f416 Tri a 19

Summary

Tri a 19, also referred to as ω-5 gliadin, is a major allergen found in wheat. It is a water insoluble, monomeric gliadin fraction of wheat seed storage protein. It is considered as a marker allergen in wheat dependent exercise-induced anaphylaxis (WDEIA) seen in adults. This condition has been suggested to be named Omega-5 gliadin allergy. Besides, this allergenic protein is also correlated with severe allergic reactions to wheat in children, including wheat-induced anaphylaxis (WIA). Further, ω-5 gliadin is found to be responsible for the cross reactivity between wheat, rye and barley due to the presence of its homologous proteins, g-35 and g-70 secalins in rye and g-3 hordein in barley. Importantly, sensitization to ω-5 gliadin could be useful for differentiating patients with or without clinically relevant wheat allergy. Moreover, ω-5 gliadin-sIgE levels could serve as indicators for positive outcomes to oral wheat challenge. Additionally, ω-5 gliadin-IgE levels have shown to serve as an indicator for the oral wheat dose, which could be tolerated by the patients undergoing oral wheat immunotherapy. It could also indicate the severity of reactions in the patients during the therapy. The ω-5 gliadin IgE levels <0.35 kUA/L may help to predict development of wheat tolerance also.

Epidemiology

Worldwide distribution

Wheat allergy (WA) occurs in many ways [food allergy in children, wheat dependent exercise-induced anaphylaxis (WDEIA), baker’s asthma or skin allergy] and its prevalence differs across the geographies and among different age groups. It is considered as the third most common allergy, after cow’s milk and egg in countries such as Japan, Korea, Finland, Germany, and the United States (US) (in preschool children) (1-3).

Tri a 19, a w-5 gliadin, is a major allergen found in wheat, that induces severe immunoglobulin (IgE)-mediated allergic reactions to wheat in children, along with WDEIA in adults (4, 5).

A recent study conducted in Hongkong determined the prevalence of w-5 gliadin allergy among 221 allergic patients visiting an allergy clinic. Anaphylaxis due to WA was observed in 17 patients, of which 94% were identified with w-5 gliadin allergy (6). Further, in a study conducted in Korea among 27 adult patients with allergic reactions to wheat (IgE-mediated wheat induced anaphylaxis, WDEIA and patients with cutaneous symptoms), specific IgE (sIgE) to w-5 gliadin (Tri a 19) was found positive in 70.4% of patients (5).

In a retrospective, multicenter study conducted in Japan among 173 children with WA (OFC-confirmed or history of anaphylaxis to wheat), 72% showed a positive IgE reactivity to w-5 gliadin (7). Another study conducted in Japan among 44 children with WA (OFC-confirmed or history of severe reaction to wheat), sIgE reactivity to w-5 gliadin was reported in 84% of children (8). 

However, in a study conducted in Finland on 57 children with WA (30 with immediate reactions and 27 with delayed reactions to OFC), only 31.6%  of children showed sIgE reactivity to w-5 gliadin (9). Whereas, in another study performed in Italy among 19 OFC-confirmed wheat-allergic children, only 26% reported sIgE reactivity to Tri a 19 (10).

Environmental Characteristics

Source and tissue

Tri a 19 is a partially water-soluble protein present in wheat (11). It is a part of wheat seed-storage protein fraction (consisting of prolamins), that is soluble in alcohol (12). These allergens are said to be a portion of monomeric gliadin content of wheat seed protein, like other prolamins, i.e. α- and g-gliadins (13).

Tri a 19 is usually purified from a mixture of gliadin proteins through reverse phase-high performance liquid chromatography (RP-HPLC). However, since there are 6 different Tri 19 proteins in wheat (as it is a hexaploid species), a homogenous preparation of Tri a 19 is hard to perform. The recombinant Tri a 19 can be generated by expressing the protein in two different vectors: Escherichia coli and pET-21a. Since the recombinant form of this allergen is soluble in both water and alcohol, a homogenous preparation can be made by RP-HPLC (11).

Risk factors

Sensitization with Tri a 19 is so close related to WDEIA so it has been suggested that this condition should be named Omega-5 gliadin allergy instead of the complicated term WDEIA (12). The most important risk factors involved in precipitating an anaphylactic reaction is physical exercise combined with wheat consumption, however, the degree or period of exercise may vary among individuals. Besides exercise, other cofactors, such as intake of alcohol, aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs), infections, menstrual period, stress, as well as heat may play an important role in the precipitation of ω-5 gliadin allergy (6, 14, 15). 

Clinical Relevance

Disease Severity

Tri a 19, also referred to as ω-5 gliadin, is a major allergen found in different types of WA, such as WDEIA in adults and adolescents, IgE-mediated WA in children, including wheat induced anaphylaxis (WIA), contact dermatitis, as well as Baker’s asthma (4, 5, 16). Moreover, sensitization to ω-5 gliadin allergens has also been reported to occur in elderly patients with WDEIA (>55 years) (a rare phenomenon) (17) as well as sporadically in adults with wheat-induced anaphylaxis (WIA) (5).

In most of the cases, the ω-5 gliadin allergy is presented as WDEIA, manifested as generalized urticaria, pruritis, angioedema, shortness of breath, gastrointestinal (GI) symptoms, bronchial obstruction as well as hypotension, and if more serious, may cause anaphylactic shock or even loss of consciousness. It has reportedly been observed, after consumption of wheat combined with certain cofactors, such as exercise, aspirin and other NSAIDs, alcohol, stress and menstrual period. Most commonly, WDEIA appear within 1 to 4 hours of consuming wheat accompanied with physical exercise. It is important to note that such severe allergic reactions are not observed in the same individuals in the presence of consumption of wheat alone (12, 15).  

The association of sIgE with ω-5 gliadin and WDEIA was studied in a group of 50 patients with WDEIA. Of these patients, 25 were healthy controls, whereas another 25 were atopic dermatitis (AD) patients sensitized to wheat, but not clinically responsive to wheat consumption. The study findings showed sIgE to ω-5 gliadin being positive in 80% of WDEIA patients and 68% of AD patients (18). Of interest, there is another subtype of WDEIA with hydrolyzed wheat proteins as an elicitor. This specific WDEIA was not found to be associated with sIgE to ω-5 gliadin (19).

Interestingly, a study was conducted on 67 adult patients with positive sIgE to ω-5 gliadin and having allergy associated with wheat. According to the results, the WDEIA was reported in 53 patients, while 7 patients showed wheat-dependent anaphylaxis induced by other cofactors (aspirin and other NSAID, alcohol or stress) and another 7 were observed to have WIA without cofactor dependence (14).  Further, in a retrospective study conducted in United Kingdom (UK) among 132 adult patients with history of allergic reactions, post intake of wheat and also sensitized to ω-5 gliadin, wheat was considered to be an elicitor in 82%. Most of the patients presented with severe reactions (66%), while 24% and 10% of patients showed mild and moderate reactions, respectively. It was interesting to note that 80% of the patients identified exercise to be a cofactor for precipitation of allergic reactions, while 25%, 9% and 5% of patients reported alcohol, NSAIDs and heat as cofactors, respectively (12).

Besides WDEIA, Tri a 19-sIgE is also correlated with severe reactions due to primary WA. This has been reported in several studies. In a study conducted among 44 wheat-allergic Japanese children, significantly higher (p<0.001) ω-5 gliadin-sIgE levels were reported in patients with severe allergic reactions (n=16) as compared to patients with mild (n=12) or moderate reactions (n=14) (8). In another study performed on 27 adult patients with WA, 17 patients observed severe reactions to wheat (WIA or WDEIA), whereas 10 observed only AD or urticaria (cutaneous symptoms). According to the results, sIgE reactivity to ω-5 gliadin was found positive in 100% of patients with severe reactions, while it was positive in only 20% of patients with cutaneous symptoms alone (5).

Further, in a study conducted in Finland among 108 children with suspected WA and subjected to OFC to wheat, ω-5 gliadin was found positive in 50% of patients with immediate reactions in OFC to wheat, while it was reported to be 18% in patients with negative OFC outcome (9). 

Cross-reactive molecules

Wheat has been found to be cross-reactive with rye and barley, mainly due to the cross-reactivity observed between ω-5 gliadin and secalins (g-35 and g-70, rye) as well as g-3 hordein (barley). This was found in a study conducted in Finland among 23 patients with WDEIA who showed IgE sensitization as well as positive skin prick test (SPT) to ω-5 gliadin. According to the results, the g-70 secalins were found to be IgE-reactive in 91%, g-35 secalin in 83% and g-3 hordein in 91% of sera of adult patients. Further, the cross-reactivity was also depicted by SPT conducted in 15 WDEIA patients who were positive for ω-5 gliadin-SPT. The test showed positive results in 20%, 67% and 47% of patients with g-35 secalins, g-70 secalins, and g-3 hordein, respectively (20).  

Prevention and Therapy

Avoiding the combination of wheat intake (or gluten-containing diet) along with physical exercise may be advised in individuals in order to decrease or prevent the severe allergic reactions associated with it (12, 21). This was reported in a study where 91% of 12 adult patients (history of anaphylaxis due to wheat consumption along with exercise/stress/NSAIDs/alcohol) did not show symptoms because of the avoidance of this combination (21). In addition to this, another study reported that 69% of 26 ω-5 gliadin-allergic patients could avoid the occurrence of WDEIA, post wheat-free diet with exercise avoidance, while 67% of 21 patients could avoid WDEIA, post gluten-free diet (12).

Interestingly, it has been suggested that patients with WDEIA may normally ingest wheat without allergic reactions. However, the combination of ingestion of wheat products in conjunction with exercise could elicit the symptoms in such individuals, which could be more severe too. This may be due to lowering of the clinical threshold required to induce a reaction in such individuals by the presence of cofactors (especially exercise). This was proposed since allergic reactions could be triggered in WDEIA patients, even at rest, if challenged with a sufficiently high dose of gluten proteins (22). Further, patients with WDEIA may be suggested to follow wheat-containing diet keeping a gap of 4 hours before or after the exposure to the cofactors (based on history and challenge outcomes of the patient) as clinical threshold is found to be reduced in patients who are advised to follow wheat-free diet (23, 24).   

Molecular Aspects

Biochemistry

Tri a 19 or ω-5 gliadin belongs to the gliadin group of monomeric proteins, a fraction of water-insoluble gluten proteins of wheat (13, 25). Gliadins are very complex and exist as diverse sets of proteins. They are segregated into 4 sets, based on their mobility on an electrophoresis gel. The ω-gliadins are slow-moving proteins as compared to the α, β, and g-gliadins. These ω-gliadins are further sub grouped into ω-1.2 and ω-5 gliadin. Furthermore, all the four gliadins (α/β/g/w gliadins) vary in their polypeptide content (25).

Tri a 19 belongs to prolamin superfamily of proteins (21) and possesses a molecular weight of 65 kDa (26). It constitutes of 3-7% of wheat seed-storage protein (15) and 5-10% of wheat flour protein (27). In addition to this, Tri a 19 allergen also consists of large quantities of glutamine and proline (68% to 73%), however it is devoid of cysteine as well as methionine (12, 13, 27).

Structurally, ω-5 gliadin consists of short C-and N-terminal chains of 10 and 13 amino acids, that protect the 200 residue long, amino-acid central region. This region has a number of repetitive sequences, identified as epitopes, that possess allergenic potential (25).

Isoforms, epitopes, antibodies

As of 5th February 2021, only one isoallergen of Tri a 19 i.e. Tri a 19.0101 has been identified and officially published by the World Health Organization (WHO) and International Union of Immunological Societies (IUIS) Allergen Nomenclature Subcommittee (26).

The primary protein sequence of ω-5 gliadin has shown the presence of 7 epitopes (11). Since there are repetitive sequences seen in Tri a 19 protein, it displays a unique characteristic of obtaining several copies of IgE-reactive epitopes. The immunoreactivity of this protein depends on the frequency and arrangement of these epitope sequences (25, 27). 

Diagnostic Relevance

Disease Severity

Sensitization to ω-5 gliadin allergen has been reported to be useful for differentiating patients with or without clinically relevant WA. Moreover, ω-5 gliadin-sIgE levels could also serve as indicator for positive outcomes to OFC. This was evidenced by the following mentioned studies (7, 8, 16, 28, 29).

A study conducted on 21 Japanese children with positive OFC and 14 children with negative OFC, the ω-5 gliadin-sIgE levels were found to be significantly (p<0.001) elevated in children who showed positive OFC, suggesting that it could be useful in determining the oral wheat challenge outcomes (8). In a retrospective, cohort study conducted on 311 Japanese children and adolescents with suspected wheat intolerance, sIgE antibodies to ω-5 gliadin was found to be correlated with the probability of the presence of WA. It was further concluded from the study that sIgE antibodies to ω-5 gliadin could be useful in predicting OFC outcomes in children (<1 year) (7). Furthermore, a study performed on 88 wheat-sensitized children found higher levels of ω-5 gliadin sIgE in 88% of WA and 47% of non-WA children, as confirmed by OFC. This suggested a positive link between ω-5 gliadin sIgE and OFC outcomes (28).

In a study conducted in Sweden among 63 wheat-sensitized children pursuing a diet devoid of wheat, an OFC performed resulted in 26 positive challenges. The study results showed a total of 32 children being wheat allergic, which included 6 children with a history of severe reactions to wheat. Further, the ω-5 gliadin-sIgE levels were found to be significantly higher in wheat-allergic children as compared to non-wheat allergic children (p<0.001). The specificity (84%) of ω-5 gliadin-sIgE was found to be the highest, among all other wheat components (gliadin, high molecular and low molecular weight glutenin) (16). Further, another study conducted on 31 wheat-allergic and 72 wheat-tolerant individuals demonstrated that ω-5 gliadin sensitivity could serve as a tool for identifying patients with WA (29). Similar results were observed from 19 allergic patients with sensitization to wheat in a cross-sectional study conducted in Italy. The confirmed WA by OFC was observed in 6 patients. Serum IgE to ω-5 gliadin was observed to be significantly higher (p<0.05) in WA patients as compared to sIgE to wheat and also showed a specificity of 100% in determining WA (10). Furthermore, probability curves plotted using ω-5 gliadin-sIgE levels could serve as a good tool for appraising the risk of developing severe reactions during OFC to wheat in young children (30).

Importantly, it was found in a study conducted on 132 patients with ω-5 gliadin allergy, that ω-5 gliadin-sIgE test should be conducted in all individuals who present with anaphylaxis due to unexplained reasons. Anaphylaxis due to WA may go unnoticed in such individuals since every wheat consumption may not be involved with allergic reactions. The precipitation of allergic reactions may be due to high dose of wheat or may be due to presence of cofactors. However, majority of patients with unexplained anaphylaxis have antibodies to ω-5 gliadin. Thus, ω-5 gliadin-sIgE test could help in identifying the allergy caused by ω-5 gliadin without any delay (12). This was also observed in another study conducted among 22 patients with idiopathic anaphylaxis (anaphylaxis due to unrecognized allergen not detected by SPT or sIgE of recognized allergens). An immuno-solid phase allergen chip (ISAC) array of 35 allergens was conducted to detect the cause of anaphylaxis, which reported ω-5 gliadin to be the cause in 20% of the patients. Further, the study suggested that sIgE to ω-5 gliadin along with shrimp should be considered for evaluation of patients with idiopathic anaphylaxis before an ISAC array could be performed, if diagnostic uncertainty persists (31).

Interestingly, a prospective study conducted among 283 WDEIA patients in China found that IgE reactivity to gluten had better diagnostic value as compared to IgE reactivity to ω-5 gliadin. Also, the best diagnostic ability was shown by combination of both gluten-IgE and ω-5 gliadin-IgE (32). In support of this, a study performed in Germany could not demonstrate the differentiating ability of ω-5 gliadin between wheat allergic and non-wheat allergic patients. This study was carried out in 106 children (44 wheat allergic and 62 wheat tolerant) with a suspected WA, where ω-5 gliadin-sIgE levels were not found to be significantly different between wheat allergic and wheat tolerant children (33).

Furthermore, the role of sIgE to ω-5 gliadin was also evaluated in wheat-allergic patients undergoing oral immunotherapy. It was found that the ω-5 gliadin-sIgE levels at the baseline (before the start of therapy) and the follow-up levels (during the oral therapy) in patients undergoing oral immunotherapy could serve as an indicator for the oral wheat dose which could be tolerated as well as the severity of reactions in the patients during the therapy. This was observed in a study conducted in Finland among 100 wheat-allergic children undergoing oral immunotherapy (34). Moreover, sIgE to ω-5 gliadin is also considered as a good predictor for the age-related resolution to WA. A prospective study conducted on 55 children evaluated the predictors for the development of wheat tolerance and found an 83.6% accuracy for ω-5 gliadin-sIgE levels<0.35 kUA/L. This was considered to be the best predictor for the outgrown allergy to wheat (35). 

Compiled By

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Magnus Borres

Last reviewed: June 1 2023