In vitro effects on macrophages induced by noncytotoxic doses of silica particles possibly relevant to ambient exposure

Abstract

The RAW 246.7 macrophage cell line was exposed in vitro to aged crystalline silica particles of respirable size for 24 h at a range of doses starting from 15 μg/2×10 6 cells, which is a realistic exposure level of macrophages in the airways of ambiently exposed individuals. The particle sample used for the experiments was prepared to mimic some aspects of ambient crystalline silica particles: size distribution, morphology, and surface reactivity. Our purpose was to determine whether a nontoxic quartz load comparable to that of ambient exposure would be able to induce macrophage activation and impairment of the phagocytic ability, factors altering the lung's capacity to deal with increased particle loads (as occurs during high-pollution episodes) or infections and affecting the local and systemic responses through the release of biologically active compounds (cytokines, reactive oxygen species, NO, isoprostanes). Exposure of RAW 264.7 cells to aged silica particles induced macrophage activation (evidenced by the morphological features observed with scanning electron microscopy and by the release of TNF- α and IL-6) and impairment of phagocytosis of test particles, even at noncytotoxic doses. The reduction of the phagocytic function of the cells after silica treatment was dose-dependent, as evidenced by an increase of the population of unphagocytic cells, paralleled by a decrease of the actively phagocytizing cell population. We evaluated the oxidative stress induced by aged silica particles, quantifying the peroxidation products (8-isoprostanes) in the culture media of treated cells, and found a strong release at low doses. Isoprostanes are a complex family of compounds which have been used as in vivo markers of lipid peroxidation in human disorders, but that, as far as we know, have never been evaluated in relation to airborne particulate matter exposure. Lipid peroxides are involved in various cellular events in the inflammatory response, and isoprostanes are also supposed to exert important biological actions on airway and pulmonary vascular smooth muscles and on platelets.