Introduction: In preclinical and future clinical cardiac xenotransplantations an effective co-stimulation blockade of the CD40/CD40L pathway is essential. Blockage of CD40 or CD40L receptors has been successful in primates in the past. When blocking CD40, high antibody (ab) dosages were necessary which might not be easily feasible in future patients. Thrombotic side effects were described when first-generation anti-CD40L abs were used. To counteract these negative effects, we administered both antibodies aiming at a final low-dose combination therapy.

Method: We used genetically modified piglets (GGTA1-KO, hCD46/hTBM transgenic) for orthotopic cardiac xenotransplantation in captive-bred baboons (n=6). ATG and Rituximab served as induction therapy. Both anti-CD40L PAS Fab (XL-protein, Germany) and chimeric anti-CD40 Mab were given: anti-CD40L PAS Fab was depreciated within 60 days from 20 to 10 mg/kg BW; anti-CD40 Mab was decreased within 30 days from 50 to 30 mg/kg BW. Steroids and MMF were given additionally.

Results: Two out of six experiments failed early due to technical reasons. Two further transplantations were deliberately terminated when the targeted period of 90 postoperative days was reached with the recipients in excellent clinical condition; heart, liver and renal functions remained within normal ranges throughout the observation times. There were no histologic signs of thrombotic microangiopathy or rejection. The final two experiments were extended beyond 90 days. One had to be terminated due to recalcitrant pleural effusions after 119 days. There were no signs of thrombotic microangiopathy or rejection in post-mortem histology. The sixth animal presented in good clinical condition till day 165. Thereafter, the cardiac function deteriorated rapidly and the animal had to be euthanized four days later. Post-mortem examination revealed full-blown signs of acute humoral rejection.

Conclusion: To our knowledge, combination of anti-CD40 and anti-CD40L antibodies as immunosuppressive regimen in organ transplantation has never been described. Initially, we administered both antibodies at high dosages, which were then reduced after 30 respectively 60 days in order to mimic a more practicable future clinical situation. Three out of four animals survived rejection-free up to 119 days and there were no large-vessel thrombogenic complications. The fourth animal succumbed to acute humoral rejection. Our experience proved that the combination therapy is effective, may be in the future with different antibody combinations and at a higher final dosage.