Association of all three types of Fc gamma R (CD64, CD32, and CD16) with a gamma-chain homodimer in cultured human monocytes.

Abstract

Receptors for the Fc region of IgG (Fc gamma R) on mononuclear phagocytes have been shown to play an important role in the removal of IgG-opsonized particles from the circulation. We found that all three types of Fc gamma R (CD64, CD32, and CD16) in cultured human monocytes are associated with the gamma-chain homodimer that is also present in the high affinity receptor for IgE. Immunoprecipitates of each of these Fc gamma R, prepared from 1% digitonin lysates of cultured human monocytes, incorporated phosphate into a gamma-chain homodimer when incubated with [gamma-32P]ATP. Fc gamma RII immunoprecipitates also incorporated phosphate into Fc gamma RII itself. When human alveolar macrophages were used, similar results were obtained. Although to a minor extent, each anti-Fc gamma R immunoprecipitate from freshly purified monocytes also coprecipitated gamma-chains. These Fc gamma R and gamma-chains did not constitute one large complex, because anti-Fc gamma RI or anti-Fc gamma RIII immunoprecipitates did not coprecipitate Fc gamma RII. In addition, F (ab')2 fragments of anti-Fc gamma R mAb bound to intact cells were recovered in the anti-gamma-chain immunoprecipitates but not in immunoprecipitates made with anti-Fc gamma RIII or anti-Fc gamma RII mAb. When recovery of radioactivity in anti-gamma-chain immunoprecipitates was compared with that in anti-mouse-Ig immunoprecipitates, approximately 25% of the Fc gamma RI and 20% of the Fc gamma RII expressed at the cell surface were associated with gamma-chains. The gamma-chains may play an important role in signal transduction via Fc gamma R in human macrophages.