Omalizumab-induced reductions in mast cell FcεRI expression and function

Background: By design, omalizumab binds free IgE in the circulation and prevents its attachment to the surface of mast cells and basophils, thereby preventing them from responding to allergens. Previously, omalizumab rapidly reduced free IgE levels, as well as basophil high-affinity IgE receptors, leading to significant reductions in basophil mediator response to allergen. It is assumed that tissue mast cells are similarly altered in their FcepsilonRI density and function. Objective: We examined the phenotypic shift of skin mast cells in parallel to that of blood basophils in 3 subjects infused with omalizumab. Methods: Three subjects with allergic rhinitis underwent intradermal skin test titration with house dust mite antigen at days 0, 7, 70, and 196 of omalizumab treatment. As control subjects, 5 untreated subjects with allergic rhinitis were evaluated at similar time points. All subjects underwent skin biopsy 18 to 24 hours later at the site of allergen injection. Biopsy specimens were characterized by means of immunohistochemisty for tryptase and FcepsilonRIalpha immunoreactivity, as well other markers (CD3, CD45RO, CD68, cutaneous lymphocyte antigen, and major basic protein). Results: Omalizumab recipients, but not control subjects, demonstrated reductions in FcepsilonRIalpha immunoreactivity at days 70 and 196 in parallel with reductions in the acute wheal response to allergen. However, no reductions in tryptase-positive cells were noted at these time points. Conclusion: Reductions in free IgE levels by omalizumab leads to a rapid reduction in basophil FcepsilonRI receptor expression. In contrast, the time course for the decrease of FcepsilonRI expression in skin mast cells is slower and associated with decreased acute allergen wheal size.