The effects of clinical grade serine protease inhibitors on natural killer (NK) activity were compared. Cytotoxicity was measured with the Calcein-AM release method, using K562, Raji as a target. There is a significant correlation between measurements of NK activity by the Calcein-AM method and the 51Cr release assay. Cytotoxicity was inhibited with a calcium chelating agent or a perform inhibitor. Although up to 65% of cytotoxicity was inhibited by nafamostat mesilate with an E/T ratio of 10:1, and by 55% by ulinastatin, neither gabexate mesilate nor antithrombin III inhibited any cytotoxicity. None of these agents inhibited lymphokine-activated killer cell activity. In clinical applications, it should be noted that some protease inhibitors have been proven to have immunosuppressive effects.
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