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Role of polymorphonuclear leukocytes in silica-induced pulmonary fibrosis
Abstract:
Silicosis is usually attributed to fibroblast stimulation by secretion of damaged alveolar macrophages (AMs), but the role of polymorphonuclear leukocytes (PMNs) and of continuing cell injury in the pathogenesis has not been fully studied. Mice given intratracheal injections of 2 mg of silica received 3H-thymidine 1 hour before death at intervals to 20 weeks. Cellular populations and lysosomal content of lavage fluids were correlated with morphology, DNA synthesis, and collagen content of the lung. The initial response involved rapid PMN and AM recruitment to the alveoli. Some free particles crossed Type 1 epithelial cells, and silica was found in interstitial macrophages. Focal Type 1 cell damage was rapidly repaired by Type 2 cell proliferation. Although PMN numbers dropped after a few days, they never reached control levels and rose again after 8 weeks; the number of AMs fell to control values from 2 to 8 weeks, then increased again. Glucosaminidase and glucuronidase levels in the lavage fluid were much higher than control levels throughout the study. Increased DNA synthesis by interstitial cells occurred from 2 days to 20 weeks; increased collagen synthesis was found from 4 weeks onward. The continuing inflammatory response of the lung to silica suggests may contribute to fibroblastic stimulation.
Insights
This study reveals that ongoing lung inflammation and cell injury, involving polymorphonuclear leukocytes (PMNs) and alveolar macrophages (AMs), contribute to silicosis pathogenesis. Continued inflammatory responses in the lung suggest a role in fibroblast stimulation.
Area of Science:
- Pulmonary Medicine
- Toxicology
- Cell Biology
Background:
- Silicosis pathogenesis is primarily linked to fibroblast stimulation by damaged alveolar macrophages (AMs).
- The roles of polymorphonuclear leukocytes (PMNs) and persistent cellular injury in silicosis remain incompletely understood.
Purpose of the Study:
- To investigate the roles of PMNs and continuing cell injury in silicosis development.
- To correlate lung cellular responses and inflammatory markers with disease progression.
Main Methods:
- Mice received intratracheal silica injections and were analyzed up to 20 weeks.
- Cellular populations, lysosomal enzyme levels, DNA synthesis, and collagen content in lung lavage fluids and tissues were measured.
Main Results:
- Silica exposure triggered rapid PMN and AM recruitment, with particles crossing epithelial cells.
- Persistent PMN elevation and fluctuating AM levels were observed, alongside elevated lysosomal enzymes.
- Increased interstitial cell DNA synthesis and collagen production occurred throughout the study period.
Conclusions:
- Continuing inflammatory responses, involving PMNs and AMs, are implicated in silicosis.
- Ongoing lung cell injury and inflammation likely contribute to fibroblast stimulation and disease progression.