Two different approaches were used to evaluate the role of type 1 IFNs in stimulating NK cell IFN-␥ within mice blocked in their responsiveness to the cytokines. The first examined responses in cells from uninfected and 30-h LCMV-infected immunocompetent mice that had been treated with control antibodies or antibodies blocking access to the type 1 IFN receptor, anti-IFNAR (23) . As shown in Fig. 3A , the infected control-treated mice had the expected 25 to 30% of their NK cells expressing IFN-␥, but this response was reduced to Ͻ1% in the infected mice treated with antibodies against IFNAR. The second approach examined responses in mice that were blocked in type 1 IFN responsiveness as a result of genetic mutation of the type 1 IFN receptor (IFNAR Ϫ/Ϫ ) (24) . In comparison to the up to 36% of NK cells expressing IFN-␥ in wild-type (WT)-infected mice, Ͻ1% of the NK cells in the infected IFNAR Ϫ/Ϫ mice expressed IFN-␥ at 30 h after LCMV infection (Fig. 3B ). Thus, blocking type 1 IFN respon- Because type 1 IFNs stimulate a wide range of effects, including those important for antiviral defense, the direct consequences of NK cell responsiveness to these cytokines required examination in the context of a WT environment. Therefore, experiments evaluated IFN-␥ induction in peritoneal WT (IFNAR ϩ/ϩ ) and IF-NAR Ϫ/Ϫ cells after their isolation and adoptive transfer into WT recipient mice (Fig. 3C ). Cells from recipient and donor mice were distinguished by expression of the CD45.1 or CD45.2 allele, with recipient mice being CD45.1 ϩ and donor mice being CD45.1 Ϫ (Fig. 3C ). For these studies, WT and IFNAR-deficient PECs were prepared and transferred into WT recipient mice prior to LCMV infection. None of the populations expressed IFN-␥ when recipients remained uninfected ( Fig. 3C ; 0 h after LCMV). When WT PECs were transferred into WT recipients, both donor and recipient NK cells expressed IFN-␥ at 30 h after infection, with overall mean expression of 23 and 21%, respectively (Fig. 3C ). In contrast, transferred NK cells isolated from IFNAR-deficient mice were significantly decreased in their ability to express IFN-␥, with expression averaging approximately 4% compared to 22% of WT recipient NK cells (Fig. 3C ). Taken together, the studies conclusively prove that type 1 IFN signaling within peritoneal NK cells is required for the induction of IFN-␥ expression during LCMV infection.