MiR-146a modulates IgA class-switch DNA recombination by targeting Smad2, Smad3 and Smad4 genes

Abstract

Abstract IgA is the predominant antibody isotype at mucosal surfaces. It plays a critical role in mucosal homeostasis and provides a first line of immune protection. Class-switch DNA recombination (CSR) to IgA is a strictly controlled process that involves cytokine induction of transcription factors and expression induced cytidine deaminase (AID). CSR to IgA is initiated by TGF-β activation of Smad2, Smad3, Smad4, and Runx3, which bind to the Iα promotor region to activate germline Iα-Cα transcription. This critically opens up the Igα locus and is greatly enhanced by retinoic acid (RA). As we have suggested, CSR is modulated by epigenetic modifications, including microRNA-mediated gene regulation. miR-146a has been shown to regulate T cell, dendritic cell and macrophage functions in innate and adaptive immune responses. Its role, however, in B cells and in the antibody response, is unknown. Here we showed that, in B cells, miR-146a expression is downregulated by LPS/CD154, TGF-β, IL-4, anti-BCR plus RA, the stimuli that induce a robust CSR to IgA. Deficiency of miR-146a in miR-146a−/− B cells resulted in upregulated Smad2, Smad3 and Smad4 expression, increased recruitment of Smad2, Smad3 and Smad4 to Ia promoter, as well as elevated germline Iα-Cα transcription and CSR to IgA, without a significant alteration in AID expression and CSR to other isotypes, including IgG1. Indeed, as we also showed, miR-146a targets the 3′UTRs of Smad2, Smad3 and Smad4 mRNAs, thereby inhibiting expression of these genes. Thus, by downregulating the level of Smad2, Smad3 and Smad4 in B cells, miR-146a plays an important role in the modulation of the IgA response.