Human Neutrophils Store Type II 14-kDa Phospholipase A2 in Granules and Secrete Active Enzyme in Response to Soluble Stimuli

Abstract Although "secretory" type II 14-kDa phospholipase A 2 (sPLA 2 ) activity has been described in neutrophils, direct evidence of enzyme secretion has been elusive. We have used immunogold electron microscopy with polyclonal and monoclonal antibodies to sPLA 2 to demonstrate localization of the enzyme to granules of resting human neutrophils and translocation to phagolysosomes. Soluble stimuli such as calcium ionophore A23187 stimulate loss of cell-associated enzymatic activity. Supernatant fluids from stimulated neutrophils lack measurable PLA 2 but contain proteases which inactivate exogenous sPLA 2 . The use of alpha-1-antitrypsin as a protease inhibitor permitted this first demonstration of secretion of PLA 2 activity from stimulated human neutrophils.