Regulation of B-cell differentiation: anti-mu antibodies have opposite effects on differentiation stimulated by bacterial lipopolysaccharide and 8-mercaptoguanosine.

The mechanisms by which proliferation and differentiation are independently regulated are among the most interesting and complex problems in cell biology. Polyclonal activation of mouse B cells by bacterial lipopolysaccharide (LPS) has served as a useful model for study of these phenomena. Treatment of LPS-stimulated cells with high concentrations of bivalent antibodies to the IgM receptor uncouples these normally linked processes, enhancing proliferation while suppressing differentiation. A consensus summary of recent results from several laboratories suggests that modulation of the IgM receptor greatly reduces mRNA levels for mu and k chains, primarily by blocking the increased rate of transcription usually triggered by LPS. The expression of other differentiation-linked proteins, for example J chain and endogenous retroviral proteins, is similarly downregulated. Basal transcription of the mu-delta complex and other constitutively expressed genes, such as Class I and Class II MHC genes, is not affected. Both suppression of differentiation and enhancement of proliferation in this system depend upon the simultaneous presence of anti-mu and LPS--cells treated with saturating concentrations of anti-mu, washed, and then cultured in LPS are not suppressed, while cells pulsed briefly with both agents before culture with LPS are suppressed. These observations have led us to examine interactions of anti-mu antibody with another potent polyclonal B cell activator, 8-mercaptoguanosine (8SGuo). In this report, we show that anti-mu antibodies have polar effects on B-cell differentiation induced by 8SGuo and LPS. Differentiation induced by the former is strongly enhanced, while that induced by the latter is suppressed. The signal induced by co-stimulation with LPS and anti-mu is dominant, as suppression occurs when LPS is added to cells stimulated with 8SGuo and anti-mu at initiation or as late as 48 hours of a 96-hour culture. We present preliminary evidence that augmented B-cell differentiation caused by combined stimulation with 8SGuo and anti-mu is dependent upon a soluble factor released during the first 24 hours of culture. These results provide additional evidence that suppression of LPS-driven B-cell differentiation is an active process, probably mediated by a trans-acting repressor of transcription. The mechanisms by which 8SGuo and anti-mu interact to enhance B-cell differentiation remain to be determined.