Ultrafine Particles Cross Cellular Membranes by Nonphagocytic Mechanisms in Lungs and in Cultured Cells

TLDR

High concentrations of airborne ultrafine particles (<0.1 µm) are linked to increased pulmonary and cardiovascular mortality, and they rapidly disseminate to organs such as liver, heart, and nervous system. The study aims to elucidate how UFPs cross boundary membranes and map their distribution within primary and secondary target tissues. In vivo, rats inhaled 22‑nm TiO₂ aerosols and lung distribution was examined 1 hr and 24 hr post‑exposure by energy‑filtering TEM; in vitro, pulmonary macrophages and red blood cells were exposed to fluorescent polystyrene beads (1 µm, 0.2 µm, 0.078 µm) and uptake was visualized with confocal microscopy. Inhaled TiO₂ particles were detected on airway lumens, throughout lung tissues, and within capillaries; in vitro, cells internalized particles via diffusion or adhesion rather than endocytosis, and the particles were not membrane‑bound, granting direct access to intracellular targets and potentially increasing toxicity.

Abstract

High concentrations of airborne particles have been associated with increased pulmonary and cardiovascular mortality, with indications of a specific toxicologic role for ultrafine particles (UFPs; particles < 0.1 microm). Within hours after the respiratory system is exposed to UFPs, the UFPs may appear in many compartments of the body, including the liver, heart, and nervous system. To date, the mechanisms by which UFPs penetrate boundary membranes and the distribution of UFPs within tissue compartments of their primary and secondary target organs are largely unknown. We combined different experimental approaches to study the distribution of UFPs in lungs and their uptake by cells. In the in vivo experiments, rats inhaled an ultrafine titanium dioxide aerosol of 22 nm count median diameter. The intrapulmonary distribution of particles was analyzed 1 hr or 24 hr after the end of exposure, using energy-filtering transmission electron microscopy for elemental microanalysis of individual particles. In an in vitro study, we exposed pulmonary macrophages and red blood cells to fluorescent polystyrene microspheres (1, 0.2, and 0.078 microm) and assessed particle uptake by confocal laser scanning microscopy. Inhaled ultrafine titanium dioxide particles were found on the luminal side of airways and alveoli, in all major lung tissue compartments and cells, and within capillaries. Particle uptake in vitro into cells did not occur by any of the expected endocytic processes, but rather by diffusion or adhesive interactions. Particles within cells are not membrane bound and hence have direct access to intracellular proteins, organelles, and DNA, which may greatly enhance their toxic potential.