We recently reported that human NK cells express, in addition to CD16 [Fcγ receptor (FcγR) IIIA], a second type of FcγR, namely CD32 (FcγRII). Molecular characterization of CD32 transcripts expressed by highly purified NK cells revealed that they predominantly express products of the FcγRIIC gene. Using stable Jurkat transfectants we have analyzed the functional properties of two FcγRIIc‐specific isoforms isolated from NK cells, namely FcγRIIc1 and FcγRIIc3, which differ in their cytoplasmic tails. The ligand binding specificity for both murine and human IgG isotypes was found to be similar to that observed for FcγRIIb isoforms. Immunoprecipitation studies of FcγRIIc isoforms expressed in Jurkat cells revealed a protein of around 40 kDa for FcγRIIc1, and a protein of around 32 kDa for FcγRIIc3. Signal transduction studies performed on FcγRIIc1‐expressing Jurkat cells indicated that this molecule is functional, i.  e. capable of Ca2+ mobilization and activation of Lck, Zap‐70 and Syk protein tyrosine kinases, although the CD3 ζ chain was not found to functionally associate with FcγRIIc1. In contrast, FcγRIIc3 transfectants showed an impaired ability of this molecule to mobilize Ca2+, but activation of Lck was detected following activation via FcγRIIc3. These studies demonstrate the functional activity of FcγRIIc isoforms and suggest that the presence of CD32, in addition to CD16, on NK cells may have functional relevance.

Receptors for the Fc region of IgG (Fc + RIIIa, Fc + RIIc) and IgM (Fc ? R) were previously described on NK cells. In this work the expression of Fc receptors for IgA (Fc § R) on human NK cells and the signaling events were investigated. The Fc § R was demonstrated by flow cytometry using secretory IgA (sIgA) and anti-human IgA antibody.

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