Identification of a unique porcine Fc gamma RIIIA alpha molecular complex.

Receptors for the Fc portion of IgG (Fc gamma R) evoke a variety of immune responses upon attachment of the immune complex to this cell surface molecule. Enhancement of tumor cell cytotoxicity is one essential response mediated by Fc gamma Rs. We are studying two mAbs, G7 and PNK-E, which bind to a cytolytic triggering molecular complex on porcine NK cells and phagocytes and function in the enhancement and induction of tumor cell cytotoxicity, respectively. Through biochemical characterization and molecular cloning, we have identified the G7 molecule as the porcine homologue of human Fc gamma RIIIA alpha. The G7 and PNK-E molecules have previously been shown to exist as a complex on the cell surface, suggesting that PNK-E represents an Fc gamma RIIIA alpha-associated molecule in the porcine system. We used the mild detergent Brij 96 to identify associated molecules such as the gamma and zeta subunits. G7 and PNK-E mAbs immunoprecipitate the identical multimolecular complex which includes the gamma subunit on porcine PMN. In addition to the G7 molecule at 40 kDa and the gamma subunit at 7 kDa, Brij 96 immunoprecipitation with G7 mAb, PNK-E mAb, or anti-gamma Ab shows association of unidentified molecules of approximately 15, 20, and 25 kDa on reducing SDS-PAGE, none of which appear to be the zeta subunit. Large-scale immunoprecipitation of the G7 complex from porcine PMN will be performed to isolate sufficient quantities of these unidentified proteins to obtain amino acid sequences. Since these molecules seem to be distinct from Fc gamma R-associated subunits described to date, these studies may lead to the identification of unique Fc gamma R-associated molecules and suggest the presence of a novel Fc gamma RIIIA alpha molecular complex in the porcine system.