Infection with the dengue RNA virus activates TLR9 signaling in human dendritic cells

Abstract

Toll-like receptors (TLRs) are important sensors that recognize pathogen-associated molecular patterns. Generally, TLR9 is known to recognize bacterial or viral DNA but not viral RNA and initiate an immune response. Herein, we demonstrate that infection with dengue virus (DENV), an RNA virus, activates TLR9 in human dendritic cells (DCs). DENV infection induces release of mitochondrial DNA (mtDNA) into the cytosol and activates TLR9 signaling pathways, leading to production of interferons (IFNs). The DENV-induced mtDNA release involves reactive oxygen species generation and inflammasome activation. DENV infection disrupts the association between transcription factor A mitochondria (TFAM) and mtDNA and activates the mitochondrial permeability transition pores. The side-by-side comparison of TLR9 and cyclic GMP-AMP synthase (cGAS) knockdown reveals that both cGAS and TLR9 comparably contribute to DENV-induced immune activation. The significance of TLR9 in DENV-induced immune response is also confirmed in examination with the bone marrow-derived DCs prepared from -knockout mice. Our study unravels a previously unrecognized phenomenon in which infection with an RNA virus, DENV, activates TLR9 signaling by inducing mtDNA release in human DCs.

Dengue virus and the host innate immune response

“…2 ). This release of mtDNA has been shown to activate TLR9, an endosomal PRR that recognizes DNA containing nonmethylated CpG motifs, in human DCs in vitro 40 .…”

Innate Immune DNA Sensing of Flaviviruses

“…The current list of proposed cytosolic DNA sensors will eventually include proteins that are previously known to have been involved in DNA damage response pathways, transcription, inflammasome assemblies, and other cellular functions [56][57][58][59][60]. Although there are a few publications on some of these DNA sensors and their proposed roles in controlling Flavivirus infection in different biological systems, there is yet to be any evidence of antagonism of these pathways by flaviviruses, other than the cGAS/STING pathway [61][62][63][64].…”

“…Beyond cGAS/STING, researchers from the Ho group have also reported that the TLR9 signaling pathway is activated during DENV infection in human dendritic cells [62]. Using immunoprecipitation and qPCR assays, they have shown that TLR9 can detect leaked mitochondria DNA (mtDNA) during DENV infection.…”

Humanized Mice in Dengue Research: A Comparison with Other Mouse Models

“…Its double-stranded RNA (dsRNA) activates endosomal toll-like receptor 3 (TLR3), and its single-stranded RNA (ssRNA) activates TLR7 and TLR8 [33]. DENV also activates the complement system and TLR9/cGAS [34][35][36]. The activation of these PRRs initiates the antiviral response with a strong and early production of interferon-β (IFN-β), a weak production of interferon-γ (IFN-γ), and antimicrobial peptides (AMP) in cells such as keratinocytes [37].…”