Autophagy is activated in MΦs by various stress, including infection, inflammation and IFNs. It is typically characterized by LC3 protein lipidation followed by autophagosome formation, which leads to the fusion with lysosomes and protein degradation. Each of the above processes is controlled by autophagy specific genes conserved from yeast to humans. Our initial microarray and ChIP-on chip analyses indicated that some of autophagy related genes are downregulated in Irf8-/- DCs relative to IRF8+/+ DCs. The follow-up study confirmed that IRF8 is required for expression of more than 15 autophagy specific genes in both MΦs and DCs. Some of these genes are induced by IFN-gamma and Toll like receptor ligands, bacterial infection, starvation and by MΦs colony-stimulating factor (M-CSF). IRF8 bound to and activated many genes involved in various steps of autophagy, promoting autophagosome formation and lysosomal fusion. We found that IRF8 binds to the upstream promoter regions of a number of autophagy genes. Consistent with these findings, Irf8 -/- MΦs were defective in LC3 lipidation, autophagosome formation as well as the subsequent fuion with lysosomes and degradation of captured proteins. As a result, Irf8-/- MΦs accumulated SQSTM1 and ubiquitin-bound proteins due to defective autophagy. Transfer of Irf8 cDNA, but no Irf8 mutant cDNA, led to a rescue of some of autophagy gene expression, and increasedn LCII lipidation. We show that clearance of Listeria monocytogenes in MΦs requires IRF8-dependent activation of autophagy genes and subsequent autophagic capturing and degradation of Listeria antigens. These processes were defective in Irf8-/- macrophages where uninhibited bacterial growth ensued. Together, IRF8 is a major autophagy regulator in MΦs, essential for MΦ maturation, survival and innate immune responses. Recent epidemiological studies have reported that IRF8 is a risk factor for certain autoimmune diseases, including multiple sclerosis (MS) and systemic lupus erythematosus (SLE). TGF- signaling is thought to play an important role in the onset of MS, and the disease progression is associated with the presence of proinflammatory cytokines, including IL17 and IFN-gamma. However, etiology of the disease has not been fully understood. To elucidate a molecular basis of the linkage between IRF8 and MS, we investigated a mouse model of MS, experimental allergic encephalomyelitis (EAE). We showed that Irf8-/- mice do not develop EAE following antigen challenge and do not suffer from characteristic limb paralysis. Accordingly, Irf8-/- mice failed to produce MS-signature cytokines IL17 and IFN produced by inflammatory T cells. The subsequent analyses with Irf8 conditional knockout mice that lack IRF8 in macrophages or T cells found that IRF8 acting in MΦs and DCs is primarily responsible for the disease, and IRF8 in T cells is not. It has been recently shown that a specific integrin molecule, v8 expressed on macrophages and DCs is required for activating latent TGFβ that then transmits TGFβ signaling to naive T cells. We demonstrated that IRF8 stimulates vβ8 transcription in MΦs and DCs, and facilitates activation of inflammatory T cells. Additional experiments showed that IRF8 also promotes other inflammatory pathways by stimulating the cytokine, IL23 and activating microglia in the central nervous system. Together, our study provides mechanistic insight on how IRF8 acts as a risk factor for NS and how it exacerbates neuroinflammation.
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