CHARACTERIZATION OF HUMAN ANTIPORCINE NATURAL ANTIBODIES RECOVERED FROM EXVIVO PERFUSED HEARTS - PREDOMINANCE OF IGM AND IGG2

Hyperacute rejection is a major obstacle to successful transplantation of vascularized xenogeneic organs and is believed to be mediated at l east in part by preformed xenoreactive ''natural antibodies'' (NAb). I n this study, human NAb that could be involved in hyperacute rejection of pig heart xenografts were identified and characterized using an ex vivo model in which pig hearts were perfused with whole blood from in dividual human or pig donors. This ex vivo perfusion model allows for the continuous monitoring of physiologic parameters of cardiac functio n as well as sequential sampling of tissue and blood. Pig hearts perfu sed with allogeneic pig blood maintained normal function for at least 4 hr, whereas those perfused with xenogeneic AB+ human blood never ach ieved normal function and rejected completely after 30 min. In three s eparate experiments involving different human blood donors and pig hea rts, sequential samples of perfused blood revealed a progressive deple tion of antiporcine NAb. Samples of all three rejected cardiac xenogra fts were homogenized, and the specifically bound human anti-porcine an tibodies were eluted with citric acid. The eluted antibodies were enri ched approximately 50-120-fold for anti-porcine reactivity compared wi th serum from the corresponding donor. Eluates contained NAb of predom inately IgM and IgG2 isotypes. Immunofluorescence histology confirmed the deposition of IgM and IgG2 but not other IgG subclasses in the rej ected pig hearts. Since IgG2 utilized predominantly in response to bac terial polysaccharide antigens, our findings are consistent with the p ossibility that some NAb arise via crossreactivity with microbial anti gens and are predominantly directed against carbohydrate rather than p rotein antigens.