The pre-B cell receptor signaling for apoptosis is negatively regulated by FcγRIIB

Many studies have shown that FcγRIIB is a negative regulator of B cell receptor signaling, and even though FcγRIIB is expressed through all developmental stages of the B cell lineage, its involvement in pre-B cell receptor (pre-BCR) signaling has not been examined. To investigate FcγRIIB function at the pre-B cell stage, we have established pre-BCR positive pre-B cell lines from normal mice and FcγRIIB-deficient mice, named PreBR and Fcγ^-/-PreBR, respectively. These cell lines are able to differentiate into immature B cells in vitro by removal of IL-7. In PreBR, apoptosis was moderately induced by F(ab')₂ anti-μ Ab, but not by intact anti-μ Ab. Phosphorylation of SH2-containing inositol 5-phosphatase (SHIP) and Dok, which are involved in FcγRIIB signaling, was induced by anti-μ cross-linking in PreBR. In contrast, apoptosis was strongly induced by both the F(ab')₂ and intact anti-μ Abs in Fcγ^-/-PreBR, and the level of phosphorylation of SHIP or Dok was much lower in Fcγ^-/-PreBR than those observed in PreBR. Restoration of FcγRIIB to Fcγ^-/-PreBR followed by anti-μ cross-linking blocked severe apoptosis, and up-regulated SHIP and Dok phosphorylation. The results demonstrate that FcγRIIB negatively regulates pre-BCR-mediated signaling for apoptosis.