THYMIC STROMAL LYMPHOPOIETIN (TSLP)

TSLP is an IL-7 like cytokine and has been first discovered in the culture supernatants of mouse thymic stromal cells which gave rise for this nomenclature.

TSLP supports the growth and differentiation of B cells but also the proliferation of T cells^96,97. Different groups throughout the world demonstrated that high affinity TSLP binding requires the combined binding to the IL-7 receptor α-chain and TSLP receptor (TSLPR)^97-99. TSLP is mainly expressed by epithelial cells from the thymus, the skin, the lung, the intestine and tonsils as well as by stromal cells and mast cells^100-103. In the thymus, TSLP is responsible for the differentiation of Treg cells by instructing thymic DCs¹⁰⁴. Interestingly, human TSLP does not exert the same functions as its murine counterparts; however it does activate immature CD11c+

myeloid DCs^101,103. Thus, DCs can activate naïve CD4+ T cell proliferation and initiate the production of IL-4, IL-5, IL-13 and TNF-α (Fig. 7). In contrast, the production of the anti-inflammatory cytokines IL-10 and IFN-γ is inhibited by TSLP-induced DCs¹⁰³. TSLP is known to activate the upstream component of JAK1 and JAK2, which bind to IL-7Rα and TSLPR chain⁸. Subsequently JAK1/2 are phosphorylated and activate STAT5¹⁰⁵. TSLP binding may also lead to an activation of the subsequent STAT family members 1, 3, 4 and 6^106,107. Recent 27

phosphoproteomic data show that TSLP is also involved in a number of additional signalling pathways. It was shown that often signal transduction like Erk1/2, JNK1/2and p38 were phosphorylated after TSLP dependent activation¹⁰⁸. TSLP exerts its effects on a broad range of cells. Therefore it has been implicated to play an important role in many diseases like infections, cancer and inflammatory bowel diseases^109-111. However, an even more important role of TSLP has been anticipated in allergic diseases like AD and asthma¹¹². TSLP has been shown to be upregulated in an OVA-driven mouse model of airway inflammation¹¹³. These observations were confirmed in an OVA-induced murine model of allergic asthma and AD with TSLPR -/-mice which show a defective airway inflammation and allergic skin inflammation^114,115.

1.5.1 Role of TSLP in skin irritation

An acute insult against the stratum corneum results in perturbation of the barrier integrity and induces a process of positive and negative alarm signals which initiate both homeostatic and proinflammatory responses in the skin^22,116. The compromised barrier integrity further triggers the production of critical cytokines to initiate skin inflammation^117-119. TSLP is one of the cytokines which is expressed by keratinocytes in response to physical injury and inflammatory cytokine stimulation (Fig. 7)⁷⁴. The crucial role of TSLP in allergic inflammation is well established but the underlying mechanisms behind the trigger of TSLP production by different factors are still unknown^50,120,121. Primary human keratinocytes and skin explants were shown to produce TSLP upon bacterial, viral or inflammatory stimuli or physical trauma ^122,123. Angelova-Fischer et al. (2010) investigated the role of tape stripping and SLS on skin irritation and show that the stratum corneum of the epidermis is damaged, which is associated with an increased TSLP expression¹¹⁷. They also observed that keratinocytes express TSLP in the suprabasal cell layers of the epidermis. Among these layers it is mainly localised in the granular and spinous

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layer and is not expressed by keratinocytes in the basal layer. These data are in alignment with previous observations which have shown that TSLP expression is a characteristic sign of keratinocytes which are undergoing a differentiation process^103,124. As previously described, human TSLP can induce synergistic effects between proinflammatory and Th2 cytokines¹²³. On the other hand keratinocytes from Notch-deficient mice show an increased level of TSLP expression and an eczema-like phenotype in skin upon barrier disruption 123,125,126 indicating that there is a link between barrier integrity and TSLP production.

Figure 7: TSLP induction in keratinocytes.

Skin barrier disruption, allergen or Th2 derived cytokines triggers the epithelium cells for TSLP production.

TSLP activates DCs for the further recruitment of T cells for further production of proinflammatory cytokines or chemokine’s such as 4, 5, and TNF-α. TSLP also activates mast cells to produce other cytokines e.g. IL-13, IL-5 and TSLP itself (not shown).

Reprinted from Hamida Hammad et al. 2008: DCs and epithelial cells: linking innate and adaptive immunity in asthma: Nature Reviews Immunology 8, 193-204 (March 2008), Copyright © 2008, with permission from Nature Publishing Group (2014).

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1.5.2 Role of TSLP in AD

Many factors can elicit AD when overexpressed, though not being absolutely essential. The role of TSLP in AD development was not clear until studies showed that an overexpression of TSLP in the skin of mice leads to the development of a

“spontaneous” dermatitis, the most characteristics feature of human AD^49,103. Since TSLP is primarily produced by epithelial cells, this provided further evidence to the theory of KCs as the “initiators” of AD (Fig. 7)¹²⁷. Later on various groups confirmed TSLP as a major initiator of AD^50,51,128. Another study has shown that a direct administration of TSLP into the skin leads to AD-like lesions⁷⁴. Although this thesis is focusing on the skin, similar results were obtained for atopic asthma models^60,129. TSLP is involved in the proliferation and differentiation of Th2 cells and the subsequent production of IL-4, IL-5, IL-13 and TNF-α¹⁰³. Moreover, it was found that TSLP is highly expressed in keratinocytes from AD patients with acute and chronic lesions. Additionally it is associated with the activation and migration of DCs within the dermis¹⁰³. Therefore, TSLP was suspected to be one of the initiating factors for the development of AD.

Yoo et al. (2005) reported that keratinocyte specific overexpression of TSLP elicited skin disease with all the characteristic features of human AD, such as edema hyperkeratosisa, dermal mononuclear cell infiltrate⁴⁹. Mice lacking T cells, but overexpressing keratinocyte-specific TSLP still develop skin inflammation, indicating that T cells are not required for disease progression⁴⁹. Other studies with different AD models using TSLPR^-/- mice show that TSLP is necessary to induce AD i.e.

TSLP^-/- mice failed to develop AD^115,130.

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