Antibody-dependent cell-mediated cytotoxicity against porcine endothelium induced by a majority of human sera.

Preformed natural antibodies seem to play a predominant role in hyperacute xenograft rejection. The potential of these antibodies in antibody-dependent cell-mediated rejection has not been elucidated, yet. This study was performed to assess a possible role of human antibodies in antibody-dependent cell-mediated cytotoxicity (ADCC) in pig to human transplantation using cultured porcine endothelial cells (PEC). Sera from 100 healthy blood donors were used to determine their ability to lyse PEC in a 51Cr release assay using human PBMC as effector cells. To reveal the underlying mechanism of the observed lysis, binding of human IgM, IgA, and IgG (IgG1, IgG2, IgG3, and IgG4) antibodies and F(ab')2 fragments to PEC was studied. Sixty-four percent of the sera showed a positive ADCC with 12-56% specific lysis of PEC using 1/16 diluted serum. Blocking of FcRIII (CD16) on effector cells did prevent PEC lysis. Most sera contained IgM, IgG, and IgA antibodies to PEC. IgG F(ab')2 fragments isolated from these sera specifically recognized PEC antigens without displaying ADCC activity. With regard to IgG subclass distribution of these antibodies, all sera contained IgG antibodies reacting with PEC, whereas most sera showed binding of IgG1 or IgG3 antibodies to PEC. No binding of IgG4 antibodies to PEC was found. Binding of total IgG, IgG1, IgG2, or IgG3 antibodies to PEC was not directly correlated with serum cytotoxicity. In conclusion, most human sera have the ability to lyse PEC through non-complement-fixing IgG antibodies via binding to FcRIII (CD16) on killer cells. Thus, human IgG antibodies may contribute to porcine xenograft rejection. The in vivo consequences of the predominant recognition of PEC antigens by IgG2 and IgA antibodies remain to be studied.