Microassay for interferon, using [3H]uridine, microculture plates, and a multiple automated sample harvester

A microassay for interferon is described which uses target cells grown in microculture wells, [3H]uridine to measure vesicular stomatitis virus replication in target cells, and a multiple automated sample harvester to collect the radioactively labeled viral ribonucleic acid onto glass fiber filter disks. The disks were placed in minivials, and radioactivity was counted in a liquid scintillation spectrophotometer. Interferon activity was calculated as the reciprocal of the highest titer which inhibited the incorporation of [3H]uridine into viral ribonucleic acid by 50%. Interferon titers determined by the microassay were similar to the plaque reduction assay when 100 plaque-forming units of challenge vesicular stomatitis virus was used. However, it was found that the interferon titers decreased approximately 2-fold for each 10-fold increase in the concentration of challenge vesicular stomatitis virus when tested in the range of 10(2) to 10(5) plaque-forming units. Interferon titers determined by the microassay show a high degree of repeatability, and the assay can be used to measure small and large numbers of interferon samples.