Interleukin (IL)-17–producing CD8 + T (Tc17) cells have emerged as key players in host-microbiota interactions, infection, and cancer. The factors that drive their development, in contrast to interferon (IFN)-γ–producing effector CD8 + T cells, are not clear. Here we demonstrate that the transcription factor TCF-1 ( Tcf7 ) regulates CD8 + T cell fate decisions in double-positive (DP) thymocytes through the sequential suppression of MAF and RORγt, in parallel with TCF-1–driven modulation of chromatin state. Ablation of TCF-1 resulted in enhanced Tc17 cell development and exposed a gene set signature to drive tissue repair and lipid metabolism, which was distinct from other CD8 + T cell subsets. IL-17–producing CD8 + T cells isolated from healthy humans were also distinct from CD8 + IL-17 − T cells and enriched in pathways driven by MAF and RORγt . Overall, our study reveals how TCF-1 exerts central control of T cell differentiation in the thymus by normally repressing Tc17 differentiation and promoting an effector fate outcome.

Mucosal-associated invariant T cells (MAIT cells) express a semi-invariant T cell receptor (TCR) α-chain, TRAV1-2–TRAJ33, and are activated by vitamin B metabolites bound by the major histocompatibility complex (MHC)–related class I–like molecule, MR1. Understanding MAIT cell biology has been restrained by the lack of reagents to specifically identify and characterize these cells. Furthermore, the use of surrogate markers may misrepresent the MAIT cell population. We show that modified human MR1 tetramers loaded with the potent MAIT cell ligand, reduced 6-hydroxymethyl-8- d -ribityllumazine (rRL-6-CH 2 OH), specifically detect all human MAIT cells. Tetramer + MAIT subsets were predominantly CD8 + or CD4 − CD8 − , although a small subset of CD4 + MAIT cells was also detected. Notably, most human CD8 + MAIT cells were CD8α + CD8β −/lo , implying predominant expression of CD8αα homodimers. Tetramer-sorted MAIT cells displayed a T H 1 cytokine phenotype upon antigen-specific activation. Similarly, mouse MR1–rRL-6-CH 2 OH tetramers detected CD4 + , CD4 − CD8 − and CD8 + MAIT cells in Vα19 transgenic mice. Both human and mouse MAIT cells expressed a broad TCR-β repertoire, and although the majority of human MAIT cells expressed TRAV1-2–TRAJ33, some expressed TRAJ12 or TRAJ20 genes in conjunction with TRAV1-2. Accordingly, MR1 tetramers allow precise phenotypic characterization of human and mouse MAIT cells and revealed unanticipated TCR heterogeneity in this population.