Function of human Fc gamma RIIA and Fc gamma RIIIB.

Fc gamma RIIA (CD32), a conventional type I transmembrane protein, and Fc gamma RIIIB (CD16B), which has a glycan phosphatidylinositol (GPI) membrane anchor, are both expressed on human neutrophils. Although some details remain to be elucidated, signaling following crosslinking of Fc gamma RIIA requires the activation of tyrosine kinases of both Src-family kinases and Syk, resulting in tyrosine phosphorylation of Shc, phospholipase C gamma isozymes, and a [Ca2+]i transient. Ligation of neutrophil Fc gamma RIIIB triggers a [Ca2+]i transient, and degranulation, although probably not ADCC or an oxidative burst. However, the mechanism for signal transduction by Fc gamma RIIIB, which lacks a transmembrane domain, is not known. Fc gamma RIIA and Fc gamma RIIIB appear to synergize with each other, leading to suggestions that the GPI-anchored Fc gamma RIIIB utilizes the Fc gamma RIIA signaling apparatus. The relevance of proposed specialized membrane domains enriched in GPI-anchored proteins, sphingomyelin and glycolipids to the signaling properties of Fc gamma RIIIB likewise remains to be explored.