Absence of hyperacute rejection in pig-to-primate orthotopic pulmonary xenografts

The shortage of organ donors for transplantation is more pronounced for the lung than for any other solid organ. To address this problem, we evaluated the feasibility of pulmonary xenotransplantation. Preliminary investigations demonstrated that orthotopically placed pig lungs in cynomologous monkey recipients could be engrafted up to 9 hr after reperfusion without evidence of hyperacute rejection. In this study, the rejection reaction of pig lungs transplanted orthotopically into baboons (n = 6) was further investigated by ELISA and immunohistochemistry. Four baboon recipients were killed at 24 hr and 2 recipients were killed at 72 hr after transplantation. Pulmonary arterial flow measurements demonstrated flow to the grafts, and systemic arterial and xenograft pulmonary venous blood gas analysis suggested function of the donor lungs during the course of engraftment. Serum levels of baboon anti-pig endothelial cell xenoantibody were normal and decreased minimally over time. Immunohistochemical staining of biopsies demonstrated trace IgG and IgM along graft endothelium 2 hr after reperfusion. At 8 hr, biopsy samples showed no immunoglobulin bound to endothelial cells. Staining for complement was negative. Fibrin and platelets were detected along xenograft endothelium. Despite these findings, the lung xenografts appeared injured and clinically rejected. During the first 8 hr after reperfusion, the grafts were hyperemic and subsequently became focally ecchymotic. Chest x-rays showed progressive pulmonary congestion. These findings suggest that the lung may be relatively resistant to antibody-mediated hyperacute rejection and efforts are being directed toward identifying the mechanism of the observed xenograft lung injury.