Prevalence and peculiarities of IgE reactivity to kiwifruit pectin methylesterase and its inhibitor, Act d 7 and Act d 6, in subjects allergic to kiwifruit

- Act d 6 and Act d 7 are two new kiwifruit allergens.
- They are homologs of the pollen allergens Pla a 1 and Sal k1.
- Few kiwifruit allergic subjects reacted by in vivo test.
- Specific IgE binding was observed in allergics and non-allergics by in vitro tests.
- A multiplex test system discriminated CCD specific IgE and glycosylated allergens.

Kiwifruit is an important cause of allergy. Although several allergens have been identified in this food (www.allergome.org), data on the number and clinical relevance of kiwifruit allergenic proteins are still fragmentary. With the aim of providing a contribution to the description of the complete kiwifruit allergome, we have investigated the immunological features of pectin methylesterase (PME) and its inhibitor (PMEI) purified from the natural source. Specific IgE to PME (Act d 7) and PMEI (Act d 6) were recorded in patients with kiwifruit allergies by immunoblotting, multiplex microarray-based immunoassay and skin testing (ST). Compared to the results obtained by the microarray system, the percentage of subjects detected positive to Act d 6 was much higher in the immunoblotting. Conversely, a similar percentage of subjects positive to Act d 7 was estimated in both the in vitro tests. Few subjects compared to in vitro testing gave a positive response on ST. In the case of Act d 7 this could be ascribed to the glycan side chain recognized by cross-reactive carbohydrate determinants (CCD)-specific IgE. In fact, Act d 7 fell in the cluster of glycoallergens (including Api m 1, Cup a 1, Hor v 17, Ole e 1, Pla a 2) immobilized on the microarray and recognized by CCD-specific IgE. A comparative analysis of different kiwifruit allergens (Act d 1, 2, 5, 6, 7, 11) revealed that only Act d 7 fell in this cluster, whereas Act d 2, although reported to be glycosylated, fell outside, thus suggesting the lack of sugars bound to this molecule. The subsequent biochemical characterization confirmed that Act d 2 is not a glycoallergen. In conclusion, Act d 7 and 6 seem to be rarely involved in causing symptoms in kiwifruit allergic patients. Nevertheless they are useful to understand a single patient profile and the mechanism of protein allergenicity. In addition, in this study the use of a multiplex test system has provided more information than expected, thus contributing to both the immunological and biochemical characterization of the immobilized allergens.