Pulmonary inflammation following intratracheal instillation of cellulose nanofibrils in rats: comparison with multi-walled carbon nanotubes

Safety assessment of cellulose nanofibrils (CNFs) is required to accelerate the utilization of these materials in industrial applications. The present study aimed to characterize the effects on rat pulmonary inflammation over a period of 90 days following intratracheal instillation of three types of CNFs or multi-walled carbon nanotubes (MWCNTs) at doses of 0.5, 1.0, or 2.0 mg/kg. The pulmonary inflammatory responses induced by phosphorylated CNFs (CNF1), 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized CNFs (CNF2), CNFs produced via mechanical defibrillation (CNF3), and MWCNTs were investigated using bronchoalveolar lavage fluid analysis, histopathological findings, and comprehensive gene expression profiling of rat lungs. CNF1 and CNF2 with approximately equal diameter (7.0-8.0 nm) and length (0.8-1.0 mu m) distributions induced inflammation after dosing, which was attenuated 90 days post-instillation. CNF3 of relatively greater thickness (21.2 nm) and longer length (1.7 mu m) deposited around the terminal bronchioles were observed after instillation. Acute inflammatory responses in the alveoli induced by CNF3 were mild compared with those induced by other materials and attenuated 90 days post-instillation. MWCNTs induced severe pulmonary inflammatory responses that continued during the test period. The inflammation failed to resolve within 90 days post-instillation. A hierarchical cluster analysis revealed comparable gene expression profiles for CNF1, CNF2, and CNF3, whereas profiles of MWCNTs were different from those of other test substances. This study suggests that pulmonary inflammation is associated with the diameter and length distributions of CNFs and that the pulmonary inflammation caused by CNFs is mild compared with that caused by MWCNTs.