The role of plasma adenosine deaminase in chemoattractant-stimulated oxygen radical production in neutrophils

ObjectivesAdenosine deaminase (ADA) has a role in many immunity mediated disorders, such as asthma, tuberculosis and coronary artery disease. This study aims to investigate the ability of plasma ADA to modulate reactive oxygen species (ROS) production in neutrophils, and examine the involvement of adenosine and the cyclic AMP signaling pathway in this process.MethodsNeutrophils were stimulated, in the absence or presence of plasma, with the chemotactic peptide fMLP (formyl-methionyl-leucyl-phenylalanine), and the ROS production was determined with luminol-enhanced chemiluminescence. Activity of ADA was measured spectrophotometrically.ResultsPlasma dose-dependently amplified the ROS generation in fMLP-stimulated neutrophils. In parallel, incubation of neutrophils in plasma elevated the total ADA-activity approximately 10 times from 1.3 U/ml to 12 U/ml. Inhibition of ADA, or type IV phosphodiesterases, significantly lowered the plasma-mediated ROS production. Furthermore, the high-affinity adenosine A1 receptor antagonists DPCPX and 8-phenyltheophylline markedly inhibited the plasma-induced respiratory burst in neutrophils, suggesting an A1 receptor–mediated mechanism.ConclusionsThis study suggests that plasma ADA amplifies the release of toxic oxygen radicals from neutrophils through a downregulation of the inhibitory adenosine/cAMP-system and an enhanced activation of the stimulatory adenosine A1-receptor. This mechanism has probably a crucial role in regulating neutrophil function and in the defence against microbial infections. However, a sustained neutrophil activation could also contribute to inflammatory disorders such as atherosclerosis.