Polycythemia- and anemia-inducing erythroleukemia viruses exhibit differential erythroid transforming effects in vitro

The transforming capabilities of FVA, RLV and FVP have been examined using an in vitro transformation assay. Treatment of bone marrow cells with FVP in vitro led to the formation of hemoglobinized erythroid bursts even when these cells were cultured in methylcellulose for 5 days without added erythropoietin (Epo). A variety of FVA and RLV preparations also produced erythroid bursts without Epo but these bursts contained significantly less hemoglobin than those induced by FVP. When very low levels of Epo were added to cultures of FVA- and RLV-infected cells, the bursts were hemoglobinized, that is, similar to FVP-induced bursts. The burst-inducing agent in FVA preparations was shown to be a virus and not Epo. Spleen focus-forming virus (SFFV) pseudotypes, derived from FVA or FVP, also produced erythroid bursts in vitro, whereas four helper murine leukemia viruses did not. These studies indicated that the SFFV component was essential for erythroid burst transformation and specified the degree of hemoglobinization in the bursts formed.