Pneumococci Stimulate the Production of Inducible Nitric Oxide Synthase and Nitric Oxide by Murine Macrophages † 887

Abstract

The role of nitric oxide (NO) in the pathophysiology of gram-positive sepsis is uncertain. In inflammatory conditions, high-output NO production is catalyzed by the enzyme inducible nitric oxide synthase (iNOS). The ability of 2 strains of pneumococci, pneumococcal cell wall preparations, and purified pneumococcal capsule (Pnu-Imune 23) to trigger the production of iNOS protein and NO in RAW 264.7 murine macrophages was tested. Live pneumococci, oxacillin-killed pneumococci, and pneumococcal cell wall preparations stimulated the production of iNOS and NO by RAW 264.7 cells in the presence, but not the absence, of low concentrations of recombinant murine interferon-g. In contrast, purified pneumococcal capsule induced little or no iNOS or NO production by these cells. Thus, pneumococci stimulate high-output NO production by murine macrophages. The potential role of NO in the pathogenesis of pneumococcal sepsis deserves further study. Nitric oxide (NO) and tumor necrosis factor (TNF) have been found to play important roles in the pathogenesis of gramnegative septic shock. During inflammatory states, the enzyme inducible nitric oxide synthase (iNOS) catalyzes the hyperproduction of NO by various mammalian cells [1]. Murine macrophages produce large amounts of NO in response to inflammatory stimuli, but the existence of the high-output NO pathway in isolated human monocytes and macrophages has been controversial (reviewed in [2]). However, biosynthesis of iNOS protein [3, 4] and iNOS mRNA [3, 5] has recently been unequivocally demonstrated in human mononuclear cells in vivo during infection. In addition, nitrite accumulation has been detected in human monocyte-derived macrophages after a polyribonucleotide stimulus and was enhanced by pretreatment with interferon-g (IFN-g) [6]. In both humans and experimental animals, symptomatology of hypotension, vasoplegia to exogenous catecholamines, and end-organ dysfunction correlate with increased levels of NO during sepsis [7]. Most of the previous studies of NO production by macrophages have used gram-negative organisms or their major virulence factor, lipopolysaccharide (LPS), as the stimulus.