Figure 2. Mice deficient in IRF3 have an enhanced ability to clear in- fection by L. monocytogenes. (A) IRF3 / (n 8) and wild-type control mice (n 8) were infected intravenously with 1.5 106 L. monocytogenes and viability was assayed daily for 9 d. (B) IRF3 / (n 4) and wild-type mice (n 4) were infected intravenously with 5 105 L. monocytogenes for 3 d, at which time the L. monocytogenes titer in the spleen and liver was assayed and presented as CFUs. (C) A portion of the IRF3 / and wild- type livers from B were fixed, sectioned, and subjected to a histological analysis after hematoxylin and eosin staining. All data are representative of at least two independent experiments. proximately one tenth the LD50 (not depicted). Histological analysis showed that the L. monocytogenes–infected wild-type livers displayed more severe tissue pathology relative to IRF3 / livers at 3 d after infection (Fig. 2 C). Livers from wild-type mice demonstrated extensive perivascular infiltrate and multiple intravascular thrombi, whereas IRF3 / livers were relatively clear. Collectively, these data demonstrate that IRF3 activation inhibits the host’s ability to control L. monocytogenes growth during the early stages of infection. Type I IFNs Sensitize Mice to L. monocytogenes Infection. It has been clearly shown that mice lacking the IFNAR are extremely susceptible to many viruses (22); however, initial studies using mice on a 129SV/EV genetic background did not find a significant difference in survival between L. monocytogenes–infected wild-type and IFNAR1 / mice (23). To determine if the L. monocytogenes–resistant pheno- type observed in the IRF3 / mice, which are on a C57BL/6 background, was due to the absence of type I IFN signaling or by means of a type I IFN–independent pathway, we used IFNAR1 / mice that were crossed onto the C57BL/6 genetic background for five generations. These mice, along with their littermate wild-type and het- erozygous controls, were challenged with L. monocytogenes. Only 7% of IFNAR1 / and 33% of IFNAR1 / mice re- mained viable, whereas 77% of IFNAR1 / mice sur- vived the infection (Fig. 3 A). These data strongly indicate that IRF3-mediated induction of type I IFNs results in en- hanced severity of L. monocytogenes infection. Furthermore, functional analysis of type I IFNs during L. monocytogenes infection in various genetic backgrounds may improve our understanding of both antibacterial and antiviral immune responses in heterogenous human populations. To further challenge the hypothesis that type I IFNs in- terfere with the host’s ability to control an L. monocytogenes infection, we chose to investigate whether elevated type I IFN levels could enhance susceptibility to an L. monocyto- genes infection. We and others have found that intravenous injection of poly I:C results in strong induction of type I IFNs, including both IFN- and IFN- 4 (24 and unpub- lished data). Therefore, wild-type mice were inoculated with a combination of poly I:C and L. monocytogenes, or ei- ther agent alone. Mice given a sublethal dose of L. monocy- togenes remained healthy and viable (Fig. 3 B). Likewise, the administration of poly I:C alone did not appear to in- fluence the health status of the mice by day 9 (Fig. 3 B). However, when mice were given the combination of a sublethal dose of L. monocytogenes along with poly I:C, all of the mice died by day 4 (Fig. 3 B). To test whether the increased L. monocytogenes resistance observed in the IFNAR1 / versus wild-type mice was due to improved L. monocytogenes clearance, we infected both wild-type and IFNAR1 / mice with a sublethal dose of L. monocytogenes. After 3 d of infection, both the liver and spleen of the IFNAR1 / mice had a substantially lower L. monocytogenes titer compared with wild-type controls (Fig. 3 C). Furthermore, wild-type livers suffered from signifi- cant L. monocytogenes–induced tissue pathology, whereas IFNAR1 / livers appeared relatively clear from insult (Fig. 3 D). To determine if the decreased survival observed in mice receiving poly I:C in addition to L. monocytogenes was mediated by type I IFNs, we gave both wild-type and IFNAR1 / mice the same poly I:C L. monocytogenes treatment and monitored L. monocytogenes clearance from the spleen and liver by day 3. As shown in Fig. 3 C, wild- type mice receiving L. monocytogenes plus poly I:C exhib- ited drastically higher bacterial titers in both the liver and spleen compared with wild-type mice receiving L. monocy- togenes alone. When these same studies were performed in IFNAR1 / mice, poly I:C had almost no effect on the levels of bacteria in the liver or spleen (Fig. 3 C). Histolog- ical analysis of wild-type livers from poly I:C and L. mono- cytogenes coadministered mice revealed multiple parenchy- mal abscesses compared with the less severe lesions found in the those mice injected with L. monocytogenes alone (Fig. 3 D). Consistent with the CFU analysis, livers from IFNAR1 / mice with or without coadministration of poly I:C were relatively clear of tissue abscesses (Fig. 3 D). These findings indicate that the adverse effects elicited by 440 Type I Interferons Enhance Listeria monocytogenes Infection