We have shown previously that cross-linking receptors for the Fc of IgG (Fc gamma R) to membrane Ig will inhibit membrane Ig-dependent mouse B cell activation. In this report we have determined which mouse IgG subclasses will mediate this inhibition by using monoclonal mouse anti-p-azophenylarsonate (anti-ars) antibodies to inhibit B cell activation by arsanilated rabbit Fab'2 anti-mouse IgM (arsFab'2 anti-mu). Cell recovery and Ig secretion after 4 days of culture with soluble helper factors and arsFab'2 anti-mu was inhibited by IgG1, IgG2b, and IgG2a anti-ars, but not IgM or Fab'2 anti-ars. In addition, we determined which of the inhibitory IgG subclasses are blocked by the monoclonal anti-mouse Fc gamma R antibody 2.4G2, which we have previously shown prevents inhibition by rabbit IgG anti-mu. These experiments demonstrated that 2.4G2 Fab could block inhibition mediated by IgG2a as well as that mediated by IgG1 and IgG2b. Because it has been reported that 2.4G2 does not bind the IgG2a receptor on macrophages, these data support our previous experiments, which indicated that this inhibition is not macrophage dependent.

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