S263 is critical for FcαRI activation by cytokines.

(A) Schematic diagrams of FcαRI and different S263 substitution mutants are shown. (B) The expression of the different receptors was analyzed by flow cytometry. (C) Ba/F3 cells expressing FcαRI mutants were used for IgA-rosette assays. Cells were cytokine starved and subsequently stimulated for 15 minutes with either buffer (■) or IL-3 (gray bars). IgA binding to the cells was measured by the formation of rosettes between cells and IgA-coated beads. Results are expressed as rosette index and as means ± SE (n = 3).