Apple allergy: Causes and factors influencing fruits allergenic properties–Review

Abstract Background Apple tree fruits (Malus × domestica Borkh.) are a rich source of nutrients and nutraceuticals and are recommended as a part of the healthy, staple diet. However, apples could be also the cause of allergies including severe reactions. Allergies to fruits like apples are predominantly associated with pollinosis. In North and Central Europe, sensitisation to apples is caused mainly by cross‐reactive birch pollen aeroallergen, whereas in the Mediterranean area of Europe, apple allergy is mostly associated with allergies to peach. The allergenicity of apples differ across cultivars but only a few varieties were studied. Some factors changing apples allergenicity were identified, including unmodifiable and potentially modifiable factors for example cultivation method, ripening stage and storage conditions. Aim This review presents current knowledge about the molecular basis of apple allergenicity and factors influencing its level. Conclusions Selecting cultivars with low potential of allergenicity, removing apple peel and heat treatment could reduce the risk of severe allergy reaction incidence and presumably can be used in birch pollen immunotherapy.


| BACKGROUND
Apple tree fruits (Malus � domestica Borkh.) are a rich source of nutrients and nutraceuticals like polyphenols and other phytochemicals. The main components of apple phytochemicals are phenolic acids, dihydrochalcones, flavonoids (quercetin glycosides), catechins and oligomeric procyanidins as well as cyanidin glucosides in red fruits. 1 Due to the listed ingredients, apple may reduce risk of chronic diseases, through various mechanisms, including antioxidant or antiproliferative. 2 They may also improve the functioning of the digestive tract, regulate body mass and increase the respiratory efficiency of the body. 3 Unfortunately, apples could also be the cause of allergies including severe reactions.
About 5%-8% children and 2%-3% adults suffer from food allergy. 4 Allergies to fruits like apple, pear, peach, apricot, cherry, and to vegetables such as carrot, celery and potato are more frequent in older children and adults and they are predominantly associated with cross-reactivity between aeroallergens like tree pollens, grass or ragweed pollens and food allergens due to structural homology of some allergenic proteins. 5 In North and Central Europe, the most frequent example is the symptomatic response to raw apple in patients sensitised to birch tree pollens. 6 The primary sensitisation to allergenic molecules of Betula verrucosa (e.g., Bet v1) triggers the synthesis of specific IgE antibodies which are capable to cross-react with its homologues in apple (e.g., Mal d1). The clinical expression of such immune-mediated reaction includes rapid-onset pruritus of the oropharynx, angio-oedema, ears' pruritus and sometimes larynx constriction. These symptoms known as an Oral Allergy Syndrome (OAS) are usually mild and occur directly after exposure to the allergens. The apple allergens are heat-labile and susceptible to digestion thus the symptoms are rarely con-  Mal d 1 is identified as a 17-18 kDa protein of 158-159 amino acids encoded by 480-483 nucleotides. 9 Its biological function is connected with fungal and bacterial infection response due to the ribonuclease activity of proteins belonging to the pathogenesisrelated protein family (PR-10). Mal d 1 may also be involved in binding and transport of plant steroids and intracellular signalling. [10][11][12] The abiotic and biotic stress affects the content of Mal d 1 allergen.
Time and conditions of apple fruits storage may quantitatively alter the allergenic properties of their proteins. 13 Moreover, patients with birch pollen-related food allergies report the severity of their symptoms strongly dependent on apple variety and the degree of maturity. 14  Mal d 1 is encoded by 18 genes, seven of these are clustered into linkage group 13 (LG13), nine genes clustered into LG16 and one of them is unclustered. 9 The gene family was divided into five groups depending on number and size of introns and analysis of EST (expressed sequence tag). 15 9 Gao et al. 16  Proteins from that family are major allergens sensitising patients with non-pollen related allergies to Rosaceae fruits. [24][25][26][27] In Mediterranean countries, patients allergic to apples, but not sensitised to Mal d 4 is a cytosolic protein 12-15 kDa, 24 playing essential role in plant growth and development by participating in the regulation of actin filament polymerisation. 20 Allergy to Mal d 4 occurs mostly in the Mediterranean, with minor role in apple sensitisation. 26 This allergen is involved in sensitisation to fruits of other species and strongly cross-reacts with birch pollen Bet v 2 profilin. 13 The widespread presence of fucose and xylose on N-linked glycans of plants and in invertebrates may explain the high degree of crossreactivity that has been reported for CCD-specific IgE antibodies. 30 The clinical relevance of IgE antibodies to CCDs relies on the composition of the allergen-monovalent or multivalent with respect to the carbohydrate determinant.
To avoid misdiagnosis, an investigation of the presence of CCD antibodies should be conduct. Determination of anti-CCD IgE antibody in blood can be tested with bromelain or horseradish peroxidase, and also by the use of a test specific to MUXF3, a common plant glycan structure. 31 A positive in vitro test and a negative skin prick test to the same plant food allergen may indicate presence of noncross-linking CCD-specific IgE antibodies to that allergen. However, this phenomenon does not exclude cross-linking to other allergens with multivalent CCD epitopes or the presence of concomitant IgE antibodies to peptide epitopes. 32

| Allergenicity varies regards to apple tree varieties and cultivation method
Despite common allergies to apples, only a few studies assessing the amount of different apple allergens were conducted in commonly  (Tables 1 and 2). The extensive use of these popular cultivars has resulted in uniformity of commercial apple orchards and the limitation of genetic biodiversity. 33 The issue of the expression of genes encoding allergens in apples is also poorly understood. Even limited number of publications regarding the gene expression encoding apple allergens showed an association among gene expression and degree of maturity, storage conditions mainly with respect to the Mal d 1 gene (Table 1). Therefore, it is expected to expand similar research to a larger number of varieties, in particular with regard to old ones. There are only a few papers 40,44 describing allergen gene expression in old varieties, which are valuable in terms of taste, nutritional value, processing or for breeding. Nevertheless, cultivation of apple varieties with low allergenic potential is not developed. Currently, only Santana, Topaz and Elise cultivars, are considered hypoallergenic, and are quite well tolerated by patients with allergies. It has been shown that the Santana variety is characterised by considerable resistance to apple scab, thanks to which it is possible to significantly reduce the use of fungicides in its cultivation. 51 There is some evidence that pesticide treatment may lead to an even more robust response inducing higher expression of Mal d 1 than biotic factor. 40

| Biotic factors
Plants react to pathogen attacks (Table 3), wounding, UV-B radiation, osmotic shock, low temperature, water deficit, chemicals like ethylene or salicylic acid, inter alia by producing proteins belonging to the PR (Pathogen Resistance Proteins) family. Three of the four main apple allergens belongs to PR, which are connected with natural resistance to powdery mildew or/and to apple scab or to other stressors and chemicals. 53

| Allergenicity modifying factors
The allergenicity of apples is more complex due to the interactions of Mal d 1 protein with polyphenols (catechin) and enzymatic antioxidant system. The reaction between Mal d 1 and oxidised polyphenols can result in decrease of IgE binding as shown in Braeburn cultivar. 39 On the other hand, in Topaz, with high polyphenols content and low activity of PPO (polyphenol oxidase enzyme) conferring a high total anti-oxidative capacity, IgE binding to Mal d 1 is also reduced. 56 According to Schmits-Eiberger and Matthes, 39 in the cv. Braeburn, cv. Golden Delicious and cv. Topaz amount of total polyphenols were stable during maturation; however during storage, polyphenol content significantly decreased.
Traditional cultivation of apple varieties with low allergenic potentials is not well developed. Currently, Santana, Topaz and Elise cultivars, are considered as hypoallergenic, and are quite well tolerated in patients experiencing OAS syndrome. 40 Furthermore, it has been demonstrated that Santana is characterised by considerable resistance to apple scab, thanks to which it is possible to significantly  Currently, only the Topaz, Elise and Santana varieties are considered to be well tolerated by apple allergic patients.
Selecting cultivars with low potential of allergenicity, removing apple peel and heat treatment could reduce the risk of severe allergy reaction incidence and presumably can be used in birch pollen immunotherapy. Knowledge of the molecular mechanism of apples allergenicity and factors that modify the reaction severity could facilitate medical counselling and improve patients' care with allergies related with apple fruits.