Differential induction of NO synthesis by Gram-positive and Gram-negative bacteria and their components in bovine monocyte-derived macrophages

The ability of Gram-positive and Gram-negative bacteria to promote the induction of NO synthesis in bovine monocyte-derived macrophages (MDM) was tested. Heat-killed Gram-negative organisms induced NO synthesis at low concentrations (optimum 0.2 to 2 μg/ml wet weight), regardless of the strain, and the response was only moderately enhanced by co-administration of recombinant bovine interferon-γ (rboIFN-γ). The activity was largely, but not exclusively, due to lipopolysaccharide (LPS), since it was largely abrogated by co-incubation with polymyxin-B. Diphosphoryl-lipid-A and rough-strain LPS were two orders of magnitude more active than monophosphoryl-lipid A, but two orders of magnitude less active than smooth-strain LPS, suggesting that O side chains contribute to increasing the affinity of LPS or to act as a costimulus. Gram-positive bacteria as single stimuli were four orders of magnitude less potent in inducing NO synthesis than Gram-negative organisms, but upon costimulation with rboIFN-γ, some of them were excellent inducers of NO synthesis. A similar rboIFN-γ-enhanced NO synthesis induction was also observed for zymosan, muramyl dipeptide, lipoteichoic acid and lipoarabinomannan, although to a lesser extent than for the whole heat-inactivated prototypic organisms. Thus, bovine macrophages exposed to rboIFN-γ have mechanisms by which they universally react to bacterial compounds distinct from LPS by induction of NO synthesis.