Tissue Culture Assay of Helper Activity of Murine Leukemia Virus for Murine Sarcoma Virus

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Abstract
Coinfection of mouse embryo fibroblast (MEF) cultures with murine leukemia virus (LV) and stocks of murine sarcoma virus (MSV), containing defective sarcoma virion, led to increased numbers of foci of morphologically altered cells, as compared to infection of the MEF culture with the defective MSV stock alone. A quantitative procedure for the assay in MEF cultures of this “helper activity” of LV was developed and used for assay of stocks of the Moloney, Rauscher, Friend, and Rich strains of LV. Stocks of MSV obtained as early viral harvests after infection of the 3T3 mouse cell line with MSV were particularly useful as indicator MSV stocks in this assay because they had a high content of defective MSV. Certain stocks of MSV which showed no evidence of defectiveness as judged by their focus assay titration pattern indicated no measurable helper activity when plated over a considerable range of dilutions on MEF cultures together with a defective indicator MSV stock. This finding and consideration of the quantitative formulation of helper activity indicate that the titration patterns of such MSV stocks did not primarily reflect any helper activity of endogenous LV. This further supports the concept of the presence of a “competent” MSV in such stocks.