Effects of IL-2 and/ or Anti-IL6R Therapy on Long-term Cardiac Allograft Survival in Non-Human Primates (NHPs)

Purpose Cardiac allograft tolerance in non-human primates (NHPs) has been achieved by our group using a mixed chimerism model with allogeneic bone marrow transplant after non-myeloablative conditioning but not without kidney co-transplantation. Transplantation of the heart alone results in graft rejection within 150-180 days. Our results suggest that the ability of the kidney to confer tolerance on the co-transplanted heart relates to its ability to induce the expansion/activation of host regulatory T cells (Tregs). We investigated whether IL-2 and/ or anti-IL6R therapy could substitute for the kidney and induce cardiac allograft tolerance by augmenting Tregs. Methods Sixteen cynomolgus NHPs underwent heart and donor bone marrow transplant with mixed chimerism conditioning including total body and thymic irradiation, ATGAM, anti-CD154 mAb, and cyclosporine until post-operative day (POD) 28. Recipients in Group A (n=5) received low dose IL-2 therapy (1M IU/m2 SC, POD -6 to 5) and an anti-IL6R mAb (tocilizumab, 10 mg/kg IV, POD 0, 7, 14, 21, 28, 56, 84, 112). Recipients in Group B (n=11) received tocilizumab alone (10 mg/kg IV, POD 0, 7, 14, 21, 28, 56, 84). Results In Group A, 3/5 recipients rejected at POD 127, 198, and 254 with acute cellular rejection (ACR), antibody mediated rejection (AMR), and donor specific antibodies (DSA). 2/5 died due to sepsis (POD11) or technical complications (POD7). In Group B, 4/11 recipients are ongoing, of which 2 are \u003e430 days post-transplant without ACR. 1/11 rejected at POD 621 but only after donor skin transplant on POD 526. 1/11 failed to develop chimerism and rejected at POD 169. Graft failure/ death occurred in 5/11 due to technical or sedation complications (POD3, POD5 [n=2], POD9) and pancytopenia (POD65). Group A recipients exhibited an average 15-fold expansion in percent CD25+FoxP3+ of peripheral CD3+CD4+ cells compared to an average 3-fold expansion in Group B. Conclusion Despite its ability to promote Tregs, combined IL-2/ aIL6R treatment failed to achieve long-term allograft survival. We surmise that this was due to IL-2 stimulation of alloreactive memory T cells. In contrast, aIL6R mAb treatment alone achieved long-term allograft survival (\u003e1 year) off immunosuppression in 3 recipients and represents a promising strategy towards clinical cardiac allograft tolerance.