Humoral human xenoreactivity against isolated pig pancreatic islets

It is widely believed that the hyperacute rejection of vascularized xenogra fts in the pig-to-human combination is triggered by the binding of human pr eformed natural antibodies (PNAbs) to the Gal alpha.(1,3)Gal epitope in pig endothelium and subsequent activation of complement. However, remains poor ly defined whether xenogeneic pig pancreatic islets are damaged by antibody and complement-mediated mechanisms. We examined the expression of Gal alph a(1,3)Gal on isolated adult pig islets and the presence of PNAbs in normal human sera directed against islets, using immunofluorescence staining and c onfocal laser scanning microscopy. The pig islets were not stained with Gal alpha(1,3)Gal-specific lectin GSIB-4; however, the exocrine cells reacted strongly with GSIB4, indicating that the Gal alpha(1,3)Gal epitope was high ly expressed on exocrine cells, but not on islets. Human sera showed weak r eactivity of IgM and IgG class PNAbs to the islets, but strong reactivity t o the exocrine cells. Furthermore. we investigated the cytotoxic effect of human serum on pig islets using an in vitro model of pig-to-human islet tra nsplantation. The incubation of pig islets with normal human sera for 45 mi n resulted in less than 10% specific lysis despite the binding of PNAbs, wh ereas exposure of porcine aortic endothelial cells to the same human sera c aused 56% complement-mediated lysis, determined using a MTT cytotoxic assay . These results support the view that pig islets might not undergo early an tibody and complement-mediated rejection in humans.