Unique role for ATG5 in neutrophil-mediated immunopathology during M. tuberculosis infection

Genetic engineering in mice reveals that autophagy is not an essential mechanism in myeloid cells for controlling Mycobacterium tuberculosis infection, and that autophagy factor ATG5 protects organisms by regulating neutrophil influx and tissue damage. It's widely assumed — based on data from cell culture and mouse models — that autophagy is an essential mechanism in controlling Mycobacterium tuberculosis infection. Christina Stallings and colleagues now show that this is not the case. They find that although the autophagy gene Atg5 is key to host immunity, other autophagy genes do not affect the outcome of tuberculosis. Rather, the autophagy factor ATG5 protects mice from M. tuberculosis infection by regulating neutrophil influx and tissue damage. Mycobacterium tuberculosis, a major global health threat, replicates in macrophages in part by inhibiting phagosome–lysosome fusion, until interferon-γ (IFNγ) activates the macrophage to traffic M. tuberculosis to the lysosome. How IFNγ elicits this effect is unknown, but many studies suggest a role for macroautophagy (herein termed autophagy), a process by which cytoplasmic contents are targeted for lysosomal degradation1. The involvement of autophagy has been defined based on studies in cultured cells where M. tuberculosis co-localizes with autophagy factors ATG5, ATG12, ATG16L1, p62, NDP52, BECN1 and LC3 (refs 2, 3, 4, 5, 6), stimulation of autophagy increases bacterial killing6,7,8, and inhibition of autophagy increases bacterial survival1,2,4,6,7. Notably, these studies reveal modest (~1.5–3-fold change) effects on M. tuberculosis replication. By contrast, mice lacking ATG5 in monocyte-derived cells and neutrophils (polymorponuclear cells, PMNs) succumb to M. tuberculosis within 30 days4,9, an extremely severe phenotype similar to mice lacking IFNγ signalling10,11. Importantly, ATG5 is the only autophagy factor that has been studied during M. tuberculosis infection in vivo and autophagy-independent functions of ATG5 have been described12,13,14,15,16,17,18. For this reason, we used a genetic approach to elucidate the role for multiple autophagy-related genes and the requirement for autophagy in resistance to M. tuberculosis infection in vivo. Here we show that, contrary to expectation, autophagic capacity does not correlate with the outcome of M. tuberculosis infection. Instead, ATG5 plays a unique role in protection against M. tuberculosis by preventing PMN-mediated immunopathology. Furthermore, while Atg5 is dispensable in alveolar macrophages during M. tuberculosis infection, loss of Atg5 in PMNs can sensitize mice to M. tuberculosis. These findings shift our understanding of the role of ATG5 during M. tuberculosis infection, reveal new outcomes of ATG5 activity, and shed light on early events in innate immunity that are required to regulate disease pathology and bacterial replication.