Immunological Aspects of SARS-CoV-2 Infection and the Putative Beneficial Role of Vitamin-D

Abstract

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is still an ongoing global health crisis. Immediately after the inhalation of SARS-CoV-2 viral particles, alveolar type II epithelial cells harbor and initiate local innate immunity. These particles can infect circulating macrophages, which then present the coronavirus antigens to T cells. Subsequently, the activation and differentiation of various types of T cells, as well as uncontrollable cytokine release (also known as cytokine storms), result in tissue destruction and amplification of the immune response. Vitamin D enhances the innate immunity required for combating COVID-19 by activating toll-like receptor 2. It also enhances antimicrobial peptide synthesis, such as through the promotion of the expression and secretion of cathelicidin and β-defensin; promotes autophagy through autophagosome formation; and increases the synthesis of lysosomal degradation enzymes within macrophages. Regarding adaptive immunity, vitamin D enhances CD4⁺ T cells, suppresses T helper 17 cells, and promotes the production of virus-specific antibodies by activating T cell-dependent B cells. Moreover, vitamin D attenuates the release of pro-inflammatory cytokines by CD4⁺ T cells through nuclear factor κB signaling, thereby inhibiting the development of a cytokine storm. SARS-CoV-2 enters cells after its spike proteins are bound to angiotensin-converting enzyme 2 (ACE2) receptors. Vitamin D increases the bioavailability and expression of ACE2, which may be responsible for trapping and inactivating the virus. Activation of the renin-angiotensin-aldosterone system (RAS) is responsible for tissue destruction, inflammation, and organ failure related to SARS-CoV-2. Vitamin D inhibits renin expression and serves as a negative RAS regulator. In conclusion, vitamin D defends the body against SARS-CoV-2 through a novel complex mechanism that operates through interactions between the activation of both innate and adaptive immunity, ACE2 expression, and inhibition of the RAS system. Multiple observation studies have shown that serum concentrations of 25 hydroxyvitamin D are inversely correlated with the incidence or severity of COVID-19. The evidence gathered thus far, generally meets Hill's causality criteria in a biological system, although experimental verification is not sufficient. We speculated that adequate vitamin D supplementation may be essential for mitigating the progression and severity of COVID-19. Future studies are warranted to determine the dosage and effectiveness of vitamin D supplementation among different populations of individuals with COVID-19.

Keywords: adaptive immunity; angiotensin-converting enzyme 2; coronavirus disease 2019; innate immunity; renin–angiotensin–aldosterone system; severe acute respiratory syndrome coronavirus 2; vitamin D.

Figure 1

Possible pathophysiological pathways after the entry of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 infects both alveolar macrophages and type II alveolar cells by binding to angiotensin-converting 2 (ACE2) receptors. Before SRAS-CoV-2 enters the host cells, the spike protein 1 (S1) should be pre-activated by the host furin, a convertase proprotein, which will expose the receptor binding domain (RBD) of S1. RBD has a strong binding affinity for the host cell membrane ACE2 for effective entry. After binding of RBD and ACE2, the type 2 transmembrane protease (TMPRSS2) will proteolytically activated the S1/S2 boundary through cleavage the S1–S2 protein which will cause drastic structural changes with further expose the fusion peptide (FP) of S2 which will facilitate the processing of viral-host cell fusion [44]. Immediately after entry, S protein activation is mediated by lysosomal cathepsins and/or furin within the TGN [14,15]. SARS-CoV-2 replication is suppressed by synthetic furin inhibitors [16]. After entry into the host cell, the virus downregulates ACE2 expression, which in turn upregulates angiotensin II (Ang II). Ang II interacts with its receptor, Ang II receptor type 1, and modulates the gene expression of several inflammatory cytokines via nuclear factor κB signaling. This interaction also promotes macrophage activation and results in the production of inflammatory cytokines that may cause acute respiratory distress syndrome or macrophage activation syndrome. Some metalloproteases, such as ADAM metallopeptidase domain 17, cleave these pro-inflammatory cytokines and ACE2 receptors, resulting in their release as soluble forms. This contributes to the loss of the protective function of surface ACE2 and potentially exacerbates SARS-CoV-2 pathogenesis [39]. Monocytes and macrophages in the mononuclear phagocyte system, infected with SARS-CoV-2, produce various pro-inflammatory cytokines and chemokines, a process critical for the induction of local and systemic inflammatory responses known as cytokine storms [18].

Figure 2

Putative vitamin D-related innate immunity (anti-infection activity) and autophagy responses to coronavirus disease 2019 (COVID-19) infection. The activation of monocyte toll-like receptors (TLR1/TLR2) by pathogen-associated molecular patterns (PAMPs) induces the expression of the cytokine interleukin-1 (IL-1) and suppresses the expression of the IL-1 receptor antagonist, thereby enhancing intracrine signaling by IL-1 and increasing the activity of nuclear factor кB (NF-кB). Pathogen phagocytosis increases the intracellular concentrations of muramyl dipeptide (MDP), which can then bind to the intracellular pathogen recognition receptor NOD2 and increase NF-кB activity. In addition, the activation of TLR1/TLR2 by PAMP results in the transcriptional induction of vitamin D receptor (VDR) and the activation of 1α-hydroxylase expression. Circulating 25-hydroxyvitamin D [25(OH)D] bound to serum vitamin D-binding protein enters monocytes in its free form and is converted to active 1,25-dihydroxyvitamin D [1,25(OH)2D] by mitochondrial 1α-hydroxylase. It then binds to VDR and acts as a transcription factor, induces the expression of cathelicidin and β-defensin 4A, and promotes autophagy through autophagosome formation. NF-кB also enhances the transcriptional induction of cathelicidin and β-defensin 4A. In the presence of increased cathelicidin, immune cells induce the activity of NOD2/CARD15-β-defensin 2, autophagy-related protein 5 (ATG5), and BECLIN1, and they then induce autophagy. Cathelicidin, β-defensin 4A, and mature autophagosomes then work in concert to eliminate bacteria. Cytoplasm SNAP receptor proteins mediate fusion between autophagosomes and lysosomes, and various lysosomal enzymes further hydrolyze proteins, lipids, and nucleic acids. Digestive nutrients may be recycled and utilized by the cells. The net efficacy of such a response is highly dependent on vitamin D status, as well as the availability of circulating 25(OH)D for intracrine conversion to active 1,25(OH)2D by the enzyme 1α-hydroxylase. Activation of TLR1 and TLR2 by PAMP induces the expression of cytokines and inflammatory pathways. Adequate vitamin D supplementation may strengthen the innate immune response against COVID-19 through TLR activation and autophagy, enhance antimicrobial peptide synthesis, and increase the generation of lysosomal degradation enzymes within macrophages.

Figure 3

Vitamin D-related adaptive immune responses to COVID-19. Dendritic cells expressing 1α-hydroxylase and the vitamin D receptor (VDR) can utilize circulating 25-hydroxyvitamin D [25(OH)D] for intracrine responses through localized conversion to active vitamin D [1,25(OH)2D]. Intracrine synthesis of 1,25(OH)2D inhibits the maturation of dendritic cells, thereby modulating CD4⁺ T cell function. CD4⁺ T cell responses to 25(OH)D may also be mediated in a paracrine manner, with 1,25(OH)2D acting on VDR-expressing CD4⁺ T cells. VDR-expressing CD4⁺ T cells are also potential targets for systemic 1,25(OH)2D (endocrine effect). Vitamin D acts on dendritic cells to stimulate effector CD4⁺ cells to differentiate into one of the four types of CD4⁺ cells. Activated T cells also express VDR. Under normal circumstances, vitamin D increases T helper (Th) 2 (Th2) cytokines (e.g., IL-10) and the efficiency of regulatory T (Treg) lymphocytes. Vitamin D inhibits the development of Th1 cells, which are associated with the cellular immune response. In addition, vitamin D promotes the association of Th2 cells with humorally mediated immunity. Thus, vitamin D promotes the shift from Th1 to Th2 cells. Vitamin D also inhibits the development of Th17 cells, which play roles in tissue damage and inflammation. The fourth group of CD4⁺ T cells, Tregs, suppress the function of vitamin D. Circulating and local active vitamin D acts through intracrine, paracrine, and endocrine effects to regulate adaptive immunity in SARS-CoV infection. First, it suppresses the maturation of dendritic cells and weakens the antigenic presentation. Second, it increases cytokine production by CD4⁺ T cells and promotes the efficiency of Treg lymphocytes. Finally, it suppresses Th1 and Th17 cytokine secretion, as well as related tissue destruction [82].

Figure 4

Effects of vitamin D on angiotensin-converting enzyme 2 (ACE2) and the renin–angiotensin–aldosterone system (RAS) in response to the coronavirus disease 2019 (COVID-19). (A) Schematic of the RAS under normal circumstances, with physiological steps of the generation of angiotensin (Ang) II and Ang 1–7 shown, as well as their activity on specific receptors. (B) Interaction of the severe acute respiratory syndrome coronavirus 2 with the RAS. (C) Possible therapeutic effects of vitamin D for COVID-19 and related acute respiratory distress syndrome or macrophage activation syndrome. The ACE2 molecule, besides being a receptor of SARS-CoV-2, reduces the activity of the renin–angiotensin system by converting Ang I and Ang II into Ang 1–9 and Ang 1–7 respectively [33]. Thus, the ACE2 protein has been shown to play an important role in protecting against some disorders such as cardiovascular complications, chronic obstructive pulmonary disease (COPD) and diabetes, among other COVID-19 comorbidities [34]. The ACE2/Ang 1–7 axis counterbalances the ACE/Ang II-I axis by decreasing Ang II levels, the activation of angiotensin type 1 receptors (AT1Rs) and, thus, leads to decreased pathophysiological effects on tissues, such as inflammation and fibrosis [35].