Hepatobiliary clearance of a long and thick carbon nanotube following pulmonary exposure in mice

Long and thick carbon nanotubes (CNTs), such as Mitsui-7, can translocate from the lungs to secondary organs, including the liver, following pulmonary exposure. Although Mitsui-7 CNT is biopersistent, previous studies have suggested clearance from the liver. The mechanisms underlying this process remain poorly understood. Fiber-shaped nanomaterials interact differently with hepatic cell types, including hepatocytes, compared with spherical nanoparticles. Based on these observations, this study aimed to investigate whether long and thick CNTs can be eliminated from the liver via the hepatobiliary system. In this study, female C57BL/6 J mice (n = 6 per group) were exposed by a single intratracheal instillation to 54 μg Mitsui-7 or vehicle (2% mouse serum). Twenty-eight days after exposure, the animals were euthanized, and tissue samples (lung, kidney, spleen, urinary bladder, liver, gallbladder, bile, and hepatic lymph nodes) were collected. Samples were formalin-fixed, paraffin-embedded, and sectioned. Sections were stained with hematoxylin and eosin, and the presence of CNTs within tissue sections was assessed using enhanced darkfield microscopy. CNTs were detected in the liver, gallbladder, and bile, indicating that long, thick CNTs were excreted from liver via the biliary route, suggesting hepatobiliary clearance. CNTs were also observed in hepatic lymph nodes, suggesting possible hepatic clearance; however, the hepatic lymph nodes do not contribute to systemic elimination. No CNTs were detected in the urinary bladder, indicating that long and thick CNTs were not cleared by glomerular filtration. These results indicate that the hepatobiliary system serves as a route for systemic elimination of long and thick CNTs.