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|Biological Function||Lipid transfer protein.|
|Source Material||Apple extract|
|Latin Name||Malus x domestica|
|Other Names||Lipid transfer protein|
|Categories||Fruits, Food Of Plant Origin|
|Molecular Weight||9 kDa|
Mal d 3 is a 9 kDa, non-specific lipid transfer protein (nsLTP) and is recognized as an important pan-allergen of apple (Malus domestica). The prevalence of apple allergy is reported commonly in European countries, with Mal d 3 sensitization mostly reported in the Mediterranean regions. Mal d 3 sensitization in apple-allergic patients can cause severe systemic allergic reactions like diarrhea, vomiting, urticaria, hypotension, asthma, and even anaphylaxis. Mal d 3 is mainly found in the apple peel and is a highly stable allergen as it is resistant to heat as well as pepsin digestion. Two isoallergens of Mal d 3 are reported i.e., Mal d 3.01 and Mal d 3.02. Mal d 3 sensitization can serve as a marker for primary sensitization to apple allergy and can differentiate true apple allergy from the birch-apple syndrome. Extensive cross-reactivity may be observed between Mal d 3 and LTP-containing foods (Rosaceae family members like peach) or pollen-LTP due to high structural homology and sequence identity. Furthermore, the sequence identity of Mal d 3 is also observed with other LTP like mustard (Sin a 3), lettuce (Lac s 1) and raspberry (Rub i 3).
Allergy to various fruits of the Rosaceae family is commonly seen (1). The European Community Respiratory Health Survey involving sera of 4522 individuals from 13 countries stated the overall prevalence of sensitization to apple was more common in European countries (Germany, Belgium, Italy, Norway, Sweden, France, and Spain) as compared to the United Kingdom, the United States and Australia (2).
Mal d 3, a potential allergen of Malus domestica (apple), is a well-known non-specific lipid-transfer protein (nsLTP) and an important pan-allergen, from apple (3, 4). Apple allergy due to Mal d 3 sensitization is mostly seen in the Mediterranean countries of Europe (4).
A prospective cross-sectional study was conducted among 389 apple-allergic patients from the Netherlands, Austria, Italy, and Spain. Mal d 3 sensitization was found to be highest in Spain as compared to other countries (1). Interestingly, another study conducted among 81 apple-hypersensitive patients in Spain demonstrated Mal d 3 sensitization in 37% of these patients (5). Similarly, high immunoglobulin E (IgE) sensitization to Mal d 3 (68%) was observed in a study on 19 Spanish apple-allergic patients (6).
Furthermore, in a prospective study conducted among 13 children with food and inhalant allergy in Poland, 7 children showed a positive response to oral food challenge (OFC) to apple. All 13 children including the 7 OFC positive children were sensitized to Mal d 3 (7). Also, another prospective study on 21 children with birch and apple allergy found Mal d 3-IgE reactivity in 95% of children (8).
Mal d 3, is restricted in the epidermis of the apple (peel), portraying a distinct pattern of distribution compared to other apple allergens. The allergen content in the peel depends on the type of apple cultivar, position on the tree, storage after harvesting, and apple maturity. The levels of Mal d 3 were found to be more when harvested from the shady site as compared to the sunny site when picked from the lower part of the tree as compared to the upper part or in fully mature fruit ready for consumption (9).
The molecule can be generated by recombinant expression systems. Mal d 3 has been reported to be expressed in the Escherichia coli or Pichia pastoris. Among different methods of chromatography, ion-exchange chromatography is one of the methods used for the purification of Mal d 3 (3, 10).
Mal d 3, like other LTPs, is a highly stable allergen and can cause severe systemic allergic reactions like diarrhea, vomiting, urticaria, hypotension, asthma, and even anaphylaxis (11).
Mal d 3 sensitization can be considered to be a risk factor for severe systemic reactions to apple. This is demonstrated by a prospective, cross-sectional study which was conducted among 389 apple-allergic patients from the Netherlands, Austria, Italy, and Spain. More than 35% of systemic reactions (urticaria, anaphylaxis, anaphylaxis shock) in Spain may be attributed to sensitization to Mal d 3 which was found to be significantly associated (p<0.0001) (1).
Interestingly, in a study among 81 apple-hypersensitive patients, specific IgE (sIgE) to Mal d 3 was found to be similar in patients with mild symptoms (oral allergy syndrome, 38% of 35) and patients with generalized symptoms (37.5% of 46). OAS was observed in 43.2% (35 out of 81) patients and generalized symptoms were observed in 56.8% (46 out of 81) patients. (5).
Furthermore, an interesting case of anaphylaxis was reported in a 12-year-old male attributed to sensitization to apple LTP (Mal d 3). The patient had a history of sensitization to profilin of fruits of Rosaceae family, grasses, and pollens. However, serious severe reactions were reported after consumption of apple before physical exercise. This was ascribed to LTP sensitization following exposure to a co-factor like physical exercise. Due to the co-factor presence, the gastrointestinal absorption of food was accelerated which revamped the symptom severity of the allergic reactions and led to anaphylaxis (12).
LTPs are one of the important allergens found common in Rosaceae fruit family, various vegetables, and nuts (3, 13). nsLTPs are referred to as plant pan-allergens (3) due to their widespread distribution among plant-foods and pollens (13, 14). The LTPs from different plant-foods and pollens can cross-react with each other, which may cause multiple plant-food sensitizations, and lead to LTP syndrome. However, it may be confounded with other factors (13, 14).
Further, a study on 389 apple-allergic patients found a significant association of Mal d 3-sIgE reactivity to IgE reactivity to Bet v 1 (birch pollen), and pollens of mugwort and plane tree. Bet v 1 sensitization was found to be negatively correlated (p<0.0001) while pollens of mugwort (p=0.07) and plane tree (p=0.001) were positively correlated. Thus, Bet v 1 sensitization in apple-allergic patients can offer a protective effect against the development of severe reactions to apple due to Mal d 3. Moreover, the study also found a significant correlation of Mal d 3-IgE levels with peach-IgE levels (correlation coefficient r=0.60) (1).
LTPs are pan-allergens belonging to the group of PR-14 proteins. They are small-sized (9 kDa), single polypeptide chain proteins with ~90 amino acid residues. Structurally, they form a core of 4 α-helices extending into a coiled C-terminal tail. This compact fold is stabilized by 4 disulfide bridges formed from 8 conserved cysteine residues (3). These disulfide linkages may be responsible for their extreme stability against heat and pepsin digestion. They are mainly concentrated in the epidermis of fruit (peels) to transfer lipids between membranes. Besides, they also have a role in the defense mechanism, growth, and development of plants (14).
Mal d 3 is a 9 kDa protein in apple peel (15). Two isoallergens of Mal d 3 namely; Mal d 3.01 and Mal d 3.02 are reported based on the amino acids and DNA sequence similarity (16).
Several isoforms of Mal d 3 isoallergens have been identified. The isoallergen Mal d 3.01 has 2 isoforms, Mal d 3.02 has 3 isoforms. These isoforms have been officially listed in the database of the WHO/IUIS Allergen Nomenclature Sub-Committee (16, 17).
Mal d 3 is found to cross-react with LTPs from several members of the Rosaceae family due to structural homology (11, 18). The LTP from apple (Mal d 3) and peach (Pru p 3) exhibit 80% sequence homology (14). Further, the similarity between Mal d 3.01 and Pru p 3 is noted owing to amino acid sequence, epitope regions, and antibody recognition sites. Moreover, Mal d 3.02 varies at a single position with Pru p 3. The sequence identity of Pru p 3 with Mal d 3.01 and Mal d 3.02 was found to be 80% and 77%, respectively. However, Mal d 3.01 may be indicated as more relevant to apple LTP allergenicity (16).
Furthermore, Sin a 3 (LTP from mustard) demonstrated a sequence identity in a range of 51-55% with Mal d 3. The similarity between Sin a 3 and Mal d 3 was observed to 64% (19). Also, Lac s 1 (LTP from lettuce) reported sequence identity of 67% and similarity of 92% with Mal d 1 (18). Besides, Rub i 3 (LTP from raspberry) demonstrated 74% sequence identity with Mal d 3 (15).
Mal d 3 is resistant to heat and pepsin digestion and hence can lead to severe systemic reactions and act as true food allergens (14).
LTPs can be a reliable diagnostic tool in diagnosing true apple allergy. A prospective study was conducted among 21 children with birch pollen and apple allergy. The birch-apple syndrome is mainly due to inhalant allergens. However, in this study, Mal d 3 sensitization was found in 93.3% of 15 birch-negative patients. Thus, it could be suggested that LTP sensitization can be helpful to identify a true allergy to apple rather than birch-apple syndrome (8).
A study was conducted among 182 patients sensitized to Mal d 3 and/or Pru p 3 from Antwerp (Belgium) and Barcelona (Spain) to evaluate the diagnostic sensitivity of the allergen components to identify clinically relevant allergies. Mal d 3 sensitization was observed in 130 patients. It was found that the ratio of sIgG4/sIgE to Mal d 3 demonstrated a sensitivity of 60% and specificity of 75% (p=0.04) to diagnose clinically relevant LTP related allergy attributed to Mal d 3 sensitization (20).
The main exposure route for this allergen is through ingestion (21).
Author: Turacoz Healthcare Solutions
Reviewer: Dr. Fabio Iachetti
Last reviewed: February 2021