MAGE-A3 is a frequent tumor antigen of metastasized melanoma
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MAGE-3 or MAGE-A3 is one of the best-characterized tumor antigens. Due to its tumor-restricted expression pattern and its recognition by both cytotoxic and helper T cells it constitutes a promising tumor antigen for anticancer immunotherapy, notably of malignant melanoma. Surprisingly, however, only very limited information is available on the frequency and consistency of its expression in metastatic melanoma lesions. We have now investigated the presence of MAGE-A3 mRNA in 316 tumor samples from 147 melanoma patients by RT-PCR. MAGE-A3 mRNA was detectable in 62% of metastases, and expression did not depend on the site of the metastases (skin, lymph node, and internal organs), age, sex, or duration of disease. Southern blot hybridization of the PCR product enhanced sensitivity of detection, and 26% more samples (13/50 samples tested) scored positive, indicating an even higher MAGE-A3 mRNA frequency than determined by simple ethidium bromide gel analysis. In 62 patients, we were able to investigate MAGE-A3 expression in several metastases from the same patient, and unexpectedly, both MAGE-A3-positive and MAGE-A3-negative metastases were found in 32% of these patients (20 of 62). Immunohistochemistry (using mAb 57B) demonstrated that the expression pattern was usually also heterogeneous with positively and negatively stained tumor cells within one metastasis. However, most (90%) of the metastases (47/52) gave a partially positive signal. Taken together, MAGE-A3 is a common and frequent tumor antigen in metastasized melanoma, but its expression is often heterogeneous.