LEF1 and niche factors determine T cell stemness across chronic diseases.

In settings of persistent (self or foreign) antigen, such as autoimmunity and chronic infection, immune responses are sustained by stem-like T cells. Although TCF1 has emerged as a key transcription factor (TF) associated with stemness, the TCF1hi population is heterogeneous, raising the question of whether TCF1 exclusively defines the stem T cell (TSC) pool. Using preclinical models of autoimmune type 1 diabetes and chronic infection, we discover that a small subset of TCF1hi T cells express the TF LEF1. LEF1+ TCF1hi T cells define a true self-renewing TSC pool. TSC give rise to LEF1- TCF1hi progenitor T cells (TPRO), which lack stem functions and generate terminally differentiated TCF1lo T cells (TDIFF) (TSC→TPRO→TDIFF). We show that LEF1 is essential for T cell stemness. Autoimmune and exhausted LEF1+ TSC share a unique epigenetically encoded core program enriched for genes and pathways characteristic of embryonic and adult stem cells, including WNT/β-catenin and Notch signaling. Spatial positioning, niche signals, and migration regulate stem-cell fate; accordingly, targeting integrins or Notch signaling impairs T cell stemness and prevents disease. Our studies identify LEF1 and niche-derived factors as fundamental regulators of T cell stemness across chronic diseases.