Abstract
The inflammatory and IFN pathways of innate immunity play a key role in the resistance and pathogenesis of coronavirus disease 2019 (COVID-19). Innate sensors and SARS-CoV-2–associated molecular patterns (SAMPs) remain to be completely defined. Here, we identified single-stranded RNA (ssRNA) fragments from the SARS-CoV-2 genome as direct activators of endosomal TLR7/8 and MyD88 pathway. The same sequences induced human DC activation in terms of phenotype and function, such as IFN and cytokine production and Th1 polarization. A bioinformatic scan of the viral genome identified several hundreds of fragments potentially activating TLR7/8, suggesting that products of virus endosomal processing potently activate the IFN and inflammatory responses downstream of these receptors. In vivo, SAMPs induced MyD88-dependent lung inflammation characterized by accumulation of proinflammatory and cytotoxic mediators and immune cell infiltration, as well as splenic DC phenotypical maturation. These results identified TLR7/8 as a crucial cellular sensor of ssRNAs encoded by SARS-CoV-2 involved in host resistance and the disease pathogenesis of COVID-19.
Highlights
SARS-CoV-2 is a positive-sense single-stranded RNA virus belonging to the family of Coronaviridae, including the closely related Middle East respiratory syndrome coronavirus (MERS-CoV) and SARS-CoV [1]
Within DC heterogeneity, plasmacytoid DCs play an important role as the major source of type I IFN in response to viral infection, while conventional DCs respond to a vast variety of pathogens by producing proinflammatory cytokines and are the main cells responsible for T cell activation [9,10,11]. pDCs sense single-stranded RNA (ssRNA) viruses through TLR7 [12], an endosomal receptor activated by genomic fragments rich in guanine and uracil (GU rich), derived by endosomal processing of the virus independently of infection [13]
We report that 2 short sequences within the ssRNA genome of SARS-CoV-2 activate the production of type I IFNs and the T cell–activating ability of human DCs by triggering endosomal TLR7 and TLR8
Summary
SARS-CoV-2 is a positive-sense single-stranded RNA (ssRNA) virus belonging to the family of Coronaviridae, including the closely related Middle East respiratory syndrome coronavirus (MERS-CoV) and SARS-CoV [1]. TLR7 and TLR8 display high structural and functional homology and similar ligand specificity [15] and recruit the same signaling intracellular adaptor molecule, MyD88 [16], the signaling pathways of these 2 TLRs diverge in functional significance, with TLR7 more involved in the antiviral immune response and TLR8 mastering the production of proinflammatory cytokines Both cDCs and pDCs were shown to be reduced in the blood of patients with severe acute COVID-19 [17, 18] as a possible result of cell activation [19], but the mechanisms of SARS-CoV-2 recognition and activation by innate immune cells still need to be identified. This study characterized SARS-CoV-2–associated molecular patterns (SAMPs) and identified the TLR7/8/MyD88 axis as a crucial pathway in the activation of human pDCs and cDCs
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