D2.290 - Curcubitaceae allergy, a new enemy identificated in Watermelon

Cucurbitaceae allergy is uncommon and frequently attributed to profilin or lipid transfer protein. However, some patients show reactions within the family despite negative results for these panallergens, suggesting the involvement of alternative allergens. Watermelon (Citrullus lanatus) has only one officially recognized allergen (Citr l 2, profilin), but previous studies indicate the presence of additional allergens. This study aimed to identify the responsible allergen in a patient reacting selectively to Cucurbitaceae.

We present the case of a 26-year-old woman who experienced oral allergy symptoms with melon, watermelon, and cucumber, which all belong to the Cucurbitaceae family. She also experienced oral allergy symptoms when eating chufa, peanuts, walnuts, and kiwis, and suffered from springtime rhinitis. Skin prick testing was performed using watermelon, melon, cucumber, chufa, kiwi, peanut, and walnut. We analysed the patient´s specific IgE (sIgE) levels in response to these foods and common inhalant allergens.

To expand the molecular study, protein extracts were prepared from watermelon, melon and cucumber using validated methods. SDS-PAGE and IgE-Western blotting were performed in reducing and non-reducing conditions. A watermelon band was identified by LC-MS/MS.

SPTs were positive only for watermelon, melon, and cucumber. Serum sIgE was weakly positive for melon (0.23 kU/L) and Ole e 1, but negative for profilin and LTP. Western blot revealed a single and intense ~25 kDa band exclusively in watermelon (Figure 1). Proteomic analysis showed sequence homology with Glutathione-S-transferases V (GSTs) from Cucumis melo and Cucumis sativus, although no GST have been reported for Citrullus lanatus in UniProt. Minor interspecies sequence variations are sufficient to modify conformational epitopes, which explains why IgE binding occurred only in watermelon, despite the presence of homologous proteins in related fruits.

This case identifies a 25 kDa Glutathione-S-transferase–like protein as the probable allergen responsible for watermelon allergy in a patient sensitized to Cucurbitaceae. The combination of Western blot and proteomic analysis was essential to detect an allergen not included in current databases, highlighting the need to expand molecular characterization of Cucurbitaceae allergens.