Quantitation of a Murine Leukemia Virus With a Spleen Colony Assay2

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Abstract
A method is described for the rapid quantitation of the Rauscher leukemia virus with a spleen colony assay. Virus-induced cell colonies in the spleen can be scored macroscopically at 6 to 8 days after virus inoculation into adult mice, and the colonies consist predominantly of nucleated erythrocytes in various stages of differentiation. It was shown that there was a linear relationship between virus concentration and number of colonies in the spleen. The serial isolation of virus from single colonies thus provides a method for the production of genetically homogeneous lines of this virus. Six strains of mice were tested for susceptibility to the colony-forming unit (CFU) assay. The highest CFU titers were obtained with SWR and Swiss, somewhat lower titers with BALB/c, DBA/2, and C3H, and the lowest with C57BL/6 mice. Intravenous inoculation gave an average of 10 times higher CFU titers than intraperitoneal inoculation. Virus titers expressed as endpoint for leukemia in an observation period of 150 days were higher than virus titers expressed as CFU in an observation period of 7 days, by a factor of 13 and 71 in two experiments with spleen extracts, and by a factor of 6 when plasma was used as the source of virus. With the use of the CFU assay, the half-life of the Rauscher virus at 37° C was found to be about 70 minutes.

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