# Regulation of FoxP3+ Regulatory T Cells and

Authors: Chang H. Kim
Journal: Clinical and Developmental Immunology
 (2008-02-27)

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## Abstract

Vitamin A has both positive and negative regulatory functions in the immune
system. While vitamin A is required for normal formation of immune cells and epithelial
cell barriers, vitamin A deficiency can lead to increased inflammatory responses and tissue damage.
The mechanism with which vitamin A and its metabolites such as retinoids negatively regulate
inflammatory responses has not been clearly defined. Recently, it has been established that retinoids
promote the generation of immune-suppressive FoxP3+ regulatory
T cells while they suppress the T cell differentiation into inflammatory Th17 cells in the periphery
such as intestine. These novel functions of retinoids provide a potentially important immune
regulatory mechanism. In this review, we discuss the functions of retinoids in the development
of the FoxP3+ cells and Th17 cells, the phenotype and functions of
retinoid-induced FoxP3+ T cells, and the impact of
retinoid-induced FoxP3+ T cells on the immune tolerance.


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## Body

## 1. Introduction

The immune system is regulated by various
types of cells and the factors that are produced by these cells. The cells of
the immune system sense the presence of the antigens and other signals, generated
from pathogens and commensals, and respond to the stimuli in positive or
negative ways. Among the immune cells, CD4+ T cells play central
roles in regulation of immune responses by activating or suppressing immune
cells and tissue cells. Naïve CD4+ T cells, produced in the thymus,
can become Th1, Th2, or Th17 cells [1–5], which act as effector cells
to stimulate the immune system to clear pathogens and tumor cells.  On the other hand, regulatory T cells,
exemplified by FoxP3+ T cells, suppress the immune system to prevent
overactive responses and inflammation induced by T cells and their downstream (non-T)
effector cells [6–11]. Humans and mice that do not
have functional FoxP3+ T cells as the result of mutations in the FoxP3 gene suffer from inflammatory and
autoimmune diseases and die young [12–16], which serves as the evidence
for the essential roles of these T cells in maintaining immune tolerance. FoxP3+ T cells are made in the thymus from T cell progenitors and in the periphery
from naïve CD4+ T cells. Naïve FoxP3+ T cells, made in
the thymus, exclusively migrate to secondary lymphoid tissues for activation
and differentiation into memory type FoxP3+ T cells that can migrate to B cell sites, nonlymphoid
tissues, or sites of Th1 or Th2 inflammation [11, 17–19]. A number of factors in the
periphery can regulate the generation or differentiation of FoxP3+ T
cells. These are antigens, antigen-presenting cells such as dendritic cells,
cytokines, and tissue-specific factors that are produced in the cells of
certain tissue sites. One group of such tissue-specific factors are retinoids,
which can induce FoxP3+ T cells with a gut-specific tissue tropism
from naïve FoxP3− T cells [20–26]. Reciprocally,
retinoids suppress the generation of inflammatory Th17 cells. We will
discuss in this review the general properties and functions of retinoids, their
roles in regulation of T cell differentiation, and the impact of this process
on regulation of the immune system.


## 2. Synthesis, Receptors, and Biological Functions of Retinoids

Preformed vitamin A in food digesta
is absorbed in the form of retinol, which can be made into retinal and retinoic
acid (RA) [27] (Figure 1). Also retinol can
be generated from provitamin A carotenoids such as betacarotene [28–31]. Plasma retinol binding
proteins and cellular retinol binding proteins transport and stabilize retinol [32]. Some antigen-presenting
cells in gut-associated lymphoid tissues have a unique feature, which is the
ability to produce retinoic acid from vitamin A [33] (Figure 1). The conversion of
retinol (vitamin A) to retinal is catalyzed by a subfamily of alcohol
dehydrogenases (ADH), and the subsequent conversion of retinal to RA is
catalyzed by retinal dehydrogenases (RALDH) [34]. While ADH5 is ubiquitously
expressed by dendritic cells isolated from all
of the secondary lymphoid organs, the ADH1 and ADH4 isoenzymes are
preferentially expressed by only gut-associated lymphoid tissue (GALT) dendritic cells. Peyer’s patch dendritic cells express RALDH1, while mesenteric
lymph node dendritic cells express RALDH2 [33]. The retinoic acid, produced
by GALT dendritic cells, induces T cell expression
of the two gut homing receptors CCR9 and α4β7 [33]. CCR9 is the chemokine
receptor for TECK/CCL25, which is expressed in the small intestine [35], and α4β7
acts as an adhesion molecule for lymphocyte rolling and firm adhesion on the
surface of mucosal endothelial cells by binding to its ligand mucosal addressin
cell adhesion molecule (MAdCAM-1) [36, 37].Figure 1
Synthesis of retinoic acid in the dendritic cells of the
intestine.
Vitamin A is consumed as retinyl
ester and hydrolyzed by retinyl ester hydrolase (REH). Retinol, entered into
cells, is oxidized to retinaldehyde by alcohol dehydrogenase (ADH) or short-chain
dehydrogenase/reductase (SDR). The reverse reaction is mediated by aldo-keto
reductase (AKR) or SDR. Retinaldehyde can be converted to retinoic acid (all-trans
retinoic acid or At-RA) by retinaldehyde dehydrogenase (RALDH). Retinoic acid is
degraded in cells by cytochrome P450 (CYP26). It is known that dendritic cells in the gut-associated lymphoid
tissues (GALT) such as Peyer’s patches and MLN express high levels of ADH1,
ADH4, RALDH1, and/or RALDH2.  The
retinoic acid produced by dendritic cells can be exported for other cells such
as T cells during the cognate interaction between dendritic cells and T cells.Among the metabolites
of vitamin A, 11-cis-retinal, all-trans-retinoic acid (At-RA) and 9-cis-RA
mediate the major biological functions of the vitamin [29, 34]. Vitamin A has pleiotropic
functions in the body. 11-cis-retinal functions as chromophores in light
absorption for vision, and retinoic acid participates in bone formation,
reproduction, and differentiation of many cell types during embryogenesis [28–30]. In this regard, retinoid
deficiency or overdoses can cause teratogenesis [38]. Vitamin A plays important
roles in the immune system. One such a function is its role in the formation of
epithelial linings of the body including eyes, membranes of mucosal tissues in
the respiratory, urinary and intestinal tracts, and skin, which serve as
barriers to prevent the invasion by pathogens [39]. Through their effects on
cell differentiation and death, retinoids inhibit or reverse the carcinogenic
process in some types of cancers in oral cavity, head and neck, breast, skin,
liver, and blood cells [40, 41].At-RA
and 9-cis-RA are the ligands for retinoid nuclear receptors that act as
transcription factors for gene expression. As retinoid nuclear receptors, RA
receptor (RAR) isotypes (α, β, and γ) and the retinoid X receptor (RXR)
isotypes (α, β, and γ) have been identified [29]. Each receptor has multiple
isoforms, generated through alternative mRNA splicing and/or transcription [30, 31]. At-RA preferentially binds
RARs, whereas 9-cis-RA binds equally well to both RARs and RXRs [29, 42]. The two groups of retinoid
nuclear receptors form RXR/RAR heterodimers, which can function as either
transcriptional repressors or activators depending on the availability and type
of retinoid ligands [43]. In the absence of RAR
ligands, aporeceptors recruit corepressors such as nuclear receptor corepressor
(N-CoR) or silencing mediator for retinoid and thyroid hormone receptor (SMRT),
which then recruit histone deacetylases (HDACs). Binding of RAR agonists to the
receptors induces active conformation of the receptors (Figure 2). This
decreases the affinity for corepressors and creates a binding surface for
histone acetyltransferase (HAT) coactivators, such as CREB-binding protein
(CBP) and p160 (e.g., TIF2). Transcriptional coactivators are involved in
chromatin remodeling (decondensation due to histone acetylation) or recruitment
of the basal transcription machinery. RXR agonists alone cannot dissociate the corepressors
from heterodimers of RAR-RXR, and retinoid
receptor antagonists would prevent the formation of the holoconformation.
Please see the reviews [44, 45] for more details on retinoid
receptor-mediated gene expression.Figure 2
Control of gene expression by retinoic acid.
RAR-RXR heterodimers serve as the
nuclear receptors for retinoic acid. When RAR ligands (e.g., At-RA) are not
available, the RAR-RXR heterodimers attract corepressors and histone acetyl
deacetylases to the genes which are under the control of retinoic acid response
elements (RARE: direct repeats of AGGTCA with a 5-bp spacer; also called DR-5), resulting in a closed
conformation of the chromosome and blocked transcription. When RAR ligands are
available in a tissue microenvironment (e.g., produced from dendritic cells in the intestine), they would enter into
T cells and activate RAR-RXR heterodimers. This is followed by the release of
corepressors and attraction of coactivators, which, in turn, recruit histone
acetyl transferases, resulting in an open conformation of the chromosome and
transcription of the genes. In this regard, the human and mouse FoxP3 promoters
have several RARE's and become acetylated at histones in response to retinoic
acid.Many functions of
retinoids have been identified in regulation of immune responses. As positive
roles in regulation of the immune system, retinoids enhance the numbers and
effector functions of neutrophils, NK cells, B cells, and Th2 cells [28, 46–52]. Retinoids induce the gut
homing receptor expression in T and B cells [19, 33, 53–55] and B cell Ig switch to IgA [51, 55], and, therefore, are
important regulators of mucosal immunity. Vitamin A is required for antibody
responses to T-dependent and bacterial polysaccharide antigens [56, 57], normal IgA levels in the
intestinal fluid [58], prevention of
activation-induced T cell apoptosis [59, 60], and normal phagocytic
functions and resistance to bacterial pathogens [61]. Vitamin A supplementation
can  decrease serum concentrations of inflammatory cytokines such as TNF-α and
IL-1 but increase suppressive cytokines such as IL-10 [42, 62]. In this regard,
supplementation with vitamin A can suppress inflammation in experimental
autoimmune encephalomyelitis [47, 63–66]. Paradoxically, vitamin A
deficiency is linked to both defective delayed type hypersensitivity [67–69] and excessive Th1 cell
function [46–50, 70, 71]. Vitamin A is also linked to
both lower and higher incidences of several types of cancers in humans [72, 73].Retinoids are
being used to treat acne, psoriasis, and selected cancer types. The mechanisms
of action for the retinoids in treatment of psoriasis and acne are poorly
elucidated [74, 75]. The skin cells can be the
direct therapeutic target for retinoids. It is equally possible that the therapeutic
effect of retinoids can be indirect though regulation of the immune system that mediates inflammation in the skin. Additionally, retinoids have therapeutic effects
on acute promyelocytic  leukemia (APL) cells and AIDS-related Kaposi’s sarcoma
cells by regulating cellular differentiation and/or proliferation [76, 77]. APL is a specific form of
myelocytic leukemia characterized by the t(15;17) chromosomal translocation and
formation of the abnormal PML-RARα
fusion protein [78, 79]. Retinoic acid, when
administered at pharmacological doses, can induce leukemic cell differentiation
and cell-cycle arrest by reversing the dominant-negative effects of PML-RARα
fusion protein on the functions of the wild-type PML and RARα proteins
[41, 80, 81].


## 3. Regulation of the Foxp3+ T Cell Generation by Retinoids

FoxP3+ T cells are
regulatory T cells that can suppress immune responses. FoxP3+ T
cells are made largely in two different ways: they are made in the thymus from
T cell progenitors or they can be made from FoxP3− naïve T cells in
the periphery during their activation with antigen-presenting cells such as
dendritic cells. In the periphery, many factors can affect the generation of
FoxP3+ T cells from naïve T cells. One of the factors is the route
of antigen administration. Immunization through the oral route has the tendency
to induce FoxP3+ T cells [82, 83].  Another factor is the cytokine milieu. TGF-β1 is
a very effective cytokine that induces FoxP3+ T cells from naïve T
cells [84]. In this regard, the
induction of FoxP3+ T cells by mucosal immunization is dependent on
TGF-β1 [82, 83].  Another cytokine, IL-2, is important for
peripheral induction and maintenance of FoxP3+ T cells [85–88]. IL-2 signaling is dispensable
for the induction of FoxP3+ T cells in the thymus but required for
maintenance of FoxP3+ T cells in the periphery [87, 89]. It is not just a coincidence
that FoxP3+ T cells highly express CD25 (the alpha chain of the
heterotrimeric high-affinity IL-2 receptor) to receive the IL-2 signal.  Another factor is the type of antigen-presenting
cells. Both monocyte-derived dendritic cells and
plasmacytoid dendritic cells can induce CD4+ CD25+ T cells or FoxP3+ T cells [90–92]. Dendritic
cells in tumors are particularly efficient in inducing FoxP3+ T cells [93], which provides an
explanation for the high prevalence of FoxP3+ T cells in many types
of tumors. Yet another factor is the presence of certain pathogens or toll-like
receptor (TLR) ligands [94, 95]. TLR ligands can either
expand FoxP3+ T cells or suppress their generation and functions [96, 97]. The FoxP3+ T cell
expansion by dendritic cells can be augmented in the presence of TLR ligands
such as LPS [98].Recently, a number
of groups including our group reported that retinoic acid regulates the
peripheral induction of FoxP3+ CD4+ T cells [20–26]. Retinoic acid, together with
TGF-β1, induces not only CD4+ but
also CD8+ FoxP3+ T cells [20]. As mentioned before, the dendritic cells in gut-associated lymphoid tissues have
the capacity to provide retinoic acid to T cells undergoing antigen priming [33]. While splenic dendritic cells could not induce FoxP3+ T
cells from naïve T cells, the dendritic cells
isolated from Peyer’s patches and small intestinal lamina propria were able to
induce FoxP3+ T cells (Figure 3) [22–24]. In this regard, Coombes et
al. observed that CD103+ mesenteric lymph node (MLN) DCs, but not
CD103− MLN DCs, were efficient in conversion of naïve CD4+ T cells into FoxP3+ T cells [23]. Consistently, Sun et al.
observed that the conversion rate of naïve T cells into FoxP3+ T
cells was high in intestinal lamina propria and MLN but not PLN [22]. This conversion was
dependent on TGF-β1 as neutralizing anti-TGF-β1
was able to block the induction while exogenous TGF-β1 further
enhanced the conversion. CD103+ DCs express TGF-β2,
tissue plasminogen activator, and TGF-β
binding protein 3 [23]. Tissue plasminogen activator
activates latent TGF-β, and TGF-v
binding protein 3 regulates secretion and localization of TGF-β [99–101]. Another group independently reported
that CD103+ DCs are CD11b− and induce IL-10 but fail to
induce IL-17 production in T cells [102]. Not only DCs but also some
macrophages in the gut can induce FoxP3+ T cells: CD11b+ F4/80+ CD11c− macrophages express anti-inflammatory molecules such as IL-10, TGF-β1,
TGF-β3, program death-ligand 1 (PD-L1 or also
called B7H-1), and PD-L2 (B7-DC), and are efficient in inducing FoxP3+ T cells [102]. The capacity of these antigen-presenting
cells in induction of FoxP3+ T cells is associated with their
ability to produce retinoic acid. Kang et al. showed that diethylaminobenzaldehyde
(DEAB), an inhibitor of retinaldehyde dehydrogenases, completely suppressed the
induction of FoxP3+ T cells by the mucosal DCs [24]. Mucida et al., Sun et al.,
and Kang et al. demonstrated that the mucosal DC-dependent induction of FoxP3+ T cells can be blocked by RAR antagonists such as LE540, LE135, and Ro41-5253 [20, 22, 24]. The mucosal DC-dependent
induction of FoxP3+ T cells can be mimicked by activating naïve T
cells with polyclonal T cell activators in the presence of retinoic acid and
TGF-β1 [20–26].Figure 3
Induction and homing of retinoid-induced FoxP3+ T cells. Gut homing FoxP3+ T cells are made in gut-associated lymphoid tissues (GALT) but not in the thymus. FoxP3+ T cells made in the thymus have the naïve T cell phenotype in
expression of homing receptors and migrate to secondary lymphoid tissues. In
the presence of retinoic acid, these naïve FoxP3+ T cells become gut
homing memory FoxP3+ T cells. Also, naïve FoxP3− T cells
become FoxP3+ T cells in the GALT to become gut homing FoxP3+ T cells. These FoxP3+ T cells express CCR9 and α4β7
among other trafficking receptors and are highly efficient in migration to the small
intestine. The differentiation of these T cells into gut homing FoxP3+ T cells is mediated by subsets of dendritic cells and macrophages because they
can present antigens and produce retinoic acid. Additionally, the epithelial
cell interacting adhesion molecule αEβ7 is
upregulated on FoxP3+ T cells if the tissue microenvironments have
high levels of TGF-β1. Intestine is such a tissue site that produces
both retinoic acid and TGF-β1.FoxP3+ T cells of humans and mice behave somewhat differently. T cell activation
readily upregulate FoxP3 in some human T cells [103, 104]. In this  regard, the human
FoxP3 promoter contains  six NF-AT and
AP-1 binding sites [104]. These molecules act as the
major transcription factors that are activated downstream of TCR activation.
However, the expression levels of FoxP3 in the activated T cells are relatively
low and transient [105], and these activated T cells cannot
suppress the proliferation of target T cells. Enforced expression of FoxP3 in
human T cells resulted in induction of hyporesponsiveness and suppression of
IL-2 production but failed to turn the T cells into T regulatory cells that can
suppress target T cells [106], suggesting that additional
signals are required to differentiate the FoxP3-expressing cells into fully functional
FoxP3+ T regulatory cells. In contrast, simple activation of mouse
FoxP3− T cells in the absence of T regulatory cell-inducing factors
such as TGF-β1 does not readily generate FoxP3-expressing
T cells [20–26], implying that the expression
of FoxP3 is more tightly regulated in mouse T cells. Contrary to human T cells,
enforced expression of FoxP3 in mouse T cells was sufficient to turn FoxP3− T cells into functional T regulatory cells which can suppress target
cells [107, 108]. There is another difference
between mouse and human T cells. Retinoic acid alone is sufficient to convert human
naïve T cells undergoing activation into FoxP3+ T cells [24]. However, the TGF-β1
signal is additionally required to reliably induce FoxP3 in mouse T cells in
vitro [20–26]. It is possible that human T
cells are intrinsically different from mouse T cells in expression of certain
genes such as FoxP3 as discussed above. It is also possible that human naïve
T cells, but not mouse T cells, would produce TGF-β1 [109] at levels high enough for their conversion into FoxP3+ T cells in
response to retinoic acid.Retinoic acid
induces histone acetylation in the human FoxP3 promoter [24]. This acetylation is thought
to open the FoxP3 promoter for active transcription. It is believed that
retinoic acid decreases the affinity for nuclear receptor corepressor and
recruits histone acetyltransferase coactivators [110], inducing chromatin
decondensation and recruitment of the basal transcription machinery to the
FoxP3 promoter (Figure 2).In
a manner similar to the regular FoxP3+ T cells, IL-2 is required for
induction of retinoid-induced FoxP3+ T cells [20, 24]. IL-2 at high concentrations
can increase the induction of retinoid-induced human FoxP3+ T cells [111]. IL-2 signaling activates
STAT5a and b [112], which in turn are involved
in the expression of FoxP3 [113]. It has been reported that
RARα can interact with STAT5a and b and
coregulate gene expression [114]. This implies that IL-2
signaling and STAT5 are required for the retinoic acid-induced FoxP3 expression
and generation of functional T regulatory cells. This, however, turned out to
be not true. Elias et al. reported that STAT5-deficient
naïve T cells, although less efficient than wild-type T cells,  can still be converted to FoxP3+ T
cells in response to retinoic acid and TGF-β1 [26]. Therefore, the retinoic acid
& TGF-β1-induced induction of FoxP3 expression
can be enhanced by the IL-2 signaling pathway but does not absolutely require the
IL-2/STAT5 signaling pathway.The T cells
stimulated by RA preferentially express RARα,
and RARα antagonists can completely block the retinoic
acid-induced expression of FoxP3 in human T cells [24, 25]. RARα
agonists can mimic retinoic acid in induction of FoxP3 [24, 25]. Therefore, RARα is
considered a central receptor that mediates the function of retinoic acid. In
this regard, Schambach et al. overexpressed RAR-α in
mouse T cells by retroviral gene transfer and observed an increase in the FoxP3+ T cell induction [25]. The specific role of RARα is
further supported by the information that methoprenic acid (a pan-RXR agonist)
was hardly able to induce human FoxP3+ T cells [24].In vivo,
repetitive injection of retinoic acid subcutaneous induced small intestine
homing FoxP3+ T cells in peripheral lymph nodes, which are not,
normally, the sites for generation of these T cells [111]. Also, injection of RAR
antagonists decreased the frequencies of FoxP3+ T cells in the mouse
small intestine [20]. Moreover, the expression of RARα
itself is induced in T cells activated by retinoic acid [24], which is considered a
positive feedback mechanism for generation of retinoid-induced FoxP3+ T cells.


## 4. Regulation of Homing Receptor Expression by Retinoids

It is striking that the retinoid-induced
FoxP3+ T cells in mice and humans have a highly specific tissue
tropism for the small intestine. Human retinoid-induced FoxP3+ T
cells highly express CCR9 and α4β7 and efficiently migrate toward the
small intestine chemokine TECK/CCL25 [24].  On the other hand, loss of CD62L expression on
the retinoid-induced FoxP3+ T cells would decrease their migration
into peripheral lymph nodes [24] but further promote their
migration into the small intestine. The mouse or human retinoid & TGF-β1-induced
FoxP3+ T cells are similar to human retinoid alone-induced FoxP3+ T cells in high expression of CCR9 and α4β7 [24]. Interestingly, CD103 (the alpha chain of the integrin αEβ7)
is more highly expressed by the retinoid & TGF-β1-induced
FoxP3+ T cells than TGF-β1
alone-induced FoxP3+ T cells [20, 111]. Another pathway to generate
gut homing FoxP3+ T cells is to antigen-prime preexisting FoxP3+ T cells in the presence of retinoic acid (retinoid-conditioned FoxP3+ T cells) [19, 111]. In this case, no TGF-β1 is
required to induce FoxP3+ T cells. Retinoid & TGF-β1-induced
FoxP3+ T cells and retinoid alone-induced (or conditioned) FoxP3+ T cells would migrate to the small intestine (Figure 3) but they may localize
in different locations within the small intestine due to the difference in
CD103 expression.


## 5. Effector Functions of Retinoid-induced Foxp3+ T Cells

One can
ask if the retinoid-induced FoxP3+ T cells are really regulatory T
cells in terms of functionality because simple expression of FoxP3 would not
necessarily indicate full maturation into T regulatory cells. In this regard,
the retinoid-induced FoxP3+ T cells, induced by retinoic acid from
naïve human T cells or by retinoic acid plus TGF-β1 from mouse or human T cells, appear to
have all of the features of the regulatory T cells.  These retinoid-induced human and mouse FoxP3+ T cells are hypoproliferative and can suppress the proliferation of other T
cells [21, 23–25]. When injected in vivo,
retinoid-induced FoxP3+ T cells efficiently suppressed naïve T-cell-induced
colitis in recombinase-activating gene- (RAG-) deficient mice [20, 24]. The efficacy of retinoid
& TGF-β1-treated T cells in suppression of
colitis was higher than TGF-β1 alone-treated T cells [20, 24]. However, it is unknown if
retinoid-induced FoxP3+ T cells are more efficient than TGF-β1
alone-induced FoxP3+ T cells in suppression of diseases in vivo
because it is likely that the retinoid & TGF-β1-treated
T cells contain more FoxP3+ T cells than TGF-β1
alone-treated T cells. Another explanation is that the retinoic acid-induced FoxP3+ T cells are more stable than TGF-β1
alone-induced FoxP3+ T cells in vivo even after antigenic challenge [20–22]. It has been reported that some
human and mouse FoxP3+ T cells express granzymes and can kill target
cells [115–117]. We observed a difference in
expression of effector molecules between retinoid-induced human FoxP3+ T cells and retinoid & TGF-β1-induced human FoxP3+ T
cells [24]. Retinoid-induced human FoxP3+ T cells express granzymes but the retinoid & TGF-β1-induced
FoxP3+ T cells do not express these cytotoxic molecules because TGF-β1
effectively suppresses their expression [24]. While the retinoid-induced FoxP3+ T cells have the potential to kill target cells for immunological tolerance in
the intestine, it is unknown if the FoxP3+ T cells, present in the
intestine of mice and humans, can express granzymes and kill target cells.


## 6. Regulation of Th17 Cells by Retinoids

It was believed that Th1 cells were
the major T cells to induce autoimmune diseases [118]. An increasing body of
evidence, however, suggests that Th1 cells (and their cytokine product IFN-γ or
transcription factor T-bet) play a protective role in certain autoimmune
diseases such as experimental allergic encephalomyelitis and collagen-induced
arthritis [119–121]. This implies that there are
additional cell subsets that would cause autoimmune diseases, and Th1 cells
would suppress the generation or function of the inflammatory cells. Many
groups have reported that blocking of IL-23 or IL-17 prevented the development of autoimmune diseases  [122–127]. Now, it is well established
that Th17 cells constitute a newly identified subset of inflammatory T cells [3–5]. Th17 cells characteristically
express IL-17A, IL-17F, IL-21, and IL-22 [128–130]. IL17A and IL-17F play
important roles in inflammation and host defense by inducing IL-6, GM-CSF,
G-CSF, and chemokines [131–133]. IL-21 promotes the
generation of Th17 cells [134, 135]. Interestingly, IL-22 can
play anti-inflammatory roles and induces antimicrobial proteins and
lipopolysaccharide-binding protein [136–139]. It is well established that IL-6 and TGF-β1 are
the major cytokines that induce mouse Th17 cells [140, 141]. In this regard, mouse Th17
cells share TGF-β1 with FoxP3+ T cells as an
inductive cytokine. However, TGF-β1 alone
or together with IL-6 does not promote the generation of human Th17 cells [142]. Also, there is an
IL-6-independent pathway to generate Th17 cells [143]. IL-6-independent generation
of Th17 cells is mediated, in part, by IL-21 and IL-23 [134, 135, 144]. Moreover, IL-6 induces
expression of more IL-21 and IL-23 receptors on T cells to promote Th17 cell
generation [144]. Important transcription
factors for generation of Th17 cells include STAT3 (activated by IL-6 and IL-23),
ROR-γt, and ROR-α [145–148]. ROR-γt is
an orphan nuclear receptor important also for survival of CD4+8+ thymocytes and the biology of lymphoid tissue inducer cells [149–151]. ROR-α is another
related orphan nuclear receptor, which is induced by TGF-β1
and IL-6 in T cells [148]. Yang et al. demonstrated
that overexpression of ROR-α promoted Th17 differentiation, while ROR-α
deficiency suppressed the process. Moreover, ROR-α and
ROR-γt, when overexpressed together, synergistically
enhanced Th17 differentiation [148]. As negative regulators of
Th17 cell generation, IL-27 (a Th1 promoting cytokine) [152, 153] and IL-2 [154] have been identified. Amadi-Obi et al.,
however, reported that IL-2 can expand human Th17 cells [155]. In this regard, use of IL-2
in expansion of human Th17 cells has been reported by others too [142]. Thus, whether inflammatory
Th17 cells or suppressive FoxP3+ T cells are generated in the
periphery would be determined by the cytokine milieu determined by IL-2, IL-6,
IL-21, IL-27, TGF-β1, and other Th1/2 cytokines present in
the tissue microenvironment. Additionally, there seems to be some differences in
induction of Th17 cells between mice and
humans.Antigen-presenting
cells would play critical roles in generation of Th17 cells because these cells
can present antigens and produce certain inflammatory cytokines that can induce
Th17 cells. In this regard, it was reported that MLN CD103− DCs
produce inflammatory cytokines such as TNF-α,
IL-6, IL-12p40, and IL-23p19 upon stimulation with LPS [23].  While Coombes et al. did not examine if the
CD103− DCs could induce Th17 cells, another group examined the
potential of mucosal dendritic cells in induction of Th17 cells [102]. Denning et al. reported that
intestinal lamina propria CD11b+ DCs were able to induce Th17 cells
very well, while CD11b− DCs were not efficient. CD11b+ DCs do not express CD103, and thus they are the CD103− DCs that
Coombes et al. described in a separate study [23]. In contrast, CD11b− DCs highly express CD103 and fail to induce IL-17 but are efficient in
induction of the anti-inflammatory cytokine IL-10 and FoxP3+ regulatory
T cells [102].At-RA, 9-cis RA or
other RAR agonists have potent suppressive activities on generation of Th17
cells (Figure4) [20, 24, 25]. Reciprocally, RAR antagonists can enhance
the generation of Th17 cells. In this regard, Elias et al. reported that
retinoic acid can suppress ROR-γt expression in T cells [26]. It is paradoxical, however,
that Th17 cells are present in the intestine at very high frequencies (~10% of
CD4+ T cells) in the body [147]. It is likely that the antigen-presenting
cells that induce Th17 cells would not be able to produce retinoic acid and are
different from the cells that induce gut homing FoxP3+ T cells. More
studies are required to fully understand the maturation process of Th17 cells
in terms of tissue tropism and effector function. Another concern is that most groups
used retinoic acid at pharmacological concentrations (~1 μM) to
demonstrate the function of retinoic acid in suppression of Th17 cells in vitro.
Thus, the in vivo relevance of the function of retinoic acid remains unclear.Figure 4
Reciprocal regulation of FoxP3+ T cells and Th17 cells by retinoic acid.
IL-2 and TGF-β1
promote the generation of FoxP3+ T cells from naïve T cells in the
periphery, while IL-6 and TGF-β1 promote the generation of Th17 cells, an
inflammatory T cell subset that produces IL-17A, IL-17F,
IL-21, and IL-22 as the major effector cytokines. IL-2 and the cytokines that
promote T cell proliferation into Th1 or Th2 cells (IL-4, IL-12, IFN-γ,
and IL-27) would suppress the generation of Th17 cells, while the pro-Th17 cell
cytokines, IL-6 and IL-21, can suppress the generation of FoxP3+ T
cells. Importantly, retinoic acid suppresses the
generation of Th17 cells but promotes the induction of FoxP3+ T cells.


## 7. Conclusion

Intestine is a large organ that by
design is in contact with food digesta, with commensals, and, sometimes, with pathogens.
Maintaining the balance between immunity (to pathogens) and tolerance (to food
antigens and commensals) is critical to healthy and functional intestine. The
recent literature [20–26] supports the view that retinoic
acid is an important signal that defines the “microenvironmental cue” of the
small intestine. It can be produced by gut epithelial cells and
antigen-presenting
cells, and it promotes
the generation of a gut homing subset of FoxP3+ T cells.  Since FoxP3+ T cells are important
for maintaining tolerance in the body, it is likely that retinoic acid is a
major signal that confers immunological tolerance to the small intestine. On
the other hand, retinoic acid positively regulates also B cell IgA production
and induction of gut homing effector T cells, which play essential roles in
conferring immunity against pathogens. Thus, we conclude that retinoic acid is
a common regulator of both immunity and tolerance in the gut. We still do not
completely understand all of the biological functions of retinoid-induced FoxP3+ T cells in regulation of immunity and tolerance. Moreover, the readers should be
aware of the fact that humans and mice are not identical in this regulation. Also
the in vivo relevance of these functions and their impacts on immune responses
to pathogens and cancer cells need to be verified in physiological settings.  It is also important to investigate if there are
additional tissue-specific signals that regulate the generation and migration of
FoxP3+ and Th17 cells in the gut.



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*Source: 416910-2008-03-19.xml*