Molecular characterization of a human immunoglobulin G4 antibody specific for the major birch pollen allergen, Bet v 1

Background Allergen‐specific IgG4 antibodies induced by specific immunotherapy are thought to represent a protective immune response. Objective Our aim was the molecular characterization of a human IgG4 antibody (BAB5) specific for the major birch pollen allergen Bet v 1 that was derived from an immunotherapy‐treated patient. Methods The cDNA coding for BAB5 was obtained by reverse transcriptase‐PCR from the BAB5‐producing cell line, compared with the germ line sequences and was expressed as a soluble antibody fragment in Escherichia coli. The epitope specificity and cross‐reactivity of BAB5 were investigated with recombinant and synthetic Bet v 1 fragments and Bet v 1 homologous allergens from pollen. The ability of BAB5 to block allergic patients IgE was determined by competition experiments and sandwich ELISA. Results BAB5 is an affinity‐matured Bet v 1‐specific IgG4 antibody that reacts exclusively with Bet v 1 but not with Bet v 1‐related allergens. Unlike an earlier‐described monoclonal IgG1‐blocking antibody, BAB1, which had been isolated from the same patient, BAB5 did not block allergic patients' IgE reactivity to Bet v 1. Conclusion Our study demonstrates that not all allergen‐specific IgG antibodies inhibit IgE recognition of allergens and can contribute to the success of immunotherapy. The epitope specificity and affinity of IgG antibodies but not their isotype are decisive for their protective activity.