Maintenance of the normal rat alveolar macrophage cell population. The roles of monocyte influx and alveolar macrophage proliferation in situ.

We investigated the relative importance of monocyte influx and alveolar macrophage proliferation in maintenance of the alveolar macrophage population in normal rats. Two experimental approaches were used. To detect possible monocyte influx, we radiolabeled blood monocytes by a continuous infusion of tritiated thymidine for 7 days and assayed lavaged alveolar macrophages for radiolabel by autoradiography. Although heavily labeled monocytes occurred in the blood, no heavily labeled cells were detectable among lavaged alveolar macrophages by cohort analysis. To evaluate macrophage proliferation in vivo, we injected normal unlabeled rats with colchicine and examined lavaged alveolar macrophages for arrested mitoses. Mitotic figures were observed among lavaged alveolar macrophages, and calculations at serial times after colchicine injection indicated that 1.83% of alveolar macrophages entered mitosis each day. On the basis of these 2 lines of evidence, we conclude that the alveolar macrophage cell population in normal rats is supported mainly by cellular proliferation in situ rather than direct monocyte influx from the blood compartment.