Ström L, Laurencikiené J, Miskiniené A, Severinson E. Characterization of CD40-Dependent Immunoglobulin Class Switching. Scand J Immunol 1999;49;523-532 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 ␥1, ␥2b and low levels of ⑀ transcripts. No detectable ␥2a or ␣RNA were found and the level of ␥3 transcripts was high both in stimulated and unstimulated cells. Although cells treated with anti-CD40 MoAb had high levels of GL ␥1 and ␥3 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 ␥1 promoter and that NF-B motifs are important. We show here that both LPS and anti-CD40 activated NF-B proteins binding to the ␥1 promoter. The bound NF-B 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.