Smad2 and Smad3 Inversely Regulate TGF-β Autoinduction in Clostridium butyricum-Activated Dendritic Cells

Abstract

Colonization with a mixture of Clostridium species has been shown to induce accumulation of induced regulatory T (iTreg) cells in the colon. Transforming growth factor-β (TGF-β) is an essential factor for iTreg cell induction; however, the relationship between Clostridium species and TGF-β remains to be clarified. Here we demonstrated that a gram-positive probiotic bacterial strain, Clostridium butyricum (C. butyricum), promoted iTreg cell generation in the intestine through induction of TGF-β1 from lamina propria dendritic cells (LPDCs). C. butyricum-mediated TGF-β1 induction was mainly Toll-like receptor 2 (TLR2) dependent, and the ERK-AP-1 kinase pathway played an important role. In addition, the autocrine TGF-β-Smad3 transcription factor signal was necessary for robust TGF-β expression in DCs, whereas Smad2 negatively regulated TGF-β expression. Smad2-deficient DCs expressed higher concentrations of TGF-β and were tolerogenic for colitis models. This study reveals a novel mechanism of TGF-β induction by Clostridia through a cooperation between TLR2-AP-1 and TGF-β-Smad signaling pathways.

Regulation of the Immune Response by TGF-β: From Conception to Autoimmunity and Infection

“…In addition, TGFb signaling increases recruitment of the transcription factor PU.1 to the promoter and intronic regions of the Runx3 gene (Chopin et al 2013), which encodes a critical transcription factor in Langerhans cell development (Fainaru et al 2004). It has been proposed that, in the intestinal lamina propria, a combination of TGF-b and bacterial sensing regulates the tolerogenic properties of gut DCs, largely by controlling DC production of TGF-b, which is suggested to direct pTreg-cell generation (Kashiwagi et al 2015). Notably, although autocrine TGF-b signaling confers enhanced TGF-b expression in DCs, the TGF-b-activated Smads appear to play opposing roles in this regulation with Smad3 promoting and Smad2 inhibiting TGF-b production.…”

“…DCs that lack only Smad2 express higher levels of mRNA for TGF-b1 and IL-10, and lower levels of mRNA for inflammatory cytokines, for example, TNF-a, IL-6, and IL-12, and show tolerogenic activity. However, despite the indication that autocrine TGF-b signaling promotes TGF-b production in DCs, whether DCs represent the critical source of TGF-b for pTreg-cell differentiation, as proposed (Kashiwagi et al 2015), remains to be validated by genetic methods, for example, using CD11c-Cre-mediated deletion of floxed Tgfb1. Indeed, it has been shown that T cells themselves are the essential source of TGF-b for Th17 differentiation Gutcher et al 2011).…”

TGF-β in inflammatory bowel disease: a key regulator of immune cells, epithelium, and the intestinal microbiota

“…1a). Although the mechanism underlying modulation of TGF-b production in the human intestine remains to be elucidated, TGF-b production is upregulated by various factors, such as bacteria, viruses, cytokines, apoptotic cells, and the autocrine/paracrine loop [15,16]. A study using laser-captured micro-dissection reported that TGF-b expression was higher in the lamina propria than the epithelium in a healthy human colon [9].…”

“…Clostridium cluster IV and XIVa were reported to be less abundant in IBD patients than in healthy controls [76]. Another study showed that administration of Clostridium butyricum (cluster I) as a probiotic [77] promoted Treg differentiation through TGF-b1 produced by lamina propria DCs in a TGF-b-Smad and TLR-ERK-AP1 pathway-dependent manner [15].…”

Recent advances in understanding contextual TGFβ signaling

“…Previously, TGFβ signaling was shown to prevent inflammation of the bowel upon activation in the associated dendritic cells 44 . New data shed further light on this process by suggesting that Clostridium species within the microbiota induce TGFβ1 expression in colonic dendritic cells in a model of experimental colitis 45 . This in turn stimulates the generation of immunosuppressive regulatory T (Treg) cells, thereby ameliorating the bowel colitis.…”