Stimulus strength determines the BTK-dependence of the SHIP1-deficient phenotype in IgE/antigen-triggered mast cells

Antigen (Ag)-mediated crosslinking of IgE-loaded high-affinity receptors for IgE (Fc epsilon RI) on mast cells (MCs) triggers activation of proinflammatory effector functions relevant for IgE-associated allergic disorders. The cytosolic tyrosine kinase BTK and the SH2-containing inositol-5'-phosphatase SHIP1 are central positive and negative regulators of Ag-triggered MC activation, respectively, contrarily controlling Ca2+ mobilisation, degranulation, and cytokine production. Using genetic and pharmacological techniques, we examined whether BTK activation in Shipl-/- MCs is mandatory for the manifestation of the well-known hyperactive phenotype of Shipl-/- MCs. We demonstrate the prominence of BTK for the Shipl-/- phenotype in a manner strictly dependent on the strength of the initial Ag stimulus; particular importance for BTK was identified in Shipl-/- bone marrow-derived MCs in response to stimulation with suboptimal Ag concentrations. With respect to MAPK activation, BTK showed particular importance at suboptimal Ag concentrations, allowing for an analogous-to-digital switch resulting in full activation of ERK1/2 already at low Ag concentrations. Our data allow for a more precise definition of the role of BTK in Fc epsilon RI-mediated signal transduction and effector function in MCs. Moreover, they suggest that reduced activation or curtate expression of SHIP1 can be compensated by pharmacological inhibition of BTK and vice versa.