The role of mast cells and mast cell mediators in the development of atopic dermatitis in a mouse model

Sammanfattning: Atopic dermatitis (AD) is a complex, often lifelong allergic disease affecting around 10 % of both dogs and humans. The hallmark symptom is severe pruritus, causing a lowered quality of life. Mast cells (MCs) are known to play an important part of the immunopathogenesis, promoting a faulty T helper cell type 2 (Th-2) response which follows by a production of specific immunoglobulin E (IgE) antibodies towards environmental allergens (Ag). To further investigate the role of MCs and its mediators in the progression of AD, a low-calcemic vitamin D3 analog (MC903) was used to induce AD-like symptoms locally on the ears of two different knock-out (KO) mouse strains. The first strain was Wsh-/- mice deficient in MCs. The second strain was mice deficient in mouse mast cell protease 4 (mMCP-4-/-), a homolog to the human and canine MC chymase (MCC). The hypothesis was to observe an increase in inflammatory parameters following MC903-treatment, such as migration of inflammatory cells, higher amount of inflammatory mediators and clinically observed ear thickening. Thereby, MC903 would be proven to induce a functioning AD animal model. The results showed that daily topical MC903 application on the left ear induced changes mimicking AD macroscopically, microscopically and biochemically, including a Th-2 response represented by associated cytokines. No changes were seen in the vehicle-treated ears (treated with ethanol) used as an internal control. The epithelial cell-derived thymic stromal lymphopoietin (TSLP) appeared to be an important cytokine in promoting the skin inflammation since the amount of TSLP was markedly high locally in the MC903-treated ears. In MC-deficient mice, TSLP-levels were significantly lower, thus implicating a central role of MCs in this model. Although the presence of MCs caused higher levels of both TSLP and interleukin 33 (IL-33) after MC903-treatment, the clinical appearance in Wsh+/- mice was milder compared to mice lacking MCs. This indicates that MCs do promote the AD-like skin inflammation caused by MC903, but at the same time prohibit the inflammation from being too excessive. The results from a small in vitro study performed as a part of this study, proved that MC903 also activates MCs directly to degranulate. The MCC homolog mMCP-4 did on the contrary only contribute to the lowering of IL-33 levels, most likely through cleavage of the cytokine by mMCP- 4, therefore no apparent role of MCC could be observed. In the cleavage study of TSLP, a more rapid cleavage of TSLP was mediated by tryptase than chymase. Thus, as a continuation of this study, the investigation of the role of tryptase in the MC903-induced AD-model would be of high interest. In conclusion, MC903 application induced an AD-like local inflammation, thus showing that the method provides a functioning AD-model. MCs contribute to disease progression through cytokine production, but have at the same time a limiting role in the course of inflammation.