A fundamental question in developmental immunology is how bipotential thymocyte precursors generate both CD4⁺helper and CD8⁺cytotoxic T cell lineages. The MHC specificity of αβ T cell receptors (TCRs) on precursors is closely correlated with cell fate–determining processes, prompting studies to characterize how variations in TCR signaling are linked with genetic programs establishing lineage-specific gene expression signatures, such as exclusive CD4 or CD8 expression. The key transcription factors ThPOK and Runx3 have been identified as mediating development of helper and cytotoxic T cell lineages, respectively. Together with increasing knowledge of epigenetic regulators, these findings have advanced our understanding of the transcription factor network regulating the CD4/CD8 dichotomy. It has also become apparent that CD4⁺T cells retain developmental plasticity, allowing them to acquire cytotoxic activity in the periphery. Despite such advances, further studies are necessary to identify the molecular links between TCR signaling and the nuclear machinery regulating expression of ThPOK and Runx3.

T cell receptor (TCR) signaling by MHC class I and II induces thymocytes to acquire cytotoxic and helper fates via the induction of Runx3 and ThPOK transcription factors, respectively. The mechanisms by which TCR signaling is translated into transcriptional programs for each cell fate remain elusive. Here, we show that, in post-selection thymocytes, a genome organizer, SATB1, activates genes for lineage-specifying factors, including ThPOK, Runx3, CD4, CD8, and Treg factor Foxp3, via regulating enhancers in these genes in a locus-specific manner. Indeed, SATB1-deficient thymocytes are partially re-directed into inappropriate T lineages after both MHC class I- and II-mediated selection, and they fail to generate NKT and Treg subsets. Despite its essential role in activating enhancers for the gene encoding ThPOK in TCR-signaled thymocytes, SATB1 becomes dispensable for maintaining ThPOK in CD4+ T cells. Collectively, our findings demonstrate that SATB1 shapes the primary T cell pool by directing lineage-specific transcriptional programs in the thymus.

LAG3 shedding from conventional CD4 ⁺ T cells drives responsiveness to anti-PD1 immunotherapy (see the related Focus by Seidel and Bengsch ).

CCL5 is a unique chemokine with distinct stage and cell-type specificities for regulating inflammation, but how these specificities are achieved and how CCL5 modulates immune responses is not well understood. Here we identify two stage-specific enhancers: the proximal enhancer mediates the constitutive CCL5 expression during the steady state, while the distal enhancer located 1.35 Mb from the promoter induces CCL5 expression in activated cells. Both enhancers are antagonized by RUNX/CBFβ complexes, and SATB1 further mediates the long-distance interaction of the distal enhancer with the promoter. Deletion of the proximal enhancer decreases CCL5 expression and augments the cytotoxic activity of tissue-resident T and NK cells, which coincides with reduced melanoma metastasis in mouse models. By contrast, increased CCL5 expression resulting from RUNX3 mutation is associated with more tumor metastasis in the lung. Collectively, our results suggest that RUNX3-mediated CCL5 repression is critical for modulating anti-tumor immunity.