Both IgG and IgM anti-pig antibodies induce complement activation and cytotoxicity

Hyperacute rejection of pig xenografts transplanted in humans is caused by endothelial cell binding of pre-formed xenoreactive antibodies (XAb) and ac tivation of the classical pathway of complement. Human XAb mainly consist o f anti-Gal alpha1-3Gal antibodies, which occur in IgM, Ige and IgA classes. Whereas IgM anti-Gal alpha1-3Gal antibodies have an established role in hy peracute rejection, the potential role of IgG XAb in this process is still controversial. The aim of the present study was to assess the specificity a nd functional properties of IgG and IgM XAb. Both classes were present in a ll human plasma samples tested, with a high inter-individual variability. L evels of IgG XAb did not correlate with levels of IgM XAb. Binding to Gal a lpha1-3Gal is strongly correlated with binding to the pig cell line PK15, b oth for IgG and for IgM, pointing to Gal alpha1-3Gal as the major antigen r ecognized. Both purified IgM and IgG induced C3 deposition on PK15 cells an d complement-dependent cytotoxicity in a dose-dependent way. The combinatio n of IgG and IgM XAb resulted in an additive effect on cytotoxicity. Affini ty-purified IgG anti-Gal alpha1-3Gal antibodies were 22 times less potent t han IgM in induction of cytotoxicity. These results indicate a quantitative , but not a qualitative, difference between IgM and Ige anti-pig antibodies concerning their complement-activating properties. Therefore, both classes of XAb are of importance in the pathogenesis of hyperacute rejection, and the relative importance of each class may differ considerably between indiv idual patients, depending on the ratio of IgG and IgM XAb present in serum.