Self-Renewing Islet TCF1⁺ CD8 T Cells Undergo IL-27-Controlled Differentiation to Become TCF1^- Terminal Effectors during the Progression of Type 1 Diabetes

In type 1 diabetes (T1D) autoreactive CD8 T cells infiltrate pancreatic islets and destroy insulin-producing 13 cells. Progression to T1D onset is a chronic process, which suggests that the effector activity of 13-cell autoreactive CD8 T cells needs to be maintained throughout the course of disease development. The mechanism that sustains diabetogenic CD8 T cell effectors during the course of T1D progression has not been completely defined. Here we used single-cell RNA sequencing to gain further insight into the phenotypic complexity of islet-infiltrating CD8 T cells in NOD mice. We identified two functionally distinct subsets of activated CD8 T cells, CD44(high)TCF1(+)CXCR6(-) and CD44(high)TCF1(-)CXCR6(+), in islets of prediabetic NOD mice. Compared with CD44(high)TCF1(+)CXCR6(-) CD8 T cells, the CD44(high)TCF1(-)CXCR6(+) subset expressed higher levels of inhibitory and cytotoxic molecules and was more prone to apoptosis. Adoptive cell transfer experiments revealed that CD44(high)TCF1(+)CXCR6(-) CD8 T cells, through continuous generation of the CD44(high)TCF1(-)CXCR6(+) subset, were more capable than the latter population to promote insulitis and the development of T1D. We further showed that direct IL-27 signaling in CD8 T cells promoted the generation of terminal effectors from the CD44(high)TCF1(+)CXCR6(-) population. These results indicate that islet CD44(high)TCF1(+)CXCR6(-) CD8 T cells are a progenitor-like subset with self-renewing capacity, and, under an IL-27-controlled mechanism, they differentiate into the CD44(high)TCF1(-)CXCR6(+) terminal effector population. Our study provides new insight into the sustainability of the CD8 T cell response in the pathogenesis of T1D.