Assessment of the correlation between nitrite concentration and listericidal activity in cultures of resident and elicited murine macrophages.

Reactive nitrogen intermediates (RNI) derived from L-arginine have been implicated as important anti-bacterial agents in the control of Listeria monocytogenes by murine macrophages. However, not all evidence is consistent with this conclusion. In the present study, this issue was examined using a simple experimental system to assess the correlation between macrophages Listericidal activity and production of nitrite (a stable end product of RNI) in culture. Various levels of nitrite production were achieved by activating macrophages with interferon-gamma (IFN-gamma) (20 or 500 U/ml) with or without lipopolysaccharide (LPS) (10 ng/ml) for 20 h before the Listericidal assay, and by using normal and arginine-free culture medium during the Listericidal assay. Nitrite concentration was measured for the same wells used to assess Listericidal activity. There was essentially no correlation between initial or final nitrite concentration and Listericidal activity in resident peritoneal macrophages. Significant correlations were noted between initial and final nitrite concentration and Listericidal activity in proteose peptone-elicited peritoneal macrophages. However, the correlation coefficients (0.34 and 0.52) suggested marginal biological relevance. In addition, no correlation was noted when LPS-activated macrophages were omitted from analysis. A previous study suggested that the enhanced Listericidal activity of LPS-treated macrophages could be accounted for by an enhanced rate of phagocytosis during the initial phase of the assay. These results suggest RNI are probably not the predominant bactericidal agents used by macrophages from female CD-1 mice to kill L. monocytogenes. However, it remains possible that RNI are important anti-bacterial agents in highly activated (LPS-treated) macrophages, and that there are other mechanisms whereby RNI contribute to host resistance to L. monocytogenes.