t241 Pla a 1

Summary

Pla a 1, a non-glycosylated protein, is a major allergen of London plane (Platanus acerifolia) tree pollen. Pla a 1 belongs to the pectine methyl esterase/invertase inhibitor family. Relevant cross-reactivity to Pla a 1 is limited to pollen homologues from other Platanaceae trees. Therefore, Pla a 1 sensitization is a marker of genuine sensitization to pollens of the Platanaceae family.

Epidemiology

Worldwide distribution

The London plane tree (Platanus acerifolia), a cosmopolitan, wind-pollinated, mainly urban and ornamental species, is a major cause of pollen sensitization in exposed populations [1, 2].

Pla a 1 was identified as a major plane tree pollen allergen, binding to IgE from up to 92% of monosensitized Platanus-allergic patients [3]. However, most patients sensitized to P. acerifolia are also sensitized to other pollens, and the molecular profile of sensitization to P. acerifolia differs according to climate conditions [1, 4]. The prevalence of Pla a 1 IgE is lower in subjects with positive skin prick tests to P. acerifolia pollen allergenic extracts but lacking a clear history of Platanus pollen allergy and/or presenting with multiple pollen elicitors, often as a result of polysensitization and panallergen involvement [4-6]. In patients with P. acerifolia pollinosis confirmed by nasal challenge with the culprit extract, Pla a 1 IgE was positive in 58.8% (10/17) [7].

Overall, studies in general population and unselected, either suspected or confirmed, allergic patients have found prevalence figures for Pla a 1 sensitization at less than 5% in different parts of Europe and Asia [8-10]. As an example, the prevalence of Pla a 1 sensitization was 1.77% and ranked 46th among 75 common allergenic molecules in a large scale study of Italian subjects with a suspicion of airborne or food allergy [11].

Monosensitization to Pla a 1 is infrequent, seen in 2-10% of Pla a 1 sensitized subjects [4, 11].

Environmental Characteristics

Source and tissue

Pla a 1 is present in low amounts in plane tree pollen, 0.08% of PBS-extracted total protein content [3]. Its main subcellular location in mature pollen grains is the endoplasmic reticulum [12]. Pollen grain hydration and activation results in rapid diffusion and extracellular release of Pla a 1 [12]. 

Risk factors

The main risk factor for developing sensitization to Pla a 1 is exposure to P. acerifolia pollen [4]. Gender bias in Pla a 1 sensitization was not observed [11].

Clinical Relevance

Detailed information regarding P. acerifolia pollen is available in the whole allergen section. P. acerifolia pollinosis manifests as allergic rhinitis and asthma, which are often associated [3, 7]. Food allergy associated to plane tree pollinosis is not related to Pla a 1 sensitization, but rather to panallergens such as profilins and lipid transfer proteins [5-7].

Disease severity and prediction

Pla a 1 sensitization is associated with intermittent allergic rhinitis, rhinoconjunctivitis, and asthma with a seasonal pattern [7].

Cross-reactive molecules

London tree pollen has been implicated in cross-reactivity with various plant allergenic sources, including pollen from other trees, from grass, and weeds, and more importantly plant foods, e.g. peach [1, 7]. To date, Pla a 1-induced cross-reactivity is not considered relevant for species outside the Platanaceae family [1].

Prevention and Therapy

Experimental trials

Not relevant.

Molecular Aspects

Biochemistry

Pla a 1 is a basic protein with a molecular weight of 18 kDa, found in monomeric form and non-glycosylated in its native form [3]. It belongs to the biochemical family of plant invertase/pectin methylesterase inhibitors, however, an invertase inhibition activity was not demonstrated [13]. The rapid release of Pla a 1 outside the pollen grain after hydration suggests a role in pollen growth and germination [12].

Isoforms, epitopes, antibodies

As of December 1st, 2021, one isoallergen of Pla a 1 i.e., Pla a 1.0101, has been identified and officially published by the World Health Organization (WHO) and International Union of Immunological Societies (IUIS) Allergen Nomenclature [14].

Cross-reactivity due to structural similarity

Pla a 1 exhibits sequence identity of 92% with Pla or 1, its homologue found in Platanus orientalis pollen, and potentially relevant sequence identity of 30-40% with homologues from grape and tomato [13, 15]. 

Diagnostic Relevance

Marker allergen for genuine sensitization to Platanaceae pollens

Pla a 1 is recommended as a marker of genuine sensitization to Platanaceae pollen [1]. Among 28 patients with evocative symptoms and positive skin prick tests to plane extract receiving a nasal challenge test to plane tree pollen, Pla a 1 IgE detection exhibited a sensitivity of 58.8% (10/17) and a specificity of 90.9% (10/11) [7]. Testing for Pla a 1 and Pla a 2 IgE has been suggested as a means of detecting genuine plane tree sensitization, instead of the plane pollen extract [16].

Cross-Reactivity

Pla a 1 cross-reactivity is limited to similar allergens from Platanaceae pollen [1].

AIT Prescription

Detectable Pla a 1 IgE confirms genuine sensitization to P. acerifolia, hence assisting with AIT selection [1].

Exposure

The main route of exposure is through inhalation of P. acerifolia and other Platanaceae pollen.

Compiled By

Author: Joana Vitte

Reviewer: Dr. Christian Fischer

Last reviewed: February  2022