Augmentation of helper virus replication in the presence of defective retrovirus

The interaction between defective spleen focus-forming virus (SFFV) and helper virus(es) in Friend virus (FV) complex has been assumed to be one-way, with the helper virus complementing SFFV by supplying necessary virion components. To test this assumption the expression of both SFFV and helper virus in partially congenic mice which differ at the Fv-2 locus, a gene that specifically controls susceptibility to SFFV, was analysed. When the mice were infected with LLV (a strain of Friend SFFV-free helper virus), there was no detectable effect of Fv-2 genotype on LLV expression as tested by virus infectivity in the XC plaque assay or by quantitative viral antigen analysis in an immunoprecipitation assay. However, after infection with FV complex there was an amplification of LLV (as well as SFFV) synthesis in Fv-2s as compared with Fv-2r hosts. To determine whether the increased LLV synthesis in Fv-2s mice was due to an increased population of susceptible target cells as a result of SFFV infection and/or transformation, the ratios of LLV-infected cells in the spleens of LLV- and FV-infected Fv-2s hosts in an infectious centre assay, were compared. Since the percentage of LLV-infected cells was equivalent in both instances, the higher rate of LLV synthesis after infection with FV complex was presumably due to intrinsic properties of SFFV-infected erythroid cells.