The pathogenesis of COVID-19 involves cell death caused by SARS-CoV-2 infection and hyperinflammatory response. Therefore, understanding the mechanism of inflammatory response during SARS-CoV-2 is the key to develop strategies to reduce COVID-19 complications. We investigated what viral ligand is responsible for activation of innate immune cells by stimulating human and mouse macrophages with recombinant spike (S), membrane (M), envelope (E), and neucleocapsid (N) proteins. Interestingly, we observed that S protein, its subunits and S-trimer, but not M, N, and E proteins, induced inflammatory cytokines and chemokines. Similar to immune cells, human and mouse lung epithelial cells also responded to S protein to produce inflammatory molecules. Since SARS-CoV-2 dose not infect immune cells, we interrogated whether virally infected epithelial cells stimulate macrophages. We observed that S protein-expressed lung epithelial cells induced inflammatory responses in macrophages when cocultured. Notably, S protein induced inflammation independent of ACE2 receptor. Digging dipper, we wanted to explore what pattern recognition receptor senses S protein to trigger inflammation. Our investigation revealed that TLR2, but not TLR4, is the immune sensor for S protein which in association with either TLR1 or TLR6 activates the NF-kB pathway via MyD88. Consistently, S-induced inflammatory response was completely absent in Tlr2/ mice. Blocking TLR2 chemically or though gene manipulation also abrogated S-induced inflammation in vitro. Together these data demonstrate the mechanism of cytokine storm during SARS-CoV-2 infection, and suggest that TLR2 could be a potential therapeutic target for COVID-19.

Supported by R01 DK125352