Inhibition of neutrophil oxidative burst and phagocytosis by meconium

OBJECTIVE: Meconium in amniotic fluid has been associated with an increased prevalence of chorioamnionitis. In an effort to delineate the mechanism of this association, we determined the effect of meconium on the neutrophilʹs capacity for phagocytosis and microbial killing by oxidative burst in vitro. STUDY DESIGN: Sterile meconium samples were obtained from four fetuses at the time of breech delivery and were then pooled and lyophilized. Neutrophils were purified from whole blood of each of 13 pregnant nonlaboring patients. Phagocytosis and the oxidative burst of neutrophils in the presence and absence of meconium were assessed by single-cell analysis with flow cytometry. Phagocytosis was measured as the mean fluorescence intensity produced after 30 minutes of incubation with fluorescein-labeled Escherichia coli. Oxidative burst was measured as the mean fluorescence intensity resulting from the oxidation of internalized reduced dichlorodihydrofluorescein after 15 minutes of stimulation with phorbol myristate acetate. Oxidative burst was expressed as the neutrophil oxidative index and the net fluorescence intensity. Neutrophil oxidative index was equivalent to the quotient of the mean fluorescence intensity for phorbol myristate acetate-stimulated and unstimulated cells. Net fluorescence intensity was equivalent to the absolute difference between stimulated and unstimulated cells. RESULTS: Exposure of neutrophils to light and very light meconium each resulted in significantly lower mean neutrophil oxidative index compared 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 lower mean net fluorescence intensity than that of control cells (112 ± 220 and 188 ± 294 vs 613 ± 328, p = 0.0001 and p = 0.005, respectively). Phagocytosis was significantly impaired in the presence of moderate meconium compared with control cells (2239 ± 393 vs 4645 ± 2071, p = 0.0001). Light meconium did not significantly affect phagocytosis. CONCLUSION: Meconium has significant effects on neutrophil function in vitro. Both light and very light meconium inhibit the oxidative burst. Moderate meconium inhibits phagocytosis.