Nonviral helper factors for leukemia virus expression in mice.

: Nonviral helper factors as well as murine leukemia viruses (MuLV) can increase the infectivity of Friend spleen focus-forming virus (SFFV) in mice which restrict its natural helper virus. Such factors have been isolated from cell-free extracts of human leukemic spleens, bacteria, and mycoplasmata. Unlike immunodepressants and target cell stimulators (which can also increase the pathogenicity of SFFV if applied before infection), nonviral helper factors are most efficient when they are injected with SFFV into helper virus-restrictive mice. In these respects the nonviral factors resemble MuLV. However, helper factors differ from MuLV by their inability to replicate in neonatal mice and by their greater stability to heat. Although the human leukemic helper (HuLV) extracts examined lack detectable mycoplasma or bacterial contamination, the helper assay in mice will be of limited value as a possible detection system for human leukemogenic viruses until a unique marker is found by which to distinguish HuLH activity from that of prokaryotic origin. This makes a systematic comparison among the nonviral helper factors all the more important.