On testing the activity of proteases from human polymorphonuclear neutrophils on blood smears

A cytochemical method is presented for the demonstration of proteases in human polymorphonuclear (PMN) neutrophils on fixed blood smears. This new technique is based on solubilization of proteases from PMN neutrophils by incubation with 0.25 M NaCl in borate buffer at pH 8.5 which leads to degradation of erythrocytes and plasma in a disclike zone (halo) around centrally situated PMN neutrophils, an effect that is visualized by staining smears using a modified colloidal iron reaction. Halo formation is inhibited by trypsin inhibitors of soya-bean as well as of chicken egg white mucoid and by phenylmethyl-sulfonylfluoride.