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BACKGROUND Although the ascites of patients with ovarian carcinoma has been reported to contain immunosuppressive factors, the identity and source of this activity has not been fully elucidated. The objective of this study was to describe the purification of a single immunosuppressive protein, α-1 acid glycoprotein, from ovarian carcinoma ascites, identify its site of production, and describe a possible mechanism by which it inhibits lymphocytes. METHODS Ascites from proteins from five patients with epithelial ovarian carcinoma first were differentially precipitated by size with different concentrations of polyethylene glycol and then separated on the basis of isoelectric focusing. The protein factions then were placed in a lymphocyte proliferation assay to determine immunosuppressive activity. Western blot analysis was used to identify α-1 acid glycoprotein as an immunosuppressive protein in ascites. Total RNA was extracted from ovarian and hepatic cell lines as well as primary and recurrent ovarian tumor samples. Reverse-transcriptase polymerase chain reaction then was utilized to identify the site of production of this protein. Purified α-1 acid glycoprotein was placed in lymphocyte culture and its effects on lymphocyte interleukin-2 (IL-2) production were measured by enzyme-linked immunoadsorbent assay. RESULTS Addition of purified α-1 acid glycoprotein to the lymphocyte assay resulted in a 60% decrease in lymphocyte proliferation (P < 0.05). α-1 acid glycoprotein transcript was not identified in ovarian tumor cells. The addition of purified α-1 acid glycoprotein to the lymphocyte culture resulted in a 65% decrease in IL-2 secretion into the media (P < 0.05). CONCLUSIONS α-1 acid glycoprotein is an immunosuppressive protein purified from ovarian carcinoma ascites. It is not expressed primarily by ovarian carcinoma cells. It appears to inhibit IL-2 secretion by lymphocytes. Cancer 1997; 80:1448-56. © 1997 American Cancer Society.