Retinoic Acid Induces the Expression of Germ-Line Cα Transcript Mainly by a TGF-β-Independent Mechanism

The effects of retinoic acid (RA) on expression of germ-line transcripts, IαCα and Iγ1Cγ1, and of IgA and IgG1 mRNAs by murine surface IgM-positive B-cells were examined by reverse transcriptase–polymerase chain reaction (RT-PCR). LPS-stimulated B-cells were cultured for 2–3 days in the presence of IL-4 and IL-5 with or without RA. Total RNA was extracted from the cells, and RT-PCR specific for the germ-line transcripts was carried out. RA strongly induced mature germ-line Cα transcript (IαCα) at concentrations between 10 and 100 nM.On the other hand, RA completely inhibited IL-4-induced Iγ1Cγ1 expression. Significant induction of IαCα was observed even at a low RA concentration (0.2 nM) in the presence of LPS (1.5–5 μg/ml) and without cytokines, and three- to fourfold stimulation of IαCα induction was seen at 5 nM.IαCα expression induced by RA (10 nM) and LPS (1.5 μg/ml) was not significantly affected by addition of anti-TGF-β1 and anti-TGF-β2 neutralizing antibodies, although that induced by TGF-β1 or TGF-β2 was completely inhibited by these antibodies. These results suggest that the major induction pathway of IαCα was not mediated by active TGF-β and that RA at physiological concentrations may be involved in IgA isotype switchingin vivoin a TGF-β-independent manner.