Elevated Levels of IL-9 Fail to Suppress Pathogenic T helper 17 cells in Sjögren’s Disease

Sjögren's disease (SjD) is a chronic autoimmune disorder characterized by inflammation of the exocrine glands, leading to dry mouth and dry eyes. This study investigates the role of interleukin-9 (IL-9) and T helper 9 (Th9) cells in the pathogenesis of SjD. We found that serum IL-9 levels were significantly elevated in SjD patients and correlated with clinical laboratory parameters, including autoantibody production. In a mouse model of SjD, IL-9 and Th9-associated cytokines were also elevated, and Th9 cells were enriched in the salivary glands. Our results suggest that IL-9 is produced by multiple cell types, including macrophages, CD4 ⁺ T cells, and NK cells, and that Th9 cells contribute to the development of SjD by promoting inflammation and autoantibody production. We also found that Th9 and Th17 polarization conditions increased Th2 and Th17 cells in SjD mice, indicating a shared epigenetic program that renders T cells permissive to multiple differentiation pathways. Anti-IL-9 treatment had a sex-dependent effect, reducing autoantibody production in male mice but worsening focal glandular infiltration in female mice. Our findings suggest that IL-9 plays a complex role in SjD pathobiology, contributing to both local immunoregulation and systemic autoantibody response. Overall, this study offers new insights into the role of IL-9 and Th9 cells in SjD, highlighting the potential for therapeutic targeting of the IL-9/Th9 axis in the treatment of this disease.