Molecular cloning and characterization of a porcine Fc gamma RIIb sub-isoform(FcγRIIb1)

Receptors for the Fc fragment of IgG (FcγRs) constitute one of the main effector mechanisms through which IgG immune complexes exert their action. Four FcγRs, FcγRI (CD64) with high affinity, FcγRI with intermediate affinity, FcγRII (CD32) and FcγRIII (CD16) with low affinity, have been identified. There are three FcγRII isoforms (activating FcγRIIa and FcγRIIc, and inhibiting FcγRIIb) existing in humans, one isoform in mice (inhibiting FcγRIIb), and two isoforms in cattle (inhibiting FcγRIIb, activating FcγRIIc). Two splice sub-isoforms of FcγRIIb, FcγRIIb1(b1) and FcγRIIb2(b2), have been identified in humans, mice and cattle, however, few of FcγRIIb sub-isoforms have been investigated in pig. In this study, we describe the molecular cloning, sequencing and characterization of a porcine FcγRIIb sub-isoform, FcγRIIb1. The cDNA encoding porcine FcγRIIb1 was isolated from peripheral blood leucocytes RNA with RT-PCR. The porcine FcγRIIb1 cDNA contains a 951 bp open-reading frame, encoding a 316 amino acid transmembrane glycoprotein composed of two immunoglobulin (Ig)-like extracellular domains, a transmembrane region and a cytoplasmic tail with an immunoreceptor tyrosine-based inhibiting motif (ITIM). The porcine FcγRIIb1 shares 98.3% homology and has a 19 amino acid in-frame insertion in cytoplasmic tail when compared with amino acid sequence of DQ026064. Immunofluorescence analysis showed that the glycoprotein encoded by the porcine FcγRIIb1 cDNA was expressed in the stable transfected COS-7 cells, and an immunoglobulin-binding assay showed that it had binding activity for IgG immune complexes. Identification of the porcine FcγRIIb1 will help our understanding of the molecular basis of IgG–FcγR interaction in the porcine immune response.