Association of IgA-Fc receptors (Fc alpha R) with Fc epsilon RI gamma 2 subunits in U937 cells. Aggregation induces the tyrosine phosphorylation of gamma 2.

Abstract

We investigated the possibility that IgA-binding chains of Fc alpha R on monocytic cells are physically associated with gamma 2 subunits of Fc epsilon RI (Fc epsilon RI gamma 2 or gamma 2). Fc alpha R was precipitated from lysates of IFN gamma-treated U937 cells, subclone 10.6, and probed by immunoblotting with Ab against human gamma 2. Fc alpha R was precipitated through anti-Fc alpha R mAbs A59 or A62, through A62 from lysates that had been exhaustively precleared of high affinity IgG-Fc receptors (Fc gamma RI) and of low affinity Fc gamma RII, and through anti-Fc alpha R mAb A77 from Fc gamma RI-precleared lysates of untreated 10.6 cells. precipitation was also performed through F(ab')2 A77 and through the native ligand of the receptor, hlgA. In all cases, Fc alpha R precipitates contained co-isolated 22-kDa gamma 2 (unreduced). The Fc alpha R alpha-chain/gamma 2 complex did not readily dissociate in 1% Nonidet P-40 as did Fc gamma RI alpha-chain/gamma 2, suggesting a novel aspect to the Fc alpha R subunit interaction. Specific Fc alpha R aggregation on cells triggered a robust respiratory burst and the tyrosine phosphorylation of several proteins. Among them was phospho-gamma 2, which migrated as a 24- to 28-kDa gamma 2 phosphoprotein on gels and was detected as a 28-kDa phosphoprotein by anti-phosphotyrosine immunoblot. Aggregated Fc alpha Rs that were precipitated from Fc alpha R-triggered cells also contained a phosphoprotein of the same mobility and immunoreactivity, as did aggregated Fc gamma RI from which the 28-kDa phosphoprotein could be more readily eluted and identified (as phospho-gamma 2). We concluded that myelocytic Fc alpha Rs are multichain complexes containing gamma 2 subunits that are tyrosine phosphorylated upon Fc alpha R aggregation. As IgA is the predominant Ig on mucosal surfaces, gamma-subunits may play an important role in mucosal immunity involving leukocytic Fc alpha R.