Background Human basophils and mast cells express the low-affinity immunoglobulin (Ig)G receptor FcγRIIB. It has previously been shown in artificial model systems that cross-linking of the high-affinity IgE receptor FcɛRI and FcγRIIB leads to inhibition of FcɛRI signalling.
Objective The aim of the present study was to investigate whether cross-linking of FcɛRI and FcγRIIB contributes to IgG-mediated inhibition of histamine release in human basophils in a system using the sera from specific immunotherapy (SIT) patients and the major allergen from birch pollen, Bet v 1. As IgG4 furthermore has been proposed to have special blocking properties, we investigated the significance of IgG subclass specificity for this inhibition.
Methods Binding of recombinant Bet v 1–IgG complexes to FcγRII and IgG-binding activities in the sera from 25 birch pollen-allergic patients treated with SIT were measured using 125I-rBet v 1. Inhibition of basophil histamine release was assessed by incubating washed leucocytes with complexes of rBet v 1–IgG with or without blocking of FcγRII.
Results We observed low binding of rBet v 1–IgG complexes to FcγRII, which was negatively correlated with the relative IgG4-binding activities. Blocking of FcγRII did not reverse the SIT–IgG-induced inhibition of basophil histamine release. However, IgG-binding activities correlated significantly with the ability of the SIT sera to inhibit basophil histamine release.
Conclusion We suggest that at least in birch pollen SIT, the contribution of FcγRIIB-mediated inhibitory signalling to SIT–IgG-induced inhibition of human basophil histamine release is of minor importance. The main contributor to the inhibitory effect of SIT-induced IgG seems to be blocking of the allergen–IgE interaction.