Single-walled carbon nanotube (SWCNT)-induced interstitial fibrosis in the lungs of rats is associated with increased levels of PDGF mRNA and the formation of unique intercellular carbon structures that bridge alveolar macrophages in situ.

Background: Nanotechnology is a rapidly advancing industry with many new products already available to the public. Therefore, it is essential to gain an understanding of the possible health risks associated with exposure to nanomaterials and to identify biomarkers of exposure. In this study, we investigated the fibrogenic potential of SWCNT synthesized by chemical vapor deposition using cobalt (Co) and molybdenum (Mo) as catalysts. Following a single oropharyngeal aspiration of SWCNT in rats, we evaluated lung histopathology, cell proliferation, and growth factor mRNAs at 1 and 21 days post-exposure. Comparisons were made to vehicle alone (saline containing a biocompatible nonionic surfactant), inert carbon black (CB) nanoparticles, or vanadium pentoxide (V2O5) as a known inducer of fibrosis.

Results: SWCNT or CB caused no overt inflammatory response at 1 or 21 days post-exposure as determined by histopathology and evaluation of cells (>95% macrophages) in bronchoalveolar lavage (BAL) fluid. However, SWCNT induced the formation of small, focal interstitial fibrotic lesions within the alveolar region of the lung at 21 days. A small fraction of alveolar macrophages harvested by BAL from the lungs of SWCNT-exposed rats at 21 days were bridged by unique intercellular carbon structures that extended into the cytoplasm of each macrophage. These "carbon bridge" structures between macrophages were also observed in situ in the lungs of SWCNT-exposed rats. No carbon bridges were observed in CB-exposed rats. SWCNT caused cell proliferation only at sites of fibrotic lesion formation as measured by bromodeoxyuridine uptake into alveolar cells. SWCNT increased platelet-derived growth factor (PDGF)-A, PDGF-B, and PDGF-C mRNA levels significantly at 1 day as measured by Taqman quantitative real-time RT-PCR. At 21 days, SWCNT did not increase any mRNAs evaluated, while V2O5 significantly increased mRNAs encoding PDGF-A, -B, and -C chains, PDGF-R alpha, osteopontin (OPN), connective tissue growth factor (CTGF), and transforming growth factor (TGF)-beta1.

Conclusion: Our findings indicate that SWCNT do not cause lung inflammation and yet induce the formation of small, focal interstitial fibrotic lesions in the alveolar region of the lungs of rats. Of greatest interest was the discovery of unique intercellular carbon structures composed of SWCNT that bridged lung macrophages. These "carbon bridges" offer a novel and easily identifiable biomarker of exposure.