When Nanoparticles Get in the Way: Impact of Projected Area on In Vivo and In Vitro Macrophage Function

Previous reports by others establish that particle surface area is related to a change in macrophage function as measured by the ability to clear particles from the alveolar spaces. However, for nanoparticles the relation may not be strictly due to surface chemistry: The cumulative projected area of the particles may reflect the degree to which the inner or outer surface of the macrophage is shielded from other objects or molecules. We apply this alternative interpretation to in vitro measurements of macrophage uptake of 26-nm-diameter fluorescent beads and to in vivo data presented in a classic inhalation toxicology paper on nano-sized TiO₂ particles. In their paper, Oberdörster et al. (Environ. Health Perspect. 102[suppl. 5]:173–179, 1994) reported that following inhalation exposure to 20-nm or 250-nm TiO₂ particles, the half-times for alveolar clearance of polystyrene test particles were proportional to square centimeters of TiO₂ particle surface per million macrophages; macrophage toxicity from TiO₂ particle surface was assumed to be the cause of the decrease in the clearance rate of polystyrene test particles. When TiO₂ particle projected area was incorporated into the in vivo macrophage dosimetry calculations, particle projected areas ranged in value from covering only a fraction (0.1) of the macrophage surface to covering the cell surface 4 times over. The observed decrease in macrophage mediated alveolar clearance of polystyrene test particles was directly related to the potential for TiO₂ particles to mask the surface of the macrophage—a possibility that was visualized in vitro with confocal laser scanning microscopy.