g206 Phl p 2

Summary

Phl p 2 is a major respiratory pollen allergen present in Timothy grass (Phleum pratense), which is being recognized by more than 200 million people suffering from allergy worldwide (1). Phl p 2 is a representative of the large family of cross-reacting grass pollen allergens classified as grass allergens group 2/3. (2). Phl p 2 is a 10- to 12-kDa non-glycosylated protein of 95–98 amino acid residue and has an average sequence identity of 60% with group 2 allergens from rye grass (Lolp II, LoZp III) (2).

Recombinant Phl p 2 (rPhl p 2) was produced in Escherichia coli (E. coli). It was observed that around 60% of grass pollen allergic patients displayed specific IgE reactivity with the recombinant allergen and elicit histamine release from the sensitized individuals (3). Recombinant form of Phl p 2 had been found to compete with the natural protein for binding to specific IgEs indicating that it is immunologically equivalent to the natural allergens (2, 3). IgE antibodies of allergic patients actually recognize the conformational epitopes present on Phl p 2 (1). Potential cross-reactivity of Phl p 2 with other grass species has not been reported due to the lack of the presence of conserved epitopes (1).

Pediatric populations suffering from moderate allergic rhinitis (AR) are found to contain higher levels of Phl p 2 than those suffering from mild AR (4). 

Epidemiology

Worldwide distribution

Phl p 2 is an allergen present in timothy grass pollen, which is being recognized by more than 200 million people suffering from allergy worldwide (1). A study used sera from European, Asian, and American subjects to assess the percentage of IgE directed to pollen extracts from nine different monocot species including rPhl p 1, rPhl p 2, rPhlp 5 of timothy grass and rBet v 2 of birch pollen. The study detected that among 68% grass pollen allergic patients, a high level of grass pollen-specific IgE (59%) sensitization occurred for the four recombinant allergens only (5). In a study in Sweden, ARC was found to be associated with timothy grass pollen allergens in adolescents (aged 13-15 years) with Phl p 1 being the key allergen sensitizer followed by Phl p 5, Phl p 4, Phl p 6, and Phl p 2 (6). Similarly, a multicenter study on children in Germany detected the level of IgE sensitization of timothy grass allergens to be highest in Phl p1, followed by Phl p 5, and Phlp 2 (7). In a pediatric European population, 32% were found to have positive skin prick test for Phl p 2 (8). Among grass pollen allergy patients in Brazil, 76% were found to be sensitized for Phl p 2 (9).

Environmental Characteristics

Source and tissue

Localization study detected that allergens of timothy grass pollen (including Phl p 2) are mainly located in the interior of the pollen grain, such as in the cytoplasm and sometimes within ribosome-rich areas (10).

Clinical Relevance

Specific molecules

Phl p 2 is one of the principal allergen components of timothy grass that is considered to be the major elicitors of grass pollen allergy symptoms (11). A study in Greece assessing specific IgE levels to the clinically relevant timothy grass pollen allergens detected the level of Phl p 2 to be significantly higher (p= 0.043) in children with moderate AR than in children with mild AR indicating the ability of Phl p 2 to induce allergic symptoms in AR patients (4).

Successful expression of Phl p 2 was done in E. coli to produce rPhl p 2, which competes with the natural Phl p 2 for binding to specific IgEs suggesting that rPhl p 2 is immunologically equivalent to the natural allergens (2, 3). rPhl p 2 is known to induce production of IgE specific to Phl p 2 in about 60% of grass pollen allergic patients leading to histamine release from these sensitized individuals (3, 12). Another study with 95% of the sera containing detectable IgE to timothy extract determined increased prevalence of IgE reactivity towards different recombinant allergen components (rPhl p 1, 2, 4, 5, 6, 7, 11,12) with prevalence for rPhl p 2 being 55% (13).

Studies reported about development of vaccine using non-allergenic peptides from the IgE-binding sites of four major timothy grass pollen allergens (Phl p 1, Phl p 2, Phl p 5, Phl p 6), since these 4 allergens are found to be the most relevant allergens in patients with grass pollen allergy (11, 14).

Molecular Aspects

Biochemistry

Timothy grass Phl p 2 is a 10- to 12-kD non-glycosylated protein of 95–98 amino acid residue exhibiting 85–90% sequence identity between species. There is about 60% and 65% sequence identity between Phl p 2 and allergens of rye grass Lollium perenne and orchard grass Dactylis glomerate, respectively (2, 15). Recently a homologous molecule of timothy grass Phl p 2 known as Sor h 2 had been identified in Johnson grass (which is a member of Panicoideae family) (16).

Successful expression of Phl p 2 was done in E. coli along with the determination of the crystal and solutions structures of rPhl p 2. This recombinant molecule has an ellipsoid shape with a stable 9-stranded β-sandwich similar to an Ig-fold. However, the molecule lacks the specific features of the core packing found in the immunoglobulin family. Phl p 2 has a C-terminal strand, which is missing in the Ig family; moreover, Phl p2 is devoid of the Ca2+- binding motif, a salient feature of C2 domain. This indicates that Phl p 2 allergen belongs to a unique structural family (15).

Isoforms, epitopes, antibodies

Phl p 2 has 1 isoallergen Phl p 2.0101 (17).

X-ray crystallographic study of three-dimensional structure of the complex between Phl p 2 and its specific human IgE-derived Fab revealed a conformational epitope made up of the planar surface of the four-stranded anti-parallel β-sheet of Phl p 2 and several isolated residues (1, 18). This indicated that IgE antibodies of allergic patients actually recognize the conformational epitopes present on Phl p 2 (1).

A study on pediatric population determined that in the preclinical sensitization phase, Phl p1, Phl p 5, and Phl p 4 were most frequently recognized; whereas Phl p 2 was recognized mostly in the post-onset years. Therefore during IgE sensitization process, Phl p 2 was recognized later than Phl p 1, Phl p 4, and Phl p 5 (19).

Cross-reactivity

Timothy grass pollen allergens (Phl p 1 and Phl p 2) and corn allergen (Zea m1) contain similar expansin structural motif that mediate cell wall expansion in plant. Common epitopes of Phl p 1, Phl p 2, and Zea m 1 are located on the protein portions of these expansins (18). This indicates that the structural similarity of conserved surface patches can explain the clinically observed cross-reactivities of allergens. Timothy grass pollen allergen Phl p 2 is dominated by charged and polar residues that form tight contacts (some of them by hydrogen bonds) to the antibody (18).

The alignment of Phl p 2 sequence with nine homologous sequences present in the C-terminal domains of group 1 grass pollen allergens portrayed that only three epitope residues are fully conserved indicating the lack of probable cross-reactivity between Phl p 2 with other group 1 grass pollen allergens (1).

Phl p2 and Phl p 3 show moderate sequence identity and extremely different isoelectric points (Phl p 2: 4.6; pI Phl p 3: 8.9). However, the three-dimensional structure of both allergens is highly similar and there exists variable cross-reactivity between these two allergens and the IgE of the allergic patients (20). In fact, Phl p 2 and Phl p 3 allergens signify a family of cross-reactive allergens that are able to replace one another for diagnosis as well as for use in allergen-specific immunotherapy (20).

Compiled By

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: October  2020