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International audience; Hepatic fibrosis arises from inflammation in the liver initiated by resident macrophage activation and massive leukocyte accumulation. Hepatic macrophages hold a central position in maintaining homeostasis in the liver and in the pathogenesis of acute and chronic liver injury linked to fibrogenesis. Interferon regulatory factor 5 (IRF5) has recently emerged as an important proinflammatory transcription factor involved in macrophage activation under acute and chronic inflammation. Here, we revealed that IRF5 is significantly induced in liver macrophages from human subjects developing liver fibrosis from nonalcoholic fatty liver disease or hepatitis C virus infection. Furthermore, IRF5 expression positively correlated with clinical markers of liver damage, such as plasma transaminase and bilirubin levels. Interestingly, mice lacking IRF5 in myeloid cells (MKO) were protected from hepatic fibrosis induced by metabolic or toxic stresses. Transcriptional reprogramming of macrophages lacking IRF5 was characterized by immunosuppressive and antiapoptotic properties. Consequently, IRF5 MKO mice respond to hepatocellular stress by promoting hepatocyte survival, leading to complete protection from hepatic fibrogenesis. Our findings reveal a regulatory network, governed by IRF5, that mediates hepatocyte death and liver fibrosis in mice and humans. Therefore, modulating IRF5 function may be an attractive approach to experimental therapeutics in fibroinflammatory liver disease.

Interferon Regulatory Factor 5 (IRF5) is a key transcription factor that regulates inflammatory responses by acting as a mediator of macrophage plasticity. IRF5 is expressed at high levels in pro-inflammatory (M1) macrophages where it drives expression of characteristic M1 markers, such as IL-12p40, IL-12p35 and IL-23p19, and inhibits expression of typical markers of anti-inflammatory (M2) macrophages, like IL-10. In this way, IRF5 polarises macrophages towards a pro-inflammatory phenotype. There are two modes of IRF5 activity: 1) direct binding to Interferon Stimulated Response Element (ISRE) sites in the DNA, and 2) indirect recruitment via protein- protein interactions. IRF5 can be indirectly recruited via interactions with the RelA subunit of Nuclear Factor kappa B (NFκB) at multiple inflammatory loci genome- wide, including Tnf. This suggests protein-protein interactions between the two factors are of great importance for driving inflammatory gene expression. My research shows the IRF Association Domain (IAD) of IRF5 and the Dimerisation Domain (DD) of RelA form an interaction interface between the two proteins. I have identified a short peptide, a region of IRF5 IAD sequence, that can bind to RelA, and hypothesise this peptide could block IRF5-RelA interactions, potentially dampening expression of inflammatory mediators co-regulated by these two factors. I have also identified a novel co-factor of IRF5: Krüppel Associated Protein 1 (KAP1) by a proteomic screen consisting of affinity purification coupled to mass spectrometry. The IRF5-KAP1 complex is present in M1 macrophages, and absence of KAP1 in this cell type results in prolonged TNF secretion, suggesting that KAP1 is important for ‘switching off’ Tnf gene expression. Further investigation showed KAP1 is important for regulation of a heterochromatin (H3K9me3) environment downstream of the Tnf locus. By understanding the IRF5 interactome, we hope to unravel and potentially manipulate the determinants of IRF5’s key role in inflammation in a cell-type and activity-specific manner. Open Access