Benign epithelial inclusions in renal hilar lymph nodes of a cynomolgus monkey (Macaca fascicularis).

Epithelial cell clusters found ectopically in lymph nodes are generally considered metastatic lesions from malignant tumors; however, benign epithelial inclusions (BEIs) are a known exception, and distinguishing them from malignancy is often challenging in human medicine. We report the presence of BEIs in a cynomolgus monkey with severe renal damage but no evidence of neoplastic lesions. The inclusions were observed in the subcapsular sinuses of the lymph nodes and were positive for cytokeratin and aquaporin-1 by immunohistochemistry. Similar epithelial clusters were also found within the lymphatic vessels of the renal hilum. Furthermore, migration of epithelial cells into the lymphatic vessels within the renal parenchyma was observed, suggesting that the BEIs in this case originated from renal tubular epithelium. To our knowledge, this is the first report of BEIs of renal tubular origin in an animal, providing valuable insights into their histological and ultrastructural features, as well as their underlying pathogenesis.