BACKGROUND: The success of many pathogens relies on their ability to circumvent the innate and adaptive immune defenses. How bacterial pathogens subvert adaptive immune defenses is not clear. Cholesterol-dependent cytolysins (CDCs) represent an expansive family of homologous pore-forming toxins that are produced by more than 20 gram-positive bacterial species. Listeriolysin O (LLO), a prototype CDC, is the main virulence factor of Listeria monocytogenes. METHODS: We employed flow cytometric and microarray techniques to analyze the effect of LLO on T cell activation in vitro and in vivo. RESULTS: In vivo and in vitro proliferation of CD4(+) T cells upon T cell receptor (TCR) activation was highly diminished in the presence of LLO or wild-type L. monocytogenes but not in the presence of LLO-deficient L. monocytogenes. This block in T cell proliferation was specific to T cell activation via the TCR and not by phorbol 12-myristate 13-acetate-ionomycin, which bypasses the proximal TCR signaling event. The results of microarray analysis suggest that LLO-induced T cell unresponsiveness is due to the induction of a calcium-nuclear factor of activated T cells-dependent transcriptional program that drives the expression of negative regulators of TCR signaling. CONCLUSION: These findings provide important insights into how bacterial toxins silence adaptive immune responses and thus enable prolonged survival of the pathogen in the host.
BACKGROUND: The success of many pathogens relies on their ability to circumvent the innate and adaptive immune defenses. How bacterial pathogens subvert adaptive immune defenses is not clear. Cholesterol-dependent cytolysins (CDCs) represent an expansive family of homologous pore-forming toxins that are produced by more than 20 gram-positive bacterial species. Listeriolysin O (LLO), a prototype CDC, is the main virulence factor of Listeria monocytogenes. METHODS: We employed flow cytometric and microarray techniques to analyze the effect of LLO on T cell activation in vitro and in vivo. RESULTS: In vivo and in vitro proliferation of CD4(+) T cells upon T cell receptor (TCR) activation was highly diminished in the presence of LLO or wild-type L. monocytogenes but not in the presence of LLO-deficient L. monocytogenes. This block in T cell proliferation was specific to T cell activation via the TCR and not by phorbol 12-myristate 13-acetate-ionomycin, which bypasses the proximal TCR signaling event. The results of microarray analysis suggest that LLO-induced T cell unresponsiveness is due to the induction of a calcium-nuclear factor of activated T cells-dependent transcriptional program that drives the expression of negative regulators of TCR signaling. CONCLUSION: These findings provide important insights into how bacterial toxins silence adaptive immune responses and thus enable prolonged survival of the pathogen in the host.