Effect of lung surfactant on the release of factor increasing monocytopoiesis by macrophages

Upon ingestion of particulate and soluble material, at the site of an inflammation macrophages release the factor increasing monocytopoiesis (FIM), which accelerates the rate of division of the monoblasts and promonocytes in the bone marrow. It is not known, however, whether FIM is released by macrophages present at noninflamed sites. Since FIM is secreted only during phagocytosis and alveolar macrophages ingest surfactant in vivo, the present study was performed to find out whether surfactant induces the release of FIM by alveolar macrophages. Resident alveolar macrophages were found to contain FIM and secrete this factor in vitro in the absence of an introduced phagocytable particle. Resident peritoneal macrophages also contained FIM and released this factor after exposure to surfactant. These findings suggest that in the absence of an inflammatory stimulus in vivo, alveolar macrophages that have ingested surfactant release FIM to maintain the normal production of monocytes in the bone marrow.