The Nucleocapsid Protein of SARS-CoV-2 Does Not Induce Inflammation in Endothelial Cells or Monocytes

COVID-19 is caused by SARS-CoV-2 and has led to over seven million deaths worldwide. It can lead to severe complications like ARDS and sepsis with a systemic inflammatory response. The N-protein is a key structural protein of SARS-CoV-2 and is critical for viral replication and genome packaging. However, its role in COVID-19 induced inflammation remains controversial. One aspect that has been largely overlooked is the consideration of residual endotoxins in recombinant proteins when conducting in vitro immune response studies. We hypothesized that the N-protein will be proinflammatory in endothelial cells and monocytes, independent of potential endotoxin contamination. Our results showed that endotoxin depleted N-proteins did not lead to an increase in inflammatory response in these cells. Furthermore, any inflammatory effects seen were reduced to control levels with polymyxin B treatment. Overall, this showed that the inflammatory effects of recombinant N-proteins that have been observed in current literature may be due to endotoxin contamination, suggesting that it does not directly induce inflammation.