Pseudotyping of dual-tropic recombinant viruses generated by infection of mice with different ecotropic murine leukemia viruses.

Using a new focal immunofluorescence assay (FIA) with monoclonal antibodies, dual-tropic recombinant mink cell focus-inducing (MCF) retroviruses were specifically detected directly in mouse cells. With the FIA, MCF and ecotropic murine leukemia virus (MuLV) infectious centers were quantitated in noninoculated AKR/J mice and in mice inoculated with either ecotropic Friend- or Moloney-MuLV. Comparison of the titers obtained in mouse and mink cells suggested that pseudotyping of MCF viruses with ecotropic MuLV envelopes occurred at different levels in these three mouse models. Adult and newborn IRW mice inoculated with Friend-MuLV produced MCF viruses which were mostly pseudotyped with ecotropic MuLV envelopes. Newborn IRW mice inoculated with Moloney-MuLV produced MCF viruses in both spleen and thymus. Most Moloney-MCF viruses from spleens were pseudotyped with ecotropic MuLV envelopes, whereas thymic Moloney-MCF viruses were not. Noninoculated AKR/J mice spontaneously produced MCF virus at least 2 months before the onset of organ enlargement, and the majority of MCF viruses in both spleens and thymuses of these mice did not appear to be pseudotyped.