Evaluation of <scp>MAGE</scp>‐<scp>A4</scp> expression in breast cancer and its impact on prognosis

Melanoma-associated antigen (MAGE)-A4, a cancer testis antigen, presents a promising target for chimeric antigen receptor T cell therapy in refractory solid tumors, including breast cancer (BC). However, the lack of highly specific Abs against MAGE-A4 is a major challenge for the development of MAGE-A4-targeted immunotherapies. This study aimed to validate the specificity of a novel MAGE-A4 Ab (E701U) and examine MAGE-A4 expression in clinical BC samples. MAGE-A1, -A2B, -A3, -A4, -A6, -A9, -A10, and -A12 genes were transfected into HEK293 cells. MAGE-A4 expression in each inserted cell block was evaluated using an E701U Ab. Subsequently, we evaluated MAGE-A4 expression in 403 primary BC tissue samples by immunohistochemistry using E701U and analyzed the clinical impact of MAGE-A4 in patients with early BC. The results showed that MAGE-A4 expression was limited to cells transduced with the MAGE-A4 gene. MAGE-A4 expression was observed in 5.7% of the BC samples. Positivity in triple-negative BC was significantly higher than in the other subtypes. The 5-year overall survival rate of patients with MAGE-A4(+) was significantly worse than those with MAGE-A4(-) BC. Moreover, the 5-year recurrence-free survival (RFS) rate of patients with MAGE-A4(+) BC was significantly lower than that of patients with MAGE-A4(-) BC. MAGE-A4 expression was an independent prognostic factor for RFS. In conclusion, the E701U Ab showed reliable specificity for MAGE-A4 expression among MAGE family genes. Patients with MAGE-A4(+) BC have an unfavorable prognosis and represent potential candidates for MAGE-A4-specific immunotherapy.