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Rachel Wilson.
Is barrier dysfunction the be-all and end-all of atopic eczema?
The functional epidermal barrier limits passive water loss from the body, reduces the
absorption of external toxins, defends against microbial infection and protects deeper layers
of differentiating epidermal cells from trauma [1]. The recognition of barrier dysfunction as
the ‘be-all’ and ‘end-all’ of atopic eczema would drive progress in diagnosis and treatment,
and compel researchers to direct their investigations towards obtaining a better understanding
of the epidermal barrier and away from other factors thought to contribute to this condition.
In order to determine whether barrier dysfunction has superior significance in the
pathogenesis of atopic eczema, evidence in favour of this must be weighed against evidence
for the role of these other factors; the local and systemic immune responses, the environment
and microbiome, and the genotype and heritability patterns of this skin disease. In advance of
this discussion, I will first outline the structure of the epidermal barrier, and mechanisms for
its dysfunction.
The principal component of the epidermal barrier is the stratum corneum, which Elias likened
structurally to a brick wall. In this model corneocytes represent bricks embedded in a mortar
of intercellular lipid lamellae, which contains ceramides, cholesterols, and fatty acids (Fig.1)
[2] [3]. Corneocytes are stabilised by linking corneodesmosomes which lend tensile strength
to the barrier. Beneath this, the stratum granulosum reinforces the barrier with tight junctions
between keratinocytes. The process of epidermal desquamation is controlled by serine
proteases, predominantly of the Kallikrein (KLK) subfamily, and protease inhibitors [4].
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Fig 1. Brick wall structure of epidermal barrier. [5]
There are many mechanisms by which barrier dysfunction can be said to occur, including
cutaneous immune dysregulation, microbial barrier dysfunction, and structural abnormalities
in the epidermis. In this essay I will accept the premise that barrier dysfunction refers to
structural abnormalities in the epidermis, which may be inherited or acquired. Defects in the
skin barrier are thought to permit epicutaneous sensitization through trans-epidermal allergen
transfer. This describes the ‘Outside-In’ hypothesis for atopic sensitization, in which barrier
dysfunction precedes the onset of atopic eczema [6].
One of the most compelling arguments in favour of barrier dysfunction being the be-all and
end-all of atopic eczema stems from work done on the function of the protein filaggrin,
(filament-aggregating protein). Profilaggrin is encoded by the gene FLG, and is expressed in
the keratohyalin bodies of the granular layer. Serine proteases cleave profilaggrin into
filaggrin which functions in the organisation of the cornified envelope through promoting the
compaction of corneocytes, by aggregation of keratin microfibrils. Eventually, filaggrin is
broken down to produce Natural Moisturising Factor (NMF) which maintains the hydration
and acidic pH of the upper stratum corneum [7].
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A profoundly significant association between two FLG null mutations, (R510X and
2282del4) and atopic eczema has been demonstrated [8]. A recent meta-analysis has
estimated an odds ratio of 4.09 (95% confidence interval 2.64-6.33) for the association of
these FLG null mutations with atopic eczema, and current literature suggests that these
mutations represent the strongest genetic risk factors for atopic eczema to date [9]. Abnormal
filaggrin may produce a dysfunctional barrier sufficient to permit transfer of exogenous
allergens into sites where innate immune mechanisms can be activated through antigen
presentation. FLG mutations have been shown to significantly increase the likelihood of
developing atopic eczema, as infants with FLG mutations demonstrated barrier dysfunction,
assessed as increased trans epidermal water loss (TEWL), prior to the development of clinical
disease [10].
Despite many compelling studies, it is important to note that skin barrier dysfunction has
been demonstrated in atopic eczema irrespective of FLG genotype, suggesting that barrier
dysfunction due to filaggrin mutations does not singularly account for atopic eczema. Firstly,
it has been shown that at least 50% of atopic eczema patients do not have FLG mutations.
Secondly, it is known that individuals with FLG mutations may never experience eczema,
and many who do will resolve these symptoms during childhood [11]. This suggests that
other factors which may cause barrier defects are important, and compensatory mechanisms
may operate to restore adequate barrier function and enable one to ‘grow out’ of eczema
symptoms.
The converse argument to barrier dysfunction preceding atopic eczema is the ‘Inside-Out’
theory which suggests that a hyper reactive or dysfunctional immune system drives atopic
sensitization with subsequent barrier breakdown [4]. In the discussion of immune factors, I
will focus on the role of cytokines in driving an ‘Inside-Out’ mechanism for atopic eczema,
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and consider whether these might represent a more important factor than barrier dysfunction
in this process.
Epidermal immunity is strongly dependent on the function of Langerhans Cells, the dendritic
cells of the skin. Recent work suggests that Langerhans Cells, present in increased numbers
in eczematous skin, produce the cytokine Il-25 which drives the Th2 response, priming local
immunity to mediate atopic sensitization and eczematous skin lesions [12]. In addition, it is
noted that Il-25 may reduce FLG expression, suggesting that acquired defects in filaggrin can
be induced independently of whether FLG mutations are pre-existent [12]. This may be
relevant as a source of confounding in previous studies which have interpreted findings of
reduced epidermal filaggrin as being wholly due to an FLG mutation.
TSLP (thymic stromal lymphopoietin) is an Il-7 like cytokine expressed by keratinocytes, and
other epithelial cells at barrier surfaces. TSLP has been described as a ‘master switch’ for
atopy and atopic eczema in particular. Several studies have demonstrated increased TSLP
expression in eczematous skin and generalised increased serum TSLP in atopic individuals
[13]. Animal studies have shown that TSLP receptor knockout mice do not develop
eczematous lesions when subjected to sensitization with ovalbumin, while the over
expression of TSLP is sufficient to produce severe eczematous lesions without allergen
exposure [13]. TSLP stimulated dendritic cells produced the Il-25 receptor, indicating an
interplay between several cytokine pathways in producing atopic eczema which may
subsequently cause an acquired primary barrier defect by depleting filaggrin [14]. This
suggests that in addition to heritable barrier defects through FLG mutations, barrier defects
are frequently acquired as a result of the interaction between cytokine pathways and innate
immune cells. This supports the notion that barrier dysfunction is apparent in multiple
mechanisms causing atopic eczema, but that it is not necessarily the primary event in the
causal pathway. When the epidermal protease KLK5 binds to its receptor (Par2), TSLP, in
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addition to other proinflammatory cytokines, is produced. Blocking the KLK5 receptor
appears to inhibit the early production of TSLP, but does not stop the development of atopic
sensitization and eczematous lesions. This suggests that other processes in addition to those
mediated by TSLP drive atopic eczema from the inside, out [15].
Recently, IgE auto-antibodies against human epidermal proteins (Homs 1) were identified in
atopic eczema patients [16]. This has led to a revision of the Th1/Th2 paradigm explanation
for atopic sensitization, such that a new hypothesis has emerged in which it is suggested that
an initial Th2 reaction is necessary for the first sensitization, Th1 mediated inflammation
sustains chronic lesions, and an autoimmune mechanism perpetuates severe cases [17]. This
supports the ‘Inside-Out’ theory and indicates that immune pathways play an important role
in initiating and perpetuating atopic eczema, potentially more so than the role of barrier
dysfunction.
Some environmental factors are independent precipitants of atopic eczema, in the context of
healthy epidermis. House dust mite proteases (Der p1, Der p2) promote the degradation of the
epidermal barrier, and Staphylococcus Aureus exotoxins appear to break down
corneodesmosomes [18]. Many other factors have been implicated in the pathogenesis of
atopic eczema, despite a functioning epidermal barrier. These include soap and detergents,
hard versus soft water, topical corticosteroids, differences according to birth order, socioeconomic stratification, geographic distribution and seasonality of symptom severity [18].
Other environmental factors exacerbate atopic eczema when barrier dysfunction is
established, or may themselves cause barrier defects. It has been shown that children with
FLG mutations had an increased risk of developing atopic eczema, but this risk was higher if
children had been exposed to cat allergen at birth [19].
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In addition, some environmental factors mediate short term, acute effects which disappear if
the exposure is removed, whereas others may elicit a long term change in barrier or immune
constitution, through epigenetic mechanisms. The human microbiome could be considered
one such ‘environmental’ factor.
The ‘Old Friends’ hypothesis suggests that the disappearance of certain immunomodulatory
intestinal microbe species has made the western human immune system inclined towards
inappropriate activation characteristic of atopic diseases, with the concurrent increase of an
‘atopy associated microbial consortium’ [20]. Evidence that probiotic supplementation during
infancy significantly decreased the incidence of atopic eczema in infants and young children
adds weight to this theory [21]. The difficulty faced in differentiating between environmental
factors which precipitate and those which exacerbate atopic eczema complicates the
evaluation of the significance of these contributors, in comparison to barrier dysfunction.
Atopic eczema is a polygenic condition, for which multiple susceptibility loci have been
identified. Most studies have focused on adaptive and innate immune response genes, but
skin barrier dysfunction genes are now of increasing interest. It is likely that individuals
carrying mutations in more than one susceptibility gene demonstrate an additive effect
(epistasis) in the context of epigenetic factors. This has become apparent in the case of
immunity-associated genes, namely Il-10 and Il-13 polymorphisms which have been reported
to show an epistatic effect with FLG mutations on the risk of developing atopic eczema [22].
A novel strategy for comparing the roles of immunity and barrier dysfunction in the
pathogenesis of atopic eczema may lie in the interrogation of gene pathway interactions over
single gene interactions. Resources such as the ‘Ingenuity Pathways Analysis’ (IPA) system
can be used to evaluate the role of immunity and barrier dysfunction gene polymorphisms by
investigating the extent to which genes are in related networks.
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Of 81 immunity genes identified in more than 100 previous studies on atopic eczema, IPA
clustered the majority of these genes into two major networks, an antigen presentation
pathway, and a cell signalling pathway, both associated with immune dysregulation [23].
Following from this, the use of immunity versus barrier dysfunction gene pathway
association studies may reveal which truly represents the be-all and end-all of atopic eczema.
The examination of atopic skin conditions which conform to monogenic patterns of
inheritance is another mode by which immune or barrier dysfunction genes can be evaluated.
Netherton Syndrome (NS) is an autosomal recessive skin condition characterised by
dermatitis, asthma, and allergic rhinitis. NS is caused by a mutation in the gene SPINK5
which encodes LEKTI, a kallikrein protease inhibitor. LEKTI deficiency leads to increased
KLK5 activity in stratum corneum layers and hence the breakdown of corneodesmosomes,
the over-degradation of profilaggrin and subsequent loss of barrier function [24]. SPINK5
polymorphisms have a demonstrated association with atopic eczema, yet do not appear to
relate to serum IgE levels [25]. This suggests that barrier dysfunction can result from an
‘Inside-Out’ mechanism independent of systemic immunity.
Wiskott-Aldrich syndrome is an X linked recessive condition which incorporates features of
severe eczema, asthma and food allergy with a raised systemic IgE [26]. Hyper IgE syndrome
is also characterised by eczematous dermatitis. Epidermal barrier dysfunction has not been
described in these conditions, indicating that atopic eczema can emerge in the context of
immunity-associated genetic dysfunction in addition to barrier dysfunction.
It would be interesting to ask if atopic sensitization and eczema follows acute barrier
dysfunction. We know that the entire epidermis is lost in Epidermolysis Bullosa, but these
patients do not usually go on to develop atopic features [27].
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This suggests that the complete loss of epidermis is not conducive to the development of
atopic features, and rather, milder conditions in which living layers of the lower epidermis
and the function of Langerhans Cells are maintained is more likely to permit atopic
sensitization. Therefore, again, it seems that barrier dysfunction is critical, but must not be so
severe as to prevent innate immune cells from being able to access allergens which have
breached the damaged barrier. It would be interesting to observe what happens to eczema in
conditions where skin is damaged by other acute mechanisms, such as in the case of
Staphylococcal Scalded Skin Syndrome or due to mechanical trauma from scarring, skin graft
harvesting and tattoos. Indeed- if an allogeneic graft of healthy epidermis was transplanted
onto barrier compromised skin- would eczematous lesions develop in the graft?
What determines whether a dysfunctional epidermal barrier will manifest features indicative
of eczema over another condition where barrier dysfunction is implicated, such as psoriasis,
acne or rosacea? The ‘common variant/multiple disease’ hypothesis proposes that common
genetic variants found at a high frequency in the population play a role in related clinical
phenotypes in the context of different genetic backgrounds and under different environmental
conditions [28]. Barrier dysfunction may be a prerequisite for atopic eczema, but could also
signify the tip of the disease iceberg when we consider the vast complement of genetic and
environmental factors which interplay to produce a climate in which atopic eczema will
manifest.
Pruritis may represent a critical link between the environment, epidermal barrier, systemic
immunity and the CNS. Barrier dysfunction could be caused by damage from scratching
rather than an inherent structural barrier abnormality. Par2 agonists and Il-31 have been cited
as epidermal itch mediators [29] [30]. Relief of itch by scratching activates frontal brain areas
involved in reward, decision making and the hedonic experience [31].
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It establishes a compulsive behavioural pathway, and leads to the remodelling of pain
processing systems in the mechanism of central sensitization. In addition, both acute and
chronic psycho-emotional stress can trigger and enhance pruritis [32]. Therefore, it is
apparent that psycho-neuro-immunological pathways exist and that they may drive barrier
dysfunction through scratching. The role of barrier dysfunction is again apparent.
In conclusion, the epidermal barrier represents an interface for the converging and
interrelated effects of external (environmental) and internal (immune, neuro-psychological,
genetic) factors such that it is difficult to extract the significance of any individual component
without the effect of confounding. The extent of barrier dysfunction has been shown to
parallel the severity of atopic eczema symptoms [33], and the mainstay of current eczema
therapies are topical creams aimed at ‘restoring’ the skin barrier.
The advent of gene
pathway association studies will elucidate the respective roles of barrier and immunity genes
and bring the contribution of environmental factors under renewed scrutiny. So far, it appears
that barrier dysfunction is ubiquitous in the pathogenesis of atopic eczema regardless of
which of many potential causal mechanisms is considered. In some pathways it holds the
position of a start point for disease, in others it may be the culmination of effects from several
interacting systems. It might be reasonable, therefore, to concede that barrier dysfunction (at
the moment) is the be-all and end-all of atopic eczema. In fact, this could even be something
to celebrate, as it has been suggested that heterozygote carriers of FLG mutations may be
advantaged by a process of natural vaccination across their mildly perturbed epidermal
barrier (Irvine & McLean, 2006).
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