Phosphorylation of type II Fcγ receptor on activated human B lymphocytes

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Abstract
Activation of resting human B lymphocytes either by cross-linking their membranal IgM or by phorbol esters has been previously demonstrated to modulate the type II receptor for Fcγ domains (FcγRII) shortly after stimulation a decrease in IgG binding capacity and an enhancement of FcγRII expression were observed. These were followed by the release of FcγRII fragments from the cell membrane. Since protein phosphorylation is a well-established signal transduction element, we examined whether FcγRII may be a target of such activation induced phosphorylation. Resting (high density) and activated (low density) human tonsil B lymphocytes were stimulated either by cross-linking their surface IgM (sIgM) or by the phorbol ester TPA. This treatment induced specific phosphorylation of a 36 kd membrane protein. This polypeptide was shown to specifically bind to IgG-coated Sepharose beads or to monoclonal FcγRII-antibody-coated Affi-Gel 10 beads; thus, it most probably corresponds to the FcγRII of these cells. In addition, phosphorylation of a 20 kd protein with similar binding characteristics was also observed in several experiments. Both serine and tyrosine were the amino acids that underwent phosphorylation in the 36 kd FcγRII. The extent of FcγRII phosphorylation correlated with the increase in receptor expression as monitored by specific mAb binding and, at the same time, with the decrease in the capacity to bind IgG-sensitized erythrocytes. These results suggest that stimulation-induced phosphorylation of FcγRII on B cells is an early signal transduction element involved in controlling B cell response.

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