Fv-1 restriction of xenotropic and amphotropic murine leukemia virus genomes phenotypically mixed with ecotropic virus

Abstract

Xenotropic murine leukemia virus (MuLV) genomes rendered capable of infecting mouse cells by phenotypic mixing with an ecotropic MuLV exhibited the Fv-1 restriction pattern of that ecotropic virus. Like Fv-1-restricted ecotropic viruses, xenotropic genomes phenotypically mixed with N-tropic ecotropic MuLV showed two-hit dose-response kinetics when titrated on restrictive Fv-1 b cells and the kinetics were converted to one-hit by the addition of N-tropic ecotropic virus. When SC-1 cells chronically infected with N-tropic amphotropic MuLV were superinfected with B-tropic ecotropic virus, each genome tended to maintain its homologous tropism, but with intermediate N:B infectivity ratios, and these ratios changed between the acute and chronic phases of the infection. In clonal lines chronically producing both viruses, the relative N- or B-tropism of the two genomes tended to vary in the same direction, and this correlated with which genome was produced in greater abundance. It is concluded that there is some recognition of amphotropic and ecotropic genomes by their homologous Fv-1 tropism determinants, and that the tropism of the virions in the mixed harvest is determined by this recognition factor and by the relative abundance of the two types of determinant.