mTOR-dependent translation drives tumor infiltrating CD8+ effector and CD4+ Treg cells expansion

We performed a systematic analysis of the translation rate of tumor-infiltrating lymphocytes (TILs) and the microenvironment inputs affecting it, both in humans and in mice. Measurement of puromycin incorporation, a proxy of protein synthesis, revealed an increase of translating CD4(+) and CD8(+) cells in tumors, compared to normal tissues. High translation levels are associated with phospho-S6 labeling downstream of mTORC1 activation, whereas low levels correlate with hypoxic areas, in agreement with data showing that T cell receptor stimulation and hypoxia act as translation stimulators and inhibitors, respectively. Additional analyses revealed the specific phenotype of translating TILs. CD8(+) translating cells have enriched expression of IFN-gamma and CD-39, and reduced SLAMF6, pointing to a cytotoxic phenotype. CD4(+) translating cells are mostly regulatory T cells (Tregs) with enriched levels of CTLA-4 and Ki67, suggesting an expanding immunosuppressive phenotype. In conclusion, the majority of translationally active TILs is represented by cytotoxic CD8(+) and suppressive CD4(+) Tregs, implying that other subsets may be largely composed by inactive bystanders.