ReviewThe physiological role and pharmacological potential of nitric oxide in neutrophil activation
Introduction
Neutrophils comprise a fundamental component of the non-specific immune response to bacterial infection. The neutrophil response is characterised by adhesion to the vascular endothelium, followed by migration into tissues, oxygen radical-dependent killing of microbes, and elimination of microbes and tissue debris by phagocytosis. Since the agents released by activated neutrophils are potentially toxic to host tissues, neutrophils are subsequently removed by the process of apoptosis and are engulfed by macrophages to resolve the inflammatory response. Thus, modulation of the activation status of the neutrophil is of key importance in determining the balance between immune defense and host injury.
The aim of this review is to examine the role of nitric oxide (NO) in modulating neutrophil function. Effects of NO on the neutrophil functions detailed above will be discussed to elucidate the physiological role of endogenous NO, as well as its pharmacological potential in clinical situations.
Section snippets
NO production by neutrophils
Several studies have reported NO production by neutrophils, but others have claimed little or no NO production by these cells. For example, NO synthesis coincident with neutrophil chemotaxis has been detected by functional NO activity [1], [2], chemically [3], [4], [5] and by ADP ribosylation of proteins [6]. Human blood neutrophils stimulated in vitro with monocyte-derived cytokines and neutrophils from inflamed exudates are reported to express inducible NO synthase (iNOS) [7], [8].
Role of NO in neutrophil chemotaxis
There is considerable evidence that NO and cyclic GMP act as endogenous mediators of the chemotactic response of neutrophils. NOS inhibitors such as NG-monomethyl l-arginine (l-NMMA), NG-nitro-l-arginine methyl ester (l-NAME) and canavanine (l-CAN) all inhibited neutrophil chemotaxis induced by the bacterial peptide n-formyl-methionyl-leucyl-phenylalanine (fMLP) (Table 1), supporting a role for NO as a mediator of chemotaxis. Nω-nitro l-arginine (l-NNA) was ineffective in this respect, perhaps
NO in neutrophil adhesion
The migration of neutrophils from the blood though the endothelium to the site of inflammation requires highly coordinated cell–cell adhesive interactions between neutrophils and the endothelium. Selectins mediate the initial, low affinity adherence of neutrophils when rolling along the endothelium [24], whereas activation of neutrophil integrin adhesion molecules, and their interaction with endothelial immunoglobulins, enables transmigration of neutrophils to occur [25].
Neutrophil aggregation
NO and generation of reactive oxygen species by neutrophils
Generation of superoxide (O2−) during the respiratory burst results from the assembly and activation of the NADPH oxidase-dependent transmembrane electron transport chain. This oxidase activation is essential for the bactericidal function of neutrophils [45] and consequently, their toxic potential. Several studies have looked at the effects of NOS inhibitors on the neutrophil respiratory burst (Table 2) but only one [104] revealed the involvement of NO. This cannot relate to lack of NOS
NO and neutrophil phagocytosis
Less work has been carried out looking at NO and neutrophil phagocytosis. An early study suggested that endogenous NO could play a role in human neutrophil phagocytosis since anucleate, granule-poor neutrophil cytoplasts decreased their uptake and killing of staphylococci from supernatants in the presence of the NOS inhibitor l-NMMA, an effect that was reversed by l-arginine [68]. Subsequently l-arginine supplementation was shown to increase phagocytosis of staphylococcus by human neutrophils
NO and neutrophil apoptosis
The role of endogenous NO in neutrophil apoptosis is less clear. The guanylate cyclase inhibitor LY 83583 increased the rate of spontaneous neutrophil apoptosis suggesting that cyclic GMP may limit neutrophil apoptosis [71]. However, it has not been shown that this cyclic GMP effect is subsequent to stimulation of guanylate cyclase by NO. Levels of neutrophil nitrite, a stable oxidation product of NO, increase as neutrophils undergo spontaneous apoptosis in culture [72]. However, since nitrite
Clinical significance of NO and neutrophils
In therapeutic terms, inhaled exogenous NO is being evaluated as a novel treatment for pulmonary arterial hypertension [76], [77] in adult respiratory distress syndrome. In addition to its vasodilator action, NO has anti-inflammatory action by reducing neutrophil superoxide anion generation [78], [79]. Similarly, inhaled NO prevents neutrophil-related lung injury in several animal models [80], [81].
Since exposure of neutrophils to clinically relevant concentrations of NO has been found to
Acknowledgements
I would like to thank Payong Wanikiat for performing the experiments on isolated neutrophils and with a rat model of ischaemia reperfusion injury, as part of her PhD thesis and for her thesis itself, which provided a starting point for the neutrophil and NO literature.
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