Quantitative colony method for tumorigenic cells transformed by two distinct strains of Friend leukemia virus

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Abstract
An in vitro colony method capable of detecting spleen cells malignantly transformed by Friend leukemia virus is described. These colony-forming cells, which form large erythroid colonies (104-105 cells) in methylcellulose, can be detected late after infection with the anemia-inducing (FV-A) or polycythemia-inducing (FV-P) isolates of Friend virus. Colony formation by these cells is dependent only on fetal calf serum as an exogeneous growth factor. These colony-forming cells in FV-P-infected spleens could not be detected until at least 3 wk after virus infection, even though the most rapid increase in spleen weight occurred between 1 and 2 wk after infection. Thereafter, the numbers of colony-forming cells increased sharply up to 5 wk after infection with FV-P, beyond which time the mice generally did not survive. After infection with FV-A, colony-forming cells were detected only at 8-12 wk and their numbers generally increased thereafter. Permanent cell lines were established from a significant fraction of FV-P and FV-A-induced colonies and these cell lines could be chemically induced to synthesize Hb. All individual colonies produced complete Friend virus complex. Virus production appeared to decline in at least some cell lines. FV-P- and FV-A-induced colonies contained cells capable of forming spleen colonies in irradiated recipients and s.c. tumors in unirradiated mice. The assay method described here appears to detect a unique class of malignant Friend virus-transformed cells that can be detected only in the advanced stages of Friend virus-induced erythroleukemia.

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