Major Cat Allergen Fel d 4: Structure and Identification of a Cross‐Reactive IgE ‐Epitope‐Containing Area

Background Allergic sensitization to cats and other furry animals is a major cause of asthma and allergic rhinitis in more than 200 million people worldwide. According to the frequency of IgE recognition, allergen‐specific IgE levels, and allergenic activity, Fel d 4 is a major allergen in the cat ( Felis domesticus ). The lipocalin allergen Fel d 4 is highly homologous to dog (Can f 6) and major horse (Equ c 1) allergens. Accordingly, IgE cross‐reactivity to these allergens contributes to polysensitization and allergic responses upon exposure to different animals. Methods Fel d 4 was recombinantly produced in two systems, E. coli and Expi293F mammalian cells. Recombinant forms were characterized by circular dichroism and mass spectrometry. The Fel d 4 3D structure was determined using X‐ray crystallography. Immunoreactivity, epitope analyses, and cross‐reactive properties were assessed by ELISA and basophil release assays using allergic patients’ sera. Results We reveal the rFel d 4 crystal structures and demonstrate that mammalian cells produce an N‐glycosylated recombinant Fel d 4 allergen. The C‐terminal regions of Fel d 4, Can f 6, and Equ c 1 constitute conformational IgE ‐epitope‐containing areas responsible for cross‐reactivity. Conclusion Uncovering the IgE ‐binding sites of Fel d 4 and cross‐reactive allergens contributes to future rational design of active and passive allergen‐specific treatment forms.