Molecular and cellular mechanisms of allergic sensitization

Tree and weed pollen are common inducers of IgE-mediated allergies. Despite a detailed biochemical, structural and immunological characterization, it is still not entirely clear how sensitization to major pollen allergens occurs. The processes following inhalation of pollen, e.g. how pollen interact with the nasal mucosa, how and during which time period allergens pass nasal epithelia and induce the production of allergen-specific IgE, are still unsolved. Recent findings indicated that pollen allergens need to interact with other compounds in pollen, e.g. low molecular pollen-associated lipids, in order to initiate an allergic immune response. One major goal of this Austrian-Russian joint research project is the development of an animal model of allergic sensitization which mimics natural allergen exposure in man, i.e. inhalation of pollen grains. In contrast, most current animal models are based on intraperitoneal or subcutaneous injection of pollen allergens or pollen extracts. We will employ reporter mice which indicate allergy-promoting reactions by the appearance of fluorescent cells. Thereby, the analysis of the interaction of pollen compounds with epithelial cells, antigen-presenting cells and T cells in vivo is facilitated. In parallel, human epithelial cells, dendritic cells and T cells will be stimulated with pollen compounds and cellular responses will be assessed. Moreover, a special transwell system will be employed to study the transition of pollen compounds in vitro. It has been shown that pollen allergens can bind to low molecular lipids in pollen. Thus, emphasis will be laid on the role of protein-lipid-complexes in allergic sensitization. Recombinant candidate allergens will be produced and characterized for binding to different lipids. Binding constants, binding sites and the consequence of binding on structural, biochemical and immunological features of the allergens will be studied. All these experiments will be done by our Russian partners. We will employ the allergens and allergen-lipid-complexes in our models and assess the cellular reactions in vitro and in vivo. Hence, this joint project combines cellular allergology with insights into protein structure and lipid-protein interactions. Thereby, molecular features will be linked to clinical relevance. Together, this cooperation will enhance our understanding of the pathophysiology of IgE- mediated allergy which is not only relevant to better understand the onset of allergic disorders but also to develop improved concepts for their prevention and effective treatment.

IgE-mediated allergies affect approximately 25% of the population in industrialized countries such as Austria. The development of allergic diseases occurs in two main phases. The first phase, called sensitization, proceeds unnoticed and is completed when an individual produces allergen-specific IgE antibodies. The subsequent "effector phase" is characterized by the onset of typical allergic symptoms, such as hay fever, when allergen is encountered. Birch pollen is a common cause of IgE-mediated allergies in Northern and Central Europe. The most important sensitizing allergen in birch pollen is Bet v 1, and 93% of birch pollen allergic individuals exhibit Bet v 1-specific IgE antibodies. Although Bet v 1 is one of the most extensively studied allergens, the reason why this protein has such a strong sensitizing activity has not yet been elucidated. This research project sought to study special characteristics of Bet v 1 that may explain why it is a potent allergen. A human in vitro model was developed for this purpose, representing crucial steps in the sensitization process: allergen transport through the respiratory epithelium, uptake and processing by antigen-presenting cells, and subsequent activation of allergen-specific T cell cells. Our molecular and cellular studies indicated that immediately upon contact with the nasal epithelium, large amounts of Bet v 1 are released from birch pollen grains and rapidly pass through it. Dendritic cells internalize and process the transferred Bet v 1 and induce allergen-specific T cell responses. T cell activation was enhanced by additional components in birch pollen, as they altered the gene expression of nasal epithelia cells. We have also identified so-called "dominant" epitopes in Bet v 1 which induce T cell and antibody responses in many different birch pollen-allergic individuals. In summary, the results of this project will contribute to better understand the development of allergic diseases. A better understanding of the basic processes that trigger allergies is important for developing strategies for their prevention.