SARS-CoV-2 infection induces functional impairment of vitamin D receptor signaling to drive interleukin 6-dependent hyperinflammation in mononuclear phagocytes

The dysregulated inflammatory response, particularly the interleukin 6 (IL-6)-driven cytokine storm, is a hallmark of severe COVID-19. Mononuclear phagocytes are key cellular populations in the pathogenesis of SARS-CoV-2 infection, but the underlying regulatory mechanisms remain incompletely defined. Based on existing evidence of vitamin D immunomodulatory properties in viral infections, we investigated its role during SARS-CoV-2 infection. We integrated transcriptional profiling of monocytes and myeloid dendritic cells (mDCs) from healthy individuals and COVID-19 patients (moderate/severe, with/without viremia) with an in vitro model of SARS-CoV-2-infected monocyte-derived macrophages (MDMs). Additionally, a functional assay using U937-derived monocytes and macrophages challenged with inactivated SARS-CoV-2 particles was used to assess the immunomodulatory effect of calcitriol (vitamin D) treatment. We found that SARS-CoV-2 infection triggers an NF-κB-dependent inflammatory signature associated with disease severity, both in monocytes from COVID-19 patients and in MDMs infected in vitro. This signature is characterized by hyperproduction of IL-6 and upregulation of its signaling components, including IL6R, JAK1/2, STAT3, and SOCS3. Moreover, we found some evidence of a novel viral-dependent suppression of the Vitamin D Receptor (VDR) pathway, demonstrated by downregulation of both VDR expression and its target genes, including CAMP, LYZ, and IRF5, in monocytes from patients with COVID-19 and in vitro SARS-CoV-2-infected MDMs. This resulted in functional impairment of VDR signaling. Importantly, calcitriol treatment potently suppressed SARS-CoV-2-induced IL-6 production in our model of U937 cell line, suggesting that restoration of VDR signaling could temper this key inflammatory axis. Our findings reveal a dual-hit mechanism in severe COVID-19, in which SARS-CoV-2 infection simultaneously hyperactivates the pro-inflammatory pattern-recognition receptors/NF-κB/IL-6 axis and suppresses the anti-inflammatory VDR pathway in mononuclear phagocytes. The effective inhibition of IL-6 by calcitriol provides a potential role of vitamin D in mitigating pathological inflammation, positioning it as a plausible immunomodulatory strategy for severe COVID-19.