Molecular Interactions between a Recombinant IgE Antibody and the β-Lactoglobulin Allergen

Abstract

Allergies are caused by the immune reaction to commonly harmless proteins, allergens. This reaction is typified by immunoglobulin E (IgE) antibodies. We report the crystal structure of an IgE Fab fragment in complex with beta-lactoglobulin (BLG), one of the major allergens of bovine milk. The solved structure shows how two IgE/Fab molecules bind the dimeric BLG. The epitope of BLG consists of six different short fragments of the polypeptide chain, which are located especially in the beta strands, covering a flat area on the allergen surface. All six CDR (complementary-determining region) loops of the IgE Fab participate in the binding of BLG. The light chain CDR loops are responsible for the binding of the flat beta sheet region of BLG. The IgE epitope is different from common IgG epitopes that are normally located in the exposed loop regions of antigens and observed also in the two recently determined allergen-IgG complexes.

Crystal Structure of a Dimerized Cockroach Allergen Bla g 2 Complexed with a Monoclonal Antibody

“…The size of the allergen-antibody interface (ϳ870 Å 2 ) is similar to the interfaces in a variety of other antibody-antigen complexes (32). In particular, the size of the interface is comparable to those found in three other allergen-antibody complexes (10,11,33).…”

“…In fact, because of dimerization, one epitope in each allergen would be enough to cross-link IgE antibodies. Recently, the crystal structure of a complex of ␤-lactoglobulin and Fab from IgE antibodies of a combinatorial scFv phage display library showed that the allergen is a dimer, and that the Fab fragments are located in positions such that the dimeric ␤-lactoglobulin would be able to cross-link two IgE antibodies (11). Our results suggest that allergen dimerization plays a role in allergenicity to Bla g 2, by increasing IgE antibody cross-linking, in agreement with a study with engineered dimers of carrot allergen Dau c 1 (28).…”

“…Only three allergen-antibody complex structures have been resolved to date: birch pollen allergen, Bet v 1 (9) and bee venom allergen, Api m 2 (hyaluronidase) (10), both in the complexes with recombinant Fab fragments of IgG antibodies, and ␤-lactoglobulin in complex with Fab fragment of IgE (11). A majority of reports describing epitope mapping have not been based on the tertiary structure, but rather have used different approaches, such as libraries of overlapping synthetic peptides, e.g.…”

Close-up of the Immunogenic α1,3-Galactose Epitope as Defined by a Monoclonal Chimeric Immunoglobulin E and Human Serum Using Saturation Transfer Difference (STD) NMR

“…Moreover, the recognition of the ␣-Gal epitope by nature clearly contradicts the hypothesis deduced from crystal structures with proteins that IgE epitopes are preferably defined by large surface areas comprising several structural elements (45,46). This epitope and probably all types of CCD epitopes represent classical IgG epitopes.…”

“…Although at first realized using murine antibodies (42)(43)(44), co-crystals revealing the nature of authentic IgE epitopes remain scarce due to the lack of human monoclonal IgE antibodies. As a result, only two structures of allergen-IgE complexes are available so far (45,46). A peculiarity of anti-CCD antibodies is that effectual epitope is constituted by the carbohydrate itself and that it is closely related in humans and other animals.…”