Peripheral Regulatory T Cells in Major Depressive Disorder Exhibit a Th1-like Feature with Compromised Suppressive Function

Background: Major depressive disorder (MDD) is pathologically linked to inflammatory-immune dysregulation. Although regulatory T cells (Tregs) are crucial for immune homeostasis, clinical studies have demonstrated a paradoxical dynamic (deficiency vs. expansion) feature of peripheral Tregs in patients with MDD. However, disease-specific plasticity and microenvironmental reprogramming mechanisms remain elusive, highlighting the need for pathology-driven biomarkers and targeted therapies. Methods: We integrated mass cytometry (CyTOF; n=37), single-cell RNA/T cell receptor sequencing (n=10), and flow cytometry (n=123) to profile Tregs in MDD. Functional assays assessed the suppression capacity, interleukin (IL)-6/interferon (IFN)-γ-driven plasticity, and biomarker potential using a receiver-operating characteristic (ROC) analysis. Results: Patients with MDD exhibited significant peripheral immune dysregulation, notably expanded peripheral Tregs (1.2-fold vs. controls, p=0.001). Single-cell analysis revealed that the expanded Tregs were mainly T-bet+ Th1-like subpopulations (1.5-fold vs. controls, p=0.01) with metabolic abnormalities, featuring mitochondrial dysfunction (reduced activity of the oxidative phosphorylation pathway, FDR<0.001) and abnormally high expression of IFN-γ (gene set enrichment score NES=2.55), along with a significant reduction in the immunosuppressive capacity (16.7% vs. 62.8% inhibition, p<0.001). Further investigation revealed that the IL-6 signaling pathway drove the abnormal differentiation of Th1-like Tregs, and IL-6 blockade reversed their inflammatory phenotype (1.8-fold reduction, p<0.001). Clinically, T-bet+ Th1-like Tregs distinguished drug-naïve MDD patients with high accuracy (AUC=0.776, 95% CI: 0.668-0.884), correlating with symptom severity (HAMD: r=0.49, p<0.001). Conclusions: MDD-associated IL-6-driven Th1-like Treg polarization underpins MDD immunopathology, combining metabolic failure and inflammatory effector conversion. These cells serve as dual diagnostic biomarkers and therapeutic targets, supporting IL-6/IFN-γ axis modulation for depression treatment.