Figure 4. Type I, but not Type II, IFN signaling is in- hibited in L. monocytogenes–infected IFNAR1 mice. (A) The spleens and serum from IFNAR1 / and wild-type mice infected with 1.5 106 L. monocytogenes were re- moved 24 h after infection and serum IFN- and IL-6 lev- els were assayed by ELISA. (B) The splenocyte mRNA was subjected to a Q-PCR analysis to assay IP10, IRF1, TGTP, IL-6, Mx1, TRAIL, PKR, and Daxx expression. (C) IFNAR1 / and wild-type BMMs were infected with L. monocytogenes at a multiplicity of infection of 1 and as- sayed at the indicated time points for IL-1 , Mx1, PKR, Daxx, TRAIL, and PML gene expression by Q-PCR. Re- sults shown are representative of at least two independent experiments. Q-PCR data are presented in relative expres- sion units and have been normalized to L32 mRNA levels. determine whether they might be contributing to the type I IFN–dependent expression of apoptotic genes during the L. monocytogenes infection. Wild-type and IFNAR / BMMs were challenged with L. monocytogenes for up to 8 h. Gene expression was analyzed at the indi- cated time points by Q-PCR (Fig. 4 C). We found that macrophage cells, similar to the heterogenous splenocyte population, respond to L. monocytogenes by up-regulating numerous proapoptotic genes, such as PKR, Daxx, TRAIL, and PML, in a type I IFN–dependent manner. As a con- trol, type I IFN was not required for the up-regulation of the proinflammatory cytokine IL-1 (Fig. 4 C). These data indicate that L. monocytogenes induction of type I IFN– dependent proapoptotic genes observed in the spleen likely involves splenic macrophages. L. monocytogenes–induced Splenic Apoptosis Is Dependent on Type I IFNs. One of the hallmark effects mediated by type I IFNs is the induction of apoptosis in infected cells, which has been shown to drastically inhibit the viral life cycle (28). To assess whether L. monocytogenes can also induce apoptosis, mice were infected with L. monocytogenes and splenocyte apoptosis was assessed 48 h later by TUNEL assay. We ob- served massive apoptosis, as indicated by TUNEL cells, in the splenic tissue from infected mice compared with unin- fected controls (Fig. 5 A). TUNEL cells were widely dis- tributed and highly prevalent throughout the spleens in a fol- licular pattern. To determine whether the type I IFN pathway contributes to splenic apoptosis after infection by L. monocytogenes, wild-type, IFNAR1 / , and IRF3 / mice were infected with L. monocytogenes and TUNEL assays were performed 48 h later. Despite roughly equivalent bacterial loads at this time point (Fig. 5 B), L. monocytogenes–infected IFNAR1 / and IRF3 / spleens exhibited a drastic reduc- tion in splenic apoptosis compared with wild-type controls (Fig. 5 A). These results demonstrate that L. monocytogenes– induced apoptosis in vivo is largely dependent on type I IFN production and signaling. Discussion Although type I IFN up-regulation plays an important role in fighting viral infection (23), our results demonstrate that 442 Type I Interferons Enhance Listeria monocytogenes Infection