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 bee n implicated as important anti-bacterial agents in the control of List eria 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 macrophage Listericidal activity and production of nitrite (a stable end product of RNI) in culture. Various levels of nitrite prod uction 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. Nitrit e concentration was measured for the same wells used to assess Listeri cidal activity. There was essentially no correlation between initial o r final nitrite concentration and Listericidal activity in resident pe ritoneal macrophages. Significant correlations were noted between init ial and final nitrite concentration and Listericidal activity in prote ose 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 w ere omitted from analysis. A previous study suggested that the enhance d Listericidal activity of LPS-treated macrophages could be accounted for by an enhanced rate of phagocytosis during the initial phase of th e assay. These results suggest RNI are probably not the predominant ba ctericidal agents used by macrophages from female CD-1 mice to kill L. monocytogenes. However, it remains possible that RNI are important an ti-bacterial agents in highly activated (LPS-treated) macrophages, and that there are other mechanisms whereby RNI contribute to host resist ance to L. monocytogenes.