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.
Valentina Salvi, Hoang Oanh Nguyen, Francesca Sozio, Tiziana Schioppa, Carolina Gaudenzi, Mattia Laffranchi, Patrizia Scapini, Mauro Passari, Ilaria Barbazza, Laura Tiberio, Nicola Tamassia, Cecilia Garlanda, Annalisa Del Prete, Marco A. Cassatella, Alberto Mantovani, Silvano Sozzani, Daniela Bosisio
TLR7 and TLR8 are responsible for primary DC activation by SAMPs.