SUMMARY Exhausted CD8+ T cells (TEX) in chronic infections and cancer have limited effector function, high inhibitory receptor co-expression and extensive transcriptional changes compared to effector (TEFF) or memory (TMEM) CD8+ T cells. TEX are important clinical targets of checkpoint blockade and other immunotherapies. Epigenetically, TEX are a distinct immune subset, with a unique chromatin landscape compared to TEFF and TMEM. However, the mechanisms governing the transcriptional and epigenetic development of TEX remain unknown. Here, we identify the HMG-box transcription factor TOX as a central regulator of TEX. TOX is largely dispensable for TEFF and TMEM formation, but is critical for exhaustion and without TOX TEX do not form. TOX is induced by calcineurin and NFAT2 and operates in a feed-forward loop to become calcineurin independent and sustained in TEX. Thus, robust TOX expression results in commitment to TEX by translating persistent stimulation into a distinct TEX transcriptional and epigenetic developmental program.

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We report on the development of the PennPET Explorer whole-body imager. Methods: The PennPET Explorer is a multiring system designed with a long axial field of view. The imager is scalable and comprises multiple 22.9-cm-long ring segments, each with 18 detector modules based on a commercial digital silicon photomultiplier. A prototype 3-segment imager has been completed and tested with an active 64-cm axial field of view. Results: The instrument design is described, and its physical performance measurements are presented. These include sensitivity of 55 kcps/MBq, spatial resolution of 4.0 mm, energy resolution of 12%, timing resolution of 256 ps, and a noise-equivalent count rate above 1,000 kcps beyond 30 kBq/mL. After an evaluation of lesion torso phantoms to characterize quantitative accuracy, human studies were performed on healthy volunteers. Conclusion: The physical performance measurements validated the system design and led to highquality human studies.

Summary BET (bromodomain and extraterminal motif) proteins are pharmacologic targets for the treatment of diverse diseases, yet the roles of individual BET family members remain unclear. We find that BRD2 but not BRD4 colocalizes with the architectural/insulator protein CCCTC-binding factor (CTCF) genome-wide. CTCF recruits BRD2 to co-bound sites, whereas BRD2 is dispensable for CTCF occupancy. Disruption of a CTCF/BRD2-occupied element positioned between two unrelated genes enables regulatory influence to spread from one gene to another, suggesting that CTCF and BRD2 form a transcriptional boundary. Accordingly, single molecule mRNA FISH reveals that upon site-specific CTCF disruption or BRD2 depletion, expression of the two genes becomes increasingly correlated. HiC shows that BRD2 depletion weakens boundaries co-occupied by CTCF and BRD2, but not those that lack BRD2. These findings indicate that BRD2 supports boundary activity and raise the possibility that pharmacologic BET inhibitors can influence gene expression in part by perturbing domain boundary function.