Olive (Olea europaea L.) pollen produces respiratory allergy in the populations of geographical areas where this plant is widely cultivated, mainly the countries surrounding the Mediterranean Sea and several regions of North, South America and Australia 1 2 . Up to date, 10 allergens, i.e. Ole e 1 to Ole e 10, have been purified and characterized in Olea europaea L. pollen 3 4 . The major allergen Ole e 1 is a protein of 18–22 kDa, displaying acidic pI and different forms of N-glycosylation 5 6 . Besides its high homology to its counterparts in other members of the Oleaceae family, such as lilac and privet 7 8 , Ole e 1 exhibits relevant similarity with the products of genes Lat52 from tomato, Zm13 from maize and several extensins from Arabidopsis thaliana and Nicotiana tabacum among other proteins. These proteins have been indexed like members of the denominated "pollen proteins of the Ole e 1 family" (Accession number: PF01190) within the Pfam protein families database 9 . These plant pollen proteins are structurally related. They are most probably secreted and consist of about 145 residues. There are 6 cysteines conserved in the sequence of these proteins which seem to be involved in disulphide bonds. Although their biological function is not yet known, they have been suggested to be involved in important events during pollen formation, such as hydration, germination and/or pollen tube growth 10 11 12 13 .

Pollen from different olive cultivars considerably differ in their capacity to bind IgE antibodies and to elicit an allergenic response, as tested by different in vivo and in vitro techniques 14 15 16 17 . Noteworthy, quantitative differences in the content of Ole e 1 have been described in the pollen of several olive cultivars, which are positively correlated with the amounts of Ole e 1 transcripts present in these pollens 17 . Although these quantitative differences are likely responsible for the different reactivity of patients to the pollen from different cultivars, the concomitances of other factors can not be discarded. In this context, the role of other major and secondary allergens from olive pollen, and the presence of divergent epitopes of Ole e 1 in the different cultivars should be evaluated.

Ole e 1 presents numerous microheterogeneities in its nucleotide sequence 18 19 . These nucleotide substitutions result in many cases in amino acid changes in the natural protein 20 , which have been reported to influence the binding of IgE molecules to Ole e 1 proteins 21 . However, up to date, the presence of such polymorphism has never been related to the genetic origin of the pollen tested. On the contrary, commercially available pollen from uncertain origin or pollen mixes from different sources seems to be the common material used for biochemical, molecular and immunological characterization of allergens. The presence of an extremely wide germplasm in the olive, with over 1200 varieties cultivated over the world 22 clearly point to this genetic variability as a putative cause of inconsistency for Ole e 1 sequence. This work explores the molecular variability of Ole e 1 allergen throughout a number of olive cultivars and discusses putative implications of such polymorphism in both the biology of the plant and the development of the allergic symptoms.

Polymorphism is a frequent feature for allergens molecules. Different degrees of polymorphism have been described in the allergens of many different sources, including house dust mites 26 , foods 27 28 and allergens from different grass and tree pollens 29 30 . However, although allergen polymorphism is beginning to be deeply characterized at the molecular level, little is known in many cases regarding the origin of such polymorphism. In several cases, allergen polymorphism has been attributed to the presence of multigene protein families 31 . In other allergens, the presence of post-translational modifications may also determine the presence of multiple forms of the allergen, as is the case of Ole e 1 32 . In apple (Malus domestica), up to 18 Mal d 1 genes have been characterized. Allelic diversity regarding this allergen has been considered as a major explanation for the considerable differences in allergenicity, widely described among apple cultivars 28 . The high rate of nucleotide substitutions detected in Ole e 1 indicates that these substitutions are unlikely to represent in vitro artefacts as the result of error incorporations by the Pfu polymerase used. The present paper demonstrates that Ole e 1 polymorphism is clearly linked to the cultivar (genetic) origin of the pollen analyzed, and therefore represents the basis for further research in relation to the biological function and the allergenicity of this gene product.

It was previously reported by us 17 that pollen from different olive cultivars exhibit quantitative differences in the content of Ole e 1, which were positively correlated with the amounts of Ole e 1 transcripts present in these pollens. The results shown here also indicate that qualitative differences in the Ole e 1 allergen also occur in these pollens. Three major parameters have been demonstrated to influence the reactivity of Ole e 1 to IgG and IgE antibodies in a higher or lower degree 21 : three-dimensional (3D)-folding, the glycan component and several point changes of the amino acid sequence. The present paper reports, on the basis of the sequences obtained, that modifications in these features can be related to the genetic origin of the pollen analyzed.

Sequence substitutions in Ole e 1 from different varieties are not equally distributed over the whole sequence. Conserved regions include those corresponding to the conserved Cysteine residues at positions 22, 43, 78, 90 and 131. The absence up to date of a 3D model for Ole e 1 or any Ole e 1-like protein with significant identity to this protein within the PDB databases makes impossible to determine precisely the putative involvement of amino acid substitutions into conformational changes of the protein, which may affect either the biological function or the IgG/IgE binding ability of the protein. However, we may speculate about the potential changes. Substitution of 19C may dramatically alter tertiary structure of the Ole e 1 protein, as the presence of a disulphide bond between 19C–90C in Ole e 1 was previously established 33 . The conformation proposed by these authors is supposed to be shared by most members of proteins of the Ole e 1 family, where an identical distribution of the 6 cysteine residues is widely maintained, no matter their phylogenetic distances or relationships. As a consequence, important modifications in the reactivity of antibodies to conformational epitopes might be expected, as the binding of Ole e 1 to human IgE antibodies has been shown to be highly dependent on the integrity of the disulphide bridges 21 . The putative absence of the 19C–90C disulfide bridge in some Ole e 1 forms, could also represent an additional hint against the proposed homology between the Kunitz protein family and Ole e 1-related pollen proteins 34 35 , since disulfide arrangement is a typical feature for the classification of the protease inhibitor protein family 36 . The presence of free – SH groups (corresponding to the 90C) in these Ole e 1 forms with substitutions in the 19C residue, and therefore unable to form the 19C–90C intramolecular bridge, could alternatively promote the formation of dimeric forms of the protein throughout the formation of intermolecular disulphide bridges. It has been described that the SDS-PAGE pattern of Ole e 1 frequently exhibits a faint band of 40 kDa as a result of the presence of a 20 kD dimer 20 , which may account up to 5% of Ole e 1. Our observations show that these dimeric forms of Ole e 1 are relatively more abundant in the extracts of Bella de España and Arbequina than in the rest of cultivars examined. Monomeric and dimeric forms of Ole e 1 also react differentially to a panel of monoclonal antibodies to Ole e 1 and to sera from olive-allergic patients (unpublished results).

The role of the glycan moiety of Ole e 1 in antibody binding has also been established 6 7 21 37 . These authors concluded that carbohydrate moieties may constitute determinants possessing a relative significance in the binding of IgE from hypersensitive patients, not only in the Ole e 1 allergen but also in other allergenic glycoproteins. In this context, the presence of modifications, such as the putative presence of an extra site for N-glycosylation which is reported here for the cultivar Bella de España may represent an important modification in the allergenic capability of the Ole e 1 protein of this pollen, which should be further tested.

Several examples are available throughout the literature demonstrating that sequence changes need not be extensive to significantly alter the immunological properties (both antigenicity and allergenicity) of an allergen: genetic variation of Der p 2 produces striking effects on T cell responses and IgE binding 38 . Regarding pollen allergens, recombinant Bet v 1 allergen shows reduced IgE binding compared with its highly allergenic Bet v 1a counterpart, from which only differs in nine residues 39 40 . On the opposite hand, recombinant rOle e 1/3c isoform of Ole e 1 21 displays the highest degree of IgE-binding in comparison with rOle e 1/5c, rSir v 1 and rLig v 1 forms (the latest from Syringa vulgaris and Ligustrum vulgare, respectively), which present a progressively higher number of substitutions with respect to the amino acid sequence determined for Ole e 1 by Edman degradation 18 19 20 . Experimental evidence of the importance of certain positions of the amino acid sequence of Ole e 1 for both IgG and IgE binding and the elicitation of the allergenic response has been obtained since the nineties 21 24 25 41 . As the result of these studies, B-cell, T-cell epitopes and IgE and IgG B-cell immunodominant regions of Ole e 1 have been determined. It is foreseeable that the described substitutions affecting these regions will have a particular contribution in putative changes in the allergenicity/immunogenicity of the protein.

Furthermore, the induction of minor changes and the study of variants within an allergen is a commonly proposed approach to the engineering of hypoallergens. In accordance with the results showed here, some modified allergens, or even hypoallergenic forms of Ole e 1 could naturally occur in some olive varieties. An important evidence of this approach has recently emerged after the description of the generation and further immunological testing of three hypoallergenic mutants of Ole e 1 42 . Two out of the three mutants generated by site-directed mutagenesis of Ole e 1-specific cDNA and produced in Pichia pastoris (135Δ10 and 140Δ5 deletion mutants) were able to strongly reduce the IgE-binding capability of sera from olive pollen-allergic patients. In addition, the 135Δ10 mutant was able to maintain intact its T cell reactivity and to induce blocking antibodies. From the sequences analyzed in the present paper, those three corresponding to Bella de España showed point substitutions in 135Y, and five sequences (three from Bella de España, one from Picual, and one from acebuche) displayed substitutions throughout the 144N and the 145M positions. On the basis of this coincidence in the altered regions of Ole e 1, the allergen isoforms of Bella de España could be considered as potential hypoallergenic forms of natural origin. Experimental data obtained in our laboratory have shown that the Ole e 1 variants from Bella de España pollen display reduced binding to IgEs from olive-allergic patients in Western blotting experiments, when compared to Ole e 1 forms from other olive cultivars (unpublished results). Future analysis of epitope reactivity should include putative modifications in other immuno-relevant regions of the protein which should also comprise the N-terminus of Ole e 1, whose sequence has not been analyzed in this work.

The presence of differences in the reactivity to Ole e 1 displayed by patients from different geographical origin in Spain 3 43 44 could be explained by the presence of Ole e 1 polymorphism linked to the varietal origin of the pollen present in the atmosphere, as the major olive varieties are precisely and discretely distributed over geographic regions 45 . A similar finding has been described for major house dust mite allergens, where the predominant variants of Der p 2 have been found to be distinct in different regions 26 .

The analysis of the sequences showed here indicates that the Ole e 1 sequences from varieties like Bella de España display a higher degree of identity with Ole e 1-like proteins from Oleaceae than with the "canonical" sequence of Ole e 1 obtained after Edman degradation 20 or other Ole e 1 sequences described in the literature or present in databases. Important implications of such relationships, particularly regarding the presence of cross-reaction phenomena are expected: i.e. the exposure to pollen from such olive varieties (mainly anemophilous, and therefore highly represented like an aeroallergen in broad areas) may spread, or even trigger patient's reactivity to pollens from mainly entomophilous, and only locally represented plants species with low frequency of allergic sensitization like Ligustrum, Fraxinus, Syringa and other Oleaceae 46 47 .

A large variety of morphological, biometric, biochemical and molecular characters have been widely used to describe and characterize olive germplasm 48 . Molecular markers include RFLPs, RAPDs, AFLPs, micro- and mini-satellite sequences. The low level of intra-varietal polymorphism showed by Ole e 1 sequences, compared to the higher polymorphism present among cultivars could be derived to the use of allergen sequences as molecular marker for olive breeding purposes. As an example, Ole e 1 sequences could be used as an additional marker in order to complement the discrimination between closely related olive cultivars, where other markers produce no or little differential patterns. The present study shows how two very closely related cultivars are recognized like different, on the basis of Ole e 1 similarity. This is the case of cultivars Picholine marocaine and Menara, the later considered a clonal selection of the former, raised in Morocco on the basis of better production, sooner come into bearing, and easier propagation from cuttings 49 . The increasing number of molecular markers available will help to elucidate important questions regarding the olive culture, which remain unanswered. Conflicting hypotheses exist about the phylogenetic relationships between Olea europaea L. and Olea-related species, between wild and cultivated forms of Olea europaea L. and about the origin of the culture 50 . Other biological implications of the allergen polymorphism in olive pollen with regard to pollen production and performance have been recently addressed 51 .

Although much effort must yet be put into the characterization of olive pollen allergen polymorphism in relationship to the varietal origin, the advantages of the knowledge generated are obvious, and a number of applications are foreseeable. The study of natural variants will lead to the development of methods for improving efficacy and safety of the extracts currently in use for diagnostic and therapeutic purposes, as well as the routine trials for quality control of many laboratories. New strategies for the design of hypoallergenic variants will be considered and natural pollen extracts from different varietal sources could be used in order to further personalize patient's reactivity or like natural hypoallergenic extracts, as an alternative or in addition to the use of recombinant allergens. Finally, advances in the breeding of hypoallergenic cultivars might also be directed by a broader knowledge of the genetic basis of this variability. The clinical implications of allergen polymorphism in the olive pollen are examined in detail in 51 , in particularly how cultivar differences may affect extract quality, diagnostic and therapeutic efficacy and safety, and the development of new vaccines.

The sequence polymorphism present in the olive pollen major allergen Ole e 1 is highly dependent on the genetic origin of the pollen analyzed. Predicted Ole e 1 proteins from different olive cultivars are likely to display different qualitative properties. These differences have a number of implications in olive pollen biology. They also should be considered for optimal design of allergen formulations, including the design of recombinant allergens, in order to improve both their efficiency and safety.

JDA and MIRG designed the study. AMHK and AJC designed the experiments. AMHK, AJC and JCJL cloned and obtained the sequences. JDA and AMHK analyzed the sequences. JDA, MIRG and AMHK prepared the manuscript.