IL-6 Receptor Blockade but Not IL-2 Treatment Contributes to Long Term NHP Cardiac Allograft Survival in Transient Mixed Chimeras

PURPOSE Long term tolerance of cardiac allografts has been achieved in non-human primates (NHPs) using a mixed chimerism protocol and by co-transplanting a kidney from the same heart and bone marrow donor. In stark contrast, heart alone recipients rejected in 150-180 days. T cell subset analyses showed a greater expansion of regulatory T cells (Treg) in heart/kidney recipients compared to heart alone controls. To achieve tolerance of isolated heart allografts, we added IL-2 and/or anti-IL6R therapy to enhance host Treg activity. METHODS Twelve cynomolgus heart alone transplant recipients underwent donor bone marrow transplantation with mixed chimerism conditioning that included non-myeloablative total body irradiation, thymic irradiation, horse anti-thymocyte globulin, anti-CD154 mAb, and cyclosporine until POD 28. Five animals in Group A received low dose IL-2 (1MIU/m2 SC from POD -6 to 5) and IL-6 receptor blockade with tocilizumab (10 mg/kg IV on POD 0, 7, 14, 21, 28, 56, 84, and 112). Seven recipients in Group B, received tocilizumab alone (10 mg/kg IV on POD 0, 7, 14, 21, 28, 56, and 84). RESULTS In Group A, 3/5 animals rejected their allografts at days 127, 198, and 254 post-transplant with evidence of acute cellular rejection (ACR), antibody mediated rejection (AMR), and donor specific antibodies (DSA). Two animals died on days 7 and 11 post-transplant from sepsis and pancytopenia, respectively. In Group B, 3/7 recipients are ongoing (POD 111 and 62) with one animal over 509 days with no evidence of ACR. 1/7 rejected at day 169 post-transplant with ACR 3R, pAMR3, and DSA and 3/7 animals died on days 5, 9, and 65 post-transplant from a technical failure, sedation complication, and pancytopenia, respectively. Recipients in Group A exhibited an average 15-fold expansion in the percent CD25+FoxP3+ of CD3+CD4+ cells in the periphery compared to an average 3-fold expansion in Group B recipients. CONCLUSION Adding IL-6 receptor blockade to a mixed chimerism protocol has achieved tolerance of an isolated heart allograft for the first time. This protocol or a version thereof has the potential to be trialed in humans. The surprising deleterious effects of low dose IL-2 may be related to its ability to activate alloreactive T cells despite augmenting Tregs.