Live-cell imaging and in vitro assays study of macrophage phagocytosis of mineral fibers

The interaction between inhaled mineral fibres and alveolar macrophage provokes many adverse effects including cell damage. The mechanisms by which macrophage phagocytosis of mineral fibres exert cytotoxic activity are not fully understood. Our work focuses on the monitoring of the early steps of the interaction between chrysotile, crocidolite and erionite fibres and M0-THP-1 macrophages (8 h of exposure to 25 μg/mL of fibres) using time-lapse video microscopy (TLM) coupled with in vitro assays. TLM movies showed that macrophages easily phagocytose erionite fibres (mean L<10 µm) while numerous chrysotile and crocidolite fibres are partially phagocytosed due to their extreme length (mean L> 10 µm). The cytotoxic action of fibrous erionite occurs rapidly (2 h) while that of asbestos increases gradually. Asbestos fibres trigger significant apoptotic phenomena while fibrous erionite is associated with a necrotic-like effect. Asbestos toxicity is linked to their ability to stimulate production of Reactive Oxygen Species (ROS). ROS are generated by iron on the fibre surface (primary) or by M0-THP-1 cells as a result of frustrated phagocytosis induced by the long asbestos fibres (secondary). An alternative mechanism by which fibrous erionite can induce cytotoxicity may occur during phagocytosis due to the absorption of ions present in the cytosol of cells leading to dysregulation of ion homeostasis, swelling and cell lysis. At the same time, engulfed erionite fibres can reduce the level of cytosolic Ca2+ and interfere with endoplasmic reticulum-mitochondria crosstalk causing failure of M0-THP-1 cells apoptosis.