Epigenetic regulation of IgA class-switching by miR-146a through selective downregulation of Smad2, Smad3 and Smad4

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 defense in immune protection. Class switch DNA recombination (CSR) to IgA is initiated by TGF-β, which induces Smad2, Smad3, Smad4 and Runx3 binding to the Iα promoter. This promotes germline Iα-Cα transcription, thereby increasing accessibility to the Igα locus. Epigenetic modifications, such as microRNAs, have been shown to modulate the antibody response, although the role of microRNAs in B cell differentiaton processes, such as guiding CSR to specific Ig isotypes, remains unexplored. miR146a has been shown to regulate T cell, dendritic cell and macrophage functions in innate and adaptive immune responses. Here we show that miR146a, which targets Smad2, Smad3 and Smad4 3′UTRs and is the most abundant miRNA in resting B cells, is significantly downregulated in B cells undergoing CSR to IgA. Deletion of miR146a, as in miR146a−/− B cells, frees Smad2, Smad3 and Smad4 from transcriptional repression, thereby increasing recruitment of these transcription factors to the Iα promoter. This results in increased germline Iα-Cα transcription and CSR to IgA without alterations to Aicda and Prdm1 expression. Accordingly, miR146a−/− mice displayed an elevated gut, systemic IgA responses and kidney IgA deposition, a hallmark of hyper IgA nephropathy. This was recapitulated in μMT/miR146a−/− mixed bone marrow chimera mice, further demonstrating the B cell-intrinsic role of miR146a in the modulation of CSR to IgA. Thus, by silencing Smad2, Smad3 and Smad4 transcription, miR-146a plays an important B cell intrinsic role in the modulation of the IgA response.