Pulmonary Macrophages Can Stimulate Cell Growth of Bovine Bronchial Epithelial Cells

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Abstract
Macrophages are thought to participate in tissue repair following injury by releasing growth factors into the local environment. To evaluate whether pulmonary macrophages can mediate airway epithelial repair, we attempted to determine if pulmonary macrophages can stimulate growth of bovine bronchial epithelial cells in vitro. Bronchial epithelial cells isolated by protease digestion of the bovine bronchi were plated into tissue culture dishes with and without macrophage-conditioned medium. Bronchial epithelial cells cultured with macrophage-conditioned medium showed a significantly greater cell growth than those without macrophage-conditioned medium when assessed by direct enumeration of the cell numbers and by clonal growth assay. Stimulation of proliferation was confirmed by autoradiography using [3H]thymidine uptake into cell nuclei. Co-culture of pulmonary macrophages with bronchial epithelial cells also led to an increase in cell number. Immunohistochemical staining of the proliferating cells showed that these cells were positively stained by anti-keratin antibodies confirming that they were bronchial epithelial cells. Partial characterization of the activity in macrophage-conditioned medium showed that it was nondialyzable, pepsin- and acid-labile, and lipid-inextractable. Sephadex G-75 column fractionation indicated this activity existed in a high molecular fraction, thus suggesting a peptide. DEAE ion exchange chromatography revealed 3 peaks of stimulating activity. One peak resulted in a decrease in cell number, suggesting a possible inhibitory activity. The DEAE results thus suggest that macrophages may release several factors that can affect bronchial epithelial cell proliferation. In conclusion, pulmonary macrophages stimulate cell proliferation of bronchial epithelial cells in vitro. The stimulatory activity that may be heterogeneous appears to have the properties of a peptide.