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FcγRII expression in resting and activated B lymphocytes

In this report, we analyze the expression of the type II receptor for the Fc region of IgG (FcγRII) in resting and lipopolysaccharide (LPS)‐activated murine B lymphocytes. FcγRII is encoded by two genes, α and β. The β gene encodes two mRNA, β1 and β2, which are generated by alternative splicing. Using an S1 nuclease protection assay, we found that resting and activated B lymphocytes express predominantly the β1 transcript. Very low levels of the β2 mRNA were detected in this assay, while no expression of the α transcript could be detected. Quantitative Northern blot analysis showed that the amount of FcγRII β mRNA was increased 9‐fold in LPS‐activated B lymphocytes. The expression of FcγRII during the various phases of B cell activation was then studied by immunofluorescence using the monoclonal antibody 2.4G2. LPS stimulation induced an increase of the FcγRII cellular pool as well as of its expression at the surface of B lymphocytes. The rise in FcγRII surface expression occurred after the induction of class II antigens (la) and before transferrin receptor induction. FcγRII expression was found to be enhanced during the G1 phase of the cell cycle since (a) only large cells (i.e. those that had entered the G1 phase) expressed an increased amount of FcγRII and (b) blocking the entry of activated cells into the S phase (with the ion channel blocker quinine) did not affect the FcγRII induction by LPS. Furthermore, only B cell activators that induced cells to enter into G1 [LPS and F(ab')2 anti‐IgM antibodies, but not interleukin 4] caused an increase in the expression of FcγRII. These results show that the increase in the membrane expression of FcγRII occurs during the early G1 phase, establishing it as a marker for the entry of B lymphocytes into the cell cycle.