Inhibition of Calpain Blocks the Phagosomal Escape of Listeria monocytogenes

Abstract

Listeria monocytogenes is a Gram-positive facultative intracellular bacterium responsible for the food borne infection listeriosis, affecting principally the immunocompromised, the old, neonates and pregnant women. Following invasion L. monocytogenes escapes the phagosome and replicates in the cytoplasm. Phagosome escape is central to L. monocytogenes virulence and is required for initiating innate host-defence responses such as the secretion of the cytokine interleukin-1. Phagosome escape of L. monocytogenes is reported to depend upon host proteins such as γ-interferon-inducible lysosomal thiol reductase and the cystic fibrosis transmembrane conductance regulator. The host cytosolic cysteine protease calpain is required in the life cycle of numerous pathogens, and previous research reports an activation of calpain by L. monocytogenes infection. Thus we sought to determine whether host calpain was required for the virulence of L. monocytogenes. Treatment of macrophages with calpain inhibitors blocked escape of L. monocytogenes from the phagosome and consequently its proliferation within the cytosol. This was independent of any direct effect on the production of bacterial virulence factors or of a bactericidal effect. Furthermore, the secretion of interleukin-1β, a host cytokine whose secretion induced by L. monocytogenes depends upon phagosome escape, was also blocked by calpain inhibition. These data indicate that L. monocytogenes co-opts host calpain to facilitate its escape from the phagosome, and more generally, that calpain may represent a cellular Achilles heel exploited by pathogens.

Listeria monocytogenes Alters Mast Cell Phenotype, Mediator and Osteopontin Secretion in a Listeriolysin-Dependent Manner

“…Listeria monocytogenes strain EGDe, EGDe InlA m (termed WT) [20], and its Δhly derivative [54] were used. Cells were cultured in tryptone soya broth (TSB) or agar (Fisher) at 37°C.…”

Listeria monocytogenes: cell biology of invasion and intracellular growth

“…Consequently, within mammalian hosts the LLO poreforming activity is compartmentalized to slightly acidified bacterial-containing compartments (110,111) and cytoplasmic LLO activity, which is cytotoxic to host cells and detrimental to intracellular inhibiting its activation and reverting its bactericidal functions (143). The activity of the cytosolic cysteine protease calpain has been shown to be required for efficient Lm vacuolar escape, but the targets of this protease remain to be identified (144).…”

Calpains promote neutrophil recruitment and bacterial clearance in an acute bacterial peritonitis model

“…These results suggest that calpain activation leads to enhanced bacterial killing, likely by promoting phagolysosome maturation. Calpains are also required for phagosomal escape of Listeria monocytogene infected macrophages, and enhance release of both IL-1α and IL-1β [60]. Although we did not observe this in our FIP model in vivo, it will be important to define the contributions of calpains to phagocytosis of a number of bacterial pathogens using this calpain KO model in future.…”