IL induces STAT activation in myeloid

IL-10 induces STAT3 activation in myeloid serine protease resulting in the regulation of proinflammatory gene expression in these cells, and myeloid cell specific deletion of STAT3 develop colitis in a phenomena similar to IL-10 deficient mice (Kobayashi et al., 2003), suggesting that STAT3 is key immediate of IL-10 signaling cascade. Interestingly, colitis developed in myeloid-specific STAT3 KO mice can be prevented by TLR4 deficiency (Kobayashi et al., 2003). In addition, MyD88 deficiency completely rescues colitis in IL-10 deficient mice (Rakoff-Nahoum et al., 2006). Recently, Hoshi et al. reported that MyD88 signaling in colonic mononuclear phagocytes drives colitis development in IL-10 deficient mice (Hoshi et al., 2012). They demonstrated that Villin-MyD88/IL-10 KO mice developed colitis comparable to IL-10 deficient mice. However, CD11c-MyD88/IL-10 or LysM-MyD88/IL-10 KO mice showed minimal signs of intestinal inflammation. Furthermore, recent reports suggest IL-10-IL-10R signaling cascade restricted to macrophage is important for colitis development (Shouval et al., 2014; Zigmond et al., 2014). Taken together, these observations suggest that IL-10 induced by TLR-MyD88 signaling pathways is essential for the control of colitis development. It is well-known that IL-10 can suppress proinflammatory cytokine gene expressions from different cell types including T cells and macrophages; however, the molecular mechanisms for the inhibitory effects of IL-10 have not been fully defined. Previously, Zhou et al. reported that IL-10 abolished recruitment of RNA polymerase II to the IL-12/23 p40 promoter in LPS-stimulated macrophages resulted in the suppressing of IL-12/23 p40 gene transcription (Zhou et al., 2004). Driessler et al. demonstrated that IL-10 selectively induced nuclear translocation and DNA-binding of the repressive NF-κB p50/p50 homodimer is an important mechanism for IL-10 to repress inflammatory gene transcription (Driessler et al., 2004). Kabayashi et al. found that acetylated histone H4 transiently associated with the IL-12/23 p40 promoter in WT macrophages, whereas association of these factors was prolonged in IL-10 deficient macrophages (Kobayashi et al., 2012). In addition, experiments using histone deacetylase inhibitors and HDAC3 short hairpin RNA indicates that HDACs are involved in histone deacetylation of the IL-12/23 p40 promoter by IL-10(Kobayashi et al., 2012). They suggest that histone acetylation on IL-12/23 p40 promoter by HDAC3 mediates homeostatic effects of IL-10 in macrophages. Taken together, these observations indicate that IL-10 may act through diverse mechanisms in the suppression of proinflammatory gene expressions. Recently, it has been demonstrated that IL-10 can regulate miR expressions resulting in the anti-inflammatory functions. McCoy et al. have shown that IL-10 suppressed the expression of miR-155 induced by TLR activation, enhancing the expression of the miR-155 target gene SH2 domain-containing inositol-5-phosphatase 1 (SHIP1) (McCoy et al., 2010). In addition, Curtale et al. have demonstrated that LPS induces expression of miR-146b via an IL-10-dependent and miR-146b modulated the TLR4 signaling pathway by direct targeting of multiple elements, including TLR4, MyD88, IRAK1, and TRAF6 (Curtale et al., 2013). The enforced expression of miR-146b in human monocytes leads to a significant reduction in the LPS-dependent production of proinflammatory cytokines and chemokines (Curtale et al., 2013). In the present study, we found that miR-146b was induced in murine M1 macrophages after stimulation with LPS and miR-146b expression was impaired in IL-10 deficient cells. Importantly, we demonstrated that enforced expression of miR-146b significantly suppressed M1 macrophage differentiation by suppressing M1 macrophage signature gene expression. In addition, we identified that the miR-146b targets IRF5, a key transcription factor for M1 macrophage differentiation. Furthermore, miR-146b deficient mice exhibit enhanced M1 macrophage polarization. Thus it may be concluded that IL-10 induces the expression of miR-146b, which targets IRF5 in the modulation of M1 macrophage differentiation.