Sarcoidosis: can a murine model help define a role for silica?

Both genetic and environmental factors are thought to play a role in the etiology of sarcoidosis. An association of the condition with exposure to environmental microbes has been recognized for many years, and has become stronger in the last 10-15 years with the advent of newer investigative techniques. A body of literature now is accumulating suggesting that silica may be yet another trigger in genetically predisposed persons. Impressive support for an etiologic role of mycobacteria derives from earlier studies by several investigators in Japan and in Europe and more recently from the US in Baltimore and Nashville. Other investigators have produced evidence that propionibacteria and fungi can also act as environmental triggers in sarcoidosis patients. We propose that, in an animal model that has been previously sensitized to microbial antigens, the introduction of silica through the gastrointestinal route, or intravenously, may have a granuloma-worsening effect, if the strain of animals is already predisposed to develop granulomatous disease. Here the silica may add a "second hit" to the "first hit" given by the exposure to microbial antigens.