Rat basophil leukaemia (RBL) cells sensitized with low affinity IgE respond to high valency antigen

Background The very low concentrations of IgE antibodies in serum tnake investigations of the affinity of allergen-specific antibodies extremely difficult. In the absence of such studies, the fact that low IgE concentrations are capable of inducing powerful effector function has encouraged the view that IgE antibodies are typically high affinity antibodies. Yet the phenomenon of allergic cross-reactivity suggests that lower affinity IgE antibodies may sometitnes be of clinical significance. Objectives To investigate the effect of antibody affinity upon mast cell sensitivity in an in vitro model. Methods Rat basophil leukaemia (RBL) cells were sensitized with one of three monoclonal IgE antibodies which bind to trinitrophenyiated proteins with varying affinity. Serotonin release was measured after challenge of sensitized cells with trinitrophenyiated human serum albumin (TNP-HSA). Results Low valency TNP₃-HSA failed to stimulate degranulation of RBL cells sensitized with SPE-7 anti-DNP IgE, which binds TNP with low affinity. However, upon challenge with high concentrations (1250 ng/mL) of TNP₈-HSA, or as little as 10 ng/mL of highly substituted TNP₂₃-HSA, low levels of degranulation were seen. A similar relationship between antigen valency and cell sensitivity was seen with cells sensitized with the H-le-DNP anti-DNP IgE. which binds with moderate affinity to TNP proteins. Conclusion High valency antigen is capable of activating RBL cells sensitized with low affinity antibody. This has important implications for our understanding of allergic sensitization. It also suggests that the long-rccognized relationship between antigen valency and RBL cell sensitivity may partly reflect the high functional affinity of cell-bound IgE when directed against multivalent antigen.