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 function23-26. Vitamin D inhibits SARS-CoV-2 replication in vitro13,20, mitigates lung inflammation, damage, and lethality in mice with an MHV-3 model for β-CoV respiratory infections13,20, reduces SARS-CoV-2 replication in nasal epithelial cells via increased type I interferon expression16, downregulates proinflammatory cytokines IL-1β and TNF-α in SARS-CoV-2 spike protein-stimulated cells12, attenuates nucleocapsid protein-induced hyperinflammation by inactivating the NLRP3 inflammasome through the VDR-BRCC3 signaling pathway17, may be neuroprotective by protecting the blood-brain barrier, reducing neuroinflammation, and via immunomodulatory effects27, downregulates ACE2 and TMPRSS2 in human trophoblasts and minimizes spike protein-induced inflammation15, may minimize cytokine storm by dampening excessive cytokine production1, minimizes platelet aggregation mediated by SARS-CoV-2 spike protein via inhibiting integrin αIIbβ3 outside-in signaling11, and improves regulatory immune cell levels and control of proinflammatory cytokines in severe COVID-1928. Calcifediol inhibits SARS-CoV-2 papain-like protease (PLpro), a critical enzyme for viral replication10. Symptomatic COVID-19 is associated with a lower frequency of natural killer (NK) cells and vitamin D has been shown to improve NK cell activity29,30.