Balb/c mouse embryonic fibroblasts were transformed by the murine sarcoma virus (MSV) ( strain). These cells have a disorderly growth pattern. When grown in soft agar 61% of them formed colonies. They contained ‘C’ type MSV particles and the transplantation antigen of the murine leukosis group of viruses. After 200 passages with interferon in the tissue culture medium, a cell line (MSV-IF) emerged. This cell line recovered orderly growth and only 0.5% of the cells gave rise to colonies in soft agar. It contained about ten times more ‘C’ type particles than the original MSV cells. These particles harboured the MSV and leukaemia genome. The cells also retained the group and transplantation antigens. MSV-IF cells were completely resistant to exogenous interferon. When challenged with Newcastle disease virus they produced about ten times more interferon than the original MSV cells. Newcastle disease virus did not induce a refractory state to further interferon induction. When challenged five times every 48 hr the cells always produced a significant amount of interferon. It was postulated that this MSV-interferon cell line, unlike the original cell population, was not able to produce or use a repressor of interferon synthesis. In addition, the transformed cells recovered, to some degree, properties of the original normal cells.


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