INTRACELLULAR KILLING OF LISTERIA-MONOCYTOGENES IN THE J774.1 MACROPHAGE-LIKE CELL-LINE AND THE LIPOPOLYSACCHARIDE (LPS)-RESISTANT MUTANT LPS1916 CELL-LINE DEFECTIVE IN THE GENERATION OF REACTIVE OXYGEN INTERMEDIATES AFTER LPS TREATMENT

Listeria monocytogenes is a facultative intracellular pathogen and sur vives within phagocytic cells by escaping from phagosomes into the cyt oplasm. It has been reported that, in vivo, L. monocytogenes is effect ively eliminated through cell-mediated immunity, especially by macroph ages which have been immunologically activated by cytokines such as ga mma interferon (IFN-gamma). However, this killing mechanism for L. mon ocytogenes and the role of macrophage activation in this bacterial kil ling are unclear. We demonstrated the listericidal effect of oxidative radicals induced by lipopolysaccharide (LPS) and IFN-gamma, using a m acrophage-like cell line, J774.1, and a mutant cell line, LPS1916. LPS 1916 cells do not exhibit normal generation of O-2(-) and H2O2 after t reatment with 0.1 mu g of LPS per ml, although J774.1 cells generate 1 00 times the normal level of oxidative radicals with the same LPS trea tment. The growth of L. monocytogenes was strongly inhibited in J774.1 cells pretreated with 0.1 mu g of LPS per ml or the combination of 0. 1 mu g of LPS per ml and 10 U of IFN-gamma per ml. On the other hand, in LPS1916 cells, the growth of L. monocytogenes was not inhibited by treatment with LPS only, although LPS1916 cells pretreated with the co mbination of LPS and IFN-gamma shelved moderate inhibition of listeria l growth. This killing was not influenced by treatment with N-G-monome thyl-L-arginine, which is a strong inhibitor of nitrite oxide generati on. Interestingly, J774.1 cells treated with LPS did not show enhanced intraphagosomal killing of a nonhemolytic strain of avirulent L. mono cytogenes that lacks the ability to escape from phagosomes, and this k illing was not influenced by treatment with N-G-monomethyl-L-arginine either. These results suggest that the reactive oxygen radicals are mo re important than nitric oxide in the mechanism underlying the intrace llular killing of virulent L. monocytogenes and that there seem to be different killing mechanisms for virulent and avirulent strains of L. monocytogenes in activated-macrophage cell lines.