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Characterization of CD40‐Dependent Immunoglobulin Class Switching

Previous studies have shown that signalling via CD40 together with cytokines stimulates immunoglobulin (Ig) class switching in B cells. This process includes induction of germline (GL) transcripts and switch recombination. Using an agonistic rat anti‐mouse CD40 monoclonal antibody (MoAb), we investigated the role of CD40 signalling in these molecular events. We found that stimulation of murine B cells induced high steady‐state levels of germline γ1, γ2b and low levels of ε transcripts. No detectable γ2a or αRNA were found and the level of γ3 transcripts was high both in stimulated and unstimulated cells. Although cells treated with anti‐CD40 MoAb had high levels of GL γ1 and γ3 transcripts, we failed to detect switching to IgG1 or IgG3. However, anti‐CD40 MoAb‐stimulated cells increased expression of IgG2b. Interestingly, anti‐CD40 plus interleukin (IL)‐5 induced switching to IgG1. Previous work has demonstrated that CD40 signalling, but not lipopolysaccharide (LPS), induces the γ1 promoter and that NF‐κB motifs are important. We show here that both LPS and anti‐CD40 activated NF‐κB proteins binding to the γ1 promoter. The bound NF‐κB complexes were different with regard to total concentration and subunit composition. In the light of our data, the mechanism of CD40‐mediated Ig class switching is discussed.