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

TLDR

Plasmacytoid dendritic cells are potent secretors of type I interferons in response to CpG motifs and various viruses. This study aimed to elucidate the molecular mechanism by which plasmacytoid dendritic cells recognize viruses. The authors used TLR knockout mice and pharmacological inhibitors to show that HSV‑2–induced IFN‑α secretion requires TLR9 and occurs via an endocytic pathway blocked by chloroquine or bafilomycin A1. They demonstrated that HSV‑2 DNA triggers IFN‑α secretion through a MyD88‑dependent, TLR9‑mediated, endocytic pathway, with CpG oligonucleotides dose‑dependently inhibiting the response and confirming the mechanism in vivo.

Abstract

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-α in response to either live or UV-inactivated Herpes simplex virus-2 (HSV-2). Next, we identified that IFN-α 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-α 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-α secretion from pDCs and that inhibitory CpG oligonucleotide treatment diminished HSV-induced IFN-α 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-α 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-α by pDCs.