Listeriolysin O is a target of the immune response to Listeria monocytogenes

The immunologic mechanism of protective immunity to the intracellular parasite Listeria monocytogenes (Lm) is not well understood, however, antilisterial immunity can be adoptively transferred with T lymphocytes from Lm-immune donors. The Lm-immune cells are believed to produce macrophage-activating lymphokines, which leads to the eventual macrophage-dependent eradication of the bacterium. Increasing evidence suggests that immunity to Lm resides exclusively within the CD8+ T cell subset. It is possible that the Lm-immune CD8+ T cells function to release sequestered Lm from nonprofessional phagocytes to awaiting activated macrophage populations. This study was conducted to determine if listeriolysin O (LLO), which is an essential determinant of Lm pathogenicity, is also a target of the antilisterial immune response. We have found that target cells infected with a LLO+ Lm strain are lysed by Lm-immune cytotoxic cells, whereas target cells infected with a LLO- Lm mutant, or pulsed with a heat-killed Lm preparation, are not lysed by the Lm-immune effector cells. We have used a Bacillus subtilis (Bs) construct that expresses the LLO gene product and found that target cells infected with the LLO+ Bs construct are lysed by antilisterial cytotoxic cells. The antilisterial cytotoxic response is targeted against LLO, in that we have also used a Bs construct that expresses the perfringolysin (PLO) gene product and found that target cells infected with the PLO+ Bs are not lysed by antilisterial cytotoxic effector cells. These data strongly suggest that LLO is a target antigen of antilisterial immunity and may represent the dominant target during the expression of the immune response to Lm.