Activation of murine macrophages by silica particles in vitro is a process independent of silica-induced cell death.

We have tested the murine macrophagic cell line RAW 264.7 for its ability to undergo activation after exposure to silica particles in vitro. When exposed to silica under controlled conditions (each cell having access to about 10 silica particles), RAW 264.7 cells were able to phagocytose the particles. Concomitantly, there was a significant increase in tumor necrosis factor alpha (TNF alpha) mRNA accumulation and TNF alpha secretion. The level of TNF alpha production by RAW 264.7 cells increased up to 5-fold 48 h after phagocytosis of silica particles with very low cell toxicity. The phagocytic stimulus did not induce nitric oxide production. When cells were exposed to a higher number of silica particles, cell activation was attained at shorter times but a substantial number of cells were damaged at 48 h. Interferon gamma (IFN gamma) alone induced an increased production of TNF alpha in RAW 264.7 cells, not further augmented by a subsequent exposure to silica of the IFN gamma-treated cells. Other macrophage-like cell lines as well as primary peritoneal macrophages were able to phagocytose silica particles but showed different abilities to produce and secrete TNF alpha once phagocytosis took place. Therefore, RAW 264.7 cells were chosen as a model for in vitro studies of the long-term response of macrophages to silica.