Dual susceptibility of a 3T3 mouse cell line to infection by N‐ and B‐tropic murine leukemia virus: Apparent lack of expression of the FV‐1 gene

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Abstract
A murine sarcoma virus‐transformed S+L—3T3FL cell line has previously been shown to be equally susceptible to infection with both N‐ and B‐tropic variants of murine leukemia virus, unlike all other mouse cells so far described (Krontiris et al., 1973). Transformed S+L—‐BALB/3T3 and S+L‐ NIH/3T3 cell lines were isolated and were found to retain the same resistance to N‐ or B‐tropic viruses as that exhibited by their non‐transformed parent lines. This, coupled with the fact that normal, uninfected 3T3FL cells also show a dual susceptibility, indicates that the presence of the S+L—MSV genome is not responsible for the enhanced permissiveness. Clones of 3T3FL cells are themselves dually susceptible, indicating that this cell line is not simply a mixture of B‐ and N‐type cells. Apparently, the Fv‐1 gene, which is responsible for MuLV host‐range restriction in this system, is either missing or not expressed in 3T3FL cells. The enhanced susceptibility of normal and S+L—3T3FL cells makes these cell lines ideal for titrations of murine leukemia viruses and their MSV “pseudotypes”.