Prolonged IL-33 Exposure Switches the Secreted Mast Cell Cytokine Profile From Pro-Inflammatory to Pro-Tolerant

Background: During desensitization, the immune response to an allergen switches from a type 2 inflammation towards immunological tolerance. Subcutaneous Hymenoptera venom and alum injections induce IL-33 release during venom immunotherapy (VIT). To elucidate whether mast cells (MCs) contribute to this switch in immune response, we investigated the effect of IL-33 and Hymenoptera venom on MC cytokine profiles under VIT-like conditions.
Methods: LAD2 cells or primary human skin MCs were stimulated with IL-33 and Hymenoptera venom or Compound 48/80, with or without IL-33 pre-exposure (priming) for up to four days. In addition, c-kit was stimulated or inhibited. Cell surface marker expression was monitored by flow cytometry. Secreted cytokines were measured in supernatants by a bead-based multiplex assay and ELISA. Phosphorylated c-kit was determined by Western blot.
Results: LAD2 cell stimulation led to the secretion of IL-2, IL-5, IL-6, IL-13, TNF-α, and TGF-β1, phosphorylation of c-kit, and a reduced c-kit surface expression after 24 h. IL-33 primed LAD2 cells secreted mainly the tolerance-promoting cytokine TGF-β1 but little pro-inflammatory/type 2 cytokines, showed less c-kit phosphorylation, and a higher c-kit surface expression following stimulation. Inhibition of c-kit before stimulation led to a similar secreted cytokine profile phenotype as IL-33 priming. IL-33 primed skin MCs showed a comparable cytokine switch and receptor expression pattern.
Conclusion: Changes in the interaction of IL-33 receptor (ST2) and c-kit after prolonged IL-33 exposure skewed MCs towards tolerance-promoting cytokines during desensitization therapy. These findings suggest a trendsetting role of MCs for the type of response towards Hymenoptera venom.