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The role of CD8+ T-cells in the
pathogenesis of Atopic Dermatitis
Masterthesis
Maarten Hillen
December 2009
Supervision
Edward Knol, PhD.
Department of Dermatology/Allergology, UMC Utrecht
[email protected]
Personal information
Maarten Hillen, BSc.
Master student Infection & Immunity, Utrecht University
[email protected]
2
Index
Summary
page 3
Introduction
page 4
Pro-inflammatory role of CD8+ T-cells in AD
page 6
Anti-inflammatory role of CD8+ T-cells in atopic diseases
page 8
Conclusions and discussion
page 10
References
page 13
Summary
Atopic Dermatitis (AD) is a chronic T-cell mediated inflammatory disease. Unlike most allergic
diseases, Th2 polarization is only associated to acute AD lesions, while a Th1 environment is present
in chronic lesions, including very high IFN-γ levels. CD4+ T-cells have always been regarded pivotal
in the pathogenesis of AD, but interest in the role of CD8 + T-cells has sparked as several papers have
found evidence for their involvement in the disease. Expression of CD30 and CD69 on CD8+ T-cells in
AD lesions has been stated to correlate with disease severity. Furthermore, correlations between
disease severity and the amount of IL-22 producing CD8+ T-cells in AD lesions and TREC levels in
CD8+ T-cells from the peripheral blood of AD patients have been described. Additionally, CD8 + Tcells were shown to be essential for AD lesions formation in a mouse model of AD. CD8+ T-cells
could also have an anti-inflammatory role in AD pathogenesis, as they can kill invading pathogens,
preventing further inflammation and tissue damage. They are also able to restore the barrier function
of the skin by inducing upregulation of components of the stratum corneum. The role of CD8 + T-cells
in AD pathogenesis has been underestimated for many years and increasing amounts of evidence for
their role will be published. The net effect that CD8+ T-cells have on AD pathogenesis remains
difficult to assess, as there are so many different processes they can influence. But, judging from the
evidence that is available at this time, they appear to attribute to the disease rather than decrease it.
3
Introduction
The human immune system is the product of aeons of evolution, which have brought forth a system
that is both delicate in its regulatory mechanisms and powerful in its actions. In order to remain
functional and effective, the immune system must be kept in balance. An immune response that is too
harsh can do more damage than the infection against which it is mounted would. Immune responses
against auto-antigens or harmless allergens can be detrimental if not kept in check. Allergic responses
are caused by activation of the immune system upon exposure to external substances that should
normally be regarded as harmless and invoke tolerance. Allergy is defined as a hypersensitivity
reaction initiated by specific immunologic mechanisms [1]. The atopic diseases that arise in patients
with allergy have a prevalence of up to 30% in developed countries and a high overall morbidity [2,3].
A large part of these diseases are attributed to atopic dermatitis (AD) and asthma.
The classic allergy model involves mast-cells with IgE class antibodies against the allergen on their
surface. Upon allergen contact, IgE antibodies are cross-linked which causes the mast-cells to burst
and release histamine into the surrounding tissue, with the allergic symptoms such as red eyes and
wheezing as a result [1,4]. An important factor is the Th1/Th2 balance, largely determining the amount
of IgE produced in a patient. A delicate balance between Th1 and Th2 cells exists in the body. Th1
cells primarily secrete IFN-γ, a very potent pro-inflammatory cytokine that can stimulate an array of
cells including neutrophils and effector CD8+ T-cells. In addition, it causes a positive feedback-loop
via upregulation of IFN- γ production in macrophages which in turn creates an environment in which
more CD4+ T-cells maturate into Th1 cells. Th2 cells secrete cytokines such as IL-4 and IL-5 that
stimulate B-cells to produce certain antibody types, including IgE, induces eosinophil maturation and
the maturation of CD4+ T-cells into Th2 cells. Additionally, Th1 and Th2 cytokines trigger a negative
feedback-loop in cells of the opposite phenotype, preventing them from secreting their cytokines.
Thus, the environment in which a CD4+ T-cell maturates determines the phenotype it will develop.
Furthermore, if the Th1/Th2 balance shifts towards either of the phenotypes, this will not naturally
revert to an even distribution due to the positive and negative feedback-loops present. The balance is
influenced by external factors such as infections. Upon infection with viruses or intra-cellular bacteria,
the balance can tilt towards a Th1 response due to additional secretion of IFN- γ by APCs and T-cells.
Infection with parasites can cause a shift towards a Th2 environment [5].
As allergy has been associated with high IgE levels in the blood, which is Th2 dependent, it has been
considered a condition that arises when the Th1/Th2 balance shifts too much towards Th2. As the
percentage of allergic children has been rising for the last decades, while hygiene has been improving
in conjunction, it was proposed that these two processes were causally correlated; the Hygiene
Hypothesis. The rise in hygiene level supposedly caused children to encounter fewer infections during
early childhood, resulting in a shift towards a Th2 environment and a subsequent predisposition to
allergy. Many epidemiologic studies have been conducted to investigate the effects of hygiene and
infections during childhood on development of allergy, but a clear-cut conclusion has yet to be formed
[6]. Interesting details include indications that BCG vaccination is associated with prevalence of atopic
diseases [7].
CD8+ T-cells have long been regarded as predominant IFN-g producers that did not produce Th2
cytokines. However, when cultured in the presence of IL-2 and IL-4, CD8+ T-cells can produce
significant amounts of IL-4 [8]. In fact, CD8+ T-cells can form subsets similar to Th1 and Th2 cells
under certain conditions, called Tc1 and Tc2 cells. The subtype differentiation is less drastic compared
to Th1 and Th2 cells, as unlike Th2 cells Tc2 cells can still produce some IFN-γ and also produce
lower amounts of IL-4 [9].
The human immune system has several built-in systems to prevent over-activations of the immune
system like in the case of allergy. Immune cells have to receive several stimuli from different cells
before they are activated, and even then a single activated cell will not be able to mount an effective
response against a target without the help of other activated cells. For instance, T-cells require signals
from both the T-cell receptor and co-stimulation via CD28 and DCs require stimulation via TLRs and
CD40 to achieve optimal licensing [10]. In addition, there are cells that suppress activation of immune
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cells, either by secreting suppressive cytokines or suppressing activity using cell-cell contact. The
most well-known suppressive cells are the Natural Regulatory T-cells or Natural Tregs. These
CD4+FoxP3+ T-cells can suppress immune responses by secreting IL-10 and TGF-β and can also bind
cells with the CTLA-4 on their membrane to suppress activity. Several other CD4+ regulatory cells are
known, their activity is comparable to that of Natural Tregs but can differ in cytokines secreted and
surface-marker expression [11]. In addition to CD4+ T-regs, CD8+FoxP3+ Tregs have also been
identified in recent years. They are able to suppress immune responses via cellular and humoral
pathways comparable to those influenced by CD4+ Tregs [12]. However, only CD4+ Tregs have been
associated with suppression of allergic responses, including supposed correlations between low Treg
levels and predisposition for certain atopic diseases [13,14,15].
Asthma
As the lungs exist of only a small layer of mucosal tissue, they are vulnerable to infection and damage.
The immune system in the lungs is specialized in maintaining gaseous exchange and as such,
preventing aberrant inflammation is essential. Alveolar macrophages and a large amount of Tregs
make sure that immune responses are only mounted upon encountering pathogens. However, in
individuals with genetic predisposition, allergen sensitization can still trigger an immune response,
giving rise to asthma [13]. Asthma is an allergic disease mediated by local T-cells in a Th2 cytokine
milieu. CD4+ Th2 cells in the lungs of asthma patients produce IL-5, which attracts eosinophils that
cause bronchoconstriction by leukotriene production and tissue damage through release of toxic
granule proteins [16,17]. The number of asthma patients is still increasing world-wide and prevalence
can be as high as 34.8% in Costa-Rica, followed by Austria with 23.6% [17].
As most allergic diseases, asthma has long been regarded mainly CD4+ T-cell mediated. However,
recent publications have implicated that also CD8+ T-cells play a role in asthma pathogenesis. CD8+ Tcells can indirectly affect the Th1/Th2 balance and IgE production. IFN-γ from CD8+ T-cells induces
production of IL-12 and IL-18 by Dendritic Cells (DCs), triggering IFN- γ production by CD4+ Tcells, which results in moderation of the Th2 polarisation. As such, CD8+ T-cell activation can reduce
tissue damage in the lungs of asthma patients [18]. This process will be further clarified later on.
Atopic Dermatitis
AD is a T-cell mediated chronic inflammatory disease caused by hyper-reactivity of the skin to
exogenous antigens, primarily from dust mites, often giving rise to eczema. The subsequent loss of
skin barrier function due to damage to the stratum corneum can allow normally inhaled allergens and
pathogens to penetrate the skin, further aggravating the disease [19]. CD4+ T-cells have always been
regarded essential in the development of atopic eczema by producing Th2 cytokines that induce classswitching to IgE antibodies and eosinophil infiltration [20]. AD is different from most other atopic
diseases like asthma, where the CD4+ T-cells mostly retain a Th2-cytokine profile, Th1 cells undergo
preferential apoptosis and IFN-γ levels are low [21]. Acute AD lesions largely follow this model of
Th2 polarisation with high IgE levels. In chronic AD lesions however, the CD4+ T-cells appear to
switch to a Th1 cytokine pattern including high IFN-γ secretion levels [22].
The supposed pivotal role of CD4+ T-cells in AD pathology is mostly based on indirect evidence from
AD skin histology and data on PBMCs, instead of looking at the cells in the AD lesions themselves
and their function [21,23,24]. In addition, several papers have been published over the years that do
describe the presence of CD8+ T-cells in AD lesions and also their ability to induce IgE production and
stimulate eosinophil survival [25,26,27]. Though the described percentages of CD8+ T-cells present in
lesions are highly variable, it is debatable whether the lack of research into their possible function in
AD is justified. Perhaps CD8+ T-cells do indeed play a role in the pathogenesis of AD, as has been
shown for asthma, and have these cells been underappreciated for many years. In this thesis, the
possibility of an essential function of CD8+ T-cells in AD pathogenesis is discussed. Indications for the
existing of pro- and anti-inflammatory roles of CD8+ T-cells in AD pathogenesis will be reviewed,
followed by a discussion on implications this has on our view of the disease and therapeutic strategies.
5
Pro-inflammatory role of CD8+ T-cells in AD
Several published papers link a high CD8+ T-cell activity in peripheral blood or AD lesions to
increased AD disease severity. Seneviratne et al. describe a correlation between the amount of dustmite allergen specific CD8+ T-cells present in the peripheral blood of AD patients and disease severity
[28]. In addition, it was shown that IFN-γ production by peripheral blood CD8+ T-cells of AD patients
is decreased after successful immunotherapy [29]. These data indicate that CD8+ T-cells are not only
involved but are actually very important in AD pathogenesis. However, as these data are based on
peripheral blood samples, this evidence is only circumstantial.
By depleting CD8+ T-cells in a mouse model for AD, it was shown that CD8+ T-cells are required for
AD lesion formation in mice. Mice that were depleted of CD4+ T-cells using the same method showed
severe eczema formation upon allergenic sensitization. In addition, adoptive transfer of CD8+ T-cells
from sensitized mice into naïve mice lead to rapid lesion formation, indicating that not the CD4+ Tcells but actually the CD8+ T-cells are pivotal in the process of AD lesion formation. It was proposed
that the CD8+ T-cells are recruited into the skin the first 24 hours after allergen exposure, causing
upregulation of Fas on keratinocytes via IFN-γ and subsequent apoptosis of keratinocytes and
infiltration of eosinophils and CD4+ T-cells upon secretion of chemotactic factors by the skin cells,
after which the CD8+ T-cells largely disappear [30]. This model could explain the problems
encountered with measuring CD8+ T-cells in AD lesions; perhaps the amount of CD8+ T-cells in the
lesion peaks early after allergen exposure and decreases shortly thereafter, resulting in varying CD8 +
T-cell numbers measured at different time points.
Though data from mouse models and peripheral blood of AD patients can give indications on the
function of CD8+ T-cells in the disease, information on the cells from the lesions themselves is less
indirect and more informative. In skin biopsies from human patients with chronic AD, a high
percentage of IL-22 producing CD4+ and CD8+ T-cells are present. IL-22 mRNA levels were increased
20-fold compared to healthy controls and the number of CD8+IL-22+ cells present correlated with
disease severity. In addition, lowered percentages of IL-17 producing CD4+ T-cells, called Th-17 cells,
were found, while the number of IL-17 producing CD8+ T-cells, or Tc-17 cells, in the skin of AD
patients did not differ. Material from patients with psoriasis was used as control, as psoriasis skin
shows large histological resemblance to AD skin and involves a lot of the same pathways and
cytokines [31]. In psoriasis, elevated levels of IL-17 are present, as well as higher levels of IL-23.
These two cytokines are thought to contribute to psoriasis pathogenesis by attracting neutrophils to the
site of inflammation and inducing expression of anti-microbial peptides [32,33]. IL-22 is also thought
to play a role in psoriasis, as it mediates several processes involved in its pathogenesis. Th-17 and Tc17 cells produce both IL-17 and IL-22, but there are also T-cells that produce only IL-22, know as Th22 and Tc-22 cells [34]. (Figure 1). The fact that in the chronic AD lesions examined, IL-22 producing
T-cells were very frequent, while the IL-17 producing cells were more scarce indicates that perhaps
Th-22 and Tc-22 cells are involved in the psoriasis-resembling histological features seen in AD, while
the differences between the diseases are due to missing signals from IL-17 cells, probably because
they are suppressed by the Th2 cells which are far more frequent in AD when compared to psoriasis.
Regardless of the specific mechanisms that are involved, only the CD8+IL-22+ T-cell numbers
correlated with disease severity [31], which is a very strong indication for an important role of CD8+
T-cells in AD pathogenesis.
In addition to data on IL-17 and IL-22 producing cells, experiments have been conducted on the
phenotype of CD8+ T-cells present in AD lesions and possible correlations to disease severity. In
peripheral blood from children with AD, high expression of the activation marker CD69 on
CD3+CD8+ T-cells has been found to correlate with disease severity measured by the “Scoring Atopic
Dermatitis” (SCORAD) index [35]. Though this does not necessarily mean that activated CD8+ T-cells
induce more severe AD symptoms, it does indicate that CD8+ T-cells are somehow involved in the
process, in either a causal relation or an indirect one. In a different paper, a correlation between AD
disease severity and the number of CD30+CD8+ T-cells in both peripheral blood and skin biopsies
from AD patients was described [36]. CD30 is a TNFR-family member and its ligand CD30L is
expressed on mast cells and T-cells. Upon ligation of CD30, mast cells carrying CD30L are stimulated
6
via reverse-signalling, a common feature in TNFR signalling, in an antigen-independent manner to
become activated, which adds to the allergic response. CD30 is also expressed on CD1α + Langerhans
cells in the skin [37]. It appears that CD8+ T-cells are activated by Langerhans cells upon allergenic
sensitization, which triggers a cascade of CD8+ T-cell and mast-cell activation, partially via CD30, that
does not require the presence of additional allergen. B-cells are also indirectly stimulated by cytokines
produced by the Langerhans cells in addition to CD40/CD40L stimulation, which triggers IL-4
production by Th2 cells, inducing class-switching to IgE. The activated mast cells with IgE against the
allergen release histamine, IL-5 and other inflammatory molecules, which attract eosinophils and give
rise to AD lesions [38]. CD8+ T-cell appear to play an important role in this process.
Figure 1. Production of IL-17 and IL-22 in Psoriasis (left) and Atopic Dermatitis (right). In the Th1
environment present in psoriasis, Th17 cells produce both IL-17 and IL-22 and IL-22 is also produced by
Th22 cells. IL-17 and IL-22 combined give rise to Psoriasis symptoms. In AD, the Th2 environment blocks
Th17 function. Thus, only IL-22 is produced by both Th22 and Tc22 cells and the lack of IL-17 gives rise to
symptoms specific for AD [31].
Though no experiments with T-cells from AD lesions were conducted, only peripheral blood T-cells
were used, a study on T-cell receptor excision circles (TRECs) in T-cells from AD and psoriasis
patients describes a very novel way of looking at T-cell role in AD. Measurements of TRECs in
peripheral blood cells of Psoriasis and AD patients showed that men with AD have less TREC content
7
in their CD8+ T-cells compared to healthy controls and that the TREC content correlated with disease
severity and IgE levels [39]. TRECS are small circles of DNA that are excised out of the T-cell
genome during TCR generation in the thymus. Every cell that exits the thymus contains a TREC.
When a cell with a TREC divides, the TREC does not multiply and is transferred to only one of the
daughter cells. Every multiplication further dilutes the TREC content of the daughter cells. Thus, the
TREC content of a sample, usually represented in TREC/mL blood or TREC/amount of cells, can be
seen as a measure for the combination of thymic output and peripheral proliferation. As it is
impossible to differentiate between these two parameters with only data on TREC content, Ki67
measurements are usually conducted in parallel to assess proliferation of the cells. Thymic function
can be calculated by correcting the amount of TREC positive cells for the amount of divided Ki67+
cells [40,41]. The fact that TREC content in CD8+ T-cells of male AD patients is supposedly lowered
indicates that either thymic output is thwarted or there is increased peripheral proliferation in these
patients. No additional measurements were done by the authors except for telomere length assessment,
which only provides indirect information in T-cells [42], so a clear conclusion can not be drawn. The
authors propose that thymic function in men with AD is thwarted because they have measured highly
variable TREC contents in these patients. They conclude that the thymus “fires” bursts of naïve T-cells
after disease flares instead of a constant flow of new cells. They even hypothesize that thymic function
plays an important role in AD pathogenesis [39]. However, it is equally possible that disease flares
trigger additional proliferation of the CD8+ T-cells and thus a lower TREC content is measured, which
in time is compensated by the constant flow of new naïve T-cells from the thymus, resulting in a
meandering pattern in TREC content measurements. A model in which disease flares trigger additional
proliferation of T-cells rather than thwarted thymic function would also make AD more comparable to
other chronic inflammatory diseases like psoriasis, rheumatoid arthritis and systemic lupus
erythematosus, in which very low TREC levels in both CD4+ and CD8+ T-cells are present but no
effect on thymic function is known [39,41,43]. The fact that the correlation between TREC content in
CD8+ T-cells and disease severity could only be found in men is not particularly peculiar, as men are
known to have lower TREC levels in general and usually develop AD earlier in their life, their CD8 +
T-cell populations could be worn down more compared to females [44]. Though the research on
TREC contents in AD patients in this paper is not particularly elegant, the chosen approach is rather
unique for this field. The apparent correlation between the number of CD8+ T-cells that proliferate or
are newly produced and disease severity is another indication that these cells are indeed an important
factor in AD pathogenesis, though the fact that the TRECs are measured in peripheral blood makes
this evidence relatively circumstantial.
Anti-inflammatory role of CD8+ T-cells in atopic diseases
CD4+ T-cells are generally regarded as the most important suppressive cells, but CD8+ T-cells are also
able to suppress immune responses. Here, a short overview of literature on suppressive function of
CD8+ T-cells in general is given, followed by data on CD8+ T-cell suppression in asthma and
mechanisms possibly involved in AD.
Immune suppression by CD8+ T-cells
Suppressive CD8+ T-cells were first recognized in 1970 and were called suppressor T-cells at the time
[45]. They are currently called CD8+ Tregs. A lack of identified specific surface markers has made
research into suppressor cells in general very difficult, causing an inability to determine detailed
mechanisms of suppression [46]. Natural CD4+ Tregs express CD25 on their surface and have
intracellular FoxP3 expression [11]. However, no definitive surface expression pattern for CD8+ Tcells has been established yet. As in CD4+ Tregs, CD8+ Tregs often express FoxP3 and activation or
differentiation markers such as CD25, IL-122 and CD45RC [12,47].
CD8+ Tregs deploy three methods of suppression. First, they can induce suppression by cytokine
secretion and by cell-cell contact like their CD4+ counterparts do [12]. In addition, CD8+ Tregs are able
to reduce inflammation levels by killing target cells; in auto-immune disease settings, they
preferentially kill Th1 cells, which can result in a switch to a less pathogenic Th2 response to autoantigens [48]. The exact mechanism by which CD8+ Tregs kill their targets is not yet clear, though it is
8
possible that they use the same mechanism for this as effector CD8+ T-cells. FasL does not appear to
be involved, but perforine could possibly play a role [49]. Papers published on regulatory CD8+ T-cells
use auto-immune settings as background for their research [48,50], while data on a possible role in
allergy is not yet available. In addition to the function of CD8+ Tregs, effector CD8+ T-cells can
regulate immune responses in various other ways. They can produce and secrete various cytokines that
can modulate immune responses. IFN-γ is the most apparent cytokine secreted by CD8+ T-cells and
has various effects on the immune system, including modulation of APC cytokine secretion, activation
of T- B- and NK-cells and some anti-viral effects [51]. Though it is a very potent pro-inflammatory
cytokine, it can have indirect anti-inflammatory effects [52].
Suppression of the immune response in asthma by CD8+ T-cells
An example of suppressive CD8+ T-cell activity not related to Treg-like mechanics is found in asthma.
It was described that CD8+ Tc1-cells can indirectly decrease inflammation by modulating the highly
polarised Th2 environment, decreasing IgE production of B-cells. The interesting fact in this system is
that IFN-γ from the Tc1-cells is not directly involved. Tc2 cells that produce much lower amounts of
IFN-γ are still able to suppress IgE production in a mouse model with OVA-induced allergy.
Furthermore, adoptive transfer of CD8+ T-cells from IFN-γ -/- mice into Wt mice inhibited IgE
production in the acceptor mice just as effective as cells transferred from Wt mice. However, when
cells were transferred from Wt mice into sensitized IFN-γ -/- mice, IgE production was not affected.
Additional transfer of CD4+ T-cells from naïve Wt mice restored suppression of IgE production. Thus,
it is the IFN-γ from CD4+ T-cells suppressing IgE production in asthma, the CD8+ T-cells merely
stimulate DCs to induce CD4+ T-cell differentiation into Th1 cells (Figure 2) [18,52]. However, it
should be noted that Tc2 cells exacerbate asthma in a way similar to Th2 cells. As most of the CD8+ Tcells found in the lungs of asthma patients are of a Tc2 phenotype, the net contribution of CD8+ T-cells
to the disease is difficult to assess [53,54].
Figure 2. Indirect effect of CD8+ T-cells on IgE production of B-cells in asthma. The CD8+ T-cells trigger DCs
to produce IL-12 and IL-18, which induces naïve CD4+ T-cells to maturate into Th1 cells. IFN-γ from these
Th1Suppression
cells attenuates
IgE
production
by B-cells
of the
immune
response
in AD [18].
by CD8+ T-cells
9
It is unlikely that the process of immune suppression by CD8+ Tc1-cells as seen in asthma is identical
in AD, as asthma differs from AD in various crucial ways. Asthma is very dependent on a Th2
environment and IgE production. Strong IgE sensitization is a very important risk factor for the
development of childhood and life-long asthma [55] and in patients with established asthma, serum
IgE levels are correlated with disease severity [56]. In AD, especially in chronic lesions, a Th1
environment with high IFN-γ levels secreted by both CD4+ and CD8+ T-cells is common and as such a
process in which CD8+ T-cells indirectly modulate the Th1/Th2 balance polarization towards Th1
would probably not decrease at least chronic AD symptoms efficiently. The Th2 environment and high
IgE levels are still important in the onset and early stages of AD lesions [21], but as it was shown that
CD4+ and CD8+ T-cells in the peripheral blood of AD patients are equipotent in inducing IgE
production by B-cells and eosinophil survival, it is questionable whether CD8+ T-cells are able to
suppress even the early AD stages [57]. However, it is possible that additional IFN-γ decreases AD
symptoms in a different way. It was shown that Th2 cytokines IL-4 and IL-13 induce skin barrier
destruction by downregulating components of the stratum corneum. In contrast, IFN-γ induces
upregulation of these components and thus additional IFN-γ levels could partly restore skin barrier
function, decreasing the amount of allergens and pathogens that can invade the skin [19].
Perhaps CD8+ Tregs are able to suppress inflammation in AD lesions. FoxP3+CD8+ T-cells have been
shown to be able to suppress immune responses via humoral and cellular pathways and could be
involved in suppression of allergic inflammation [12]. No relevant data on the role of CD8+ Tregs in
AD is available yet, but data on CD4+ Treg activity in AD is abundant. Though CD8+ Tregs differ from
CD4+ Tregs in mechanisms of suppression and expression of several proteins [47,48], they have
enough similarities to justify a comparison. Surprisingly, the amount of CD4+ Tregs in the peripheral
blood of AD patients is increased when compared to healthy controls [58]. In AD lesions, CD4+ Tregs
are able to proliferate efficiently and CD4+ Tregs isolated from AD lesions are able to suppress
proliferation of target cells ex vivo [59]. Thus, it seems that even though there are large amount of
functionally potent Tregs present in AD lesions, they are still unable to efficiently regulate the
vigorous allergic response. It can be proposed that the amount of immune activation present in AD
lesions is so large that the Tregs are unable to suppress it, even though their numbers and potency are
large. There is no reason to assume that CD8+ T-cells are able to regulate the response in AD lesions
when their CD4+ counterparts are not capable of this, unless they use a different yet unknown strategy
for this.
It is possible that CD8+ T-cells are able to decrease inflammation in AD lesions by preventing
infection with pathogens. AD lesions have frequent interaction with pathogens which can increase the
inflammation and possibly decrease the effectiveness of any suppressive signals. AD patients are
known to be highly susceptible to certain bacterial, viral and fungal infections of the skin, due to both
a disrupted barrier function of the skin and an environment in which pathogens can thrive [60]. In
early AD lesions, the Th2 environment causes a decrease in secretion of anti-microbial peptides by
epithelial cells [61]. In addition, less plasmacytoid DCs are present in AD patients, compared to both
healthy controls and patients that suffer from different skin diseases such as psoriasis. These cells
carry many pattern recognition receptors such as Toll-like receptors (TLRs) on their membrane and
play a pivotal role in mounting an efficient response against many pathogens [62]. The infections
exacerbate the allergic response and result in more severe disease. Infection with S. Aureus is common
in AD patients and the toxins secreted by the bacterium trigger production of Toxin-specific IgE and
activation of eosinophils and Th2 cells, which provides a bacterial survival advantage and causes
disease flares in conjunction [19,63]. If CD8+ T-cells are able to kill pathogens infecting the lesions,
they could prevent the added Th2 polarisation and disease flares that the infection causes, thus
suppressing the disease severity.
Conclusions and discussion
Even though CD4+ T-cells have been regarded essential in the development and further pathogenesis
of AD, an increasing amount of evidence shows that CD8+ T-cells are also very important. Data from
the peripheral blood of AD patients indicates that there is a correlation between the amount of
10
allergen-specific CD8+ T-cells and disease severity, as well as a decrease in peripheral blood CD8 + Tcell IFN-γ production upon immunotherapy [28,29]. In a mouse model, CD8+ T-cells are essential for
AD lesion formation [30]. Furthermore, the phenotype and TREC levels of CD8+ T-cells present in
lesions from AD patients correlate with disease severity [31,35,36]. Several papers show evidence for
a pro-inflammatory role of CD8+ T-cells in AD pathogenesis. It is a feasible hypothesis, as CD8+ Tcells are known to be able to induce skin tissue damage, indirectly by activating mast cells [36] and
preventing differentiation of skin cells [15], directly by causing upregulation of Fas on skin cells and
apoptosis of skin cells via proteases, FasL, Perforin and Granzyme B [21,64].
However, some evidence for an anti-inflammatory role of CD8+ T-cells in AD exists as well. Data
from asthma studies shows an anti-inflammatory role of CD8+ Tc1-cells at the site of infection [18]. It
is unlikely that a similar process influences AD pathogenesis, as AD is far less Th2 dependent than
asthma [55,56]. It is also improbable that CD8+ Tregs can efficiently suppress AD disease severity, as
data on CD4+ Tregs present in AD lesions shows that these cells are unable to decrease inflammation,
regardless of their high numbers and potency [59]. It can be argued that CD8+ Tregs could be able to
suppress the inflammation in AD lesions using their potential to kill target cells, which CD4+ Tregs do
not possess. For instance, killing eosinophils could prevent large scale tissue damage. No evidence for
such a mechanism exists however. Furthermore, it is possible that CD8+ T-cells are able to partly
restore skin barrier function, as IFN-γ induces upregulation of components of the stratum corneum
[19]. Restoration of the barrier function of the skin prevents aeroallergens and pathogens from
penetrating the skin and causing additional inflammation. Additionally, CD8+ T-cells can prevent
infection of the lesions with pathogens by killing pathogens and infected cells. S. Aureus is well
known to cause superinfection in AD patients, which causes enhanced inflammation and disease. In
conjunction, S. Aureus toxins cause production of IgE, extra maturation of Th2 cells and also
activation of eosinophils [63].
Figure 3. Schematic overview of the processes CD8+ T-cells could influence that affect tissue
damage in AD lesions. They prevent tissue damage by killing pathogens and promoting tissue repair
via IFN-γ. CD8+ Tregs might kill eosinophils, indirectly preventing tissue damage. In conjunction,
CD8+ T-cells promote tissue damage directly by inducing apoptosis of skin cells and indirectly by
11
activating mast-cells and blocking skin cell differentiation.
CD8+ T-cells appear to be able to influence AD pathogenesis on many levels (See Figure 3) and with
both pro-inflammatory and anti-inflammatory activity making their net role in the disease difficult to
assess. In addition, the processes which they affect are influenced by many different factors such as
infections of the lesions and stage of the disease. But, when taking all reviewed evidence into account,
one is prone to conclude that CD8+ T-cells attribute to AD pathogenesis rather than decrease it.
Especially the evidence from mouse models and on the phenotype of CD8+ T-cells in lesions of AD
patients is very convincing, while evidence for an anti-inflammatory role can only truly convince in
the presence of a superinfection. It seems unlikely that the same cells that are essential for the
formation of lesions in mice have a direct suppressive function in human AD patients, unless multiple
distinct phenotypes exist with separate effects on AD pathogenesis. Additional research is definitely
required in order to fully understand the role of CD8+ T-cells in AD pathogenesis. Data from AD
patients will be important but not sufficient to answer questions that have arisen, as the CD8+ T-cells
are in the centre of an intricate web of interactions with different cells and processes, making it
difficult to single out their effect on specific processes without interference of their other functions in
humans. This makes transgenic animal models a very important tool for the future. Especially data
from AD mouse models on the effects of different subtypes of CD8+ T-cells on AD pathogenesis could
prove invaluable to differentiate between pro- and anti-inflammatory roles of CD8+ T-cells in the
disease. For instance, adoptive transfer of CD8+ Tregs into sensitized AD mice could show whether
these cells are able to suppress the inflammation, in contrast to their CD4+ counterparts.
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