The Cytokinetic Behavior of Pulmonary Alveolar Macrophages in Monocytopenic Mice

The cytokinetic behavior of pulmonary alveolar macrophages (PAM) was studied by pulse labeling with 3HTdR in mice made monocytopenic by a single intravenous injection of the bone‐seeking isotope strontium‐89 (89Sr). In the presence or absence of blood monocytes, PAM population size was unchanged for up to 1 month of chronic, severe monocytopenia. Pulse‐labeling studies performed during monocytopenia show that in control mice PAM population half‐times were 17.8 days with a potential doubling time of 38 days, whereas T½ was 14.8 days with a 28.5 day population doubling time for PAM in 89Sr‐treated mice. Analysis of the halving times of the PAM mean grain count and the halving times of the most highly pulse‐labeled cohorts suggested that PAM cell cycle times (Tc) were 5.1 days with a PAM rate of disappearance of 10.8%/day in 88Sr‐treated mice and Tc of 6.6 days with a PAM rate of disappearance of 11.4%/day in 89Sr‐treated mice. As measured by 3HTdR‐labeling techniques, these cytokinetic values are in close approximation to each other, suggesting that 89Sr treatment did not significantly alter either PAM population size or cytokinetic behavior. Employing experimental values it was possible to construct a simple model of PAM population growth that supports the concept that the PAM population is self‐renewing in the adult mouse. Taken together, the data show that a major portion of the resident PAM need not depend on the daily influx of peripheral blood monocytes to maintain themselves in a kinetically steady state.