AbstractProteinase 3 (PR3), a serine proteinase contained in neutrophil azurophilic granules, is considered a risk factor for vasculitides and rheumatoid arthritis when expressed on the outer leaflet of neutrophil plasma membrane and is the preferred target of antineutrophil cytoplasm autoantibodies (ANCA) in Wegener granulomatosis. ANCA binding to PR3 expressed at the surface of neutrophils activates them. Evidence is provided that neutrophil apoptosis induced significantly more membrane PR3 expression without degranulation (but no enhanced membrane CD35, CD66b, CD63, myeloperoxidase, or elastase expression). This observation was confirmed on cytoplasts, a model of granule-free neutrophils. We hypothesized that PR3 could interact with proteins involved in membrane flip-flop (eg, phospholipid scramblase 1 [PLSCR1]). PR3-PLSCR1 interaction in neutrophils was demonstrated by confocal microscopy and coimmunoprecipitation. In the RBL-2H3 rat mast-cell line stably transfected with PR3 or its inactive mutant (PR3S203A), PR3 externalization depended on PLSCR1, as shown by less PR3 externalization in the presence of rPLSCR1 siRNA, but independently of its serine-proteinase activity. Finally, apoptosis-externalized PR3 decreased the human macrophage-phagocytosis rate of apoptotic PR3 transfectants. Therefore, in addition to ANCA binding in vasculitis, the proinflammatory role of membrane PR3 expression may involve interference with macrophage clearance of apoptotic neutrophils.