Human IgG xenoreactive antibodies mediate damage to porcine endothelial cells in vitro by both humoral and cellular mechanisms

A major barrier to the transplantation of a porcine organ into a human recipient is the hyperacute rejection response which has been shown to be mediated by xenoreactive antibodies (XAb) and complement proteins. Here, evidence is presented that normal human sera contain IgG XAb that are able to mediate increased adhesion of polymorphonuclear leucocytes (PMN) to porcine aortic endothelial cells (PAEC) in vitro, to initiate damage to endothelial cells via antibody-dependent cellular cytotoxicity mechanisms (ADCC) along with peripheral blood mononuclear cells (PBMC) and to activate the classical complement cascade. PMN exhibit a 2.8-fold increase in adhesion to PAEC from 19.9 +/- 4% to 56 +/- 1.7% at an IgG concentration of 16 mg/ml. This increase is independent of treatment of the PMN with interferon-gamma. PAEC are lysed in the presence of complement: 42.8 +/- 0.7% lysis occurs with a 1/8 dilution of human serum as a source of immunoglobulin of all classes, while 39.3 +/- 3% lysis occurs with purified IgG at 13 mg/ml in the presence of baby rabbit complement. PAEC are also lysed by PBMC in the presence of human IgG XAb, a maximum of 52.0 +/- 5% being observed at effector-to-target (E:T) ratios of 30:1. PBMC bearing Fc gamma RIII receptors for the Fc portion of the IgG molecule mediate the endothelial cell damage since the anti-Fc gamma RIII monoclonal antibody, 3G8, can inhibit lysis by up to 77 +/- 5%. We conclude that human IgG are able to damage porcine endothelial cells using cellular and humoral mechanisms and that IgG XAb can efficiently activate the classical complement cascade in this model system.