The aim of this study was to determine whether the use of combined immunotherapy with a brief course of humanized anti-CD4Ig and hCTLA4Ig would prolong heterotopic cardiac allograft survival in primates (rhesus monkeys). This model was based on work in “high responder” rats where a brief course of depletive anti-CD4 mAb plus hCTLA4Ig was successful in inducing transplantation tolerance.

Heterotopic cardiac transplants were performed in rhesus recipients. Donor/recipient pairs between groups were confirmed to be reactive prior to transplantation by MLR matching. Humanized anti-CD4Ig, a recently developed anti-CD4 mAb, was given at a dose of 20 mg/kg i.v. on days −3, −2, −1, and 0. hCTLA4Ig was administered at 6 mg/kg/dose i.v. on days 0 and 2 for the first recipient and days 0, 2, 4, and 6 for the second recipient. No further immunosuppression was administered. The treated (n= 2) or untreated (n= 5) recipients were followed for graft function by daily palpitation.

Treatment with anti-CD4Ig plus hCTLA4Ig resulted in a significant prolongation of heart graft survival (42 days for the first recipient and 52 days for the second recipient) compared to untreated recipients (7 days × 4, 11 days × 1). FACS analysis demonstrated CD4 depletion of anti-CD4 treated animals to <2% on posttransplant day 1. The CD4⁺T cells gradually repopulated to 50–70% pretransplant levels just prior to rejection. No adverse responses (fever, tachypnea, tachycardia, infections) were observed.

These are the first results demonstrating that a brief course of combined specific induction immunotherapy with humanized anti-CD4Ig plus hCTLA4Ig, in the absence of adjuvant posttransplant immunosuppression, was well tolerated and resulted in marked prolongation of cardiac allograft survival in primates.