In recent years there has been an effort to understand possible NonCalcemic roles of Vitamin D, including its role in the Immune System and, in particular, on T-Cell-mediated Immunity.

Vitamin D receptors are found in significant concentrations in the T-Lymphocyte and Macrophage populations. However, their highest concentration is in the immature Immune Cells of the Thymus and the mature CD-8⁺ T-Lymphocytes.

The significant role of Vitamin D compounds as selective ImmunoSuppressants is illustrated by their ability to either prevent or markedly suppress animal models of AutoImmune Disease.

Results show that 1,25-DiHydroxyVitamin D3 can either prevent or markedly suppress Experimental AutoImmune Encephalomyelitis, Rheumatoid Arthritis, Systemic Lupus Erythematosus, Type I Diabetes, and Inflammatory Bowel Disease.

In almost every case, the action of the Vitamin D Hormone requires that the animals be maintained on a normal or high Calcium diet. Possible mechanisms of suppression of these AutoImmune Disorders by the Vitamin D Hormone have been presented.

The Vitamin D Hormone stimulates Transforming Growth Factor TGFß-1 and InterLeukin-4 (IL-4) production, which in turn may suppress inflammatory T-Cell activity.

In support of this, the Vitamin D Hormone is unable to suppress a murine model of the human disease Multiple Sclerosis in IL-4-deficient mice.

The results suggest an important role for Vitamin D in AutoImmune Disorders and provide a fertile and interesting area of research that may yield important new therapies.

Vitamin D is one of the oldest Hormones that have been made in the earliest life forms for over 750 million years. Phytoplankton, Zooplankton, and most plants and animals that are exposed to sunlight have the capacity to make Vitamin D.

Vitamin D is critically important for the development, growth, and maintenance of a healthy skeleton from birth until death.

The major function of Vitamin D is to maintain Calcium homeostasis. It accomplishes this by increasing the efficiency of the Intestine to absorb dietary Calcium.

When there is inadequate Calcium in the diet to satisfy the body's Calcium requirement, Vitamin D communicates to the Osteoblasts that signal Osteoclast Precursors to mature and dissolve the Calcium stored in the bone.

Vitamin D is metabolized in the Liver and then in the Kidney to 1,25-DiHydroxyVitamin D [1,25(OH)(2)D]. 1,25(OH)(2)D receptors (VDR) are present not only in the Intestine and bone, but in a wide variety of other tissues.

Including the Brain, Heart, Stomach, Pancreas, activated T and B-Lymphocytes, Skin, Gonads, etc. 1,25(OH)(2)D is one of the most potent substances to inhibit proliferation of both normal and hyperproliferative cells and induce them to mature.

It is also recognized that a wide variety of tissues, including Colon, Prostate, Breast, and Skin have the enzymatic machinery to produce 1,25(OH)(2)D. 1,25(OH)(2)D and its analogs have been developed for treating the HyperProliferative disease Psoriasis.

Vitamin D deficiency is a major unrecognized health problem. Not only does it cause Rickets in children, Osteomalacia and Osteoporosis in adults, but may have long lasting effects.

Chronic Vitamin D deficiency may have serious adverse consequences, including increased risk of Hypertension, Multiple Sclerosis, Cancers of the Colon, Prostate, Breast, and Ovary, and type 1 Diabetes.

There needs to be a better appreciation of the importance of Vitamin D for overall health and well being.

Copyright 2002 Wiley-Liss, Inc.

MS is a chronic, Immune-mediated Inflammatory and NeuroDegenerative Disease of the Central Nervous System (CNS), with an Etiology that is not yet fully understood. The prevalence of MS is highest where environmental supplies of Vitamin D are lowest.

It is well recognized that the active Hormonal form of Vitamin D, 1,25-DiHydroxyVitamin D (1,25-(OH)(2)D), is a natural ImmunoRegulator with anti-inflammatory action.

The mechanism by which Vitamin D nutrition is thought to influence MS involves Paracrine or Autocrine metabolism of 25OHD by cells expressing the enzyme 1 -OHase in peripheral tissues involved in Immune and Neural function.

Administration of the active metabolite 1,25-(OH)(2)D in mice and rats with Experimental Allergic Encephalomyelitis (EAE, an animal model of MS) not only prevented, but also reduced disease activity.

1,25-(OH)(2)D alters Dendritic Cell and T-Cell function and regulates Macrophages in EAE. Interestingly, 1,25-(OH)(2)D is thought to be operating on CNS constituent cells as well.

Vitamin D deficiency is caused by insufficient sunlight exposure or low dietary Vitamin (D3) intake. Subtle defects in Vitamin D metabolism, including genetic polymorphisms related to Vitamin D, might possibly be involved as well.

Optimal 25OHD Serum concentrations, throughout the year, may be beneficial for patients with MS, both to obtain Immune-mediated suppression of disease activity, and also to decrease disease-related complications, including increased bone resorption, fractures, and muscle weakness.

Treatment from weaning until old age with 1,25-DiHydroxyVitamin D (1,25(OH)(2)(D3)) prevents Diabetes in NOD mice. It is mainly through its actions on Dendritic Cells (DCs), that 1,25(OH)(2)(D3) changes the function of potentially AutoReactive T-Lymphocytes.

In contrast, early life treatment (from 3 to 70 days of age) of NOD mice with Vitamin D or 1,25(OH)(2)(D3) did not influence final Diabetes incidence at 200 days of age.

Also in spontaneous diabetic BB rats, Diabetes could not be prevented by early life treatment (from 3 to 50 days of age) with Vitamin D (1000 IU per day) or 1,25(OH)(2)(D3) (0.2 microg/kg per day or 1 microg/kg per 2 days).

However, when NOD mice were made Vitamin D deficient in early life (until 100 days of age), Diabetes onset occurred earlier and final incidence was increased. These data further support a role for Vitamin D and its metabolites in the pathogenesis of type 1 Diabetes in NOD mice.

Low Vitamin D status has been implicated in the Etiology of Autoimmune Diseases such as Multiple Sclerosis, Rheumatoid Arthritis, Insulin-Dependent Diabetes Mellitus, and Inflammatory Bowel Disease.

The optimal level of Vitamin D intake required to support optimal Immune function is not known but is likely to be at least that required for healthy bones.

Experimentally, Vitamin D deficiency results in the increased incidence of Autoimmune Disease. Mechanistically, the data point to a role for Vitamin D in the development of Self-Tolerance.

The Vitamin D Hormone (1,25-Dihydroxy Vitamin (D3)) regulates T-Helper Cell (Th1) and Dendritic Cell function while inducing Regulatory T-Cell function. The net result is a decrease in the Th1-driven Autoimmune Response and decreased severity of symptoms.

This review discusses the accumulating evidence pointing to a link between Vitamin D and Autoimmunity. Increased Vitamin D intakes might decrease the incidence and severity of Autoimmune Diseases and the rate of bone fracture.

The discoveries that activated Macrophages produce 125-DiHydroxyVitamin D3 (1,25-(OH)2D3), and that Immune System cells express the Vitamin D Receptor (VDR), suggested that the Vitamin D Endocrine System influences Immune System function.

In this review, we compare and contrast how 1,25-(OH)2D3 synthesis and degradation is regulated in Kidney Cells and activated Macrophages, summarize data on Hormone Receptor function and expression in Lymphocytes and Myeloid lineage cells, and discuss how locally-produced 1,25-(OH)2D3 may activate a negative feed-back loop at sites of inflammation.

Studies of Immunity in humans and animals lacking VDR function, or lacking Vitamin D, are viewed to gain insight into the Immunological functions of the Vitamin D Endocrine System.

The strong associations between poor Vitamin D nutrition, particular VDR Alleles, and susceptibility to chronic Mycobacterial infections, together with evidence that 1,25-(OH)2D3 served as a vaccine adjuvant enhancing AntiBody-Mediated Immunity.

Suggest a model wherein high levels of 1,25-(OH)2D3-Liganded VDR Transcriptional activity may promote the CD4⁺ T-Helper 2 (Th2) Cell-Mediated and Mucosal AntiBody Responses to cutaneous Antigens in vivo.

We also review a diverse and rapidly growing body of Epidemiological, Climatological, Genetic, Nutritional and Biological evidence indicating that the Vitamin D Endocrine System functions in the establishment and/or maintenance of Immunological Self Tolerance.

Studies done in animal models of Multiple Sclerosis (MS), Insulin-Dependent Diabetes Mellitus (IDDM), Inflammatory Bowel Disease (IBD), and Transplantation support a model wherein the 1,25-(OH)2D3 may augment the function of Suppressor T-Cells that maintain Self Tolerance to organ-specific self Antigens.

The recent progress in Infectious Disease, Autoimmunity and Transplantation has stimulated a gratifying renaissance of interest in the Vitamin D Endocrine System and its role in Immunological health.

It has been a little more than 20 years since the first appreciation that the biologically active Hormonal form of the secosteroid Vitamin D-classically categorized as a regulator of Calcium/Phosphorous metabolism and bone mineralization-can exert effects on cells of the Immune System.

Since then a substantial literature has accumulated to suggest that these effects are exerted on multiple Immune Cell types, are predominantly suppressive at pharmacologic levels, and are potent enough to have true therapeutic potential in the management or prevention of Immune-Mediated Diseases.

Less clear at present, however, are the physiological roles played by the Vitamin D Endocrine System in the regulation of normal and abnormal Immune Responses.

In this review, an appraisal of the current understanding of Vitamin D-mediated Immune regulation is presented.

That emphasizes progress towards its clinical application, as well as the manner in which emerging models of normal Immune function may facilitate a more complete understanding of its physiologic significance.

Following their activation, naive CD4⁺ T-Cells can differentiate into one of two Effector Cell subsets, Th1 and Th2. These two subsets have different Cytokine secretion patterns and thus mediate separate arms of the Immune Response.

It has been established that the fat-soluble Vitamin (D3) metabolite 1,25-DiHydroxyVitamin (D3) (1,25(OH)(2)(D3)) and its Nuclear Receptor, the Vitamin D Receptor, play an important role in the Immune System.

Primarily through the transcriptional inhibition of Cytokine Genes that either are required for Th1 differentiation or are products of differentiated Th1 Cells.

Therefore, we wanted to test directly the ability of 1,25(OH)(2)(D3) to alter the Th differentiation process.

Our results indicate that 1,25(OH)(2)(D3) inhibits not only the Th1 Cytokine IFN-γ; but also the Th2 Cytokine IL-4 in naive CD62 ligand+CD4⁺ T-Cells during their in vitro polarization.

This effect is most dramatic when the Ligand is present from the onset of the differentiation process. If the Ligand is added after the polarization has ensued, the inhibition is significantly diminished.

In activated (CD62 Ligand-CD4⁺) T-Cells, 1,25(OH)(2)(D3) is still able to inhibit IFN-γ but has no effect on IL-4 production.

Our results also indicate that inhibition of these two Cytokines in naive cells by Vitamin D Receptor and its Ligand is neither a result of a cell cycle block nor an inhibition of Th1 or Th2 transcription factor expression.

But, rather, at least in the case of Th2 differentiation, an attenuation of IL-4 transcription by the Receptor.

In addition to its role in Calcium and Phosphorous homeostasis, 1 ,25-DiHydroxyVitamin (D3) [1,25-(OH)(2)(D3)] appears to be a modulator of the Immune System.

Administration of 1,25-(OH)(2)(D3) prevents disease in several Autoimmune animal models, including Experimental Autoimmune Encephalomyelitis (EAE). The Vitamin D Receptor is believed to mediate this activity.

Among cells of the Immune System, CD8CD8⁺ T-Cells have the highest levels of the Vitamin D Receptor. Because CD8⁺ T-Cells have been implicated as both Suppressors and Effectors of the inflammation associated with Multiple Sclerosis and EAE.

We examined the question of whether the 1,25-(OH)(2)(D3) suppression of EAE occurs through a CD8⁺ T-Cell-dependent mechanism.

To test this hypothesis, mice that are homozygous knockouts for the chain of the CD8 Receptor and have been characterized as lacking functional CD8⁺ T-Cells (CD8⁺ -/-) were provided 1,25-(OH)(2)(D3) in their diet before EAE induction.

Although CD8⁺ -/- mice fed the same diet lacking 1,25-(OH)(2)(D3) have a high incidence of EAE, EAE did not occur in CD8⁺ -/- mice fed the diet containing 1,25-(OH)(2)(D3).

We conclude that CD8⁺ T-Cells neither are needed nor do they play a role in the prevention of EAE by 1,25-(OH)(2)(D3).

IL-2 plays an important and complex role in the Immune System, serving as a Growth Factor, a Differentiation Factor, and a regulator of cell death. It shares many of these functions with other Cytokines such as IL-15, which complicates the interpretation of the IL-2-deficient phenotype.

Nonetheless, it is clear that a major indispensable role of IL-2 signaling, at least in mice, is to limit the number of activated T-Cells in the periphery after exposure to self- or environmental Antigens.

Although we still do not understand the mechanism by which this occurs, there emerge several general conclusions that provide a foundation for future work on this issue.

The Autoimmune phenotype associated with IL-2 deficiency results from the dysregulated activity of Thymus-derived TCR ß CD4⁺ and/or CD8⁺ T-Cells.

The process requires an initial Antigenic stimulus (e.g., from Intestinal Flora or Self-Antigens), but can then spread to naive bystander T-Cells by an undefined mechanism.

A key observation is that neighboring T-Cells with intact IL-2 signaling can suppress the dysregulated activity of IL-2- or IL-2R-deficient T-Cells in a dominant manner.

Indicating that IL-2-based Immune Regulation is a systemic rather than cell autonomous property.

The IL-2R signal responsible for maintaining homeostasis is not dependent on either the Shc or Stat5 pathways, therefore AICD, which is Stat5-dependent in vitro, cannot be the critical mechanism.

Finally, expression of the IL-2R in the Thymus and not the periphery appears sufficient to suppress the Autoimmune phenotype, suggesting that IL-2 signaling may be required for some aspect of Thymocyte selection and/or differentiation that has so far gone undetected.

These observations can be combined into a unified model wherein IL-2 signaling is required for the development of a Regulatory T-Cell subset that serves to terminate Antigen-induced responses of TCR ß T-Cells.

On the other hand, the role of IL-2 in the maintenance of Self-Tolerance could be multifaceted, with no single model accounting for all experimental findings.

Of note, most of our knowledge regarding IL-2 signaling and Tolerance has come from studies of knockout mice.

Conceptually, it is important to remember that the function of a molecule such as IL-2 is not simply the inverse of the phenotype seen when that molecule is absent, since necessity and sufficiency are not always coupled in physiological processes.

In this regard, perhaps new insights into this important issue can be gained by better defining when and where IL-2 signaling normally occurs in wild-type animals and the functional status of the T-Cells involved.

InterLeukin-2 (IL-2) knockout (KO) mice, which spontaneously develop symptoms of Inflammatory Bowel Disease similar to Ulcerative Colitis in humans, were made Vitamin D deficient (D-) or Vitamin D sufficient (D+) or were supplemented with 1,25-DiHydroxyVitamin (D3) (1,25D3).

1,25-DiHydroxyVitamin D3 supplementation, but not Vitamin D supplementation, reduced the early mortality of IL-2 KO mice. However, Colitis severity was not different in D-, D+, or 1,25D3 IL-2 KO mice.

Cells from D- IL-2 KO mice produced more Interferon-gamma (IFN-γ) than cells from all other mice. Con A-induced proliferation was upregulated in IL-2 KO mice and downregulated in wildtype (WT) mice fed 1,25D3.

All other measured Immune Responses in cells from IL-2 KO mice were unchanged by Vitamin D status. In vitro addition of 1,25-DiHydroxyVitamin D3 significantly reduced the production of IL-10 and IFN-γ in cells from D- and D+ WT mice.

Conversely, IFN-γ and IL-10 production in cells from IL-2 KO mice were refractory to in vitro 1,25-DiHydroxyVitamin D3 treatments.

In the absence of IL-2, Vitamin D was ineffective for suppressing Colitis and ineffective for the in vitro downregulation of IL-10 or IFN-γ production. One target of 1,25-DiHydroxyVitamin D3 in the Immune System is the IL-2 gene.

The active metabolite of Vitamin D, 1,25-DiHydroxyVitamin (D3), suppresses Autoimmune Disease in several animal models including Experimental Autoimmune Encephalomyelitis (EAE), a model of Multiple Sclerosis.

The molecular mechanism of this ImmunoSuppression is at present unknown. While 1,25-DiHydroxyVitamin (D3) is believed to function through a single Vitamin D receptor, there are reports of other Vitamin D Receptors as well as a "nongenomic" mode of action.

We have prepared the EAE model possessing the Vitamin D Receptor null mutation and determined if 1,25-DiHydroxyVitamin (D3) can suppress this disease in the absence of a functional Vitamin D Receptor.

Vitamin D Receptor null mice develop EAE although the incidence rate is one-half that of wild-type controls.

The administration of 1,25-DiHydroxyVitamin (D3) had no significant effect on the incidence of EAE in the Vitamin D Receptor null mice, while it completely blocked EAE in the wild-type mice.

We conclude that 1,25-DiHydroxyVitamin (D3) functions to suppress EAE through the well-known VDR and not through an undiscovered Receptor or through a "nongenomic" mechanism.