Requirements of a costimulus for IL-4-induced IgE class switching in murine B cells activated via antigen receptors: effectiveness of 8-mercaptoguanosine.

In the present paper, we investigated the requirements for the induction of IgE responses in murine B cells stimulated via surface Ig receptors by trinitrophenyl (TNP)-LPS or F(ab')2 of anti-mouse IgM (anti-mu) Abs. When murine B cells were cultured with 0.2 microgram/ml TNP-LPS, the secretion of anti-TNP IgM, but not that of IgG1 or IgE, was induced. It should be noted that IL-4, alone or in combination with IL-2 or IL-5, was not sufficient for inducing IgE class switching in these B cells, in contrast to the cells activated polyclonally with 20 micrograms/ml LPS, suggesting that an additional signal is required under the former conditions. After extensive screening of costimuli for inducing IgE in cooperation with IL-4, we found that 8-mercaptoguanosine (8-SGuo), a potent B cell activator, was effective. The addition of 8-SGuo to TNP-LPS-stimulated B cell cultures resulted in the secretion of both anti-TNP IgM and IgG1, but not anti-TNP IgE. Anti-TNP IgE formation was induced only when 8-SGuo was added in combination with IL-4, whereas 8-SGuo plus IL-4 failed to induce IgE secretion in the absence of TNP-LPS. Similar requirements for 8-SGuo and IL-4 for IgE formation were observed when B cells were stimulated with anti-mu. It was revealed that induction of the IgE responses was accompanied by increased frequency of surface IgE+ B cells and the expression of germline and productive epsilon transcripts. In B cells stimulated with anti-mu plus IL-4, 8-SGuo was shown to enhance the expression of the B cell-specific activator protein gene that is thought to be required in IL-4-dependent germline epsilon transcription.