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Interstitial pulmonary macrophages produce platelet-derived growth factor that stimulates rat lung fibroblast
A R Brody1, J C Bonner, L H Overby
1Laboratory of Pulmonary Pathobiology, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina.
Abstract:
Alveolar macrophages from humans and several animal species produce factors in vitro that modulate fibroblast growth and have been proposed as mediators of interstitial pulmonary fibrosis. Pulmonary interstitial macrophages (IMs) have not been studied previously in this regard. Pulmonary IMs were isolated from prelavaged rat lungs by enzymatic digestion of tissue and subsequent differential adherence of cells to culture dishes. The ability of IMs to release modulators of fibroblast growth into the culture medium was assessed by measuring [3H]thymidine incorporation into DNA and/or nuclear labeling of early-passage rat lung fibroblasts exposed to medium conditioned by IMs. The percentages of nuclei labeled in fibroblast cultures exposed to interstitial macrophage-conditioned medium (IMCM) alone did not significantly differ from that observed in controls, but fibroblasts exposed to IMCM supplemented with 2% platelet-poor plasma showed a 2.6-fold increase in labeling, indicating that IMCM contains predominantly "competence" growth factor activity. Similar results were obtained using purified human platelet-derived growth factor (PDGF). The level of growth factor activity released by IMs increased in cells that had phagocytized iron spheres during the culture period. In addition, fractionation of IMCM by high-performance liquid chromatography demonstrated most of the growth factor activity at a relative molecular mass of about 35 kd. Subsequent quantitative analysis of the fractions by an enzyme immunoassay for PDGF demonstrated that IMCM contains a homologue of human PDGF. These results show that IMs are capable of producing a PDGF-like growth factor for autologous fibroblasts and that release of this factor is enhanced by exposure to an insoluble inorganic particle. Because PDGF is a potent growth factor for fibroblasts and is released by IMs, it is essential to ask in future studies whether this or similar macrophage products play a significant role in mediating fibroblast proliferation in vivo.
Insights
Pulmonary interstitial macrophages (IMs) release a platelet-derived growth factor (PDGF)-like molecule that stimulates fibroblast growth. This release is enhanced by particle phagocytosis, suggesting a role in pulmonary fibrosis.
Area of Science:
- Pulmonary immunology
- Cell biology
- Fibrosis research
Background:
- Alveolar macrophages are known to produce factors that influence fibroblast growth, potentially mediating interstitial pulmonary fibrosis.
- The role of pulmonary interstitial macrophages (IMs) in this process has not been previously investigated.
Purpose of the Study:
- To investigate the capacity of rat pulmonary interstitial macrophages (IMs) to release fibroblast growth modulators.
- To characterize the nature of these growth factors and identify potential mediators of pulmonary fibrosis.
Main Methods:
- Isolation of pulmonary IMs from rat lungs via enzymatic digestion and differential adherence.
- Assessment of fibroblast growth modulation using [3H]thymidine incorporation and nuclear labeling assays on rat lung fibroblasts.
- Characterization of growth factor activity using high-performance liquid chromatography (HPLC) and enzyme immunoassay for platelet-derived growth factor (PDGF).
Main Results:
- Interstitial macrophage-conditioned medium (IMCM) demonstrated significant "competence" growth factor activity for fibroblasts, particularly when supplemented with plasma.
- The growth factor activity released by IMs increased after phagocytosis of iron spheres.
- HPLC and immunoassay revealed that IMCM contains a homologue of human PDGF, with activity concentrated around 35 kDa.
Conclusions:
- Rat pulmonary interstitial macrophages (IMs) produce and release a PDGF-like growth factor capable of stimulating autologous fibroblast proliferation.
- The release of this factor is enhanced by exposure to insoluble inorganic particles.
- These findings highlight a potential role for IM-derived PDGF in mediating fibroblast proliferation in vivo, warranting further investigation in the context of pulmonary fibrosis.