Cell immortalization enhances Listeria monocytogenes invasion

Recent outbreaks of human listeriosis have emphasized the importance of food in the etiology of epidemic listeriosis, suggesting that the gastrointestinal tract is the natural site of entry for Listeria monocytogenes into the organism. L. monocytogenes invasion of finite cell lines derived from the porcine ileum exhibited a 100-fold lower penetration level, without any intracellular multiplication, when compared to CaCo-2 cells, a widely used in vitro model for L. monocytogenes invasion. Same results were obtained with both pig kidney primary cells and mouse kidney finite cell lines. To demonstrate that cell immortalization enhances L. monocytogenes invasion, finite cell lines from porcine ileum and from murine kidney were immortalized by Simian virus 40 (SV40) large T oncogene. Unlike their untransformed counterparts, the immortal cells obtained were invaded by L. monocytogenes, as observed for CaCo-2 cells as well as for spontaneously immortal human (HeLa) and murine (3T3) cell lines. Extensive electron microscopy examinations of porcine epithelioid cells infected by L. monocytogenes showed numerous bacteria within the immortal cells, whereas neither intracellular bacteria nor any bacterial antigen were revealed inside finite cell lines. These data suggested that L. monocytogenes were not destroyed inside finite cell lines but only poorly entered the finite or primary cells. Speculating that L. monocytogenes invasion is under control of differentiation or proliferation of the cells, only an enterocyte subset at a defined state of differentiation or expressing particular receptors could be invaded in vivo.