Inhibition of human eosinophil activation by a cysteinyl leukotriene receptor antagonist (pranlukast; ONO-1078).

Eosinophils produce cysteinyl leukotrienes such as leukotriene C4 and D4 upon stimulation by platelet-activating factor or other mediators, and these cells themselves express cysteinyl leukotriene receptors. Pranlukast, a compound developed in Japan, antagonizes cysteinyl leukotriene receptors and inhibits contraction of airway smooth muscle, microvascular leakage into airways, and eosinophil infiltration. This agent can decrease symptoms of bronchial asthma, but its specific influences on effector functions of eosinophils important to the pathogenesis and exacerbation of asthma remain unknown. In the present study, we investigated the effect of pranlukast on human eosinophil functions. Eosinophils obtained from peripheral blood of normal volunteers were stimulated by platelet-activating factor, leukotriene D4, or phorbol ester. Superoxide anion generation was measured by reduction of cytochrome c. Expression of alphaMbeta2 was analyzed by flow cytometry. To evaluate eosinophil degranulation, eosinophil protein X, a toxic granule constituent, was measured by radioimmunoassay in sample supernatants. Pranlukast partially inhibited major eosinophil effector functions of superoxide anion generation and degranulation induced by platelet-activating factor, although at concentrations tested pranlukast failed to significantly reduce platelet-activating factor-induced alphaMbeta2 expression. Pranlukast completely inhibited leukotriene D4-induced superoxide generation and alphaMbeta2 expression. In contrast, pranlukast at 10(-6)M did not affect phorbol ester-induced superoxide generation at 120 minutes, degranulation, or alphaMbeta2 expression. The results suggested that inhibition by pranlukast of platelet-activating, factor-induced eosinophil effector functions such as superoxide generation and degranulation might result at least partly from antagonism of autocrine mechanisms involving cysteinyl leukotrienes produced in response to platelet-activating factor.