CELL IMMORTALIZATION ENHANCES LISTERIA-MONOCYTOGENES INVASION

Recent outbreaks of human listeriosis have emphasized the importance o f food in the etiology of epidemic listeriosis, suggesting that the ga strointestinal tract is the natural site of entry for Listeria monoocy togenes into the organism. L. monocytogenes invasion of finite cell li nes derived from the porcine ileum exhibited a 100-fold lower penetrat ion level, without any intracellular multiplication, when compared to CaCo-2 cells, a widely used in vitro model for L. monocytogenes invasi on. Same results were obtained with both pig kidney primary cells and mouse kidney finite cell lines. To demonstrate that cell immortalizati on enhances L. monocytogenes invasion, finite cell lines from porcine ileum and from murine kidney were immortalized by Simian virus 40 (SV4 0) large T oncogene. Unlike their untransformed counterparts, the immo rtal cells obtained were invaded by L. monocytogenes, as observed for CaCo-2 cells as well as for spontaneously immortal human (HeLa) and mu rine (3T3) cell lines. Extensive electron microscopy examinations of p orcine epithelioid cells infected by L. monocytogenes showed numerous bacteria within the immortal cells, whereas neither intracellular bact eria 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 differ entiation or proliferation of the cells, only an enterocyte subset at a defined state of differentiation or expressing particular receptors could be invaded in vivo.