Modulation of erythropoiesis by the helper-independent Friend leukemia virus F-MuLV.

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Abstract
Friend leukemia virus (FLV) is a complex of 2 viruses, a defective spleen focus-forming component (SFFV) and a helper-independent virus Friend murine leukemia virus (F-MuLV). Although erythropoietic changes in susceptible adult mice after FLV infection have been attributed to the SFFV component, F-MuLV alone can induce certain erythropoietic changes in susceptible newborn mice. In this study, very different erythropoietic changes were observed after infection with F-MuLV. Unlike the FLV complexes, F-MuLV did not induce in newborn BALB/c mice an increase in the erythroid progenitor cells (CFU-E*). Instead, a dramatic increase in the bursts (BFU-E*) was detected in the infected spleens and marrow. The frequency of BFU-E* in the infected spleens increased to a level of .apprx. 1500/105 cells or 5 .times. 107/spleen. In the marrow, frequency increased to a level of 300 BFU-E*/105 cells or .apprx. 3 .times. 105/femur. These F-MuLV-induced BFU-E* differ from the bursts in uninfected mice; e.g., although the morphology of the erythroid bursts in F-MuLV infected mice was similar to that in uninfected mice, colonies were less hemoglobinized when compared with those in the uninfected mice. When adult BALB/c or DBA/2 mice were administered phenylhydrazine (PHZ), infection of cloned F-MuLV also induced certain erythropoietic changes. Although F-MuLV alone had little effect on the spleen weights (0.1-0.15 g) of adult mice and PHZ only moderately increased the spleen weights (0.3 g), F-MuLV infection in PHZ-treated mice induced severe splenomegaly (.apprx. 1 g), severe anemia and an increase in the frequency of CFU-E*. Thus, F-MuLV itself modulates erythropoesis in newborn and adult mice. That F-MuLV in newborn BALB/c mice elicits very different erythropoietic changes also has interesting implications; the induction of a dramatic increase in BFU-E* with no increase (possibly even a slight decrease) in CFU-E* implies that the leukemic block in F-MuLV-infected newborn mice is at a stage between BFU-E and CFU-E and that F-MuLV infects different, possibly more immature, erythroid target cells than FLV complexes.

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