Modulation of the tumor-associated shedding antigen by antitumor drugs and analysis of the shedding mechanism

We have previously reported that the 3-methylcolanthrene-induced rat fibrosarcoma KMT-17 cell line and its subclone A3 shed a tumor-associated antigen (TAA), termed CE7, from the cell surface and that this phenomenon might play a role in an immunological escape mechanism. We also reported that CE7-antigen shedding was inhibited by low dose (30 Gy) X-irradiation in vivo or in vitro, the antigen was expressed on the cell surface and the immunogenicity of CE7-antigen expressing cells was enhanced. The purpose of this study was to examine whether this CE7-antigen shedding is inhibited by the antitumor d rugs bleomycin (BLM), mitomycin C (MMC) and cyclophosphamide (4H-CY) and to clarify the mechanism of antigen shedding. We found that CE7-antigen shedding was inhibited following treatment with these antitumor drugs in vitro. When FCS concentration in media was changed, CE7-antigen expression was enhanced parallel to the extent of inhibiti on of cell growth (prolonged doubling time). We demonstrated CE7-antigen in A3 cells grown in 10% FCS medium by Western blotting. A large amount of CE7-antigen was localized in the cytoplasm of the cells. These results suggest that A3 cells in 10% FCS medium produce CE7-antigen, store it in the cytoplasm and shed it from the cell surface in vesicular form and that antitumor drug treatment inhibits vesicle production, enhancing CE7-antigen expression on the cell surface.