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) ca n induce activation of porcine endothelial cells independent of comple ment. Therefore we hypothesized that pretreatment of porcine endotheli al cells with anti-pig nAbs may affect the ability of the endothelial cells, when subsequently incubated with a source of nabs and complemen t, to bind antibodies and complement components and to undergo complem ent-mediated cytotoxicity. We preincubated porcine endothelial cells a t 37 degrees C for 1 hr or 40 hr with a source of nAbs. We then incuba ted these pretreated endothelial cells with a complement sourer that c ontained a normal complement level and a low level of IgM nabs for 1 h r to measure bound IgM and IgG and complement components, and for 4 hr to measure cytotoxicity. We found that preincubation for as long as 4 0 hr did not impair the binding of IgM and IgG, implying no antibody-i nduced loss of membrane antigens from the endothelial cells, or the bi nding 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 co uld be induced with purified human IgM but not with purified Ige or Ig M-depleted human serum. The ability of purified IgM to induce resistan ce was abrogated by removal of anti-pig xenoreactive nAbs by absorptio n with pig endothelial cells, and induction of resistance required pro tein synthesis. We conclude that prolonged incubation of human anti-pi g nAbs with pig endothelial cells does not cause loss of endothelial c ell membrane antigens or impairment in binding of nAbs or complement c omponents: instead, it induces marked resistance to complement-mediate d cytotoxicity. These observations may be of value to develop strategi es that enhance survival of a xenograft.