Lack of correlation between in vivo and in vitro assays for the detection of virus released from clones of Friend erythroleukemia cells.

The biological properties of the virus synthesized by 18 clones of a line of mouse bone-marrow hematopoietic cells transformed in vitro by the polycythemic strain of Friend leukemia virus (FLV-P) were compared. In vitro assays were performed to determine whether the virus released into the culture fluids was ecotropic or xenotropic, and in vivo assays were carried out to determine spleen focus formation and leukemogenicity in susceptible DBA/2J and BALB/c mice. A number of clones released virus which reproduced the entire range of effects typical of the FLY-P complex. However, in other clones, there appeared to be no correlation between the assays for leukemogenicity and the assays for either ecotropic virus, reverse transcriptase activity, or virus antigens. Xenotropic virus was not detected in any of the cultures. These results suggest that the FLV-P complex contains a heterogeneous population of viruses, but the possibility that the differences observed may be due to the inability of FLV-P to be expressed fully in some clones cannot be excluded.