Abstract
Large eosinophilic cytoplasmic inclusions (ECIs) are occasionally seen in the bronchiolar epithelium of control rats. Increased ECls were recently observed in rats treated with an inhaled corticosteroid. Little information is available about these ECIs. They were characterized by H&E and PAS stains, immunohistochemistry (IHC) for Clara cell specific protein (CCSP), cytochrome P450 enzymes and surfactant-associated proteins (SP-A, B and D), in situ hybridization (ISH) for CCSP mRNA and electron microscopy (EM). The ECls were present in the Clara cells and stained with eosin and PAS. All IHC markers and ISH positively stained the cytoplasm of Clara cells. The ECls were strongly positive for SP-D and either negative or weakly positive for CCSP. All other markers were negative by IHC. EM revealed large secretory granules up to 17 μm, showing irregular limiting membrane. The ECI number was quantified on H&E sections (4 lobes/animal) in control and treated rats after inhalation of the corticosteroid. There was a positive dose-related trend (p < 0.0001) in ECI counts in all studies of 3 months duration and greater. The greatest observed change showed an increase in mean from 0.6 (Control group) to 43.1 (high-dose group). There was no evidence of degeneration/necrosis of the Clara cells. In conclusion, the ECls were enlarged secretory granules containing SP-D and possibly a small amount of CCSP, which increased following treatment with an inhaled corticosteroid. We have demonstrated in the worst case that less than 2% of the total Clara cell population is affected, so careful microscopic examination is recommended.