HUMAN-COMPLEMENT ACTIVATION VIA THE ALTERNATIVE PATHWAY ON PORCINE ENDOTHELIUM INITIATED BY IGA ANTIBODIES

The role of the classical pathway (CP) and the alternative pathway (AP ) of complement activation in hyperacute xenograft rejection remains a matter of considerable debate, In addition, it is unknown whether IgG and IgA antibodies activate complement, although these antibodies hav e been found in hyperacutely rejected xenografts. This study was initi ated to assess a possible role of the AP of complement activation in a pig-to-human transplantation model using fresh human sera and isolate d antibodies with cultured porcine endothelial cells (PEC) as targets. IgM, IgG, monomeric IgA, and dimeric IgA (dIgA) antibodies with react ivity toward PEC as determined by ELISA were isolated from pooled norm al human sera. Serum from patients with agammaglobulinemia was used as a source of human complement. C3 and C4 deposition on nonfixed PEC du ring CP (1% serum) or AP activation (10% serum with MgEGTA) was analyz ed using an ELISA. Complement-mediated PEC lysis was tested in a Cr-51 release assay. Using normal human sera as the source of antibodies an d complement, C3 and C4 deposition was already found after 10 min of i ncubation in the CP, whereas an increasing amount of C3 was found in t he AP. During AP activation, no C4 deposition was observed, indicating that CP activation did not contribute to the observed AP mediated C3 deposition, Moreover, dIgA antibodies caused deposition of C3 and not C4, Purified IgM and dIgA antibodies (1 mg/ml) in the presence of 10% agammaglobulinemic serum showed a mean specific PEC lysis of 31% and 2 8%, respectively. Agammaglobulinemic serum alone or with IgG; or monom eric IgA antibodies had no detectable lytic activity. In conclusion, d IgA antibodies might play an additional role in pig-to-human xenograft rejection by activating human complement via the AP.