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Exploring the role of the CTL epitope region of listeriolysin O in the pathogenesis of Listeria monocytogenes

Marie-Annick Lety

Claude Frehel

Faculté de Médecine Necker-Enfants Malades, INSERM U-570, 156, rue de Vaugirard, 75730 Paris Cedex 15, France

Correspondence
Alain Charbit
charbit{at}necker.fr

Listeria monocytogenes is a facultative intracellular bacterial pathogen responsible for severe opportunistic infections in humans and animals. The secreted cholesterol-dependent cytolysin, listeriolysin O (LLO), mediates phagosomal escape and allows bacterial growth in the cytosol of infected cells. In order to identify new LLO determinants participating in bacterial pathogenesis, this study focused on a major target of LLO proteolytic cleavage in vitro, the CTL epitope region (residues 91–99). Mutations were generated by site-directed mutagenesis in the epitope or in the two clusters of positive charges flanking the epitope. Two LLO mutants (a single mutation K103A and a double mutation R89G, K90G) were normally and stably secreted by L. monocytogenes. In contrast, a mutant carrying four amino acid substitutions in the epitope itself (Y92K, D94A, E97K, Y98F) was highly susceptible to proteolytic degradation. While these three LLO mutant proteins showed a reduced haemolytic activity, they all promoted efficient phagosomal escape and intracellular multiplication in different cell types, and were non-cytotoxic. The deletion of the epitope (91–99), as well as the substitution of two, three or four of the four lysine residues (K103 to K106) by alanine residues did not lead to the production of a detectable protein. These results confirm the lack of correlation between haemolytic activity and phagosomal membrane disruption. They reveal the importance of the 91–99 region in the production of a stable and functional LLO. LD₅₀ determinations in the mouse model suggest a possible link between LLO stability and virulence.

A supplementary table of primer sequences is available with the online version of this paper.

These authors contributed equally to this work.