Transforming growth factor (TGF)-beta1 directs class switch recombination (CSR) to IgG2b as well as to IgA. Smad3/4, Runx3 and p300 mediate TGF-beta1-induced germ-line (GL) alpha transcription leading to IgA expression. However, the molecular mechanisms by which TGF-beta1 induces IgG2b CSR are unknown. We used luciferase reporter plasmids to investigate how TGF-beta1 regulates the activity of the promoter for GL transcripts of IgG2b constant gene (GLgamma2b promoter). Similarly to the GLalpha promoter, overexpression of Smad3/4 and Runx3 enhances TGF-beta1-induced GLgamma2b promoter activity. Mutation analysis of the promoter identified likely Smad- and Runx3-binding sites. Also similar to the GLalpha promoter, overexpression of p300 enhances Smad3/4-mediated promoter activity, whereas E1A represses promoter activity. Since these regulation mechanisms underlying both GLalpha and GLgamma2b transcription are similar, we explored the possibility that TGF-beta1 induces IgA CSR via transitional IgG2b CSR. TGF-beta1 enhances the expression of both Ialpha-Cmu and Ialpha-Cgamma2b circle transcripts, indicative of direct (Smu-->Salpha) and sequential CSR (Smu-->Sgamma2b-->Salpha).