Histological and immunohistochemical characterization of salivary pleomorphic adenomas in aged rhesus macaques ( Macaca mulatta )

While pleomorphic adenoma (PA) is the most common salivary neoplasm in human medicine, it appears to be rare in animals and has only recently been reported in a rhesus macaque. We gathered 6 cases of PAs affecting aged rhesus macaques and summarized their gross, histological, and immunohistochemical characteristics. Affected animals ranged from 19 to 32 years, were predominantly male, and the neoplasm was an incidental finding at necropsy. The neoplasms affected the submandibular salivary gland (5/6) and the parotid gland (1/6). Neoplasms were round to ovoid, tan, well-demarcated, and firm. Histologically, the masses had an epithelial and a myoepithelial component supported by a hyalinized fibrous stroma. Immunohistochemistry (IHC) was performed in 5 cases. Pancytokeratin expression was diffuse in the epithelial component. Pleomorphic adenoma gene 1 (PLAG1), a specific nuclear IHC marker for human PAs, was negative in all but 1 case, where rare myoepithelial cells had false-positive cytoplasmic immunolabeling. Calponin and smooth muscle actin (SMA) highlighted the myoepithelial component, with calponin showing stronger expression than SMA in both the number and distribution of immunolabeled myoepithelial cells. Three cases had less than or equal to 5% of cells expressing Ki-67; in 2 cases that were larger with areas of necrosis, expression ranged from 10% to 20% of cells. In summary, salivary PAs in rhesus macaques are usually incidental and apparently affect mainly the submandibular salivary gland in aged males. Immunohistochemistry for PLAG1 is not helpful in the diagnosis, which instead relies primarily on gross and histological findings.