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The attenuated American BT-8 strain of bluetongue virus is 5 to 10 times more potent an interferon inducer than any other viral or nonviral agent reported to date, including as much as 600,000 units/ml of plasma by 8 h after intravenous injection.

Fourteen continuous human cell lines, including nine derived from tumors and five from non-neoplastic tissues, produced interferon in response to induction with bluetongue virus (BTV), Newcastle disease virus (NDV), and poly(I) . poly(C) complexed with DEAE-dextran. The seven best interferon-producing cell lines (one from a melanoma, five derived from carcinomas, and one SV40-virus-transformed kidney cell line) responded to at least one of the viral inducers with yields of interferon over 1000 units/ml. Because the HT-1376 bladder carcinoma cell line produced high yields of interferon in this survey, and is easily propagated, the optimal conditions for interferon production were investigated, using BTV as the inducer. Interferon yields in 59 inductions over a period of about two years consistently fell within a 6-fold range, and had a geometric mean titer of about 2700 reference units (RU)/ml, representing the production of about 3 RU/10(3) cells. This yield is comparable to mean titers of 1 to 10 RU/10(3) cells obtained by others with human leukocytes, foreskin cell strains, or the Namalva lymphoblastoid cell line. UV-inactivated BTV at a multiplicity corresponding to 10 PFU/cell was as effective an inducer in the HT-1376 cell line as the fully infectious virus at a multiplicity of 1 PFU/cell. The interferon produced by the HT-1376 epithelial cell line has characteristics similar to the interferon induced by poly(I) . poly(C) in human diploid fibroblasts. These studies clearly demonstrate that many different types of tumor-derived cells have the capacity to produce interferon, and that some equal or surpass the efficiency of diploid cells.

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