Mycoplasma gallisepticum uses itaconate-associated mitochondrial inhibition to suppress host immunometabolism

Many pathogens actively suppress early host immune responses to enhance their fitness. Mitochondria function as key regulators of immune activation, yet whether pathogens suppress host immunity by manipulating mitochondrial metabolism in vivo remains largely unknown. During an innate immune response, the metabolite itaconate increases in abundance and acts as an immunomodulator, due to its inhibition of succinate dehydrogenase (SDH), a key mitochondrial regulator of cellular immunity. We hypothesized that Mycoplasma gallisepticum (MG), a recently emerged pathogen of wild songbirds, most notably house finches (Haemorhous mexicanus), suppresses early host immune responses by limiting SDH-dependent immune activation via itaconate. We tested these hypotheses using experimental 3-day infection of finches with heat-killed MG, live MG or pharmacological elevation of the SDH inhibitor itaconate. Following inoculation, we quantified intracellular itaconate and mitochondrial respiratory function in peripheral blood mononuclear cells (PBMCs) and pro-inflammatory cytokine gene expression in erythrocytes, in addition to infected tissues (trachea and conjunctiva). Heat-killed MG increased SDH-dependent mitochondrial respiration in PBMCs and cytokine gene expression in erythrocytes, but live MG did not show these increases, but revealed increased itaconate accumulation in PBMCs. Dimethyl itaconate administration reproduced the suppressed metabolic and immune phenotype in blood cells observed with live MG, suggesting an itaconate-associated mechanism. In contrast, live MG increased mitochondrial respiration and gene expression levels of cytokines in eyelid conjunctiva, whereas other treatments did not. These findings indicate that MG suppresses host metabolic and cytokine signaling in systemically circulating immune cells through a mechanism consistent with itaconate-mediated inhibition of SDH-dependent mitochondrial respiration, while still inducing an inflammatory response at the site of infection. Our data suggest that MG, like other pathogens, can commandeer host immunometabolic pathways during infection to their benefit and that mitochondria are a key site of competition between host and pathogen.