# Mycobacterium tuberculosis triggers reduced inflammatory cytokine responses and virulence in mice lacking Tax1bp1

## TL;DR

This study shows that mice lacking Tax1bp1 have reduced inflammation and bacterial growth when infected with Mycobacterium tuberculosis, revealing a cell-specific role for Tax1bp1 in immune responses.

## Contribution

The study reveals a cell type-specific role of Tax1bp1 in modulating autophagy, cell death, and inflammation during Mtb infection.

## Key findings

- Tax1bp1-deficient mice showed reduced Mtb growth and inflammatory responses compared to wild-type mice.
- Tax1bp1 deficiency reduced necrotic-like cell death in alveolar macrophages, which favors Mtb replication.
- Tax1bp1-deficient mice also showed reduced Listeria monocytogenes growth, indicating a broader role in host defense.

## Abstract

Host responses – autophagy, cell death, and inflammation – limit the growth of bacterial pathogens while minimizing tissue damage. During the early stages of infection, Mycobacterium tuberculosis (Mtb) thwarts these and other innate immune defense mechanisms in alveolar macrophages (AMs) derived from the yolk sac; in later stages, it circumvents defenses in recruited mononuclear cells (MNCs) and survives within them despite additional cytokine stimulation from recruited T cells. The mechanisms that drive variable rates of Mtb growth in different macrophage subtypes and how Mtb manipulates inflammatory responses to grow within innate immune cells remain obscure. Here we explored the role of the host factor, Tax-1 binding protein 1 (Tax1bp1), an autophagy receptor that targets pathogens for degradation through selective autophagy and terminates pro-inflammatory cytokine responses. Unexpectedly, we found that Tax1bp1-deficient mice were less susceptible to Mtb infection, and generated reduced inflammatory cytokine responses, compared to wild-type mice; the same mutant mice exhibited decreased growth of, and inflammatory cytokine responses to, Listeria monocytogenes, suggesting that Tax1bp1 plays a role in host responses to multiple intracellular pathogens. Contrary to our previous ex vivo findings in bone marrow-derived macrophages (BMDMs), in vivo growth of Mtb in AMs and a subset of recruited MNCs was more limited in mice lacking Tax1bp1 relative to wild-type mice. To better understand these differences, we performed global protein abundance measurements in mock- and Mtb-infected AM samples ex vivo from wild-type mice. These experiments revealed that Tax1bp1 protein abundance does not significantly change early after infection in AMs but does in BMDMs; moreover, early after infection, Tax1bp1-deficiency reduced necrotic-like cell death -- an outcome that favors Mtb replication -- in AMs but not BMDMs. Together, these results show that deficiency of Tax1bp1 plays a crucial, cell type-specific role in linking the regulation of autophagy, cell death, and anti-inflammatory host responses and overall reducing bacterial growth.

Macrophages are the first innate immune cells to encounter and be infected by Mycobacterium tuberculosis (Mtb) during infection. There are at least 9 different types of macrophages, and recent studies suggest that some permit M. tuberculosis replication and survival more than others, but the mechanisms for cell type-specific differences in M. tuberculosis growth are only beginning to be understood. We found that deficiency of the host factor, Tax1bp1 (Tax-1 binding protein 1), restricts M. tuberculosis growth during animal infection and in specific subsets of innate immune cells, including alveolar macrophages, while enhancing M. tuberculosis in bone marrow-derived macrophages. We also found that Tax1bp1-deficiency has a similar phenotype in abrogating the pathogenesis of another intracellular pathogen, Listeria monocytogenes. During infection, Tax1bp1-deficiency reduced inflammatory cytokine production and neutrophil and CD8+ T cell recruitment to the lungs of Mtb-infected mice. Compared to bone marrow-derived macrophages, in alveolar macrophages, Tax1bp1-deficiency decreased the release of inflammatory mediators and necrotic-like host cell death, a mode of host cell death known to enhance M. tuberculosis growth. Tax1bp1 protein level did not change significantly during Mtb infection of AMs but did increase significantly during BMDM infection, highlighting a potential mechanism that explains the different responses mediated by Tax1bp1 in BMDMs and AMs. Our research highlights that Tax1bp1 is a host target for host-directed therapy against M. tuberculosis and controls host responses to M. tuberculosis in a cell type-specific manner.