FIG. 1.

L. monocytogenes induces persistent IL-6 expression in Caco-2 cells in an LLO-dependent manner. (A) Kinetics of IL-6 expression in L monocytogenes-infected Caco-2 cells. Caco-2 cells were infected with L. monocytogenes wt (LMwt), the hly mutant, or ATCC 15313, and the expression of IL-6 was analyzed by RT-PCR. The results are representative of the results of three similar experiments. (B) Intracellular growth of L. monocytogenes. Caco-2 cells were infected with L. monocytogenes wt, the hly mutant, a fivefold-higher dose of the hly mutant (5 × hly mutant), or ATCC 15313, and the number of intracellular bacteria was determined by the CFU assay. The results are representative of the results of two similar experiments. The data are the means ± standard deviations for three determinations. (C) Cytoplasmic invasion by L. monocytogenes. Caco-2 cells were infected with L. monocytogenes wt and the hly mutant for 5 h. The cells were then fixed and stained with phalloidin-Alexa 488 to detect actin nucleation (green) and with anti-Listeria goat polyclonal antibody and sequential anti-goat immunoglobulin G-Alexa 546 to detect L. monocytogenes (red). (D) Quantitative analysis of IL-6 expression. Caco-2 cells were infected with L. monocytogenes wt in the absence or presence of cytochalashin D (cyt. D) or with a fivefold-higher dose of the hly mutant. The expression of IL-6 at 5 h after infection was analyzed by real-time RT-PCR. The results were expressed relative to the level of β-actin mRNA. The data are the means ± standard deviations from three independent experiments. Two asterisks indicate that there was a significant difference compared to uninfected cells.