i208 Api m 1

Summary

Api m 1, a glycoprotein of the phospholipase 2 (PLA2) family, is a major allergen of Apis mellifera (honeybee) venom (HBV) and a marker allergen for genuine sensitization to this venom. 

Epidemiology

Worldwide distribution

Api m 1 sensitization has a prevalence of 57% to 97% among HBV-allergic populations (reviewed in [1]). Thus, Api m 1 is consistently found as a major allergen in HBV-allergic patients.

Environmental Characteristics

Source and tissue

Api m 1 is an enzyme secreted into the venom sac of Apis mellifera, where it contributes 12-16% of venom dry weight [1, 2]. Its production exhibits seasonal variations, similar to Api m 4 (mellitin) [3].

Risk factors

Sensitization to Api m 1 occurs through injection (bee sting). Being stung simultaneously by a large number of bees increases the risk of systemic manifestations in sensitized, but also non-sensitized subjects, as Api m 1 exerts toxic effects independent of its allergenic potency [1, 4].

Clinical Relevance

Specific molecules

Of the 12 known allergens within HBV, Api m 1 is considered the most prominent allergen, in terms of prevalence of sensitization, levels of specific IgE to Api m 1, and quantitative correlation between Api m 1 and HBV specific IgE [5-10]. Api m 1 does not cross-react with phospholipases of the PLA1 family found in Vespid venoms, therefore, Api m 1 is a marker allergen allowing to discriminate between HBV and Vespid venom sensitization [1].

Besides Api m 1, sensitization to other HBV marker allergens may achieve a prevalence of 50% or higher among HBV-allergic patients:  Api m 3, Api m 4, and Api m 10 [1].

Cross-reactive molecules

Other PLA2 venom allergens have been characterized in the Apis (bee) and Bombus (bumblebee) genera, but not in Vespids [1, 11]. Api m 1 and other PLA2 allergens do not allow discrimination between HBV and bumblebee venom sensitization [1].

Disease severity

Various patterns of sensitization to HBV allergens have been described, but no clinical correlate of severity has been identified so far [1].

Prevention and Therapy

Experimental trials

In one study, recombinant hypoallergenic variants of Api m 1 were produced with altered structural folding. This prevented recognition of Api m 1 by IgE, which relies on the allergen having the correct three-dimensional structure. Unlike correctly folded Api m 1 variants, which induced IgE secretion and TH2-type cytokine production in cultures of peripheral blood mononuclear cells, the incorrectly folded variants stimulated production of IgG4 and TH1-dominant cytokines [9].

Some studies involving short, immunodominant T cell epitope peptides derived from Api m 1 suggest that these peptides may be capable of suppressing the immune response against the entire allergen in HBV-allergic patients [9].

Molecular Aspects

Biochemistry

Api m 1 is a calcium-dependent hydrolase of the secreted PLA2 family, with a molecular weight of 16 kDa (167 aminoacids), displaying disulfide bonds and an N-glycosylation site with mannose, N-acetylglucosamine and fucose lateral chains [1, 3].

Following activation by melittin [4], Api m 1 targets cellular, bacterial, or surfactant phospholipids [4, 12]. Api m 1 action leads to the release of free fatty acids and lysophospholipids, thus initiating the eicosanoid pathway of proinflammatory effects and also exerting direct toxicity [1, 4, 12]. Api m 1 displays multifaceted immune effects, including Treg induction [13, 14], corroborating an effect observed with HBV in beekeepers [15]  but also induction of type 2 responses when used an adjuvant for peripheral sensitization to a distinct antigen [12].

Isoforms, epitopes, antibodies

As of July 5, 2023, a unique isoallergen, Api m 1.0101, has been included in the World Health Organization (WHO) and International Union of Immunological Societies (IUIS) Allergen Nomenclature [11].

Two IgE-binding sequential epitopes were described on Api m 1 and were shown to induce IgG and IgE production when fused with a bacteriophage-derived carrier [16]. Later, structural and functional comparison of purified versus recombinant Api m 1 showed higher allergenic potency with basophil activation assays, suggesting the presence of carbohydrate epitopes on nApi m 1 in addition to protein-based ones on rApi m 1, and the potential contribution of these carbohydrate epitopes to in vivo allergenicity of Api m 1 [17].

Cross-reactivity due to structural similarity

 Api m 1 displays aminoacid sequence similarity of 50% or higher restricted within the Apis genus: Apis cerana (oriental or asiatic honeybee), and Apis mellifera carnica (Carniolan honeybee) [3]. However, cross-reactivity among PLA2 allergens from HBV and bumblebee venom needs to be considered, as it precludes their use for discriminating sensitization between these two venoms [1].

Diagnostic Relevance

Diagnosis of genuine sensitization to Apis mellifera venom

Api m 1 is a marker allergen for HBV sensitization. Therefore, the demonstration of specific IgE to Api m 1 confirms genuine sensitization to HBV or bumblebee venom, and supports the initiation of HBV VIT in eligible patients [1, 18].

With a prevalence of up to 97% in HBV-allergic patients, IgE to Api m 1 is a relevant clinical tool [1]. However, in populations of HBV allergic patients with lower figures of prevalence of IgE to Api m 1, using additional HBV marker allergens such as Api m 3, Api m 4 and Api m 10 increases the chance of demonstrating genuine sensitization to HBV [1, 8, 19].

Disease severity

In Hymenoptera venom IgE testing, the quantitative result of specific IgE to a molecular allergen or whole venom extract is neither predictive of, nor correlated to the severity of the reaction [1].

Sensitivity of in vitro assays

The prevalence of sensitization to individual HBV allergens, including Api m 1, in HBV-allergic patients varies depending on multiple factors such as geography, patient inclusion criteria, single or double positivity to HBV and Vespid venoms, use of a recombinant allergen from bacterial or insect cell expression versus a natural purified allergen, and assay format [1, 6, 7, 10, 17, 19-21]. Thus, the diagnostic sensitivity of specific IgE to rApi m 1 ranges from 56 to 97% in HBV-allergic patients [1, 8, 10, 19].  Using a panel of HBV allergens such as Api m 1, Api m 2, Api m 3, and Api m 10 improves the rate of confirmation of genuine HBV sensitization, albeit with important variations as a function of geography and patient clinical history [1, 8, 10, 22].

Api m 1 sensitization can be detected with commercially available singleplex and multiplex methods. Intermethod comparison showed good agreement between singleplex and multiplex methods for the detection of Api m 1 sensitization [21], and between different singleplex methods for quantitative assessment of IgE to rApi m 1 [22]. 

Diagnostic specificity

The diagnostic specificity of IgE to rApi m 1 for HBV allergy has been consistently reported at very high levels, 97 to 100% [5, 10, 19, 22]. The use of recombinant Api m 1 retains IgE reactivity comparable to the native protein but eliminates CCD binding, thus improving the identification of species-specific sensitization [1, 7, 17].

AIT Prescription

Demonstrated sensitization to Api m 1 confirms genuine sensitization to HBV, thus supporting the choice of HBV AIT in eligible patients [1].

Given the high prevalence of Api m 1 sensitization in HBV allergic patients, this allergen is the best candidate for designing AIT candidates such as mimotopes or hypoallergenic derivatives [9, 16].

Compiled By

Author: Prof. Joana Vitte

Reviewer: Dr. Merima Mehic

Last reviewed: 2023-07-17