Virus, infections and treatments. For LCMV infection experiments, mice were either uninfected (0 h after infection) or infected i.p. with 5 ϫ 10 4 PFU of LCMV Armstrong strain, clone E350 (7, 9, 17) . For IL-12 blocking experiments, animals were injected with either 750 g or 1 mg of anti-IL-12p40 (anti-IL-12/23, clone C17.8; BioXCell, West Lebanon, NH) or control rat IgG2a (clone 2A3; BioXCell) antibodies 12 h prior to LCMV infection and 36 h prior to treatment with 20 g of LPS (from Salmonella enterica serovar Enteritidis; Sigma, St. Louis, MO). Samples from LPStreated mice were harvested 6 h after treatment (22) . (The levels of efficacy of treatment with either 750 g or 1 mg of anti-IL-12 were similar.) For IFNAR blocking experiments, animals were injected with 250 g of anti-IFNAR1 (clone MAR1-5A3; Leinco Technologies, St. Louis, MO) or control IgG antibodies 12 h prior to LCMV infection. For NK depletion experiments, animals were injected with 500 g of anti-NK1.1 (clone PK136; BioXCell) or control IgG2a antibodies at 36 or 12 h prior to LCMV infection.