IL-33 fine tunes mast cell degranulation and chemokine production at the single-cell level

Background: Mast cells are versatile key components of allergy and inflammation known to respond to both innate and adaptive immunologic stimuli. However, the response of individual mast cells to cumulative stimuli remains poorly understood. Objectives: We sought to dissect mast cell responses at the single-cell level and their potentiation by IL-33. Methods: We monitored mast cell degranulation in real time by exploiting the capacity of fluorochrome-labeled avidin to stain degranulating cells. During the degranulation process, the granule matrix is externalized and immediately bound by fluorochrome-labeled avidin present in the culture medium. The degranulation process is monitored by using either time-lapse microscopy or fluorescence-activated cell sorting analysis. Results: Single-cell analysis revealed a strong heterogeneity of individual mast cell degranulation responses. We observed that the number of degranulating mast cells was graded according to the FceRI stimulation strength, whereas the magnitude of individual mast cell degranulation remained unchanged, suggesting an all-or-none response of mast cells after Fc epsilon RI triggering. IL-33 pretreatment increased not only the number of degranulating and chemokine-producing mast cells but also the magnitude of individual mast cell degranulation and chemokine production. Conclusion: We illustrate the effect of IL-33 on mast cell biology at the single-cell level by showing that IL-33 potentiates IgE-mediated mast cell responses by both increasing the number of responding cells and enhancing the responses of individual mast cells.