Basophils contribute to allergic reactions when surface-bound IgE is cross-linked by allergens. How they respond to innate stimuli to produce cytokines during tissue inflammation and homeostasis is less known. Here, we comprehensively identify and compare which innate and homeostatic stimuli, among cytokines, growth factors, hormones, metabolites, and bacterial components, can activate primary basophils. We find that activated basophils display phenotypic heterogeneity at the single-cell level, suggesting that mutually exclusive functional basophil subsets exist in vivo. Furthermore, using selective kinase inhibitors we show that distinct receptor families, including IL-1R, GPCR, FcR, CD200R, and cytokine receptors, mediate basophil activation through Syk and IκB kinases. We propose these pathways have a rheostat function controlling basophil activation to diverse stimuli.
Mature basophils play critical inflammatory roles during helminthic, autoimmune, and allergic diseases through their secretion of histamine and the type 2 cytokines interleukin 4 (IL-4) and IL-13. Basophils are activated typically by allergen-mediated IgE cross-linking but also by endogenous “innate” factors. The aim of this study was to identify the innate stimuli (cytokines, chemokines, growth factors, hormones, neuropeptides, metabolites, and bacterial products) and signaling pathways inducing primary basophil activation. Basophils from naïve mice or helminth-infected mice were cultured with up to 96 distinct stimuli and their influence on basophil survival, activation, degranulation, and IL-4 or IL-13 expression were investigated. Activated basophils show a heterogeneous phenotype and segregate into distinct subsets expressing IL-4, IL-13, activation, or degranulation markers. We find that several innate stimuli including epithelial derived inflammatory cytokines (IL-33, IL-18, TSLP, and GM-CSF), growth factors (IL-3, IL-7, TGFβ, and VEGF), eicosanoids, metabolites, TLR ligands, and type I IFN exert significant direct effects on basophils. Basophil activation mediated by distinct upstream signaling pathways is always sensitive to Syk and IκB kinases-specific inhibitors but not necessarily to NFAT, STAT5, adenylate cyclase, or c-fos/AP-1 inhibitors. Thus, basophils are activated by very diverse mediators, but their activation seem controlled by a core checkpoint involving Syk and IκB kinases.