The growing commerce in micro- and nanotechnology is expected to increase human exposure to submicrometer and nanoscale particles, including certain forms of amorphous silica. When inhaled, these particles are likely to reach the alveoli, where alveolar type II epithelial cells that are distinguished by apical microvilli are found. These cells play critical roles in the function of the alveoli and participate in the immune response to amorphous silica and other particles by releasing chemokines. The cellular interactions of the particles, which drive the cellular responses, are still unclear. Adverse effects of nanoparticles have been attributed, in part, to the unique properties of materials at the nanoscale. However, little is known about the cellular interactions of individual or small nanoparticle aggregates, mostly because of their tendency to agglomerate under experimental conditions. Here we investigate the interaction and internalization pathway of individual precipitated amorphous silica particles with specific surface properties and size, by following one particle at a time. We find that both 100 and 500 nm particles can take advantage of the actin turnover machinery within filopodia and microvilli-like structures to advance their way into alveolar type II epithelial cells. This pathway is strictly dependent on the positive surface charge of the particle and on the integrity of the actin filaments, unraveling the coupling of the particle with the intracellular environment across the cell membrane. The retrograde pathway brings a new mechanism by which positive surface charge supports particle recruitment, and potential subsequent toxicity, by polarized epithelial cells bearing microvilli.

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