Maintenance triple immunosuppression with cyclosporin A, mycophenolate sodium and steroids allows prolonged survival of primate recipients of hDAF porcine renal xenografts

To date, the best results in life-supporting pig-to-primate renal xenotransplantation have been obtained in recipients exposed to long-term immunosuppression with cyclophosphamide. As this agent is frequently associated with side-effects, we have explored the potential of a mycophenolate sodium-based maintenance immunosuppression in this model. Human decay-accelerating factor (hDAF) transgenic kidneys were transplanted into splenectomized and bilaterally nephrectomized cynomolgus monkeys immunosuppressed with mycophenolate sodium, cyclosporin A and steroids, and exposed to a brief induction course with cyclophosphamide (up to four doses). After transplantation, the primates were monitored daily for biochemical and haematological evaluations and for the measurements of haemolytic anti-pig antibodies (APA). A detailed histological analysis of each explanted graft was also performed. All the animals showed very poor initial graft function but survived for up to 51 days. In contrast to our previous studies in xenograft recipients on long-term immunosuppression with cyclophosphamide, minimal or no circulating xeno-directed antibodies, as measured by the evaluation of APA titres, were detected in this series although some degree of acute humoral rejection was observed in all the explanted grafts and was the primary cause of graft failure. Furthermore, in addition to areas of humorally mediated graft damage, we have observed for the first time areas with exclusive and prominent infiltration by CD2+ and CD8+ mononuclear cells presenting patterns compatible with tubulitis, glomerulitis and arteritis, which we have called acute cellular xenograft rejection (ACXR). In addition, CD68+ infiltrating macrophages and CD20+ B-cells were also present. This study demonstrates that a triple maintenance immunosuppression with mycophenolate sodium, cyclosporin A and steroids is a viable alternative to a cyclophosphamide-based immunosuppression to obtain prolonged survival of porcine organs transplanted into primates. However, a more stringent control of antibody forming cells remains essential to further extend the survival of xenografts in this model. In addition, the use of the immunosuppressive regimen reported here in the primate is associated with the occurrence of a new category of cell-mediated xenograft injury (ACXR) whose significance has yet to be clarified.