Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• CD8 T cell exhaustion

• Epigenetic reprogramming

• Immune checkpoint blockade

Main Findings

CD8 T cell exhaustion represents one of the biggest obstacles for the success of current immunotherapy approaches. Despite the vast knowledge regarding the immunophenotypic characterisation of CD8+ T cell exhausted phenotypes, currently little is known regarding the transcriptional and epigenetic programming that drives and fixates T cell exhaustion, and if such programming steps can be reversed. In this preprint, the authors identify STAT5a as a critical transcription factor that participates in a reciprocally antagonistic circuit with Tox, a well-known transcription factor involved in the establishment of terminally-exhausted T cells.

Using the LCMV clone 13 model of chronic infection in mice, and the subsequent adoptive transfer of TCR-transgenic LCMV-specific P14 T cells, the authors show that STAT5a overexpression leads to the accumulation of T cells with “effector-like” phenotypes which was evident since the first week of chronic infection, characterised by low expression of exhausting-associated transcription factors (i.e. Tcf1 and Tox), and high expression of effector molecules (granzyme B). The epigenetic landscape of P14 T cells overexpressing STAT5a also showed a preference in chromatin accessibility towards gene encoding effector molecules (including FasL, granzyme A, granzyme B, IFNγ, IFNγR1, IL-2Rα, IL-2Rβ, and perforin-1) and significant reduced accessibility of genes involved in T cell exhaustion (such as Tcf7, Tox, and Eomes). Furthermore, STAT5a overexpression in these cells showed a direct chromatin remodelling in the Tox locus similar to the epigenetic landscape of effector T cells, characterised by a decreased in active H3K27 acetylation in the binding sites of NFAT1 and NFAT2, which are critical transcription factors involved in Tox induction.

In later stages of chronic infection, where the majority of antigen-specific T cells become terminally-exhausted, the authors show that STAT5a overexpression leads to the preferential accumulation of intermediate-exhausted T cells (Ly108- CD69-), which upregulates markers of effector T and NK cells, including Granzyme A, Klre1, Klrb1c and T-bet, while downregulating markers of exhaustion such as PD-1, LAG-3, and Tox.

Lastly, using an orthogonal receptor/ligand system where ortho-IL-2 activates P14 cells transduced with ortho-IL-2Rβ which selectively binds STAT5a, the authors demonstrate that ortho-IL-2 treatment induces a preferential expansion of intermediate-exhausted T cells, and that this expansion is potentiated when ortho-IL-2 treatment is combined with anti-PD-L1. Furthermore, the adoptive transfer of P14 cells overexpressing STAT5a in animals challenged with B16 melanoma cells expressing the LCMV antigen showed a significant decrease in tumour growth compared to animals receiving WT P14 cells.

Limitations

• Although the role of Tox in the establishment of terminal T cell exhaustion is widely described, Tcf1 also plays a pivotal role, specifically in the first stages of exhaustion differentiation. The authors found several links between STAT5a overactivation and Tcf1 suppression, but no further experiments were described in order to better explore the relationship between these two critical transcription factors.

• This dataset derives from a very manipulated adoptive transfer model using a TCR-transgenic T cell line. Therefore, it is imperative to determine if these results can also be translated to naturally-occurring T cells in chronic settings, such as in viral infections and cancer.

Significance/Novelty

This preprint demonstrates a previously undescribed molecular circuit between the transcription factors STAT5a and Tox in the settings of a chronic viral infection, thus shedding more lights into the key transitional stages that occur from T cell activation towards T cell exhaustion.

In addition, the manipulation of STAT5a signalling in these settings can induce the accumulation of intermediate-exhausted T cells which can be preferentially boosted in checkpoint blockade strategies, such as anti-PD-1/PD-L1 therapy, thus providing a potentially novel therapeutic pathway that can be targeted in several clinical settings.

Credit

Reviewed by David Arcia-Anaya as part of the cross-institutional journal club of the Immunology Institute of the Icahn School of Medicine, Mount Sinai and the Kennedy Institute of Rheumatology, University of Oxford.