**3.1 IL-4 gene**

42 Atopic Dermatitis – Disease Etiology and Clinical Management

skin lesions within 12 months. Inflammatory skin lesions show the presence of *Staphylococcus aureus*, a common infectious complication and exacerbating factor in human AD patients. Histological analyses of the lesions in these mice show spongiosis, acanthosis, dermal and epidermal infiltration of mononuclear cells and eosinophils, and degranulating mast cells in the dermis. Expression of Th2-type cytokines (IL-5, IL-13), Th1-type cytokines (IL-12p40, IFN-γ), and inflammatory cytokines (TNF-α, IL-1β) is upregulated in the skin lesions of these mice. Enhancement of these cytokines except IL-12p40 is even observed in the skin, before the disease onset or in the intact skin (17). Correspondingly, serum levels of IgE and IgG1 are elevated even before the onset, which reflects high expression of IL-4 and IL-13. Afterward, up-regulated serum levels of IgG2a are observed, reflecting high expression of IFN-γ (19). T cells in these mice possess higher proliferating capacity *in vitro* spontaneously or induced by stimulants such as T cell receptor triggering or Staphylococcus enterotoxins A and B compared to T cells in wildtype mice (18). It is of note that the numbers of dendritic cells, macrophages, and NK cells increase in the skin tissues and lymphoid organs of these mice, which suggests that

overexpression of IL-4 in skin tissues modulates antigen-presenting activity *in vivo*.

Zhu's group established IL-13 transgenic mice in which withdrawal of doxycycline induces expression of IL-13 under the control of the promoter region of the keratin 5 gene (20). Six to eight weeks after the withdrawal of doxycycline, AD-like skin lesions (i.e. pruritis, loss of hair, erythema, crusting, excoriation, bacterial pyoderma, and erosions, thereafter dry lichenified skin lesion) appear in these mice. All of the mice develop these features within four months. In the skin lesions, F4/80+ cells including macrophages and dendritic cells, eosinophils, CD4+ cells, and mast cells are accumulated. Expression of various chemokines (CCL2/MCP-1, CCL5/RANTES, CCL11/Eotaxin, CCL17/TARC, CCL22/MDC, CCL27/CTACK), TSLP (a critical cytokine correlated with AD at the interface of epidermis and immune cells), and VEGF (a potent cytokine for angiogenesis), is up-regulated in these mice. Serum levels of IgE and IgG1 are elevated. Furthermore, systemic Th2 skewing is enhanced because the lymphocytes in the lymph nodes of these mice produce more IL-4 and IL-13 upon stimulation of anti-CD3/CD28 antibody (Ab)

An initial linkage analysis in Amish families showed the association of 5q31.1 with serum IgE levels (21). This chromosome region contains a cluster of Th2-type cytokine genes. Therefore, much attention has been paid to the genetic association of Th2-type cytokine genes with atopy (defined as high serum IgE level) or with each specific phenotype of allergic diseases. As a result, several single nucleotide polymorphisms (SNPs) on the *IL4*, *IL5*, and *IL13* genes have been shown to be associated with atopy or AD (Table 2). Furthermore, SNPs located on the *IL4RA* and *IL13RA1* genes encoding the IL-4 receptor α chain (IL-4Rα) and the IL-13R α1 chain (IL-13Rα1) and on the *STAT6* gene encoding STAT6 were also reported to be associated with atopy or AD. IL-4Rα is a component of both type I IL-4R and type II IL-4R/IL-13, whereas IL-13Rα1 is a component of type II IL-4R/IL-13

**2.2.2 IL-13 transgenic mice** 

than do those in the wild-type mice.

**3. Genetic associations of Th2-type cytokines with AD** 

There exists a SNP in the promoter of the *IL4* gene (-590C/T). This SNP was originally reported to be associated with bronchial asthma (24) and is thought to affect the binding activities to NFAT, resulting in modulating IL-4 production. Transmission disequilibrium testing shows a significantly preferential transmission of the T allele in AD patients (25). Another group confirmed the association of this SNP with AD (26).

The Roles of Th2-Type Cytokines in the Pathogenesis of Atopic Dermatitis 45


The type II IL-4R or the IL-13R is composed of IL-4Rα and IL-13Rα1. Both IL-4 and IL-13 bind to this receptor on cell surface, transducing their signals intracellularly (5, 6). Several SNPs on the *IL4RA* gene are known to be genetically associated with atopy or allergic diseases. The genetic association of Gln576Arg, also referred as Gln551Arg, was originally found in hyper IgE syndrome patients, some of who had severe AD (41). Position 576 is adjacent to the tyrosine residue at 575, a binding site for SHP-1, a phosphotyrosine phosphatase. Exchange of glutamine for arginine decreases the binding activities for SHP-1, resulting in up-regulation of STAT6 activation. The genetic association of this SNP with AD

IL-13Rα1 is another component of type II IL-4R/IL-13R, together with IL-4Rα. A SNP in the coding region of the *IL13RA1* gene, 1398A/G, was reported to be genetically associated with serum IgE levels (27). The function of this SNP is unclear because this SNP is a silent one.

STAT6 is a transcriptional factor, critical for both IL-4 and IL-13 signals (5, 6). Heterodimerization of type I IL-4R or type II IL-4R/IL-13R by binding of IL-4 or IL-13 activates Jak1, Jak3, or Tyk2, tyrosine kinases associated with IL-4Rα, common γ, and IL-13Rα1 respectively, followed by activation of STAT6. Several SNPs on the *STAT6* gene, 2964G/A, in18SNP1C/T, and GT repeat in exon1 were reported to be associated with serum

A SNP on the IL5 gene (-703C/T) was reported to be associated with the numbers of

Because of the importance of Th2-type cytokines in the pathogenesis of AD, it has been thought that Th2-type cytokines have the potential to be targeted to develop novel agents against AD. A clinical trial to administer a humanized anti–IL-5 monoclonal Ab (mepolizumab) to AD patients was performed, with a disappointing result (46). To the best of our knowledge, no trial targeting IL-4 or IL-13 for AD patients has thus far been

AD patients received 750 mg of humanized anti–IL-5 monoclonal Abs (mepolizumab) intravenously twice (at day 0 and 7) (46). Efficacy assessed by SCORAD was observed at

been observed.

**3.3 IL-4Rα gene** 

**3.4 IL-13α1 gene** 

**3.5 STAT6 gene** 

**3.6 IL-5 gene** 

performed.

was confirmed by another group (42).

IgE levels or with eosinophil numbers (43, 44).

**4.1 Antagonist of IL-5 for treatment of AD** 

eosinophils in blood and serum IgE levels, but not with AD (45).

**4. Th2-type cytokine – Targeted treatment for AD** 

Fig. 1. The receptor structures and signal pathways of IL-4 and IL-13. IL-4 binds to type I IL-4R or type II IL-4R/IL-13R, and IL-13 binds to type II IL-4R/IL-13R. Type I IL-4R and type II IL-4R/IL-13R are composed of IL-4Rα and the IL-2R γ chain (common γ) or IL-4Rα and IL-13Rα1, respectively. Engagement of the receptors activates Jak kinases followed by activation of STAT6.
