Biliary Glycoprotein (BGPa, CD66a, CEACAM1) Mediates Inhibitory Signals

Biliary glycoprotein (BGP, CD66a, CEACAM1) is a member of the carcinoembryonic antigen family (CEA, CD66), a group of transmembrane proteins belonging to the immunoglobulin superfamily. The structural features surrounding the tyrosine residues in the cytoplasmic domain of BGP share similarity with the consensus sequence of the immunoreceptor tyrosine-based inhibition motif (ITIM), the docking site for SHIP, SHP-1, and SHP-2 molecules. Using the well-characterized inhibitory receptor, FcgammaRIIB, we constructed a FcgammaRIIB-BGPa chimeric molecule that contained the extracellular and transmembrane domain of FcgammaRIIB and the cytoplasmic tail of BGPa and expressed it in DT40 B cells. Our results showed that FcgammaRIIB-BGPa, just like the unmodified FcgammaRIIB molecule, inhibited calcium influx in activated DT40 B cells. Substitution of tyrosine with phenylalanine (Y459F) in FcgammaRIIB-BGPa completely abrogated its ability to inhibit calcium influx, indicating that the motif surrounding Y459 is ITIM. The presence of ITIM was also supported by showing that the FcgammaRIIB-BGPa-mediated inhibitory effect was reduced in SHP-1and SHP-2 mutant DT40 B cells and further diminished in a SHP-1/-2 double-deficient mutant line. The results suggest that SHP-1 and SHP-2 are required for the FcgammaRIIB-BGPa-mediated inhibitory signals.