Herpes simplex virus type 1 activates murine natural interferon-producing cells through toll-like receptor 9

Natural interferon-producing cells (IPCs) specialize in the production of high levels of type 1 interferons (IFNs) in response to encapsulated DNA and RNA viruses. Here we demonstrate that the secretion of type 1 IFN in response to herpes simplex virus type 1 (HSV-1) in vitro is mediated by the toll-like receptor 9 (TLR9)/MyD88 pathway. Moreover, IPCs produce interleukin-12 (IL-12) in response to HSV-1 in vitro, which is also dependent on TLR9/MyD88 signaling. Remarkably, though TLR9/MyD88-deficiency abrogates IPC responses to HSV-1 in vitro, mice lacking either MyD88 or TLR9 are capable of controlling HSV-1 replication in vivo after local infection, demonstrating that TLR9- and MyD88-independent pathways in cells other than IPCs can effectively compensate for defective IPC responses to HSV-1.