Cytolysin-Dependent Escape of the Bacterium from the Phagosome Is Required but Not Sufficient for Induction of the Th1 Immune Response against Listeria monocytogenes Infection: Distinct Role of Listeriolysin O Determined by Cytolysin Gene Replacement

Abstract

ABSTRACT Listeria monocytogenes evades the antimicrobial mechanisms of macrophages by escaping from the phagosome into the cytosolic space via a unique cytolysin that targets the phagosomal membrane, listeriolysin O (LLO), encoded by hly . Gamma interferon (IFN-γ), which is known to play a pivotal role in the induction of Th1-dependent protective immunity in mice, appears to be produced, depending on the bacterial virulence factor. To determine whether the LLO molecule (the major virulence factor of L. monocytogenes ) is indispensable or the escape of bacteria from the phagosome is sufficient to induce IFN-γ production, we first constructed an hly -deleted mutant of L. monocytogenes and then established isogenic L. monocytogenes mutants expressing LLO or ivanolysin O (ILO), encoded by ilo from Listeria ivanovii . LLO-expressing L. monocytogenes was highly capable of inducing IFN-γ production and Listeria -specific protective immunity, while the hly -deleted mutant was not. In contrast, the level of IFN-γ induced by ILO-expressing L. monocytogenes was significantly lower both in vitro and in vivo, despite the ability of this strain to escape the phagosome and the intracellular multiplication at a level equivalent to that of LLO-expressing L. monocytogenes . Only a negligible level of protective immunity was induced in mice against challenge with LLO- and ILO-expressing L. monocytogenes . These results clearly show that escape of the bacterium from the phagosome is a prerequisite but is not sufficient for the IFN-γ-dependent Th1 response against L. monocytogenes , and some distinct molecular nature of LLO is indispensable for the final induction of IFN-γ that is essentially required to generate a Th1-dependent immune response.