Biological, Immunological, and Molecular Properties of Revertants of Cat Cells Transformed by Murine Sarcoma Virus

Cloned cat cells (CCC) transformed by the ml isolate of Moloney murine sarcoma virus (MSV) have the properties of sarcoma-positive, leukemia-negative (S+L-) cells. Over the past 5 years, partially flat as well as very contact-inhibited variant cloneswere isolated from several clonal generations of S+l— cat cells. Partially flat subclones still contained the MSV genome and could serve as useful cell systems for quantal assays for a number of replicating retroviruses. The frequency of isolation of rescue-negative, very flat subclones diminished with sequential cloning cycles of S+l—cells. The rescue-negative revertant cells grew to low density in liquid media, were more similar to normal cells in soft-agar colony formation, and were intermediate in concanavalin A agglutinability when compared to normal or S+l- cells. Revertants could be retransformed by pseudotypes of MSV other than MSV coated with feline endogenous xenotropic virus. Feline endogenous xenotropic virus was readily induced from S+L-cells but was not inducible from some revertants by halogenated pyrimidines. Rescue-negative revertants could support the growth of a number of helper viruses. Chromosomes of parental CCC normal cells, MSV producer cells, S+l— cells, or revertants were all significantly hypodiploid. No stable pathognomonic changes were observed relative to type and chromosome number among the above. The MSV-coded precursor polyprotein with a molecular weight of 60,000 was detected in producer and S+l— cells but not detected in revertants. Producer and S+l- cells had multiple MSV src copies in their DNA, whereas revertants did not contain residual src DNA. The number of either feline endogenous xenotropic virus or feline leukemia virus-like DNA copies was not different amongproducer, S + L-, or revertant cells. Accordingly, all rescue-negative cat cell revertants tested have lost multiple copies of MSV proviral DNA.