Deletion of galactose-a1,3-galactose (Gal) and/or transgenic expression of human complement regulatory proteins have been shown to prolong the survival of vascularised xenografts, but the combined effect of these modifications on islet xenograft survival is unknown.
Aim: We examined the fate of wild type (WT) and combined effect of removal of Gal and expression of human CD55 and CD59 porcine neonatal islet cell clusters (NICC), and determined the potential of these NICC to avoid hyperacute rejection when transplanted intraportally into baboons.
Method: 1-5 day old wild type (WT) or transgenic (GTKO/CD55-CD59-HT) piglets were used as islet donors. Recipient baboons received by intraportal infusion 10,000 IEQ/kg of either wild type (n=4) or GTKO/CD55-CD59-HT (n=5) NICC. Graft function was evaluated in the immediate transplant period and at one month. Recipients of WT grafts received heparin but no immunosuppression and those with GTKO/CD55-CD59-HT grafts received heparin, ATG induction with tacrolimus and mycophenolate mofetil.
Results: Within the first 12 hours of transplantation, WT NICC showed widespread thrombosis with substantial neutrophil and mononuclear cell infiltrate. There were transient decreases in systemic platelet counts and fibrinogen levels and increased D-dimer levels. In contrast, GTKO/CD55-CD59-HT NICC exhibited no signs of early thrombosis nor infiltrate, and platelet counts, D-dimer and fibrinogen levels were unchanged from baseline within the early transplant period. Histological assessment of GTKO/CD55-CD59-HT NICC grafts at one month revealed T and B cell infiltrates although insulin, glucagon and somatostatin positive cells were still present in all grafts.
Conclusion: Wild type porcine NICCs initiate a rapid innate immune response with immediate thrombosis and rapid destruction of NICC grafts. Removal of Gal and expression of human CD55 and CD59 in the setting of immune suppressive therapy prevents early thrombotic destruction of NICC, but T cell-mediated rejection remains a barrier to long-term islet xenograft survival.