Differential activation of human monocyte-derived and plasmacytoid dendritic cells by West Nile virus generated in different host cells.

Dendritic cells (DCs) play a central role in innate immunity and antiviral responses. In this study, we investigated the production of alpha interferon (IFN-alpha) and inducible chemokines by human monocyte-derived dendritic cells (mDCs) and plasmacytoid dendritic cells (pDCs) infected with West Nile virus (WNV), an emergent pathogen whose infection can lead to severe cases of encephalitis in the elderly, children, and immunocompromised individuals. Our experiments demonstrated that WNV grown in mammalian cells (WNV(Vero)) was a potent inducer of IFN-alpha secretion in pDCs and, to a lesser degree, in mDCs. The ability of WNV(Vero) to induce IFN-alpha in pDCs did not require viral replication and was prevented by the treatment of cells with bafilomycin A1 and chloroquine, suggesting that it was dependent on endosomal Toll-like receptor recognition. On the other hand, IFN-alpha production in mDCs required viral replication and was associated with the nuclear translocation of IRF3 and viral antigen expression. Strikingly, pDCs failed to produce IFN-alpha when stimulated with WNV grown in mosquito cells (WNV(C7/10)), while mDCs responded similarly to WNV(Vero) or WNV(C7/10). Moreover, the IFN-dependent chemokine IP-10 was produced in substantial amounts by pDCs in response to WNV(Vero) but not WNV(C7/10), while interleukin-8 was produced in greater amounts by mDCs infected with WNV(C7/10) than in those infected with WNV(Vero). These findings suggest that cell-specific mechanisms of WNV recognition leading to the production of type I IFN and inflammatory chemokines by DCs may contribute to both the innate immune response and disease pathogenesis in human infections.

Early production of type I interferon during West Nile virus infection: role for lymphoid tissues in IRF3-independent interferon production.

Infection of cells with flaviviruses in vitro is reduced by pretreatment with small amounts of type I interferon (IFN-alpha/beta). Similarly, pretreatment of animals with IFN and experiments using mice defective in IFN signaling have indicated a role for IFN in controlling flavivirus disease in vivo. These data, along with findings that flavivirus-infected cells block IFN signaling, suggest that flavivirus infection can trigger an IFN response. To investigate IFN gene induction by the very first cells infected during in vivo infection with the flavivirus West Nile virus (WNV), we infected mice with high-titer preparations of WNV virus-like particles (VLPs), which initiate viral genome replication in cells but fail to spread. These studies demonstrated a brisk production of IFN in vivo, with peak levels of over 1,000 units/ml detected in sera between 8 and 24 h after inoculation by either the intraperitoneal or footpad route. The IFN response was dependent on genome replication, and WNV genomes and WNV antigen-positive cells were readily detected in the popliteal lymph nodes (pLN) of VLP-inoculated mice. High levels of IFN mRNA transcripts and functional IFN were also produced in VLP-inoculated IFN regulatory factor 3 null (IRF3(-/-)) mice, indicating that IFN production was independent of the IRF3 pathways to IFN gene transcription, consistent with the IFN type produced (predominantly alpha).