RELEASE OF OXYGEN METABOLITES FROM CHEMOATTRACTANT-STIMULATED NEUTROPHILS IS INHIBITED BY RESTING PLATELETS - ROLE OF EXTRACELLULAR ADENOSINE AND ACTIN POLYMERIZATION

The effect of human platelets on chemoattractant-induced generation of oxygen metabolites in neutrophils was investigated, using luminol-enh anced chemiluminescence (CL). Resting platelets inhibited the extracel lular, but not the intracellular, production of oxygen radicals in ony l-leucyl-phenylalanine(fMet-Leu-Phe)-stimulated neutrophils. Maximal e ffect was obtained at the physiological neutrophil/platelet ratio of 1 /50. Similar results were acquired by adding supernatants of platelets , indicating a role for a soluble factor. Removal of extracellular ade nosine by adenosine deaminase (ADA), or blocking of adenosine-receptor s by theophylline, antagonized the inhibitory effects of platelets (or the equivalent supernatant) on the neutrophil respiratory burst. In c ontrast, accumulation of adenosine by apyrase enhanced the inhibition. Exogenous adenosine mimicked the effects of platelets on the fMet-Leu -Phe-induced respiratory burst. To further assess the role of platelet -derived adenosine, the platelets were fixed with paraformaldehyde. We found that fixed platelets, as well as their supernatant, inhibited t he fMet-Leu-Phe-induced CL-response to the same extent as viable cells . These effects were also reversed by ADA and theophylline, respective ly. A prior removal of adenosine in the platelet suspension by ADA, fo llowed by treatment with erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) t o inactivate ADA, did not reverse the inhibitory action of platelets o n the fMet-Leu-Phe-induced CL-response in neutrophils. However, if ade nosine receptors of the neutrophil at the same time were blocked with theophylline, the inhibition was significantly reduced. Platelets mark edly increased the generation of adenosine in a neutrophil suspension. This effect was antagonized by S-(4-Nitrobenzyl)-6-thioguanosine (NBT G), but unaffected by alpha,beta-methyleneadenosine5'diphosphate (AMP- CP), indicating that the platelet-dependent accumulation of adenosine is due to an increased release of endogenous adenosine from neutrophil s and not to a degradation of extracellular AMP. in correlation, NBTG, but not AMP-CP, reversed the platelet-mediated inhibition of the fMet -Leu-Phe-induced CL-response in neutrophils. Consequently, these data suggest that a platelet-derived factor increases the release of endoge nously formed adenosine from neutrophils, terminating the production o f oxygen radicals. The inhibition of oxidase activity was also associa ted with a platelet-induced polymerization of actin in the margin of t he neutrophils. Treatment of neutrophils with cytochalasin B reversed the effects of platelets, both on F-actin content and CL-response. In summary, resting platelets limit the release of oxygen radicals from c hemoattractant-stimulate neutrophils, thus preventing excessive damage to host tissues in the vascular space. This effect is suggested to be associated with an increased generation of neutrophil-derived adenosi ne enhancing an autoregulatory inhibitory pathway, and a peripheral ac cumulation of actin filaments forming a barrier for extracellular rele ase of reactive oxygen radicals. (C) 1996 by The American Society of H ematology.