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At present several cross-reactive groups of pollen allergens have been identified: 1. The PR 10 Family 2. Profilins 3. Pollen Calcium-binding Proteins or Polcalins 4. Oleaceae Pollen Group 1 Allergen 5. Thaumatin-like Protein or the PR-5 Group 6. Grass Pollen 1 Allergen or the β-Expansins 7. Group 1 Allergen of the Ragweed Pollen 8. PR-12 Group 1 allergens of the Compositae Pollen 9. Cyclophilins 10. Other Allergens.
1. The PR 10 Family:
These are the group I allergens of the Fagales family. Several allergens have been identified namely Bet vI(birch), CoraI(hazel), Aln gI(alder), Car bI(hornbean), etc. These allergens exist in multiple isoforms showing a high degree of sequence homology. They are responsible for cross-reactivity with homologous proteins in apple, cherry, apricot, pear, hazelnut, carrot, celery and other vegetables.
Bet vl is the best characterized protein of this group. Its B-cell epitopes are deduced, which consists of a seven- stranded antiparallel β-sheet that wraps around a 25-residue long C-terminal α-helix. The β-sheet and the C-terminal part of the long helix are separated by two consecutive helices. Bet v I allergen shows a significant homologies to the tree pathogenesis-related proteins which are induced in response to stress conditions of the plant.
2. Profilins:
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Profilins are involved in the regulation of actin polymerization and signal transduction of phosphatidyl inositol pathway. Profilin is considered as a ubiquitous cross-reactive plant allergen. It is believed that IgE cross-reactivity is mostly because of the highly conserved 3D structure of the Profilins, rather than similarity at the amino-acid level.
Despite extensive cross-reactivity among themselves and with the human homologue as Well; it seems that a large proportion of IgE reactivity to profilins is clinically irrelevant. This has partially been explained that Chymase exocytosed by the mast cell cleaves human and birch pollen profilins, thus reduce the IgE- binding activity.
3. Pollen Calcium-binding Proteins or Polcalins:
Calcium-binding allergens containing 2 EF- hands ( Bet vIV, Aln gIV, Ole e 111, Cyn dVII, Phi pVII and Bra rl) and 4 EF-hands (Jun o IV and Ole e VIII) domains has been identified as pollen- specific cross-reactive proteins. The IgE recognition of Calcium-binding allergens seems to be conformational and influenced by the bound Calcium.
4. Oleaceae Pollen Group 1 Allergen:
Ole el, the major allergen of the Olive pollen, is a polymorphic glycoprotein with three disulphide bonds. It has several homologues in other pollen such as Lig vl, Fra el, etc., responsible for wide-range cross-reactivity among Oleaceae.
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The IgE-recognition of Ole el depends on the integrity of the disulphide bonds. Moreover, the glycan moiety is able to bind IgE and seems to induce the release of histamine from basophils.
5. Thaumatin-like Protein or the PR-5 Group:
This group of allergens has been characterized from several fruits and from pollen like mountain Cedar (Jun a III). Sequence similarities among thaumatin-like allergens indicate a potential for IgE-cross-reactivity.
6. Grass Pollen 1 Allergen or the β-Expansins:
Groupl pollen allergens (β-expansins) of the grass family (Poaceae) are the most prominent allergens in the pollen of grasses and have been characterized in at least 19 grass species. They are polymorphic, N-glycosylatcd and contain seven conserved cysteine residues in the N- terminal of the protein. Biologically, expansions are a family of protein involved in cell wall loosening and extension.
Purified natural and recombinant group 1 grass pollen allergens (Phi pi, Lol pi) show extensive IgE-mediated cross-reactivity among the members of this group.
7. Group 1 Allergen of the Ragweed Pollen:
These are a group of closely related proteins. Allergens, Amb a II-IV from short ragweed and Amb t V from giant ragweed, are the most extensively studied pollen allergens. Recently a third homologue, Amb p V has been identified from western ragweed. The Amb Vs are the smallest pollen allergens so far recorded, consisting of a single polypeptide chain with no detectable carbohydrate or lipid.
The allergen contains a small segment of antiparallel β-sheet, a C-terminal α-helix, and several loops. A rigid core is present at the interface of the helix and sheet which is comprised of hydrophobic residue and two disulphide bonds. These allergens along with other homologous allergens in Cypress and Cedar pollen (Cryjl) show sequence homology to Pectate lyase enzyme (wall degrading protein).
8. PR-12 Group 1 allergens of the Compositae Pollen:
The major allergen of the mugwort (Artimisia vulgaris) pollen Art vl, is an example of this group. It has a group of secreted proteins with an N-terminal cysteine-rich domain homologous to plant defensins and a C-terminal hydroxyproline-rich region.
9. Cyclophilins:
Cyclophilins are cyclosporin-binding proteins, present in almost all organisms, from archebacteria to plants and mammals. Several fungal allergens (Mai sVI, Aspfll) and Bet vVII, a minor allergen of the birch (Betula verrucosa) pollen are included in this group. These allergens are highly conserved in relation to their phylogeny and have been designated as a growing family of cross-reactive pan allergens.
10. Other Allergens:
In addition to the above mentioned pollen allergens, there are other groups of allergens.
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They are:
1) Enolases;
2) Manganese-superoxide dismutase;
3) β-1,3-Glicanases (PR-2);
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4) Class 1 (basic) chitinase or PR-3 proteins;
5) Non-specific lipid-transfer proteins (PR- 14);
6) Plant cysteine proteases;
7) Plant albumins and globulins;
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8) Avian proteins, like Gal d 1-IV,
9) Mammalian proteins, like Bos dIV, Bos dV, Bos dVII, α- lactalbumin, β-lactalbumin and Casein;
10) Mite group I allergens (Cysteine protease);
11) Mite group II allergens;
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12) Tropomyosine, e.g., Shrimp allergen (Pen al), Cockroach allergen (Per a VII);
13) Parvalbumin(Calcium-binding protein allergen of fishes);
14) Hyaluronidases(Phospholipase Al and A2 of Vespids).
A list of important and well-characterized pollen allergens are summarized in the Table 10.7.
