Metformin promotes the expansion of CD8+ central memory T cells by offsetting Pentose phosphate pathway

CD8⁺ central memory T (Tcm) cells underpin long-lived antitumor and antiviral immune responses, but their numbers wane with advancing age. Metformin promotes CD8⁺ memory T-cell generation and tumor infiltration; however, its effect on the distribution of circulating CD8⁺ memory subsets, especially CD8⁺Tcm cells, has not been established. In this study, we isolated peripheral blood mononuclear cells (PBMCs) from middle-aged healthy donors (45–65 years) undergoing routine health screening. In vitro treatment with 20 mM metformin for 24 hours increased the frequency of CD8⁺Tcm cells and promoted a metabolic shift characterized by enhanced expression of CPT1a, a rate-limiting enzyme of fatty acid oxidation (FAO), together with elevated ATP5a expression, increased mitochondrial mass and mitochondrial ROS (mtROS) production. Metformin also upregulated glucose-6-phosphate dehydrogenase (G6PD) while reducing the expression of glycolysis-associated enzymes HK1 and HK2. Collectively, these findings implicate the FAO–OXPHOS–ROS–PPP axis as a regulator of metformin-induced CD8⁺ Tcm expansion and reveal the metabolic plasticity of human CD8⁺ memory T cells during midlife. This suggests that metabolic modulation may offer a strategy to support T-cell homeostasis across the ageing trajectory.