TOX expression on HBV specific CD8+ T cells is linked to clinical stage of chronic HBV infection

T-cell exhaustion or dysfunction is a common feature of chronic viral infections such as a chronic HBV infection. The mechanisms controlling the differentiation of exhausted T cells are poorly understood. Recently, the transcription factor and epigenetic modifier TOX was identified as a master regulator of the T-cell exhaustion program in mice. However, the role of TOX in T-cell responses to HBV and different human viral antigens remains unclear.

To address this important question, we analyzed the phenotype and function of HLA-A*02:01 restricted virus-specific CD8+ T cells from 23 HBeAg negative and 7 HBeAg positive chronically HBV-infected patients, as well as patients chronically infected with HCV, HIV, and healthy controls using flow cytometry and high-parametric mass cytometry.

We found that TOX expression on virus-specific CD8+ T cells was strongly increased in persistent viral infections. Interestingly, TOX was significantly higher expressed on HBV-specific T cells facing high levels of antigen isolated from HBeAg positive patients compared to patients with HBeAg negative infection (inactive carrier) indicating that TOX is linked to the different clinical stages of HBV infection and to high antigen load. In agreement with this assumption, we found that virus-specific CD8+ T cells obtained from chronically HCV- and HIV-infected patients with high viral loads also expressed high levels of TOX. Functional analysis linked TOX to a diminished polyfunctionality of exhausted HBV-specific CD8+ T cells suggesting a TOX-dependent programming of CD8+ T-cell dysfunction. Despite these clear links of TOX to severe T-cell exhaustion, we also observed a high TOX expression on memory CD8+ T cells, in particular senescent cells with irreversible arrest of cell proliferation and cell function in patients with chronic viral infection but also healthy individuals. Indeed, differential patterns of Eomes, T-bet and TCF1 co-expression with TOX were transcriptionally linked to exhaustion vs. senescence programs.

In sum, these results reveal a context-dependent role for TOX tied into differential antigen-dependent exhaustion and senescence programs in humans. The links between TOX and differential exhaustion of HBV-specific T cells during different clinical stages of HBV infection has implications for immunotherapeutic approaches in chronic HBV infection.