The alternatively spliced CD64 transcript FcgammaRIb2 does not specify a surface-expressed isoform

Three highly homologous genes (A, B and C) and six transcripts have been identified for the class I human IgG receptor (CD64). The hFcgammaRIa1 isoform encodes the prototypic high-affinity receptor for IgG. The alternatively spliced hFcgammaRIb2 transcript was postulated to exist as a second surface-expressed CD64 isoform on myeloid cells. In this report we assessed this proposed role for hFcgammaRIb2 in detail. As CD64 monoclonal antibodies might not recognize hFcgammaRIb2, we tagged the receptor with an hemagglutinin tag and transfected hFcgammaRIb2tag in the presence of FcR gamma-chain into IIA1.6 cells. Both transcript and protein of hFcgammaRIb2tag were clearly present in transfectants. However, in contrast to the (control) hFcgammaRIa1tag, no surface expression of hFcgammaRIb2tag was detectable with a tag-specific monoclonal antibody. Confocal scan laser microscopy revealed hFcgammaRIb2tag to be retained in the endoplasmic reticulum, resulting in absent plasma membrane expression. These results show hFcgammaRIb2 neither to be surface expressed, nor to represent a separate CD64 isoform. This finding, furthermore, implicates that other FcR transcripts defined at the mRNA level may not represent true FcR isoforms either.