Porcine Melanotic Cutaneous Lesions and Lymph Nodes

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.

Immunohistochemical Expression of Melanocytic Antigen PNL2, Melan A, S100, and PGP 9.5 in Equine Melanocytic Neoplasms

The immunoreactivity of PNL2, Melan A, and protein gene product (PGP) 9.5 was compared with that of S100 protein in 50 formalin-fixed, paraffin-embedded equine melanocytic neoplasms. PNL2, PGP 9.5, and S100 protein were detected in all 50 neoplasms; none expressed Melan A. PNL2 was not expressed in 62 nonmelanocytic tumors (equine sarcoids, schwannomas, carcinomas, sarcomas, endocrine tumors, sex-cord stromal tumors, germ cell tumors, and leukocytic tumors) or in normal tissues other than epidermis. In summary, antibody PNL2 is a sensitive marker of equine melanocytic neoplasms and is more specific than S100 protein or PGP 9.5. In contrast, the monoclonal antibody to Melan A did not react with any of the equine melanomas.

Immunohistochemical Identification of Canine Melanocytic Neoplasms With Antibodies to Melanocytic Antigen PNL2 and Tyrosinase

The immunoreactivity of PNL2 and antityrosinase in formalin-fixed, paraffin-embedded canine melanocytic neoplasms ( n = 101) was compared with that of Melan A. Of the 113 samples overall, 106 were positive for PNL2, 101 for Melan A, and 90 for tyrosinase. Six melanomas that were positive for PNL2 were negative for Melan A; 1 melanoma that was negative for PNL2 was positive for Melan A. Eighty tumors were positive for all 3 markers; 111 reacted with at least 1 the 3 antibodies. Decalcification with formic acid for up to 1 week did not affect immunoreactivity of any of the markers; however, decalcification with HCl for 1 day or 1 week notably decreased or completely abrogated immunoreactivity for Melan A and PNL2. There was only minor loss of immunoreactivity for tyrosinase in tissues decalcified with HCl for 1 week. Prolonged fixation (up to 2 months) did not affect PNL2 or tyrosinase immunoreactivity; however, Melan A immunoreactivity was reduced after 1 month of fixation. PNL2 was not expressed in 120 nonmelanocytic tumors (carcinomas, sarcomas, steroid-producing tumors, and leukocytic tumors). In summary, antibody PNL2 is slightly more sensitive than Melan A and more sensitive than tyrosinase in the identification of canine melanocytic neoplasms. Furthermore, PNL2 does not appear to cross-react with nonmelanocytic neoplasms. PNL2 is resistant to prolonged fixation but sensitive to strong decalcification. Results indicate that PNL2 is an excellent marker in the identification of canine melanomas and that the sensitivity is close to 100% when used in conjunction with Melan A and tyrosinase.