The opportunistic intracellular bacterial pathogen Rhodococcusequi elicits type I interferon by engaging cytosolic DNA sensing in macrophages

Rhodococcusequi is a major cause of foal pneumonia and an opportunistic pathogen in immunocompromised humans. While alveolar macrophages constitute the primary replicative niche for R. equi, little is known about how intracellularR. equi is sensed by macrophages. Here, we discovered that that in addition to previously characterized pro-inflammatory cytokines (e.g., Tnfa, Il6, Il1b), macrophages infected with R. equi induce a robust type I IFN response, including Ifnband interferon-stimulated genes (ISGs), similar to the evolutionarily related pathogen, Mycobacterium tuberculosis. Follow up studies using a combination of mammalian and bacterial genetics demonstrated that induction of this type I IFN expression program is largely dependent on the cGAS/STING/TBK1 axis of the cytosolic DNA sensing pathway, suggesting that R. equi perturbs the phagosomal membrane and causes DNA release into the cytosol following phagocytosis. Consistent with this, we found that a population of ~12% of R. equi phagosomes recruits the galectin-3,-8 and -9 danger receptors. Interestingly, neither phagosomal damage nor induction of type I IFN require the R. equi's virulence-associated plasmid. Importantly, R. equi infection of both mice and foals stimulates ISG expression, in organs (mice) and circulating monocytes (foals). By demonstrating that R. equi activates cytosolic DNA sensing in macrophages and elicits type I IFN responses in animal models, our work provides novel insights into how R. equi engages the innate immune system and furthers our understanding how this zoonotic pathogen causes inflammation and disease. Author summary Rhodococcusequi is a facultative intracellular bacterial pathogen of horses and other domestic animals, as well as an opportunistic pathogen of humans. In human patients, Rhodococcus pneumonia bears some pathological similarities to pulmonary tuberculosis, and poses a risk for misdiagnosis. In horses, R. equi infection has a major detrimental impact on the equine breeding industry due to a lack of an efficacious vaccine and its ubiquitous distribution in soil. Given the prevalence of subclinical infection and high false positive rate in current screening methods, there exists a critical need to identify factors contributing to host susceptibility. Here, we use a combination of bacterial genetics and animal models toinvestigate innate immune responses duringR. equiinfection. We found that R. equi modulates host immune sensing to elicit a type I interferon response in a manner resembling that of M. tuberculosis. We also found that the danger sensors galectin-3, -8, and -9 are recruited to a population of R. equi-containing vacuoles, independent of expression of VapA. Our research identifies innate immune sensing events and immune transcriptional signatures that may lead to biomarkers for clinical disease, more accurate screening methods, and insight into susceptibility to infection.

Automated summary

Rhodococcusequi is a major cause of foal pneumonia and an opportunistic pathogen in immunocompromised humans. Macrophages infected with R. equi induce a robust type I IFN response, similar to Mycobacterium tuberculosis. R. equi activates cytosolic DNA sensing in macrophages and elicits type I IFN responses in animal models.