Presence of proteinase 3 in secretory vesicles: Evidence of a novel, highly mobilizable intracellular pool distinct from azurophil granules

Proteinase 3 (PR3), which is also called myeloblastin, the target autoantig en for antineutrophil cytoplasmic antibodies (ANCA) in Wegener's granulomat osis, is a serine proteinase stored in azurophil granules of human neutroph ils. We have previously shown that, in contrast to elastase or myeloperoxid ase, PR3 is also expressed at the plasma membrane of a subset of unactivate d neutrophils and that a high proportion of neutrophils expressing membrane PR3 is a risk factor for vasculitis, The present study demonstrates that t he association of PR3 with the plasma membrane is not an ionic interaction and seems to be covalent. Fractionation of neutrophils shows that, besides the azurophil granules, PR3 could be detected both in specific granules and in the plasma membrane-enriched fraction containing secretory vesicles, wh ereas elastase and myeloperoxidase were exclusively located in azurophil gr anules. Electron microscopy confirms that PR3 is present along with CR1 in secretory vesicles as well as in some specific granules. In neutrophils sti mulated with an increasing dose of FMLP, membrane PR3 expression increased with the degranulation of secretory vesicles, followed by specific granules , and culminated after azurophil granules mobilization. The presence of a r eadily plasma membrane-mobilizable pool of PR3 contained in the secretory v esicles might play a relevant role in the pathophysiological mechanisms of ANCA-associated vasculitis. (C) 1999 by The American Society of Hematology.