The discovery of a physiological role of vitamin D in the regulation of the innate immune response began with the finding that activated macrophages and dendritic cells express CYP27B1 (the gene encoding for 1-α hydroxylase) and the VDR gene [21,48]. In 2006, a pivotal study demonstrated that toll-like receptor (TLR) activation of human monocytes and macrophages with a synthetic M. Tuberculosis-derived lipopeptide led to a shift in the gene expression profile of these cells enhancing the expression of VDR and CYP27B1, with an increase in the endogenous production of 1,25(OH)₂D from circulating 25(OH)D [20]. In the same study, 25(OH)D₃ (calcifediol) supplementation on TLR-stimulated human monocytes in culture strongly enhanced the expression of cathelicidin, a natural antibacterial peptide, thus supporting the idea that 1,25(OH)₂D through binding to VDR could regulate the innate immune response. These observations, together with an IL-15-dependent, CYP27B1-induced stimulation of macrophage development and differentiation and the well-known excessive 1,25(OH)₂D₃ production by macrophages in several granulomatous diseases, led to the assumption that vitamin D can act as a potent stimulator of mechanisms associated with pathogen elimination. Furthermore, 1,25(OH)₂D in monocytes and hepatocytes inhibit the expression of hepcidin, another natural antibacterial protein directed to suppress the transmembrane protein, ferroportin, thus inhibiting iron export from cells and limiting iron supply to microorganisms to sustain their growth [49].

Dendritic antigen-presenting cells express VDR and are modulated by 1,25(OH)₂D towards a more tolerogenic phenotype through a delay in their maturation, thus limiting the ability to present antigen to T cells. In mature dendritic cells, VDR expression is reduced [50] with a decrease in sensitivity to 1,25(OH)₂D, allowing an initial presentation of antigen to T cells. The inhibition of cell maturation acts as a barrier to the overstimulation of T cells. Neutrophils express VDR but lack CYP27B1 [51], indicating that these cells may preferentially function as a hormonal target for 1,25(OH)₂D. Existing data indicate that vitamin D can induce the formation of neutrophil extracellular traps (NETs) and acts on primary human neutrophils as a modulator of the inflammatory response by lowering inflammatory neutrophil-derived cytokine production [52]. Collectively, these data indicate that vitamin D reinforces bacterial killing by these cells, while restricting a neutrophil-induced inflammatory response [53]. Furthermore, 1,25(OH)₂D is also able to influence eosinophil function through the downregulation of interleukin (IL)-15, the pivotal cytokine involved in the recruitment of these cells [54], and lowers immunoglobulin-E (IgE)-dependent mast cell activation [55]. Natural killer (NK) cells play an intermediate role between innate and adaptive immunity [56]. Several studies confirm that vitamin D also exerts a regulatory role on this family of T lymphocytes by modulating their cytotoxicity, cytokine secretion, and degranulation [57].

Other evidence suggests that 1,25(OH)₂D may directly modulate the expression of several cytokines implicated in innate immunity. In co-cultures of infected macrophages, vitamin D induces the expression of IL-1-beta and IL-8 [58] and, in infected human peripheral blood mononuclear cells, downregulates the expression of other proinflammatory cytokines such as IL-6, tumor necrosis factor-(TNF)-α, and interferon (IFN)-γ [59]. These data confirm an important role of 1,25(OH)₂D in enhancing a prompt response to infection together with a modulating effect on the acute inflammatory response.

Besides the indirect consequences on T-cell differentiation and function derived from the effects of vitamin D on innate immune cells, 1,25(OH)₂D acts as a regulator of mature T cells by altering the balance between T helper (Th)1 and Th2 cell differentiation. Specifically, 1,25(OH)₂D enhances the expression of IL-4 and strongly inhibits IFN-γ production from naïve CD4+ T cells in culture, thus showing the capacity to inhibit Th1 and reciprocally promote Th2 cell differentiation [44]. This effect may suggest a possible preventive or therapeutic role of 1,25(OH)₂D in several Th1 cell-driven diseases, such as multiple sclerosis, type 1 diabetes mellitus (T1D), inflammatory bowel diseases, and rheumatoid arthritis (RA). Another study performed in a mouse model of human Crohn disease confirmed that calcitriol with or without dexamethasone upregulates Th2 markers and is able to reduce not only Th1 differentiation but also the Th17-driven response, indicating that vitamin D may exert some of its effects on inflammation and autoimmunity through the regulation of IL-17 expression in T cells [45]. A reduction of Th17 cells is usually counterbalanced by an increase in T-regulatory cells’ (T-regs) reciprocal axis. The 1,25(OH)₂D₃ together with transforming growth factor beta (TGF-β) is able to induce forkhead box P3 expression in naïve CD4+ T cells, promoting T-regs’ differentiation [46], and to increase the production of the anti-inflammatory cytokine IL-10 from CD4+/CD25+ T-regs [60] with potential beneficial effects in several autoimmune diseases. With the aim to better understand the molecular mechanisms involved in the orderly shutdown and retraction of CD4+ type Th1 cell response, Chauss et al. recently analyzed the bronchoalveolar lavage fluid CD4+ T cells of patients with COVID-19 and identified an autocrine/paracrine vitamin D loop that allows Th1 cells to activate and respond to vitamin D as part of a complex shutdown program repressing IFN-γ and enhancing IL-10 [61]. Indeed, the molecular pathways identified in this study may yield important information to aid in the development of novel therapeutic approaches to accelerate the shutdown program of hyper-inflammatory cells in COVID-19 patients.

Human active B-lymphocytes carry VDR as well as 1-α-hydroxylase, suggesting that vitamin D may strongly also influence this family of immune cells [47]. B-cell response is influenced by vitamin D via an inhibition of the ongoing proliferation of activated B-cells, induction of B-cell apoptosis, and inhibition of the generation of plasma cells. As a net result of these effects, vitamin D acts as an inhibitor of immunoglobulin secretion, also suggesting a possible role of vitamin D in B-cells-related disorders.

Experimental data accumulated in the past three decades provide convincing evidence for an immunomodulatory effect of vitamin D either on innate or on adaptive immunity [14,62]. To mention a paradigmatic example, in RA, vitamin D exerts effects against the intrinsic disease mechanisms, which include Th1 polarization, lower expression of Th2 cytokines, higher immunoglobulin production, enhanced Th17 differentiation, and lower T-regs’ activation and function. Based on these anti-inflammatory properties of vitamin D, a protective effect on several inflammatory and autoimmune diseases has been hypothesized by numerous reports demonstrating (mainly retrospective studies) low circulating levels of 25(OH)D in a high proportion of patients with chronic inflammatory conditions. In this respect, it is important to address two critical points. First of all, a mechanism of reverse causality induced by chronic diseases cannot be ruled out. Secondly, data derived from several recent studies support the hypothesis that systemic inflammation lowers 25(OH)D concentrations, thus contributing to the low circulating levels observed in patients suffering from chronic inflammatory diseases. In this context, in a recent experimental study based on a human endotoxemia model reproducing systemic inflammation in vivo, circulating levels of 25(OH)D significantly decreased shortly after initiation of a bolus of E. coli-derived lipopolysaccharide [63].