Are Ca2+-binding motifs involved in the immunoglobin E-binding of allergens? Olive pollen allergens as model of study

Several Ca2+-binding proteins, which possess EF-hand sites with a high sequence similarity, have been found to be able to induce Type-I allergy.

To study whether the common EF-hand sequential motifs can be involved in the IgE-reactivity of these proteins, thus being responsible of a degree of cross-reactivity among different Ca2+-binding proteins.

Two olive pollen allergens, Ole e 3 and Ole e 8, have been used in the study. Parvalbumin and calmodulin were included in immunological analyses. Sera from patients allergic to olive pollen, as well as Ole e 3- and Ole e 8-specific rabbit antisera were used in indirect enzyme-linked immunosorbent assay (ELISA), ELISA inhibition assays and immunoblotting. Conformational analyses (circular dichroism spectra and thermal stability) and specific immunodetection assays were performed in the presence and the absence of Ca2+. Chemical breakdown and high-performance liquid chromatography (HPLC) was used to obtain fragments from Ole e 3 containing a single EF-hand motif.

Thirty-four (17%) and 16 (8.2%) out of 195 sera from patients allergic to olive pollen contained specific IgE against Ole e 3 and Ole e 8, respectively. The IgE-binding of 12 allergic sera diminished up to 22% for Ole e 3 and to 82% for Ole e 8, when depleted Ca2+. A pool of these sera recognized the two olive allergens and parvalbumin, but at very different extent. Inhibition of the IgE-binding was only achieved between two olive allergens. No structural relationships between Ole e 3 and Ole e 8 were established when specific polyclonal antisera against both proteins were used.

EF-hand Ca2+-binding sites can not be considered as general allergenic motifs responsible for the cross-reactivity between Ca2+-binding allergens. Different families of Ca2+-binding allergens have specific epitopes that could be involved in the cross-reactivity among members of the same family.