A quantitative comparison between in vivo-and in vitro-derived Friend leukemia virus

The biological activities of RNA viruses derived from Friend leukemia cells in culture (TCV) were compared with those of viruses derived from the plasma (PV) of mice infected with Friend leukemia virus (FLV). The comparison was quantitatively based on the actual number of viruses used in each experiment as determined by counting under the electron microscope. Electron microscopy also provided a qualitative assessment of the structural integrity of the concentrated virus particles used in various bioassays. The data shows that the leukemogenic and spleen-focus-forming (SFF) activities of TCV, although demonstrable, are respectively 10(5) and 10(4) lower than those of PV. Moreover, TCV has 10(4) less helper activity (S+L- test) than PV. The level of reverse transcriptase activity is ten times lower in TCV than in PV which indicates that there is little correlation between polymerase activity and the other biological activities measured. The decreased biological activity of the in vitro grown virus is thought to be intrinsic to this type of virus although all extrinsic factors have not been ruled out.