Listeria monocytogenes triggers noncanonical autophagy upon phagocytosis, but avoids subsequent growth-restricting xenophagy

Abstract

Xenophagy is a selective macroautophagic process that protects the host cytosol by entrapping and delivering microbes to a degradative compartment. Both noncanonical autophagic pathways and xenophagy are activated by microbes during infection, but the relative importance and function of these distinct processes are not clear. In this study, we used bacterial and host mutants to dissect the contribution of autophagic processes responsible for bacterial growth restriction of Listeria monocytogenes. L. monocytogenes is a facultative intracellular pathogen that escapes from phagosomes, grows in the host cytosol, and avoids autophagy by expressing three determinants of pathogenesis: two secreted phospholipases C (PLCs; PlcA and PlcB) and a surface protein (ActA). We found that shortly after phagocytosis, wild-type (WT) L. monocytogenes escaped from a noncanonical autophagic process that targets damaged vacuoles. During this process, the autophagy marker LC3 localized to single-membrane phagosomes independently of the ULK complex, which is required for initiation of macroautophagy. However, growth restriction of bacteria lacking PlcA, PlcB, and ActA required FIP200 and TBK1, both involved in the engulfment of microbes by xenophagy. Time-lapse video microscopy revealed that deposition of LC3 on L. monocytogenes-containing vacuoles via noncanonical autophagy had no apparent role in restricting bacterial growth and that, upon access to the host cytosol, WT L. monocytogenes utilized PLCs and ActA to avoid subsequent xenophagy. In conclusion, although noncanonical autophagy targets phagosomes, xenophagy was required to restrict the growth of L. monocytogenes, an intracellular pathogen that damages the entry vacuole.