PLASMA ANTIPROTEINASE SCREEN AND NEUTROPHIL-MEDIATED PLATELET ACTIVATION - A MAJOR ROLE PLAYED BY ALPHA(1) ANTITRYPSIN

Upon activation, human polymorphonuclear neutrophils (PMN) release two serine proteinases, cathepsin G (Cat.G) and elastase (HLE), which in turn synergize to activate nearby platelets. We looked for the inhibitory effect of plasma and the involvement of alpha(1), antichymotrypsin (alpha(1) ACT) and alpha(1) antitrypsin (alpha(1) AT), on this cell-to-cell cooperation. It was observed that inhibition by plasma of PMN-mediated platelet activation was rather correlated with an effect on HLE (r = 0.95) than on Cat.G (r = 0.65) enzymatic activity. Purified alpha(1) AT suppressed in a concentration-dependent manner HE activity present in the supernatant of activated PMN. When HLE was fully blocked, alpha(1) AT started to inhibit Cat.G activity. By contrast and as expected, purified alpha(1) ACT inhibited only Cat.G activity. Using specific blocking polyclonal antibodies against alpha(1) AT and alpha(1) ACT, it was demonstrated that the inhibitory effect of plasma vs. HLE was entirely mediated by alpha(1) AT. By contrast, blockade of Cat.G activity was only partly due to plasma alpha(1) ACT and around 50% was attributable to alpha(1) AT. When plasma from patients with an acute inflammatory state was used in place of plasma from normal subjects, the inhibitory effect was more pronounced, while plasma depleted in alpha(1) AT and alpha(1) ACT was less effective. These data indicate a predominant role of alpha(1) AT in the inhibition by plasma of the PMN-mediated platelet activation.