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Capacity for epithelial differentiation in synovial sarcoma: analysis of a new human cell line

Aim—To analyse the capacity for epithelial differentiation in synovial sarcoma using a new human cell line. Methods—A new human cell line, KU-SS-1, was established from a monophasic, spindle cell type of synovial sarcoma by grafting those cells on to severe combined immunodeficient (SCID) mice and then transferring them to in vitro culture systems. The KU-SS-1 cells were characterised by light and electron microscopy, and by immunohistochemical, flow cytometric, and cytogenetic analysis. Results—Primary tumour and cultured cells at passage 20 showed a positive reaction for vimentin, which is a mesenchymal marker. After 40 passages, subcultured cells were injected into SCID mice to induce further tumours. These advanced subcultured cells and the tumour cells that they induced were positive for cytokeratin, an epithelial marker, and exhibited epithelial ultrastructural features such as intermediate junctions. Furthermore, two colour immunofluorescent analysis for proliferating nuclear cell antigen (PCNA) and intermediate filaments showed that a large number of PCNA expressing cells were positive for vimentin, and that part of this fraction also expressed cytokeratin. The existence of cells with reactivity for these three markers indicated that, in this cell line, a fraction with high proliferating capacity had both mesenchymal and epithelial markers. In addition, cytogenetically, this cell line expressed the SYT–SSX chimaeric transcript as a result of the t(X;18)(p11;q11) translocation. Conclusions—A human synovial sarcoma cell line was established and stably maintained in cell culture for more than 70 passages. In addition, this cell line showed epithelial differentiation, which supports the hypothesis that synovial sarcoma is a carcinosarcoma like tumour with true epithelial differentiation. This cell line will be a useful tool for investigating the nature of this tumour and will contribute to clinical studies.