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 have been found in hyperacutely rejected xenografts. This study was initiated to assess a possible role of the AP of complement activation in a pig-to-human transplantation model using fresh human sera and isolated antibodies with cultured porcine endothelial cells (PEC) as targets. IgM, IgG, monomeric IgA, and dimeric IgA (dIgA) antibodies with reactivity toward PEC as determined by ELISA were isolated from pooled normal human sera. Serum from patients with agammaglobulinemia was used as a source of human complement. C3 and C4 deposition on nonfixed PEC during CP (1% serum) or AP activation (10% serum with MgEGTA) was analyzed using an ELISA. Complement-mediated PEC lysis was tested in a 51Cr release assay. Using normal human sera as the source of antibodies and complement, C3 and C4 deposition was already found after 10 min of incubation in the CP, whereas an increasing amount of C3 was found in the 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 28%, respectively. Agammaglobulinemic serum alone or with IgG or monomeric IgA antibodies had no detectable lytic activity. In conclusion, dIgA antibodies might play an additional role in pig-to-human xenograft rejection by activating human complement via the AP.