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Dimerization of the Major Birch Pollen Allergen Bet v 1 Is Important for its In Vivo IgE-Cross-Linking Potential in Mice¹

Isabella Schöll^*, Narayana Kalkura, Yuliya Shedziankova, Alexander Bergmann, Petra Verdino, Regina Knittelfelder^*, Tamara Kopp, Brigitte Hantusch^*, Christian Betzel, Karsten Dierks^¶, Otto Scheiner^*, George Boltz-Nitulescu^*, Walter Keller and Erika Jensen-Jarolim^2,*

^* Center of Physiology and Pathophysiology and Division of Immunology, Allergy, and Infectious Diseases, Department of Dermatology, Medical University of Vienna, Vienna, Austria; Institute of Medical Biochemistry and Molecular Biology, University of Hamburg, Hamburg, Germany; Division of Structural Biology, Institute of Chemistry, Karl Franzens University, Graz, Austria; and ^¶ Dierks & Partner, System Techniques, Hamburg, Germany

In type I allergy, the cross-linking of membrane IgE on B lymphocytes and of cytophilic IgE on effector cells by their respective allergens are key events. For cross-linking two IgE molecules, allergens need at least two epitopes. On large molecules, these could be different epitopes in a multivalent, or identical epitopes in a symmetrical, fashion. However, the availability of epitopes may be limited on small allergens such as Bet v 1, the major birch pollen allergen. The present work analyzes whether dimerization is required for the cross-linking capacity of this allergen. In immunoblots, murine monoclonal and polyclonal human Bet v 1-specific Abs detected, besides a Bet v 1 monomer of 17 kDa, a dimer of 34 kDa. In dynamic light scattering, Bet v 1 appeared as dimers and even multimers, but a single condition could be defined where it behaved exclusively monomerically. Small-angle x-ray scattering of the monomeric and dimeric samples resulted in diagrams agreeing with the calculated models. Circular dichroism measurements indicated that the structure of Bet v 1 was preserved under monomeric conditions. Skin tests in Bet v 1-allergic mice were positive with Bet v 1 dimer, but remained negative using the monomer. Furthermore, in contrast to dimeric Bet v 1, the monomer was less capable of activating murine memory B cells for IgE production in vivo. Our data indicate that the presentation of two identical epitopes by dimerized allergens is a precondition for cross-linking of IgE on mast cells and B lymphocytes.