Phagocytosis-associated oxidative metabolism in human milk macrophages

Evidence exists that human milk macrophages, which are the most abundant cells in milk, play an important role in the protection of the newborn infant against infection. We investigated the oxidative metabolism of milk macrophages by measuring luminol-dependent chemiluminescence, generation of superoxide anion (O2-) and production of hydrogen peroxide (H2O2) in the resting state and after stimulation with opsonized zymosan or phorbol myristate acetate (PMA). Generally, macrophages generated luminol-dependent luminescence, O2- and H2O2 with either stimulus to a similar extent as did blood monocytes. In addition, macrophages killed Escherichia coli and Staphylococcus aureus as effectively as did blood monocytes (approximately 75%, 120 min). Acidification of macrophages in milk (pH 1, 30 min) only slightly reduced their PMA-stimulated production of oxygen radicals. Bacterial killing by macrophages preincubated at pH 1 was about 70% that from controls maintained at pH 7. When macrophages were cultured for several days in endotoxin-free medium, their ability to produce oxygen metabolites declined. By continuous treatment with bacterial LPS (10 ng/ml), milk-macrophages could be "primed" to release large amounts of oxygen intermediates. In addition, the O2- response of macrophages cultured without bacterial products could be partially restored by the addition of LPS to the culture. We conclude that milk macrophages are capable of releasing large amounts of oxygen metabolites, and could contribute to the protection of neonates against bacterial and fungal infections.