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Vitamin D and the
Immune System: New
Perspectives on an
Martin Hewison, PhD
Nonclassic actions of vitamin D were first recognized 30 years ago when receptors for
active 1,25-dihydroxyvitamin D3(1,25(OH)2D3) were detected in various neoplastic
cells lines.1,2Other studies immediately following this showed that binding of
1,25(OH)2D3to the vitamin D receptor (VDR) promoted antiproliferative and prodiffer-
entiation responses in cancer cells,3,4highlighting an entirely new facet of vitamin D
action. The spectrum of nonclassic responses to vitamin D was then extended to
include actions on cells from the immune system.5,6This interaction was further
endorsed by the observation that some patients with the granulomatous disease
sarcoidosis present with increased circulating levels of 1,25(OH)2D3and associated
hypercalcemia.7,8In these patients the high serum level of 1,25(OH)2D3is caused
by increased activity of the enzyme 25-hydroxyvitamin D-1a-hydrooxylase (1a-
hydroxylase). However, in contrast to normal subjects in whom 1a-hydroxylase is
classically localized in the kidney, the increased synthesis of 1,25(OH)2D3in patients
with sarcoidosis involves 1a-hydroxylase activity in disease-associated macro-
phages.9–11Thus, it was concluded that the immune system had the potential to
synthesize 1,25(OH)2D3and elicit autocrine or paracrine responses from immune cells
expressing the VDR.12
Despite these early advances, the precise nature of the interaction between vitamin
D and the immune system remained unresolved for many years. Some pieces of the
puzzle were easier to complete than others. For example, it became evident that dys-
regulation of 1,25(OH)2D3was not restricted to sarcoidosis but was a common feature
of many granulomatous disorders and some forms of cancer.13Likewise, at least in
This work was supported by NIH grant RO1AR050626 to M.H.
Department of Orthopaedic Surgery, Molecular Biology Institute, David Geffen School of
Medicine at UCLA, 615 Charles East Young Drive South, Los Angeles, CA 90095, USA
E-mail address: firstname.lastname@example.org
? Vitamin D ? CYP27b1 ? Toll-like receptor ? Macrophage
?Cathelicidin ?Regulatory T cells
Endocrinol Metab Clin N Am 39 (2010) 365–379
0889-8529/10/$ – see front matter ª 2010 Elsevier Inc. All rights reserved.
Author's personal copy
vitro, it was possible to potently regulate a range of immune cell functions using
1,25(OH)2D3or its synthetic analogs.14,15However, the key remaining question was
whether or not vitamin D could act as a physiologic regulator of normal immune
responses. Answers to this question began to appear about 5 years ago and new
information on the fundamental nature of vitamin D sufficiency/insufficiency has
provided a fresh perspective on nonclassic actions of vitamin D. As a consequence,
there is now a much broader acceptance that vitamin D plays an active role in regu-
lating specific facets of human immunity. Details of this are reviewed and the possible
effect of vitamin D insufficiency and vitamin D supplementation on normal immune
function and human disease are discussed in this article.
VITAMIN D AND INNATE IMMUNITY
Macrophages, Vitamin D, and Cathelicidin
Consistent with the earlier seminal observations of extrarenal 1a-hydroxylase activity
in patients with sarcoidosis, the effects of vitamin D on macrophage function have
been central to many of the new observations implicating vitamin D in the regulation
of immune responses. In common with natural killer cells (NK) and cytotoxic T lympho-
cytes (cytotoxic T cells), macrophages, and their monocyte precursors play a central
role in initial nonspecific immune responses to pathogenic organisms or tissue
damage, so-called cell-mediated immunity. Their role is to phagocytose pathogens
or cell debris and then eliminate or assimilate the resulting waste material. In addition,
macrophages can interface with the adaptive immune system by using phagocytic
material for antigen presentation to T lymphocytes (T cells).
For many years, the key action of vitamin D on macrophages was believed to be its
ability to stimulate differentiation of precursor monocytes to more mature phagocytic
macrophages.3,5,16,17This concept was supported by observations showing differen-
tial expression of VDR and 1a-hydroxylase during the differentiation of human mono-
cytes macrophages.18The latter report also emphasized early studies showing that
normal human macrophages were able to synthesize 1,25(OH)2D3when stimulated
with interferon gamma (IFNg).19Localized activation of vitamin D, coupled with
expression of endogenous VDR was strongly suggestive of an autocrine or intracrine
system for vitamin D action in normal monocytes/macrophages.
However, confirmation of such a mechanism was only obtained in 2006 when Liu
and colleagues20carried out DNA array analyses to define innate immunity genes
that were specifically modulated in monocytes by Mycobacterium tuberculosis. In
a seminal investigation both the VDR and the gene for 1a-hydroxylase (CYP27B1)
were shown to be induced following activation of the principal pathogen recognition
receptor for M tuberculosis, toll-like receptor 2/1 (TLR2/1). Subsequent experiments
confirmed that precursor 25-hydroxyvitamin D3(25OHD3) was able to induce intra-
crine VDR responses in monocytes that had been treated with a TLR2/1 activator.
In particular, the TLR2/1-25OHD3combination stimulated expression of the antibac-
terial protein cathelicidin, so that vitamin D was able to promote monocyte killing of
M tuberculosis.20Notably, the ability to promote expression of the antibacterial protein
following a TLR2/1 challenge was directly influenced by the 25OHD3status of the
donor serum used for monocyte culture.20More recently, the authors have shown
that vitamin D supplementation in vivo can also enhance TLR2/1-induced cathelicidin
expression.21Cathelicidin was identified several years ago as a target for transcrip-
tional regulation by 1,25(OH)2D3-liganded VDR, in that its gene promoter contains
a functional vitamin D response element (VDRE).22,23This VDRE occurs within a small
interchangeable nuclear element (SINE) sequence which only seems to be present in
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the cathelicidin gene promoter of higher primates, suggesting that vitamin D regulation
of this facet of innate immunity is a relatively recent evolutionary development.22
Recent reports have underlined the importance of cathelicidin as a target for vitamin
D but also suggest that this mechanism may be more complex than initially believed.
As yet, the precise signal system by which TLR activation induces expression of VDR
and 1a-hydroxylase remains unclear. Promoter-reporter analysis of the events
involved in transcriptional regulation of CYP27B1 suggest that TLR4-mediated induc-
tion of the enzyme involves JAK-STAT, MAP kinase and nuclear factor kappaB
(NF-kB) pathways, and that these synergize with IFNg-mediated induction of
CYP27B1.24However, other studies have proposed that TLR2/1 induction of 1a-
hydroxylase occurs indirectly as a consequence of TLR2/1-induced interleukin (IL)-
15, which is a potent inducer of CYP27B1 and 1a-hydroxylase activity.25In a similar
fashion, IL-17A has been shown to enhance 1,25(OH)2D3-mediated induction of cath-
elicidin, although this response does not seem to involve transcriptional regulation of
1a-hydroxylase or increased VDR sensitivity.26One pathway that has been poorly
studied in this regard concerns the enzyme 24-hydroxylase, which is conventionally
considered to function by inactivating 1,25(OH)2D3. The gene for 24-hydroxylase
(CYP24) is potently induced by 25OHD3following TLR2/1 activation of monocytes20
but, as yet, it is unclear whether this involves the nonmetabolic splice variant form
of CYP24 known to be expressed by macrophages.27
Regulation ofthe antibacterial protein by 1,25(OH)2D3has been described for a wide
variety of cell types other than macrophages, including keratinocytes,23,28,29lung
epithelial cells,30myeloid cell lines,22,23,29and placental trophoblasts.31In some
cases,28,31this seems to involve an intracrine response similar to that reported for
monocytes. However, the mechanisms controlling local synthesis of 1,25(OH)2D3in
these cells vary considerably. In keratinocytes, low baseline expression of 1a-hydrox-
ylase is enhanced following epidermal wounding by transforming growth factor beta
(TGFb).28The resulting increase in 1,25(OH)2D3concentration up-regulates expres-
sion of TLR2 and TLR4 by keratinocytes, thereby priming these cells for further innate
immune responses to pathogens or tissue damage.28By contrast, in trophoblasts,
induction of cathelicidin and subsequent bacterial killing by 25OHD3seems to be
caused by constitutive 1a-hydroxylase activity, which is not further enhanced by
TLR activation.31The latter may be a result of the rapid nonimmune induction of 1a-
hydroxylase and VDR that occurs within the placenta during early gestation.32
Although most of the studies on vitamin D-mediated innate immunity have focused
on the role of 1,25(OH)2D3-bound VDR as a pivotal transcriptional regulator of catheli-
example, recent studies of biliary epithelial cells have shown that cathelicidin expres-
sion can be induced in a VDR-dependent fashion by bile salts.34This provides a mech-
anism for maintaining biliary sterility, although additive effects of 1,25(OH)2D3also
highlight a novel therapeutic application for vitamin D in the treatment of primary biliary
cirrhosis. Conversely, other compounds may act to disrupt normal 1,25(OH)2D3-VDR-
product of cigarette smoking, has been shown to attenuate vitamin D-mediated induc-
tion of macrophage cathelcidin in a VDR-dependent fashion by stimulating expression
of 24-hydroxylase and vitamin D catabolism.35The precise mechanism by which this
occurs has yet to be determined but these data suggest that some toxic compounds
are actively detrimental to vitamin D-mediated immunity.
The observations described earlier show clearly that vitamin D is a potent stimulator
of mechanisms associated with pathogen elimination. In subsequent sections the clin-
ical importance of this with respect to vitamin D insufficiency and immune-related
Vitamin D and the Immune System
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diseases is discussed in more detail. However, 1 key question that immediately arises
from the current observations is why there is a need to involve the vitamin D system in
the TLR-induction of innate immunity. As previously described, VDR-mediated tran-
scriptional regulation of cathelicidin is a relatively recent evolutionary change and
was presumably advantageous when primates (including early Homo sapiens) were
exposed to abundant sunlight, thereby priming high serum levels of vitamin D. Other
benefits of incorporating vitamin D into innateimmune regulation include the fact that it
is associated with key feedback control pathways. As already mentioned, vitamin D
has its own catabolic enzyme in the form of 24-hydroxylase, which sensitively atten-
uates responses to 1,25(OH)2D3and, in the case of the CYP24 splice variant, may
also attenuate synthesis of this vitamin D metabolite.27However, vitamin D may
1,25(OH)2D3has been shown to potently down-regulate expression of monocyte
TLR2 and TLR4, thereby suppressing inflammatory responses that are normally acti-
vated by these receptors.36Thus, by using CYP24 and TLR regulatory mechanisms,
vitamin D may help to promote appropriate innate immune responses while preventing
an over elaboration of innate immune responses and the tissue damage frequently
associated with this.
immuneactivationpathways in that
Dendritic Cells and Antigen Presentation
In addition to the phagocytic acquisition and elimination of pathogens and cell debris,
innate immunity also involves the presentation of resultant antigen to cells involved in
the adaptive arm of the immune system (Fig. 1). Although several cells are able to do
this, the most well-recognized group of professional antigen-presenting cells (APCs)
are dendritic cells (DCs). Expression of VDR by purified tissue DCs was first reported
cells) provided evidence that 1,25(OH)2D3could act to attenuate antigen presenta-
proposed that vitamin D could act to promote tolerance and this was endorsed by
studies of pancreatic islet transplantation in which lower rejection rates were observed
in 1,25(OH)2D3-treated mice.41Crucially this response to 1,25(OH)2D3appeared to be
caused by decreased DC maturation and concomitant enhancement of suppressor or
regulatory T cells (Treg).41Further studies have underlined the importance of Treg
generation42as part of the interaction between vitamin D and the immune system,
and this is discussed in greater detail in later sections of this article.
Although regulation of DC maturation represents a potential target for 1,25(OH)2D3
and its synthetic analogs as treatment of autoimmune disease and host-graft rejec-
tion, another perspective was provided by the observation that DCs express 1a-
hydroxylase in a similar fashion to macrophages.43,44Data from monocyte-derived
DCs showed that 1a-hydroxylase expression and activity increases as DCs differen-
tiate towards a mature phenotype.44Functional analyses showed that treatment
with 25OHD3suppresses DC maturation and inhibits T-cell proliferation, confirming
the existence of an intracrine pathway for vitamin D similar to that observed for macro-
phages.44Mature DCs showed lower levels of VDR than immature DCs or mono-
cytes.44This reciprocal organization of 1a-hydroxylase and VDR expression may be
advantageous in that mature antigen-presenting DCs may be relatively insensitive to
1,25(OH)2D3, thereby allowing induction of an initial T-cell response. However, the
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high levels of 1,25(OH)2D3being synthesized by these cells can to act on VDR-
expressing immature DCs and thus prevent their further development.45In this way,
paracrine action of locally produced 1,25(OH)2D3allows initial presentation of antigen
to T cells while preventing continued maturation of DCs and over stimulation of T cells.
Although DCs are heterogeneous in terms of their location, phenotype, and function,
they are broadly divided into 2 groups based on their origin. Myeloid (mDCs) and plas-
macytoid (pDCs) express different types of cytokines and chemokines and seem to
exert complementary effects on T-cell responses, with mDCs being the most effective
APCs46and pDCs being more closely associated with immune tolerance.47Therefore
1,25(OH)2D3preferentially regulates mDCs, suggesting that the key effect of vitamin D
in this instance is to suppress activation of naive T cells. Although in this study pDCs
showed no apparent immune response to 1,25(OH)2D3, this does not preclude a role
for vitamin D in the regulation of tolerogenic responses. One possibility is that local
intracrine synthesis of 1,25(OH)2D3is more effective in achieving these responses.
Alternatively, 1,25(OH)2D3synthesized by pDCs may regulate tolerance through para-
crine effects on VDR-expressing T cells. This is discussed in further detail in the
VITAMIN D AND ADAPTIVE IMMUNITY
Vitamin D and T-cell Function
Resting T cells express almost undetectable levels of VDR, but levels of the
receptor increase as T cells proliferate following antigenic activation.48–50As
Fig. 1. Effects of vitamin D on innate and adaptive immunity. The principal innate and adap-
tive immune responses to a pathogenic challenge, and the positive or negative regulation of
these responses by vitamin D. B-cell, B lymphocyte; cyto T-cell, cytotoxic T cell; DC, dendritic
cell; MF, macrophage; T-cell, T lymphocyte; TLR, toll-like receptor; Treg, regulatory T cell.
Vitamin D and the Immune System
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a consequence, initial studies of the effects of vitamin D on T cells focused on the
ability of 1,25(OH)2D3to suppress T-cell proliferation.48–50However, the recogni-
tion that CD41effector T cells were capable of considerable phenotypic plasticity,
suggested that vitamin D might also influence the phenotype of T cells. Lemire
and colleagues51first reported that 1,25(OH)2D3 preferentially inhibited T helper
1 (Th1) cells, which are a subset of CD41effector T cells closely associated
with cellular, rather than humoral, immune responses. Subsequent studies
confirmed this observation and demonstrated that the cytokine profile of
1,25(OH)2D3-treated human T cells was consistent with Th2 cells, a subset of
CD41T cells associated with humoral (antibody)-mediated immunity.52,53The
conclusion from these observations was that vitamin D promotes a T-cell shift
from Th1 to Th2 and thus might help to limit the potential tissue damage associ-
ated with Th1 cellular immune responses. However, the validity of this generaliza-
tion was called into question by studies using mouse T cells in which 1,25(OH)2D3
was shown to inhibit cytokines associated with Th1 (IFNg) and Th2 (IL-4). Subse-
quent analysis of immune cells from the VDR gene knockout mouse added further
confusion by showing that these animals had reduced (rather than the predicted
increased) levels of Th1 cells.54Thus, although in vitro vitamin D seems to broadly
support a shift from Th1 to Th2 in CD41cells, it seems likely that in vivo its
effects on T cells are more complex.
The T-cell repertoire has continued to expand with the characterization of another
effector T-cell lineage distinct from Th1 or Th2 cells, termed Th17 cells because of
their capacity to synthesize IL-17.55,56Th17 cells play an essential role in combating
certain pathogens but they have also been linked to tissue damage and inflamma-
tion.57,58The precise role of vitamin D as a regulator of Th17 cells has yet to be fully
elucidated but studies of animal models of the gastrointestinal inflammatory disease
colitis have shown that treatment with 1,25(OH)2D3reduces expression of IL-17,59
and loss of 1,25(OH)2D3as a result of CYP27b1 gene ablation leads to increased levels
of this cytokine.60Thus, it possible that vitamin D exerts some of its effects on inflam-
mation and autoimmune disease through the regulation of Th17 cells.
A fourth group of CD41T cells exert suppressor rather than effector functions and
areknownasregulatoryTcellsor Tregs.Inview ofitsearly recognition asasuppressor
of T-cell proliferation, it was anticipated that vitamin D would have effects on Tregs,
and indeed in 2002 Barrat and colleagues61showed that 1,25(OH)2D3, in conjunction
with glucocorticoids, potently stimulated the generation of IL-10–producing CD41/
CD251Tregs. Subsequent reports indicated that 1,25(OH)2D3 alone can induce
Tregs,62and it seems that preferential differentiation of Tregs is a pivotal mechanism
linking vitamin D and adaptive immunity, with potential beneficial effects for autoim-
mune disease and host-graft rejection.63–65This immunosuppressive mechanism is
likely to be mediated by the induction of tolerogenic DCs as described in the previous
section of the review,41,66,67but direct effects on T cells may also be important.68In
this latter study, it was notable that 1,25(OH)2D3 increased IL-10 secretion and
TLR9 expression by Tregs, suggesting a novel link between innate and adaptive
Relative to the amount of literature on CD41effector cells, our understanding of the
effects of vitamin D on CD81suppressor T cells remains somewhat limited. In contrast
to CD41cells, CD81cells show poor antiproliferative response to 1,25(OH)2D3.50,69,70
However, VDR expression seems to be abundant in CD81cells suggesting that they
are still potential targets for 1,25(OH)2D3. Indeed subsequent reports have shown that
1,25(OH)2D3actively regulates cytokine production by CD81cells,71and can also
regulate the proliferation of CD81cells following specific immune stimuli.72Despite
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this, 1,25(OH)2D3does not seem to have a significant effect on animal disease models
such as experimental autoimmune encephalomyelitis in which CD81cells have been
Although many of the studies linking 1,25(OH)2D3with adaptive immunity have
focused on changes in T-cell proliferation and phenotype, it is important to recognize
that other facets of T-cell function may also be affected by the hormone. In particular,
recent studies have shown that vitamin D can exert powerful effects on the homing of
T cells to specific tissues. Initial studies suggested that 1,25(OH)2D3acts to inhibit
migration of T cells to lymph nodes.74However, more recent reports have shown an
active role for vitamin D in promoting homing of T cells to the skin via up-regulation
of chemokine receptor 10 (CCR10), the ligand for which, CCL27, is expressed by
epidermal keratinocytes.75This T-cell homing response was induced by 25OHD3as
well as 1,25(OH)2D3and the author suggested that DCs and T cells were possible
sources of the local 1a-hydroxylase activity.75In contrast to its positive effect on
epidermal T-cell homing, vitamin D seems to exert a negative effect on chemokines
and chemokine receptors associated with the gastrointestinal tract.75However, it
seems likely that this is highly T-cell selective as newer studies using the VDR gene
knockout mouse have shown aberrant gastrointestinal migration of a subset of
CD81cells, and this effect seems to be closely linked to the increased risk of colitis
in VDR knockout mice.76
Vitamin D and B-cell Function
Like T cells, active but not inactive B cells express the VDR.77Consequently, initial
studies indicated that 1,25(OH)2D3could directly regulate B-cell proliferation78and
immunoglobulin (Ig) production.77Subsequent work contradicted this, suggesting
instead that the ability of 1,25(OH)2D3to suppress proliferation and Ig production
was caused by indirect effects mediated via helper T cells.79However, more recent
reports have shown that 1,25(OH)2D3does indeed exert direct effects on B-cell
homeostasis.80In addition to confirming direct VDR-mediated effects on B-cell prolif-
eration and Ig production, this study also highlighted the ability of 1,25(OH)2D3to
inhibit the differentiation of plasma cells and class-switched memory cells, suggesting
a potential role for vitamin D in B-cell–related disorders such as systemic lupus eryth-
ematosus. Expression of CYP27b1 was also detected in B cells, indicating that B cells
may be capable of autocrine/intracrine responses to vitamin D.80Indeed, this may be
common to lymphocytes in general as CYP27b1 expression has also been detected in
VITAMIN D, THE IMMUNE SYSTEM AND HUMAN HEALTH
For many years vitamin D status was defined simply by whether or not the patient had
symptoms of the bone disease rickets (osteomalacia in adults). However, an entirely
new perspective on vitamin D status has arisen from the observation that serum levels
of the main circulating form of vitamin D (25OHD3) as high as 75 nM correlate inversely
with parathyroid hormone.81This, has prompted the introduction of a new term,
vitamin D insufficiency, defined by serum levels of 25OHD3that are suboptimal (<75
nM) but not necessarily rachitic (<20 nM).82Unlike serum concentrations of
1,25(OH)2D3, which are primarily defined by the endocrine regulators of the vitamin
D–activating enzyme, 1a-hydroxylase, circulating levels of 25OHD3are a direct reflec-
tion of vitamin D status, which for any given individual depends on access to vitamin D
either through exposure to sunlight or through dietary intake. The net effect of this is
that vitamin D status can vary significantly in populations depending on geographic,
Vitamin D and the Immune System
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social, or economic factors. As a result of these new parameters for vitamin D status,
a consensus statement from the 13th Workshop on Vitamin D concluded that vitamin
D insufficiency was a worldwide epidemic. Moreover, recent studies have shown that
in the last 10 years alone, serum vitamin D levels have on average fallen by 20%.83The
key question now being considered is what is the physiologic and clinical effect of
global vitamin D insufficiency beyond classic bone diseases such as rickets? Epide-
miologic studies have highlighted possible links between vitamin D insufficiency and
a wide range of human diseases.82The final section of the article describes 4 of the
key clinical problems that have been linked to the immunomodulatory properties of
Vitamin D and Tuberculosis
The observation that vitamin D acts to promote innate immune responses to TLR acti-
vation by M tuberculosis20has provided a new perspective on observations made
many decades ago on the beneficial effects of ultraviolet light exposure on tubercu-
losis (TB). As a consequence this has become the most well-studied facet of the inter-
action between vitamin D and innate immunity.84Initial studies to assess the effects of
25OHD status on ex vivo macrophage function have shown that supplementation with
a single oral dose of 2.5 mg of vitamin D enhances the ability of recipient macrophages
to combat Bacille Calmette Gu? erin infection in vitro.85The potential benefits of vitamin
D as treatment of TB have been further endorsed by a study that showed that adjunct
vitamin D supplementation (0.25 mg vitamin D/d) of TB patients receiving conventional
therapy for the disease reduced the time for sputum smear conversion from acid-fast
bacteria (AFB)–positive to AFB-negative status.86A recent, double-blind, randomized,
placebo-controlled trial showed that vitamin D supplementation had no effect on clin-
ical outcomes or mortality amongst TB patients, although it should be emphasized
that none of the supplemented patients in this study showed a significant increase
in serum vitamin D levels.87
Vitamin D and Multiple Sclerosis
Several epidemiology studies have reported an association between vitamin D insuf-
ficiency and the incidence and/or severity of the autoimmune disease multiple scle-
rosis (MS) (reviewed in Ref.88). These observations have been supported by analysis
of animal models, such as the experimental autoimmune encephalomyelitis (EAE)
mouse, which show increased disease severity under dietary vitamin D restriction.89
Conversely, administration of 1,25(OH)2D3to EAE mice confers disease protection
through effects on cytokine synthesis and apoptosis of inflammatory cells.90,91
Some effects of 1,25(OH)2D3on EAE seem to be dependent on IL-10 activity.65
Vitamin D and Type 1 Diabetes
In common with MS, published reports suggest that there is a link between vitamin D
deficiency and another autoimmune disease, type 1 diabetes (reviewed in Ref.92). Low
circulating levels of 25OHD3have been reported in adolescents at the time of diag-
nosis of type 1 diabetes,93and other data have documented the beneficial effects
of vitamin D supplementation in protecting against type 1 diabetes.94Another strand
of evidence linking vitamin D with type 1 diabetes stems from extensive genetic anal-
yses on the physiologic effect of inherited variations in the genes for various compo-
nents of the vitamin D metabolic and signaling system. Previous studies have
indicated that some VDR gene haplotypes confer protection against diabetes,95and
more recently this has been expanded to show that genetic variants of the
CYP27b1 gene also affect susceptibility to type 1 diabetes.96Similar to animal model
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studies for MS, in vivo use of the nonobese diabetic (NOD) mouse as a model for type
1 diabetes has shown increased disease severity under conditions of dietary vitamin D
Vitamin D and Crohn Disease
Several strands of evidence have linked vitamin D to the dysregulated immune
responses observed with inflammatory bowel diseases such as Crohn disease. First,
epidemiology suggests that patients with Crohn disease have decreased serum levels
of 25OHD3.98–100Second, studies in vivo using various animal models indicate that
1,25(OH)2D3plays a crucial role in the pathophysiology of experimentally induced
forms of inflammatory bowel disease.60,101–103Third, expression of 1a-hydroxylase
has been detected in the human colon,104with the vitamin D–activating enzyme being
up-regulated in disease-affected tissue from patients with Crohn disease.105In the
case of the latter, dysregulated colonic expression of 1a-hydroxylase was associated
with increased circulating levels of 1,25(OH)2D3indicating that, as with sarcoidosis,
localized synthesis of this vitamin D metabolite can spill over into the general circula-
tion under conditions of persistent disease.105Current studies have implicated aber-
rant innate immune handling of enteric microbiota as an initiator of the adaptive
immune damage associated with Crohn disease.106It is thus tempting to speculate
that effects of vitamin D on this disease may involve the activation of innate immunity,
together with the suppression of adaptive immunity and associated inflammation.
It is almost 30 years since an interaction between vitamin D and the immune system
was first documented. Although this was initially proposed as a nonclassic effect of
vitamin D associated with granulomatous diseases, our current view is now changed
considerably. Recent studies have shown a potential physiologic role for vitamin D in
regulating normal innate and adaptive immunity. Future studies now need to focus on
the clinical implications of vitamin D–mediated immunity and, in particular, the
possible beneficial effects of supplementary vitamin D with respect to infectious
and autoimmune diseases.
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