Atopic dermatitis as Th2 disease revisited

T helper 1 (Th1) and T helper 2 (Th2) populations are a classical dichotomy to understand the immunological disorders, and this concept remains vital in atopic dermatitis (AD). While IL‐4 and IL‐13 are produced mainly by Th2 cells, mast cells, and basophils, these cytokines target these producers, indicating the presence of autocrine and paracrine stimulation. There are two types of receptors for IL‐4 and IL‐13, type I and type II. IL‐4 binds to both type I and II receptors, while IL‐13 has affinity to type II. Dupilumab is an IL‐4Rα‐antagonist that inhibits IL‐4 and IL‐13 signaling through blockade of the shared IL‐4α subunit. The Th2‐stimulating effect of thymic stromal lymphopoietin (TSLP) is triggered by its binding to TSLP receptor (TSLPR) on Th2 cells. The frequency of TSLPR+CD4+ T cells correlates with disease activity. CD4+ T cells directly interact with TSLP to produce a high amount of IL‐4. Conversely, IL‐4 induces TSLPR expression in T cells. Moreover, TSLP and IL‐4 promote CCR4 expression, which is a chemokine receptor for CCL17/TARC. Langerhans cells initiate epicutaneous sensitization with protein antigens and induce Th2 cell–mediated immune responses via TSLP signaling. Filaggrin deficiency, which cornified layer‐damaged skin allows protein antigens to penetrate through the stratum corneum, leads to allergy where IL‐4 and IL‐13 are overproduced. Subsequently, IL‐4 and IL‐13 further depress filaggrin expression with vicious cycle. AD is generally a Th2 disease, but there exist several exceptional and undeniable facts to be considered, including Th1‐activated intrinsic AD, Th1‐infiltrating chronic AD skin lesions, and Th17 involvement.


| INTRODUC TI ON
Atopic dermatitis (AD) is a chronic-intermittent, eczematous dermatitis that starts at infancy or early childhood and persists for a large part of life. A large number of clinical, laboratory, and experimental studies have been performed, but the pathophysiology of AD remains to be clarified.
Th1 and Th2 populations are a classical dichotomy to understand the immunological disorders, 1 and this concept remains vital in clinical and basic research of allergic diseases. When we see the diagnostic criteria of AD by Hanifin and Rejka, 2 we can realize that there are a considerable number of manifestations presumably associated with Th2 cells, including xerosis, ichthyosis, palmar hyperlinearity, immediate skin test reaction, elevated serum immunoglobulin E (IgE), tendency toward cutaneous infection, and impaired cell-mediated immunity. In addition, we have indicated that chondrodermatitis of the auricle 3  Recent use of anti-Th2 cytokine or receptor antibody therapies has been providing important information on Th2-skewing condition of AD. The clinical efficacies of these treatments are mandatory for evaluation of Th2 cytokines in AD pathogenesis. In this review, we revisit the essential role of Th2 cells for AD, but also several issues that are as yet unelucidated merely with Th2 dogma.

| PERTURBATI ON OF S TR AT U M CORNEUM BARRIER IN AD
The barrier function is usually assessed by transepidermal water loss (TEWL) and skin surface hydration. AD patients, especially common, serum IgE-high, extrinsic AD patients, have increased TEWL and lower skin surface hydration, compared with healthy subjects. Even at the non-lesional forearm and lower leg of patients and normal volunteers, the level of skin surface hydration was significantly lower in AD than in normal subjects. 5 The identification of loss-of-function mutations in filaggrin gene (FLG) has shed new light on the mechanisms of AD. 6 When we examined the amount of filaggrin in stratum corneum of AD patients, ichthyosis vulgaris patients, and normal subjects, we confirmed that filaggrin is apparently reduced in ichthyosis vulgaris and AD compared with normal subjects. 7 Patients with FLG mutations have various extents of ichthyosis vulgaris (Figure 1), palmar hyperlinearity ( Figure 2), and atopic dermatitis. Different FLG mutations were F I G U R E 1 Vicious cycle between barrier impairment and allergic disorder in atopic dermatitis. Filaggrin deficiency is a typical cause of barrier impairment (as seen in ichthyosis vulgaris), which allows protein antigens to penetrate through stratum corneum, leading to Langerhans cell-mediated Th2 allergic responses. The resultant upregulation of IL-4/IL-13 production reduces filaggrin expression, resulting in further reduction in barrier and promotion of allergic reaction F I G U R E 2 Palmar hyperlinearity associated with FLG mutations. The intensity of hyperlinearity depends on the age of patients reported depending on races, and the frequency of FLG mutations is 24.3% in AD patients in Hamamatsu University Hospital. Profilaggrin is the major component of the keratohyalin granules within epidermal granular cells. During epidermal terminal differentiation, the profilaggrin polyprotein is dephosphorylated and rapidly cleaved by serine proteases, such as kallikrein-5, 8 to form monomeric filaggrin, which is further degraded into natural moisturizing factors.
Perturbation of skin barrier function as a result of reduction or complete loss of filaggrin expression enhances percutaneous transfer of allergens ( Figure 1). The association of the FLG mutations in particular with the extrinsic type of AD was observed. 9 Thus, the skin barrier is perturbed in AD, and FLG mutation represents a typical cause of barrier impairment. The allergic conditions of AD may be preceded by skin barrier impairment, which allows protein antigens to penetrate through disrupted barrier.

| TH2-P OL ARIZED IMMUNOLOG IC AL S TATE OF AD
In general, skin responses to exogenous antigens can be divided into three types, depending on the evoked timing after exposure, the eruption type, and the immunocompetent participants ( Figure 3).
The responses comprise immediate-, late-phase, and delayed-type reactions. AD is well known as a Th2-polarized disease. Th2 cells are involved mainly in the late-phase reaction, 10 but the Th2 reaction may extend to parts of the immediate-and delayed-type hypersensitivity, because of graduation between these three categories. In the chronic lesion of AD, Th1 cells (additionally Th22 and Tc1 cells) are considered to induce chronic eczema with thick epidermis.
As expected with elevated total serum IgE and circulating eosinophils, AD patients show high levels of Th2 cytokines, such as IL-4, IL-5, and IL-13. 11 Along with the elevation of IL-5, eosinophil counts and eosinophil cationic protein levels are increased in AD. 12 In addition to AD, there are many Th2-polarized skin diseases such as eosinophilic pustular folliculitis, prurigo, angiolymphoid hyperplasia with eosinophilia (Kimura's disease), eosinophilic cellulitis (Wells' syndrome), bullous pemphigoid, and cutaneous T-cell lymphoma.
Among them, AD is a disorder where the epidermis and dermis afford Th2-associated inflammatory sites.
To identify Th2 cells, several methods are used. Intracellular cytokine staining and subsequent flow cytometry are a standard method, and IL-4-or IL-5-positive CD4 + cells represent Th2 cells.
The surface marker of CD4 + CD7 − CCR4 + phenotype is also employed. Chemotaxis toward CCL17/TARC represents functional identification. 13  In this sense, it is an issue which cell type is the initial producer of IL-4. One candidate for this "early IL-4" producer is basophil. It is noted that IL-33 stimulates basophils to secrete IL-4. 16 There are two types of receptors for IL-4 and IL-13 ( Figure 4).

| IL-4/IL-13 AND THEIR RECEP TOR S
Both receptors are comprised of heterodimers. Type I consists of IL-4Rα and Common γ, and type II is composed of IL-4Rα and IL-13Rα1.
IL-13Rα2 is another IL-13 receptor, serving as either a decoy or a key mediator of fibrosis. IL-4 binds to both type I and II receptors, while IL-13 has affinity to type II and IL-13Rα2. Dupilumab is an IL-4 Rαantagonist that inhibits IL-4 and IL-13 signaling through blockade of the shared IL-4α subunit. 17 In addition to allergy, IL-4/IL-13 regulates the immune microenvironment in cancer. Signaling via type II receptor induces tumor proliferation, cell survival, cell adhesion, and metastasis. 18 Thus, type II expression in tumors is associated with poor prognosis.

| PROMOTI ON OF IL-4 S TIMUL ATI ON/ PRODUC TION BY AD -AGG R AVATING FAC TOR THYMIC S TROMAL LYMPHOP OIE TIN IN TH2 CELL S
CD4 + CCR4 + CXCR3 − CCR7 − CCR10 + CLA + Th2 cells in AD patients exhibit enhanced TSLPR expression. 20 The frequency of TSLPR + CD4 + T cells correlates with disease activity. CD4 + T cells directly interact with TSLP to produce a high amount of IL-4.

Conversely, IL-4 induces TSLPR expression in T cells. Moreover,
TSLP and IL-4 promote CCR4 expression, which is a chemokine receptor for CCL17/TARC. Thus, the scheme shows mutual activation of IL-4 production, TSLP receptor expression, and CCR4 expression by IL-4 and TSLP in Th2 cells.
Likewise, many AD-aggravating factors directly or indirectly induce stimulation of Th2 cells and resultant IL-4 production ( Figure 6).
Protein antigens induce Th2 responses, while metals and haptens usually evoke Th1 responses. 21 In the sweat, Malassezia antigen may deteriorate Th2 aspect of AD, 22 while nickel, cobalt, and chromium are usually Th1 inducers. 9 Mental stress or anxiety is associated with Th2 status. 23 In addition, future studies on microbiota might shed light for its association with Th2 condition. 24

| REL ATIONS HIP B E T WEEN BARRIER S TATUS AND TH2 RE S P ON S E S IN AD
The skin immune status is closely associated with the disordered condition of skin barrier. 28 Barrier disruption induces Th2 cytokine production and eosinophil infiltration in mice. 25 Topical protein antigen application further promotes Th2 responses. 29 Therefore, barrier impairment results in elevation of Th2 status.
Filaggrin deficiency, which cornified layer-damaged skin allows protein antigens to penetrate through the stratum corneum, leads to allergy where IL-4 and IL-13 are overproduced. Subsequently, IL-4 and IL-13 further depress filaggrin expression. 30 This cascade yields vicious cycle, and therefore, even heterozygous FLG mutations may result in development of AD. In addition, IL-4 suppresses the enhancement of ceramide synthesis and cutaneous permeability barrier functions, which further aggravates the barrier. 31 Neutralization of the normally acidic stratum corneum has deleterious consequences for permeability barrier homeostasis and stratum corneum integrity/cohesion attributable to serine proteases activation, leading to deactivation/degradation of lipid-processing enzymes and corneodesmosomes. 32

| L ACK OF PURIT Y OF TH2 P OL ARIZ ATI ON IN AD
Although several criteria for its definition have been widely approved, there still exist variations in the diagnosis of AD because of its heterogeneous aspects. AD is generally a Th2 disease, but there exist several exceptional and undeniable facts to be considered.

| Intrinsic AD and Th1 activation
The first issue is the presence of intrinsic type among AD patients.
AD can be categorized into the IgE-high, extrinsic type and the IgEnormal, intrinsic type. 21 While extrinsic AD is the classical type with high prevalence, the incidence of intrinsic AD is approximately 20% with female predominance. The skin barrier is perturbed in the extrinsic, and FLG mutation represents a typical cause of barrier impairment. Intrinsic AD is immunologically characterized by the higher expression of IFNγ and non-protein antigens, such as metals and haptens, may induce dermatitis. 9,21 The overproduction of IFNγ may further downregulate IgE production in intrinsic AD, as suggested by our in vitro study, 9 and intrinsic AD is linked with much lower levels of IL-4 and IL-13. 33

| Chronic AD lesion and Th1 infiltration
It is known that Th2 cells infiltrate in the acute lesion of AD, but Th1 cells, and presumably Tc1 cells, are involved in the chronic lesion.
The acute skin lesion corresponds to the late-phase reaction evoked by Th2 cells and eosinophils, while the chronic skin lesion corresponds to the delayed-type hypersensitivity induced by Th1/Tc1 cells. 34 Accordingly, IFNγ generally plays an essential role for occurrence of dermatitis. 26 Clinically, the therapeutic effectiveness of systemic IFNγ, which suppresses Th2 cells in vitro, is limited, further supporting the notion that AD does not have a pure Th2 property.

| Involvement of Th17 cells
Th17 cells, producing IL-17A and IL-22, are increased in the peripheral blood of AD, and Th17 cells infiltrate in the acute skin lesion F I G U R E 6 Causative and aggravating factors in atopic dermatitis more markedly than in the chronic lesion. 35 There is a tendency that the frequency of circulating Th17 cells is higher in intrinsic AD than in extrinsic AD. 9 In the lesional skin, another group of investigators found positive correlations between Th17-related molecules and SCORAD scores in patients with intrinsic AD, whereas only patients with extrinsic AD show positive correlations between SCORAD scores and Th2 cytokine (IL-4 and IL-5) levels. 36 Asian patients may be more prone to have Th17-participating AD. 37

| THER APIES FOR IMMUNOMODUL ATION OF AD
Recent cytokine-targeting biologic therapies have been proving the disease mechanisms underlying atopic dermatitis as well as psoriasis.
The essential involvement of IL-4 and IL-13 in AD pathogenesis is strongly supported by the therapeutic effectiveness of anti-IL-4Rα antibody dupilumab, which block IL-4 binding to type I and type II IL-4 receptors and IL-13 binding to type II receptor ( Figure 4).
Dupilumab depresses Th2-centered inflammatory axis 38 and exerts a high clinical efficacy. Dupilumab was approved in Japan in 2018, and the number of patients treated with this biologic has been increasing. Before dupilumab was marketed, many patients with refractory AD had been treated with cyclosporine A. However, because of its renal toxicity, careful attention is necessary for its long term use.
Dupilumab is beneficial for such cases.
There are possible Th2 cytokine-targeting therapies, including anti-IL-13 antibody 39 and anti-IL-31 antibody. 40 Furthermore, semi-Th2 targeting JAK inhibitors are promising therapeutic modalities for AD. Further clarification of the immunological mechanism of AD may lead to development of novel therapies. 19

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.