Effect of the shape of mica particles on the production of tumor necrosis factor alpha in mouse macrophages.

OBJECTIVES

Micas are thin, sheetlike minerals that are abundant in rocks and sand. They are common components of resuspended road dust in spring in Finland. The proinflammatory potential of mica particles was studied.

METHODS

Respirable-size particles were prepared from phlogopite mica either by milling (milled phlogopite) or water elutriation (elutriated phlogopite). The cations were extracted with acids from one phlogopite sample (acid-treated phlogopite). Minusil (alpha-quartz) and titanium dioxide were used as positive and negative controls, respectively. Cultured mouse macrophage cells (RAW264.7) were exposed for 24 hours to the mineral particles at concentrations between 10 and 500 microg/ml. The mineral particle concentration of 100 microg/ml and the time range from 3 to 48 hours were used to study the time-dependency of the responses. Cell viability and the production of nitric oxide and proinflammatory cytokines [interleukin-6 (IL-6), tumor necrosis factor alpha (TNF-alpha)] were studied in the macrophages.

RESULTS

Elutriated phlogopite particles were larger, and they had a smaller surface area and a more regular, plate-like shape than milled phlogopite. The mineral particles showed the following order of potency to induce TNF-alpha production in macrophages: elutriated phlogopite > Minusil > acid-treated phlogopite > milled phlogopite > titanium dioxide. Only elutriated phlogopite induced a small, dose-dependent IL-6 response. The mineral particle samples did not induce nitric oxide production in the macrophages.

CONCLUSIONS

Phlogopite mica stimulates macrophages to produce proinflammatory cytokines. The platelike shape of mica particles rather than surface area seems to be important for cytokine production.