Pharmacological inhibition of USP18 improves antibacterial responses and the intracellular control of Mycobacterium tuberculosis in macrophages

Introduction Type I interferon (IFN-I) responses correlate with the severity of Tuberculosis. ISG15, a ubiquitin-like protein, regulates immune responses intracellularly by protein posttranslational modification (ISGylation) or extracellularly, in a cytokine-like manner. The ubiquitin specific protease USP18 deconjugates ISG15 from target proteins and is a major negative regulator of IFN-I signaling. Methods Here we show that M. tuberculosis infection induces ISGylation associated transcripts in murine bone marrow-derived macrophages (BMM). The expression of these transcripts was dependent on IFN-I signaling and was further enhanced by supplementation of IFN-β, but not with IFN-γ or IL-1β. While stimulation with IFN-β impaired the intracellular control of M. tuberculosis in BMM, the bacterial growth was not altered by addition of extracellular ISG15. Treatment with two USP18 inhibitors, increased the ISGylation in uninfected and M. tuberculosis -infected BMM. Results The inhibition of USP18 enhanced both IFN-I responses and antimicrobial responses in BMM and increased NF-κB-activation and IL-1β expression. Moreover, the USP18 inhibitors improved the intracellular macrophage control of M. tuberculosis in an IFN-I independent manner. Instead, the addition of IFN-β to the BMM cultures at least partially reversed the bacterial control conferred by the USP18 inhibition. Discussion Our results suggest that the pharmacological inhibition of USP18 can be further explored as a therapeutic tool in the control of tuberculosis.