Fate of antigen in xenotransplantation: implications for acute vascular rejection and accommodation

Antigen down-modulation plays a critical role in xenotransplants involving humoral responses against the Forssman antigen and may play a role in the long-term survival of ABO-incompatible allografts. The present study investigates the fate of porcine antigens in pig-to-primate xenotransplantation. Human antibodies bound to the glycocalyx of cultured porcine aortic endothelial cells as judged by electron microscopy and were shed from the cell surface in a complex with fibronectin, a glycoprotein that is found in the apical membrane glycocalyx of cultured cells. Antibody was shed in a metabolically dependent process with a t(1/2) of 2 to 3 hours. However, the amount of antigen on the cell surface did not change appreciably within 24 hours, suggesting that antigen modulation did not occur. Over the ensuing days, antigen expression decreased, although the change was always less than 50% of baseline. Changes in antigen expression were due for the most part to changes in expression of alpha-galactosyl residues. Consistent with results obtained in vitro, antigen expression in porcine organ transplants remained at approximately the baseline level as determined by immunofluorescence analysis of IgM binding to graft endothelium. If, as these results suggest, antigen is not down-modulated in pig-to-primate xenotransplantation, then therapies aimed at prolonged xenograft survival must focus on antibody or genetic manipulation of antigen expression.