Relationship between Moloney MSV tumor resistance and endogenous virogene expression in AKR mouse strain and its hybrids.

We have examined the induction of Moloney-MSV tumors in AKR and other mouse strains in relation to endogenous virus expression. All virus-free strains so far tested were tumor-susceptible, while AKR was resistant. The selectivity of MSV tumor resistance, characteristic of AKR mice, was associated with AKR virogens segregation in first backcross to MuLV-negative/MSV-susceptible mice, and in a few second backcross families. Stronger evidence that AKR ecotropic virogene expression is the major, but not the sole, determinant of M-MSV tumor resistance in AKR mice was obtained in strains partially congeneic for akv-2 viral gene and in recombinant inbred lines derived from an original cross between AKR and C57L mice. These mice or lines, in which inheritance of endogenous viral genes had occurred, at the same time inherited the ability to show strong resistance to tumor induction by an exogenous oncogenic virus. This finding suggests that ecotropic endogenous viruses can exert a beneficial effect in their hosts providing some protective functions. Although the association between AKR-MuLV and M-MSV resistance is definite, the mechanisms by which MuLV+ mice are refractory to M-MSV tumors have still to be elucidated.