Characteristics of the In Vitro Assay of Murine Sarcoma Virus (Moloney) and Virus-Infected Cells

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Abstract
A number of conditions are critical to the sensitivity and reliability of in vitro assay of murine sarcoma virus (MSV) (Moloney) and MSV (Moloney) infected cells. The age of the mouse embryos used as the source of indicator cells, the ability of the cells to multiply, the time of virus adsorption, and volume of inoculum are all important and must be standardixed if results are to be reproducible. Optimum conditions now in routine use in our laboratory are described, including the use of 15-day embryos as a source of primary cells, infection at the time the secondary cultures are planted, and allowing the inoculum to be left in contact with the indicator cells. Results indicate that virus adsorption to normal cells and release from infected cells are slow and inefficient. Infected cells are incapable of the sustained division necessary to form foci, and the most important factors in focus formation appear to be virus release and reinfection of neighboring cells. Over 90% of tumor cells up to the twelfth transplant generation and in vitro transformed cells to the sixth generation maintain the diploid chromosome number, although as many as 61% of the latter register as infectious centers.