IL-33 is unlikely an independent trigger of anaphylaxis even in subjects with high IgE levels. However, the rapid release of IL-33 upon allergen provocation in vivo supports its role as a mediator of immediate allergic responses.

Cytokines of the GM-CSF family signal via the same receptor subunit (βc) and, thus, have overlapping effects on cells that express all cytokine-specific α-chains (IL-3Rα, IL-5Rα, GM-CSFRα), such as human basophils, whose rapid effector functions are similarly enhanced by IL-3, IL-5, and GM-CSF. However, previous work has shown that IL-3, but not IL-5 and GM-CSF, supports and induces allergy-associated functions of human basophils at later time points. This includes induction of Th2 cytokine and chemokine secretion, high-affinity IgE receptor-independent leukotriene C4 (LTC4) formation, expression of enzymes (e.g., RALDH2, granzyme B), and kinases (e.g., Pim1). Here, we address the question of why IL-3, but not IL-5 or GM-CSF, is capable of inducing these late responses in human basophils, and we investigate the mechanism that underlies the unique regulatory capacity of IL-3. We find that IL-3, IL-5, and GM-CSF rapidly activate the same canonical signaling cascades in a qualitatively identical manner with comparable strength, but we identify signaling duration as major discriminating factor. IL-5 and GM-CSF rapidly down-regulate surface levels of their receptors within minutes, concomitant with a rapid decay in signaling molecule activation and time-dependent loss of ability of these cytokines to prime basophils for functional responses. By contrast, IL-3 hardly down-regulates the α-chain of its receptor without depleting the common β-chain, which enables extraordinarily sustained signaling events, predominantly the activation of Stat5. Of interest, acute IL-3 signaling is not sufficient to induce persistent phenotypical and functional changes in human basophils. Induction of these functional late responses depends on continuous IL-3 receptor activation and signaling.

This study identifies IL-3 as a potent inducer of RANKL expression in human basophils, suggesting them to interact with bone physiology and activation of immune cells. IgER-dependent and IgER-independent stimuli modulate the IL-3-mediated RANKL expression in a time- and stimulus-dependent fashion.

Human basophils are essential effector cells of chronic allergic inflammation. IL‐1 family cytokines such as interleukin (IL)‐33 and IL‐1β are elevated in serum and bronchoalveolar lavage fluid of allergic asthmatics. IL‐33 is known to be a critical regulator of basophil's T2 immune responses. However, the effect of IL‐1β on the function of basophils has not been well investigated. Here, we elucidate whether IL‐1β regulates the function of human basophils and compared the effects of IL‐1β and IL‐33 on basophils of healthy and allergic subjects. We found that IL‐1β activates the p38 MAPK signaling pathway and promotes IL‐8 release in basophils of healthy donors, while FcεRI‐mediated LCT4and histamine secretion is not affected. Strikingly, in the presence of IL‐3, IL‐1β shows more potency than IL‐33, as evidenced by the enhanced p38 phosphorylation and NF‐κB activation, as well as the release of both IL‐13 and IL‐8. We found that the enhanced basophil responsiveness is achieved through IL‐3‐induced IL‐1RI surface expression. Importantly, basophils of allergic donors release significantly higher amounts of IL‐8 compared to those from healthy donors upon IL‐33 and IL‐1β stimulation. Consistently, we detected increased IL‐1RI and decreased IL‐3 receptor alpha‐chain (CD123) and CCR3 expression on basophils of allergic subjects compared to healthy controls, suggesting an in vivo IL‐3 priming in allergic donors. In summary, our results suggest enhanced sensitivity of basophils toward IL‐33 and IL‐1β in allergic subjects compared to those from healthy controls.

SummaryBackground In contrast to eosinophils and neutrophils, the regulation of the lifespan of human basophils is poorly defined, with the exception of the potent anti-apoptotic effect of IL-3 that also promotes pro-inflammatory effector functions and phenotypic changes. Type I IFNs (IFN-a, IFN-b), which are well known for their anti-viral activities, have the capacity to inhibit allergic inflammation. Objective To elucidate whether type I IFNs have the potential to abrogate the lifespan and/or effector functions of human basophils. Methods We cultured human basophils, and for comparison, eosinophils and neutrophils, with IL-3, interferons, FasL and TRAIL, alone or in combination, and studied cell survival, effector functions and signalling pathways involved. Results Despite an identical pattern of early signalling in basophils, eosinophils and neutrophils in response to different types of interferons, only basophils displayed enhanced apoptosis after type I IFN treatment. IFN-c prolonged survival of eosinophils but did not affect the lifespan of basophils. IFN-a-mediated apoptosis required STAT1-STAT2 heterodimers and the contribution of constitutive p38 MAPK activity. Whereas the death ligands FasL and TRAIL-induced apoptosis in basophils per se, IFN-a-mediated apoptosis did neither involve autocrine TRAIL signalling nor did it sensitize basophils to FasL-induced apoptosis. However, IFN-a and FasL displayed an additive effect in killing basophils. Interestingly, IL-3, which protected basophils from IFN-a-, TRAIL-or FasL-mediated apoptosis, did not completely block the additive effect of combined IFN-a and FasL treatment. Moreover, we demonstrate that IFN-a suppressed IL-3-induced release of IL-8 and IL-13. In contrast to IFN-a-mediated apoptosis, these inhibitory effects of IFN-a were not dependent on p38 MAPK signalling. Conclusions and Clinical Relevance Our study defines the unique and granulocyte-typespecific inhibitory and pro-apoptotic function of type I IFNs and their cooperation with death ligands in human blood basophils, which may be relevant for the anti-allergic properties of type I IFNs.