The OX40/OX40L Axis Promotes Th2 Activity and Impairs Regulatory T Cell Function in Atopic Dermatitis

Background: Clinical trials using monoclonal antibodies targeting the OX40/OX40L axis (AMG451, GBR830, KHK4083, and KY1005) have shown promise in long-term disease modification of atopic dermatitis (AD). OX40/OX40L enhances survival of effector T cells and inhibits regulatory T cells (Tregs) function in other inflammatory diseases, but its mechanism of action in AD is unelucidated. We aimed to evaluate OX40- and OX40L-expressing cells in the skin and blood of AD patients to understand how the axis promotes T cell activity and inflammation. Methods: Skin samples from AD and healthy controls (HCs) were evaluated using immunohistochemistry (IHC) and immunofluorescence (IF). Flow cytometry was used to analyze peripheral blood mononuclear cells (PBMCs), and RT-qPCR and ELISA were performed on purified effector CD4⁺ T cells and from AD patients and HCs. RNA-seq analysis was conducted on effector CD4⁺ T cells and Tregs stimulated with anti-CD3/28, with and without OX40L, in both AD patients and HCs. Results: IHC showed increased OX40⁺ and OX40L⁺ cell expression in AD skin lesions. Circulating CLA⁺CD4⁺ T cells and CLA⁺ Tregs upregulate OX40 expression in AD patients compared to HCs. OX40⁺CLA⁺CD4⁺ Tregs correlated with SCORAD. RNA-seq, RT-qPCR, and ELISA revealed that the OX40/OX40L axis maintained the Th2 phenotype of effector CD4+ T cells and decreased IL-10 produced by Tregs. Conclusion: AD patients exhibit significant upregulation of OX40 expression in effector and regulatory CD4⁺ T cells. Both subsets interact with OX40L-expressing cells in the skin, leading to the promotion of skin inflammation by downregulation of function and anti-inflammatory capacity of Tregs.