Quantitation of Friend spleen focus forming virus by a nine day 59Fe assay

A previously described 3 day ⁵⁹Fe assay for quantitation of Friend spleen focus forming virus has been modified to produce a 200 fold more sensitive 9 day ⁵⁹Fe assay. A characterization of this assay is reported here. Male BALB/c mice received intravenous injections of appropriately diluted Friend polycythemia virus (FVP); control mice received virus diluent. All mice were allowed food and water ad libitum for 6 days, and on day 6 after virus injection were fasted by removal of food but not water. On day 3 of the fast (the 9th day after virus injection) each mouse received an intraperitoneal injection of 1 μCi of ⁵⁹Fe. Six hours later the mice were sacrificed and the splenic radioactivity was determined. The percent splenic incorporation of ⁵⁹Fe was directly related to the logarithm of spleen focus forming units (SFFU) of FVP injected in a range of approximately 25 to 1,000 SFFU. Using a standard FVP preparation in a dose range of 25 to 1,000 SFFU, it was possible to determine the SFFU titers of unknown samples by extrapolation of the percent splenic ⁵⁹Fe incorporation to the logarithm of SFFU. SFFU titers obtained by the 9 day ⁵⁹Fe assay were similar to those obtained by the enumerative response assay. Advantages of the 9 day ⁵⁹Fe assay over the enumerative response assay include a 50 fold greater virus dose range, an easier and a more objective counting procedure, and a reduced coefficient of variation.