High levels of naturally occurring IgG antibodies inhibit IgE-facilitated antigen-presentation in adults with cow's milk allergy

Background: Cow's milk allergy (CMA) in adults is rare, sometimes severe, and in >60% adult-onset. In addition to CM-specific IgE, also high levels of IgG1 and IgG4 are observed in these patients, which are even higher than birch pollen (BP)-specific IgG levels after BP-specific immunotherapy (SIT). Induction of naturally occurring IgG antibodies to CM is related to CM exposure in early life. However, little is known about the role of these IgG antibodies in CMA. In BP allergy, IgG antibodies induced by SIT decrease IgE-facilitated antigen-presentation (IgE-FAP), thereby inhibiting T cell activation. In this study, the functional role of IgG in IgE-FAP in CMA was analyzed. Methods: Serum of CMA patients was incubated with CM-allergen at several concentrations to generate immunoglobulin (Ig)-allergen complexes. Complexes were incubated with EBV-transformed B cells or PBMC of healthy individuals to allow binding. Composition of complexes on B cells or PBMC was determined by FACS using anti-IgE, IgG1, IgG4 antibodies. Binding of complexes to CD23 or CD32 on B cells or PBMC was analyzed, using CD23- or CD32-blocking antibodies. The potential to induce IgE-FAP was studied by stimulation of CM-specific T cells with EBV-B cells after binding of complexes, before and after depletion of IgG antibodies using IgG-depletion columns. Results: In the presence of CM-specific IgG, IgE-FAP was inefficient, and only occurred at high allergen concentrations (10-100x higher than BP complexes in BP allergy). Complexes consisted of IgE, IgG1 and IgG4 antibodies. Blocking CD23 and CD32 showed that binding occurred primarily through CD23. In PBMC, complexes bound mostly to B cells via CD23, but not to monocytes. Ig-allergen complexes hardly enhanced stimulation of CM-specific T cells as compared to pinocytosis of CM allergen. Depletion of IgG from the serum resulted in more efficient complex formation and enhanced T cell proliferation at lower allergen concentrations, confirming its blocking capacity. Conclusion: Naturally occurring IgG antibodies in CMA patients inhibit IgE-FAP by B cells. IgE-FAP has been proposed to play a role in enhancement of T cell memory during exposure to very low concentrations of allergens. So, IgG may play a functional role in controlling the T cell response towards CM. Despite the presence of blocking IgG, patients are still allergic to CM, indicating that additional factors play a role in the induction and maintenance of tolerance to CM.