A specific Fcγ receptor on cultured rat mesangial cells

Mesangial cells represent specialized pericytes in the renal glomerulus that contribute to the regulation of a variety of glomerular functions. Recently we and others have shown that cultured mesangial cells bind and take up immunocomplexes in an Fc-dependent manner leading in turn to generation of PGE₂, reactive oxygen, and platelet-activating factor. The present studies were designed to further characterize potential Fc-γR on mesangial cells. Binding assays with either monomeric or heat aggregated (HA) [¹²⁵I]labeled rat subclass-specific IgG were performed at 4°C for 2 h on subcultured rat mesangial cells. Monomeric rat IgG2a, IgG2b, IgG₁ and HA IgG2a bound only nonspecifically. Saturable Fc-dependent binding occurred for HA IgG(2b) and HA IgG₁ though maximal binding and affinity were much higher for IgG(2b). The presence of an Fc-γR was confirmed by surface protein iodination of mesangial cells (MC) and immunoprecipitation with either a polyclonal or mAb 2.4G2 prepared against murine Fc-γR. Both antibodies precipitated a 45-kDa iodinated protein band from cultured rat MC that comigrated with that from murine macrophage J774 cells on SDS-PAGE. This protein band also reacted with the polyclonal anti Fc-γR antibody on immunoblots. In contrast rat renal papillary epithelial cells were negative. The 45-kDa protein recognized by the rat anti-Fc-γR antibody 2.4G2 probably represents the binding site for HA IgG(2b), as the 2.4G2 antibody also blocked binding of HA IgG(2b). By immunofluorescence microscopy all MC stained positively with the polyclonal anti-Fc-γR antibody. A cDNA probe for the Fc-γRII-α on murine macrophages hybridized to mRNA from cultured rat MC which was of the same size (though less abundant) as that from J774 macrophages. These results further characterize the Fc-γR on cultured rat MC, and raise the possibility that the mesangial Fc-γR may play a role in the handling of immune-complexes by the renal glomerulus.