1. Introduction
Although the amount of vitamin C required to prevent scurvy is relatively low (i.e., ~10 mg/day) [9], the recommended dietary intakes for vitamin C are up to one hundred-fold higher than that for many other vitamins [10]. A diet that supplies 100–200 mg/day of vitamin C provides adequate to saturating plasma concentrations in healthy individuals and should cover general requirements for the reduction of chronic disease risk [11,12]. Due to the low storage capacity of the body for the water-soluble vitamin, a regular and adequate intake is required to prevent hypovitaminosis C. Epidemiological studies have indicated that hypovitaminosis C (plasma vitamin C < 23 μmol/L) is relatively common in Western populations, and vitamin C deficiency (<11 μmol/L) is the fourth leading nutrient deficiency in the United States [13,14].
Vitamin C has a number of activities that could conceivably contribute to its immune-modulating effects. It is a highly effective antioxidant, due to its ability to readily donate electrons, thus protecting important biomolecules (proteins, lipids, carbohydrates, and nucleic acids) from damage by oxidants generated during normal cell metabolism and through exposure to toxins and pollutants (e.g., cigarette smoke) [17]. Vitamin C is also a cofactor for a family of biosynthetic and gene regulatory monooxygenase and dioxygenase enzymes [18,19]. The vitamin has long been known as a cofactor for the lysyl and prolyl hydroxylases required for stabilization of the tertiary structure of collagen, and is a cofactor for the two hydroxylases involved in carnitine biosynthesis, a molecule required for transport of fatty acids into mitochondria for generation of metabolic energy (Figure 1) [19].
Vitamin C is also a cofactor for the hydroxylase enzymes involved in the synthesis of catecholamine hormones, e.g., norepinephrine, and amidated peptide hormones e.g., vasopressin, which are central to the cardiovascular response to severe infection [20]. Furthermore, research over the past 15 years or so has uncovered new roles for vitamin C in the regulation of gene transcription and cell signaling pathways through regulation of transcription factor activity and epigenetic marks (Figure 1) [21,22]. For example, the asparagyl and prolyl hydroylases required for the downregulation of the pleiotropic transcription factor hypoxia-inducible factor-1α (HIF-1α) utilize vitamin C as a cofactor [21]. Recent research has also indicated an important role for vitamin C in regulation of DNA and histone methylation by acting as a cofactor for enzymes which hydoxylate these epigenetic marks [22].
2. Barrier Integrity and Wound Healing
Vitamin C is actively accumulated into the epidermal and dermal cells via the two sodium-dependent vitamin C transporter (SVCT) isoforms 1 and 2 [27], suggesting that the vitamin has crucial functions within the skin. Clues to the role of vitamin C in the skin come from the symptoms of the vitamin C deficiency disease scurvy, which is characterized by bleeding gums, bruising, and impaired wound healing [28,29].
Vitamin C intervention studies in humans (using both dietary and gram doses of vitamin C) have shown enhanced vitamin C uptake into skin cells [26,36] and enhanced oxidant scavenging activity of the skin [36,37]. The elevated antioxidant status of the skin following vitamin C supplementation could potentially protect against oxidative stress induced by environmental pollutants [38,39]. The antioxidant effects of vitamin C are likely to be enhanced in combination with vitamin E [40,102].
Following surgery, patients require relatively high intakes of vitamin C in order to normalize their plasma vitamin C status (e.g., ≥500 mg/day) [105], and administration of antioxidant micronutrients, including vitamin C, to patients with disorders in wound healing can shorten the time to wound closure [48,49,106,107].
3. Vitamin C and Leukocyte Function
Leukocytes, such as neutrophils and monocytes, actively accumulate vitamin C against a concentration gradient, resulting in values that are 50- to 100-fold higher than plasma concentrations [111,112,113]. These cells accumulate maximal vitamin C concentrations at dietary intakes of ~100 mg/day [114,115], although other body tissues likely require higher intakes for saturation [116,117].
Accumulation of millimolar concentrations of vitamin C into neutrophils, particularly following activation of their oxidative burst, is thought to protect these cells from oxidative damage [119].
An alteration in the balance between oxidant generation and antioxidant defenses can lead to alterations in multiple signaling pathways, with the pro-inflammatory transcription factor nuclear factor кB (NFкB) playing a central role [121]. Oxidants can activate NFкB, which triggers a signaling cascade leading to continued synthesis of oxidative species and other inflammatory mediators [122,123]. Vitamin C has been shown to attenuate both oxidant generation and NFкB activation in dendritic cells in vitro, and NFкB activation in neutrophils isolated from septic Gulo knockout mice [75,124].
3.1. Neutrophil Chemotaxis
Neutrophil infiltration into infected tissues is an early step in innate immunity.
Neutrophils express more than 30 different chemokine and chemoattractant receptors in order to sense and rapidly respond to tissue damage signals [128]. Early studies carried out in scorbutic guinea pigs indicated impaired leukocyte chemotactic response compared with leukocytes isolated from guinea pigs supplemented with adequate vitamin C in their diet (Table 1) [54,55,56,64]. These findings suggest that vitamin C deficiency may impact on the ability of phagocytes to migrate to sites of infection.
Supplementation of healthy volunteers with dietary or gram doses of vitamin C has also been shown to enhance neutrophil chemotactic ability [61,62,63,147].
3.3. Neutrophil Apoptosis and Clearance
Following microbial phagocytosis and killing, neutrophils undergo a process of programmed cell death called apoptosis [161]. This process facilitates subsequent phagocytosis and clearance of the spent neutrophils from sites of inflammation by macrophages, thus supporting resolution of inflammation and preventing excessive tissue damage (Figure 2).
Thus, vitamin C may be expected to protect the oxidant-sensitive caspase-dependent apoptotic process following activation of neutrophils. In support of this premise, in vitro studies have shown that loading human neutrophils with vitamin C can enhance Escherichia coli-mediated apoptosis of the neutrophils (Table 1) [71].
To date, only one study has investigated the effect of vitamin C supplementation on neutrophil apoptosis in septic patients [178]. Intravenous supplementation of septic abdominal surgery patients with 450 mg/day vitamin C was found to decrease caspase-3 protein levels and, thus was presumed to have an anti-apoptotic effect on peripheral blood neutrophils.
3.4. Neutrophil Necrosis and NETosis
Neutrophils that fail to undergo apoptosis instead undergo necrotic cell death (Figure 2). The subsequent release of toxic intracellular components, such as proteases, can cause extensive tissue damage [179,180]. One recently discovered form of neutrophil death has been termed NETosis. This results from the release of ‘neutrophil extracellular traps’ (NETs) comprising neutrophil DNA, histones, and enzymes [181]. Although NETs have been proposed to comprise a unique method of microbial killing [182,183], they have also been implicated in tissue damage and organ failure [184,185]. NET-associated histones can act as damage-associated molecular pattern proteins, activating the immune system and causing further damage [186]. Patients with sepsis, or who go on to develop sepsis, have significantly elevated levels of circulating cell-free DNA, which is thought to indicate NET formation [184,187].
3.5. Lymphocyte Function
Like phagocytes, B- and T-lymphocytes accumulate vitamin C to high levels via SVCT [192,193]. The role of vitamin C within these cells is less clear, although antioxidant protection has been suggested [194]. In vitro studies have indicated that incubation of vitamin C with lymphocytes promotes proliferation [76,77], resulting in enhanced antibody generation [78], and also provides resistance to various cell death stimuli [195]. Furthermore, vitamin C appears to have an important role in developmental differentiation and maturation of immature T-cells (Table 1) [76,79]. Similar proliferative and differentiation/maturation effects have been observed with mature and immature natural killer cells, respectively [196].
Administration of vitamin C to elderly people was also shown to enhance ex vivo lymphocyte proliferation [80], a finding confirmed using combinations of vitamin C with vitamins A and/or E [148,197].
Vitamin C administration enhanced Treg proliferation and inhibited the negative immunoregulation of Tregs by inhibiting the expression of specific transcription factors, antigens, and cytokines [89]. The mechanisms involved likely rely on the gene regulatory effects of vitamin C [79,89,199,200].
3.6. Inflammatory Mediators
Incubation of vitamin C with peripheral blood lymphocytes decreased lipopolysaccharide (LPS)-induced generation of the pro-inflammatory cytokines TNF-α and IFN-γ, and increased anti-inflammatory IL-10 production, while having no effect on IL-1β levels [77]. Furthermore, in vitro addition of vitamin C to peripheral blood monocytes isolated from pneumonia patients decreased the generation of the pro-inflammatory cytokines TNF-α and IL-6 [86].
Supplementation of healthy human volunteers with 1 g/day vitamin C (with and without vitamin E) was shown to enhance peripheral blood mononuclear cell-derived IL-10, IL-1, and TNF-α following stimulation with LPS [87,94]. Thus, the effect of vitamin C on cytokine generation appears to depend on the cell type and/or the inflammatory stimulant.
4. Vitamin C Insufficiency Conditions
Numerous environmental and health conditions can have an impact on vitamin C status. In this section we discuss examples which also have a link with impaired immunity and increased susceptibility to infection. For example, exposure to air pollution containing oxidants, such as ozone and nitrogen dioxide, can upset the oxidant-antioxidant balance within the body and cause oxidative stress [204]. Oxidative stress can also occur if antioxidant defenses are impaired, which may be the case when vitamin C levels are insufficient [205]. Air pollution can damage respiratory tract lining fluid and increase the risk of respiratory disease, particularly in children and the elderly [204,206] who are at risk of both impaired immunity and vitamin C insufficiency [14,204]. Vitamin C is a free-radical scavenger that can scavenge superoxide and peroxyl radicals, hydrogen peroxide, hypochlorous acid, and oxidant air pollutants [207,208]. The antioxidant properties of vitamin C enable it to protect lung cells exposed to oxidants and oxidant-mediated damage caused by various pollutants, heavy metals, pesticides, and xenobiotics [204,209].
Tobacco smoke is an underestimated pollutant in many parts of the world. Both smokers and passive smokers have lower plasma and leukocyte vitamin C levels than non-smokers [10,210,211], partly due to increased oxidative stress and to both a lower intake and a higher metabolic turnover of vitamin C compared to non-smokers [10,211,212,213]. Mean serum concentrations of vitamin C in adults who smoke have been found to be one-third lower than those of non-smokers, and it has been recommended that smokers should consume an additional 35 mg/day of vitamin C to ensure there is sufficient ascorbic acid to repair oxidant damage [10,14]. Vitamin C levels are also lower in children and adolescents exposed to environmental tobacco smoke [214].
A decrease in plasma vitamin C levels has been observed in studies of type 2 diabetes [18,226], and a major cause of increased need for vitamin C in type 2 diabetes is thought to be the high level of oxidative stress caused by hyperglycemia [10,227,228]. Inverse correlations have been reported between plasma vitamin C concentrations and the risk of diabetes, hemoglobin A1c concentrations (an index of glucose tolerance), fasting and postprandial blood glucose, and oxidative stress [219,229,230,231,232]. Meta-analysis of interventional studies has indicted that supplementation with vitamin C can improve glycemic control in type 2 diabetes [233].
Elderly people are particularly susceptible to infections due to immunosenescence and decreased immune cell function [234]. For example, common viral infections such as respiratory illnesses, that are usually self-limiting in healthy young people, can lead to the development of complications such as pneumonia, resulting in increased morbidity and mortality in elderly people. A lower mean vitamin C status has been observed in free-living or institutionalized elderly people, indicated by lowered plasma and leukocyte concentrations [10,235,236], which is of concern because low vitamin C concentrations (<17 µmol/L) in older people (aged 75–82 years) are strongly predictive of all-cause mortality [237].
5. Vitamin C and Infection
A major symptom of the vitamin C deficiency disease scurvy is the marked susceptibility to infections, particularly of the respiratory tract, with pneumonia being one of the most frequent complications of scurvy and a major cause of death [7]. Patients with acute respiratory infections, such as pulmonary tuberculosis and pneumonia, have decreased plasma vitamin C concentrations relative to control subjects [245]. Administration of vitamin C to patients with acute respiratory infections returns their plasma vitamin C levels to normal and ameliorates the severity of the respiratory symptoms [246].
Meta-analysis has indicated that vitamin C supplementation with doses of 200 mg or more daily is effective in ameliorating the severity and duration of the common cold, and the incidence of the common cold if also exposed to physical stress [249].
Referência :
Nutrients. 2017 Nov 3;9(11):1211.
