Organelle-associated innate immune responses to self-DNA in mammals

Abstract

Since early in evolution, mammalian cells have been equipped with pattern recognition receptors (PRRs) that detect the presence of pathogen-associated molecular pattern (PAMPs) or damage-associated molecular pattern (DAMPs) to elicit innate immune responses. Despite recent advances in the understanding of PRR signaling and its regulation, structurally homologous PRRs are often considered to operate similarly, and therefore the functional diversity of PRRs among mammals has not been explored. Also, in contrast to the increasing knowledge about the roles of mitochondria in innate immunity, the roles of peroxisomes, the other dynamic and metabolic organelles, remain elusive. Our first study in this thesis addresses the evolutionary diversity of cyclic GMP-AMP synthase (cGAS) activity. cGAS is the enzyme PRR that detects DNA in the cytosol. As opposed to the assumption that cGAS is similarly regulated in mammals, we identify three distinct classes of regulation of cGAS self-DNA reactivity. Class 1 cGAS, which includes human, contains N-terminal domain which restricts otherwise intrinsically self-DNA-reactive C-terminal catalytic domain. N-terminus of Class 2 cGAS (mouse) rather promotes self-DNA reactivity, and Class 3 cGAS (including chimpanzee) is not reactive to self-DNA. The self-DNA reactivity of Class 1 cGAS is linked to mitochondrial localization, while other cGAS classes do not follow this rule. These findings provide new insights to the field that demand the careful consideration into species-specific functions when studying cGAS and other PRRs. The second study in this thesis examines the roles of peroxisomes in PRR signaling. We found that peroxisomal matrix proteins are generally required for PRR responses in macrophages, including TLRs, RLRs, and NLRP3 inflammasomes. We further discover that pristanic acid, a branched-chain fatty acid that is the substrate for peroxisomal -oxidation, reprograms macrophages from an inflammatory state to an antiviral state through self-DNA-mediated cGAS activation. This cGAS activation by pristanic acid results not only in the induction of type I interferon (IFN) responses but also the proliferation of peroxisomes. Pristanic acid promotes histone deacetylase (HDAC) activity, which is required for cytokine responses. These results altogether demonstrate the important roles of peroxisomes in innate immune responses. Overall, our thesis work identifies organelle-associated cGAS responses to self-DNA from two perspectives: Class 1 cGAS catalytic domain reacts with self-DNA in mitochondria; and an intermediate product of peroxisomal metabolism induces self-DNA-mediated cGAS activation.