Examination of Cytopathic Effect and Apoptosis in Listeria monocytogenes-Infected Hybridoma B-Lymphocyte (Ped-2E9) Line In Vitro

In our previous studies, we reported that hybridoma B-lymphocytes can be used to determine the virulence of Listeria species in an in vitro cytotoxicity assay. Here, we examined the cytopathic effect, i.e., membrane damage and the nature of cell death induced by Listeria monocytogenes on murine hybridoma B-lymphocytes (Ped-2E9). Membrane damage was assessed by microscopic analyses and by measuring the release of intracellular alkaline phosphatase(AP) and lactate dehydrogenase (LDH). Cell death was determined by DNA fragmentation analyses using agarose gel electrophoresis. Infection by listeriolysin O (LLO)-producing L. monocytogenes strains induced substantial amounts of AP and LDH release from Ped-2E9 hybridoma B-cells, suggesting severe membrane damage in these cells, while an LLO-negative L. monocytogenes mutant strain had no effect. An LLO-producing recombinant L. innocua ($prifA^+hly^+$) strain also induced high AP and LDH release and cytopathic changes in Ped-2E9 cells. Light or scanning electron microscopic examination revealed L. monocytogenes mediated membrane destabilization, pore formation, intense cytoplasmic granulation, bleb formation, and lysis of Ped-2E9 cells. LLO-producing L. monocytogenes and L. innocua ($prifA^{+}hly{^}+$) also induced ladder-like DNA fragmentation in Ped-2E9 cells. Collectively, these results suggest that L. monocytogenes, specifically LLO-producing strains, can induce a severe cytopathic effect leading to apoptosis in hybridoma B-lymphocytes (Ped-2E9).