We have demonstrated that one Fc receptor for IgG (FcR) (CD16) on cultured human monocytes appears to be a developmentally regulated membrane protein. This receptor appears to contain less carbohydrate (if any) than does its counterpart on human neutrophils. Expression of CD16 on cultured monocytes increases with respect to both percentage of positive cells and numbers of sites per cell with length of time in culture. This was in contrast to expression of other types of FcRs that either decreased (CDw32) or did not change (FcRp72). Unlike an FcR that binds monomeric IgG (FcRp72), expression of CD16 on monocytes from most normal individuals was not influenced by IFN-gamma. After 14 d in culture, CD16 appeared to be the predominant FcR on cultured monocytes, and was capable of mediating both ligand attachment and phagocytosis. These findings support the hypothesis that CD16 plays an important role in mediating immunophagocytosis.

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