Iron-Dependent allergenicity of Alt a 1: A link between fungal nutritional immunity and allergic sensitization

The major fungal allergen Alt a 1 from Alternaria alternata is linked to allergic asthma. We assessed its biological role in nutritional immunity to iron and its allergenic potential using in silico, in vitro, and in vivo approaches. Alternaria was cultured with or without iron and with quercetin to promote iron-quercetin (FeQ2) complexes, followed by Alt a 1 expression analysis, which revealed enhanced expression under iron deficiency and reduced levels by addition of quercetin. In silico and RBLsx38 mast cell degranulation assays showed that FeQ2 binding to Alt a 1 (holoAlt a 1) masked the IgE epitope Y87-D96 by promoting tetramer-formation of Alt a 1, reducing IgE binding and antigen-specific degranulation compared to ligand-free Alt a 1 (apoAlt a 1). In BALB/c mice, intranasal exposure to apo- or holoAlt a 1 before intraperitoneal Alt a 1/alum sensitization demonstrated that holoAlt a 1 exposure led to lower allergen-specific antibody titers, fewer mature antigen-presenting cells, increased regulatory T cells, and reduced allergic symptoms upon challenge compared to mice exposed to apoAlt a 1. Additionally, human PBMCs incubated with holoAlt a 1 exhibited fewer Th cells and plasmablasts, but more immature B cells with a higher iron content than those exposed to apoAlt a 1. These findings demonstrate that iron-poor conditions trigger nutritional immunity in Alternaria, increasing Alt a 1 expression, and that apoAlt a 1, through iron scavenging, has greater sensitizing capacity than the holo form, influencing immune responses relevant to allergic asthma. One Sentence Summary: Iron-poor conditions activate nutritional immunity in Alternaria, increasing the expression of allergenic dimeric Alt a 1, whereas its iron-bound tetrameric form does not trigger an immune response and thus prevents allergy development.