Human IgM xenoreactive natural antibodies can induce resistance of porcine endothelial cells to complement-mediated injury

It is thought that human IgM xenoreactive natural antibodies (nAbs) can induce activation of porcine endothelial cells independent of complement. Therefore we hypothesized that pretreatment of porcine endothelial cells with anti-pig nAbs may affect the ability of the endothelial cells, when subsequently incubated with a source of nabs and complement, to bind antibodies and complement components and to undergo complement-mediated cytotoxicity. We preincubated porcine endothelial cells at 37 degrees C for 1 hr or 40 hr with a source of nAbs. We then incubated these pretreated endothelial cells with a complement sourer that contained a normal complement level and a low level of IgM nabs for 1 hr to measure bound IgM and IgG and complement components, and for 4 hr to measure cytotoxicity. We found that preincubation for as long as 40 hr did not impair the binding of IgM and IgG, implying no antibody-induced loss of membrane antigens from the endothelial cells, or the binding of C3bi, C4d, C6, and C9 upon complement activation. In contrast, preincubation for 40 hr with a nAb source induced in the endothelial cells marked resistance to complement-mediated killing. Resistance could be induced with purified human IgM but not with purified Ige or IgM-depleted human serum. The ability of purified IgM to induce resistance was abrogated by removal of anti-pig xenoreactive nAbs by absorption with pig endothelial cells, and induction of resistance required protein synthesis. We conclude that prolonged incubation of human anti-pig nAbs with pig endothelial cells does not cause loss of endothelial cell membrane antigens or impairment in binding of nAbs or complement components: instead, it induces marked resistance to complement-mediated cytotoxicity. These observations may be of value to develop strategies that enhance survival of a xenograft.