The ontogeny of pulmonary alveolar macrophages in parabiotic mice.

A bilateral distribution of labeled blood monocytes was achieved by unilaterally labeling donor members of surgically parabiosed B6D2F1/J mice with tritiated thymidine (3HTdR). Labeled pulmonary alveolar macrophages (PAM) were found in lung lavage fluid of donors. By contrast, no labeled PAM were found in lung lavage fluid from unstimulated recipients, even 12 days after labeling. Contralateral 3HTdR flash labeling showed that in recipients 3%, or about 10(4), PAM were in S phase of the cell cycle. Contralateral intratracheal challenge of recipient members with either Corynebacterium parvum vaccine or with Salmonella typhimurium lipopolysaccharide induced the emigration of labeled blood monocytes of donor origin into recipient alveoli. These data show that in parabiotic mice, blood monocytes do not regularly enter the alveolar compartment. It may be concluded that monocytes are not needed on a daily basis to sustain PAM populations.