In Vitro Characterization of Inhibition Function of Calcifediol to the Protease Activity of SARS-COV-2 PLpro

Vitamin D has been identified by the European Food Safety Authority (EFSA) as having sufficient evidence for a causal relationship between intake and optimal immune system function25-28. Vitamin D inhibits SARS-CoV-2 replication in vitro15,22, mitigates lung inflammation, damage, and lethality in mice with an MHV-3 model for β-CoV respiratory infections15,22, reduces SARS-CoV-2 replication in nasal epithelial cells via increased type I interferon expression18, downregulates proinflammatory cytokines IL-1β and TNF-α in SARS-CoV-2 spike protein-stimulated cells14, attenuates nucleocapsid protein-induced hyperinflammation by inactivating the NLRP3 inflammasome through the VDR-BRCC3 signaling pathway19, may be neuroprotective by protecting the blood-brain barrier, reducing neuroinflammation, and via immunomodulatory effects29, may mitigate hyperinflammation and cytokine storm by upregulating TLR10 expression which downregulates proinflammatory cytokines11, downregulates ACE2 and TMPRSS2 in human trophoblasts and minimizes spike protein-induced inflammation17, may minimize cytokine storm by dampening excessive cytokine production1, may suppress post-entry viral replication and minimize cathepsin-L-dependent entry via LL-37 induction10, minimizes platelet aggregation mediated by SARS-CoV-2 spike protein via inhibiting integrin αIIbβ3 outside-in signaling13, and improves regulatory immune cell levels and control of proinflammatory cytokines in severe COVID-1930. Calcifediol inhibits SARS-CoV-2 papain-like protease (PLpro), a critical enzyme for viral replication12. Symptomatic COVID-19 is associated with a lower frequency of natural killer (NK) cells and vitamin D has been shown to improve NK cell activity31,32.