Inhalation exposure studies of mice were conducted with multiwalled carbon nanotubes (MWCNTs) to assess the fibrotic potential of this manufactured carbon nanomaterial. To address the hypothesis that MWCNTs cause persistent morphologic changes, male C57BL/6J mice were exposed in a whole-body inhalation system to a 5 mg/m3 MWCNT aerosol for 5 hours/day for 12 days (4 times/week for 3 weeks). At the end of inhalation exposures, lungs were preserved by vascular perfusion of fixative while inflated with air at 1, 14, and 84 days post inhalation exposure. A separate, clean-air control group was also studied. Sections were prepared to analyze the distribution of lung burden following inhalation exposure. Morphometric measurements of Sirius Red staining were used to assess the connective tissue response. At day 1 post-exposure 86+/-4 and 14+/-6 percent of the lung burden (mean+/-SE, N=5) were in the alveolar and airway regions, respectively. Distribution within the alveolar region was 57+/-6, 7+/-5 and 20+/-4 percent in alveolar macrophages, alveolar airspaces and alveolar tissue, respectively. The mean linear intercept, a measure of the degree of alveolar expansion, was not significantly different between groups being 29.5+/-0.5, 29.6+/-0.6, 29.1 +/-0.5 and 29.4+/-0.4 microns for clean-air controls, 1, 14 and 84 days MWCNT groups, respectively. The connective tissue in the alveolar region of MWCNT-exposed mice demonstrated a progressive increase in thickness over time (0.17+/-0.02, 0.22 +/-0.02 and 0.26+/-0.03 for 1, 14 and 84 days) and was significantly different from clean-air controls (0.16+/-0.02) at 84 days. Despite the relatively low fraction of the lung burden being delivered to the alveolar tissue, the average thickness of connective tissue in the alveolar region increased by 53% in the 84 days after inhalation exposure. These results demonstrate that inhaled MWCNTs have the potential to produce a progressive, fibrotic response in the alveolar tissues of the lungs.
The Toxicologist. Society of Toxicology 51st Annual Meeting and ToxExpo, March 11-15, 2012, San Francisco, California