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

## TL;DR

Blocking USP18 improves immune responses and helps control tuberculosis bacteria in macrophages.

## Contribution

Pharmacological inhibition of USP18 enhances antibacterial responses in macrophages independently of IFN-I.

## Key findings

- M. tuberculosis infection induces ISGylation transcripts dependent on IFN-I signaling.
- USP18 inhibition increases ISGylation and antimicrobial responses in infected macrophages.
- USP18 inhibition improves intracellular control of M. tuberculosis in an IFN-I-independent manner.

## Abstract

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.

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.

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.

Our results suggest that the pharmacological inhibition of USP18 can be further explored as a therapeutic tool in the control of tuberculosis.