Inhalant Mammal-Derived Lipocalin Allergens and the Innate Immunity

Abstract

A major part of important mammalian respiratory allergens belongs to the lipocalin family of proteins. By this time, 19 respiratory mammalian lipocalin allergens have been registered in the WHO/IUIS Allergen Nomenclature Database. Originally, lipocalins, small extracellular proteins (molecular mass ca. 20 kDa), were characterized as transport proteins but they are currently known to exert a variety of biological functions. The three-dimensional structure of lipocalins is well-preserved, and lipocalin allergens can exhibit high amino acid identities, in several cases more than 50%. Lipocalins contain an internal ligand-binding site where they can harbor small principally hydrophobic molecules. Another characteristic feature is their capacity to bind to specific cell-surface receptors. In all, the physicochemical properties of lipocalin allergens do not offer any straightforward explanations for their allergenicity. Allergic sensitization begins at epithelial barriers where diverse insults through pattern recognition receptors awaken innate immunity. This front-line response is manifested by epithelial barrier-associated cytokines which together with other components of immunity can initiate the sensitization process. In the following, the crucial factor in allergic sensitization is interleukin (IL)-4 which is needed for stabilizing and promoting the type 2 immune response. The source for IL-4 has been searched widely. Candidates for it may be non-professional antigen-presenting cells, such as basophils or mast cells, as well as CD4+ T cells. The synthesis of IL-4 by CD4+ T cells requires T cell receptor engagement, i.e., the recognition of allergen peptides, which also provides the specificity for sensitization. Lipocalin and innate immunity-associated cell-surface receptors are implicated in facilitating the access of lipocalin allergens into the immune system. However, the significance of this for allergic sensitization is unclear, as the recognition by these receptors has been found to produce conflicting results. As to potential adjuvants associated with mammalian lipocalin allergens, the hydrophobic ligands transported by lipocalins have not been reported to enhance sensitization while it is justified to suppose that lipopolysaccharide plays a role in it. Taken together, type 2 immunity to lipocalin allergens appears to be a harmful immune response resulting from a combination of signals involving both the innate and adaptive immunities.

Structural and ligand binding analysis of the pet allergens Can f 1 and Fel d 7

“…We were unable to see melting temperature change and proteolytic degradation of Can f 1 and Fel d 7 in the absence and presence of fatty acids (data not shown) likely due to the high intrinsic stability which has been noted for other lipocalins (35,36). In other allergens like Bla g 1, the bound ligand can further stabilize these proteins potentially affecting antigen processing (37, 38).…”

Decoding the immune landscape: a comprehensive analysis of immune-associated biomarkers in cervical carcinoma and their implications for immunotherapy strategies

“…It is increasingly evident that lipocalins (LCNs) play multifaceted biological roles in regulating cell proliferation, differentiation, apoptosis, and senescence ( Flower, 1995 ; 1996 ; Akerstrom et al, 2000 ; Grzyb et al, 2006 ). They are also associated with the regulation of immune responses/inflammation, odor reception, reproduction, cancer development, and metabolic disorders, as well as cardiovascular remodeling ( Virtanen, 2021 ; Ganfornina et al, 2022 ; Redl and Habeler, 2022 ). Our study establishes a positive correlation between LCN10 and PD-1 expression while showing a negative correlation with PD-L1 expression, further underscoring the connection of LCN10 with immune responses.…”

EAACIMolecular Allergology User's Guide 2.0

“…Lipocalin allergens were found to elicit weak adaptive cellular immune responses, e.g., T cell epitopes of Bos d 2 and Can f 1 were only recognized suboptimally by human T cells. It is thus likely that a major contribution to allergenicity may be based on innate immunity, receptor binding, or their role in ligand binding 464 . The milk allergen Bos d 5, ß‐lactoglobulin, was shown to bind many ligands, mainly long‐chain fatty acids, resulting in changes in structure and resistance to denaturation 465 .…”

“…It is thus likely that a major contribution to allergenicity may be based on innate immunity, receptor binding, or their role in ligand binding. 464 The milk allergen Bos d 5, ß-lactoglobulin, was shown to bind many ligands, mainly long-chain fatty acids, resulting in changes in structure and resistance to denaturation. 465 However, Bos d 5 also binds quercetin-iron complexes and in this case the ligand load seems to provide an immune-regulatory effect and protection against allergic sensitization to birch pollen allergens in mice.…”