Disentangling the Immunomodulatory Effects of Vitamin D on the SARS-CoV-2 Virus by In Vitro Approaches

Vitamin D is a fat-soluble vitamin with multiple functions, including the modulation of the immune response, amongst others. Earlier studies have demonstrated that the active form of vitamin D, 1,25-dihydroxivitamin D, inhibits LPS-induced IL-6 and TNF-α production by human monocytes in a dose-dependent manner. On the other hand, some in vitro studies support that this vitamin has immune modulatory effects on viral infections. However, it remains unclear whether vitamin D regulates the immune response in infectious diseases triggered by viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes COVID-19. This study aimed to evaluate the anti-inflammatory properties of vitamin D against the spike protein of the SARS-CoV-2 virus. For this purpose, vitamin D was used in two different doses of 10 and 25 nM on the THP-1 cell line, which was stimulated with low doses of the SARS-CoV-2 virus spike protein. The THP-1 cell line, which is derived from human monocytic cells, was chosen since it contains the ACE2 transporter of the spike protein. Moreover, it is a widely used model to examine inflammatory processes due to its potential to stimulate inflammation and the release of inflammatory cytokines. The THP-1 cells were incubated for 1 h with the spike protein, subsequently treated with the two selected doses of vitamin D and incubated for 24 h. ELISA and RT-qPCR techniques were used to quantify the levels of inflammatory cytokines. Our results showed that vitamin D had no effect on the mRNA transcriptional levels of cytokine IL-6, but it was able to down-regulate the transcriptional levels of the pro-inflammatory cytokines IL-1β and TNF-α. There was no dose–response relationship between vitamin D and the expression of these genes. In conclusion, vitamin D inhibited inflammatory cytokine production on spike protein-stimulated inflammation in the THP1 cell line. The study is being completed by testing higher doses of vitamin D and of the spike protein. Additionally, other markers of inflammation are being measured through the use of transcriptomic analyses of the control vs. treated THP1 cells.