Cathepsin G enhances human natural killer cytotoxicity

Cathepsin G is a serine protease located in the azurophil granules of neutrophils. In this study, we investigated the effect of cathepsin G on the functions of human natural killer (NK) cells in vitro. Cathepsin G enhanced NK cytotoxicity rapidly in a dose-dependent fashion. The ability to augment NK cytotoxicity was markedly reduced in the presence of the inhibitor, phenylmethanesulphonyl fluoride (PMSF) or chymostatin, demonstrating that the proteolytic activity of cathepsin G is essential for the induction of NK cytotoxicity. Granulocyte exocytosis is required for NK cell-dependent target killing. Cathepsin G induced the release of the granule enzyme, N-acetyl-beta-D-glucosaminidase, from human NK cells. Moreover, an increase in the cytosolic-free Ca2+ concentration was observed in NK cells after stimulation with cathepsin G. When human granulocytes were stimulated with cytochalasin B and N-formyl-methionyl-leucyl-phenylalanine (fMLP), cathepsin G was released. The cathepsin G released from granulocytes also caused enhancement of NK cytotoxicity. In the presence of serine protease inhibitor the supernatant including cathepsin G obtained from stimulated granulocytes did not enhance NK cytotoxicity, but the stimulated granulocytes did. Highly purified human NK cells treated with cathepsin G enhanced NK cytotoxicity, but NK-depleted lymphocytes did not, demonstrating that cathepsin G regulates NK cytotoxicity independently of other factors. We have shown recently that human cathepsin G binds to human NK cells. These combined data indicate that cathepsin G released from granulocytes binds to NK cells and augments NK cytotoxicity through its protease activity.