Ana o 3
Summary
Ana o 3 is a water-soluble, 2S albumin, seed-storage protein in cashew nuts (Anacardium occidentale), having a molecular weight of 14 kDa. The prevalence of cashew nut allergy is widespread across the globe, with Ana o 3 being the major allergen component of cashew nut. The sensitization of Ana o 3 is responsible for severe systemic reactions and anaphylaxis in cashew-allergic individuals. This protein is resistant to gastric and intestinal digestion by pepsin and trypsin. The immunoreactivity of Ana o 3 is reduced by thermal processing methods, like blanching, autoclaving or roasting (10 min), however it may increase when subjected to high temperature processing (170℃ for 20 min). Further, Ana o 3 can serve as a marker for primary sensitization to cashew nuts. Also, monosensitization to Ana o 3 without sensitization to other food allergens could be the most potent predictor of severe anaphylaxis. Extensive cross-reactivity is reported among cashew nuts and pistachio due to cross-reactivity among their 2S albumins i.e. Ana o 3 and Pis v 1 (pistachio). Besides, Ana o 3 is also cross-reactive with Jug r 1 (walnut) and to some extent with Cor a 14 (hazelnut).
Epidemiology
Worldwide distribution
Allergy to cashew nuts (Anacardium occidentale) is a typical immunoglobulin (IgE)-mediated allergic disorder, being reported from the United States, Australia and Europe and sparsely from Asia and Africa. Ana o 3 is considered as one of the major allergens of cashew nut belonging to the prolamin superfamily of proteins.
A study conducted in Germany found that 81% of sera obtained from cashew-allergic patients were reactive to Ana o 3 on immunoblot analysis. Another study performed in Germany reported increased levels of serum-specific immunoglobulin E (sIgE) towards Ana o 3 in 69% of cashew-allergic patients (42 out of 61), while in only 5% of cashew sensitized but tolerant patients (1 out of 19). Furthermore, Ana o 3-IgE sensitization was found in 27% of cashew and peanut-sensitized individuals in a study in Finland (26 out of 98).
In a Japanese study conducted on 95 patients with suspected cashew allergy, sensitization to Ana o 3 (based on sIgE levels) was found in 100% (n=26) of cashew-allergic patients and in 51% (n=69) of tolerant, but cashew-sensitized patients.
Clinical Relevance
Disease severity
Ana o 3 is one of the major allergens of cashew nut and is responsible for severe systemic reactions and anaphylaxis in allergic individuals. In a study conducted on 227 children with severe allergic reactions or anaphylaxis, Ana o 3 (Ana o 3) was found as the most frequent cause of severe anaphylaxis (14.3%). Furthermore, the severe anaphylaxis rate with systemic reactions was found to be 84.6%, due to Ana o 3.
A study performed among 100 Greek children reported recombinant Ana o 3-sIgE sensitization in 93% of cashew-allergic children and in only 6% of cashew-tolerant children.
Sensitization to Ana o 3 was found in all (100%) of the cashew-allergic patients based on oral food challenge (OFC), while it was found in only 51% of cashew-tolerant patients. The OFC-positive patients depicted moderate-to-severe reactions, which included gastrointestinal reactions (73%) as most common, followed by oral mucosa (69%), skin reactions (50%), respiratory reactions (38%) and cardiovascular reactions (15%).
Cross-reactive molecules
Cross-reactivity between cashew and pistachio has been widely reported and this is attributed to their homologous seed storage proteins. A study conducted on 2 groups of patients evaluated the cross-sensitization profiles of the cashew-sensitized patients among Anacardiaceae family members (pistachio, mango and pink peppercorn). The first group (n=7) consisted of patients having co-sensitization to only Anacardiaceae species, while the second group (n=11) consisted of patients with co-sensitization to other tree nuts in addition to Anacardiaceae species. It was reported that, in both the patient groups, the 2S albumins of cashew nut (Ana o 3) and pistachio (Pis v 1) were identified on the western blots.
A study conducted in Germany reported that 87% of cashew-allergic patients sensitized to Ana o 3 also suffered from walnut allergy (13 out of 15), and this may be due to cross-reactivity between Ana o 3 and Jug r 1.
Co-sensitization between cashew nut and hazelnut allergy has been reported in a large study conducted in the US, however, the correlation between the sIgE values of Ana o 3 and Cor a 14 was found to be low (correlation coefficient=0.43). Furthermore, a low-to-moderate correlation was reported between the 2S albumin of cashew nut (Ana o 3) and 11S globulin of hazelnut (Cor a 9) (correlation coefficient=0.50). Another study observed that the cross-reactivity was found to be rare between the 2S albumins of cashew nut and hazelnut (Ana o 3 and Cor a 14 respectively).
Diagnostics
Disease Severity
A study conducted on 227 children with systemic allergic reactions or anaphylaxis reported that sensitization to Ana o 3 can display good predictive ability for identifying cashew nut allergies, however this sensitization has shown to increase the risk of severe anaphylaxis in cashew-allergic individuals. The study further suggested that monosensitization to Ana o 3 without sensitization to other food allergens could be the most potent predictor of severe anaphylaxis.
In a study conducted on 100 Greek cashew-allergic children, Ana o 3-sIgE levels of 0.1 kUA/l displayed a diagnostic sensitivity and specificity of 98% and 94%, respectively, thus proving it to be of high diagnostic value. Further, a study performed on 61 cashew-sensitized subjects in Germany, reported differentiating ability of Ana o 3-sIgE between clinically allergic and tolerant children to cashew nuts. The predictive ability of Ana o 3-sIgE (95% at 2.0 kUA/l) for clinically relevant cashew nut allergy was found to be significantly higher (p<0.005) than sIgE for whole cashew extract having AUC (area under the curve) value of 0.9 as compared to 0.7 for cashew extract.
In a prospective, multicenter study performed on 173 children with suspected cashew nut allergy, sIgE to Ana o 3 was able to better discriminate between clinically allergic or tolerant individuals in comparison with sIgE to whole cashew extract or skin prick test.
Further, a Japanese study conducted on 95 cashew-sensitized children also confirmed the predictive ability of Ana o 3 sensitization for clinically relevant cashew nut allergy based on OFC. The study reported a clinical sensitivity and specificity of 69% and 96%, respectively for Ana o 3-sIgE levels of 2.2 kUA/l.
Cross-reactivity
A study conducted to isolate and purify cashew allergens: Ana o 1, Ana o 2 and Ana o 3, have reported cross-reactivity between Ana o 1 and Ana o 3. Ana o 1 was able to inhibit the IgE-binding to Ana o 3, and to lesser extent with Ana o 2.
Exposure
The main route of exposure is through ingestion.
Cross-Reactivity
A high degree of cross-reactivity has been observed between cashew nuts and pistachios, due to structural similarity between their homologous proteins. A similarity in amino acid sequences of Ana o 3 with its homologous 2S albumin protein i.e. Pis v 1 (pistachio) has been found to be 66%.
The sequence identity between 2S albumin of cashew (Ana o 3) and hazelnut (Cor a 14) has been reported to be 43%. Further, Ana o 3 IgE-epitope was found to overlap with the corresponding epitope of walnut allergen Jug r 1, that showed 81% sequence homology with Ana o 3.
References
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- Lange L, Lasota L, Finger A, Vlajnic D, Büsing S, Meister J, et al. Ana o 3-specific IgE is a good predictor for clinically relevant cashew allergy in children. Allergy. 2017;72(4):598-603.
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- Bastiaan-Net S, Reitsma M, Cordewener JHG, van der Valk JPM, America T, Dubois AEJ, et al. IgE Cross-Reactivity of Cashew Nut Allergens. International archives of allergy and immunology. 2019;178(1):19-32.
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- Costa J, Silva I, Vicente AA, Oliveira M, Mafra I. Pistachio nut allergy: An updated overview. Critical reviews in food science and nutrition. 2019;59(4):546-62.
- van der Valk JP, Gerth van Wijk R, Vergouwe Y, Steyerberg EW, Reitsma M, Wichers HJ, et al. sIgE Ana o 1, 2 and 3 accurately distinguish tolerant from allergic children sensitized to cashew nuts. Clinical and experimental allergy : journal of the British Society for Allergy and Clinical Immunology. 2017;47(1):113-20.