We have identified an immunosuppressive activity from the conditioned medium of an androgen-independent human prostatic carcinoma cell line, JCA-1. This activity is constitutively produced by JCA-1 cells and is capable of suppressing normal human peripheral blood lymphocyte proliferation irreversibly in a dose-dependent manner. Immunosuppressive activity was semi-purified by a combination of ion-exchange chromatography and gel filtration with an apparent molecular weight of 40-55 kDa. The immunosuppressive activity was not cytolytic to lymphocytes and was sensitive to 56 degrees C, reducing agent as well as to protease digestion. Cell cycle analysis revealed that the suppressive activity did not induce apoptosis, but it prevented some G(1) lymphocytes from entering into the S phase of the cell cycle.

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