Studies on the expression and immunogenicity of the B50 melanoma antigen and its relationship with calreticulin

B50 is a 50 kDa protein antigen originally identified and isolated from cultured B16 murine melanoma cells; it is found in close association with a melanoma-specific antigen termed B700. Using a specific rabbit antiserum, B50 (or B50 cross-reactive molecules) has been shown to be expressed by 35 out of 36 cell lines, including melanomas, sarcomas, fibrosarcomas, carcinomas, gliomas, immortalized and primary fibroblasts, melanocyte and keratinocyte cell lines obtained from murine, human, hamster, swine, and canine donors. B50 expression is localized on the cellular membrane and in the cytoplasm in varying amounts in seven of the nine cell lines tested. Mice immunized to B50 demonstrated a significant tumour rejection response when subsequently challenged with B16 F10 melanoma cells. Previous studies had indicated that B50 has significant N-terminal amino acid sequence homology with calreticulin. Calreticulin, a calcium-binding protein, is part of the Ro/SS-A complex. This complex is the primary autoantigenic determinant of the autoimmune diseases systemic lupus erythematosus and primary Sjogren's syndrome. We now show that sera from patients with those diseases contain antibodies which bind B50, although B50 itself does not bind calcium. Thus, B50 and calreticulin are closely related but distinct antigens.