Allergènes moléculaires des pollens : où en sommes-nous ?

Abstract

Pollen allergens belong to various families of PR (i.e. pathogenesis-related) proteins involved in plant defense mechanisms, such as PR-2 (endo-glucanases), PR-3 (chitinases), PR-5 (thaumatin-like proteins or TLP), PR-10 (Bet v 1-like proteins), PR-12 (defensins) and PR-14 (lipid transfer proteins or LTP). They may also comprise proteins with other functions such as structural function (profilin, expansins), metabolic function (polcalcin) or enzymatic function (ribonucleases, pectate lyases, polygalacturonases) (e.g. ribonucleases, pectate lyases and polygalacturonases). The structural scaffolds of the major weed and tree pollen allergens have been identified either by X-ray crystallography or by NMR for smaller molecules such as LTP and polcalcin. Other pollen allergens of unknown structure have been successfully modelled from templates belonging to the same protein families. Epitope mapping of free allergens and analysis of allergen-IgE complexes enabled identification of IgE-binding B-cell epitopes at the surface of a few pollen allergens. A higher number of T-cell epitopes were identified along the amino acid sequence of various pollen allergens likely to interact with CD4+ T lymphocytes via T-cell receptors (TCRs). However, this low-affinity recognition of T-cell epitopes by TCRs is degenerate, allowing a single TCR to recognize different T-cell epitopes, and, conversely, enabling many different TCRs to recognize the same T-cell epitope. The identification of both B- and T-cell epitopes of pollen allergens is of paramount importance in optimizing the use of natural and recombinant pollen allergens as molecular tools for the component-resolved diagnosis of pollinoses and in improving current immunotherapy practice, for instance by reducing side effects associated with immunotherapy. In addition, identification of B- and T-cell epitopes may provide valuable assistance in the prediction of IgE-binding cross-reactivity and co-sensitization among pollen allergens of different origins, and between pollen and fruit or vegetable allergens responsible for oral allergy syndrome (OAS). Our review aims to present an overview of all these molecular aspects of pollen allergens.