A mechanistic review of silica-induced inhalation toxicity

Abstract

Crystalline forms of silica have been proposed as positive control material for the toxicity test of inhaled particulate/fibrous matter, although mechanism of silica-induced inhalation toxicity has not yet been established. Inhalation exposure of α-quartz to rodents induces severe lung inflammation and fibrosis only after a certain period of latency, despite strong surface reactivity. The delayed occurrence of inhalation toxicity by α-quartz may be largely attributed to the sequestration of α-quartz particles by alternatively activated (M2) macrophages that express abundant levels of scavenger receptors but are relatively insensitive to inflammatory stimuli. When exposure to α-quartz continues, lung dust burden reaches a particle overload level, at which M2 macrophages cannot accommodate further quartz particles. Free quartz particles distributed in the interstitium interact with another subtype of macrophages, classically activated/inflammatory (M1) macrophages, which secrete various inflammatory cytokines, but silica-laden M1 macrophages initiate granuloma formation, which sequesters silica particles, too. Furthermore, the ability of M2 macrophages to clear foreign matter, particularly bacterial endotoxins [lipopolysaccharides (LPS)], may decrease due to α-quartz cytotoxicity. When LPS concentration in the lung reaches a certain level, LPS primes M1 macrophages to prepare for interleukin-1β production in response to α-quartz and also stimulates M1 macrophages and plasmacytoid dendritic cells (pDCs) to produce tumor necrosis factor (TNF)-α and interferon (IFN)-β, respectively. Besides, IFN-β may enhance TNF-α production in LPS-stimulated M1 macrophages. The elevated levels of inflammatory cytokines produce progressive lung inflammation and fibrosis.

• α-Quartz
• granulomas
• IL-1β
• LPS
• macrophages
• silicosis
• TGF-β
• TNF-α

Acknowledgements

The author would like to thank late Dr. Norio Funahashi for his helpful review of the manuscript.

Declaration of interest

The author's affiliation is as shown on the cover page. The author is solely responsible for the content and writing of the article.