FcR γ-chain is essential for both surface expression and function of human FcγRI (CD64) in vivo

Abstract

Most Ig receptors exist as hetero-oligomeric complexes with separate ligand binding (α) and signal transducing (β, γ, or ζ) subunits. For FcγRIIIa and FcεRI, association with the FcR γ-chain is essential for surface expression. However, the human high affinity IgG receptor, hFcγRI, was found to be surface-expressed by itself in transient transfection models. We have now analyzed the integrity of hFcγRI expression in more detail in stable transfectants. In vitro we noted that, in the absence of FcR γ- chain, surface expression of hFcγRI rapidly declined to background levels, in both IIA1.6 B cells and NIH3T3 fibroblasts. The effect of FcR γ-chain on hFcγRI surface expression in vivo was evaluated by using two newly generated transgenic mouse lines, selectively expressing hFcγRI on myeloid cells. These transgenic mice were crossed with FcR γ-chain-deficient mice. Analysis of blood monocytes and peritoneal macrophages showed that surface expression of hFcγRI was reduced by ~80%. The remaining ~20% of receptors were still capable of binding IgG-opsonized RBC, suggesting FcR γ-chain not to be critical for hFcγRI ligand-binding capacity. Importantly, however, hFcγRI signaling capacity was lost in FcR γ-chain-deficient cells. No phagocytosis could be observed using either ligand sensitized (EA-IgG2a) or CD64-targeted erythrocytes (using a bispecific antibody) in both hFcγRI transgenic lines. This documents the FcR γ-chain to be indispensable for both surface membrane expression and function of human FcγRI in vivo.