Actin-based motility allows Listeria monocytogenes to avoid autophagy in the macrophage cytosol.

Listeria monocytogenes grows in the host cytosol and uses the surface protein ActA to promote actin polymerisation and mediate actin-based motility. ActA, along with two secreted bacterial phospholipases C, also mediates avoidance from autophagy, a degradative process that targets intracellular microbes. Although it is known that ActA prevents autophagic recognition of L. monocytogenes in epithelial cells by masking the bacterial surface with host factors, the relative roles of actin polymerisation and actin-based motility in autophagy avoidance are unclear in macrophages. Using pharmacological inhibition of actin polymerisation and a collection of actA mutants, we found that actin polymerisation prevented the colocalisation of L. monocytogenes with polyubiquitin, the autophagy receptor p62, and the autophagy protein LC3 during macrophage infection. In addition, the ability of L. monocytogenes to stimulate actin polymerisation promoted autophagy avoidance and growth in macrophages in the absence of phospholipases C. Time-lapse microscopy using green fluorescent protein-LC3 macrophages and a probe for filamentous actin showed that bacteria undergoing actin-based motility moved away from LC3-positive membranes. Collectively, these results suggested that although actin polymerisation protects the bacterial surface from autophagic recognition, actin-based motility allows escape of L. monocytogenes from autophagic membranes in the macrophage cytosol.