Roles of Listeria monocytogenes virulence factors in survival: virulence factors distinct from listeriolysin are needed for the organism to survive an early neutrophil-mediated host defense mechanism.
Avirulent mutant strains of Listeria monocytogenes which fail to produce phosphatidylinositol-specific phospholipase C, or which produce reduced amounts of hemolytic listeriolysin O, are incapable of causing progressive infection in normal mice. However, both strains can grow progressively in mice that have been rendered incapable of focusing neutrophils at sites of infection as a result of being treated with monoclonal antibody 5C6, specific for the type 3 complement receptor of myelomonocytic cells. In 5C6-treated mice, phospholipase C-negative and listeriolysin-defective mutant strains of L. monocytogenes, like the wild-type strain, give rise in the liver to large numbers of discrete foci of infected hepatocytes that retain their morphological integrity during the first 24 h, despite their large bacterial burden. In normal mice, in contrast, sites of infection in the liver are indicated by discrete focal accumulations of neutrophils that occupy the space originally occupied by infected hepatocytes. It is apparent that in normal mice neutrophils function to lyse infected hepatocytes and thereby to release L. monocytogenes for ingestion and killing by neutrophils themselves and by macrophages. However, whereas a proportion of wild-type organisms survive this early mechanism of defense to give rise to progressive infection, the phospholipase C-negative organisms are totally eliminated. On the basis of these and other results, it is suggested that virulence factors other than listeriolysin are needed by L. monocytogenes to counteract the early neutrophil-mediated mechanism of defense. Listeriolysin, itself, is an intrinsic virulence factor that allows L. monocytogenes to survive and multiply in a proportion of the fixed phagocytes of the liver (permissive phagocytes) and which enables the organism to go on to infect and replicate in adjacent hepatocytes. It was found that a mutant strain of L. monocytogenes incapable of producing any listeriolysin was incapable of establishing progressive infection, even in 5C6-treated mice.