Myeloid cells, including tissue and lymphoid DC and macrophages (Mw), are among the first cells to encounter WNV during infection and thus function to restrict the spread of virus to distant tissues and the CNS [2] . To define the role of IPS-1 in controlling virus replication and innate immunity in myeloid cells, we analyzed WNV infection and host responses in primary bone marrow-derived DC and Mw recovered from wild type and IPS-1 2/2 mice. DC and Mw were infected at an MOI of 1.0 (relative to viral plaque assay quantification of BHK-21 cells; see Methods) and evaluated for virus replication, IFN induction, and innate immune triggering of ISG expression (Fig 2) . IPS-1 2/2 DCs sustained significantly higher WNV replication at 36 and 48 hours pi compared to wild type infected cells (Fig 2A) . WNV infection of wild type DCs induced IFN-b secretion but this response was completely abolished in IPS-1 2/2 DCs (Fig 2B) . The lack of IFN-b induction in IPS-1 2/2 DCs correlated with a lack of ISG expression including RIG-I, MDA5, and STAT-1 ( Fig 2C) . In addition, expression of ISG54 and ISG49, which are direct IRF-3 target genes [30, 31] , were not induced during WNV infection of IPS-1 2/2 DCs (Fig 2C) . Moreover, ISG56, another IRF-3 target gene [31] , was induced late during infection and to lower levels as compared to ISG54 and ISG49 in wild type, infected DCs. WNV infection of IPS-1 2/2 Mw resulted in significantly higher virus replication between 24 and 48 hours pi as compared to infected wild type cells ( Fig 2D) . Whereas wild type infected Mw expressed IFN-b, this response was completely abolished in IPS-1 2/2 Mw (Fig 2F) . We also observed a differential expression of ISGs and IRF-3target genes within WNV-infected Mw. RIG-I, MDA5, and STAT-1 were not induced in IPS-1 2/2 Mw, whereas, ISG56, ISG49, and PKR were expressed at reduced levels and with delayed kinetics. These data establish that IPS-1-dependent RLR signaling is the major innate immune signaling pathway that controls virus replication in conventional DCs and Mw.