Abstract A197: The HMG transcription factor TOX induces a transcriptional and epigenetic program of CD8+ T-cell exhaustion in chronic infection and cancer
Authors: Khan Omar , Giles Josephine R. , McDonald Sierra , Wherry E. John 
Published: February 2019

The protective capacity of the adaptive immune system relies on efficient and coordinated transitions between cellular fates. Following initial activation by specific antigen, naïve CD8+ T-cells proliferate extensively and undergo a highly orchestrated program of molecular rewiring and differentiation into effector CD8+ T-cells (TEFF) that can mediate protection through cytotoxicity and production of inflammatory cytokines. If the infection or antigen is cleared, most of this TEFF pool dies, but a subset persists, undergoing additional differentiation to form a pool of long-lived, self-renewing memory T-cells (TMEM) capable of mounting rapid recall responses. In contrast, during chronic infections or cancer, when T-cell stimulation persists, this program of functional T-cell differentiation is diverted and T-cells fail to sustain robust effector functions, instead becoming exhausted. Exhausted CD8+ T-cells (TEX) may balance limited pathogen or tumor control while restraining damaging immunopathology, but the consequence of restrained functionality is disease persistence and possible progression. Though first described in mice infected with lymphocytic choriomeningitis virus (LCMV), it is now clear that T-cell exhaustion is a common feature of many chronic infections as well as a variety of cancers in both mice and humans. Indeed, TEX are highly therapeutically relevant since these cells are a major target of checkpoint blockade mediated immune reinvigoration in human cancer patients. T-cell exhaustion is characterized by the progressive decline in effector function including the hierarchical loss of inflammatory cytokine production (IL-2, TNFa, IFNg). TEX also sustain high co-expression of multiple inhibitory receptors (PD-1, LAG3, TIGIT, CD160, TIM-3, 2B4), have reduced glycolytic and oxidative phosphorylation capacity, and impaired proliferation and survival. Underlying these major differences in TEX compared to TEFF and TMEM, is a distinct transcriptional program highlighted by altered use of key transcription factors and altered transcriptional circuits. Moreover, recent epigenetic analyses revealed that TEX differ from TEFF and TMEM by ~6,000 open chromatin regions, similar to differences between other major hematopoietic lineages, suggesting that TEX are not simply a state of activation of TEFF or TMEM, but rather are a distinct immune lineage. Yet the mechanisms that initiate this TEX fate commitment and epigenetic and transcriptional programming have thus far remained poorly understood, and exhaustion-specific TFs or transcriptional programming activities have remained elusive. Here we identify the HMG-box protein TOX as a master regulator of the TEX lineage. We find that robust TOX expression is limited to chronic infections and cancers in both mice and humans. TOX is largely dispensable for TEFF and TMEM formation, but in the absence of TOX, TEX do not form. Instead, TOX-deficiency results in the accumulation and subsequent rapid deletion of TEFF cells. TOX is induced by calcineurin and, specifically, NFAT2 and then operates in a feed-forward loop to become calcineurin independent and is durably expressed at a high level in TEX, but not TEFF or TMEM. TOX interacts with histone modifying enzyme proteins including the HBO1 complex and the acetyl-transferase Kat7, providing a mechanism for TEX lineage-specific epigenetic changes. Thus, high and sustained induction of TOX causes lineage commitment to TEX by translating persisting TCR stimulation cues into a distinct TEX transcriptional and epigenetic developmental program.

Citation Format: Omar Khan, Josephine R. Giles, Sierra McDonald, E. John Wherry. The HMG transcription factor TOX induces a transcriptional and epigenetic program of CD8+ T-cell exhaustion in chronic infection and cancer [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A197.