A Bioassay for Serum Interferon Based on Induction of 2′5′-Oligoadenylate Synthetase Activity

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Abstract
We have developed a bioassay for interferons (IFN) based on measuring the amounts of 2′,5′ oligoadenylate synthetase (2-5AS) induced in cells of the THP-1 monocyte line in response to IFN. The assay can be completed in 20 h, gives reproducible results, and is at least 50 times more sensitive to IFN-α than conventional cytopathic effect inhibition antiviral assays. It is, respectively, less and much less sensitive to IFN-β and IFN-γ. The presence of preexisting 2-5AS activity in a sample does not influence the results. We have used this assay to measure very low levels (0.1–0.5 IU/ml) of endogenously formed IFN-α in serum samples from patients with various diseases and also to measure the residual small amounts of IFN-α still present in the serum as late as 48 h after an i.m. injection of 3 million IU, which is appreciably later than in previous methods. Thus, our highly sensitive assay offers considerable advantages, not least in relation to the clinical use of IFN.