Metabolic and Functional Study of γδ T Cells in Critically Ill Patients

This prospective observational cohort study investigates the subset-specific metabolic adaptation and functional remodeling of cytotoxic γδ T cells in critically ill patients with and without sepsis. As a bridge between innate and adaptive immunity, γδ T cells contribute to early antimicrobial defense; however, their cytotoxic function and its metabolic regulation in sepsis remain incompletely understood. Our preliminary single-cell transcriptomic analysis revealed altered expression of cytotoxicity-associated genes (GZMB, PRF1, and GNLY) and mitochondrial oxidative phosphorylation (OXPHOS) pathway genes in cytotoxic γδ T cells from patients with sepsis. In particular, COX6C expression correlated with cytotoxic effector gene expression. Across healthy volunteers, critically ill non-septic patients, and critically ill septic patients, we will integrate flow cytometry, mitochondrial function assays, and functional validation experiments to characterize circulating cytotoxic γδ T-cell abundance and function and examine the contribution of COX6C to mitochondrial dysfunction and impaired cytotoxicity. To investigate the relationship between systemic tryptophan metabolism and these cellular abnormalities, targeted tryptophan-pathway metabolomics using liquid chromatography-tandem mass spectrometry (LC-MS/MS) will quantify kynurenic acid (KYNA), tryptophan, kynurenine, and related metabolites in paired peripheral blood and fecal samples. Patients with sepsis will be stratified into older (≥65 years) and non-older (\

Inclusion Criteria: 1. Healthy Control Group (NHC): * Age ≥ 18 years. * No acute or chronic major diseases. * Provide written informed consent. 2. Non-septic Critical Illness Group (CI-NS): * Age ≥ 18 years. * Admitted to the ICU and meeting the definition of critical illness. * Excluded from sepsis according to the Sepsis-3 criteria (infection + ΔSOFA ≥ 2 points). * Written informed consent provided by the participant or legally authorized representative. 3. Septic Critical Illness Group (CI-Sep): * Age ≥ 18 years. * Admitted to the ICU and meeting the Sepsis-3 criteria (infection + ΔSOFA ≥ 2 points). * Written informed consent provided by the participant or legally authorized representative. Exclusion Criteria: * Age \< 18 years. * Known immunodeficiency, HIV infection, active hematologic malignancy, or history of hematopoietic stem cell or solid organ transplantation within the past 3 months. * Receipt of T-cell-targeted immunosuppressive therapy (e.g., antithymocyte globulin, calcineurin inhibitors, mycophenolate mofetil, methotrexate, or high-dose corticosteroids \>1 mg/kg/day prednisone equivalent) before ICU admission or within 24 hours after ICU admission. * Use of immune checkpoint inhibitors (e.g., anti-PD-1/PD-L1/CTLA-4 antibodies) within the past 6 weeks. * Expected ICU stay \< 24 hours or imminent risk of death (moribund state). * Pregnancy or breastfeeding. * Active major bleeding. * Inability to obtain informed consent.