f427 Ara h 9

Summary

Ara h 9 is non-specific lipid-transfer protein (nsLTP) peanut allergen component. Regional variation has been reported for IgE sensitization to Ara h 9 and therefore it is considered to be a predictor of clinical peanut allergy in Spanish and Asian populations. LTPs are stable and able to withstand thermal and protease treatment. Patients who are LTP-sensitized can display systemic clinical signs in addition to symptoms associated with oral allergy syndrome. A study found that patients who were monosensitized to Ara h 9 were more likely to experience symptoms of bronchospasm compared to patients who were not sensitized. Ara h 9 IgE has limited diagnostic accuracy for peanut allergy because of cross-reactivity between LTPs however, it can be valuable when diagnosing allergy symptoms due to cross-reactions with other LTPs found in pollen and fruit. Cosensitization with peach Pru p 3 and mugwort Art v 3 LTPs have been observed.

Epidemiology

Worldwide distribution

Studies have typically reported peanut allergy prevalence rates between 1–2% in Western nations. Peanut allergy appears to be less common in Asia and other global areas, although epidemiological studies in non-Western regions have been sparse (1). Peanut allergy usually begins in childhood and persists throughout the affected individual’s lifetime however, approximately 20% of young children develop tolerance (2).

IgE sensitization to Ara h 9 is estimated to affect 10–20% of the population in the USA (3), with Ara h 9 sensitization identified in 11% and 22% of peanut-allergic patients in Sweden and Austria, respectively (4, 5). Similarly, Ara h 9 sensitization was detected in 20% of 192 peanut-allergic patients in the UK (6). Regional differences have been identified with 90% (29/32) prevalence in peanut-allergic Spanish patients (Mediterranean group) whereas four out of six USA patients and two of 35 German patients were sensitized to Ara h 9 in the non-Mediterranean peanut-allergic group (7). Over 80% of peanut-sensitized patients recruited for a study in China were sensitized to Ara h 9 and 12 out of 18 peanut-allergic patients were monosensitized to this allergen component (8). In summary, Ara h 9 is considered to be a predictor of clinical peanut allergy in Spanish and Asian populations (3). 

Environmental Characteristics

Source and tissue

Ara h 9 is a non-specific lipid-transfer protein (nsLTP) peanut allergen component (9). Lipid-transfer proteins are abundantly found in most tissues (10). 

Clinical Relevance

Patients who are LTP-sensitized can display systemic clinical signs in addition to symptoms associated with oral allergy syndrome, commonly experienced with birch pollen-related food allergies (11). With LTPs being resistant to both heat and proteases, they are able to trigger systemic reactions as well as local symptoms (12).

Patients monosensitized to Ara h 9 were more likely to experience symptoms of bronchospasm compared to patients who were not sensitized (26% versus 9%; P = 0.05) (6). Ara h 9-sensitized patients were older however, the association between bronchospasm and Ara h 9 sensitization remained significant even after adjusting for age (6).

Cross-reactive Molecules

Thirteen patients who were allergic to peanuts were also allergic to peaches (Pru p 3) and presented with the same clinical symptoms. However, two of these patients experienced more severe symptoms to peanuts compared to peaches (8).

Molecular Aspects

Biochemistry

Ara h 9 has a molecular mass of 9.8 kDa and is therefore a type 1 lipid transfer protein (LTP) (10, 13). LTPs are stable and able to withstand thermal and protease treatment which is considered to be as a result of four α-helices stabilized by four disulfide bonds (11). Their biological function is to facilitate the transport of phospholipids and other fatty acids across cell membranes (14). In addition, LTPs are involved in plant defense against pathogens under a range of environmental stresses such as drought, heat, cold, or salt (14). LTPs are classified as pathogenesis-related (PR) proteins, PR-14 (14).

Isoforms, epitopes, antibodies

Two isoforms exist, Ara h 9.0101 and Ara h 9.0201, which share 90% sequence identity with Ara h 9 (9, 13).

Cross-reactivity

The two isoforms of Ara h 9 share 60–70% identical amino acid sequence with LTPs from other plant produce (15).

Peach LTP, Pru p 3 could be a sensitizing allergen in people with peanut allergy for whom the major peanut allergens did not contribute to primary sensitization (11). Ma et al. (2016) reported cosensitization of Ara h 9 with Pru p 3 and also the mugwort nsLTP, Art v 3. Art v 3 has an important role in peach sensitization, but currently the relationship between Ara h 9 and Art v 3 is yet to be fully explored (8).

Diagnostic Relevance

Cross-reactivity

The results of a meta-analysis study found that Ara h 9 sIgE has limited diagnostic accuracy because of cross-reactivity between LTPs (16). However, IgE to Ara h 9 can be valuable to use when diagnosing allergy symptoms due to cross-reactions with other LTPs found in pollen and fruit (17).

Compiled By

Author: RubyDuke Communications

Reviewer: Dr. Magnus Borres

Last reviewed: December 2020