Inhaled Multiwalled Carbon Nanotubes Potentiate Airway Fibrosis in Murine Allergic Asthma

Carbon nanotubes are gaining increasing attention due to possible health risks from occupational or environmental exposures. This study tested the hypothesis that inhaled multiwalled carbon nano-tubes (MWCNT) would increase airway fibrosis in mice with allergic asthma. Normal and ovalbumin-sensitized mice were exposed to a MWCNT aerosol (100 mg/m3) or saline aerosol for 6 hours. Lung injury, inflammation, andfibrosiswereexaminedbyhistopathology, clinical chemistry, ELISA, or RT-PCR for cytokines/chemokines, growth factors, and collagen at 1 and 14 days after inhalation. InhaledMWCNTwere distributed throughout the lung and found in macrophagesby lightmicroscopy,butwerealsoevident in epithelial cells by electron microscopy. Quantitative morphometry showed significant airway fibrosis at 14 days in mice that received a combi-nation of ovalbumin and MWCNT, but not in mice that received ovalbumin orMWCNT only. Ovalbumin-sensitizedmice that did not inhale MWCNT had elevated levels IL-13 and transforming growth factor (TGF)-b1 in lung lavage fluid, but not platelet-derivedgrowth factor (PDGF)-AA. In contrast, unsensitized mice that inhaled MWCNT had elevated PDGF-AA, but not increased levels of TGF-b1 and IL-13. This suggested that airway fibrosis resulting from com-bined ovalbumin sensitization and MWCNT inhalation requires PDGF, a potent fibroblast mitogen, and TGF-b1, which stimulates collagenproduction.CombinedovalbuminsensitizationandMWCNT inhalation also synergistically increased IL-5 mRNA levels, which could further contribute to airway fibrosis. These data indicate that inhaled MWCNT require pre-existing inflammation to cause airway fibrosis. Our findings suggest that individuals with pre-existing allergic inflammation may be susceptible to airway fibrosis from inhaled MWCNT