The Modulatory Role of Vitamin D in the Molecular Mechanisms of Sepsis and Infections with Focus on Viral Pathogenesis: A Narrative Review

Viral infections are a widely recognized cause of mortality. Recently it has been demonstrated that vitamin D plays an important immuno-modulatory role in both innate and adaptive immune responses against infections, by reducing excessive inflammation and enhancing defense mechanisms. Specifically, exacerbation of infections may cause sepsis, characterized by a dysregulated immune response with hyperinflammation and immune exhaustion. Vitamin D can modulate these processes through various mechanisms, such as enhancing antiviral protection, promoting anti-inflammatory responses, protecting endothelial barriers, and modulating T-cells. This has been demonstrated for various viral infections, including influenza viruses, respiratory syncytial virus, and severe acute respiratory syndrome coronavirus. Although vitamin D deficiency has been associated with increased susceptibility to viral infections and immune cells of infected patients are highly responsive to vitamin D, the clinical benefits of its supplementation to vitamin D-deficient infected individuals are uncertain and, most importantly, direct evidence linking vitamin D to viral sepsis specifically remains particularly limited, with most mechanistic inference extrapolated from non-septic viral infection and bacterial sepsis literature. Based on these assumptions, this review will outline the present understanding about vitamin D regulatory effects on immune system and the possible connection between its serum levels and viral infections, while also addressing controversial issues.