Insufficient Innate Immunity Contributes to the Susceptibility of the Castaneous Mouse to Orthopoxvirus Infection

The castaneous (CAST) mouse, a wild-derived inbred strain, is highly susceptible to orthopoxvirus infection by intranasal and systemic routes. The 50% lethal intraperitoneal dose of vaccinia virus (VACV) was 3 PFU for CAST mice, whereas BALB/c mice survived 106 PFU. At all times and in all organs analyzed, virus titers were higher in CAST than in BALB/c mice. In individual CAST mice, luciferaseexpressing VACV was seen to replicate rapidly leading to death, whereas virus levels increased for a few days and then declined in BALB/c mice. Increases in gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) were delayed and low in CAST mice compared to BALB/c mice following VACV infection or poly(I-C) inoculation, consistent with differences in innate immune responses. In addition, naive CAST mice had considerably lower numbers of NK and T cells than BALB/c mice. The percentage of IFN-gamma -producing CD4(+) and CD8(+) T cells increased following infection of CAST mice only after considerable virus spread, and the absolute cell numbers remained low. Administration of exogenous IFN-gamma or -alpha to CAST mice before or during the first days of infection suppressed virus replication and prolonged survival, allowing the mice to make adaptive CD4(+) and CD8(+) T cell responses that were necessary to clear the virus after cessation of interferon treatment. Thus, insufficient innate cytokine and cellular immune responses contribute to the unique susceptibility of CAST mice to VACV, whereas the adaptive immune response can be protective only if virus replication is suppressed during the first several days of infection. IMPORTANCE Most inbred mouse strains are relatively resistant to orthopoxviruses. The castaneous (CAST) mouse is a notable exception, exhibiting extreme vulnerability to monkeypox virus, cowpox virus, and vaccinia virus and thus providing a unique model for studying pathogenicity, immunity, vaccines, and antiviral drugs. To fully utilize the CAST mouse for such purposes, it is necessary to understand the basis for virus susceptibility. We showed that naive CAST mice make low IFN-gamma and TNF-alpha responses and have low levels of NK cells and CD4(+) and CD8(+) T cells compared to a resistant classical inbred mouse strain. Attenuating virus replication with one or more doses of exogenous IFN-alpha or -gamma before or during the first few days of infection enabled the development of adaptive cellular immunity and clearance of virus. Further genetic studies may reveal the basis for the low innate immunity.