Rapid Assay of Murine Leukemia Virus Helper Activity for Friend Spleen Focus-Forming Virus

Coinfection of adult BALB/c mice with a lymphatic leukemia virus (LLV) derived from Friend virus complex (FV) and with FV stocks containing spleen focus-forming virus (SFFV) resulted in greater numbers of spleen foci than those observed in BALB/c mice infected with defective SFFV alone. This ability of LLV to increase the focus-forming efficiency of SFFV in BALB/c mice was the basis for a 9-day graded assay method for quantifying LLV helper activity in vivo. Exposure of LLV to formalin, ether, ultraviolet light, or specific antiserum eliminated this activity, while serial passage of LLV (previously passaged in Swiss mice) through BALB/c mice was associated with a progressive increase in LLV helper activity. For a given LLV preparation, helper activity varied with different stocks of “indicator” SFFV and in different mouse strains, but remained constant in BALB/c mice when the same stock of indicator SFFV was used over a fivefold dose range. The method is reproducible and sensitive; it can detect murine leukemia viruses antigenically distinct from LLV, and it may be useful in the search for leukemia viruses of other species.