Nitric oxide modulates neutrophil function

• An important role of the neutrophils is to attack and destroy microbial intruders. In order to do so, these leukocytes must adhere to the blood vessel wall, pass through the endothelial cell layer, move through the tissues to the point of infection, ingest the intruder and destroy it.The aim of this thesis was to study the effects of nitric oxide (NO) on the different steps in the inflammatory response of the neutrophil.Adhesion to a collagen surface was decreased by pretreatment with a NO-releasing substance, and this was mimicked by pretreatment with a functional cGMP-analogue, indicating a role of the NO/cGMP pathway. Homotypic adhesion, aggregation, was not affected by external NO, but when the precursor of endogenous NO-production CLarginine) was added, the aggregation increased. Measurements of total F-actin content in cells showed that a NO-releasing substance decreased the total amount of F-actin, while cGMP increased it. Treatment with L-arginine had no effect.Phagocytosis was neither affected by endogenous NO, nor by a NO-releasing substance. However, if the prey itself released NO, both the adhesion to and phagocytosis of the target was decreased. This NO-particle also inhibited the production of oxygen metabolites, as measured using Luminal-dependent chemiluminescence. The inhibition was almost exclusively affecting the intracellular production of oxygen metabolites, as could be seen when neutrophils were stimulated with FMLP or PMA after an incubation with the NO-releasing particles.Treatment of neutrophils with the NO-releasing substance nitroprusside resulted in a decrease of the respiratory burst. It was primarily the extracellular release that was diminished. This effect can be explained by an inhibition of the enzyme producing oxygen metabolites. Endogenous NO-production increased the chemiluminescence whereas an inhibitor of the NO-synthase decreased it. A possible explanation for these effects is an NO-inhibition of the protection against hydrogen peroxide, i.e. catalase activity, resulting in an increased amount of oxygen metabolites.In conclusion, these results shows that NO has different effects depending on where it is produced and in what quantities. It modulates the different steps of the inflammatory response of neutrophils. Control of NO-formation could therefore be a way to control inflammation.

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