ABSTRACT
Transforming growth factor (TGF)-beta has been shown to stimulate isotype switching to IgA in cultures of lipopolysaccharide (LPS)-stimulated B cells. The induction of isotype switching is associated with the appearance of novel germline alpha transcripts that cannot be found in cultures stimulated with LPS alone. TGF-beta also increases the steady state level of productive alpha mRNA. In order to further elucidate both the role of TGF-beta and germline transcripts in isotype switching to IgA in B cells, the mechanism responsible for the changes in alpha mRNA was investigated. The increase in alpha mRNA which does not occur until day 2 of culture continues until at least day 4. Nuclear run-on analysis demonstrated that TGF-beta does not significantly increase the rate of transcription of either germline or productive alpha mRNA after 12, 24, or 48 h of culture. However, by day 2 of culture TGF-beta increases the half-life of alpha mRNA. These findings support the idea that TGF-beta acts as a secondary signal to stimulate isotype switching to IgA in a population that has already received a signal that drives it toward IgA production. In addition these studies suggest that either the germline transcripts or processing of pre-germline alpha mRNA transcripts plays a role in targeting recombination.
