Retroviruses and embryogenesis: Microinjection of Moloney leukemia virus into midgestation mouse embryos

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The interaction of Moloney leukemia virus (M-MuLV) with developing post-implantation mouse embryos was studied. First, the frequency at which embryos in utero are infected by transplacental transmission with maternal virus was explored. To exclude milk transmission from the viremic mother, embryos were delivered by cesarean section prior to birth and given to normal foster mothers. None of 72 mice raised this way developed viremia. This indicates that the placenta is an efficient barrier protecting the developing embryo against infection with exogenous retroviruses. To overcome the placental barrier and to introduce virus into embryos at defined stages of differentiation, Moloney leukemia virus was microinjected directly into embryos in utero at day 8 or 9 of gestation. Between 60 and 70% of the injected embryos survived to birth and were tested for viremia at 4 weeks of age. M-MuLVspecific sequences were quantitated in organs of viremic animals derived from midgestation embryos microinjected with virus. Molecular hybridization experiments with nucleic acids extracted from different organs of these animals indicated that every cell type carried M-MuLV-specific DNA sequences and that high concentrations of M-MuLV-specific RNA sequences were present in every organ. In contrast, M-MuLV infection and expression is restricted to lymphatic tissues when animals are exposed to virus after birth or in BALB/Mo mice. These results indicate that the most important parameter determining the “target tropism” of Moloney leukemia virus infection and expression is the stage of embryogenesis and cellular differentiation at which virus infection takes place. In viremic C57BL animals derived from microinoculated embryos, the hair color changed beginning at age 6 weeks. This was not observed in animals exposed to virus after birth. All animals succumbed to MMuLV-induced leukemia at a later age. The results suggest that expression of M-MuLV may also lead to cellular dysfunctions other than leukemic transformation.