Immunohistochemical detection of melanoma-specific antigens in spontaneous canine melanoma

Twenty-five formalin-fixed, paraffin wax-embedded canine melanomas were examined immunohistochemically by an immunoperoxidase method to assess their reactivity with three human melanoma-specific monoclonal antibodies (HMB-45, MEL-1, NK1/C3). HMB-45 and MEL-1 reacted with 22/25 (88%) and 18/25 (72%) of canine melanomas, respectively, but only after microwave antigen retrieval and pretreatment with potassium permanganate and oxalic acid (KMnO(4)/OA). Positive reactivity to HMB-45 and MEL-1 was as follows: oral melanomas, 13/16 and 9/16, respectively; cutaneous melanomas, 8/8 and 8/8; melanoma of the digit, 1/1 and 1/1; all pigmented melanomas, 16/18 and 14/18; all amelanotic tumours, 6/7 and 4/7. HMB-45 immunolabelling was characterized by a diffuse granular cytoplasmic pattern within the tumour cells, and MEL-1 labelling by a cytoplasmic pattern with sporadic nuclear localization. In most tumours the labelling was homogeneous, but some showed a multifocal distribution. Generally, a higher percentage of canine melanomas was labelled by HMB-45 than by MEL-1. NK1/C3 failed to label any of seven melanomas tested, regardless of KMnO(4)/OA-pretreatment. Of 16 non-melanocytic tumours (specificity controls), 15 showed no significant reactivity with HMB-45, the exception being one of three plasmacytomas. Immunolabelling by MEL-1 on non-melanocytic tumours was not pursued, due to the poor sensitivity of this antibody as compared with that of HMB-45.