GRP75 sustains CD8⁺ T cell homeostasis and memory generation through ensuring mitochondrial fitness

The development and functional maintenance of CD8⁺ T cells are metabolically regulated processes in which mitochondria serve as the central hub. Here, we identify glucose-regulated protein 75 (GRP75) as a critical mitochondrial regulator controlling these processes. Using T cell-specific Hspa9 (encodes GRP75) knockout mice, we demonstrate that GRP75 deficiency disrupts CD8⁺ T cell fate, leading to defective T cell homeostasis and impaired memory differentiation. Mechanistically, impaired mitochondrial function in GRP75-deficient CD8⁺ T cells leads to perturbation of IL-7R signaling and aberrant expression of effector-associated molecules. Further studies reveal that GRP75 deficiency leads to upregulation of interferon regulatory factor 4 (IRF4), a critical transcription factor for effector versus memory fate, which in turn suppresses memory CD8⁺ T cell differentiation. Our findings establish GRP75 as a pivotal mitochondrial checkpoint that coordinates metabolic state and functional fate in CD8⁺ T cells.