Involvement of lysosomal proteins in morphology-driven toxicity of (nano)fibers

Fiber morphology (length > 5 μm; respirable diameter < 3 μm) and biopersistence have been linked to their potential to cause fibrosis, lung cancer, and malignant pleural mesothelioma. Among the mechanisms involved, frustrated phagocytosis occurs when macrophages attempt but fail to fully internalize and clear long rigid fibers. Although nanofibers could meet these criteria and trigger frustrated phagocytosis, their small diameters may enable them to entangle, causing them to lose their fiber-like morphology and affecting their toxicological potential. The toxicological assessment of (nano)fibers relies on animal studies; therefore, there is an urgent need to establish in vitro alternatives. Carbon nanotubes, the most commercially prevalent class of nanofibers, have been extensively investigated, and some have demonstrated pathogenic potential, by causing inflammation initiated by cathepsin B translocation from the lysosomes into the cytosol. Independent studies have indicated that only long and rigid carbon nanofibers lead to a decrease of lysosomal enzymes (including multiple cathepsins) inside macrophages and increased levels in the extracellular environment. Thus, different roles for cathepsin B have been proposed in response to nanofiber exposure. To reconcile these observations, this review examines the underlying mechanisms by assessing in vitro studies, particularly how in vitro macrophages respond to carbon-based nanomaterials of distinct morphologies, discusses the limitations of current in vitro models, and evaluates potential approaches for assessing nanofiber toxicity.