Identification, in mouse macrophages and in serum, of a soluble receptor for the Fc portion of IgG (FcγR) encoded by an alternatively spliced transcript of the FcγRII gene

Low affinity FcγR 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 FcγR, FcγRilb1 and Fcγ;Rilb2, have been described. They result from the translation of alternatively spliced premRNA, FcγRilb2 lacking sequences of the first intracytoplasmic domain (IC1). Soluble forms of FcγR (sFcγR) have previously been shown to result from proteolysis of membrane receptors. We report here the identification, in macrophages, of a mRNA derived from the FCγRll gene by splicing exons encoding the TM and IC1 domains, i.e. corresponding to a TM-deleted FcγRllb2 mRNA. A soluble protein possibly encoded by this mRNA was identified in macrophage supernatants. In accordance with FcγR nomenclature, we propose to name this new FcγRll Isoform FcγRllb3. It is the most abundant 8FcγR present in serum, as compared with 8FcγR resulting from cleavage of membrane FcγR.