Requirement for Runx Proteins in IgA Class Switching Acting Downstream of TGF-β1 and Retinoic Acid Signaling

IgA is a specific isotype required for mucosal immunity and is the most abundant Ab produced in vivo. Recently, several inductive signals for IgA class switch recombination have been identified; however, the molecular details of the action of these signals and the specific factors acting in B cells remain elusive. In this study, we show that combination of retinoic acid (RA) and TGF-β1 with other factors induced a much higher frequency of IgA-switched cells than reported previously. In addition, IgA production is severely impaired in Runx2-Runx3 double-deficient mice. In Runx2-Runx3–deficient B cells, both RA- and TGF-β1–dependent inductions of α germline transcription are completely blocked. These data suggest that Runx proteins play an essential role in IgA class switching acting downstream of RA and TGF-β1 signaling.