Interleukin-15-mediated activation and dysregulation of CD8⁺ T cells in the pathogenesis of primary sclerosing cholangitis

Abstract

Primary sclerosing cholangitis (PSC) is a chronic cholestatic liver disease characterized by progressive bile duct inflammation and fibrosis, frequently leading to cirrhosis and cholangiocarcinoma. Despite clear immune involvement, disease-modifying therapies are lacking, and the mechanisms driving biliary injury remain incompletely understood. CD8⁺ T cells are enriched in inflamed portal tracts and localize in close proximity to cholangiocytes, yet their functional role in PSC has not been fully defined. Interleukin-15 (IL-15) is a pleiotropic cytokine capable of inducing antigen-independent bystander activation of CD8⁺ T cells, but its relevance in PSC has not previously been addressed. This thesis comprehensively characterizes CD8⁺ T cell phenotypes in PSC and investigates the role of IL-15 within the biliary microenvironment. Spectral flow cytometry demonstrated that peripheral blood CD8⁺ T cells from individuals with PSC, even at early disease stages, display increased expression of activation and exhaustion markers: HLA-DR and PD-1, together with altered granzyme profiles consistent with IL-15 exposure. Single-cell CITE-sequencing of intrahepatic CD8⁺ T cells from end-stage PSC livers revealed a predominance of effector memory populations expressing MHC class II molecules, interferon-γ, NK cell-associated receptors, and cytotoxic effector molecules, with enrichment of IL-15-associated STAT5 signaling pathways. Single-nucleus RNA sequencing across PSC stages identified expansion of bystander activated effector memory CD8⁺ T cell subsets with disease progression and revealed biliary epithelial cells as a relevant source of IL-15 and IL-15Rα. Increased IL15 expression was detectable already in early PSC, whereas IL15RA expression increased with liver disease severity. These findings were validated in vitro using human cholangiocyte organoids and biliary epithelial cell lines, in which interferon-γ robustly induced IL-15 and IL-15Rα expression. Spatial transcriptomics demonstrated close proximity of activated CD8⁺ T cells and IL-15–expressing cholangiocytes within inflamed portal areas. Finally, a murine model of acute cholangitis confirmed induction of IL-15 mediated bystander activation of endogenous CD8⁺ T cells in vivo. In conclusion, this work identifies an IL-15-associated bystander-activated CD8⁺ T cell phenotype in PSC and provides evidence that biliary epithelial cells may contribute to shaping this immune response. These findings advance the understanding of immune mediated biliary injury in PSC and highlight IL-15 signaling as a potential therapeutic target.