Epitope Mapping of Cashew Allergens (Anacardium occidentale)

Epitope mapping is a crucial process in understanding how IgE antibodies recognize specific regions of cashew allergenic proteins. This helps in identifying cross-reactivity with other nuts and potential targets for hypoallergenic food processing and immunotherapy.

Overview of Cashew Allergenic Proteins and Their Epitopes

Cashew nuts contain multiple IgE-binding proteins, mainly Ana o 1, Ana o 2, Ana o 3, which contain linear (continuous) and conformational (discontinuous) epitopes.

Epitope Mapping of Major Cashew Allergens

Ana o 1 (7S Vicilin)

• Location of IgE-binding Epitopes:
  • Regions: Amino acids 55–75, 120–140, 200–225
  • Core IgE Binding Sites: D-E-F-G motifs
• Characteristics:
  • Contains both linear and conformational epitopes.
  • Highly cross-reactive with peanuts (Ara h 1), almonds, and walnuts.
  • Resistant to digestion by pepsin and trypsin, making it a potent food allergen.

Ana o 2 (11S Legumin)

• Location of IgE-binding Epitopes:
  • Regions: Amino acids 40–65, 150–180, 270–290
  • Core IgE Binding Sites: K-E-P-L motifs
• Characteristics:
  • Linear epitopes dominate this allergen.
  • Highly cross-reactive with pistachios, walnuts, and pecans.
  • Stable under heat and digestion, meaning roasted cashews remain allergenic.

Ana o 3 (2S Albumin)

• Location of IgE-binding Epitopes:
  • Regions: Amino acids 10–30, 90–110, 130–150
  • Core IgE Binding Sites: C-C-X-X motifs (disulfide-stabilized regions)
• Characteristics:
  • Highly stable allergen, resistant to heat and digestion.
  • Strongest IgE binding affinity of all cashew allergens.
  • Cross-reacts with peanuts (Ara h 2, Ara h 6) and Brazil nuts (Ber e 1).
  • Even trace amounts in food can trigger anaphylaxis.

Heat and Enzymatic Stability of Cashew Epitopes

Key Observations:

• Ana o 3 (2S Albumin) is the most stable allergen, surviving heat and stomach digestion.
• Processing (roasting, grinding) can increase allergenicity by exposing more IgE-binding sites.

Structural Analysis of IgE-Binding Sites

3D Structural Epitope Mapping

• Using X-ray crystallography and molecular docking, scientists have identified high-affinity IgE binding pockets in Ana o 1, Ana o 2, and Ana o 3.
• Conformational Epitopes:
  • Found mainly in Ana o 1.
  • Require proper protein folding to be allergenic.
  • Denaturation (extreme heat or enzymatic cleavage) can reduce allergenicity.
• Linear Epitopes:
  • Present in Ana o 2 and Ana o 3.
  • Stable and resistant to food processing.

Cross-Reactivity with Other Allergens

Cashew epitopes share sequence homology with allergens from other tree nuts, peanuts, and pollen.

Implications for Allergen Detection and Immunotherapy

Diagnostic Testing

• Epitope-based IgE assays:
  • Advanced component-resolved diagnostics (CRD) can identify which specific cashew epitopes trigger reactions.
• Skin Prick Test (SPT) vs. Serum IgE Testing:
  • SPT: Tests whole cashew extract, less specific.
  • Serum IgE (ImmunoCAP, ELISA): Identifies specific allergenic peptide sequences.

Immunotherapy (Desensitization)

• Targeting Linear Epitopes:
  • Potential hypoallergenic variants could be engineered by modifying epitope sequences.
• Peptide-Based Vaccines:
  • Small, non-IgE-binding peptide fragments may be used in oral or sublingual immunotherapy (OIT/SLIT).
• Epitope Masking via Enzymatic Processing:
  • Certain food processing methods (fermentation, enzymatic hydrolysis) may reduce allergenicity by disrupting IgE-binding regions.

Conclusion

• Cashew allergenic proteins (Ana o 1, Ana o 2, Ana o 3) contain both linear and conformational IgE-binding epitopes.
• Ana o 3 is the most stable allergen, resistant to heat and digestion, making it a major cause of severe reactions.
• Cross-reactivity with peanuts, pistachios, and walnuts is due to shared epitope sequences.
• Epitope mapping enables improved diagnostics and potential therapies through hypoallergenic protein modifications and immunotherapy.