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

## TL;DR

This study identifies a key region in the cat allergen Fel d 4 that causes allergic reactions and cross-reacts with allergens from dogs and horses.

## Contribution

The study reveals the 3D structure of Fel d 4 and identifies a cross-reactive IgE-epitope in its C-terminal region.

## Key findings

- The C-terminal region of Fel d 4 contains a major IgE-reactive epitope.
- Fel d 4 shares cross-reactivity with allergens Can f 6 and Equ c 1 due to structural similarities.
- Mammalian cell-produced rFel d 4 is N-glycosylated and structurally characterized.

## Abstract

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.

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.

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.

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.

This study reports the three‐dimensional structure of an important cat allergen, Fel d 4, that shares high homology with Equ c 1 (i.e., major horse allergen) and Can f 6 (i.e., dog allergen).The C‐terminal region of the Fel d 4 contains a major IgE‐reactive epitope.The C‐terminal epitope‐containing area plays a role in the cross‐reactivity between Fel d 4, Equ c 1, and Can f 6.Abbreviations: ELISA, enzyme‐linked immunosorbent assay; IgE, immunoglobulin E; OD, optical density; PDB, Protein Data Bank; RBL, rat basophil leukemia; sIgG, specific IgG.

This study reports the three‐dimensional structure of an important cat allergen, Fel d 4, that shares high homology with Equ c 1 (i.e., major horse allergen) and Can f 6 (i.e., dog allergen).

The C‐terminal region of the Fel d 4 contains a major IgE‐reactive epitope.

The C‐terminal epitope‐containing area plays a role in the cross‐reactivity between Fel d 4, Equ c 1, and Can f 6.