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

Background: 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.

Objective: To decipher the role of IL-18 in Th2 responses in blood and skin of AD patients.

Methods: 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.

Results: 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.

Conclusion: 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.