Citrin: a novel food allergen in citrus seeds and citrus-derived pectin that shows cross-reactivity with cashew and pistachio.

BACKGROUND: Patients exquisitely sensitive to cashew/pistachio are at risk for allergic reactions to citrus seeds and pectin. OBJECTIVE: In this study, we sought to evaluate whether pectin is contaminated with citrus seeds, to identify a culprit antigen in citrus seeds, and to assess for cross-reactivity among allergens in citrus seeds, citrus pectin, and cashew or pistachio. METHODS: Proteins from orange seed coats, orange seed endosperms, lemon seeds, grapefruit seeds, citrus pectin, apple pectin, and grapefruit pectin were extracted. Protein concentrations in all extracts were determined and visualized using sodium dodecyl sulfate-polyacrylamide gel electrophoresis technique. Immunoglobulin E-binding capacity was determined with Western blot analyses and tandem mass spectrometry for the identification of the culprit allergen in citrus seeds and pectin. RESULTS: In subjects with citrus seed, pectin, and cashew allergies, there was strong immunoglobulin E-reactivity to bands between 17 to 28 kDa and 28 to 38 kDa. The tandem mass spectrometry analysis of these bands indicated the presence of citrin as the culprit allergen. Citrin and Ana o 2 are both 11S globulins belonging to the cupin superfamily, and significant homology was found between these proteins. CONCLUSION: Citrus pectin may be contaminated with citrus seeds. Citrin, a newly identified allergen in citrus seeds, seems to be the culprit antigen in citrus seeds and contaminated citrus pectin. Citrin is highly homologous with Ana o 2 in cashew and Pis v 2 in pistachio, suggesting potential for cross-reactivity and providing an explanation for co-allergenicity of cashew or pistachio, citrus seeds, and citrus pectin.

The major glycoprotein allergen from Arachis hypogaea, Ara h 1, is a ligand of dendritic cell-specific ICAM-grabbing nonintegrin and acts as a Th2 adjuvant in vitro.

Nonmammalian glycan structures from helminths act as Th2 adjuvants. Some of these structures are also common on plant glycoproteins. We hypothesized that glycan structures present on peanut glycoallergens act as Th2 adjuvants. Peanut Ag (PNAg), but not deglycosylated PNAg, activated monocyte-derived dendritic cells (MDDCs) as measured by MHC/costimulatory molecule up-regulation, and by their ability to drive T cell proliferation. Furthermore, PNAg-activated MDDCs induced 2- to 3-fold more IL-4- and IL-13-secreting Th2 cells than immature or TNF/IL-1-activated MDDCs when cultured with naive CD4+ T cells. Human MDDCs rapidly internalized Ag in a calcium- and glycan-dependent manner consistent with recognition by C-type lectin. Dendritic cell (DC)-specific ICAM-grabbing nonintegrin (DC-SIGN) (CD209) was shown to recognize PNAg by enhanced uptake in transfected cell lines. To identify the DC-SIGN ligand from unfractionated PNAg, we expressed the extracellular portion of DC-SIGN as an Fc-fusion protein and used it to immunoprecipitate PNAg. A single glycoprotein was pulled down in a calcium-dependent manner, and its identity as Ara h 1 was proven by immunolabeling and mass spectrometry. Purified Ara h 1 was found to be sufficient for the induction of MDDCs that prime Th2-skewed T cell responses. Both PNAg and purified Ara h 1 induced Erk 1/2 phosphorylation of MDDCs, consistent with previous reports on the effect of Th2 adjuvants on DCs.

Identification of sesame seed allergens by 2-dimensional proteomics and Edman sequencing: seed storage proteins as common food allergens.

BACKGROUND: Sesame seed allergy is becoming increasingly prevalent, probably because of its use in international fast-food and bakery products. Despite this fact, few studies have focused on the identification of its major allergenic proteins. OBJECTIVE: The aim of this study was to identify allergenic proteins of sesame seeds (Sesamum indicum). METHODS: Extracted sesame seed proteins were separated by means of SDS-PAGE and 2-dimensional (2-D) PAGE. Immunolabeling was performed with individual patient sera from 20 patients with sesame seed allergy. Selected proteins were further analyzed by means of Edman sequencing. RESULTS: IgE-binding proteins were identified at 78, 52, 45, 34, 32, 29, 25, 20, 9, and 7 kd. Analyzing internal sequences, the protein at 45 kd, which was recognized by 75% of the patients, was found to be a 7S vicilin-type globulin, a seed storage protein of sesame and named Ses i 3. The protein at 7 kd was found to be a 2S albumin, another seed storage protein of sesame and named Ses i 2. Seed storage proteins are known food allergens in peanut, walnut, Brazil nut, and soybean. Interestingly, one known IgE-binding epitope of the peanut allergen Ara h 1 has 80% homology with the corresponding area of Ses i 3. The different amino acids were previously shown not to be critical for IgE binding in Ara h 1. In addition, the proteins at 78 and 34 kd were found to be homologous to the embryonic abundant protein and the seed maturation protein of soybeans, respectively. CONCLUSION: The identification of 4 sesame seed allergens is the first step toward generating recombinant allergens for use in future immunotherapeutic approaches. In addition, the detection of conserved IgE binding epitopes in common food allergens might be a useful tool for predicting cross-reactivity to certain foods.