Development of MoAb HMSA-3 and HMSA-4 against human melanoma melanosomes and their reactivities on formalin-fixed melanoma tissues.

To elucidate the nature of melanosomal protein in normal and malignant melanocytes, two mouse monoclonal antibodies (MoAbs), designated MoAb HMSA-3 and MoAb HMSA-4, were developed by the solubilized melanosomes of human malignant melanoma. The specificity of the two MoAbs was characterized immunohistochemically by comparison with that of MoAb HMSA-2 in various forms of melanoma tissues. MoAb HMSA-3 and MoAb HMSA-4 were IgM, k subclass while MoAb HMSA-2 was IgG1, k subclass. The three MoAbs possessed many similarities; (a) positive reactivity in formalin-fixed and paraffin-processed specimens, (b) identification of cytoplasmic antigen(s) in melanoma cells, (c) negative reactivity with normal epidermal melanocytes on paraffin-sections, and (d) intense reaction with amelanotic melanoma cells, particularly in superficial spreading and acral lentiginous melanoma and in metastatic lymph nodes. The three MoAbs, however, identified the different cells on the same serial sections, suggesting that the three may recognize the different epitopes. Thus in 32 cases of primary and metastatic melanomas examined, one of the three MoAbs always showed a positive reactivity, though the other two were negatively or weakly reacted. Our study indicated that the melanosomal protein may provide a unique source to develop MoAbs which identify malignant melanocytes on routine paraffin sections.