IgE epitopes on the cat (Felis domesticus) major allergen Fel d I: a study with overlapping synthetic peptides.

BACKGROUND

The major cat allergen Fel d I is composed of two disulfide-linked polypeptide chains, chain 1 (70 amino acid residues) and chain 2 (92 amino acid residues). Reduction and alkylation of Fel d I eliminates almost all antigenic and allergenic activity, and detection of linear epitopes with synthetic peptides is therefore not expected.

METHODS

We synthesized synthetic peptides of both chains of about 14 amino acid residues, overlapping by 7 residues. The peptides were coupled to Sepharose (Pharmacia, Uppsala, Sweden) and tested with sera of patients with cat allergy.

RESULTS

Three peptides showed specific binding of human IgE, residues 25-38 and 46-59 of chain 1 and residue 15-28 of chain 2. IgE binding was inhibited by Fel d I and the corresponding peptide. Of 61 patients with cat allergy tested, 65% showed IgE binding to at least one of the peptides; 46% showed IgE binding to peptide 25-38, 11% to peptide 46-59, and 28% to peptide 15-28. Each peptide was recognized by only one of the 78 patients with negative RAST results. By affinity chromatography with peptide-Sepharose anti-Fel d I antibodies were isolated, also confirming the specificity of IgE binding to the peptides. The percentage of IgE antibodies against Fel d I reactive with the peptides varied with the serum and the peptide-Sepharose used and ranged from 2% to 55%.

CONCLUSIONS

Because the affinity of IgE binding to the peptides was very low and only serum samples with high titers of Fel d I-specific IgE antibodies (RAST 4+/5+) showed significant binding, these peptides are not suitable for diagnostic purposes. However, the peptides are useful tools for comparing IgE and IgG responses and for studying the relationship to the T-cell epitopes on this molecule.