**9. Prostanoids as inflammatory mediators**

In an established AD lesion, numerous inflammatory mediators such as cytokines (e.g., IL-4, IL-5, IL-13) and chemokines (e.g., CCL5, CCL11, CCL17) contribute to the development of AD (Guttman-Yassky et al.).

As for the inflammatory mediators in AD lesions, the role of PGD2-CRTH2 signaling has been the most frequently investigated. In the skin, PGD2 is the major prostanoid produced

(Laouini et al., 2005), it might be possible that some prostanoid signaling regulates the antibody production. *In vitro*, PGE2 drives Ig class switching to IgE by acting at EP2 and EP4 on B cells under LPS and IL-4 stimulation *in vitro* (Fedyk and Phipps, 1996). Whether such

T regulatory cells (Treg) make up one of the T cell subsets which has potent suppressive functions in various disease models. There are several reports that analyzed Treg number and function in AD patients, but those results are not necessarily consistent (Brandt et al., 2009; Ou et al., 2004; Schnopp et al., 2007; Verhagen et al., 2006). However, considering the fact that loss of Treg in skin can lead to AD-like skin lesions in both human (Ochs et al., 2005) and mouse (Brunkow et al., 2001), it is very likely that Treg play important roles in the

It has been well known that ultraviolet (UV) radiation causes immunosuppression, and it is one of the effective treatment options for AD. Although multiple suppression mechanisms have been proposed, induction of Treg is considered one of the central factors for the suppression mechanism. As blocking of prostanoid production by treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) treatment can abolish the immunosuppressive effect through UV (Chung et al., 1986; Hart et al., 2002; Walterscheid et al., 2002), it has been suspected that prostanoids play important roles in the UV-induced immunosuppression, especially in Treg induction. By UV radiation, various prostanoids are produced in the skin, with PGE2 being the most abundant prostanoids (Kuwamoto et al., 2000; Ruzicka et al., 1983; Soontrapa et al., 2011). Recently, it has been revealed that PGE2-EP4 signaling mediates the induction of Treg by UV irradiation, and regulates UV-induced immunosuppression (Soontrapa et al.). Blockade of EP4 signaling suppresses the increase of Treg in dLNs and abolishes the immunosuppressive effect of UV. Blockade of EP4 signaling also diminishes the RANKL expression on KCs after UV irradiation (Soontrapa et al., 2011). It is known that RANKL expression on UV-irradiated KCs activates LCs, and the RANKL-activated LCs function to induce Treg in dLNs (Loser et al., 2006). These results indicate that PGE2-EP4 signaling regulates RANKL expression on KCs and controls Treg induction from UV. Other than PGE2, it has been reported that PGD2 induces Treg differentiation through DP in a mouse asthma model (Hammad et al., 2007). Inhalation of a selective DP1 agonist suppressed the cardinal features of asthma by targeting the function of lung DCs. In mice treated with a DP1 agonist or receiving DP1 agonist-treated DCs, there was an increase in Tregs that suppressed inflammation in an IL-10-dependent way (Hammad et al., 2007). These effects of a DP1 agonist on DCs were mediated by cyclic AMP-dependent protein kinase A. Taken together, control of EP4 and/or DP1 signaling could represent a novel

In an established AD lesion, numerous inflammatory mediators such as cytokines (e.g., IL-4, IL-5, IL-13) and chemokines (e.g., CCL5, CCL11, CCL17) contribute to the development of

As for the inflammatory mediators in AD lesions, the role of PGD2-CRTH2 signaling has been the most frequently investigated. In the skin, PGD2 is the major prostanoid produced

actions occur *in vivo* remains unknown, and this should be clarified in future studies.

**8. Prostanoids and Treg induction** 

pathogenesis of AD.

immunosuppressive approach.

AD (Guttman-Yassky et al.).

**9. Prostanoids as inflammatory mediators** 

by activated mast cells. PGD2 has two types of receptors, DP and CRTH2. CRTH2 induces chemotaxis in Th2 cells, eosinophils and basophils with enhanced degranulation *in vitro* (Hirai et al., 2001; Yoshimura-Uchiyama et al., 2004). CRTH2 amplifies Th2 responses by preventing apoptosis of Th2 cells and enhancing their capacity to secrete cytokines (Nomiya et al., 2008; Xue et al., 2009). CRTH2 also amplifies eosinophil functions by mobilizing them from the bone marrow, preventing their apoptosis, and promoting their chemokinesis and degranulation (Gervais et al., 2001). CRTH2 mRNA expression is high in peripheral blood mononuclear cells of patients with AD (Hijnen et al., 2005), and circulating eosinophils and T cells in patients with AD have an increased surface expression of CRTH2 (Iwasaki et al., 2002), suggesting the role of CRTH2 in AD.

He et al. have recently reported that lack of CRTH2 signaling ameliorates the inflammation only in newly-challenged skin, while loss of this signaling in chronic challenged areas did not affect the inflammation (He et al., 2011). They used CRTH2 knockout mice in two types of AD models: one that was repeatedly sensitized with OVA for a total of seven weeks, which mimicked the chronic lesions of AD; and one that was challenged with OVA after the repeated sensitization of other skin areas for a total of seven weeks, which was supposed to mimic the acute lesions of AD. In the chronic lesions, the inflammatory cell infiltration and cytokine concentration was similar between wild-type and CRTH2 knockout mice, while in the acute lesions, such factors were significantly decreased in CRTH2 knockout mice compared with wild-type mice (He et al.). Consistently, the concentration of PGD2 increased significantly in the acute lesions, while the concentration of PGD2 in the chronic lesions was similar compared with that of non-affected skin. CRTH2 knockout mice also showed comparable levels of IgE production, indicating that CRTH2 signaling had little effect much on the antibody production process. Boehme et al. have previously reported that administration of a CRTH2 antagonist inhibited the development of chronic AD lesions in the same model (Boehme et al., 2009), but the CRTH2 antagonist-treated group also showed reduced IgE production, suggesting the possibility that administration of the CRTH2 antagonist affected the extent of sensitization non-specifically and thus lead to the reduced inflammatory cell infiltration in the skin. Collectively, blockage of PGD2-CRTH2 signaling might inhibit allergic skin inflammation elicited in patients with AD by re-exposure to antigens to which they have been sensitized (Figure 3).

In contrast, stimulation of EP3-signaling in KCs is reported to play an anti-inflammatory role in skin inflammation by inhibiting chemokine production from KCs (Honda et al., 2009). Administration of an EP3 specific agonist suppressed a CHS response, and EP3 knockout mice showed an enhanced CHS response, suggesting that PGE2-EP3 signaling works as a negative regulator of allergic cutaneous inflammation. An anti-inflammatory role of EP3 signaling is also reported in other allergic diseases such as mouse asthma and the allergic conjunctivitis model (Kunikata et al., 2005; Ueta et al., 2009).

#### **10. Prostanoids as itch mediators**

It is reported that some prostanoids can modulate pruritus, a significant hallmark of AD. In human studies, PGE2 is a weak pruritogen and prolongs experimentally-induced itch (Hagermark and Strandberg, 1977; Neisius et al., 2002), although injection of PGE2 alone does not elicit itch-associated response in animal experiments (Andoh and Kuraishi, 1998). TXA2 is also reported as a mediator of itch (Andoh et al., 2007). Injection of a TP agonist alone elicited itch-associated responses. TP was expressed in both KCs and nerve fibers in

The Role of Prostanoids in Atopic Dermatitis 73

Repeated antigen exposure to the skin stimulates KCs to secrete pro-inflammatory cytokines, chemokines and other mediators, which activate the endothelial activation of blood vessels. This activation attracts memory T cell infiltration into the skin. PGE2 dilates blood vessels possibly through EP2 and EP4. The PGD2-CRTH2 signaling promotes Th2 cells/eosinophils/neutrophils infiltration in skin. The PGE2-EP3 signaling inhibits KC activation and plays an anti-inflammatory role in CHS. The TP signaling mediates itch signaling through afferent nerves, while stimulation of DP signaling inhibits itch-

NSAIDs, which block the production of all prostanoids, usually have limited effects on AD (Kabashima and Miyachi, 2004) and therefore have not been given so much attention as a potential therapeutic agent. However, the analysis using the receptor knockout mice and receptor specific drugs has revealed new unexpected roles of prostanoids in the immune systems. In addition, signaling from even the same receptor can produce the opposite effect depending on the context, such as the Th1 modulating effect generated through EP2/EP4 signaling (Yao et al., 2009). Therefore, it would be necessary to reconsider the role of prostanoids in the development of AD. It is also important to correlate these immunomodulatory actions of prostanoids found in mice to their actions in immune diseases of humans. Currently, CRTH2 antagonists are on their way to being used in clinical applications for AD or asthma (Ulven and Kostenis., 2010). Further analysis of the role of each prostanoid receptor has great potential in leading to a new

Fig. 3. Prostanoids as inflammatory/itch mediators

associated responses.

therapeutic target for AD.

**11. Summary and future direction** 

skin (Andoh et al., 2007). TX synthase was also expressed in KCs. In some studies, the association of genetic polymorphisms in the TP gene with asthma and atopy has been reported (Shin et al., 2003). TXA2 may be involved in the pathogenesis of atopic disease not only as an airway constriction factor but also as an itch mediator.

On the other hand, PGD2 is reported to play anti-pruritic roles in the mouse AD model (Arai et al., 2004). Administration of PGD2 and a DP1 agonist reduced scratching behaviors in an AD model using NC/Nga mice, while administration of a DP2 agonist did not reduce such behavior (Arai et al., 2004). Inhibition of histamine release from mast cells is proposed as a possible suppression mechanism of the DP signaling (Hashimoto et al., 2005). Blockade of TP signaling or stimulation of DP1 signaling may lead to a new target for the treatment of pruritic disease, including AD.

Fig. 2. Regulation of DC migration and Th differentiation by prostanoids

During the sensitization period, antigens induce pro-inflammatory cytokine secretion by KCs, which enhances cutaneous DC (LCs and dDCs) activation and migration to dLNs. In the LNs, cutaneous DCs activate naïve T cells that differentiate into mature memory T cells. During antigen exposure to the skin, KCs produce PGE2 and mast cells produce PGD2. Moreover, *schistosomes* produce PGD2 during a helminthic infection. The PGE2-EP4 pathway promotes, but PGD2-DP and PGI2-IP pathways inhibit cutaneous DC migration and maturation. TXA2 produced by activated cutaneous DCs seems to act on naïve T cells to disrupt DC-T cell interaction. The PGE2-EP1/EP2/EP4 pathways promote Th1 cell differentiation, and the PGE2-EP4 pathway also promotes Th17 cell expansion.

skin (Andoh et al., 2007). TX synthase was also expressed in KCs. In some studies, the association of genetic polymorphisms in the TP gene with asthma and atopy has been reported (Shin et al., 2003). TXA2 may be involved in the pathogenesis of atopic disease not

On the other hand, PGD2 is reported to play anti-pruritic roles in the mouse AD model (Arai et al., 2004). Administration of PGD2 and a DP1 agonist reduced scratching behaviors in an AD model using NC/Nga mice, while administration of a DP2 agonist did not reduce such behavior (Arai et al., 2004). Inhibition of histamine release from mast cells is proposed as a possible suppression mechanism of the DP signaling (Hashimoto et al., 2005). Blockade of TP signaling or stimulation of DP1 signaling may lead to a new target for the treatment of

only as an airway constriction factor but also as an itch mediator.

Fig. 2. Regulation of DC migration and Th differentiation by prostanoids

differentiation, and the PGE2-EP4 pathway also promotes Th17 cell expansion.

During the sensitization period, antigens induce pro-inflammatory cytokine secretion by KCs, which enhances cutaneous DC (LCs and dDCs) activation and migration to dLNs. In the LNs, cutaneous DCs activate naïve T cells that differentiate into mature memory T cells. During antigen exposure to the skin, KCs produce PGE2 and mast cells produce PGD2. Moreover, *schistosomes* produce PGD2 during a helminthic infection. The PGE2-EP4 pathway promotes, but PGD2-DP and PGI2-IP pathways inhibit cutaneous DC migration and maturation. TXA2 produced by activated cutaneous DCs seems to act on naïve T cells to disrupt DC-T cell interaction. The PGE2-EP1/EP2/EP4 pathways promote Th1 cell

pruritic disease, including AD.

Fig. 3. Prostanoids as inflammatory/itch mediators

Repeated antigen exposure to the skin stimulates KCs to secrete pro-inflammatory cytokines, chemokines and other mediators, which activate the endothelial activation of blood vessels. This activation attracts memory T cell infiltration into the skin. PGE2 dilates blood vessels possibly through EP2 and EP4. The PGD2-CRTH2 signaling promotes Th2 cells/eosinophils/neutrophils infiltration in skin. The PGE2-EP3 signaling inhibits KC activation and plays an anti-inflammatory role in CHS. The TP signaling mediates itch signaling through afferent nerves, while stimulation of DP signaling inhibits itchassociated responses.
