The SH2 domain-containing inositol 5-phosphatase SHIP1 is recruited to the intracytoplasmic domain of human FcγRIIB and is mandatory for negative regulation of B cell activation

Murine FcγRIIB were demonstrated to recruit SH2 domain-containing inositol 5-phosphatases (SHIP1/2), when their ITIM is tyrosyl-phosphorylated upon co-aggregation with BCR, and SHIP1 to account for FcγRIIB-dependent negative regulation of murine B cell activation. Although human FcγRIIB share the same ITIM as murine FcγRIIB and similarly inhibit human B cell activation, which among the four known SH2 domain-containing (tyrosine or inositol) phosphatases is/are recruited by human FcγRIIB is unclear. Our recent finding that, besides the ITIM, a second tyrosine-based motif is mandatory for murine FcγRIIB to recruit SHIP1 challenged the possibility that human FcγRIIB recruit this phosphatase. Human FcγRIIB indeed lack this motif. Using an experimental model which enabled us to compare human FcγRIIB and murine FcγRIIB under strictly controlled conditions, we show that SHIP1 is recruited to the intracytoplasmic domain of human FcγRIIB and inhibits the same biological responses and intracellular signals as when recruited by murine FcγRIIB. Identical results were observed in murine and in human B cells. We demonstrate that SHIP is necessary for human FcγRIIB to inhibit BCR signaling, and cannot be replaced by SHP-1 or SHP-2. Although it contains no tyrosine, the C-terminal segment of human FcγRIIB was as mandatory as the tyrosine-containing C-terminal segment of murine FcγRIIB for SHIP1 to be recruited to the ITIM. This segment, however, did not recruit the adapters Grb2/Grap which were demonstrated to stabilize the recruitment of SHIP1 to the ITIM in murine FcγRIIB.