Api m 4
Summary
Api m 4, or melittin, is a marker allergen for genuine sensitization to Apis mellifera (honeybee) venom (HBV). Api m 4 sensitization is observed in 17 - 54% of HBV-allergic patients. Accounting for 50% of HBV dry weight, Api m 4 is also a powerful cytolytic, pain-inducing, and proinflammatory peptide acting in synergy with Api m 1, the HBV phospholipase A2 (PLA2).
Epidemiology
Worldwide distribution
The prevalence of sensitization to Api m 4 varies from 17% to 54% in HBV-allergic patients [1]. Api m 4 is usually considered a minor allergen in HBV-allergic patients but has been reported as a major allergen in studies from Spain [2,3,4].
Clinical Relevance
Specific molecules
Api m 4 is a marker allergen for genuine HBV sensitization [6]. Apparent monosensitization to Api m 4 is infrequent, reported at 1-2% in HBV-allergic patients [7].
Cross-reactive molecules
Api m 4 homologues with high sequence identity are present in other Hymenoptera venoms from the Apis (bee) and Bombus (bumblebee) genera, called bombolittins for the latter [8], as well as from Vespids, however, an allergenic activity has not been reported for these homologues [6, 9, 10].
Disease severity
An Api m 4-related phenotype with increased severity and decreased tolerance to venom immunotherapy (VIT) has been described in studies from Spain [3,4]. Further studies are warranted since these associations were not observed in subsequent studies within other locations.
Systemic reactions and Anaphylaxis
Among HBV-allergic patients with Api m 4-specific IgE at 0.98 kUA/L or greater, 83% developed systemic reactions grade 3 or 4 in Müller’s classification following field stings, compared with 42% in the control group comprising HBV-allergic patients with Api m 4-specific IgE lower than 0.98 kUA/L [4].
Diagnostics
Diagnosis of genuine sensitization to Apis mellifera venom
Api m 4 is a marker allergen for HBV sensitization. It is especially useful as an additional marker allergen, in combination with Api m 3 and Api m 10, in populations with lower prevalence of IgE to Api m 1, since demonstrated genuine sensitization to HBV supports the initiation of HBV VIT in eligible patients [2,6,7]. Since Vespid venoms lack clinically relevant Api m 4 homologues, Api m 4 is also useful for discrimination between HBV and Vespid sensitization [2].
Disease severity
In Hymenoptera venom IgE testing, the quantitative result of specific IgE to a molecular allergen or whole venom extract is currently considered as neither predictive of, nor correlated to the severity of the reaction [6]. This view may change in the future, since reports of identifiable phenotypes associated with certain molecular sensitization patterns and levels of allergen-specific IgE, such as the presence and high levels of Api m 4-specific IgE, are available for some populations [3,4]. An increased severity at diagnosis and during VIT has been reported in Spanish HBV-allergic patients [3,4].
Sensitivity of in vitro assays
The prevalence of sensitization to individual HBV allergens, including Api m 4, 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 purified, recombinant or synthetic allergen, and assay format [6]. Api m 4-specific antibodies have been measured using purified, recombinant, and synthetic peptides, with the latter reportedly displaying greater detection sensitivity, e.g. 43% with the purified peptide versus 54% with the synthetic peptide [2].
Functional studies with basophil activation assays are scarce. An early study of HBV molecular allergens reported the lack of Api m 4-induced basophil upregulation of the activation marker CD203c, however, actual Api m 4 sensitization of the recruited HBV-allergic patients had not been assessed [14].
Api m 4 sensitization can be detected with commercially available singleplex and multiplex methods, with variable availability among manufacturers and over time.
Diagnostic specificity
The diagnostic specificity of Api m 4-specific IgE for HBV sensitization has been consistently reported at very high levels, estimated at 100% since no healthy controls and no Vespid-only allergic patient displayed Api m 4-specific IgE [4,7].
AIT Prescription
Api m 4 is a marker of genuine sensitization to HBV, thus supporting the choice of HBV AIT in eligible patients [6]. An association between Api m 4 sensitization greater than 0.98 kUA/L, increased risk of adverse systemic reactions to VIT and a success rate of controlled sting challenge of 82% (compared to 100% in the control group) after one or two years of VIT were reported in Spanish HBV-allergic patients [3,4]. Thus, elevated levels of Api m 4-specific IgE before VIT initiation have been proposed as predictive for poor tolerance of HBV VIT [3].
A robust increase in Api m 4-specific IgG4 levels was observed during VIT, a finding also used to confirm the presence of Api m 4 in VIT preparations [4,7].
Prevention And Therapy
Experimental trials
Api m 4 immunogenicity is low, due to its small size and high water solubility; moreover, the safe therapeutic interval is narrow, due to its cytotoxicity. Current therapeutic applications of Api m 4 aim to harnessing its anti-tumor effects, both direct via cytotoxicity and indirect via immune response elicitation [8, 11].
Explained Results
Allergen Information
Api m 4 or melittin, a small cationic and amphiphilic defense peptide, is the most abundant HBV component, accounting for half of HBV dry weight. Api m 4 exerts IgE-dependent (allergic) and IgE-independent (Api m 1 activation, lipid membrane disruption, cytolysis, pain, immune modulation) effects.
Clinical information
Api m 4-specific IgE identifies genuine HBV sensitization and discriminates it from Vespid sensitization in patients who are double positive to HBV and Vespid venom extracts. In HBV-allergic patients, the prevalence of Api m 4 sensitization ranges from 17 to 54% and its diagnostic specificity is 100%.
Cross-reactivity
Api m 4 does not cross-react with Vespid venom homologues.
Author: Dr. Joana Vitte
References
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- Guha S, Ferrie RP, Ghimire J, Ventura CR, Wu E, Sun L, et al. Applications and evolution of melittin, the quintessential membrane active peptide. Biochem Pharmacol. 2021;193:114769.
- IUIS/WHO. IUIS/WHO Apis mellifera 2023 [Available from: http://allergen.org/search.php?allergenname=&allergensource=apis+mellifera&TaxSource=&TaxOrder=&foodallerg=all&bioname=.
- UniProt. UniProt P01501 for Api m 4 2023 [Available from: https://www.uniprot.org/uniprotkb/P01501/entry.
- Yu X, Dai Y, Zhao Y, Qi S, Liu L, Lu L, et al. Melittin-lipid nanoparticles target to lymph nodes and elicit a systemic anti-tumor immune response. Nat Commun. 2020;11(1):1110.
- Gilbert RJ, Dalla Serra M, Froelich CJ, Wallace MI, Anderluh G. Membrane pore formation at protein-lipid interfaces. Trends Biochem Sci. 2014;39(11):510-6.
- Seppala U, Francese S, Turillazzi S, Moneti G, Clench M, Barber D. In situ imaging of honeybee (Apis mellifera) venom components from aqueous and aluminum hydroxide-adsorbed venom immunotherapy preparations. J Allergy Clin Immunol. 2012;129(5):1314-20 e3.
- Binder M, Fierlbeck G, King T, Valent P, Buhring HJ. Individual hymenoptera venom compounds induce upregulation of the basophil activation marker ectonucleotide pyrophosphatase/phosphodiesterase 3 (CD203c) in sensitized patients. Int Arch Allergy Immunol. 2002;129(2):160-8.