Analysis of transforming growth factor-β1-induced Ig germ-line γ2b transcription and its implication for IgA isotype switching

Transforming growth factor (TGF)-beta 1 directs class switch recombination (CSR) to IgG2b as well as to IgA. Smad3/4, Runx3 and p300 mediate TGF-beta 1-induced germ-line (GL) alpha transcription leading to IgA expression. However, the molecular mechanisms by which TGF-beta 1 induces IgG2b CSR are unknown. We used luciferase reporter plasmids to investigate how TGF-beta 1 regulates the activity of the promoter for GL transcripts of IgG2b constant gene (GL gamma 2b promoter). Similarly to the GL alpha promoter, overexpression of Smad3/4 and Runx3 enhances TGF-beta 1-induced GL gamma 2b promoter activity. Mutation analysis of the promoter identified likely Smad- and Runx3-binding sites. Also similar to the GL alpha promoter, overexpression of p300 enhances Smad3/4-mediated promoter activity, whereas E1A represses promoter activity. Since these regulation mechanisms underlying both GL alpha and GL gamma 2b transcription are similar, we explored the possibility that TGF-beta 1 induces IgA CSR via transitional IgG2b CSR. TGF-beta 1 enhances the expression of both I alpha-C mu and I alpha-C gamma 2b circle transcripts, indicative of direct (S mu -> S alpha) and sequential CSR (S mu -> S gamma 2b -> S alpha).