Zhou, Y.; Gao, L.; Cho, R. H.; Ly, J.; Wang, H.; Narra, H.; Tsai, K.-H.; Soong, L.; Liang, Y.

Orientia tsutsugamushi (Ot) is an obligately intracellular bacterium that causes scrub typhus, a potentially severe infectious disease characterized by systemic inflammation and multiorgan dysfunction. We recently reported a protective role for IFN-{gamma} signaling in host defense against Ot infection; however, the underlying mechanisms remain obscure. Inducible nitric oxide synthase (iNOS, encoded by Nos2) is a key antimicrobial effector induced downstream of IFN-{gamma} signaling. Here, we used transgenic mouse models to further investigate the biological functions of iNOS. We first revealed the requirement of iNOS for the restriction of Ot growth in cultured bone marrow-derived macrophages. Using an intradermal mouse model, we found that while tissues of Nos2-/- and wild-type mice exhibited comparable bacterial burdens during acute infection phases, Nos2-/- mice developed eschar-like lesions similar to those observed in Ifngr1-/- mice, indicating a critical role for the IFN-{gamma}/iNOS axis in regulating skin pathology in scrub typhus. Notably, Nos2-/- mice displayed impaired bacterial clearance during the recovery phase (day 42), with persistent bacterial burdens in multiple organs accompanied by sustained immune activation and elevated inflammatory responses. Histopathological and biochemical analyses further revealed increased tissue damage and dysregulated physiological homeostasis in the Nos2-/- mice during recovery. Mechanistically, iNOS deficiency resulted in heightened myeloid cell activation and prolonged expression of proinflammatory mediators, suggesting a dual contribution of iNOS in both antimicrobial defense and inflammation resolution. Collectively, these findings provide new insight into IFN-{gamma}-mediated defense mechanisms and imply the distinct roles of iNOS during different stages of scrub typhus.