TOX is expressed by exhausted and polyfunctional human effector memory CD8⁺ T cells

CD8⁺ T cell exhaustion is a hallmark of many cancers and chronic infections. In mice, T cell factor 1 (TCF-1) maintains exhausted CD8⁺ T cell responses, whereas thymocyte selection-associated HMG box (TOX) is required for the epigenetic remodeling and survival of exhausted CD8⁺ T cells. However, it has remained unclear to what extent these transcription factors play analogous roles in humans. In this study, we mapped the expression of TOX and TCF-1 as a function of differentiation and specificity in the human CD8⁺ T cell landscape. Here, we demonstrate that circulating TOX⁺ CD8⁺ T cells exist in most humans, but that TOX is not exclusively associated with exhaustion. Effector memory CD8⁺ T cells generally expressed TOX, whereas naive and early-differentiated memory CD8⁺ T cells generally expressed TCF-1. Cytolytic gene and protein expression signatures were also defined by the expression of TOX. In the context of a relentless immune challenge, exhausted HIV-specific CD8⁺ T cells commonly expressed TOX, often in clusters with various activation markers and inhibitory receptors, and expressed less TCF-1. However, polyfunctional memory CD8⁺ T cells specific for cytomegalovirus (CMV) or Epstein-Barr virus (EBV) also expressed TOX, either with or without TCF-1. A similar phenotype was observed among HIV-specific CD8⁺ T cells from individuals who maintained exceptional immune control of viral replication. Collectively, these data demonstrate that TOX is expressed by most circulating effector memory CD8⁺ T cell subsets and not exclusively linked to exhaustion.