Rheumatology Service. University Hospital of Salamanca

The vitamin D system has extraskeletal pleiotropic functions, including the modulation of the adaptive immune response and the enhancement of the innate response[1-3]. This explains why vitamin D influence on infections has been the subject of many analyses. The implication of vitamin D deficiency in tuberculosis has been known for decades. But it has also been associated with other infections, mainly respiratory tract infections and others such as the flu, exacerbations of chronic obstructive pulmonary disease or cystic fibrosis, sepsis or human immunodeficiency virus infection. More recently, there has been interest in knowing its influence on the pathogenesis and possible therapeutic use in SARS-CoV-2 infection[4]. In this article we will review the role of vitamin D in the immune system and its influence on infectious diseases.
As is well known, the biological function of vitamin D depends mainly on its active form 1,25(OH)2D or calcitriol. Binding to the intracellular vitamin D receptor (VDR) induces the expression of target genes. This receptor is present in numerous cells, including a large part of the cells of the immune system. The synthesis of calcitriol requires two steps of hydroxylation of vitamin D3 or cholecalciferol. The first in the liver, where it is transformed into 25(OH)D, calcidiol or calcifediol, and hydroxylation in the 1-alpha position in the kidney to obtain the definitive calcitriol[5].

Relationship between vitamin D and innate immunity
The immune response to the presence of a pathogen requires a rapid response and the innate immune system provides resources to intervene in barriers such as mucosa or skin. Calcitriol enhances innate immune reactions by inducing the transcription of genes that encode proteins with destructive capacity of bacteria, viruses and fungi.
Some immune cells such as macrophages, dendritic cells and cells of the bronchial and pulmonary mucosa have transmembrane “toll-like” receptors (TLR) and recognize the “molecular patterns associated with pathogens” that are part of the structure of infectious agents. This is the starting point for various cell signaling pathways that facilitate the synthesis of cytokines, chemokines, recognition receptors and the so-called “antimicrobial peptides” that have the ability to destroy pathogens. This response increases the competence of both immune cells in their anti-infective action5. On the other hand, cells of the adaptive system are activated[1].
This response is magnified by calcitriol, since one of the consequences of TLR activation is the increase in VDR expression. The VDR-calcitriol bond translocates to the cell nucleus where it binds to the retinoic acid receptor and promotes the transcription of genes such as catechillin and several defensins in cells of the immune system and respiratory endothelial surfaces. These are antimicrobial peptides, crucial in protecting this gateway[6].

Modulation of adaptive immunity by vitamin D
Calcitriol acts at different levels endocrine, paracrine and intracrine. As a whole, it decreases Th1 and Th17 responses, increases Th2 and Th8 while promoting T-reg. The result is an anti-inflammatory effect[5].
In a first step, it regulates the Th1 response by inhibiting the production of type 1 proinflammatory cytokines such as IL-12, INF-gamma IL6, IL8, TNF-alpha and IL9. On the contrary, it increases the Th2 response and its production of anti-inflammatory cytokines such as IL4, IL5 and IL10. This regulation of cytokines is mediated by the blockade of NF-Kappa B activation. It induces a change in the polarization of macrophages, passing from the proinflammatory M1 phenotype to the anti-inflammatory M2 phenotype. It inhibits dendritic cell differentiation by reducing the expression of MHC class 2 molecules, costimulation molecules, and increasing IL12. In this way, by preventing the maturation of dendritic cells, autoimmunity is prevented and immunological tolerance is promoted. Antigen presentation by immature dendritic cells impairs the immune response and induces tolerance.
Concerning B cells, calcitriol inhibits differentiation, proliferation and promotes apoptosis[6].
The global effect of vitamin D as a modulator of the immune system is mainly anti-inflammatory and helps prevent autoimmunity2.

Infection and vitamin D
If vitamin D participates in this way on the immune system, it is logical to think that its deficiency can limit the defenses against infectious agents[3]. This hypothesis is supported by many epidemiological studies. Studies have focused mainly on respiratory infections (cold, acute respiratory infections, tuberculosis) and viral infections (flu, HIV infection, hepatitis B virus infection, as well as coronavirus infection). See table 1[7].
In an intuitive way, it is easy to associate the nadir of vitamin D levels, during the winter, with the increase in the incidence of influenza and respiratory infections. This hypothesis has been evaluated in various studies, finding an association between vitamin D deficiency and influenza. The influenza virus survives better in environmental conditions of low exposure to ultraviolet light, temperature and humidity. But when attenuated virus has been inoculated into healthy volunteers, the appearance of clinical manifestations is more frequent when it is done in winter and in subjects with poorer vitamin D status[8]. Protection against the influenza virus is related to an action that enhances innate immunity, increasing the production of defensins by cells of the immune system and the respiratory epithelium.
A clear relationship between vitamin D deficiency and the prevalence of tuberculosis or its activity has been confirmed. Mycobacterium tuberculosis has an affinity for the “toll-like” receptors of macrophages, vitamin D would reduce its capacity for growth and replication by increasing the production of cathelicidin by macrophages. From a clinical point of view, recent data indicate that low concentrations of vitamin D would be more of a risk factor than the cause of infection. Supplementation does not manage to cure the active infection, but it can reduce the inflammatory load and improve anthropometric aspects that will help the patient in long-term recovery[9]. In a recent meta-analysis, 8 clinical trials with more than 1,750 patients suffering from active pulmonary tuberculosis and who received, in addition to anti-tuberculous treatment, different vitamin D supplementation regimens were evaluated. In all of them cholecalciferol was used as a form of vitamin D for supplementation in tuberculosis. Negativization of sputum was more frequent among those who received vitamin D. Among other beneficial effects, they found an improvement in lymphocyte count and recovery on chest X-ray[10].
Many studies explore the link with upper respiratory tract infections, which would be consistent in view of its involvement in innate immunity and cellular response in the endothelium of the respiratory tract. Epidemiological data on the status of the vitamin D system and these infections are available, as well as studies on their prevention with different supplementation regimens or higher therapeutic doses[11].
Among the epidemiological studies, a survey carried out in the United Kingdom of almost 20,000 people over 12 years of age should be highlighted. More history of recent respiratory infections was detected in individuals with lower concentrations of 25(OH)VD. After adjusting for confounding factors, the chance of having had an infection was higher when vitamin levels were 10 ng/ml or less when compared to optimal levels[12]. This association is even stronger in certain risk groups such as asthmatics or chronic obstructive pulmonary disease. Similar results are found in various cohorts from various countries, especially from the Anglo-Saxon environment. When hospital data is assessed, it is also reported that hospitalizations for influenza are longer and with a worse prognosis if there is a deficit of 25(OH)VD9.
To answer the question of whether acute respiratory infections can be prevented with vitamin D supplementation, there are numerous studies available, including some randomized clinical trials versus placebo. In a systematic review that evaluated 30 articles, positive results were only found in a few, especially those that included young patients and healthy adults. The best results have been seen with doses of 400 to 2,000 IU daily. Prevention in school-age children is also effective. It does not prevent respiratory infections in newborns when supplementation is administered to pregnant women.
In patients with previous pulmonary involvement, such as adults with COPD, favorable results are only seen in patients with deficient levels of 25(OH)VD. However, for adults with cystic fibrosis given a single dose of 250,000 IU of vitamin D3, hospitalizations are reduced over the following year[13].
Children are a particularly susceptible population for respiratory infections. In a clinical trial that included children between 3 and 12 months of age, it was found that the use of high doses of vitamin D (1,200 IU/day) for 4 months had a greater preventive effect on the appearance of influenza A than doses low vitamin D (400IU/day) (26% vs 46%). This article shows that to achieve greater efficacy, higher doses of vitamin D are needed. These high doses seem safe because the adverse effects were not different between the two groups and there were no pharmacological toxicity problems[14].
A meta-analysis has recently been published that includes 46 clinical trials and more than 75,000 patients. In the assessment of the global effect on the reduction of upper respiratory tract infections, a positive effect of vitamin D supplementation was found. The result has a significant difference, although it is of little magnitude (1-2%). One of the limitations they point out is the great heterogeneity of the studies in terms of age, dose of vitamin D, duration of treatment, study follow-up time. However, the authors conclude that daily doses of 400-1,000 IU for 12 months are the most beneficial[15].
It has also been tested on other infections. Supplementation does not modify the frequency or evolution of pneumonia. In the case of some viruses, such as the human immunodeficiency virus, deficient levels of vitamin D are linked to an increase in opportunistic infections and a decrease in survival. In these patients, the high inflammatory load at the expense of TNF-alpha would interfere with the production of 1,25(OH)D, blocking PTH. In patients with hepatitis B virus infection, the prevalence of failure is very high. Lower levels correlate strongly with increased viral load and poor progression to cirrhosis. Two meta-analyses that include nearly 10,000 patients agree that low concentrations of 25(OH)VD increase susceptibility to serious infections, sepsis, and increase mortality[9].
In summary and taking into account all the data, several authors point out that concentrations of 30 ng/ml can be considered effective in preventing infections, especially respiratory ones. Levels above 40 ng/ml do not appear to provide additional benefit. Levels below 20 ng/ml are associated with a higher risk of infections. The safety of supplementation is confirmed in numerous studies. As very infrequent adverse effects, some hypercalcaemia has been reported with no known subsequent complications[7,9,15].
Vitamin D and COVID-19
The literature generated in relation to COVID-19 and possible treatments is enormous. Unfortunately, a pharmacological measure against SARS-CoV-2 is not yet available. Much has been written about the role of vitamin D since, in addition to its influence on the immune system and the response of respiratory cells, there is the fact that vitamin D inhibits the expression of the renin-angiotensin-aldosterone system. In COVID-19, there is a dysregulation of the system with a predominance of angiotensin, which can favor the serious complications of the disease. In addition, vitamin D also reduces thromboembolic events associated with this infection. The anticoagulant effect is achieved by increased expression of anticoagulant glycoproteins, such as defensins, and decreased synthesis of factors essential for coagulation, inhibition of the renin-angiotensin system, and induction of angiotensin-converting enzyme type 2 receptors. An additional effect would be the role it has in reducing the oxidative stress that appears in the infection, downregulating the expression of glutathione[8].
Epidemiological data offer a clear relationship between low levels of 25(OH)D and SARS-CoV-2 infection. In a recent meta-analysis, low levels are associated with an increased risk of infection (OR 1.64) and severity (OR 2.58), but it has not been seen to influence mortality[17]. Some authors propose a new hypothesis to explain this situation and consider the possibility that 25(OH)D is a negative marker of the severity of inflammation. This decrease has been observed in other processes with high inflammation[16].
The use of vitamin D, in the form of supplementation or in higher doses, has been used in clinical trials. The results are variable and do not exceed the results obtained with other drugs such as dexamethasone or tocilizumab. However, given the frequency of low levels in the population at greatest risk, its use is recommended in many protocols considering the few side effects and the cost.
In summary, vitamin D has an immunomodulatory role in the immune system, enhancing innate immunity and the response of the respiratory epithelium to pathogens. Its deficit is associated with an increase in infections, such as tuberculosis, influenza, human immunodeficiency or COVID-19, and its correction has a beneficial preventive effect, especially evident in upper respiratory infections.

Conflict of interest: The author declares that he has no conflicts of interest.

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