Ligand Binding of PR-10 Proteins with a Particular Focus on the Bet v 1 Allergen Family.

PURPOSE OF REVIEW: Pathogenesis-related class 10 (PR-10) proteins are highly conserved plant proteins, which are induced in response to abiotic and biotic stress factors. To date, no unique biological function could be assigned to them. Rather a more general role of PR-10 in plant development and defense mechanisms has been proposed. In addition, some PR-10 proteins act as allergens by triggering allergic symptoms in sensitized individuals. Regardless of the diversity of reported activities, all PR-10 proteins share a common fold characterized by a solvent-accessible hydrophobic cavity, which serves as a binding site for a myriad of small-molecule ligands, mostly phytohormones and flavonoids. RECENT FINDINGS: Most of available data relate to the ligand binding activity of allergenic PR-10, particularly for those belonging to Bet v 1 family of allergens. Bet v 1 and its homologues were shown to bind flavonoids with high affinity, but the specificity appears to differ between homologues from different species. The flavonoid Q3O-(Glc)-Gal was shown to specifically bind to hazelnut Cor a 1 but not to Bet v 1. Similarly, Q3OS bound only to the major isoform Bet v 1.0101 and not to other closely related isoforms. In contrast, Bet v 1 and hazelnut Cor a 1 showed very similar binding behavior towards other flavonoids such as quercetin, genistein, apigenin, daidzein, and resveratrol. Recent research findings highlighted the importance of more precise knowledge of ligand binding for understanding the functional diversification of PR-10 proteins.

When the allergy alarm bells toll: The role of Toll-like receptors in allergic diseases and treatment.

Toll-like receptors of the human immune system are specialized pathogen detectors able to link innate and adaptive immune responses. TLR ligands include among others bacteria-, mycoplasma- or virus-derived compounds such as lipids, lipo- and glycoproteins and nucleic acids. Not only are genetic variations in TLR-related genes associated with the pathogenesis of allergic diseases, including asthma and allergic rhinitis, their expression also differs between allergic and non-allergic individuals. Due to a complex interplay of genes, environmental factors, and allergen sources the interpretation of TLRs involved in immunoglobulin E-mediated diseases remains challenging. Therefore, it is imperative to dissect the role of TLRs in allergies. In this review, we discuss i) the expression of TLRs in organs and cell types involved in the allergic immune response, ii) their involvement in modulating allergy-associated or -protective immune responses, and iii) how differential activation of TLRs by environmental factors, such as microbial, viral or air pollutant exposure, results in allergy development. However, we focus on iv) allergen sources interacting with TLRs, and v) how targeting TLRs could be employed in novel therapeutic strategies. Understanding the contributions of TLRs to allergy development allow the identification of knowledge gaps, provide guidance for ongoing research efforts, and built the foundation for future exploitation of TLRs in vaccine design.

Birch Pollen Induces Toll-Like Receptor 4-Dependent Dendritic Cell Activation Favoring T Cell Responses.

Seasonal exposure to birch pollen (BP) is a major cause of pollinosis. The specific role of Toll-like receptor 4 (TLR4) in BP-induced allergic inflammation and the identification of key factors in birch pollen extracts (BPE) initiating this process remain to be explored. This study aimed to examine (i) the importance of TLR4 for dendritic cell (DC) activation by BPE, (ii) the extent of the contribution of BPE-derived lipopolysaccharide (LPS) and other potential TLR4 adjuvant(s) in BPE, and (iii) the relevance of the TLR4-dependent activation of BPE-stimulated DCs in the initiation of an adaptive immune response. In vitro, activation of murine bone marrow-derived DCs (BMDCs) and human monocyte-derived DCs by BPE or the equivalent LPS (nLPS) was analyzed by flow cytometry. Polymyxin B (PMB), a TLR4 antagonist and TLR4-deficient BMDCs were used to investigate the TLR4 signaling in DC activation. The immunostimulatory activity of BPE was compared to protein-/lipid-depleted BPE-fractions. In co-cultures of BPE-pulsed BMDCs and Bet v 1-specific hybridoma T cells, the influence of the TLR4-dependent DC activation on T cell activation was analyzed. In vivo immunization of IL-4 reporter mice was conducted to study BPE-induced Th2 polarization upon PMB pre-treatment. Murine and human DC activation induced by either BPE or nLPS was inhibited by the TLR4 antagonist or by PMB, and abrogated in TLR4-deficient BMDCs compared to wild-type BMDCs. The lipid-free but not the protein-free fraction showed a reduced capacity to activate the TLR4 signaling and murine DCs. In human DCs, nLPS only partially reproduced the BPE-induced activation intensity. BPE-primed BMDCs efficiently stimulated T cell activation, which was repressed by the TLR4 antagonist or PMB, and the addition of nLPS to Bet v 1 did not reproduce the effect of BPE. In vivo, immunization with BPE induced a significant Th2 polarization, whereas administration of BPE pre-incubated with PMB showed a decreased tendency. These findings suggest that TLR4 is a major pathway by which BPE triggers DC activation that is involved in the initiation of adaptive immune responses. Further characterization of these BP-derived TLR4 adjuvants could provide new candidates for therapeutic strategies targeting specific mechanisms in BP-induced allergic inflammation.