INHIBITION OF NEUTROPHIL OXIDATIVE BURST AND PHAGOCYTOSIS BY MECONIUM

OBJECTIVE: Meconium in amniotic fluid has been associated with an incr eased prevalence of chorioamnionitis. In an effort to delineate the me chanism of this association, we determined the effect of meconium on t he neutrophil's capacity for phagocytosis and microbial killing by oxi dative burst in vitro. STUDY DESIGN: Sterile meconium samples were obt ained from four fetuses at the time of breech delivery and were then p ooled and lyophilized. Neutrophils were purified from whole blood of e ach of 13 pregnant nonlaboring patients. Phagocytosis and the oxidativ e burst of neutrophils in the presence and absence of meconium were as sessed by single-cell analysis with flow cytometry. Phagocytosis was m easured as the mean fluoresence intensity produced after 30 minutes of incubation with fluorescein-labeled Escherichia coli. Oxidative burst was measured as the mean fluorescence intensity resulting from the ox idation of internalized reduced dichlorodihydrofluorescein after 15 mi nutes of stimulation with phorbol myristate acetate. Oxidative burst w as expressed as the neutrophil oxidative index and the net fluorescenc e intensity. Neutrophil oxidative index was equivalent to the quotient of the mean fluorescence intensity for phorbol myristate acetate-stim ulated and unstimulated cells. Net fluorescence intensity was equivale nt to the absolute difference between stimulated and unstimulated cell s. RESULTS: Exposure of neutrophils to light and very light meconium e ach resulted in significantly lower mean neutrophil oxidative index co mpared with unexposed controls (3.2 +/- 4.9 and 4.2 +/- 5.9 vs 16.2 +/ - 7.5, p = 0.00002 and p = 0.0007, respectively) and significantly low er mean net fluorescence intensity than that of control cells (112 +/- 220 and 188 +/- 294 vs 613 +/- 328, p = 0.0001 and p = 0.005, respect ively). Phagocytosis was significantly impaired in the presence of mod erate meconium compared with control cells (2239 +/- 393 vs 4645 +/- 2 071, p = 0.0001). Light meconium did not significantly affect phagocyt osis. CONCLUSION: Meconium has significant effects on neutrophil funct ion in vitro. Both light and very light meconium inhibit the oxidative burst. Moderate meconium inhibits phagocytosis.