f98 Gliadin

Summary

Gliadin, a component of wheat, is frequently identified as a key wheat allergen. Wheat is a leading food allergen after cow’s milk and egg. Specific IgE to wheat proteins is commonly detected among atopic children of all ages without true food allergy mainly due to cross-reactivity to grass pollen. The gliadins, together with the glutenins. are wheat-specific proteins and do not cross-react with grass pollen allergens. The gliadins are members of the cereal prolamin family and the most important have been identified as α/β, γ, and ω-gliadin. The clinical role of Omega-5 gliadin is the most studied among gliadins. It plays an important role in food-dependent exercise-induced anaphylaxis and it has been suggested to call this condition Omega-5 Gliadin Allergy. Omega-5 gliadin is also the major wheat allergen causing immediate allergic reactions in children. Specific IgE to Omega-5 gliadin has high specificity and low sensitivity. Testing for IgE to Gliadins has proven to be useful in the diagnosis of wheat allergy. Sensitization gives additional information especially in patients negative to omega-5 gliadin due to its higher sensitivity.

Epidemiology

Worldwide distribution

The prevalence of wheat allergy (WA) varies across geographies and among different age groups globally and is one of the leading food allergens in children (Ricci et al. 2019). It is one of the top three food allergies after cow’s milk and egg, especially in countries like Germany, Japan, and Finland, and among preschool children in the US (Burkhardt et al. 2018, Ashtari et al. 2019). The prevalence of WA was found to be 0.21% among Japanese adults (Morita et al. 2012). Wheat is now acknowledged as the leading cause of food-dependent exercise-induced anaphylaxis (FDEIA) (Kennard et al. 2018). Specific IgE (sIgE) antibodies targeting omega-5 gliadin (O-5-G) are prevalent in adults and adolescents suffering from wheat-dependent exercise-induced anaphylaxis (WDEIA) (Nilsson et al. 2013, Makela et al. 2014, Nilsson et al. 2015), while glutenin and other forms of gliadins are less common triggers of WDEIA (Kennard et al. 2018). In Japanese patients with WDEIA, it was found that 82.0% tested positive for sIgE to O-5-G using purified wheat proteins. Additionally, the incidence of positive reactions to O-5-G in WDEIA patients aged ≥20 years was significantly higher (92.8%) compared to patients <20 years old (46.1%) (Morita et al. 2009), suggesting O-5-G may not be a primary allergen in children with WDEIA (Watanabe et al. 2022).

A retrospective UK-based study was conducted in four allergy centers, focusing on 132 adult patients (>18 years) presented with O-5-G allergy. Each patient developed allergic reactions after consumption of wheat and had O-5-G sIgE antibodies. About 50% underwent skin prick testing (SPT) for wheat, with 52% showing a positive result; only 11% of these patients were tested for O-5-G and 67% showed positive results. This group of UK allergists has suggested that this allergy should be called O-5-G Allergy instead of WDEIA (Kennard et al. 2018).

Another retrospective real-world survey study conducted in Australia over eight years (2008 – 2016) found 80 adults with clinical histories matching O-5-G allergy and having serum-specific O-5-G IgE (Zubrinich et al. 2021). A Thailand-based cross-sectional pilot study involving an adult-onset IgE-mediated WA cohort with 11 patients having WDEIA and 4 patients with typical WA detected that the molecular weight of serum IgE from more than half of the bound proteins was between 35 and 55 kDa in both gliadin and glutenin extracts. The primary allergens identified were O-5-G and gamma gliadin (via ELISA); all 4 typical WA patients showed sensitization to gamma gliadin, and all 11 WDEIA patients showed sensitization to O-5-G. Additionally, alpha or beta gliadin, high molecular weight (HMW) glutenin, and possibly α-amylase inhibitor or low molecular weight (LMW) glutenin were also identified as major allergens (Piboonpocanun et al. 2021).

A study in Germany aimed to evaluate IgE reactivity to different wheat proteins in 17 patients with confirmed WDEIA and found (via immunoassay) that IgE antibodies specific to recombinant O-5-G were present in 82% of patients (14/17) and IgE antibodies specific to alpha, beta, and gamma gliadin were also present in 82% of patients. Additionally, micro-array analysis revealed O-5-G as the most common allergen found in 88% (15/17) patients, followed by gamma gliadin (76%; 13/17) (Hofmann et al. 2012).

Environmental characteristics

Source and tissue

Wheat protein, gliadin, particularly O-5-G has been identified as the major allergen of wheat involved in causing WDEIA (Nilsson et al. 2013). Gliadin makes up about 30% of the total protein found in wheat grain. Gliadins are large families of proteins with similar sequences of amino acids (Urade et al. 2018).

Risk factors

One of the primary risk factors for any food allergy is genetic predisposition (Ricci et al. 2019). Atopic disorders such as asthma, allergic rhinitis (AR), and atopic dermatitis (AD) often co-exist in children suffering from WA and most of these children show allergic reactions to other food. Sensitization to cow’s milk, eggs, and even grasses is reported to correlate with WA (Ricci et al. 2019). Besides, aspirin, consumption of alcohol, exposure to infections, stress, or the influence of female sex hormones and menstruation may act as either alternative triggers or co-factors for WDEIA reactions triggering an immediate-type hypersensitivity response following the consumption of wheat (Hofmann et al. 2012). 

Clinical relevance

Disease severity

IgE-mediated as well as non-IgE-mediated food allergies are induced by wheat. IgE-mediated reactions to what can be food allergy as well as respiratory allergy. Food allergy symptoms are urticaria/angioedema, AR, asthma, worsening of AD, abdominal pain, vomiting, and exercise-induced anaphylaxis. IgE-mediated respiratory allergy symptoms are Baker’s asthma and Baker’s rhinitis (Cianferoni 2016). A study highlighted that positive IgE responses to wheat components, including gliadins, were found in a significant majority of individuals with 93% showing immediate reactions and 41% showing delayed reactions on ingestion of wheat (Makela et al. 2014).

Non-IgE-mediated reactions include eosinophilic gastrointestinal diseases and enterocolitis syndrome (Cianferoni 2016). 

Omega-5 Gliadin Allergy (Wheat-dependent exercise-induced anaphylaxis)

WDEIA represents a comparatively uncommon yet potentially serious allergic reaction to wheat. This condition is characterized by anaphylactic symptoms ranging from urticaria to complications involving respiratory, gastrointestinal, or cardiovascular systems. These symptoms usually occur within 1-4 hours after consuming wheat and are then precipitated by engaging in physical exercise (Hofmann et al. 2012).

In a UK-based study of 132 patients with O-5-G allergy (data of 84% of patients were available), 39% reacted within 30 minutes of wheat ingestion, 56% between 30 minutes and an hour, and 5% reacted after an hour. Most symptoms were predominantly severe (66%), with fewer moderate (10%) and mild (24%) cases, as classified by the Brown anaphylaxis grading. Exercise (80%), alcohol (25%), and nonsteroidal anti-inflammatory drugs (9%) were identified as the most widespread cofactors. However, the level of sIgE to O-5-G was not found as a predictor for allergic reaction severity (Kennard et al. 2018).

In a survey-based study of 80 adult individuals with O-5-G allergy, the clinical profiles of the respondents showed allergic reactions such as asthma (9%), eczema (9%), other food allergies (47%), and multisystem anaphylaxis at least once (88%) (Zubrinich et al. 2021).  

Symptoms of WDEIA usually vary, which was evidenced in a study of 67 WDEIA patients where 38.81% had food-dependent exercise-induced allergy, 23.88% had exercise-induced symptoms, 14.93% had idiopathic anaphylaxis, 14.93% had food-induced symptoms, and 7.46% had recurrent acute urticaria (Le et al. 2015). Diverse symptoms were also observed in another study of 197 WDEIA patients, all had skin allergies; 52.8% experienced respiratory, 34.0% gastrointestinal, and 93.4% cardiovascular symptoms. Common clinical symptoms included urticaria (100%), loss of consciousness (82.7%), dyspnea (50.8%), hypotension (47.2%), and blurred vision (39.1%) (Du et al. 2022). 

Respiratory and cutaneous allergy

Baker's asthma and AR are respiratory allergies triggered by inhaling wheat flour and dust and are ranked among the most frequent occupational allergies, particularly affecting individuals in professions like baking, milling, and confectionery. Additionally, contact urticaria is also a common allergic manifestation among millers, bakers, and flour handlers (Wieser et al. 2020).

In a study involving ten baking industry workers suffering from AR and/or asthma due to wheat flour, it was found that one patient, who had no prior history of anaphylactic reactions, tested positive for O-5-G (Tri a 19) (Dubini et al. 2020). Gliadin, α-amylase inhibitors, and lipid transfer protein in wheat flour should be considered as potential allergens for patients suffering from both baker's asthma and IgE-mediated wheat food allergy (Yilmaz et al. 2017).

Atopic dermatitis

The prevalence of positive SPTs in foods is reported to be notably higher in children with AD (Ricci et al. 2019). A study was conducted to explore the role of gliadin peptides as allergens in 18 children with AD. It was found that 13 children showed positive reactions to wheat and 11 children displayed positive reactions to gliadin in SPT (Varjonen et al. 2000).

Cross-reactive molecules

The interaction between gliadin peptides of wheat and allergens from other cereals is clinically significant (Vojdani et al. 2013). In a study, OFCs with barley were conducted on patients with confirmed WA. The levels of sIgE to wheat, O-5-G, and barley were found to be considerably higher in patients who had a positive reaction to OFC compared to those with a negative reaction, indicating barley allergy in individuals with WA is likely due to cross-reactivity from wheat(Kubota et al. 2022). Significant cross-reactivity between wheat and barley was also evidenced in multiple studies (Pourpak et al. 2005, Takei et al. 2022, Yanagida et al. 2022). However, no significant cross-reactivity was observed between wheat and rice, or corn and rice (Pourpak et al. 2005).

Wheat sensitization may occur in patients with grass pollen allergy; however, this is not of clinical relevance. In a UK cohort study, cross-sensitization between grass pollen and wheat was observed. Out of 588 children, 16.3% (96 children) were sensitized to grass pollen and 13.4% (79 children) to wheat pollen, with a significant overlap where 78% (75 out of 96) were sensitized to both (Venter et al. 2016). Another study on a pediatric population (aged 5-17 years) allergic to grass pollen detected that 60% of these patients were sensitized to wheat with a median sIgE level of 0.5 kU/L. All of the included patients were able to consume wheat without having symptoms. This wheat sensitization is due to cross-reactivity between grass and wheat (Nilsson et al. 2018).

Prevention and therapy

The most frequently adopted approach for the management of IgE-mediated WA is to eliminate wheat or cross-reactive cereals from the diet in combination with exercise as well as inhaled wheat allergens (occupational exposure) (Cianferoni 2016, Kennard et al. 2018). Avoiding gluten or wheat-based foods and exercising can effectively prevent reactions in roughly 70% of cases (Kennard et al. 2018). According to a study on 197 WDEIA patients, a wheat-free diet was the most common management of WA, followed by exercise. A wheat-free diet prevented future anaphylaxis in 91.7% of cases, while avoiding wheat with exercise was effective in preventing anaphylaxis in 87.2%, and reducing wheat with exercise avoidance worked in 80.5% of cases (Du et al. 2022). 

Patients with WA need to be trained in identifying key food allergens on labels and should be given appropriate written instructions for effectively removing wheat from their diet (Cianferoni 2016). According to a questionnaire-based study children with WA experience a significant impact on their health-related quality of life (HRQoL), suggesting that families with children having WA might require more comprehensive healthcare assistance. for the management of allergic conditions (Borres et al. 2017). In case of accidental exposure to wheat and anaphylactic reactions start, then administration of epinephrine with autoinjectors and antihistamines is the life-saving treatment (Kennard et al. 2018). 

Furthermore, a multicenter, randomized, double-blind, placebo-controlled clinical trial demonstrated that vital wheat gluten (VWG) oral immunotherapy (OIT) effectively desensitized most patients with WA after one year. This study provides the safety and efficacy data for VWG OIT for the first time in a large population of WA patients, highlighting the need for further research to identify optimal dosages and treatment durations for wheat OIT (Nowak-Wegrzyn et al. 2019).

Molecular aspects    

Biochemistry

Gluten is divided into gliadins (soluble in 70% alcohol) and glutenins (soluble in acids and bases); gliadins constitute about 50% of gluten proteins. Gliadins are monomeric proteins that can be separated via gel electrophoresis into four components based on their electrophoretic mobility at low pH. The components are- alpha-gliadin (with fasted mobility), followed by beta, gamma, and omega (with slowest mobility) gliadins (Barak et al. 2015, Burkhardt et al. 2018).

Alpha, beta, and gamma gliadins have in terms of molecular weights (30-35 kDa) and disulfide bonds (3 and 4 intrachain S=S bonds). Omega gliadins, in contrast, have a larger molecular weight ranging between 44 and 88 kDa and lack cysteine, which prevents them from forming disulfide bridges. Omega gliadins are characterized by their repetitive glutamine and proline sequences (PQQPFPQQ). The secondary structure of alpha, beta, and gamma gliadins primarily consists of alpha-helices and beta-sheets, while omega gliadins predominantly feature beta-turns (Klosok et al. 2021). 

Isoforms, epitopes, antibodies

As of January 3, 2024, there are several isoallergens of different forms of gliadins. Alpha and beta gliadin (Tri a 21) has 5 isoallergens i.e. Tri a 21.0101, Tri a 21.0201, Tri a 21.0301, Tri a 21.0401, Tri a 21.0501, gamma gliadin (Tri a 20) has three isoforms Tri a 20.0101, Tri a 20.0201, Tri a 20.0301, whereas omega gliadin (Tri a 19) has only one isoallergen Tri a 19.0101. These isoallergens are identified and officially published by the World Health Organization (WHO) and International Union of Immunological Societies (IUIS) Allergen Nomenclature Subcommittee (WHO/IUIS-Tria19 2023, WHO/IUIS-Tria20 2023, WHO/IUIS-Tria21 2023). 

Cross-reactivity due to structural similarity

Individuals who are allergic to wheat are often found to be cross-reactive with other cereals such as barley and rye (Czaja-Bulsa et al. 2017). Wheat, barley, and rye belong to the same taxonomic group, which can be attributed to this cross-reactivity. The structural similarity of the homologous protein in these cereals plays an important role in this cross-reaction. It has been found that prolamins, like gamma-3 hordein in barley, gamma-70, and gamma-35 secalins in rye cross-react with O-5 gliadin in wheat. In addition, a few more proteins in these cereals exhibit a high degree of cross-reactivity. A significant sequence similarity (>80%) has been established among various proteins like alpha-purothionins from wheat, rye, and barley (Czaja-Bulsa et al. 2017). 

In a Finnish study involving 23 WDEIA patients, all showed IgE sensitization and positive SPT to O-5-G. Moreover, 91% of the adult patient’s sera reacted to gamma-70 secalins, 83% to gamma-35 secalin, and 91% to gamma-3 hordein. Additionally, cross-reactivity was evident in O-5-G-SPT positive patients with gamma-35 secalins (20%), gamma-70 secalins (67%), and gamma-3 hordein (47%) (Palosuo et al. 2001).  

Diagnostic relevance

Disease Severity

Specific IgE tests and SPTs are necessary to identify food sensitization. To definitively diagnose a food allergy, an OFC test is crucial; however, it carries the risk of anaphylaxis and is time-intensive. Consequently, SPT and sIgE, which have high accuracy and minimal to no risk of anaphylactic reactions, are significant diagnostic methods (Phisitbuntoon et al. 2023). The Component Resolved Diagnosis (CRD) can also be considered to identify sensitization toward a specific food-allergenic component (Costanzo et al. 2022).

Wheat allergy diagnosis poses significant challenges due to delayed reactions, m(Nilsson et al. 2015). Hence, WA diagnosis is difficult, despite the sophisticated component-based techniques used. However, the accuracy of identifying wheat allergies can be improved by evaluating IgE responses to a range of wheat components (Ito et al. 2008, Makela et al. 2014, Nilsson et al. 2015). Furthermore, the O-5-G-sIgE test should be performed in adults with unexplained anaphylaxis to reduce delay in allergy diagnosis as most of these patients were found to have O-5-G antibodies (Kennard et al. 2018). 

A study evaluated the clinical profiles of 63 children (aged 1-17 years) with confirmed wheat sensitization and assessed the impact of measuring IgE antibodies against wheat components. It was found that IgE levels of wheat and its components were significantly higher (p<0.001) in children with WA compared to non-WA children. Specifically, O-5-G showed 84% specificity and 62% sensitivity, while gliadin, HMW-glutenin, and LMW-glutenin exhibited sensitivities of 81–94% and specificities of 29–52%. Moreover, a strong correlation was also observed between IgE antibody levels to wheat components and the severity of WA reactions (Nilsson et al. 2015).

Several studies on the pediatric population revealed that sIgE antibodies to O-5-G are correlated with a positive response to the wheat challenge test in wheat-sensitized children. Moreover, sIgE -O-5-G antibodies could be a valuable marker in deciding clinical reactivity during challenge tests in wheat-sensitized or WA individuals (Ito et al. 2008) and is especially beneficial in diagnosing WA in infants and children suspected of having this condition (Shibata et al. 2008, Ebisawa et al. 2012). Additionally, another study concluded that a combination of markers (wheat CD-sensitivity and IgE antibodies to wheat components such as O-5-G) could be helpful in diagnosis and increase the accuracy of the outcomes of the wheat challenge test in children with WA. This might aid in early prevention and treatment (Nilsson et al. 2013).

Cross-Reactivity

Cross-reactivity among different cereals plays a significant role in managing WA. Studying in vitro cross-reactivity among these cereals could aid in understanding potential clinical cross-reactivity (Takei et al. 2022). A study was designed to explore the cross-reactivity of three fractions among cereals (albumin/globulin, gliadin, and glutenin) in children (average age 6.3 years) with immediate-type WA. These children either had positive responses in wheat OFC tests or experienced immediate allergic reactions after eating wheat. The study found that these children were sensitized to all three wheat fractions and wheat showed 100% cross-reactivity to barley and rye from the viewpoint of sensitization. Furthermore, the sIgE concentrations against wheat showed a marked elevation compared to those for barley and rye due to which the allergenic potential of wheat was found to be higher than barley and rye. Thus, wheat is the primary source of sensitization and cross-reactivity might be responsible for sensitization to barley and rye. Hence, it is important to consider the cross-reactivity of wheat with barley and rye while managing children with WA (Takei et al. 2022).

Exposure

Ingestion of wheat is commonly the primary exposure route responsible for typical IgE-mediated allergic reactions. Inhalation is a secondary exposure route; conditions like Baker's asthma and rhinitis are frequently caused by the inhalation of flour from wheat and other grains (Ricci et al. 2019). 

Explained results 

Allergen Information

The prevalence of WA varies globally, with it being a leading food allergen in children (Ricci et al. 2019). Gliadin comprises approximately 30% of the overall protein content in wheat grain. Gliadins are large families of proteins with similar sequences of amino acids and are categorized into alpha, beta, gamma, and omega types (Barak et al. 2015, Burkhardt et al. 2018, Urade et al. 2018).

Clinical relevance

Wheat triggers both IgE-mediated allergies (e.g., urticaria, asthma, anaphylaxis) and non-IgE-mediated reactions (e.g., eosinophilic gastrointestinal diseases). WDEIA is a notable IgE-mediated condition, exacerbated by exercise post-wheat consumption (Hofmann et al. 2012, Cianferoni 2016). Additionally, respiratory symptoms like asthma and AR are common among bakers as an occupational allergy to wheat flour (Wieser et al. 2020).

Cross-reactivity

Wheat showed extensive cross-reactivity with other cereals of the same family as barley, and rye due to shared protein structures. Recognizing and managing these wide-ranging cross-reactions are crucial for effective WA treatment (Czaja-Bulsa et al. 2017, Takei et al. 2022). 

Author: Turacoz

Reviewed: Dr. Magnus Borres