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.