IL-9 sensitizes human Th2 cells to pro-inflammatory IL-18 signals in atopic dermatitis

[Display omitted] T helper 2 (Th2) cells crucially contribute to the pathogenesis of atopic dermatitis (AD) by secreting high levels of IL-13 and IL-22. Yet, the upstream regulators that activate Th2 cells in AD skin remain unclear. IL-18 is a putative upstream regulator of Th2 cells as it is implicated in AD pathogenesis and has the capacity to activate T cells. To decipher the role of IL-18 in Th2 responses in blood and skin of AD patients. PBMCs and skin biopsies from AD patients and healthy donors were used. Functional assays were performed ex vivo using stimulation or blocking experiments. Analysis was performed using flow cytometry, bead-based multiplex assays, RT-qPCR, RNA-seq, western blotting, and spatial sequencing. IL-18Rα+ Th2 cells were enriched in blood and lesional skin of AD patients. Of all the cytokines for which Th2 cells express the receptor, only IL-9 was able to induce IL-18R via an IL-9R-JAK1/JAK3-STAT1 signaling pathway. Functionally, stimulation of circulating Th2 cells with IL-18 induced secretion of IL-13 and IL-22, an effect that was enhanced by co-stimulation with IL-9. Mechanistically, IL-18 induced Th2 cytokines via activation of IRAK4, NF-κB, and AP-1 signaling in Th2 cells, and neutralization of IL-18 inhibited these cytokines in cultured explants of AD skin lesions. Finally, IL-18 protein levels correlated positively with disease severity in lesional AD skin. Our data identify a novel IL-9-IL-18 axis that contributes to Th2 responses in AD. Our findings suggest that both IL-9 and IL-18 could represent upstream targets for future treatment of AD.