Persistence of early crosslink-dependent signal transduction events in human basophils after desensitization

Desensitization of human basophils with anti-IgE antibody or antigen induced an increased sensitivity to the phorbol ester TPA, evidenced as 2-5 fold increase in the potency of TPA to induce histamine release. As noted in previous publications the magnitude of the change in sensitivity to TPA was a function of the extent of cell surface IgE crosslinking. Thus, the density of cell surface antigen-specific IgE determined the magnitude of the curve shift and the multivalent antigen, BPO₂₁-HSA was found to produce a greater curve shift than the simpler bivalent hapten, BPO₂, in accord with previous studies which demonstrated that BPO₂ was a "weak" stimulus compared to BPO₂₁-HSA. Basophils which had been fully desensitized by prior treatment with anti-IgE or antigen in the absence of calcium also displayed the curve shift for at least 24 h after desensitization. However, the change induced by crosslinking which altered the cells' sensitivity to TPA required the continued presence of crosslinks and was therefore not the result of a permanent alteration induced by desensitization. Basophils which were fully desensitized to BPO₇-HSA demonstrated the curve shift provided that the antigen-induced crosslinks were maintained: treatment with monovalent hapten, BPO-EACA, rapidly returned the cell response to TPA to control, non-desensitized, levels. Since TPA is thought to act by activation of protein kinase C (PKC) we demonstrated that BPO-HSA induced crosslinking increased PKC activity and that the increased activity persisted unless antigen-induced crosslinks were dissociated by the addition of monovalent hapten. These data suggest that some early activation events are not altered by the process of desensitization and can be maintained at an increased level of activity indefinitely but require the maintenance of IgE crosslinks.