Helper virus is not required for in vitro erythroid transformation of hematopoietic cells by Friend virus

The Friend polycythemia virus complex (FVP), consisting of the replication-defective spleen focus-forming virus (SFFV) and a helper Friend murine leukemia virus (MuLV-F), produces erythroleukemia within 2-3 weeks in vivo. We have recently reported in vitro transformation of bone marrow cells by FVP, producing clusters of erythroid colonies (erythroid bursts) 4-6 days after infection. In contrast to uninfected bone marrow cells, FVP-treated cells proliferated and differentiated (synthesized hemoglobin) in the absence of added erythropoietin, the physiologic regulator of erythropoiesis. The relative roles of helper murine leukemia virus (MuLV) and SFFV in the in vitro erythroid transformation have now been examined. Pseudotype studies and the finding that cloned MuLV-F(free of SFFV)did not induce burst formation indicated that SFFV was essential for this in vitro effect of FVP. Because SFFV could not be obtained free of helper MuLV, we assessed the requirement of MuLV in the transformation by kinetic analyses of helper-deficient and helper-excess FVP preparations. Whereas helper-excess FVP gave single-hit kinetics both in vivo and in vitro, the helper-deficient FVP followed multiple-hit kinetics when titrated for spleen focus formation in vivo. Addition of MuLV-F to helper-deficient FVP prior to injection resulted in a marked enhancement of spleen focus formation and a conversion from multiple-hit to single-hit kinetics. In contrast, titration of this same preparation for erythroid burst transformation in vitro yielded single-hit kinetics, and the addition of helper MuLV-F had no effect. The time course of burst development was similar with or without added MuLV-F. Unlike burst transformation SFFV production by these infected cultures followed multiple-hit kinetics. Addition of MuLV-F at the time of infection led to an enhancement of SFFV production and conversion of the titration curve from multiple-hit to single-hit. These data are consistent with the idea that SFFV is competent for erythroid transformation in vitro, but requires helper MuLV for its replication.