CELL SURFACE-BOUND ELASTASE AND CATHEPSIN-G ON HUMAN NEUTROPHILS - A NOVEL, NONOXIDATIVE MECHANISM BY WHICH NEUTROPHILS FOCUS AND PRESERVE CATALYTIC ACTIVITY OF SERINE PROTEINASES

Serine proteinases of human polymorphonuclear neutrophils play an impo rtant role in neutrophil-mediated proteolytic events; however, the non -oxidative mechanisms by which the cells can degrade extracellular mat rix in the presence of proteinase inhibitors have not been elucidated. Herein, we provide the first report that human neutrophils express pe rsistently active cell surface-bound human leukocyte elastase and cath epsin G on their cell surface. Unstimulated neutrophils have minimal c ell surface expression of these enzymes; however, phorbol ester induce s a 30-fold increase. While exposure of neutrophils to chemoattractant s (fMLP and C5a) stimulates modest (two- to threefold) increases in ce ll surface expression of serine proteinases, priming with concentratio ns of lipopolysaccharide as low as 100 fg/ml leads to striking (up to 10-fold) increase in chemoattractant-induced cell surface expression, even in the presence of serum proteins. LPS-primed and fMLP-stimulated neutrophils have similar to 100 ng of cell surface human leukocyte el astase activity per 10(6) cells. Cell surface-bound human leukocyte el astase is catalytically active, yet is remarkably resistant to inhibit ion by naturally occurring proteinase inhibitors. These data indicate that binding of serine proteinases to the cell surface focuses and pre serves their catalytic activity, even in the presence of proteinase in hibitors. Upregulated expression of persistently active cell surface-b ound serine proteinases on activated neutrophils provides a novel mech anism to facilitate their egress from the vasculature, penetration of tissue barriers, and recruitment into sites of inflammation. Dysregula tion of the cell surface expression of these enzymes has the potential to cause tissue destruction during inflammation.