EXPRESSION OF LISTERIOLYSIN-O BY INTRACELLULAR LISTERIA-MONOCYTOGENESFOLLOWING INFECTION OF LIPOPOLYSACCHARIDE-TREATED OR UNTREATED J774.1MACROPHAGE-LIKE CELLS

Listeria monocytogenes replicates in a phagocytic cell following escap e into the host cytoplasm. Listeriolysin O, secreted by L. monocytogen es, which belongs to the thiol-activated hemolysin family, is known to play an important role in the escape of the bacterium into the host c ytoplasm. In this study, we demonstrated that expression of listerioly sin O by infecting L. monocytogenes was lightly induced in J774.1 macr ophage-like cells pretreated with lipopolysaccharide, although the gro wth of the bacteria was suppressed. The number of viable L. monocytoge nes decreased until 4 h post-infection and then increased between 3 an d 8 h post-infection in untreated J774.1 host cells, but it decreased until 8 h post-infection in lipopolysaccharide-treated host cells. How ever, expression of listeriolysin O by L. monocytogenes was not induce d in the untreated host cells, while it increased between 0 and 4 h po st-infection in the lipopolysaccharide-treated host cells. Expression of listeriolysin O mRNA in the lipopolysaccharide-treated host cells w as also induced at 2 h post-infection, suggesting that listeriolysin O was newly synthesized in the macrophage-like cells. These results sug gest that macrophage activation induced with lipopolysaccharide could lead to the expression of the listeriolysin O gene and the synthesis o f listeriolysin O protein under suppression of the intracellular growt h of L. monocytogenes.