Lung-associated lymph nodes and spleen are major secondary accumulating organs in mice following inhaled black carbon as a surrogate of ultrafine ambient particles

Despite increasing public concern about the health impact of air pollution, little is known about the biokinetics of ultrafine particles. Herein, we investigate their biokinetics and associated mechanisms underlying organ distribution and extrapulmonary translocation of ambient ultrafine particulate matter in mice. The test materials used in this study were black carbon (BC) generated by a spark discharge soot generator and two reference materials (SRM 2975 and carbon black). The test particles were intratracheally instilled into the lungs of mice at 25 μg/mouse, and the organ burden was evaluated up to 3 months post-instillation. The data showed significant BC accumulation in lung-associated lymph nodes (LALN) and spleen from 1-3 months post-exposure, with no detectable levels in other organs. The reference materials showed a similar distribution pattern, indicating a common extrapulmonary translocation pathway for ultrafine carbon particles deposited in alveoli. This pathway is unique to particles deposited in the alveoli, as direct injection into lymphatic and systemic circulation showed typical organ accumulation (e.g., liver, lung, spleen, and LALN). The results highlight that the unique extrapulmonary translocation of test particles to the LALN and spleen may be due to the particles that escape from the lung being the smallest particles, evading hepatic surveillance but physically entrapped in the spleen's open circulation.