Download Immune cellular regulation on autoantibody production in pemphigus

doi: 10.1111/1346-8138.12697
Journal of Dermatology 2015; 42: 11–17
REVIEW ARTICLE
Immune cellular regulation on autoantibody production in
pemphigus
Meng PAN, Haiqin ZHU, Renchao XU
Department of Dermatology, Rui Jin Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, China
ABSTRACT
Pemphigus is an autoantibody-mediated blistering disease in the skin and mucous membranes. The autoantibodies primarily target desmoglein (Dsg)1 or/and Dsg3, transmembrane glycoproteins of skin epidermal cells, leading
to loss of desmosome adhesion and acantholysis through signaling dependent and independent pathways. Thus,
the pemphigus autoantibodies themselves and immune processes, particularly cellular regulation of antibody production, remain as interesting topics in probing pemphigus pathogenesis. In this review, we focus on current
advances regarding how the pemphigus antibody production is highly regulated by the key immune effectors
including T cells, B cells and their secreted cytokines. Specifically targeting these immune effectors involved
in the pemphigus autoantibody production should provide better therapeutic options for disease treatment of
pemphigus.
Key words:
tolerance.
autoantibody production, autoreactive T and B cells, pemphigus, regulatory cells, T- and B-cell
INTRODUCTION
Pemphigus is an autoimmune disease, characterized by intraepidermal blistering of the skin and mucous membranes.
Recent studies have suggested that this disease is mediated
by pathogenic but not non-pathogenic autoantibodies in the
blood from patients.1 A series of prior studies and still-accumulating data have also revealed that these antibodies mainly
target desmogleins (Dsg), a critical glycoprotein found predominantly in the skin epidermis.2–4 Passive transfer of anti-Dsg3
antibody into neonatal mice could induce the disease,3,4 a
milestone work in exploring the pemphigus pathophysiology.
Conversely, removal of the autoantibodies from the sera prevents the induction of pemphigus in vivo. Taken together, the
specific autoantibodies are necessary and sufficient for development of pemphigus.
Despite the previous exciting findings, the exact mechanisms
for antibody production remain largely unclear. In antibodymediated humoral immune response, B cells could break the
self-tolerance and produce the antibodies, although the checkpoint by which loss of B-cell tolerance occurs requires further
investigation. In systemic lupus erythematosus (SLE), the point
at the level of dysfunction of the interleukin (IL)-secreting regulatory B cells has been linked to development of autoimmunity.5
In pemphigus, depletion of the CD19+ CD27+ memory B cells
and presence of more regulatory interleukin (IL)-10-producing B
cells have been attributed to the long-lasting complete
remission of the disease,6 indicating that the checkpoint may
locate at some point of B-cell maturation into memory B cells.
Cell-mediated immunity and cytokines may also be essential
components in the phenotypic expression of disease. For
instance, in an active murine model, the Dsg3–/– mice were
immunized with recombinant Dsg3, which leads to the induction
of Dsg3-specific immune response, including development of
autoreactive T and B cells.7 The Dsg3-primed splenocytes were
then transferred into the Rag2–/– mice, an immunodeficient
mouse without mature T and B cells but intact Dsg3 distribution
and expression, leading to an animal pemphigus phenotype.
The results showed that the anti-Dsg3 IgG antibody production
and clinical phenotypes were only observed from the mice that
were transferred with both T and B cells, indicating that both T
and B cells were involved in the triggering phase of the disease
and T cells may stimulate B-cell maturation during the process
of autoantibody production.7 On the other hand, it is known that
in some healthy individuals and the relatives of the patients, the
low titer-specific anti-Dsg antibody could be detected. These
antibodies are more likely to be non-pathogenic, given that the
patients have no clinical symptom, suggesting the necessity of
orchestrating multiple immune effectors to produce the pathogenic antibodies. More recently, a shared gene usage of the
antibody heavy chain has been identified for the pathogenic autoantibodies cloned from different pemphigus patients,8 suggesting a common genetic and immune mechanism of
autoantibody production and disease pathogenesis. In this
Correspondence: Meng Pan, M.D., Ph.D., Department of Dermatology, Rui Jin Hospital, School of Medicine, Shanghai Jiao Tong University,
Shanghai, China. Email: [email protected]
Received 2 October 2014; accepted 3 October 2014.
© 2015 Japanese Dermatological Association
11
M. Pan et al.
review, we attempt to focus on the immune regulating network
on antibody production in pemphigus, including the self-tolerance, regulatory cells and related cytokines.
INITIATION OF AUTOANTIBODY PRODUCTION
AND LOSS OF SELF-TOLERANCE IN B CELLS
B cells serve as one of the major autoantibody-secreting cells in
autoimmune diseases. Additionally, they may function as selfantigen-presenting cells. Furthermore, B cells may regulate the
T-cell development and function in vivo through multiple mechanisms. In treatment of pemphigus vulgaris (PV) patients, rituximab (an anti-CD20 monoclonal antibody) has been reported to
cause long-lasting remission by specifically targeting the CD20+
pre-B cells and mature B cells (memory B cells),6 supporting
that B cells are central to pemphigus initiation and development.
Further studies have showed that depletion of B cells by administration of rituximab could impair the T-cell function,9 thus
enhancing its effect on reduction of autoantibody production.
At the early phase of autoantibody production, a potential
mechanism is triggered by an exogenous antigen (e.g. from
viral infection) sharing similar epitope structures with human
Dsg, leading to autoantibody production. When exposed to this
foreign antigen, the human immune system presumably develops polyclonal antibodies against it. Because of the similarities
with Dsg, the produced antibodies may also cross-react with
Dsg that are specifically expressed in the skin epidermis,
resulting in skin acantholysis or pemphigus. In fogo selvage
(FS), known as a type of endemic pemphigus foliaceus in Brazil, immunoglobulin (Ig)G4 autoantibody against Dsg1 is
believed to mediate the disease pathogenesis.10 Interestingly,
the FS autoantibodies can bind to an LJM11 salivary protein
isolated from the sand fly salivary gland, suggesting that
molecular mimicry of the self-antigens (Dsg) and non-infectious
or infectious antigens may initiate the pathogenic autoantibody
production in pemphigus. This notion is further supported by
the fact that mice challenged with this antigen were able to
develop antibodies cross-reacting with Dsg1.10
Alternatively, B cells that produce non-pathogenic antibodies against Dsg may naturally exist in normal human individuals, as clearly demonstrated by antibody library screening
using phage display1,11 and detection of low titer-specific antiDsg antibodies in healthy individuals.12 Indeed, it was estimated that as much as 75% newly generated bone marrow B
cells express B-cell antigen receptors that are able to bind
self-antigens.13 Yet, a vast majority of the self-binding and
-destructive autoimmune reactions will be blocked possibly
due to the B-cell self-tolerance by negative selection. The
potential mechanisms of negative selection have been previously proposed, which include eliminating, editing or silencing
the autoreactive B cells, as elegantly demonstrated in the
genetically modified mouse models.14–16 However, the key
question remains how the clonal expansion of activated B cells
happens while ignoring the negative selection mechanism in
the physiologic condition, followed by differentiation into antibody-secreting plasma cells or germinal center B cells. Chan
et al. have showed that the self-reactive germinal center B
12
cells were eliminated when the self-antigen was expressed
ubiquitously or specifically in cells close to the germinal center
environment.17 In contrast, those cells that bind to rare antigens survived probably by “positive selection” and antibody
hypersomatic mutation, and were able to differentiate into
plasma cells secreting high-affinity autoantibodies. These data
clearly demonstrated that the landscape or microenvironment
of autoreactive B cells, including the regulatory T cells, dendritic cells, and dose and time of self-antigen exposure, may
be critical in pemphigus autoimmunity (Table 1).
REGULATION OF AUTOANTIBODY SECRETION
BY BREG
Regulatory B cells (Breg) are a relatively newly identified subset
of cells that have been shown to downregulate T-cell function,
which were first described by Mizoguchi and Bhan in 2006.18
Although the exact mechanism requires further investigation,
the inhibitory effects of Breg were shown to depend on production of cytokines including IL-10 and transforming growth factor
(TGF)-b.19,20 In both animal models and human diseases, Breg
have been shown to play a critical role in the development of
autoimmune diseases such as SLE and multiple sclerosis. We
recently found that the Breg function has been impaired in
pemphigus patients. Strikingly, we found that the number of
Breg is significantly increased in the blood of pemphigus
patients with active disease compared with that in the remittent
patients. However, the B cells from the patients were found to
be able to secrete less IL-10 and be defective in suppressing
interferon (IFN)-c secretion by the T-helper (Th)1 cells. Collectively, our results have demonstrated that impaired function of
Breg may contribute to pemphigus pathogenesis by relieving its
suppression on the autoreactive T cells via abnormal cytokine
Table 1. Loss of T cell or B cell tolerance in pemphigus
Cells
Central tolerance
Peripheral tolerance
T cell
Dsg3 expression
T-cell receptor signaling
AIRE
Low Dsg3 exposure
B-cell receptor signaling
Self-antigen exposure
Cytokines and signaling
Foxp3
Inactivated T cell
Antibody hypersomatic
mutation
HLA class II
B cell
Negative selection
In pemphigus, autoantibody production requires both autoreactive T
and B cells. Activation T or B cells may occur in the context of loss of
central or peripheral tolerance. Although the mechanism is still unclear,
T-cell self-tolerance is maintained by recognition of desmoglein (Dsg)3
(the autoantigen). At a low level, Dsg3 may inactivate the “natural” autoreactive T cells, while at a high level of Dsg3 or with a high-affinity
receptor autoantibody T or B cells may undergo apoptosis by negative
selection through the respective antigen receptors. The Dsg3 expression level is controlled by the transcriptional factor autoimmune regulator (AIRE). Likewise, loss of tolerance may also be controlled by
self-antigen expression that can be regulated by the transcriptional
factor FoxP3. T-cell tolerance is regulated by the signals directly (T regulatory cells) or indirectly (special cytokines) from other T cells. By contrast, loss of B cell tolerance is highly regulated by T cells and driven
by somatic mutation of B cell antigen receptors (BCR). This regulation
is presumably restricted by the pemphigus-specific HLA class II.
© 2015 Japanese Dermatological Association
Mechanism of autoantibody production in pemphigus
production such as IL-10.19–21 When treated with rituximab,
pemphigus patients with complete remission had a significantly
higher number of IL-10-secreting regulatory B cells and much
more IL-10 compared with those with no response and incomplete remission, suggesting a critical role of Breg in pemphigus.6
LOSS OF T-CELL SELF-TOLERANCE IN
PEMPHIGUS
Studies of healthy individuals with the PV-associated human
leukocyte antigen (HLA) class II have shown that there exist
Dsg3-specific T cells, which recognize the same epitopes of
Dsg3 as those detected in pemphigus patients.22,23 Thus, both
PV susceptibility HLA II alleles and break of T-cell tolerance
are not sufficient to cause pathogenic antibody production.
Yet, previous studies using animal models have clearly
revealed that loss of T-cell tolerance was necessary for pemphigus pathogenesis.7
Therefore, it may be difficult to understand how the immune
system breaks the T-cell tolerance in pemphigus. It appears that
loss of central T-cell tolerance may occur in the thymus, which
controls central tolerance by regulating the expression levels of
self-antigens. One of the key transcription factors involved in
this process is named autoimmune regulator (AIRE) and has
been shown to be essential in controlling expression of nearly all
the self-antigens, including Dsg3.24 The expression levels of
Dsg3 in the thymus define the fate of T-cell development by certain immune mechanisms such as negative selection. Yet, evidence that supports a central tolerance mechanism in
pemphigus remains extremely limited, although recent studies
of the population markers on the Dsg3-specific IgM+ or IgG+ B
cells in the patients before and after treatment with rituximab
may have provided some clues.6 Nevertheless, loss of T-cell tolerance can also occur in the peripheries, where the CD4+
CD25+ regulatory T cells (Treg) may play a critical role. These
Treg cells may mediate peripheral T-cell tolerance through
Foxp3, a transcription factor whose deficiency leads to proliferation of autoreactive T cells and subsequent development of
autoimmune diseases.25 Current studies have supported the
suggestion that multiple Treg cells that can interact with autoreactive T and B cells may play a role in pemphigus (Fig. 1).
From several clinical studies, it has been found that there
is also genetic susceptibility to PV, showing a significant correlation between several HLA class II alleles (such as
DRB1*04, DRB1*08 and DRB1*14) and the disease.23,26–28
Numerous polymorphisms of HLA-II have been identified
throughout several populations of PV patients, as this susceptibility may be variable by ethnicity.26,29–31 Following binding
to the self-antigen, Dsg3 is subsequently internalized and processed in the T cells before being presented together with the
related HLA molecules to activate B cells for secretion of
pathogenic autoantibodies. Very recently, in the humanized
HLA-DRB1*04:02-transgenic mouse model, the HLADRB1*04:02-restricted CD4+ T cells that are specifically reactive to human Dsg3 epitopes were shown to produce pathogenic pemphigus IgG antibodies.32
© 2015 Japanese Dermatological Association
Figure 1. Immune regulation of autoantibody production in
pemphigus. T cells including T-helper (Th)1, (Th)2, Th17 and follicular T helper cells (Tfh) are presented with desmogleins (Dsg),
the autoantigen in pemphigus. This process is restricted by
human leukocyte antigen (HLA) II, such as DRB1*04, DRB1*08
and DRB1*14. Upon activation, the T cells may subsequently
activate the B cells by direct interaction or secretion of cytokines. These cytokines are critical in regulation of B-cell function,
and can negatively (dotted arrow) or positively (solid arrow) regulate activation of autoreactive B cells. The B cells can further
mature and develop into plasma cells, which secrete high-affinity
autoantibodies against Dsg3. APC, antigen-presenting cells;
IFN, interferon; IL, interleukin; nTreg, natural regulatory T cells.
T-CELL REGULATION OF AUTOANTIBODY
PRODUCTION IN PEMPHIGUS
Regulation by Th1 and Th2 cells
Desmoglein autoantibody production and immune-mediated
regulation of T-cell subsets are two inseparable steps in pemphigus development. Th2 response has been implicated to play
a critical role in the pathogenesis of pemphigus, as evidenced
by the fact that in the patients, the levels of Th2 cytokines (IL-4
and IL-10) were significantly elevated and Th1 cytokines (IL-2
and IFN-c) were significantly decreased as compared with
healthy controls.33 Results from our studies showed that Th2
polarization is associated with the disease activation,34 in
which the ratio of Th1/Th2 was significantly lower in the active
stage as compared with the remission stage. By contrast,
D’Auria et al. reported that there was no significant elevation of
the Th2 cytokines in pemphigus patients compared with
healthy controls.35
Because B-cell activation and antibody production classically necessitate the involvement of CD4+ T cells, Dsg-reactive
T cells were capable of recognizing multiple epitopes of the
extracellular domain of Dsg and helping B cells to produce a
specific antibody. Previous studies have shown the presence
of Dsg3-reactive T cells in the peripheral blood of PV
patients.22,23,36 Dsg3-reactive T-cell clones were able to commit polyclonal na€ıve B cells to produce pathogenic anti-Dsg3
antibody and induce the PV phenotype.
The Th1 and Th2 cell recognition of Dsg3 peptides is
restricted by HLA-DRb1*0402 and/or HLA-DQb1*0503, and
13
M. Pan et al.
the proliferative response of the autoreactive Th cells can be
blocked by anti-DR and anti-DQ antibodies, respectively.23,27,28
A majority of studies confirmed the important role of Dsg-reactive Th2 cells in pathogenesis of pemphigus. Veldman et al.37
found that there is a predominance of autoreactive Dsg3-reactive Th1 cells in healthy individuals and Dsg3-reactive Th2 cells
in PV patients. However, Dsg3-reactive Th2 cells are detected
at similar frequencies in acute onset, chronic active and remittent PV, while the number of autoreactive Th1 cells exceeds
that of Th2 cells in chronic active PV. Furthermore, Rizzo C
et al. found this Dsg3-specific Th2 activity was significantly
correlated with the Dsg-3 antibody titer,38 which has previously
been shown to correlate with disease activity.39,40 This underscores the notion that B cells require activation by Dsg-specific
Th2 cells in order to trigger production of pemphigus autoantibodies, ultimately causing the disease (Fig. 1).
The Dsg-specific antibody subclasses are considered to be
more related to the disease activation. In both PV and PF,
patients with active disease demonstrate Dsg-reactive IgG4
and IgG1, while patients in remission and some healthy relatives of patients can demonstrate only anti-Dsg IgG1 subclass.41 We also observed that specific IgG1 was the major
component of the serum specific antibody in stable PV
patients, while IgG4 anti-Dsg were dominant in the active stage
and that there seemed to be a strong association between an
increase in IgG4 anti-Dsg levels and the severity of clinical
activity.34 This suggests that specific Dsg IgG4 is the major
pathogenic antibody subclass, playing a more important role in
causing acantholysis in pemphigus patients. However, there is
a dichotomy of Dsg-specific antibody subclass distribution in
pemphigus patients. David et al. showed that among pemphigus patients with active disease, IgG1 was found in all of them
and IgG4 in 85%, while IgG2 and IgG3 were found only
in 54% and 77% of the patients, respectively.42 For patients in
remission, the most common subclass was IgG4, found in
79% of patients. They concluded that IgG1 and IgG4 are predominant among patients with active lesions.
Because T-cell helper signals such as cytokines and co-stimulatory molecules are responsible for Ig isotype switching, such
as Th1 cytokines (such as IFN-c) able to induce the secretion of
IgG1 and IgG2, whereas the IgG4 and IgE subtypes are mediated by Th2 activation.43,44 The antibody profile of anti-Dsg antibodies of IgG1, IgG4 and IgE isotype is involved in the balance
of Th1/Th2 polarization. Also, Dsg-IgG4, the major pathogenic
subtypes of Dsg-specific IgG in PV, is thus assumed to be regulated by the Dsg-specific Th2 cells. In our previous study, we
showed that Dsg3 IgG1 production in mice (similar to human
IgG4) is Th2 cell dependent,34 consistent with our clinical findings about Th2-biased and IgG4-dominant anti-Dsg3 autoantibody response in an active group of PV patients. Our results
support the hypothesis that Dsg3-reactive Th2 cells are responsible for providing help for pathogenic IgG4 subclass production
in pemphigus patients. Given the association between specific
IgG4 in active disease and IgG1 in pemphigus remission, current evidence tends to imply that active disease showed a dominant Th2 response while patients in clinical remission displayed
Th1 dominance.
14
Regulation by Tfh cells
Follicular T helper cells (Tfh) are a recently discovered group of
CD4+ Th cells with intrinsic differences from previously characterized Th1, Th2 and Th17 cells. Localized in B-cell follicles of
secondary lymphoid tissues, Tfh cells are characterized by the
high-level expression of cell surface markers including CXCR5
controlling migration of Tfh cells towards the B-cell follicles,
programmed cell death-1 (PD-1), and inducible T-cell co-stimulator (ICOS), as well as transcription factor B-cell lymphoma 6
(BCL-6).45
The major function of Tfh cells appears to help B-cell activation and antibody production during humoral immune
responses. Tfh-cells dysregulation has been also associated
with the development of autoimmune diseases, such as the
systemic lupus erythematosus (SLE) model.46,47 The Tfh cells
express high levels of IL-21, which was demonstrated to be
important for the generation of Tfh cells and that responsiveness of Tfh cells to IL-21 drives the formation of the germinal
center reaction, and it clearly impacted on the amount of antibody production.48 Moreover, IL-21 contributes to development of a number of autoimmune diseases in the animal
models of SLE, experimental autoimmune encephalomyelitis
and rheumatoid arthritis.49,50
At least six different Tfh-cell subsets have been indentified,
according to their surface markers and/or anatomical locations.51 Significantly, Tfh memory cells reside in the lymph
nodes and regulate the memory B-cell response for long-term
immunity.52 In humans, most of the Tfh cells located in the light
zone of germinal center are CD4+CXCR5+CD57+CD69+ cells.
Recently, Simpson et al. reported a “circulating Tfh” subset with
characterized phenotype of CD4+CXCR5+ICOShigh PD-1high.53
The report showed that a population resembling Tfh cells can
€ gren’s syndrome patients and that
be detected in SLE and Sjo
the presence of this population correlated with the increased
diversity and titer of autoantibodies and severity of end-organ
involvement. However, the role of Tfh cells in the production of
anti-Dsg antibody in pemphigus has not been elucidated. In our
study, we found that the ratio of circulating Tfh was significantly
increased more in PV patients than that in healthy controls (data
not shown), but further studies are needed to determine whether
Tfh cells initiate autoantibody production in PV and to clarify the
mechanisms involved (Fig. 1).
Dysregulation of Th17/Treg balance
T-helper 17 cells are a recently discovered T-cell group with
intrinsic differences from previously characterized Th1 and Th2
cells. They are characterized by the production of cytokines
such as IL-17A (also called IL-17), IL-17F and IL-22 and require
a network of inflammatory cytokines for their own differentiation.54 In mice, TGF-b and IL-6 initiate the differentiation of
Th17 cells and activate the key transcription factor, retinoidrelated orphan receptor (ROR)ct. Then, they are amplified by
IL-21 and expanded and maintained by IL-23. In humans, the
differentiation is initiated by TGF-b and IL-21 that induce the
transcription factor RORc. IL-1b and IL-6 are important to
enhance the amplification of Th17 cells and IL-23 to maintain
the Th17 cell population.55 Th17 cells protect against fungal
© 2015 Japanese Dermatological Association
Mechanism of autoantibody production in pemphigus
and extracellular bacterial infections and participate in inflammatory reactions and autoimmunity. They can produce large
quantities of IL-17A, and most Th17-mediated effects are
attributed to this cytokine. In addition to IL-17A, Th17 cells coproduce IL-17F. They have similar effects on inducing the
production of pro-inflammatory cytokines, chemokines and
metalloproteinases from various tissues and cell types.56
Accumulating data suggest that Th17 cells play a significant
role in infectious diseases, autoimmune conditions, adoptive
immune response and mucosal immunity.57–59 Recent published
work suggests that Th17 cells may be involved in the pathogenesis of PV. In 2009, Arakawa et al. found that the skin lesions
from pemphigus patients expressed increased IL-17+ cells
(Th17) by immunohistochemical staining.60 However, they failed
to find a significant correlation between the Th17 cells and disease activity or anti-Dsg3 antibody titers, but proposed that perhaps increase of Th17 cells may not be a cause of the disease,
rather a protective feedback in an attempt at maintaining
homeostasis of the epithelial barrier. Cerrene et al. found that
the serum IL-17 level was also elevated in pemphigus patients.61
Some researchers also found that serum IL-17 and IL-6 levels
were elevated and serum IL-23 level was decreased.62 Recently,
our group found that the Th17 cell population in pemphigus
patients was significantly increased, when compared with the
healthy controls. We also found that the Th17 cell was significantly elevated in patients of acute stage, suggesting that Th17
cells and IL-17 cytokines may have a certain correlation with disease activity.63 Researchers from Mexico reported a case of PV
treated with ustekinumab (a human monoclonal antibody against
IL-23).64 After one month of treatment, good clinical improvement was observed and serum IL-12, IL-17, IFN-c and IL-6 levels showed significant decrease. Joshi et al. examined the
serum IL-17 and TGF-b levels of PV patients by enzyme-linked
immunoassay and found that all the PV patients had elevated
serum IL-17 levels, whereas it was not detectable in any healthy
controls.65 The mean serum TGF-b concentration was also lower
in patient sera as compared with healthy control, and the TGF-b/
IL-17 ratio was drastically reduced in patients.
It was also reported that IL-17 cytokine secreted from Th17
cells can stimulate B-cell survival and activation, further facilitating antibody secretion in SLE patients.66 Some other reports
showed that Th17 may need the help from B-cell activating
factor (BAFF) for their functions on B cells.55,67 However, the
correlation between the IL-17 population and anti-Dsg autoantibodies in pemphigus patients has not been found.63 The
exact function of IL-17-producing Th cells in pemphigus is still
unclear. However, it can be speculated that peripheral blood
Th17 cells can migrate to the lesional sites where large
amounts of inflammatory cytokines such as IL-17 and tumor
necrosis factor-a were secreted. It is not clear if Th17 cells are
the cause of the autoimmune mechanism or have a complimentary effect on pathogenic antibodies (Fig. 1). In addition,
the elevated Th17 cells in pemphigus may simply be a result of
disease activity, serving as a protective response to maintain
the homeostasis of the epithelial barrier. More studies are
needed to demonstrate the real importance of Th17/IL-17 as a
new therapeutic target in PV.
© 2015 Japanese Dermatological Association
Regulatory T cells, as we mentioned above, belong to a subclass of lymphocytes that mediate negative regulatory mechanisms. They play a key role in the maintenance of self-tolerance
and in the modulation of overall immune responses against infections and tumor cells. Treg secrete TGF-b and IL-10 and require
the specific cytokine TGF-b and the transcription factor Foxp3
for differentiation. The suppression function of Treg is mediated
by various mechanisms including cell–cell dependent; for example by expressing CTLA4 that is important to inhibit immune activation by competing for co-stimulatory ligands on T cells; and
cell–cell independent mechanisms, such as secretion of inhibitory cytokines, IL-10 or TGF-b.68 Treg can be divided into two
subgroups, natural Treg and inducible Treg according to the
sites of their maturation. CD4+CD25+Foxp3+ natural Treg suppress the proliferation of naive T cells and their differentiation to
effector T cells in vivo. They can also suppress effector activities
of differentiated CD4+ and CD8+ T cells and the function of natural killer cells, natural killer T cells, B cells, macrophages, osteoclasts and dendritic cells. In several autoimmune diseases with
aberrant antibody production, the function or number of Treg is
impaired. Depletion of Treg can lead to abnormal antibody production. Furthermore, administration of Treg into autoimmune
animals significantly reduced the autoantibody response.69
By far, a potential imbalance of CD4+CD25hi Treg has not
been extensively investigated in pemphigus. Sugiyama et al.
have revealed that the proportion of CD4+CD25hi Treg among
total CD4+ T cells in the blood of PV patients was markedly
reduced approximately 10-times less than that in the controls.70
They have also observed strikingly reduced gene expression as
well as protein expression of Foxp3 in the blood CD4+CD25hi Tcell population of PV patients, which contain more CD4+CD25hi
Treg, compared with that in normal controls. Severe reduction
of Treg may be involved in breaking self-tolerance against Dsg
in PV. However, Arakawa et al.60 found that in pemphigus
patient’s skin lesion and normal human skin there are no Treg.
Therefore, the authors speculated that reduction of Treg may
be due to accumulation in skin or in lymph node drainage. Our
group found that almost all the pemphigus patients have a
lower number of Treg.63 Interestingly we found in the acute
stage that the number of Treg was even lower. Therefore, we
concluded that the Treg population was defective in pemphigus
patients and thus cannot effectively inhibit function of effector T
cells, resulting in a large amount of effector T-cell activation
and proliferation, which may further activate B cells to produce
autoantibodies in pemphigus (Fig. 1).
The developmental pathways of Th17 and Treg are closely
related. They may differentiate from the same precursor T cells,
depending on the relative amount of cytokines present in their
environment. TGF-b is a critical differentiation factor for the
generation of Treg. Using mice with a reporter introduced into
the endogenous Foxp3 locus, Bettelli et al. found that IL-6
completely inhibits the generation of Foxp3+ Treg induced by
TGF-b.55 Also, combination of IL-6 and TGF-b promotes the
generation of Th17 cells. The data suggest that there is not only
a functional antagonism between Th17 and Treg but that there
is a dichotomy in their generation as well. Therefore, Treg and
Th17 effectors arise in a mutually exclusive fashion, depending
15
M. Pan et al.
on whether they are activated in the presence of TGF-b or TGFb plus IL-6. At the steady-state level or in the absence of any
inflammatory insult, TGF-b produced in the immune system will
suppress the generation of effective T cells and induce Foxp3+
Treg, and thereby maintain self-tolerance. However, with infection or inflammation, IL-6 produced by the activated innate
immune system will suppress the generation of TGF-b-induced
Treg and induce a pro-inflammatory T-cell response predominated by Th17 cells.69 Therefore, the regulation of the Th17/
Treg balance appears critical in the development of autoimmune and inflammatory diseases. In pemphigus patients, the
imbalanced expression of Th17 and Treg was observed and
there was a significantly negative correlation between them.63
SUMMARY
Pemphigus is a disease model of autoimmunity with identified
autoantigen and autoantibodies. B cells break the self-tolerance
to develop the pathogenic antibodies. During this utmost
immune process, autoreactive T cells provide the activation signal for B-cell proliferation and maturation. Th2-biased cellular
response was dominant in the disease development, especially
in the acute stage, correlated with the antibody production and
disease activity. Tfh cells could directly help B-cell activation,
showing elevated ratio in the peripheral blood from the patients.
However, the functional difference between circulating Tfh and
marginal zone localized Tfh still needs to be further addressed.
The balance of Th17/Treg was disrupted in pemphigus, sharing
a significantly negative correlation. Besides these immunoreactive and regulatory cells, the cytokines, such as IL-10, IL-21, IL17 and TGF-b, comprise the microenvironmental regulatory network in the disease and are variable during the different disease
stages. The combination of treatment by modulating the
homeostasis of the key cytokines or active immune cells may
lead to novel and effective immunotherapies for pemphigus and
other autoimmune and inflammatory diseases.
ACKNOWLEDGMENTS: We thank the National Natural Science Foundation of China for grant support (no. 81171499, 81472875)
and the Milstein Medical Asian American Partnership Foundation for the
Fellowship Award in Skin Disease. We thank Dr Xuming Mao (University
of Pennsylvania) for critical reading and valuable suggestions during our
manuscript preparation.
CONFLICT OF INTEREST:
None.
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