The presence of two Fc receptors on mouse macrophages: evidence from a variant cell line and differential trypsin sensitivity.

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Abstract
A stable variant of a clone of the P388D1 macrophage [tumor cell] line was isolated using 4 cycles of treatment with mouse Ig[immunoglobulin]G2a-rabbit anti-.kappa. complexes and rabbit complement. The variant had the same Ka [intrinsic association constant] and about the same number of sites per cell for IgG2a as as the parent line. The variant had 10% as many binding sites for rabbit IgG in soluble antigen-antibody complexes, and the affinity of binding was 3-fold higher. This change in binding of complexes to cells of a cloned line without alteration of IgG2a binding provides evidence for the presence of 2 distinct Fc receptors. The 2 receptors could also be distinguished on the P388D1 line and on thioglycollate-induced mouse peritoneal macrophages by differential sensitivity to trypsinization. The receptor that binds monomeric IgG2a, sheep erythrocytes (SRBC) convalently bound with IgG2a or rabbit IgG using glutaraldehyde, and Sephadex beads coupled with IgG2a or rabbit IgG using CNBr activation, is sensitive to trypsinization. The receptor that binds soluble rabbit antibody-antigen complexes, trinitrophenyl-SRBC and dinitrophenyl (DNP)-bovine serum albumin Sephadex beads coated with rabbit anti-DNP IgG is trypsin resistant. The observation that uncomplexed rabbit IgG does not bind to the trypsin-resistant receptor, whereas the same IgG bound to its antigen does, suggests that conformational changes induced by the binding of ligand may be of consequence in macrophage function.