The multifaceted dysfunction of tumour-infiltrating T cells, including exhaustion and mitochondrial dysfunction, remains a major obstacle in cancer immunotherapy1–6. Transcriptomic and epigenomic regulation of T cell dysfunction have been extensively studied7–9, but the role of proteostasis in regulating these obstacles remains less defined. Here we combined computational analyses of atlases of T cell exhaustion and mitochondrial fitness with performed targeted in vivo CRISPR screens, which identified the E3 ubiquitin ligase KLHL6 as a dual-negative regulator of both T cell exhaustion and mitochondrial dysfunction. Mechanistically, KLHL6 expression promoted TOX poly-ubiquitination and subsequent proteasomal degradation, thereby attenuating the transition of progenitor exhausted T cells towards terminal exhaustion. Simultaneously, KLHL6 maintained mitochondrial fitness by constraining the excessive mitochondrial fission that occurs during chronic T cell receptor stimulation by means of post-translational regulation of the PGAM5–Drp1 axis. However, KLHL6 is naturally downregulated by T cell receptor ligation, mitigating its potentially beneficial ubiquitin ligase activities during exposure to chronic stimulation. Enforcing KLHL6 expression in T cells markedly improved efficacy and long-term persistence against tumours and during viral infections in vivo. These findings uncover KLHL6 as a multifunctional, clinically actionable target for cancer immunotherapy, and highlight the potential of modulating proteostasis and ubiquitin modification to improve immunotherapy.
