Mitochondrial Fitness in Anti-Tumor T Cells: From Activation to Exhaustion and Memory Formation

Mitochondria have emerged as central regulators of anti-tumor immunity, extending beyond their canonical role in energy production. In cancer immunology, mitochondrial fitness—a multifaceted state encompassing mitochondrial quantity, quality, metabolic adaptability, and stress resilience—emerges as an important component shaping T cell fate across the continuum from activation to exhaustion. During early activation, mitochondrial remodeling supports calcium signaling, redox regulation, and metabolic reprogramming required for clonal expansion and effector differentiation. However, tumor-infiltrating T cells encounter a metabolically hostile tumor microenvironment (TME) characterized by hypoxia, nutrient deprivation, and immunosuppressive metabolite accumulation. To sustain functionality, T cells must maintain mitochondrial plasticity through coordinated regulation of oxidative phosphorylation, fatty acid oxidation, mitochondrial dynamics, biogenesis, and quality control pathways. Persistent antigen exposure and chronic metabolic stress progressively impair mitochondrial fitness, promoting defective oxidative metabolism, excessive reactive oxygen species accumulation, disrupted mitochondrial quality control, and ultimately T cell exhaustion. Mitochondrial fitness has emerged as a defining feature of stem-like progenitor exhausted (Tpex) and memory T cells, which retain self-renewal capacity and mediate durable immune checkpoint blockade responses. Recent studies uncovered novel links between mitochondrial metabolism, epigenetic regulation, and T cell differentiation, highlighting mitochondria as active drivers of immune cell fate rather than passive bioenergetic organelles. This review considers how mitochondrial fitness orchestrates T cell activation, adaptation, exhaustion, and memory formation within the TME, and highlights emerging concepts including mitochondrial control of stem-like T cell maintenance and metabolic reprogramming strategies for restoring mitochondrial function. A deeper understanding of mitochondrial fitness may provide opportunities to enhance cancer immunotherapy and improve long-term anti-tumor immune responses.