Effects of PAF, FMLP and opsonized zymosan on the release of ECP, elastase and superoxide from human granulocytes

Platelet-activating factor (PAF) is a potent chemoattractant for human eosinophils and neutrophils and causes eosinophil and neutrophil recruitment into animal airways. Since eosinophils and eosinophil cationic proteins are thought to play an important role in the pathophysiology of asthma, we have examined the hypothesis that PAF may also stimulate eosinophil cationic protein (ECP) release from human granulocytes. Granulocytes (93% neutrophils, 3% eosinophils) were isolated from the blood of normal volunteers, using metrizamide density gradients, and stimulated in vitro with PAF, L-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) or opsonized zymosan (OPZ). Superoxide generation was measured colorimetrically, granulocyte degranulation by a fluorimetric assay for elastase, and eosinophil activation by specific radioimmunoassay (RIA) for ECP. Granulocyte chemotaxis was also measured. Whilst both PAF and FMLP were potent chemoattractants for human mixed granulocytes (concentrations producing half the maximal effect (EC50s) ca 10 nM), PAF at concentrations below 10 microM was a poor stimulus to superoxide generation, elastase release or ECP release from the same cell population. In contrast, FMLP was a potent stimulus to both superoxide generation (EC50 48 nM) and ECP (EC50 ca 100 nM) and elastase release (EC50 ca 1 microM). OPZ was a potent stimulus to superoxide generation, but was a poor stimulus to ECP or elastase release. Thus, although PAF is a potent chemoattractant for human granulocytes, our results suggest that it alone may not stimulate their subsequent activation and release of cytotoxic products.