Isolation of separate Fc receptors for IgG complexed to antigen and native IgG from a murine leukaemia

Abstract

RECENT studies have suggested that membrane receptors for the Fc region of IgG (Fc receptors) may be heterogeneous structures with distinctive binding properties^1–5. Walker has reported that a single cell line may possess separate Fc receptors for IgG molecules of different subclass¹, and Heusser et al. have demonstrated that macrophage-like cell lines may possess two distinct receptors: one which binds monomeric IgG2a, and another which binds to aggregates of all subclasses². Unkeless recently reported that a variant clone of the P388D1 mouse macrophage line had the same number of binding sites for IgG2a as the parent line, but only 10% of the binding sites for rabbit IgG in the form of soluble antigen–antibody complexes, suggesting the presence of two distinct Fc receptors³. The molecular identity of Fc receptors remains uncertain since few structural studies defining these membrane components are currently available^6–8. In this study we demonstrate that several glycoproteins with Fc binding activity can be isolated from the plasma membranes of L1210 cells, and provide structural evidence that separate receptors may be responsible for binding IgG which has been complexed with antigen, and monomeric or aggregated IgG.