Based on our results, it can be anticipated that inclusion of recombinant Api g 2 in the current panel of allergens for molecule-based diagnosis will facilitate the evaluation of the clinical relevance of nsLTP sensitization in celery allergy and help clinicians in the management of food allergic patients.

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Scope Apium graveolens represents a relevant food allergen source linked with severe systemic reactions. We sought to identify an IgE‐binding nonspecific lipid transfer protein (nsLTP) in celery tuber. Methods and results A low molecular weight protein exclusively present in celery tuber was purified and designated Api g 6. The entire protein sequence was obtained by MS and classified as member of the nsLTP2 family. Api g 6 is monomeric in solution with a molecular mass of 6936 Da. The alpha‐helical disulfide bond‐stabilized structure confers tremendous thermal stability (Tm > 90°C) and high resistance to gastrointestinal digestion. Endolysosomal degradation demonstrated low susceptibility and the presence of a dominant peptide cluster at the C‐terminus. Thirty‐eight percent of A. graveolens allergic patients demonstrated IgE reactivity to purified natural Api g 6 in ELISA and heat treatment did only partially reduce its allergenic activity. No correlation in IgE binding and limited cross‐reactivity was observed with Api g 2 and Art v 3, nsLTP1 from celery stalks and mugwort pollen. Conclusion Api g 6, a novel nsLTP2 from celery tuber represents the first well‐characterized allergen in this protein family. Despite similar structural and physicochemical features as nsLTP1, immunological properties of Api g 6 are distinct which warrants its inclusion in molecule‐based diagnosis of A. graveolens allergy.

Identification of antibody binding epitopes is crucial to understand immunological mechanisms. It is of particular interest for allergenic proteins with high cross-reactivity as observed in the lipid transfer protein (LTP) syndrome that is characterized by severe allergic reactions. Art v 3, a pollen LTP from mugwort is frequently involved in this cross-reactivity, but no antibody binding epitopes have been determined so far.  To reveal human IgE binding regions of Art v 3, we produced three murine high-affinity monoclonal antibodies (mAbs), which showed 70-90% coverage of the allergenic epitopes from mugwort pollen allergic patients. As reliable methods to determine structural epitopes with tightly interacting intact antibodies under native conditions are lacking, we developed a straightforward NMR approach termed hydrogen/deuterium exchange memory (HDXMEM). It relies on the slow exchange between the invisible antigen-mAb complex and the free 15N-labeled antigen whose 1H-15N correlations are detected. Due to a memory effect, changes of NH protection during antibody binding are measured. Differences in H/D exchange rates and analyses of mAb reactivity to homologous LTPs revealed three structural epitopes: two partially cross-reactive regions around α-helices 2 and 4, as well as a novel Art v 3-specific epitope at the C-terminus. Protein variants with exchanged epitope residues confirmed the antibody-binding sites and revealed strongly reduced IgE reactivity. Using the novel HDXMEM for NMR epitope mapping allowed identification of the first structural epitopes of an allergenic pollen LTP. This knowledge enables improved cross-reactivity prediction for patients suffering from LTP-allergy and facilitates design of therapeutics.