Allergenic molecules
At the time of writing, 17 peanut allergens have been identified. The biochemical properties and molecular mass of peanut allergens are summarized below (55).
Peanut Allergen |
Biochemical property |
Molecular weight (kDa) |
---|
Ara h 1 |
Cupin (Vicilin-type, 7S globulin) |
64 |
Ara h 2 |
Conglutin (2S albumin) |
17 |
Ara h 3 |
Cupin (Legumin-type, 11S globulin, glycinin) |
60 (37 – fragment) |
Ara h 5 |
Profilin |
15 |
Ara h 6 |
Conglutin (2S albumin) |
15 |
Ara h 7 |
Conglutin (2S albumin) |
15 |
Ara h 8 |
Pathogenesis-related protein, PR-10, Bet v 1 family member |
17 |
Ara h 9 |
Non-specific lipid-transfer protein type 1 |
9.8 |
Ara h 10 |
Oleosin |
16 |
Ara h 11 |
Oleosin |
14 |
Ara h 12 |
Oleosin |
8 |
Ara h 13 |
Oleosin |
8 |
Ara h 14 |
Oleosin |
17.5 |
Ara h 15 |
Oleosin |
17 |
Ara h 16 |
Non-specific lipid-transfer protein type 2 |
8.5 |
Ara h 17 |
Non-specific lipid-transfer protein type 1 |
11 |
Ara h 18 |
Cyclophilin |
18 |
Peanut Allergen |
Biochemical property |
Molecular weight (kDa) |
---|
To date, a number of peanut allergens have been identified. Many of them have protective functions or are seed storage proteins. Peanut allergens belong to diverse protein families leading to immunochemical IgE-mediated cross-reactions among different members of the legume families but also other plant foods such as tree nuts (Bublin and Breiteneder 2014). Peanut allergens can be grouped according to their components; cupins (Ara h 1 and 3), conglutins (Ara h 2, 6 and 7), profilin (Ara h 5), PR-10 protein (Ara h 8), oleosins (Ara h 10, 11, 14, and 15), non-specific lipid transfer proteins (Ara h 9, 16, 17), defensins (Ara h 12 and 13) and cyclophilins (Ara h 18) (56, 57).
Currently, Ara h 1, 2, 3 and 6 are considered to be the major peanut allergens (56, 58). It is estimated that 97% of peanut allergy patients are sensitized to at least one of the allergens Ara h 1, 2 and 3 and these components provide >30% of the total protein content of peanuts (56, 59).
Using sera from 40 peanut-allergic patients, 14 individual recognition patterns were identified and the following frequency of specific IgE binding emerged demonstrating that Ara h 2 had the highest frequency of recognition: Ara h 1 was recognized by 65%, Ara h 2 by 85%, Ara h 4 (which is now known to be an isoform of Ara h 3, Ara h 3.02 (56)) by 53%, Ara h 5 by 13%, Ara h 6 by 38% and Ara h 7 by 43% of the sera (60).
Another study involving 30 peanut-allergic individuals, the majority of patients with a positive SPT were sensitized to Ara h 2 (83%) and Ara h 6 (87%). Sixteen patients (53%) were sensitized to Ara h 1 and half of the patients to Ara h 3. In addition, patients with a positive SPT to Ara h 1 and/or Ara h 3 were also sensitized to Ara h 2 and/or Ara h 6 (61).
Biomarkers of severity
Individuals who are sensitized to the storage proteins, Ara h 1, 2, 3, 6, and 7, are at an increased risk for more severe symptoms and anaphylactic reactions (62). A study found specific IgE was positively correlated with clinical severity for Ara h 1, 2 and 3 in adult patients but this trend was not observed in children (63). In addition, another study found that sensitization to rAra h 2 and rAra h 1 and/or rAra h 3 appeared to be predictive of more severe reactions (64). Similarly, the results of a recent cross-sectional study involving 222 children in Australia found that polysensitization to Ara 1, 2 and 3 can help to predict the severity of reaction at challenge (65).
The results of a meta-analysis study on the diagnostic accuracy of peanut components demonstrated that specific IgE to Ara h 1, 2 and 3 are highly specific for peanut allergy in children, however, Ara h 8 and 9 are of low clinical relevance (66). In addition, the authors suggest that specific IgE to Ara h 2 can reduce the number of oral food challenges in unclear cases and has high diagnostic accuracy for peanut allergy in children across geographic locations (66).
Cross-reactivity
Peanut and tree nut show cross reactivity in 25 to 50% of peanut-allergic patients because Ara h 2 shares IgE-binding epitopes with almond and Brazil nut allergens (7). However, despite 2S seed albumin allergens sharing structural similarities, a major peanut allergen, Ara h 2, showed no structural homology with the corresponding regions of Walnut Jug r 1, Pecan Car i 1 or Brazil nut Ber e 1 (67). Although there is a theory of cross reactivity being dependent on allergens sharing a similar sequence and/or structure, there is experimental data highlighting a lack of relationship between the percentage of shared identity and the ability to bind IgE (68). Similarities in physicochemical properties to known IgE epitopes of 2S albumins could account for clinically observed cross-sensitivity between peanuts and tree nuts (68).
Patients with peanut allergy may demonstrate sensitization to other legumes, for example soybean, lupin, lentil or pea, although currently limited data are available (69). Cross-reactivity exists among legumes because they have structurally homologous proteins and share common epitopes. However, peanut allergy associated with other legumes is less frequently reported compared to the cross-reactivity demonstrated for other types of legumes (7) Clinically, peanut allergen cross-reactivity can be relevant but in some cases, irrelevant with approximately half of patients having positive SPTs to other legumes, however, less than 5% show clinical signs when consuming them (58).
The Bet v 1 homologous peanut allergen Ara h 8, is thought to be involved in the cross reactivity of peanut allergy patients. Twenty patients with peanut and birch pollen allergy, and a positive double-blind, placebo-controlled food challenge result to peanut, all experienced symptoms in the oral cavity, progressing to more severe symptoms in 40% of patients (70). Recombinant Ara h 8-specific IgE was demonstrated in 85%, IgE binding to Ara h 8 was inhibited by Bet v 1 and recombinant Ara h 8 inhibited IgE binding to peanut in four of seven tested patient sera. The study concludes that peanut allergy might be mediated in a subgroup of the patients by cross-reactivity of Bet v 1 with the homologous peanut allergen Ara h 8 (70). Similarly, examination of the sera of five patients in relation to four recombinant allergens led to the conclusion that IgE cross-reactivity existed between Bet v 1 (birch pollen) and its homologues Gly m 4 from soybean, Ara h 8, and Pru av 1 from Cherry but variation in IgE specificity between patients was observed (71). In addition, Ara h 8 is involved in grass pollen-associated food allergy (69).
Lupins are an emerging cause of food allergy because of recent large-scale introduction as an additive to wheat flour or the use of lupin flour in processed foods and is frequently used in Europe and Australia (72, 73). They are legumes of the Fabaceae family with PR-10 white lupin sharing significant sequence homology and molecular similarity between the peanut allergen Ara h 8 (72, 73). Sanz et al. (2010) explained that in patients with clinical sensitization to a legume, particularly peanut, greater caution is required depending on the population with lupin cross-reactivity of approximately 5% in British and Norwegian populations, 17% to 68% for French and Belgian populations, and 32% for Danish populations (73).