Discovery of a potential biomarker for immunotherapy of melanoma: PLAC1 as an emerging target.

BACKGROUND Melanoma has increased in incidence worldwide prompting investigators to search for new biomarkers for targeted immunotherapy of this disease. Placenta specific 1 (PLAC1) is a new member of cancer-testis antigens with widespread expression in many types of cancer. Here, we aimed to study for the first time the expression pattern of PLAC1 in skin cancer samples including cutaneous melanoma, basal cell carcinoma (BCC), squamous cell carcinoma (SCC) in comparison to normal skin and nevus tissues and potential therapeutic effect of anti-PLAC1 antibody in melanoma cancer cell lines in vitro. MATERIALS AND METHODS Polyclonal and monoclonal antibodies were applied for immunohistochemical profiling of PLAC1 expression using tissue microarray. The cytotoxic action of anti-PLAC1 antibody alone or as an antibody drug conjugate (with anti-neoplastic agent SN38) was investigated in melanoma cell lines. RESULTS We observed that 100% (39 of 39) of melanoma tissues highly expressed PLAC1 with both cytoplasmic and surface expression pattern. Investigation of PLAC1 expression in BCC (n = 110) samples showed negative results. Cancer cells in SCC samples (n = 66) showed very weak staining. Normal skin tissues and nevus samples including congenital melanocytic nevus failed to express PLAC1. Anti-PLAC1-SN38 exerted a specific pattern of cytotoxicity in a dose- and time-dependent manner in melanoma cells expressing surface PLAC1. CONCLUSIONS Our findings re-inforce the concept of re-expression of embryonic/placental tissue antigens in cancer and highlight the possibility of melanoma targeted therapy by employing anti-PLAC1 antibodies. The data presented here should lead to the future research on targeted immunotherapy of patients with melanoma.