FcγRIIB Ile232Thr transmembrane polymorphism associated with human systemic lupus erythematosus decreases affinity to lipid rafts and attenuates inhibitory effects on B cell receptor signaling

The B cell inhibitory receptor Fc gamma RIIB plays crucial roles in the maintenance of self-tolerance. We have identified a polymorphism FCGR2B c.695T > C that results in the non-conservative replacement of 232Ile at the transmembrane helix to Thr and demonstrated the association of the polymorphism with susceptibility to systemic lupus erythematosus (SLE) in Asians. In this study, we examined the impact of FCGR2B c.695T > C on the functional properties of Fc gamma RIIB by expressing each allele product in a human B cell line ST486 lacking endogenous Fc gamma RIIB. Fc gamma RIIB 232Thr was found to be significantly less potent than wild-type 232Ile in inhibiting B cell receptor (BCR)-mediated phosphatidylinositol-3,4,5-trisphosphate accumulation, Akt and PLC gamma 2 activation and calcium mobilization, and to display decreased levels of tyrosine phosphorylation and SH2-containing 5'-inositolphosphate phosphatase recruitment compared with 232Ile after IgG Fc-mediated coligation with BCR. Notably, a quantitative analysis of the subcellular distribution of Fc gamma RIIB using I-125-labeled anti-Fc gamma RIIB revealed that Fc gamma RIIB 232Thr is less effectively distributed to detergent-insoluble lipid rafts than 232Ile, findings in accordance with the importance of the transmembrane amino acid residues, in particular large hydrophobic amino acids including Ile, in the association of membrane proteins with lipid rafts. Given the crucial roles of lipid rafts in integrating BCR signaling, decreased association of Fc gamma RIIB 232Thr could contribute to its impaired inhibitory potential. Collectively, the present findings indicate that the Ile232Thr substitution affects the localization and function of Fc gamma RIIB and that the molecular mechanism may link the polymorphism and susceptibility to SLE.