SARS‐CoV‐2 infection activates dendritic cells via cytosolic receptors rather than extracellular TLRs

Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) causes coronavirus disease 2019 (COVID‐19), an infectious disease characterized by strong induction of inflammatory cytokines, progressive lung inflammation, and potentially multiorgan dysfunction. It remains unclear how SARS‐CoV‐2 infection leads to immune activation. The Spike (S) protein of SARS‐CoV‐2 has been suggested to trigger TLR4 and thereby activate immunity. Here, we have investigated the role of TLR4 in SARS‐CoV‐2 infection and immunity. Neither exposure of isolated S protein, SARS‐CoV‐2 pseudovirus nor primary SARS‐CoV‐2 isolate induced TLR4 activation in a TLR4‐expressing cell line. Human monocyte‐derived DCs express TLR4 but not angiotensin converting enzyme 2 (ACE2), and DCs were not infected by SARS‐CoV‐2. Notably, neither S protein nor SARS‐CoV‐2 induced DC maturation or cytokines, indicating that both S protein and SARS‐CoV‐2 virus particles do not trigger extracellular TLRs including TLR4. Ectopic expression of ACE2 in DCs led to efficient infection by SARS‐CoV‐2 and, strikingly, efficient type I IFN and cytokine responses. These data strongly suggest that not extracellular TLRs but intracellular viral sensors are key players in sensing SARS‐CoV‐2. These data imply that SARS‐CoV‐2 escapes direct sensing by TLRs, which might underlie the lack of efficient immunity to SARS‐CoV‐2 early during infection.