TOX, through a glass, darkly.

The transcription factor TOX has attracted attention in recent years for its role in CD8 T cell exhaustion. In fact, TOX was known historically for its diverse roles in immune cell biology. Here, we inquire into the basis for this versatility, and propose that one main consideration is that TOX is an HMG-box transcription factor. We discuss some mechanisms that other HMG-box transcription factors employ to perform their cellular functions, as examples of the range of mechanisms TOX might employ in furthering T cell exhaustion. This inquiry begins with the literature placing TOX as a central player in CD8 T cell exhaustion and in other immune cell processes. An understanding of TOX as a transcription factor has to be organized around its binding to relevant target sites in DNA. Thus, we next cover the reasons that TOX is classified as an HMG-box protein, the well-defined but narrow scope of what TOX shares with other HMG-box proteins, and the unequivocal evidence that binding of HMG-box proteins stabilizes kinked or bent DNA. We consider the constant features and some variables in DNA recognition by HMG-box proteins. Since binding and bending DNA is not in isolation an explanation of any biological process, we look at biological examples highlighting specific ways that HMG-box proteins drive cellular processes. Finally, we outline some lines of research that could be informative in understanding the cellular mechanisms of TOX in T cell exhaustion.