Cloning and characterization of FcαRb, a novel Fcα receptor (CD89) isoform expressed in eosinophils and neutrophils

The Fc receptor for IgA (FcαR, CD89) is a transmembrane glycoprotein found on monocytes, macrophages, neutrophils, and eosinophils. Here we describe the characterization of a novel isoform of the FcαR cloned from a human eosinophil cDNA library. This clone, FcαRb, lacks the exon encoding the transmembrane/intracellular region of wild type FcαR, which is replaced by 23 new amino acids. Expression of FcαRb mRNA could be detected in eosinophils and neutrophils. IIA1.6 murine pro-B cells transfected with FcαRb cDNA secrete high levels of the protein, but also a substantial amount of FcαRb is expressed at the cell membrane. Membrane-bound FcαRb binds IgA- coated beads equally well as wild type FcαR. Surface expression is not affected by phosphatidyl inositol phospholipase C, indicating that glycosyl phosphatidyl inositol-linkage of FcαRb is not likely. In IIA1.6 cells expressing FcαRb and FcR γ, which is necessary for signal transduction by wild type FcαR, no tyrosine phosphorylation or Ca²⁺-mobilization could be observed after receptor cross-linking. These results indicate that FcαRb is likely to have a different function than wild-type FcαR receptor.