Tryptophan depletion impairs CD8+ T-lymphocyte proliferation by preventing the activation-induced metabolic shift towards glycolysis

Abstract Non-response in cancer immunotherapy poses a significant challenge, often attributed to poorly understood immune resistance mechanisms. Indoleamine 2,3-dioxygenase (IDO1), frequently expressed in advanced-stage cancers, contributes to this resistance by fostering an immunosuppressive environment through tryptophan degradation along the kynurenine pathway. While the association between IDO1 and CD8 T lymphocyte dysfunction is widely accepted, the precise mechanisms underlying T lymphocyte dysfunction via tryptophan catabolism remain debated. To elucidate these mechanisms, we used genome-wide CRISPR/Cas9 knockout screening in murine CD8 + T lymphocytes, searching for genes whose inactivation favored proliferation in low-tryptophan conditions. Strikingly, a majority of identified genes coded for proteins involved in mitochondrial function. By further exploring the impact of tryptophan deprivation on metabolism, we observed that CD8 + T cells, when activated under tryptophan shortage, fail to shift their metabolism towards glycolysis and instead rely on oxidative phosphorylation (OXPHOS) for energy production. Inactivating OXPHOS restored proliferation by forcing glycolysis despite tryptophan shortage. Since the activation-induced metabolic shift is essential to provide the macromolecules required for proliferation, our results provide a mechanistic rationale to explain the IDO1-induced proliferation arrest of CD8 + T lymphocytes.