Vitamin D and Streptococci: The Interface of Nutrition, Host Immune Response, and Antimicrobial Activity in Response to Infection

Abstract

Streptococcus species are common causes of human infection. These Gram-positive, encapsulated bacterial pathogens infect diverse anatomic spaces, leading to infections including skin and soft tissue infection, endocarditis, pneumonia, meningitis, sinusitis, otitis media, chorioamnionitis, sepsis, and even death. Risk for streptococcal infection is highest in low- and middle-income countries where micronutrient deficiency is common. Epidemiological data reveal that vitamin D deficiency is associated with enhanced risk of streptococcal infection and cognate disease outcomes. Additionally, vitamin D improves antibacterial defenses by stimulating innate immune processes such as phagocytosis and enhancing production of reactive oxygen species (oxidative burst) and antimicrobial peptides (including cathelicidin and lactoferrin), which are important for efficient killing of bacteria. This review presents the most recent published work that studies interactions between the micronutrient vitamin D, the host immune system, and pathogenic streptococci as well as comparisons with other relevant infection models.

Graphical Abstract

INTRODUCTION

Vitamin D is a secosteroid, a subclass of steroids with a broken ring structure, which regulates calcium and phosphorus homeostasis in a variety of organisms and maintains the skeleton in vertebrates.¹ Extraskeletal functions of vitamin D have become apparent as studies have implicated vitamin D in cellular proliferation, differentiation, and immune system regulation.¹ Studies of vitamin D deficiency and supplementation revealed its protective role against hypertension, infections, autoimmune diseases, cardiovascular diseases, diabetes, and some cancers in addition to its antirachitic activities.¹

The genus Streptococcus contains both commensal bacteria that are part of the human microbiota in nearly every part of the human body and also several important human pathogens. Individual species in this genus are capable of causing invasive infection at every stage of human life from the perinatal period to advanced age. Among different age cohorts, streptococcal species cause diverse infections from skin and soft tissue infections to meningitis and endocarditis.

Vitamin D deficiency is quite common, and prior studies have suggested an association between the frequency and severity of streptococcal infections in individuals that are deficient in vitamin D.^1,2 In this review, we will comprehensively cover vitamin D physiology with a focus on how vitamin D’s immunomodulatory activity impacts the pathogenesis of streptococcal infections and how vitamin D supplementation may play an important role to improve perinatal health outcomes as a common manifestation of streptococcal infection.

STREPTOCOCCUS

The Streptococcus genus is a diverse group of encapsulated catalase negative, Gram-positive bacteria which exhibit a round, or cocci, chained or diplo-arranged morphology. The genus is further divided into 49 species, many of which are found as part of the normal human microbiota colonizing the respiratory, gastrointestinal, and genitourinary tracts. Thirty-five of these species have been associated with diverse infections in humans ranging from limited skin/soft tissue infections to invasive infections such as endocarditis and meningitis.^3,4 Four of these species cause the majority of human infections: Group A Streptococcus (including Streptococcus pyogenes), Group B Streptococcus (including Streptococcus agalactiae), oral streptococci (including Streptococcus mutans), and Streptococcus pneumoniae (commonly called pneumococcus).

Group A Streptococcus.

Group A Streptococcus (GAS) causes human disease ranging from mild infection such as pharyngitis and impetigo to life threatening infections including necrotizing fasciitis and streptococcal toxic shock syndrome, which causes widespread systemic vasodilation and/or perfusion of vital organs. Globally, GAS causes disease in up to 7 million people annually, with most cases (6 million) presenting as pharyngitis, commonly referred to as “strep throat.”^5,6 Serious immune sequelae may be triggered after repeated GAS infections, including acute glomerulonephritis and rheumatic heart disease, leading to a significant number of deaths attributed to GAS.⁵ A report by the World Health Organization estimated that GAS is the ninth leading infectious cause of human mortality.⁶

Group B Streptococcus.

Streptococcus agalactiae or Group B Streptococcus (GBS) is a common cause of perinatal infections but also infects elderly patients or those with underlying conditions such as diabetes.^7-9 GBS is an especially large health burden to mothers and neonates in developing countries, with an estimated 21.7 million pregnant mothers colonized with GBS in 2015;¹⁰ GBS vaginal colonization increases the incidence of perinatal infections including chorioamnionitis, funisitis, neonatal sepsis, mastitis, and increases risk for infection-related preterm birth. GBS infection predominately manifests as soft tissue infections, although other complications may arise. An epidemiological study in the USA from 2008 to 2016 found increasing incidence of invasive GBS infection in nonpregnant patients, and most patients had at least one underlying condition with obesity, diabetes, and older age being common.¹¹

Oral Streptococci (Including Streptococcus mutans).

Streptococci are the dominant species in the oral cavity and upper respiratory tract, and many of the oral streptococcal species have been historically classified as “viridans streptococci” because many display only partial hemolysis when cultured on blood agar.¹² This group was further separated based on biochemical properties and now phylogenetic approaches. Despite being part of the normal commensal microbiota, oral streptococci can cause human disease ranging from dental caries to invasive diseases such as bacteremia or endocarditis.¹² S. mutans, a common oral Streptococcus species, is best known for causing human dental caries.¹³ S. mutans adapted to be a commensal member of the oral microbiota, where this organism is a component of oral biofilms present on teeth and is the prime initiator of plaque and a potent producer of acid, leading to exacerbation of tooth decay and oral disease.¹⁴ S. mutans is able to colonize injured heart valves and cause endocarditis because it readily moves into the bloodstream during dental operations.¹⁵ Dental caries are the most common and costly oral disease worldwide, with S. mutans as the etiological agent underlying a large proportion of this burden.¹⁶

Streptococcus pneumoniae.

Streptococcus pneumonia, also referred to as pneumococcus, causes up to 4 million cases of illness within the United States and 450 000 hospitalizations per year.¹⁷ These infections carry a high mortality rate as studies indicate that 10% of patients with invasive pneumococcal diseases succumb to their infection.¹⁸ While S. pneumoniae commensally colonizes the pharynx and upper respiratory tract of healthy people, invasive pneumococcal infection may establish at a variety of sites, including the lung parenchyma and meninges.¹⁹ Depending on the location, S. pneumoniae can be isolated form approximately 5–90% of the healthy human population.²⁰ S. pneumoniae has been widely linked to respiratory infections in immunocompromised and low- and middle-income populations and is the major cause of community acquired pneumonia, an important cause of bacteremia in infants and older adults.⁴

BIOSYNTHESIS OF VITAMIN D

Vitamin D is an essential nutrient that can either be biosynthesized or absorbed through dietary ingestion. Vitamin D biosynthesis begins with the cholesterol precursor 7-dehydrocholesterol, which can be found in the epidermal layer of the skin (Figure 1). When the epidermis is exposed to ultraviolet light B (UVB) irradiation, 7-dehydrocholesterol structurally undergoes a photochemical ring opening to yield the previtamin D₃ intermediate.²¹ In the lower layers of the skin, previtamin D₃ undergoes thermal isomerization to generate vitamin D₃ (cholecalciferol). Vitamin D₃ traffics throughout the body via vitamin D binding proteins (DBP).²² Subsequently, 25-hydroxylase enzymes in the liver, specifically CYP2R1 and CYP27A1, catalyze oxidation of vitamin D₃ to 25-hydroxycholecalciferol (25(OH)D). 25(OH)D is the primary storage form of vitamin D within the body.²² In the kidneys, 25(OH)D is oxidized by CYP27B1 to yield 1,25-dihydroxycholecalciferol (1,25(OH)₂D₃), the active form of the vitamin D hormone.²² Catalysis to 1,25(OH)₂D₃ is tightly controlled by calcium, phosphate, and fibroblast growth factor 23 (FGF23) in a negative-feedback control mechanism to regulate calcium homeostasis. As such, the active form of vitamin D (1,25(OH)₂D₃) aids in maintenance of serum calcium levels in conjunction with parathyroid hormone.²¹ Additionally, 1,25(OH)₂D₃ increases intestinal calcium absorption and promotes mineralization of the skeleton.²¹

Figure 1.

Diagram of vitamin D biosynthesis. Vitamin D biosynthesis begins with the cholesterol precursor 7-dehydrocholesterol, which can be found in the epidermal layer of the skin. When the epidermis is exposed to ultraviolet light B (UVB) irradiation, 7-dehydrocholesterol structurally undergoes a photochemical ring opening to yield the previtamin D₃ intermediate. In the lower layers of the skin, previtamin D₃ then undergoes thermal isomerization to generate vitamin D₃ (cholecalciferol). Vitamin D₃ is then transported throughout the body via binding to vitamin D binding proteins (DBP). Subsequently, 25-hydroxylase enzymes in the liver, specifically CYP2R1 and CYP27A1, catalyze the oxidation of vitamin D₃ to 25-hydroxycholecalciferol (25(OH)D). This 25(OH)D isoform is the primary storage form of vitamin D and is oxidized by CYP27B1 to yield the active hormone form of vitamin D (1,25-dihydroxycholecalciferol (1,25(OH)₂D₃)) in the kidneys.

Aside from biosynthesis, vitamin D₃ is absorbed in the gastrointestinal tract after ingestion of foods such as fatty fish (salmon, sardines, and mackerel) or egg yolks.²³ Comparatively, vitamin D₂ (ergocalciferol) is a plant-derived source of vitamin D, which is often supplemented into nutritional products and marketed as metabolically analogous to vitamin D₃. However, research suggests that vitamin D₂ supplementation is not as effective at increasing 25(OH)D concentrations in serum, and supplementation with vitamin D₃ remains the superior dietary vitamin D source.²⁴

THE FIVE MAJOR FORMS OF VITAMIN D

While vitamins D₂ and D₃ are the predominant forms of vitamin D, chemically, the molecule exists in five different forms, vitamin D₁ through D₅ (Table 1). First discovered by Windaus and Lisert in 1931, vitamin D₁ was later found to be a 1:1 mixture of two compounds, lumisterol and ergocalciferol (vitamin D₂).²⁵ Due to its lack of purity, vitamin D₁ is no longer recognized as a source of vitamin D supplementation. Vitamin D₂(ergocalciferol) is largely found in fungi but is also commercially fortified in foods such as cereal, bread, and milk products. Structurally, the only difference between vitamin D₂ and D₃ (cholecalciferol) is a double bond between carbons 22 and 23 and the methyl group on carbon 24 of the vitamin D₂ side chain. In contrast to vitamin D₂ which is produced in plants and fungi exposed to sunlight, vitamin D₃ is produced in the skin of humans when exposed to sunlight. A derivative of vitamin D₃, vitamin D₄ (22-dihydroergocalciferol), is found in select mushrooms after the precursor 22,23-dihydroergosterol is exposed to UV irradiation. This form of vitamin D is effective in raising serum calcium levels and known to treat hypocalcemia. Derived from 7-dehydrositosterol, vitamin D₅ (sitocalciferol) has been studied for its effect on prostate cancer.²⁶ In previous studies, vitamin D₃ has been successful as an antitumor agent in prostate cancer, but it also has the ability to cause hypercalcemia in patients.²⁶ A vitamin D₅ analogue, 1α-hydroxyvitamin D₅, has shown to have similar potency to vitamin D₃ without added side effects.²⁶

Table 1.

Five Major Forms of Vitamin D and Their Uses

Name	Notes	Structure
Vitamin D1 (1:1 mixture of lumisterol and ergocalciferol)	Precursor of vitamin D2 No longer used as a vitamin D supplement
Vitamin D2 (ergocalciferol)	Plant-derived or synthetically made Less effective at raising serum levels compared to vitamin D3
Vitamin D3 (cholecalciferol)	Produced in sun-exposed skin or found in animal sources such as egg yolk, oily fish, and liver Most effective form of vitamin D
Vitamin D4 (22-dihydroergocalciferol)	Produced in mushrooms upon UV irradiation Used to raise serum calcium levels
Vitamin D5 (sitocalciferol)	Studied for its effectiveness as an antitumor agent in prostate cancer

VITAMIN D HAS IMMUNOMODULATORY ACTIVITY

Innate Immunity and Vitamin D.

In recent years, research has shown that the active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)₂D), is not only critical for calcium and bone homeostasis, but also modulates innate immunity.²⁷ 1,25(OH)₂D represses pro-inflammatory cytokines and chemokines in vitro in response to inflammatory and/or infectious insults.^28-31 In addition to its anti-inflammatory actions, 1,25(OH)₂D stimulates phagocytosis^32-34 and enhances production of reactive oxygen species (oxidative burst)³⁵ and release of antimicrobial peptides (including cathelicidin and lactoferrin),^36,37 thereby aiding innate immune cells in efficient killing of invading bacteria. Interestingly, vitamin D can be used to differentiate monocyte-like THP.1 cells into macrophages.^38,39 1,25(OH)₂D may play an important role in limiting bacterial infection pathology and tissue damage (such as in tuberculosis) by downregulating levels of matrix metalloproteinases (MMPs) and upregulating levels of tissue inhibitor of MMP-1 (TIMP-1).⁴⁰ As such, the ability of 1,25(OH)₂D to suppress MMPs and induce TIMPs may play a crucial role in GBS induction of MMPs and TIMPs in the fetal membranes, leading to weakening of the membranes.⁴¹

Vitamin D has a number of immunomodulatory properties against Streptococcus species, in both innate and adaptive immunity. In primary human neutrophils, vitamin D upregulates expression of pattern recognition receptors (PRRs) in response to S. pneumoniae stimulation, particularly TLR2 and NOD2.⁴² In addition, vitamin D enhances neutrophil killing of S. pneumoniae and secretion of antimicrobial peptides HNP1–3 and LL-37 while also inducing an anti-inflammatory program whereby NFκB signaling is suppressed by inducing suppressor of cytokine signaling (SOCS) proteins.⁴² The vitamin D-induced secretion of LL-37 could increase capsule release by S. pneumoniae and thereby enhance neutrophil-mediated killing.⁴³ SOC proteins are induced in an IL-4-dependent manner, thus leading to the downregulation of NF-KB and TRAF6 and to the regulation of excessive production of inflammatory cytokines.⁴² This gives rise to the possibility that vitamin D can improve killing of pneumococci via neutrophils while seemingly limiting unwarranted inflammation. However, in a different study, macrophages treated with vitamin D and GAS induced antimicrobial peptide LL-37 production but exhibited a decrease in the ability of macrophages to kill bacteria.⁴⁴ This disparity may be due to upregulation of GAS virulence factors in response to LL-37 or could be due to differences in macrophage versus neutrophil biology. In stimulation of peripheral blood mononuclear cells (PBMCs) with pneumococcal antigen, Anderson et al. found that vitamin D reduced levels of the proinflammatory interferon (IFN)-γ, interleukin (IL)-17, and IL-22 while increasing IL-10 and, interestingly, IL-1β.⁴⁵ In a similar study, Hoe et al. found that vitamin D reduced proinflammatory cytokine release in response to stimulation with bacterial products including tumor necrosis factor-α, IFNγ, and IL-8 but showed that IL-1β was reduced, not induced, as was found in Anderson et al.^45,46 This discrepancy may be due to the difference in stimulation: Hoe et al. used lipopolysaccharide instead of pneumococcal whole cell antigen, as was used in Anderson et al., which may differentially activate the inflammasome necessary for IL-1β production.^45,46 Vitamin D also affects dendritic cell (DC) activation, the bridge to adaptive immunity, in response to pneumococcal antigens.⁴⁵ Similar to neutrophils,⁴² vitamin D induced greater expression of TLR2 and NOD2 in DCs treated with pneumococcal peptidoglycan and antimicrobial peptides (human beta defensin-3) and upregulated IL-1β, as found in Anderson et al.^46,47

Vitamin D (1,25(OH)₂D₃) decreases inflammation in a mouse model of multiple sclerosis, whereby symptoms of experimental autoimmune encephalomyelitis are significantly resolved. Li et al. show that using vitamin D₃ as a preventative therapy or postdisease treatment suppresses spinal cord expression of an endosomal toll-like receptor, TLR8, which is known to be upstream of inflammatory cytokine expression.⁴⁸ This group also found that spinal cord expression of TLR3, 4, and 7 is suppressed in the postdisease treatment group. Interestingly, expression of a common downstream TLR mediator, MyD88, is also lower in response to vitamin D₃. Therefore, vitamin D₃ treatment leads to decreased spinal cord expression of cognate TLR8-mediated inflammatory responses in monocytes such as TNF-α, IFN-γ, and IL-17 induction. The proposed mechanism for vitamin D₃ suppression is through direct effect on innate immune cells including monocytes. Accordingly, vitamin D₃ treatment of THP-1 monocyte-like cells not only significantly reduced TLR8 expression but also expression or activity of MyD88, IRF-4, IRF-7, and NF-kB in cells challenged with TLR8 ligands.⁴⁸ Importantly, these results suggest the TLR8 pathway is a target of 1,25(OH)₂D₃ and that TLR8 may play a role in the anti-inflammatory activity of vitamin D. This is likely important in the context of streptococcal infection because TLR8 has been implicated as a critical sensor of bacterial RNA in human monocyte cells and has been shown to facilitate host recognition of S. pyogenes, S. pneumoniae, and S. agalactiae.^49-51

Adaptive Immunity and Vitamin D.

While an IL-17-dominated CD4 T helper cell program (Th17) adaptive response is associated with streptococcal infection,⁵² DC stimulation with vitamin D suppresses Th17 programming (also seen in Anderson et al.) in favor of inducing a T regulatory phenotype. Concerning the humoral response, individuals with asthma had serum levels of 25-hydroxyvitamin D (25[OH]D) that correlated positively with antipneumococcal antibodies, but this correlation was far weaker in nonasthmatic study participants. In some non-Streptococcus bacterial infections (i.e., Mycobacterium tuberculosis), vitamin D stimulates IFNγ expression from T cells to promote autophagy, phagosomal maturation, and antimicrobial peptide production in macrophages.⁵³ However, other studies also using M. tuberculosis are more consistent with the findings in streptococcal infections and have shown that vitamin D downregulates IFNγ from CD4 T cells along with perforin, granulysin, and granzyme-B from natural killer cells while increasing IL-10 production.⁵⁴ Vitamin D has also been shown to alter the proliferation of lymphocytes stimulated with streptococci derivatives.^55,56 Additionally, vitamin D has been shown to improve immune responses to vaccines (such as those utilized against influenza) in patients that are vitamin D-insufficient at baseline.⁵⁷ However, vitamin D status did not differ between immunoglobulin responders and hypo-responders in patients infected with Streptococcus pneumoniae as well as patients vaccinated against S. pneumoniae, Neisseria meningitidis type C (MenC), and/or Haemophilus influenzae type b (Hib).⁵⁸

VITAMIN D HAS ANTIBACTERIAL AND ANTIBIOFILM ACTIVITY

Vitamin D has been recognized as a potential chemotherapeutic treatment for bacterial infections for decades.⁴⁹ Within the host, vitamin D has been shown to have immunomodulatory effects that likely prevent bacterial growth and biofilm formation. However, vitamin D has also been shown to have direct antimicrobial and antibiofilm activity against several pathogens, including streptococci. In vitro studies have shown that vitamin D₃ exhibits strong growth inhibition Streptococcus pyogenes, Klebsiella pneumoniae, and Escherichia coli.⁵⁹ Recent reports have demonstrated that vitamin D analogues induce lysis of planktonic cultures of Streptococcus mutans and inhibit biofilm formation, a critical process for full virulence. Saputo et al. reported that the vitamin D analogues, alfacalcidol, calcitriol, and doxercalciferol, induced lysis of S. mutans planktonic cells at least 2-fold greater than DMSO. Interestingly, only calcitriol and doxercalciferol inhibited biofilm formation with calcitriol preventing formation by 40-fold greater than DMSO. The same report also demonstrated that doxercalciferol sensitized bacitracin-resistant S. mutans to bacitracin, reducing the minimum inhibitory concentration (MIC) from 128 to 4 μg/mL.⁶⁰

The antimicrobial activity of vitamin D is not confined to Streptococcus species. Early reports demonstrated that vitamin D supplementation had potent antibacterial activity against Mycobacterium tuberculosis infections in vivo.⁶¹ Expression of vitamin D receptor and the vitamin D-1 hydroxylase genes by TLR activation leads to the induction of the antimicrobial peptide cathelicidin, which promotes intracellular M. tuberculosis death in human macrophages and monocytes.³⁶ Tubercle bacilli were shown to be reduced over days to weeks when vitamin D in both oil and propylene glycol were added topically to M. tuberculosis cultures. In addition, M. tuberculosis was unable to grow on media supplemented with vitamin D. Interestingly, cod fish oil (a prominent source of vitamin D) was used in Europe in the 1700s to treat tuberculosis, long before its bactericidal activity was known.⁶²

Vitamin D has been shown to inhibit the growth and expression of virulence factors of the dental pathogen Porphyrmonas gingivalis.⁶³ The expression of genes fimA, hagA, and hagB, which are virulence factors involved in colonization, were reduced by 43, 48, and 21%, respectively, when 1 μg/mL vitamin D (1,25(OH)₂D₃) was added. In addition, the same report described an additive antimicrobial effect against P. gingivalis when vitamin D was used in concert with antibiotics such as tetracycline and metronidazole.

Vitamin D degradation products have been shown to exert strong bactericidal activity against the gastric pathogen Helicobacter pylori.⁶⁴ While vitamin D (cholecalciferol) can be metabolized by CYP2R1, CYP27A1, and CYP27B1 to produce 25(OH)D and 1,25(OH)₂D₃, natural (nonbiologic) degradation can occur via high humidity and high temperature. 25(OH)D and 1,25(OH)₂D₃ produced by nonbiologic degradation were shown to reduce H. pylori growth by 10⁴- and 10⁶-fold, respectively, compared to undegraded cholecalciferol treated cultures. These vitamin D degradation products were shown to act by destabilizing the membrane of H. pylori and ultimately inducing the loss of structural integrity, causing cell lysis. Taken together, these results indicate that vitamin D can have antibacterial activity against a wide range of bacterial pathogens, including Streptococcus spp.

VITAMIN D DEFICIENCY AND SUSCEPTIBILITY TO STREPTOCOCCAL INFECTION

Vitamin D deficiency is associated with increased risk of infection and supplementation alters host–pathogen interactions. Micronutrient deficiencies, such as vitamin D inadequacy, has been associated with increased risk of infection, including those caused by Streptococcus.^65,66 Specifically, vitamin D deficiency has been correlated with increased risk of community acquired streptococcal pneumonia and tonsillopharyngitis.^1,65 Neutrophils upregulate CYP27B1 in the presence of IFN-y thus resulting in the upregulation of alpha-defensins (HNP1–3) that mediates the killing of pneumococci.⁴² A retrospective study of 54 patients with recurrent tonsillopharyngitis caused by GAS demonstrated that serum levels of vitamin D (25(OH)D) less than 20 ng/mL were associated with the disease.⁶⁷ These results further support an earlier study in which men with 25(OH)D serum levels less than 40 nmol/L were found to have more severe respiratory tract infections.^67,68 In addition, a cross-sectional study of 300 patients found that serum concentration of 1,25(OH)₂D₃, the active form of vitamin D, was inversely correlated with severity of community associated pneumonia caused by various pathogens, including pneumococcus. However, no correlation was found between serum concentration of 25(OH)D, the storage form of vitamin D, and severity of community associated pneumonia. The results from this study suggest that the inability to convert the storage form of vitamin D to the active form, leading to low serum levels of 1,25(OH)₂D₃, is a risk factor for streptococcal pneumonia.⁶⁵

Lower vitamin D levels have been associated with decreased expression of antimicrobial peptides in the placenta and increased incidence of GBS carriage in the recto-vaginal niche.^66,69 A study of pregnant adolescents revealed that maternal recto-vaginal GBS colonization was associated with significantly lower mRNA levels of vitamin D regulatory proteins (cubulin, megalin, CYP27B1, and CYP24A1) as well as lower levels of vitamin D receptor protein abundance and placental cathelicidin mRNA compared to non-GBS colonized pregnant adolescents. It was shown that this effect of GBS colonization on cathelicidin mRNA levels was largely mediated by cubulin and CYP27B1 mRNA expression, further supporting the idea that vitamin D and its metabolites support antimicrobial peptide production.⁶⁶ In another study, vitamin D levels in cord blood at birth were used to investigate the link between vaginal carriage of GBS and maternal vitamin D levels. GBS vaginal carriage was significantly greater in women whose newborns showed deficient (< 10 mg/mL) or insufficient (10–30 mg/mL) vitamin D levels at birth compared to women whose newborns showed sufficient (>30 mg/mL) vitamin D levels at birth.⁷⁰ The results from these studies demonstrate the association between vitamin D deficiency and susceptibility to infection by Streptococcus species.

VITAMIN D DEFICIENCY IS ASSOCIATED WITH PRETERM BIRTH

Invasive Group B Streptococcus infections during pregnancy are one of the major causes of adverse pregnancy outcomes such as preterm birth.^7,9,10 A variety of factors can influence vaginal colonization and invasive infection, which can lead to preterm birth. For example, epidemiological studies have frequently linked micronutrient deficiency with preterm birth. A recent meta-analysis compiling data from over 10 000 pregnant patients shows that maternal vitamin D deficiency (maternal serum 25-OH-D < 20 ng/mL) is associated with a significant increase in preterm birth with an odds ratio of 1.29.⁷¹ Another meta-analysis reveals that vitamin D supplementation may reduce a subset of preterm deliveries.⁷² Interestingly, some mothers who delivered preterm had particular vitamin D receptor (VDR) gene variants which are associated with lower serum vitamin D concentrations. Specifically, women with TaqI/AG, ApaI/AA, and FokI/AG VDR genotypes had lower serum vitamin D levels and higher rates of preterm delivery. Genotypes BsmI/TT and ApaI/AA were associated with true vitamin D deficiency, and those mothers were respectively 2.36 and 7.99 times more likely to deliver preterm.⁷³ Two other single nucleotide polymorphisms (FokI, Cdx2) in the VDR have been associated with spontaneous preterm birth.⁷⁴ Of note, mothers with both of these SNPs also had higher odds ratios of chorioamnionitis and infection during pregnancy, suggesting an intersection of nutritional predisposition for infection and preterm birth. Work in animal models further delineates an association of dietary vitamin D with gestational age at delivery. Mice maintained on a reduced vitamin D diet had a 47% increase in preterm birth compared to controls that consumed regular diets with vitamin D.⁷⁵ Further studies to elucidate the mechanism(s) of vitamin D in preterm birth are warranted.

DOES VITAMIN D SUPPLEMENTATION MITIGATE THE RISK OF STREPTOCOCCUS-ASSOCIATED DISEASES?

Vitamin D supplementation has also been shown to enhance neutrophil killing of pathogenic streptococcal bacteria while concomitantly dampening excessive inflammatory responses and apoptosis, indicating vitamin D has chemotherapeutic potential against streptococcal infections. However, most research thus far on streptococcal and vitamin D interactions focuses on in vitro cell work and very little in vivo work exists. Most studies with vitamin D supplementation focus on human diseases outside of infectious diseases, but there are few exceptions. For instance, in one study with Candida infection in mice, low levels of vitamin D decreased fungal burden and improved survival outcomes.⁷⁶

In an ex vivo study with bladder tissue from postmenopausal women, a group found that vitamin D enhanced production of cathelicidin, an antimicrobial peptide, when the tissues were infected with uropathogenic E. coli.⁷⁷ Using colocalizing experiments, Hertting et al. demonstrated that the bladder cells treated with vitamin D had increased expression of cathelicidin. This observation that vitamin D leads to increased production of cathelicidin and other antimicrobial peptides is encouraging to connect vitamin D with streptococcal infections.³⁶ In fact, cathelicidins have been linked to protection against Group A Streptococcus infection of the soft skin.⁷⁸ S. pyogenes has evolved two-component systems and virulence factors to overcome immune stress by cathelicidins.^79,80

Evidence linking vitamin D levels and infectious diseases takes form in one human study of inflammatory bowel disease patients. The research group observed an inverse correlation between serum vitamin D levels and instances of Clostridium difficile infections, suggesting that vitamin D availability is linked with risk of infection.⁸¹ With potential links between vitamin D and streptococcal infections, more research investigating the potential of using vitamin D supplementation to prevent or combat streptococcal infections is warranted.

Vitamin D supplementation is an inexpensive intervention shown to decrease adverse outcomes in a wide range of infectious diseases. Studies have demonstrated that vitamin D supplementation of mothers and infants can reduce the risk of sepsis in neonates.⁸² A double-blind individually randomized placebo-controlled trial involving young children showed that risk of a repeat episode of pneumonia within 90 days of supplementation of oral vitamin D₃ was lower in the intervention than in the placebo group.⁸³ Taken together, the available data regarding vitamin D supplementation suggest a potential cost-effective chemotherapeutic intervention to mitigate risk associated with infection-associated illnesses such as those caused by Streptococcus species.

CONCLUSIONS

Vitamin D exerts a wide range of effects on both host and pathogen cells (Figure 2). Vitamin D inhibits streptococcal growth, viability, and biofilm formation. Vitamin D likely signals through the vitamin D receptor and promotes innate immune cell functions, including phagocytosis, production of antimicrobial peptides (AMPs) and reactive oxygen species (ROS), upregulation of pattern recognition receptors (PRR), and production of TIMP-1, which negatively regulates matrix metalloproteinases (MMPs) that contribute to tissue damage. Vitamin D also represses production of MMPs and proinflammatory cytokines and leads to repression of Th17 programming, favoring a regulatory T cell phenotype instead. Together, these data indicate that vitamin D promotes innate immune clearance of streptococcal cells and immunoregulation of the inflammatory response to ameliorate disease outcome. Thus, a close review of existing literature suggests that vitamin D homeostasis influences the outcome of streptococcal infections.

Figure 2.

Conceptual model of the role of vitamin D in Streptococcus-host interactions. Vitamin D interacts with both host and pathogen cells. Vitamin D inhibits streptococcal growth, viability, and biofilm formation, which are important for colonization and persistence in the host niche. Vitamin D signals through the VDR and promotes innate immune cell functions including phagocytosis, production of AMPs such as lactoferrin, LL-37, and cathelicidin, as well as ROS, upregulation of PRR, and production of TIMP-1, which negatively regulates MMPs that contribute to tissue damage. Vitamin D also represses production of MMPs and proinflammatory cytokines and leads to repression of Th17 programming favoring a regulatory T cell phenotype instead. Thus, vitamin D can decrease inflammation and disease progression.