rBet v 1 PR-10, Birch

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Code: t215
Latin name: Betula verrucosa
Common names: PR-10 protein

Birch allergen components

Available ImmunoCAP:

  • t215 rBet v 1
  • t216 rBet v 2; profilin
  • t220 rBet v 4
  • t225 rBet v 6

Summary

Recombinant allergen components from pollen of birch are available for allergen-specific IgE antibody testing.

Recombinant allergens, which are biotechnology produced protein molecules originally identified from allergen extracts, have immunoglobulin E (IgE) antibody binding comparable to that of natural allergens and generally show excellent reactivity in in vitro and in vivo diagnostic tests (1). To date, many different recombinant allergens have been cloned, sequenced and expressed.

Recombinant allergens have a wide variety of uses, from the diagnosis and management of allergic patients to the development of immunotherapy to the standardisation of allergenic test products as tools in molecular allergology.

Recombinant allergens are particularly useful for further investigations in allergies manifesting wide cross-reactivity, such as allergy to birch pollen, which frequently involves cross-reactivity among pollens of trees belonging to the order Fagales (e.g., Fagaceae, Corylaceae, and Betulaceae) (2). Birch pollen is considered to be the most powerful allergen in this complex (3).

Allergens from Betula verrucosa listed by IUIS*
Bet v 1 Bet v 2 Bet v 3
Bet v 4 Bet v 6 Bet v 7
*International Union of Immunological Societies (www.allergen.org) Jan. 2008.
 

t215 rBet v 1

Recombinant non-glycosylated protein produced in an E. coli strain carrying a cloned cDNA encoding Betula verrucosa allergen Bet v 1

Common name: PR-10 protein
Biological function: Pathogenesis-related protein, ribonuclease (23)
Mw: 17 kDa
Other allergens isolated: rBet v 1a and rBet v 1d isoforms. These isoforms differ in their ability to bind IgE but are similar in their immunogenicity for T cells (24-25)

Allergen description

The major allergen of birch tree pollen is Bet v 1 (4-22). Recombinant Bet v 1 was among the first allergen-encoding cDNAs isolated and has significant sequence homology to a group of pathogenesis-related plant proteins and has been classified as a PR-10 protein. Recombinant Bet v 1 has been shown to bind IgE in most birch-pollen allergic patients (20-22). In one study, the accuracy of in vivo and in vitro diagnosis of birch pollen allergy by means of rBet v 1 was > 95% (49/51) (22). Nevertheless, differences in IgE antibody reactivity to rBet v 1 and rBet v 2 were demonstrated among allergic patients from 6 countries. The complexity of reactivity tended to be greater in individuals from the central and southern parts of Europe than from Sweden and Finland (26).

Several cross-reactive Bet v 1-homologoues are major allergens of Fagales pollen (Alder, Hazel, Hornbeam) and taxonomically related fruits, vegetables, and spices (e.g., Carrot, Celery, Apple, Apricot, Cherry, and Pear). This seems to be clearly related to the clinical pollen/food cross-reactivity found in oral allergy syndrome (OAS) (22,27). Studies have suggested that Bet v 1 is the initial sensitising allergen in many cases of Fagales pollen allergy and Birch pollen-related plant-food allergy (OAS) at least in areas where birch trees are common, as in Northern Europe (22,27). Population studies affirm that Bet v 1 is a marker allergen for genuine  sensitisation to Fagales pollen- and Birch pollen-related food allergy (22,27-28).

Bet v 1 is recognised by IgE antibodies from about 95%of Birch-allergic patients Bet v 2 and Bet v 3 from10% and Bet v 6 by approximately 32% 29. The sensitisation profiles to Bet v 1 and Bet v 2 differ among geographical areas. Bet v 2, a profilin and a minor allergen, has also been shown to be involved in cross-reactivity to certain foods.

It has been suggested that Bet v 1 can be a diagnostic marker allergen for identifying patients with genuine sensitisation to birch-pollen (30), whereas more highly cross-reactive allergens, such as Bet v 2 and Bet v 4, may serve as marker allergens for syndromes involving cross-reactivity with unrelated plants/plant products (3,30). Accordingly, patients who exhibit positive skin tests to birch pollen extracts but have never been exposed to Birch might be considered to have IgE antibodies to cross-reactive allergens such as profilin (3). Therefore, the use of rBet v 1 to identify patients with genuine birch pollen sensitisation and to confirm the diagnosis of birch pollen allergy before initiating immunotherapy with birch pollen extract has been recommended (3).

One example of a diagnostic application of recombinant birch allergens is found in a study examining allergen-specific serum IgE antibodies using the recombinant allergens Bet v 1, Bet v 2 and Bet v 4, as examined in birch-sensitive patients from the province of Cuneo, in northwestern Italy. It was reported that of 372 patients, 215 (58%) had serum IgE antibodies to Bet v 1, 166 (45%) to Bet v 2, and 35 (9%) to Bet v 4. Mono-sensitisation to Bet v 1 occurred in 146 (39%) of patients; in 96 (26%) to Bet v 2; and in only 4 (1%) to Bet v 4. Thirty-nine sera (11%) did not contain allergen-specific IgE antibodies to any of these three individual birch pollen allergens. All 372 sera (100%) had IgE antibodies against natural Birch pollen extract; 162 (44%) contained IgE antibodies reacting with Apple extract (75% of Bet v 1 positive sera). The study concluded that the 3 recombinant birch pollen allergens alone could identify 90% of birch pollen-sensitive patients (7).
 

t216 rBet v 2; profilin

Recombinant non-glycosylated protein produced in an E. coli strain carrying a cloned cDNA encoding Betula verrucosa allergen Bet v 1

Common name: Profilin
Biological function: Actin-binding protein
Mw: 15 kDa

Allergen description

Bet v 2 (5,7,14,31-38), a well-described minor allergen from birch pollen, belongs to the family of profilins, a group of common actin-binding proteins (37,39). Profilins can be found as cross-reactive allergens not only in pollen from unrelated plants (trees, grasses, weeds) but also in other plant tissues (of fruits, vegetables, nuts, spices, and latex) (40).

Bet v 2 and Bet v 3 are recognised by IgE from about 10% of Birch-allergic patients, Bet v 6 by approximately 32%, and Bet v 1 by 95% (29). The sensitisation patterns to Bet v 1 and Bet v 2, differ geographically; among Swedish and Finish patients, for example, approximately 5-7% were shown to be sensitised to Birch-profilin, compared to 20-38% of patients in Central and Southern Europe. Also, differences in IgE reactivity to rBet v 1 and rBet v 2 were demonstrated among allergic patients from six countries. The complexity of reactivity tended to be greater in individuals from the central and southern parts of Europe, as compared to Sweden and Finland (26,40).

The profilin Bet v 2, has also been shown to be involved in cross-reactivity to certain foods in tree pollen-sensitive patients.

It has been suggested that Bet v 1 can be a diagnostic marker allergen for identifying patients with genuine sensitisation to Birch-pollen (30), as opposed to patients reacting to highly cross-reactive allergens, such as Bet v 2 and Bet v 4; these may be considered marker allergens for syndromes involving cross-reactivity with numerous unrelated plants and plant products (3,30). In other words, patients who exhibit positive skin tests to Birch pollen extracts, but have not been exposed to Birch, might have IgE to highly cross-reactive allergens such as Bet v 2 (3).

One study gives an example of the diagnostic application of recombinant Birch pollen allergens. Specific serum IgE antibodies to recombinant allergens Bet v 1, Bet v 2 and Bet v 4 were examined in Birch-sensitive patients from the province of Cuneo, in northwest Italy. The study concluded that the 3 recombinant Birch pollen allergens alone could identify 90% of Birch pollen-sensitive patients (7).
 

t220 rBet v 4

Recombinant non-glycosylated protein produced in an E. coli strain carrying a cloned cDNA encoding Betula verrucosa allergen Bet v 1

Common names: 2 -EF-hand, Ca2+-binding protein, CBP, Polcalcin
Biological function: Calcium-binding protein
Mw: 8 kDa

Allergen description

Bet v 4 (7,41-43), a calcium-binding protein (44-46), is a minor allergen in birch pollen that reacts with IgE antibodies from approximately 10-20% of pollen-sensitised subjects (29,41,44). Bet v 4 is a 9 kDa calcium-binding protein of 2-EF-hand type, which is represented in pollen of a wide range of plant species. It is 67-90% identical in amino acid sequence to homologous pollen proteins from Phleum pratense, Cynodon dactylon, Brassica rapa, Brassica napus, Olea europea and Alnus glutinosa (43,47), and because of this extensive cross-reactivity can serve as a marker allergen for plant polysensitisation (48).

Bet v 3 and Bet v 4 have both been identified as EF-hand calcium-binding proteins primarily expressed in mature pollen (41,43-44). Unlike Bet v 4, which contains only 2 calcium-binding domains, Bet v 3 is a 23.7 kDa protein containing 3 typical calcium-binding motifs (48).

It has been suggested that Bet v 1 can be a diagnostic marker allergen for identifying patients with genuine sensitisation to Birch-pollen (30), as opposed to patients reacting to highly cross-reactive allergens, such as Bet v 2 and Bet v 4; these may be considered marker allergens for syndromes involving cross-reactivity with numerous unrelated plants and plant products (3).

A study gives an example of the diagnostic application of recombinant birch pollen allergens. Serum IgE antibodies to Bet v 1, Bet v 2 and Bet v 4 were examined in birch-sensitive patients from the province of Cuneo, in northwest Italy. The study concluded that the 3 recombinant birch pollen allergens alone could identify 90% of Birch pollen-sensitive patients (7).
 

t225 rBet v 6

Recombinant non-glycosylated protein produced in an E. coli strain carrying a cloned cDNA encoding Betula verrucosa allergen Bet v 6

Common name: IFR, Isoflavone reductase, (PCBER)
Biological function: Isoflavone reductase (29)
Mw: 34 kDa 

Allergen description

Bet v 6, a minor Birch pollen allergen, is an isoflavone reductase (IFR). IFRs have been found in apple, pear, orange, mango, lychee, carrot, banana, pea and chickpea (29). An IFR-like protein has also been isolated from maize and tobacco (29,49). IFR has been demonstrated in legumes and alfalfa, and a IFR-like protein had been documented in maize (250-51). The laboratory and clinical relevance of IFRs in other plants has yet to be determined.

Birch IFR has a sequence identity of 56% to 80% to IFR homologues proteins from various plants (29). The IFRs are plant defense proteins and appear to be induced by plant stress. A gene that is selectively induced both in roots and shoots in response to sulfur starvation has been demonstrated (50).  The role of plant stress in the induction of IFRs is demonstrated by grapefruit, which when treated to induce resistance against the mould decay, produced an isoflavone reductase-like protein which had a high homology to other isoflavone reductase-like proteins present in non-legume plants (52).

Isoflavone reductase (IFR) belongs to a family of plant proteins collectively termed as phenylcoumaran benzylic ether reductases (PCBERs) based on demonstrated catalytic activity. Bet v 6 displays a high degree of sequence identity of up to 81% with isoflavone reductase-homologous proteins (IFRH) and phenylcoumaran benzylic ether reductase (PCBER) as well as lower identities of 60% and 51% with isoflavone reductases (IFR) and pinoresinol-lariciresinol reductase (PLR), respectively. These reductases (IFR, IFRH, PCBER and PLR) all appear to be evolutionary derived from a common ancestor and each catalyzes a rather similar conversion in the isoflavonoid and lignan pathways.

Although the precise biochemical products so formed differ in each case, products of each reductase appear to be employed in plant defense. However, antibodies raised against PCBER do not cross-react with PLR (53-56). A characteristic difference between PCBER and isoflavone reductases (IFR) appears to be a 10 amino-acid insertion that is not present in PCBER, PLR and IFRH (55).

Bet v 6 has been found to share an 80% amino-acid sequence identity with Pyr c 5, a Bet v 6-related food allergen from pear. Assays with recombinant Pyr c 5 from Pear and Bet v 6 showed PCBER catalytic activity for both recombinant allergens, and both allergens had similar IgE binding characteristics and bound IgE from sera of birch-pollen-allergic and pear-allergic subjects. Inhibition experiments with Pyr c 5 suggested that homologous allergens may be present in many vegetable foods such as apple, peach, orange, lychee fruit, strawberry, persimmon, zucchini (courgette), and carrot. Laboratory tests of the recombinant Pyr c 5 using sera of a pear-allergic subject suggested that Pyr c 5 had the potential to elicit type I allergic reactions. This study’s data was reported to indicate that PCBER and IFR may represent a new family of pollen-related food allergens that occur not only in typical birch-pollen-related foods, but also in rarely allergenic fruits and vegetables such as orange, strawberry, persimmon, or zucchini (55,57).

Bet v 6 may be responsible for pollen-related oral allergy to specific foods in a minority of patients with birch pollen allergy (29). Bet v 6 is recognized by IgE from approximately 32% of Birch pollen allergic individuals. Recombinant Bet v 6 bound IgE from 32% of 28 sera from patients allergic to birch pollen with a ImmunoCAP® class of at least 3 compared to Bet v 1 binding in 89% of these patients (29).

Japanese cedar pollen, a major cause of seasonal pollinosis in Japan where more than 10% of Japanese people are affected, was shown to contain an isoflavone reductase-like protein. In contrast to Bet v 6 being reported as a minor allergen, this recombinant protein exhibited an IgE binding frequency of 76% (19/25) in Japanese cedar pollen allergic patients (58). 

Allergy to Sharon fruit (persimon) has been only rarely reported. Cross-reactivity with pollen (profilin, Bet v 1 and Bet v 6) appears to be involved. In a study of two patients with allergic reactions on first exposure to Sharon fruit, as well as 7 patients with birch-pollen-related apple allergy, found that an open challenge with Sharon fruit in 7 patients allergic to Birch pollen and Apple, who had not eaten Sharon fruit previously, was positive in 6/7 cases. The study concluded that Birch-pollen-related allergy to Sharon fruit is mediated by the known cross-reactive pollen allergens including Bet v 1 (59).

Compiled by Dr Harris Steinman, harris@zingsolutions.com

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As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.