A new class of MuLV has been detected and isolated from normal and leukemic AKR, C58, SJL, and NFS. AKV mice as well as from NFS mice inoculated with Friend or Moloney ecotropic viruses. These new viruses are XC negative and serologically cross-react with MCF env antigens but are ecotropic in host range, being able to only infect mouse cells to varying degrees and unable to infect mink or other cells infectable by MCF or xenotropic viruses. Viruses of this type from AKR mice cross-interfere with Moloney ecotropic and MCF viruses in SC-1 cells and appear to have properties similar to those of the SL3-2, GPA-V2, and R-XC- isolates. Analysis of their genomes by restriction endonuclease mapping of proviral DNA indicates structures similar to class II MCFs with the 5′ half of the genome being like ecotropic viruses and the env region exhibiting restriction sites characteristic of MCF viruses. In normal AKR mice, these ecotropic recombinant-like viruses are found in spleen and bone marrow as early as 1 week of age, but first appear in the thymus at 3-4 months of age. These viruses have not been detected in mice with no or low expression of ecotropic viruses (NFS, NZB, DBA/2, BALB/c, C57BL/6). Because of their apparent recombinant structure and ecotropic host range we have provisionally designated them ecotropic recombinant virus (ERV) to distinguish them from the MCF class of recombinant MuLV.
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