The role of Malassezia allergens and mast cells in atopic eczema

Abstract: Atopic eczema (AE) is a chronic inflammatory skin disease, characterized by intense itching, dry skin, infiltration of immune cells and skin lesions. To date, the cause of AE, a disease affecting 15-30% of children and 2-10% of adults, remains unknown and the pathomechanisms are not fully understood. Many factors, however, have been implied to contribute to this complex disorder, such as genetic predisposition, skin barrier defects, environmental allergens and inappropriate immune responses to microorganisms. The skin commensal yeast Malassezia has been suggested to contribute to the eczema, since approximately 50% of adult AE patients have specific IgE or positive skin prick test and/or atopy patch test against the yeast. This thesis has focused on the effect of the pH of AE skin on the allergenicity of Malassezia as well as the yeast s interaction with mast cells. In study I, we found that M. sympodialis produced, expressed and released enhanced amounts of allergens when cultured at a pH resembling AE skin compared to that of healthy individuals. One of the M. sympodialis allergens, designated Mala s 12, was selected for further investigation. In study II, we cloned, produced and characterized this allergen, which is expressed on the yeast s cell surface. We could determine that Mala s 12 had 30-50% sequence similarity to the glucose-methanol-choline (GMC) oxidoreductase enzyme superfamily and that recombinant Mala s 12 could be recognized by serum IgE from 62% of M. sympodialis-sensitized AE patients, indicating that Mala s 12 is a major allergen in this patient group. In the last two studies of this thesis, we investigated the interaction between M. sympodialis and mast cells. An increased number of mast cells have been found in the upper dermis of lesional AE patients skin and some mast cells even occur in the epidermis. In study III we determined that M. sympodialis can activate mast cells. More specifically M. sympodialis extract can stimulate non-sensitized and IgE-sensitized mast cells to release inflammatory mediators, increase IgE mediated degranulation, influence MAPK activation and alter the IL-6 production by signaling through the TLR-2/MyD88 pathway. In study IV we found that mast cells from AE patients contain an increased amount of granule mediators compared to mast cells from healthy individuals. AE patient derived mast cells also showed an enhanced response to M. sympodialis extract compared to mast cells from healthy individuals and were unable to up-regulate the fungal recognition receptor Dectin-1 upon IgE-receptor cross-linking. These observed differences indicate a differential role for MCs in AE patients compared to healthy individuals. In conclusion, M. sympodialis will release more allergens when cultured at a pH resembling that of AE skin, suggesting that the higher pH increases M. sympodialis allergenicity. Furthermore, mast cells can be activated by M. sympodialis and the activation is enhanced in mast cells from AE patients. Our findings will further help to elucidate the pathogenic mechanisms of AE and could contribute to the development of new treatment strategies for AE patients sensitized to M. sympodialis.