Toll-like Receptor 9–mediated Recognition of Herpes Simplex Virus-2 by Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDCs) have been identified as a potent secretor of the type I interferons (IFNs) in response to CpG as well as several viruses. In this study, we examined the molecular mechanism of virus recognition by pDCs. First, we demonstrated that the CD11c+Gr-1intB220+ pDCs from mouse bone marrow secreted high levels of IFN-alpha in response to either live or UV-inactivated Herpes simplex virus-2 (HSV-2). Next, we identified that IFN-alpha secretion by pDCs required the expression of the adaptor molecule MyD88, suggesting the involvement of a Toll-like receptor (TLR) in HSV-2 recognition. To test whether a TLR mediates HSV-2-induced IFN-alpha secretion from pDCs, various knockout mice were examined. These experiments revealed a clear requirement for TLR9 in this process. Further, we demonstrated that purified HSV-2 DNA can trigger IFN-alpha secretion from pDCs and that inhibitory CpG oligonucleotide treatment diminished HSV-induced IFN-alpha secretion by pDCs in a dose-dependent manner. The recognition of HSV-2 by TLR9 was mediated through an endocytic pathway that was inhibited by chloroquine or bafilomycin A1. The strict requirement for TLR9 in IFN-alpha secretion was further confirmed by the inoculation of HSV-2 in vivo. Therefore, these results demonstrate a novel mechanism whereby the genomic DNA of a virus can engage TLR9 and result in the secretion of IFN-alpha by pDCs.