PHOSPHORYLATION OF TYPE-II FC-GAMMA RECEPTOR ON ACTIVATED HUMAN LYMPHOCYTES-B

Activation of resting human B lymphocytes either by cross-linking their membranal lgM or by phorbol esters has been previously demonstrated to modulate the type II receptor for Fc-gamma domains (Fc-gamma-RII): shortly after stimulation a decrease in lgG binding capacity and an enhancement of Fc-gamma-RII expression were observed. These were followed by the release of Fc-gamma-RII fragments from the cell membrane. Since protein phosphorylation is a well-established signal transduction element, we examined whether Fc-gamma-RII 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 lgM (slgM) 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 lgG-coated Sepharose beads or to monoclonal Fc-gamma-RII-specific antibody-coated Affi-Gel 10 beads; thus, it most probably corresponds to the Fc-gamma-RII 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 Fc-gamma-RII. The extent of Fc-gamma-RII 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 lgG-sensitized erythrocytes. These results suggest that stimulation-induced phosphorylation of Fc-gamma-RII on B cells is an early signal transduction element involved in controlling B cell response.