Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response

Vitamin D resistance

Vitamin D is a secosteroid of nutritional origin but can also be generated in the skin by ultraviolet light. After two hydroxylations 1,25-(OH)2 vitamin D avidly binds and activates the vitamin D receptor (VDR), a nuclear transcription factor, hereby regulating a large number of genes. The generation of VDR deficient mice has expanded the knowledge on vitamin D from a calcium-regulating hormone to a humoral factor with extensive actions. The effects of the vitamin D system on calcium and bone homeostasis are largely mediated by promoting active intestinal calcium transport via the induction of the epithelial calcium channel TRPV6. Although VDR is redundant in bone, it may regulate the differentiation and function of several bone cells. In skin, VDR expression in keratinocytes is essential in a ligand-independent manner for the maintenance of the normal hair cycle. Therefore, VDR but not vitamin D deficiency results in alopecia. Moreover, 1,25-(OH)2 vitamin D impairs the proliferation not only of keratinocytes but also of many cell types by regulating the expression of cell cycle genes, leading to a G1 cell cycle arrest. In addition, VDR inactivation in mice results in high renin hypertension, cardiac hypertrophy and thrombogenesis. Finally, a dual effect of vitamin D was observed in the immune system where it stimulates the innate immune system while tapering down excessive activation of the acquired immune system. Taken together, the vitamin D endocrine system not only regulates calcium homeostasis but affects several systems mainly by altering gene expression but also by ligand-independent actions.

Epidemic influenza and vitamin D

In 1981, R. Edgar Hope-Simpson proposed that a 'seasonal stimulus' intimately associated with solar radiation explained the remarkable seasonality of epidemic influenza. Solar radiation triggers robust seasonal vitamin D production in the skin; vitamin D deficiency is common in the winter, and activated vitamin D, 1,25(OH)2D, a steroid hormone, has profound effects on human immunity. 1,25(OH)2D acts as an immune system modulator, preventing excessive expression of inflammatory cytokines and increasing the 'oxidative burst' potential of macrophages. Perhaps most importantly, it dramatically stimulates the expression of potent anti-microbial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection. Volunteers inoculated with live attenuated influenza virus are more likely to develop fever and serological evidence of an immune response in the winter. Vitamin D deficiency predisposes children to respiratory infections. Ultraviolet radiation (either from artificial sources or from sunlight) reduces the incidence of viral respiratory infections, as does cod liver oil (which contains vitamin D). An interventional study showed that vitamin D reduces the incidence of respiratory infections in children. We conclude that vitamin D, or lack of it, may be Hope-Simpson's 'seasonal stimulus'.

The vitamin D receptor as a therapeutic target

The vitamin D receptor (VDR) is a member of the large family of nuclear receptor transcription factors and specifically binds the micronutrient-derived hormone 1alpha,25(OH)2D3. A central endocrine role for this receptor in bone health was established at the beginning of the 20th century. Over the last 25 years, additional roles, perhaps through autocrine and paracrine mechanisms, have been established for VDR to regulate cell proliferation and differentiation, and more recently to exert immunomodulatory and antimicrobial functions. These findings, from in vitro and in vivo experiments, have generated considerable interest in targeting the VDR in multiple therapeutic settings. As with many potential therapeutics, it has also become clear that cells and tissues may also display de novo and acquired mechanisms of resistance to these actions. Consequently, a range of experimental and clinical options are being developed to bring about more targeted actions, overcome resistance and enhance efficacy of VDR-centred therapeutics.

Intracellular innate resistance to bacterial pathogens

Mammalian innate immunity stimulates antigen-specific immune responses and acts to control infection prior to the onset of adaptive immunity. Some bacterial pathogens replicate within the host cell and are therefore sheltered from some protective aspects of innate immunity such as complement. Here we focus on mechanisms of innate intracellular resistance encountered by bacterial pathogens and how some bacteria can evade destruction by the innate immune system. Major strategies of intracellular antibacterial defence include pathogen compartmentalization and iron limitation. Compartmentalization of pathogens within the host endocytic pathway is critical for generating high local concentrations of antimicrobial molecules, such as reactive oxygen species, and regulating concentrations of divalent cations that are essential for microbial growth. Cytosolic sensing, autophagy, sequestration of essential nutrients and membrane attack by antimicrobial peptides are also discussed.

The vitamin D slant on allergy

Oral vitamin D supplementation has been introduced into modern medicine to prevent rickets without the knowledge that this may have profound immunological consequences. The main vitamin D metabolite calcitriol suppresses dendritic cell maturation and consecutive Th(1) cell development, which has independently described as a key mechanism of allergy development. Animal studies and epidemiological surveys now provide a first link of early vitamin D supplementation and later allergy where several vitamin D regulated genes seem to be involved. A randomized clinical trial of vitamin D supplementation could be a further step to follow up the vitamin hypothesis.

Vitamin D3-mediated alterations to myeloid dendritic cell trafficking in vivo expand the scope of their antigen presenting properties

The mucosal immune system provides the host with a first line of adaptive immune defense against invasion by many species of pathogenic microorganisms and their secreted products. Calcitriol, the active form of Vitamin D3 (VD3), promotes the induction of mucosal immunity in mice when added to subcutaneously administered vaccine formulations. Dendritic cells (DCs) activated at vaccination sites where VD3 is present gain the capacity to bypass sequestration in the draining lymph node and traffic to the Peyer's Patches (PP) of immunized animals. By employing protocols that allow the effective tracking of endogenous or adoptively transferred myeloid DCs in vivo, we found that VD3 influences on the trafficking of fully differentiated immature DCs were temporary, and occur without negative effects to antigen processing or peptide presentation to CD4+ T cells. In contrast, DCs differentiated from hematopoietic precursors in the presence of VD3 (conditioned DCs), were markedly compromised in their antigen presenting properties, while manifesting clear alterations to their trafficking properties in vivo. Similar to the recent finding of VD3-mediated enhancement of innate immune protection, our findings suggest that VD3 could also play an important role in controlling the types of immune effector responses elicited subsequent to either infection or vaccination.

NODs in defence: from vulnerable antimicrobial peptides to chronic inflammation

Defensins and cathelicidins are prevalent and essential gastrointestinal cationic antimicrobial peptides (CAPs). However, these defensive peptides are not infallible because certain enteropathogens can overcome their protective function. Furthermore, impaired defensin synthesis has been linked to the occurrence of Crohn's disease (CD), a chronic inflammatory bowel disease. Recently, defective bacterial sensing through NOD1 and NOD2 has been related to reduced defensin production, CD predisposition and susceptibility to enteric infection. Hence, we propose that microbial sensors at the gut interface monitor the levels of these effector peptides, which might function as "danger" signals to confer tolerance and alert immunocytes. Further work is required to clarify how gastrointestinal CAPs are regulated and to assess their role in maintaining epithelial homeostasis and triggering adaptive immunity.

Dermal fibroblasts pretreated with a sterol Δ7-reductase inhibitor produce 25-hydroxyvitamin D3 upon UVB irradiation

As dermis is a physiological site of vitamin D3 photoproduction, the photo-endocrine vitamin D3 system was studied in dermal fibroblasts. Dermal fibroblasts contain the vitamin D receptor and induce 1alpha,25-dihydroxyvitamin D3-24-hydroxylase [CYP24] mRNA upon stimulation with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. In addition, dermal fibroblasts contain mRNA of the vitamin D3-25-hydroxylases (CYP2R1 and CYP27A1). However, we could not detect any 25-hydroxyvitamin D3 [25OHD3]-1alpha-hydroxylase mRNA in dermal fibroblasts and no CYP24 mRNA was induced upon ultraviolet [UVB] irradiation, even when endogenous 7-dehydrocholesterol content was elevated by pretreatment with the sterol Delta7-reductase inhibitor BM15766. Nevertheless, dermal fibroblasts produce inactive vitamin D3 metabolites that can be activated by epidermal keratinocytes as CYP24 mRNA is induced in epidermal keratinocytes but not in dermal fibroblasts after transfer of medium or cellular suspensions from BM15766-pretreated, UVB-irradiated fibroblasts. This CYP24 induction was UVB-dose dependent and was inhibited by ketoconazole. As revealed in a competitive binding assay, BM15766-pretreated dermal fibroblasts are able to produce 25OHD3 upon UVB irradiation, but no 1,25(OH)2D3 was detected via combined high-performance liquid chromatography radioimmunoassay. The physiological relevance of dermal vitamin D3 photoproduction and its subsequent conversion into 25OHD3 remains elusive.

Mycobacteria in Crohn's disease: a persistent hypothesis

Efforts to explore a mycobacterial origin for Crohn's disease typically have involved an epidemiological approach, searching for Mycobacterium avium subsp. paratuberculosis in patient tissue. An alternative approach involves consideration of genetic and experimental data regarding host resistance to mycobacteria. From human and mycobacterial genetics, it is known that mycobacterial diseases depend on both pathogen and host factors and that tuberculosis and leprosy are effectively genetic diseases. The discovery of a number of Crohn's susceptibility genes, including NOD2/CARD15, demonstrates that Crohn's also is a complex genetic disease. Mutations in NOD2/CARD15 do not necessarily lead to Crohn's disease, so other mitigating factors, genetic and/or environmental, probably are required to produce illness. Recent work has shown that NOD2/CARD15 serves a role in bacterial sensing and activation of innate immune responses, providing a link between Crohn's genetics and an environmental factor, potentially a bacterial trigger. In this review, we discuss the current understanding of mycobacterial and Crohn's genetic susceptibility and review the evidence that NOD2/CARD15 may mediate host resistance to mycobacterial infection.

Effects of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 on cytokine production by human decidual cells

The active form of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25[OH](2)D(3)) is a potent immunomodulatory seco-steroid. We have demonstrated that several components of vitamin D metabolism and signaling are strongly expressed in human uterine decidua from first trimester pregnancies, suggesting that locally produced 1,25(OH)(2)D(3) may exert immunosuppressive effects during early stages of gestation. To investigate this further, we used primary cultures of human decidual cells from first and third trimester pregnancies to demonstrate expression and activity of the enzyme that catalyzes synthesis of 1,25(OH)(2)D(3), 1alpha-hydroxylase (CYP27B1). Synthesis of 1,25(OH)(2)D(3) was higher in first trimester decidual cells (41 +/- 11.8 fmoles/h/mg protein) than in third trimester cells (8 +/- 4.4 fmoles/h/mg protein; P < 0.05). Purification of decidual cells followed by quantitative RT-PCR analysis showed that CYP27B1 was expressed by both CD10(+VE) stromal-enriched and CD10(-VE) stromal-depleted cells, with higher levels of mRNA in first trimester pregnancies. Expression of CYP27B1 correlated with TLR4 and IDO. Functional responses to 1,25(OH)(2)D(3) were studied using CD56(+VE) natural killer (NK) cells isolated from first trimester decidua. Decidual NK cells treated with 1,25(OH)(2)D(3) or precursor 25-hydroxyvitamin D(3) (25OHD(3)) for 28 h showed decreased synthesis of cytokines, such as granulocyte-macrophage colony stimulating factor 2 (CSF2), tumor necrosis factor, and interleukin 6, but increased expression of mRNA for the antimicrobial peptide cathelicidin antimicrobial peptide. These data indicate that human decidual cells are able to synthesize active 1,25(OH)(2)D(3), particularly in early gestation, and this may act in an autocrine/paracrine fashion to regulate both acquired and innate immune responses at the fetal-maternal interface.

Vitamin D-mediated hypercalcemia in slack skin disease: evidence for involvement of extrarenal 25-hydroxyvitamin D 1alpha-hydroxylase

A case of granulomatous slack skin disease is presented in which we studied the possible involvement of extrarenal 1,25(OH)2D in the pathogenesis of the patient's hypercalcemia. Immunolocalization of 1alpha-OH in skin showed simultaneous dysregulation in epithelial and granulomatous cells.

INTRODUCTION

Granuloma-forming diseases such as sarcoidosis are associated with extrarenal synthesis of active 1,25-dihydroxyvitamin D [1,25(OH)2D]. Here we describe a case of granulomatous slack skin disease in which we have studied the possible involvement of extrarenal synthesis of 1,25(OH)2D in the pathogenesis of the patient's hypercalcemia. The aim of the study was to clarify the etiology of hypercalcemia in this patient.

MATERIALS AND METHODS

This was a case study of a 19-year-old man with a T-cell lymphoproliferative disorder diagnosed as granulomatous slack skin disease who presented with hypercalcemia and raised serum 1,25(OH)2D. Analysis of expression of the enzyme 25-hydroxyvitamin D 1alpha-hydroxylase (1alpha-hydroxylase), which catalyzes synthesis of 1,25(OH)2D, was carried out by immunohistochemical analysis of involved and uninvolved skin. Approval was granted by the Mayo Foundation Institutional Review Board and Biospecimens Subcommittee.

RESULTS

In uninvolved skin, expression of 1alpha-hydroxylase was confined to the basal layer of the epidermis, whereas slack skin showed overexpression of the enzyme in dermal granulomata and basal cells of the epidermis.

CONCLUSIONS

Hypercalcemia associated with granulomatous slack skin syndrome seems to be caused by dysregulation of 1alpha-hydroxylase expression in both epidermal and dermal granulomatous cells. This contrasts with psoriasis and sarcoidosis of the skin, in which overexpression of the enzyme is restricted to keratinocytes and granulomata, respectively.

Resurrection of vitamin D deficiency and rickets

The epidemic scourge of rickets in the 19th century was caused by vitamin D deficiency due to inadequate sun exposure and resulted in growth retardation, muscle weakness, skeletal deformities, hypocalcemia, tetany, and seizures. The encouragement of sensible sun exposure and the fortification of milk with vitamin D resulted in almost complete eradication of the disease. Vitamin D (where D represents D2 or D3) is biologically inert and metabolized in the liver to 25-hydroxyvitamin D [25(OH)D], the major circulating form of vitamin D that is used to determine vitamin D status. 25(OH)D is activated in the kidneys to 1,25-dihydroxyvitamin D [1,25(OH)2D], which regulates calcium, phosphorus, and bone metabolism. Vitamin D deficiency has again become an epidemic in children, and rickets has become a global health issue. In addition to vitamin D deficiency, calcium deficiency and acquired and inherited disorders of vitamin D, calcium, and phosphorus metabolism cause rickets. This review summarizes the role of vitamin D in the prevention of rickets and its importance in the overall health and welfare of infants and children.

Topical vitamin D3 and low-calcemic analogs induce thymic stromal lymphopoietin in mouse keratinocytes and trigger an atopic dermatitis

We have demonstrated that cytokine thymic stromal lymphopoietin (TSLP), whose expression is rapidly induced upon keratinocyte-selective ablation of retinoid X receptors (RXRs) -alpha and -beta in the mouse (RXRalphabeta(ep-/-) mice), plays a key role in initiating a skin and systemic atopic dermatitis-like phenotype. We show here that topical application of the physiologically active ligand [1alpha,25-(OH)(2)D(3); calcitriol] of the vitamin D receptor, or of its low-calcemic analog MC903 (calcipotriol; Dovonex), induces TSLP expression in epidermal keratinocytes, which results in an atopic dermatitis-like syndrome mimicking that seen in RXRalphabeta(ep-/-) mutants and transgenic mice overexpressing TSLP in keratinocytes. Furthermore, topical application of retinoic acid receptor RARgamma-selective agonist BMS961 also induces TSLP expression either on its own or synergistically with 1alpha,25-(OH)(2)D(3). Our data demonstrate that RXR/vitamin D receptor and RXR/retinoic acid receptor-gamma heterodimers and their ligands cell-autonomously control the expression of TSLP in epidermal keratinocytes of the mouse. We propose molecular mechanisms through which vitamin D3 and retinoic acid signalings could be involved in the pathogenesis of atopic diseases.

Macrophages Acquire Neutrophil Granules for Antimicrobial Activity against Intracellular Pathogens

A key target of many intracellular pathogens is the macrophage. Although macrophages can generate antimicrobial activity, neutrophils have been shown to have a key role in host defense, presumably by their preformed granules containing antimicrobial agents. Yet the mechanism by which neutrophils can mediate antimicrobial activity against intracellular pathogens such as Mycobacterium tuberculosis has been a long-standing enigma. We demonstrate that apoptotic neutrophils and purified granules inhibit the growth of extracellular mycobacteria. Phagocytosis of apoptotic neutrophils by macrophages results in decreased viability of intracellular M. tuberculosis. Concomitant with uptake of apoptotic neutrophils, granule contents traffic to early endosomes, and colocalize with mycobacteria. Uptake of purified granules alone decreased growth of intracellular mycobacteria. Therefore, the transfer of antimicrobial peptides from neutrophils to macrophages provides a cooperative defense strategy between innate immune cells against intracellular pathogens and may complement other pathways that involve delivery of antimicrobial peptides to macrophages.

Association of HLA-DR, -DQ, and vitamin D receptor alleles and haplotypes with tuberculosis in the Venda of South Africa

The vitamin D receptor (VDR) and the human leukocyte antigen (HLA) class II complex affect innate and/or adaptive immunity against Mycobacterium tuberculosis. HLA-DRB1, HLA-DQB1, and VDR gene (VDR) polymorphisms were previously associated with tuberculosis (TB) and are here investigated as candidates for TB susceptibility in the Venda population of South Africa. Genomic DNA from 95 patients with pulmonary tuberculosis (PTB) and 117 ethnically matched, healthy controls were typed for HLA-DRB1, DRB3, DRB4, DRB5, DQB1, and VDR polymorphisms FokI, BsmI, ApaI, and TaqI using polymerase chain reaction-sequence specific primers (PCR-SSP). Allele and haplotype frequencies were calculated by the estimator maximum (EM) algorithm. DRB1*1302 phenotype was significantly associated with TB occurring at a significantly higher allele frequency in cases than controls and found in haplotype with DQB1*0602/3. DQB1*0301-0304 phenotype was significantly associated with TB and found in haplotype with DRB1*1101-1121, showing significant linkage disequilibrium (LD) in both cases and controls. Only DRB1*1101-1121-DQB1*05 was significantly associated with TB based on the sequential Bonferroni p value. VDR SNP phenotypes were not associated with TB, but the haplotype F-b-A-T significantly protected from TB. In conclusion, common African HLA-DRB1 and -DQB1 variants, previously associated with protection from malaria and hepatitis B/C virus persistence, predispose the Venda to TB, whereas the proposedly active VDR haplotype F-b-A-T showed significant protection.

Early days: genomics and human responses to infection

DNA microarray-based gene transcript-profiling of the responses of primates to infection has begun to yield new insights into host-pathogen interactions; this approach, however, remains plagued by challenges and complexities that have yet to be adequately addressed. The rapidly changing nature over time of acute infectious diseases in a host, and the genetic diversity of microbial pathogens present unique problems for the design and interpretation of functional-genomic studies in this field. In addition, there are the more common problems related to heterogeneity within clinical samples, the complex, non-standardized confounding variables associated with human subjects and the complexities posed by the analysis and validation of highly parallel data. Whereas various approaches have been developed to address each of these issues, there are significant limitations that remain to be overcome. The resolution of these problems should lead to a better understanding of the dialogue between the host and pathogen.

Vitamin D status as related to race and feeding type in preterm infants

BACKGROUND

Despite the higher prevalence of vitamin D deficiency in blacks, the vitamin D status of black preterm infants remains unknown. In addition, with the combination of parenteral and enteral nutritional support that preterm infants receive, the effect of vitamin D-deficient breast milk on vitamin D status is unknown.

OBJECTIVE

To evaluate vitamin D status of preterm infants through the first month after delivery and compare status by race and feeding type.

STUDY DESIGN

Thirty-six (36) preterm (< or =32 weeks gestation) infants (19 black, 17 white) had assessment of feeding type, vitamin D intake, and serum 25-hydroxyvitamin D [25(OH)D] as a marker of vitamin D status at three time points in the first month after delivery.

RESULTS

Black infants had a significantly lower mean 25(OH)D level on day 7-8 and day 14-15 evaluations than white infants [14.9 +/- 6.6 versus 23.3 +/- 9.3 ng/mL (p = 0.021) and 18.3 +/- 7.3 versus 25.6 +/- 10.3 ng/mL (p = 0.048), respectively], but the difference was no longer significant by day 28-30 evaluation [19.6 +/- 7.7 versus 26.2 +/- 11.6 ng/mL (p = 0.26)]. Vitamin D status was not significantly lower in infants receiving predominantly breast milk (p = 0.6). Vitamin D intake rose through the month as the amount and caloric density of enteral nutrition increased. Six infants had significant decrease in serum 25(OH)D values from day 14-15 to day 28-30 evaluation despite receiving > 400 IU/day vitamin D.

CONCLUSION

Differences in vitamin D status occurred between black and white infants and were significant through the first 2 weeks after delivery. Infants receiving predominantly breast milk did not have significantly worse vitamin D status than those receiving formula. The significant decline in serum 25(OH)D status observed in 28% of the infants was not related to breast milk intake.

The Effect of Ultraviolet Radiation on Human Viral Infections

Exposure to UV radiation is recognized to suppress cell-mediated immunity and therefore could adversely affect the course of a viral infection. Rodent models of viral infection confirm this possibility but the situation in human subjects is not so clear, apart from two exceptions. These are herpes simplex, in which sunlight exposure can cause reactivation, and certain papillomavirus types in which sunlight exposure can lead to the development of nonmelanoma skin cancer. In both cases, there are UV response elements in the viral genomes that alter the normal interactions between the viruses and the host following exposure, and UV-induced effects on the immune response occur in addition. These complex mechanisms are discussed, and the situation regarding UV radiation and viral exanthems plus other viruses, including the retroviruses, summarized. Finally viral vaccination is considered in the context of UV exposure and the importance of the host's genetic background emphasized. Further research is required to evaluate whether sunlight can significantly affect the resistance to common viral infections and vaccines.

High-dose vitamin D3 supplementation in a cohort of breastfeeding mothers and their infants: a 6-month follow-up pilot study

OBJECTIVE

To examine the effect of high-dose maternal vitamin D(3) (vitD) supplementation on the nutritional vitD status of breastfeeding (BF) women and their infants compared with maternal and infant controls receiving 400 and 300 IU vitD/day, respectively.

DESIGN

Fully lactating women (n = 19) were enrolled at 1-month postpartum into a randomized- control pilot trial. Each mother received one of two treatments for a 6-month study period: 0 or 6000 IU vitD(3) plus a prenatal vitamin containing 400 IU vitD(3). The infants of mothers assigned to the control group received 300 IU vitD(3)/day; those infants of mothers in the high-dose group received 0 IU (placebo). Maternal serum and milk vitD and 25(OH)D were measured at baseline then monthly; infant serum vitD and 25(OH)D were measured at baseline, and months 4 and 7. Urinary calcium/creatinine ratios were measured monthly in both mothers and infants. Dietary and BF history and outdoor activity questionnaires were completed at each visit. Changes in skin pigmentation were measured by spectrophotometry. Data were analyzed using chi-square, t-test, and analysis of variance (ANOVA) on an intent-to-treat basis.

RESULTS

High-dose (6400 IU/day) vitD(3) safely and significantly increased maternal circulating 25(OH)D and vitD from baseline compared to controls (p < 0.0028 and 0.0043, respectively). Mean milk antirachitic activity of mothers receiving 400 IU vitD/day decreased to a nadir of 45.6 at visit four and varied little during the study period (45.6-78.6 IU/L), whereas the mean activity in the 6400 IU/day group increased from 82 to 873 IU/L (p < 0.0003). There were no differences in circulating 25(OH)D levels of infants supplemented with oral vitD versus infants whose only source of vitD was breast milk.

CONCLUSION

With limited sun exposure, an intake of 400 IU/day vitamin D(3) did not sustain circulating maternal 25(OH)D levels, and thus, supplied only extremely limited amounts of vitamin D to the nursing infant via breast milk. Infant levels achieved exclusively through maternal supplementation were equivalent to levels in infants who received oral vitamin D supplementation. Thus, a maternal intake of 6400 IU/day vitamin D elevated circulating 25(OH)D in both mother and nursing infant.