Listeriolysin O-induced membrane permeation mediates persistent interleukin-6 production in Caco-2 cells during Listeria monocytogenes infection in vitro

Listeriolysin O (LLO), a major virulence factor of Listeria monocytogenes, is a member of the cholesterol-dependent cytolysin family and plays important roles not only in survival of this bacterium in phagocytes but also in induction of various cellular responses, including cytokine production. In this work, we examined the involvement of LLO in induction of the cytokine response in intestinal epithelial cells, the front line of host defense against food-borne listeriosis. Infection of Caco-2 cells with wild-type L. monocytogenes induced persistent expression of interleukin-6 (IL-6) mRNA. In contrast, IL-6 expression was observed only transiently during infection with non-LLO-producing strains. A sublytic dose of recombinant LLO (rLLO) induced the expression of IL-6 via formation of membrane pores. Under conditions of LLO-induced pore formation without extensive cell lysis, Ca2+ influx was observed, and the IL-6 expression induced by rLLO was inhibited by pretreatment with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM), an intracellular Ca2+ chelator. LLO secreted by cytoplasmic L. monocytogenes appeared to induce pore formation in the membrane and to enable the trafficking of intracellular and extracellular molecules. Pretreatment with BAPTA-AM inhibited persistent IL-6 expression in Caco-2 cells infected with wild-type L. monocytogenes. These results suggest that LLO is involved in IL-6 production in the late phase of infection through the formation of Ca2+-permeable pores and subsequent Ca2+-dependent modulation of signaling and gene expression.