Mechanisms of Shigella flexneri dissemination through double membrane vacuole escape

Shigella flexneri is an important human pathogen that causes the severe diarrheal disease, bacillary dysentery and accounts for 270 million and nearly 200,000 deaths per year. S. flexneri invades colonic epithelial cells and hijacks the host actin cytoskeleton to move in the cytosol of infected cells and disseminate to neighboring cells through a process called cell-to-cell spread. The bacterial type three-secretion system (T3SS) is required for cell-to-cell spread and escape from double membrane vacuoles (DMVs) in the adjacent cell. Before my thesis work, very little was known about the mechanisms that facilitate DMV escape. My thesis work has uncovered multiple factors, both host and bacterial, that contribute to S. flexneri DMV escape. We characterized two T3SS effectors, IcsB (Chapter 2) and IpgB1 (Chapter 3), during cell-to-cell spread. IcsB, via its acyltransferase activity, is required for efficient DMV escape and cell-to-cell spread through an unknown mechanism that does not involve host autophagy, a process that IcsB was proposed to modulate. IpgB1, via its guanine nucleotide exchange factor (GEF) activity, facilitates cell-to-cell spread through its target small GTPase, Rac1, to antagonize RhoA-mediated restriction of DMV escape. We also found a novel host factor, lysophospholipase 2 (LYPLA2) that plays a role in DMV escape (Chapter 4) during cell-to-cell spread. LYPLA2 hydrolyzes long chain fatty acids from membrane lysophospholipids and membrane-anchored proteins. My work has provided characterization of the first bacterial and host factors required for efficient DMV escape during S. flexneri cell-to-cell spread and offers novel insights into the mechanisms of S. flexneri pathogenesis.