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Enhanced macrophage-fibroblast interactions in the pulmonary interstitium increases fibrosis after silica injection
I Y Adamson1, H L Letourneau, D H Bowden
1Department of Pathology, University of Manitoba, Winnipeg, Canada.
Abstract:
The role of interstitial vs. alveolar macrophages in the generation of pulmonary fibrosis after silica was examined. Using whole body irradiation to delay the inflammatory response and so retard particulate clearance, many more instilled silica particles reached the interstitial macrophages in the first 2 weeks than after silica alone. This was followed by greatly increased fibroblast proliferation and deposition of collagen in the irradiation plus silica group, which developed large interstitial granulomas at the sites of silica retention. Although alveolar macrophages containing silica were seen in both silica groups, more interstitial particles were observed after combined irradiation and silica, significantly more silica was recovered in a residue from the lungs at 16 weeks, and pulmonary fibrosis at 8-16 weeks was greater than in all other groups. The results indicate that increased fibroblast growth and collagen synthesis in vivo are associated with phagocytosis of silica by interstitial macrophages rather than by free alveolar macrophages. It is suggested that transfer of a macrophages-derived growth factor to fibroblasts is more efficient when it occurs within the pulmonary interstitium.
Insights
Interstitial macrophages, not alveolar, drive silica-induced pulmonary fibrosis. Targeting these cells could offer new treatments for lung scarring and interstitial granulomas.
Area of Science:
- Pulmonary Medicine
- Immunology
- Toxicology
Background:
- Silica exposure is a known cause of lung disease.
- The specific roles of different macrophage populations in silica-induced pulmonary fibrosis remain unclear.
Purpose of the Study:
- To investigate whether interstitial or alveolar macrophages are primarily responsible for initiating pulmonary fibrosis following silica exposure.
- To elucidate the cellular mechanisms underlying silica-induced lung scarring.
Main Methods:
- Whole-body irradiation was used to delay inflammatory responses and particulate clearance.
- Silica instillation was performed in irradiated and non-irradiated control groups.
- Fibroblast proliferation, collagen deposition, and silica particle localization in lung tissues were assessed at various time points.
Main Results:
- Irradiation significantly increased silica particle retention by interstitial macrophages.
- Combined irradiation and silica exposure led to markedly increased fibroblast proliferation and collagen deposition.
- Pulmonary fibrosis was significantly greater in the group with enhanced interstitial macrophage silica uptake.
Conclusions:
- Phagocytosis of silica by interstitial macrophages, rather than alveolar macrophages, is strongly associated with increased fibroblast proliferation and collagen synthesis.
- Macrophage-derived growth factor transfer to fibroblasts may be more efficient within the pulmonary interstitium, driving fibrosis.