poly I:C during the L. monocytogenes infection are largely mediated by type I IFNs, and confirm the ability of type I IFNs to potently enhance L. monocytogenes pathogenicity. Type I, But Not Type II, IFN Signaling Is Impaired in L. monocytogenes–infected IFNAR1 / Mice. To examine the biological basis by which type I IFNs increase sensitivity to L. monocytogenes infection, we examined potential targets for type I IFNs that are known to play important roles in L. monocytogenes clearance. Both type I and II IFNs are critical in combating certain viral infections through the up-regula- tion of overlapping and nonoverlapping gene subsets (25). However, only type II IFN, or IFN- , has been shown to contribute to antibacterial immunity, as mice deficient in the IFN- receptor quickly succumb to infection by L. monocytogenes (26). Furthermore, mice deficient in various proteins required for IFN- signaling, including IRF2 and IRF8, have been shown to be highly susceptible to infec- tion by L. monocytogenes (27). To determine if the absence of type I IFN signaling can modulate the inducible levels of IFN- , we measured IFN- serum levels in mice that had been infected with L. monocytogenes. We found that IFN- serum levels in IFNAR1 / and wild-type mice were in- duced to similar levels 24 h after L. monocytogenes infection 441 O’Connell et al. Figure 3. Type I IFNs sensitize mice to L. monocyto- genes infection. (A) IFNAR1 / (n 13), IFNAR1 / (n 8), and IFNAR1 / (n 14) mice were infected intravenously with 1.5 106 L. monocytogenes and via- bility was assayed daily for 9 d. (B) Wild-type mice were challenged with 5 105 L. monocytogenes (n 6), 200 g poly I:C (n 6), or the combination of the two (n 6), and viability was assayed daily for 9 d. (C) IFNAR1 / and wild-type control mice were infected intravenously with 5 105 L. monocytogenes either in the absence or presence of 200 g poly I:C. After 3 d, the mice were killed and the L. monocytogenes titer was de- termined in the liver (n 4) and spleen (n 4), and presented as CFUs. (D) A portion of the livers from C were fixed, sectioned, and subjected to a histological analysis after hematoxylin and eosin staining. All data are representative of at least two independent experiments. (Fig. 4 A). IFN target genes such as IP10, TGTP, and IRF1 were also induced to similar levels in both wild-type and IFNAR1 / mice, suggesting that these genes are targets of IFN- signaling during L. monocytogenes infection (Fig. 4 B). The inflammatory cytokine IL-6 was also induced to similar levels in IFNAR1 / and wild-type mice (Fig. 4 A). In con- trast, induction of Mx1, PKR, TRAIL, and Daxx was com- pletely ablated in the IFNAR1 / spleen, indicating that this set of genes is specifically activated by type I IFNs (Fig. 4 B). These findings argue that the resistance to L. monocytogenes infection observed in IRF3- and IFNAR1-deficient mice compared with wild-type controls was not a result of altered IFN- production and subsequent signaling, or altered pro- duction of the proinflammatory cytokine IL-6. L. monocytogenes–infected Macrophages Up-regulate a Type I IFN–dependent Apoptotic Gene Subset. Fig. 4 B demon- strates that among the type I IFN–specific genes up-regu- lated in the spleen during infection are genes with known roles in promoting apoptosis such as TRAIL, PKR, and Daxx (Fig. 4 B and reference 28). Macrophage cells have been shown to undergo type I IFN–dependent apoptosis in response to L. monocytogenes (16). Because macrophages make up a fraction of the splenocyte population, we wanted to