Mitochondrial Mechanisms Promoting Innate and Intestinal Immunity

Project Summary/Abstract: Inflammatory bowel disease (IBD) is largely characterized by dysregulated cytokines and antimicrobial responses. Innate mechanisms are the initiating drives of host responses to microbes and the resulting cytokine and antimicrobial responses need to be carefully balanced. Mitochondrial pathways play a key role in mediating these innate responses and a dysregulation in mitochondrial mechanisms has been increasingly recognized to play a role in IBD. The focus on the mitochondrial dysregulation in IBD has been predominantly in epithelial cells. However, mitochondria contribute to innate immune outcomes through a variety of mechanisms, including metabolic pathways, reactive oxygen species, communication with the endoplasmic reticulum (ER), and mitochondrial DNA (mtDNA) release. Of the >240 IBD-associated loci a number of genes within these loci modulate host innate responses and mitochondrial function through both direct and indirect mechanisms. As such, upon encounter of human macrophages with microbial products, we have found IBD-associated genes that regulate glycolysis and in turn macrophage polarization, the mitochondrial respiratory chain and mtROS, and ER stress. We further find that upon human macrophage stimulation by a range of pattern recognition receptor (PRR) ligands, release of mtDNA is dramatically increased along with activation of the cGAS- STING pathway. The cGAS-STING pathway then serves to promote responses across the many PRRs. We have preliminary data that at least one IBD- associated gene which partially localizes to the mitochondria, LACC1, modulates PRR-induced activation of the cGAS-STING pathway, and in turn, downstream PRR-initiated downstream outcomes. We hypothesize that the cGAS-STING pathway amplifies responses across a broad range of PRRs through a variety of intracellular mechanisms, that the threshold of this regulation is important in susceptibility to intestinal inflammation and might be therapeutically targeted under conditions of intestinal inflammation, and that IBD- associated geneticvariants regulate these outcomes, thereby influencing key innate immune outcomes. Relevance: These combined human cell and mouse studies will provide insight into mitochondrial mechanisms regulating key outcomes in macrophages, the manner in which these mechanisms are altered in IBD patients and in the context of IBD risk variants, and how these mechanisms might be modulated during intestinal inflammation in order to improve outcomes in vivo. These comprehensive and mechanistic studies will establish a foundation for additional studies to therapeutically target mitochondrialpathways shared across innate immune responses so as to restore innate immune dysregulation.