Enhanced proliferation of pulmonary alveolar macrophages after carbon instillation in mice depleted of blood monocytes by strontium-89.

The purpose of this study was to evaluate the potential of pulmonary alveolar macrophage (PAM) and interstitial cell division as local sources of PAM in the absence of migratory monocytes. Male B6C3F1 mice were treated with Strontium-89 to deplete the levels of circulating monocytes. Normal and irradiated mice were then instilled intratracheally with carbon particles to increase the demand for PAM. Dividing cells were labeled with tritiated thymidine. The proportion of tritiated thymidine-labeled cells was then determined in lavaged PAM and in IC of lung tissue as an indicator of the proliferative response associated with the demand for increased number of PAM. It was hypothesized that, in the absence of circulating monocytes as a source of PAM, cell division of the local source of PAM would be greater in the monocyte-depleted mice than in the normal mice. It was found that proliferation of lavaged PAM was 112% greater in the monocyte-depleted mice, whereas proliferation of IC in the monocyte-depleted mice was 45% less than that in the normal mice. It was concluded that: cell proliferation of PAM is a significant local source of PAM, and division of cells in the interstitium could not be evaluated as a source of PAM due to the heterogenous nature of the cell types which comprise this population of cells. This study established that cell division of "free" PAM can be a significant local source of PAM and raises questions concerning the nature of the interstitial source of PAM.