Activin A activation of Smad3 confers a natural brake to innate inflammation

Phosphorylation of Smad3 is a critical mediator of TGF-b signaling, which plays an important role in regulating innate immune responses. However, whether Smad3 activation can be regulated in innate immune cells in TGF-ß-independent contexts remains unknown. Here, we show that Smad3 is activated through the phosphorylation of its C-terminal residues (pSmad3C) in murine and human macrophages in response to bacterial and viral antigens, which is mediated by Activin A in a TGF-b independent manner. Specifically, infectious ligands, such as LPS, induced secretion of Activin A through the transcription factor STAT5 in macrophages, and Activin A signaling in turn activated pSmad3C. This Activin A-Smad3 axis controlled the mitochondrial ATP production and ATP conversion into adenosine by CD73 in macrophages, enforcing an anti-inflammatory mechanism. Consequently, mice with a deletion of Activin A receptor 1b specifically in macrophages (Acvr1bf/f-Lyz2cre) succumbed more to sepsis due to uncontrolled inflammation and exhibited exacerbated skin disease in a mouse model of imiquimod-induced psoriasis. Thus, we have revealed a previously unrecognized natural brake to inflammation in macrophages that occurs through the activation of Smad3 in an Activin A-dependent manner. Overall design: Comparative gene expression profiling analysis of RNA-seq data for peritoneal elicited macrophages cultured in vitro with or without LPS and isolated from WT or Smad3 KO mice

Smad3的磷酸化是转化生长因子β（Transforming Growth Factor-β, TGF-β）信号通路的关键介导环节，该通路在调控固有免疫应答中发挥重要作用。然而，在不依赖TGF-β的情境下，固有免疫细胞中的Smad3激活是否可被调控仍未明确。本研究发现，在小鼠与人类巨噬细胞中，Smad3可在细菌及病毒抗原刺激下，通过其C端残基的磷酸化（pSmad3C）被激活，该过程由活化素A（Activin A）以不依赖TGF-β的方式介导。具体而言，脂多糖（Lipopolysaccharide, LPS）这类感染性配体，可通过巨噬细胞内的转录因子信号转导与转录激活因子5（Signal Transducer and Activator of Transcription 5, STAT5）诱导活化素A的分泌，而活化素A信号通路继而激活pSmad3C。这一活化素A-Smad3信号轴可调控巨噬细胞的线粒体ATP生成，以及CD73介导的ATP向腺苷的转化，从而强化抗炎机制。因此，在巨噬细胞中特异性敲除活化素A受体1b（Acvr1bf/f-Lyz2cre）的小鼠，会因炎症失控更易罹患败血症，且在咪喹莫特诱导的银屑病小鼠模型中表现出更严重的皮肤病变。综上，本研究揭示了一条此前未被发现的巨噬细胞抗炎自然制动通路，该通路通过活化素A依赖的Smad3激活途径实现。整体实验设计：对体外培养的、经或未经脂多糖处理的腹腔诱导巨噬细胞，以及从野生型（Wild Type, WT）或Smad3基因敲除（Knockout, KO）小鼠中分离的巨噬细胞的RNA测序（RNA Sequencing, RNA-seq）数据开展比较基因表达谱分析。