Toll-like receptors 9 and 3 as essential components of innate immune defense against mouse cytomegalovirus infection

TLDR

Dendritic cells produce type I interferon in response to viral infections, aiding natural‑killer‑cell‑mediated clearance, and this interferon can be induced by CpG‑oligodeoxynucleotides or double‑stranded RNA. The study investigates a codominant CpG‑ODN unresponsive phenotype caused by an N‑ethyl‑N‑nitrosourea‑induced missense mutation in the Tlr9 gene (Tlr9 CpG1). Homozygous Tlr9 CpG1 mice are highly susceptible to mouse cytomegalovirus, exhibiting impaired IFN‑α/β secretion and natural‑killer activation, and the work shows that both TLR9‑MyD88 and TLR3‑Trif pathways are activated in vivo, each contributing to innate defense but neither alone providing full protection, underscoring a critical leucine‑rich repeat motif in TLR9 essential for ligand response.

Abstract

Several subsets of dendritic cells have been shown to produce type I IFN in response to viral infections, thereby assisting the natural killer cell-dependent response that eliminates the pathogen. Type I IFN production can be induced both by unmethylated CpG-oligodeoxynucleotide and by double-stranded RNA. Here, we describe a codominant CpG-ODN unresponsive phenotype that results from an N -ethyl- N -nitrosourea-induced missense mutation in the Tlr9 gene ( Tlr9 CpG1 ). Mice homozygous for the Tlr9 CpG1 allele are highly susceptible to mouse cytomegalovirus infection and show impaired infection-induced secretion of IFN-α/β and natural killer cell activation. We also demonstrate that both the Toll-like receptor (TLR) 9 → MyD88 and TLR3 → Trif signaling pathways are activated in vivo on viral inoculation, and that each pathway contributes to innate defense against systemic viral infection. Whereas both pathways lead to type I IFN production, neither pathway offers full protection against mouse cytomegalovirus infection in the absence of the other. The Tlr9 CpG1 mutation alters a leucine-rich repeat motif and lies within a receptor domain that is conserved within the evolutionary cluster encompassing TLRs 7, 8, and 9. In other TLRs, including three mouse-specific TLRs described in this paper, the affected region is not represented. The phenotypic effect of the Tlr9 CpG1 allele thus points to a critical role for TLR9 in viral sensing and identifies a vulnerable amino acid within the ectodomain of three TLR proteins, essential for a ligand response.