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Evidence for the Rapid Decrease in Leukemogenic Potential of Rauscher Leukemia Virus in Cell Culture¹

Institute of Microbiology, Rutgers University, New Brunswick, New Jersey 08903 [J. S., V. G.], and Viral Leukemia and Lymphoma Branch, National Cancer Institute, NIH, Bethesda, Maryland 20014 [J. B. M.]

Changes in virulence of Rauscher murine leukemia virus (RLV) following single passage in cultures of secondary mouse embryo fibroblasts or the established cell line 3T3 were examined by comparison of culture lysates both for leukemogenic activity in mice and for their ability to enhance the focus formation by Moloney murine sarcoma virus (MSV) in vitro. A constant ratio of approximately 10 MSV helper units to 1 leukemogenic unit was observed as a result of titration of numerous stocks of RLV derived from mouse plasma. Cell culture lysates, however, obtained 4 to 13 days after infection with these standard RLV stocks, exhibited MSV helper-unit titers that were approximately 6000 times greater than those reflecting leukemogenicity in vivo.

Growth curves of RLV in secondary mouse embryo fibroblast cultures were characterized by the appearance of leukemogenic virus 2 days after infection, with maximum yields being obtained at Day 5. By comparison, however, relatively large quantities of MSV helper units could be detected as soon as 24 hr after infection. Phenotypic changes in RLV populations, manifested as a substantial loss of leukemogenic capacity, were observed after single in vitro passage and were maintained through at least three subsequent virus passages.

¹ This investigation was supported in part by USPHS Training Grant CA-05175. Portions of the data were submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Rutgers University (J. S.).

² Present address: Institute of Cancer Research, College of Physicians and Surgeons, Columbia University, New York, N.Y. 10032.

³ Chief, Viral Leukemia and Lymphoma Branch, National Cancer Institute, NIH, Bethesda, Md. 20014; Visiting Investigator and Advisor, Institute of Microbiology, Rutgers University, New Brunswick, N.J. 08903.

⁴ Present address: Life Sciences, Inc., 2900 72nd Street North, St. Petersburg, Fla. 33710.

Received 4/ 8/70. Accepted 11/13/70.