 30 days (ICAM-1-KO islets plus anti-CD154), 35 days (anti-LFA-1 plus anti-CD154), and 44 days (ICAM-1-KO islets plus anti-LFA-1 plus anti-CD154). CONCLUSIONS These data suggest that a synergistic effect for prolonged graft survival can be obtained by simultaneously targeting CD154 and LFA-1 in the challenging model of islet allotransplantation in NOD mice. The observation of similar results with anti-LFA-1 mAb and with ICAM-1-KO grafts suggests a key role of direct antigen presentation for the activation of LFA-1-driven signaling.