Abstract
<jats:p>How regulatory T cells (Tregs) lose lineage identity during chronic inflammation remains poorly understood. Here, using inducible Foxp3 lineage tracing together with single-cell transcriptomic, proteomic and T cell receptor (TCR) profiling in atherosclerosis-prone mice, we identify Treg destabilization as a staged and branching differentiation process rather than an abrupt loss of lineage identity. Conventional Tregs (cTregs) first transition through an effector Treg (eTreg) intermediate characterized by attenuation of the CD25-STAT5 axis while retaining core Treg features, before diversifying into eight transcriptionally distinct exTreg states, including Tfh-like, cytotoxic, Th1-like inflammatory, Th1-like cytotoxic and proliferative populations. Trajectory inference, TCR clonotype analysis and experimental Treg-to-exTreg conversion independently converged on this developmental framework, revealing that clonally related exTregs acquire distinct effector programs. Mechanistically, we identify Treg-intrinsic IL-6R signaling as an important driver of this process. IL-6 accelerated exTreg generation in vitro, whereas Treg-specific deletion of Il6ra reduced inflammatory exTreg differentiation and attenuated atherosclerosis in vivo. Together, these findings establish a framework for Treg destabilization during atherosclerosis and provide a conceptual basis for preserving Treg lineage stability in chronic inflammatory disease.</jats:p>