Factors influencing fetal macrophage development: II. Effects of the PDGF subfamily of protein-tyrosine kinase receptor ligands as studied in organ-cultured rat lungs.
Macrophage precursors in pseudoglandular rat lungs rapidly differentiate into phagocytes in organ culture, although this occurs only gradually in vivo. Macrophage colony-stimulating factor is vital for the process, but the possible importance of other ligands in the platelet-derived growth factor (PDGF) subfamily is scarcely appreciated. Macrophage development was compared in 15-day fetal rat lungs cultured on solid, serum-containing media with and without added stem cell factor (SCF) (100 ng/mL) or antibodies to PDGF-AA and -BB (10-15 micrograms/mL each). In addition, organ cultures and intact lungs were immunostained for PDGF-AA and -BB to confirm their presence in the tissues. Macrophage population growth was measured by coronal area assay. SCF initially stimulated macrophage production. Thereafter, results varied depending on baseline production by control cultures: where this was vigorous, SCF-exposed explants performed similarly; where this was moderate, the SCF explants outperformed them 1.5-2.6 times over (P < 0.01-0.001). Inhibition of macrophage production by pyrrolidine dithiocarbamate (100 microM) was not significantly diminished in the presence of SCF (10 ng/mL). Immunoreactivity for PDGF-AA and -BB was prevalent in cells of the airway epithelium and stroma during the period macrophage precursors were converting, and both isoforms were detected in differentiating macrophages as early as 2 days in vitro. Nonetheless, exposure of cultures to anti-PDGFs had no significant effect on macrophage population growth. Ligands of the PDGF subfamily differ greatly in their influence over development of fetal macrophages. Whereas the PDGFs are ineffective, SCF stimulates growth of macrophage precursors and early differentiating forms and enhances survival of older cells. It appears to act mainly in synergy with other growth factors present in fetal lungs. Furthermore, in the hierarchy of hematopoietic progenitors, the macrophage precursors may be ranked on a par with burst-forming units in the red cell lineage.