Porcine melanotic cutaneous lesions and lymph nodes: immunohistochemical differentiation of melanocytes and melanophages.

Porcine melanomas have proven interesting in a wider biological perspective due to a common phenomenon of spontaneous regression, which is characterized by infiltration of macrophages, among others. Separation of neoplastic melanocytes from pigment-laden macrophages may, however, be challenging as the morphology of melanocytes varies considerably and sometimes resembles macrophages. The aim of this study was correspondingly to characterize and differentiate the cells in 20 porcine melanocytomas and regional lymph nodes by histologic examination and immunohistochemistry for melan A, PNL2, S100, lysozyme, alpha-1-antitrypsin, and ionized calcium binding adaptor molecule 1 (Iba1). Grossly, the melanocytomas were divided into 2 distinct types: pigmented maculae (n = 7) and raised tumors (n = 13). In the maculae, the pigmented cells were mainly melanocytes reactive for melan A, PNL2 and S100. In contrast, the majority of the cells in the raised tumors were melanophages, which expressed Iba1, alpha-1-antitrypsin, and lysozyme. Yet, cells histomorphologically indistinguishable from the melanophages expressed melan A and PNL2. These cells were Iba1 and S100 negative, and ultrastructurally, they were devoid of lysosomal bodies and filled with stage III and IV melanosomes. In the regional lymph nodes, melanocytes were present in the trabecular sinuses. In focally or diffusely black lymph nodes, pigmentation was, however, mainly due to aggregates of melanophages, which were confined to the trabeculae, deep cortex, and peripheral lymphoreticular tissue. Normal and neoplastic porcine melanocytes express melan A and PNL2, and immunohistochemical staining for melan A, PNL2, and Iba1 was found useful to identify and distinguish melanocytes and melanophages in porcine melanotic lesions.