Epigenetic Modulation of Class-switch DNA Recombination to IgA by miR- 146a through Downregulation of Smad2, Smad3 and Smad4

Abstract

Abstract IgA is the predominant isotype at intestinal mucosae, where it plays a critical role in homeostasis and first line of immune protection. Class-switch DNA recombination (CSR) to IgA is directed by TGF-b which induces activation of Smad2 and Smad3 transcription factors. Activated Smad2/Smad3 dimers are recruited with Smad4 to IgH α locus Iα promoter to activate germline Iα-Cα transcription, the first step in CSR to IgA. Epigenetic factors, such as microRNAs, regulate immune cells and, as we have shown, modulate the antibody response in a B cell- intrinsic fashion. We found that miR-146a targets Smad2, Smad3 and Smad4 mRNA 3’UTRs, and keeps CSR to IgA in check in resting B cells. Enforced miR-146a expression in B cells aborted CSR to IgA by decreasing Smad levels. Induction of CSR to IgA, as directed by TGF-b, downregulated B cell miR-146a, thereby reversing the silencing of Smad2, Smad3 and Smad4, which, once expressed, led to recruitment of Smad2, Smad3 and Smad4 to Iα promoter for activation of germline Iα-Cα transcription. miR- 146a−/− mice significantly increased circulating levels of total IgA, but not IgM, IgG or IgE, and heightened the specific IgA antibody response to OVA. In miR-146a−/− mice, this was associated with increased IgA+ B cells in intestinal mucosae, increased amounts of fecal free and bacteria-bound IgA, kidney IgA deposition and IgA nephropathy. The B cell-intrinsic role of miR-146a in repression of Smad2, Smad3 and Smad4 and CSR to IgA was confirmed in miR-146a −/− B cells in vitro and in vivo in mixed bone marrow μMT/miR-146a−/− chimeric mice. Thus, by specific inhibition of Smad2, Smad3 and Smad4 expression, miR-146a plays an important and B cell intrinsic role in modulation of CSR to IgA and the IgA antibody response. Supported by NIH grants AI 079705, AI 105813, AAI 167416 and LRA grant 641363 to PC.