AbstractThe IFN-α cytokines belong to a multigene family. However, the in vivo biological functions of each of the IFN-α subtypes is unknown. Recently, we developed an experimental model in which the tibialis anterior muscles of mice were transfected in situ with naked DNA plasmids encoding an IFN transgene. Here we use this model to investigate the in vivo effect of the expression of three murine IFN-α subtypes (A1, A4, and A9) on murine CMV replication in C57BL/6, BALB/c, and A/J mice. CMV was shown to replicate in the tibialis anterior muscles of mice for at least 6 days and induced an inflammatory infiltrate. However, mice expressing the IFN-α transgenes showed a marked reduction in the peak titers of virus replication, with less severe inflammation in the muscles compared with control mice that were inoculated with blank vectors. Moreover, mice expressing the IFN-α1 transgene had significantly lower CMV titers in the inoculated muscle than mice expressing either the IFN-α4 or the IFN-α9 transgenes. Furthermore, IFN-α/β receptor knockout mice had markedly higher levels of CMV replication in the tibialis anterior muscles than the wild-type parental strain (129/Sv/Ev) following IFN-α1 transgene inoculation, suggesting that the protection observed is due to host cell-mediated IFN signaling. These data provide the first evidence indicating that there are in vivo differences in the antiviral efficacy of the IFN-α subtypes.