BACKGROUND: Beta-lactoglobulin (BLG) is a bovine lipocalin in milk with an innate defense function. The circumstances under which BLG is associated with tolerance of or allergy to milk are not understood. OBJECTIVE: Our aims were to assess the capacity of ligand-free apoBLG versus loaded BLG (holoBLG) to protect mice against allergy by using an iron-quercetin complex as an exemplary ligand and to study the molecular mechanisms of this protection. METHODS: Binding of iron-quercetin to BLG was modeled and confirmed by spectroscopy and docking calculations. Serum IgE binding to apoBLG and holoBLG in children allergic to milk and children tolerant of milk was assessed. Mice were intranasally treated with apoBLG versus holoBLG and analyzed immunologically after systemic challenge. Aryl hydrocarbon receptor (AhR) activation was evaluated with reporter cells and Cyp1A1 expression. Treated human PBMCs and human mast cells were assessed by fluorescence-activated cell sorting and degranulation, respectively. RESULTS: Modeling predicted masking of major IgE and T-cell epitopes of BLG by ligand binding. In line with this modeling, IgE binding in children allergic to milk was reduced toward holoBLG, which also impaired degranulation of mast cells. In mice, only treatments with holoBLG prevented allergic sensitization and anaphylaxis, while sustaining regulatory T cells. BLG facilitated quercetin-dependent AhR activation and, downstream of AhR, lung Cyp1A1 expression. HoloBLG shuttled iron into monocytic cells and impaired their antigen presentation. CONCLUSION: The cargo of holoBLG is decisive in preventing allergy in vivo. BLG without cargo acted as an allergen in vivo and further primed human mast cells for degranulation in an antigen-independent fashion. Our data provide a mechanistic explanation why the same proteins can act either as tolerogens or as allergens.

Biotechnology Department ETSIAAB Center for Plant Biotechnology and Genomics CBGP Technical University of Madrid Madrid Spain

Childrens Hospital Bambino Gesù Rome Italy

Department of Cell Biology and Genetics Faculty of Science Palacky University Olomouc Czech Republic

Faculty of Science Division of Pharmacology Department of Pharmaceutical Sciences Utrecht University Utrecht The Netherlands

Institute of Pathophysiology and Allergy Research Center of Pathophysiology Infectiology and Immunology Medical University of Vienna Vienna Austria

The Interuniversity Messerli Research Institute of the University of Veterinary Medicine Vienna Medical University Vienna and University Vienna Vienna Austria

The Interuniversity Messerli Research Institute of the University of Veterinary Medicine Vienna Medical University Vienna and University Vienna Vienna Austria; Institute of Pathophysiology and Allergy Research Center of Pathophysiology Infectiology and Immunology Medical University of Vienna Vienna Austria

The Interuniversity Messerli Research Institute of the University of Veterinary Medicine Vienna Medical University Vienna and University Vienna Vienna Austria; Laboratory Medicine and Immunology Department Faculty of Medicine Menoufia University Menoufia Egypt

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