The origin of pulmonary macrophages.

The controversy as to whether pulmonary macrophages derive from monocytes or from precursor cells in the pulmonary interstitium has been solved by quantitative analysis of 3H-thymidine labeling data on the circulating monocytes and the total population of pulmonary macrophages isolated from mice by lavage and enzyme digestion of lung tissue. It was found that in the normal steady state, during acute inflammatory reactions, and under long-term corticosteroid treatment, pulmonary macrophages derive mainly from circulating monocytes which enter the lungs and become pulmonary macrophages within a few hours. Local division of pulmonary macrophages made only a minor contribution to the maintenance of the population; a dividing precursor cell population located in the interstitium of the lung could not be demonstrated. The few mononuclear phagocytes dividing locally in the lung have most probably arrived recently from the circulation and originate in the bone marrow. These dividing cells already have the characteristics of pulmonary macrophages.