Isolation and Characterization of a Lymphatic Leukemia Virus in the Friend Virus Complex

A lymphatic leukemia virus (LLV), indigenous to Friend virus (FV) preparations that also contain spleen focus-forming virus (SFFV), was isolated free of detectable SFFV. SFFV was removed by 4 serial passages of FV in newborn rats, by 1 blind passage through newborn C57BL mice, or by endpoint dilution. LLV induced lymphatic leukemia, with predominantly splenic and hepatic involvement, and the pathogenicity of LLV was inversely related to host age. Early responses to LLV infection included moderate splenic enlargement, transient viremia (as detected by the ability of LLV to supply a helper function for defective SFFV), and transient depression of humoral immunity. LLV is antigenically related to a virus of similar origin (Rowson-Parr virus) and has a titer apparently exceeding that of SFFV in FV stocks. We conclude that FV preparations contain a helper virus that induces lymphatic leukemia in the absence of SFFV.