PROTEOLYTIC INACTIVATION OF ALPHA-1-ANTITRYPSIN BY HUMAN NEUTROPHILS - INVOLVEMENT OF MULTIPLE AND INTERLINKED CELL RESPONSES TO PHAGOCYTOSABLE TARGETS

Neutrophil polymorphonuclear leukocytes (PMN) can inactivate the PMN-e lastase inhibitor alpha-1-antitrypsin (A1AT) proteolytically, by using metalloproteinases normally stored as zymogens in myeloperoxidase (MP O)-negative granules. Supernatants from opsonized zymosan (OPZ)-trigge red human PMN cleaved and inactivated human A1AT through a process inh ibitable by metal-chelators, suggesting that the interaction of PMN wi th OPZ leads to the extracellular availability of active metalloenzyme s. During OPZ-triggering, PMN used approximately 80% of the generated hydrogen peroxide (H2O2) to produce HOCl by means of the MPO pathway, while the remainder was catabolized by PMN themselves. No H2O2 was ava ilable as free compound in the extracellular environment and hydroxyl( OH) or OH-like radicals were not generated. The selective deletion of single components of the HOCl-generating MPO pathway resulted in the generation of PMN supernatants free of active metalloenzymes but rich of the corresponding zymogens. Similar results were obtained by replac ing normal PMN with cells from a patient with hereditary MPO deficienc y. No evidence was obtained for the intervention or contribution of OH -like radicals, serine-proteinases and oxidized glutathione in the tra nsformation of the zymogens into enzymes able to inactivate A1AT. On c oncluding, PMN undergoing phagocytosis release MPO in amount sufficien t to handle the extracellular pool of the generated H2O2 entirely, lea ding to the generation of equimolar amounts of HOCl. In turn, HOCl or a similar compound derived from it interacts with concomitantly releas ed metallozymogens, switching on their A1AT inactivating potential wit hout the apparent contribution of other PMN-derived molecules. As HOCl and its derivatives are well-known to oxidize and inactivate A1AT, th e herein shown ability of PMN to transform short-living HOCl into long -acting and A1AT-inactivating enzymatic systems suggest a central role for the HOCl-generating MPO pathway in the PMN-mediated impairment of lung anti-elastase defenses.