Background: Approximately 35% to 40% of patients with chronic urticaria possess a circulating antibody directed to the alpha subunit of the high-affinity type I IgE receptor (FcepsilonRI), which is detectable by using histamine release assays or immunoblotting. Prior reports suggest that purified IgG may not directly activate basophils but rather does so through complement activation.
Objective: We sought to further elucidate the mechanism by which this antibody causes basophil histamine release, including the role of complement, and to reassess the relationship of functional versus binding assays.
Methods: We incubated human basophils with patient serum, patient IgG, or patient IgG plus normal serum as a complement source and measured histamine release for each condition. IgG fractions were neutralized with cloned alpha subunit to determine whether histamine release decreased proportionately. We also screened sera from 260 patients to compare histamine release with immunoblotting results.
Results: We initially tested 35 sera from patients with chronic urticaria by using basophils from 2 atopic donors and one nonreleaser with rabbit anti-IgE. No histamine was released from the nonreleaser, yet all donors responded identically to monocyte chemotactic protein 1, indicating a requirement for IgE or the IgE receptor. Basophil histamine release was markedly augmented by complement if release by IgG alone was low. Incubation of purified IgG with an increasing concentration of cloned alpha subunit gradually reduced the histamine-releasing capability in patients with positive or negative immunoblot results. Of 260 patients tested, 43% had positive histamine release results, and 47% had positive immunoblot results, yet there was no correlation when individual patients were assessed.
Conclusion: A subpopulation of patients with chronic urticaria possess IgG antibody directed to the alpha subunit of FcepsilonRI. This IgG activates basophils, which is dependent on or augmented by complement. Binding assays for the FcepsilonRI alpha subunit, such as immunoblotting, are not currently feasible as a screening method. A functional assay is required.