INTACT PIG PANCREATIC ISLET FUNCTION IN THE PRESENCE OF HUMAN XENOREACTIVE NATURAL ANTIBODY BINDING AND COMPLEMENT ACTIVATION1

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Abstract
The expression of xenogeneic epitopes and the activation of human complement by adult pig islets after prolonged culture have hitherto not been described. Freshly isolated and cultured islets were analyzed by fluorescence-activated cell sorter analysis, fluorescence microscopy, and immunohistology for expression of Gal(α1,3)Gal epitopes, binding of human xenoreactive natural antibodies (XNA), and complement deposition. Freshly isolated and cultured islets showed detectable Gal(α1,3)Gal expression and human XNA binding limited to intraislet capillary endothelial cells. No significant modification in Gal(α1,3)Gal expression and human XNA binding levels was detected in adult pig islets cultured for up to 4 days compared with freshly isolated islets. Incubation of pig islets with human serum demonstrated the deposition of C3, C4, and membrane attack complex, but not factor B with a similar pattern to XNA. However C3 and C4 showed a more widespread deposition. Despite complement activation, no cytotoxic effect on islets was detected after 4 hr of incubation with human serum capable of killing porcine endothelial cells. Even after 4 days of culture in 50% intact human serum, pig islets retained both their normal morphology and a normal insulin response to glucose stimulation. Neither islet cell lysis nor, more importantly, any alteration in β cell function occurred, which suggests that adult pig islets may not be directly damaged by serum after xenotransplantation in humans. Nevertheless, complement activation in vivo could trigger rapid cellular rejection mechanisms through islet cell opsonization and release of bioactive fragments.