Silicosis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Aparna Vuppala, M.B.B.S. [2]

Overview

The toxicity of crystalline silica results from the ability of crystalline silica surfaces to interact with aqueous media, to generate oxygen radicals, and to injure target pulmonary cells such as alveolar macrophages. Generation of inflammatory cytokines (eg, interleukin-1 and tumor necrosis factor beta) by target cells results in cytokine networking between inflammatory cells and resident pulmonary cells, which in turn leads to inflammation and fibrosis.

Pathophysiology

Pathogenesis

• The toxicity of crystalline silica appears to result from the ability of crystalline silica surfaces to interact with aqueous media, to generate oxygen radicals, and to injure target pulmonary cells such as alveolar macrophages.
• Generation of inflammatory cytokines (eg, interleukin-1 and tumor necrosis factor beta) by target cells results in cytokine networking between inflammatory cells and resident pulmonary cells, which in turn leads to inflammation and fibrosis.
• The alveolar macrophages are implicated as the major cell type in fibrogenesis, but other immune cells, namely neutrophils, T-lymphocytes, and mast cells are also involved.
• Following the interaction between effector immune cells (such as alveolar macrophage) and target tissue (such as bronchiolar/alveolar epithelial cells, fibroblasts), the progression of the disease is poorly understand.
  • Injury to the alveolar type I epithelial cell is regarded as an early event in fibrogenesis followed by hyperplasia and hypertrophyof type II epithelial cells.
  • Silica-induced cell hyperproliferation of mesenchymal cells is also a hallmark of the fibrotic lesion.
  • Proliferation may occur intially at sites of accumulation of inhaled minerals, but later at distal sites where particles or fibers are translocated over time.
  • Alternatively, mitogenic cytokines may mediate signaling events, leading to cell replication at sites physically remote from fibers.
  • The initiation of proliferation in epithelial cells and fibroblasts by silica may occur following the upregulation of the early response proto-oncogenes C-FOS, C-JUN, and C-MYC.
  • Increased expression of early response genes and protein products is also linked to the development of apoptosis

Low Intensity Exposure vs. High Intensity Exposure

• Lower intensity exposures to silica evoke reversible inflammatory changes characterized by focal aggregations of mineral-laden alveolar macrophages.
• In contrast, higher exposures elicit intense and protracted inflammatory changes, cell proliferation in various compartments of the lung, and excessive deposition of collagen and other extracellular matrix components by mesenchymal cells.