Identification, In Mouse Macrophages And In Serum, Of A Soluble Receptor For The Fc Portion Of Igg (Fc-Gamma-R) Encoded By An Alternatively Spliced Transcript Of The Fc-Gamma-Rii Gene

Low affinity FcgammaR are a heterogeneous group of glycoproteins which exist in transmembrane (TM) as well as in soluble forms. Two membrane isoforms of the murine type II FcgammaR, FcgammaRIIb1 and FcgammaRIIb2, have been described. They result from the translation of alternatively spliced pre-mRNA, FcgammaRIIb2 lacking sequences of the first intracytoplasmic domain (IC1). Soluble forms of FcgammaR (sFcgammaR) have previously been shown to result from proteolysis of membrane receptors. We report here the identification, in macrophages, of a mRNA derived from the FcgammaRII gene by splicing exons encoding the TM and IC1 domains, i.e. corresponding to a TM-deleted FcgammaRIIb2 mRNA. A soluble protein possibly encoded by this mRNA was identified in macrophage supernatants. In accordance with FcgammaR nomenclature, we propose to name this new FcgammaRII isoform FcgammaRIIb3. It is the most abundant sFcgammaR present in serum, as compared with sFcgammaR resulting from cleavage of membrane FcgammaR.