Beta-lactoglobulin (BLG) is a bovine lipocalin in milk with innate defense function. It is not understood under which circumstances BLG is associated with tolerance or allergy towards milk.
Here we assessed the capacity of ligand-free (apo-) versus -loaded (holo-) BLG to protect mice against allergy using iron-quercetin as exemplary ligand, and studied the molecular mechanisms.
Binding of iron-quercetin into BLG was modeled and confirmed by spectroscopy and docking calculations. Serum IgE binding of milk allergic and tolerant children to apo- and holo-BLG was assessed. Mice were intranasally treated with apo- versus holo-BLG and after systemic challenge immunologically analysed. AhR activation was evaluated with reporter cells and Cyp1A1 expression. Treated human peripheral blood mononuclear cells and human mast cells were assessed by FACS and degranulation, respectively.
Modelling predicted masking of major IgE and T cell epitopes of BLG by ligand-binding. In line, IgE binding of milk allergic children was reduced towards holo-BLG which also impaired degranulation of mast cells. In mice, only treatments with holo-BLG prevented allergic sensitization and anaphylaxis, while sustaining regulatory T cells. BLG facilitated quercetin-dependent AhR-activation and, downstream AhR, lung Cyp1A1 expression. Holo-BLG shuttled iron into monocytic cells and impaired their antigen-presentation.
The cargo of holo-BLG is decisive in preventing allergy in vivo. BLG without cargo acted as allergen in vivo and further primed human mast cells for degranulation in an antigen-independent fashion. Our data provide mechanistic explanation why the same proteins can either act as tolerogen or allergen.

Copyright © 2020. Published by Elsevier Inc.