WHEN mouse haematopoietic cells are incubated in plasma clot or methyl cellulose cultures in the presence of erythropoietin (EP), at least two kinds of colonies are observed1-4. After 2 days, randomly distributed erythroid colonies (defined as a clone of eight or more benzidine-positive cells) appear1,2, and after 3-8 d of culture with higher concentrations of EP, larger colonies arise in clusters or "bursts" 3,4. The cells of origin have been termed the colony-forming unit-erythroid (CFU-E)1 and burst-forming unit-erythroid (BFU-E) 3,4, respectively. Erythropoiesis in mice is also increased considerably by the Friend virus complex5,6, which has been studied mostly in vivo although there are systems for the study of erythroid transformation in vitro soon after infection7-10. Nooter and Bentvelzen11, using methyl cellulose cultures, showed that mouse marrow cells, previously incubated in vitro with purified Rauscher leukaemia virus, develop more erythroid colonies than untreated control cells. Clarke et al.12, using the plasma clot method, observed erythroid colonies 2 d after incubation of mouse marrow cells with culture fluid from Friend virus infected monolayers of fibroblasts. These colonies were similar to EP-induced 2-d colonies in gross morphology and random distribution throughout the clot. Virus-induced bursts were not present in later stages of culture and erythroid colonies were not induced after incubation of marrow cells with plasma from Friend virus infected mice. We now report that when mouse marrow cells are incubated in vitro with infectious plasma or tissue culture fluids containing the Friend virus complex, a substantial number of erythroid bursts appear 5 d after the initiation of the cultures. The Friend virus complex consists of at least two biologically active viruses- the spleen focus-forming virus (SFFV) which is replication defective and essential for increased erythropoiesis in vivo, and a helper murine leukaemia virus (MuLV-F)13,14.
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