Gfi1, a newcomer to the realm of T cell exhaustion

Persistent stimulation by chronic viral and tumor antigens drives T cells "exhausted"⁽¹⁾. While it is widely accepted that these "exhausted" T cells are comprised of self-renewing progenitors and their progenies (i.e., effector-like and terminally exhausted cells), the underlying mechanisms orchestrating this process remain incompletely understood, delineation of which could be of utmost importance to bolster therapeutic efficacy of  immune checkpoint blockade (ICB) therapies such as anti-CTLA-4 and anti-PD-1/L1, as they rely on the transition of progenitors to effector-like cells for tumor and viral control⁽²⁾.

Growth factor-independent 1 (Gfi1) is a transcriptional repressor^{(3, 4)}, suppressing target genes via recruiting histone deacetylases (HDACs), demethylases (KDMs), and other chromatin modifying complexes. We and others show that Gfi1 orchestrates the development of thymocytes^{(4, 5)};  absence of Gfi1 favors the formation of single positive CD8⁺ T thymocytes ⁽⁵⁾ and naturally occurring regulatory T cells^{(6, 7)}. These findings, together with the reported importance of chromatin imprinting in shaping  exhausted CD8⁺ T cells, prompt us to study whether Gfi1 could act as a key regulator of T cell exhaustion.

Using genetic mouse models with Gfi1 reporter or specific Gfi1 deletion in T cells that were infected with chronic LCMV Clone-13 or challenged with syngeneic tumor models, we found that Gfi1 expression is dynamically regulated in exhausted CD8⁺ T cells and indispensable for the formation of effector-like and terminally exhausted cells. As such, tumor-bearing mice lacking Gfi1 in T cells did not respond to anti-CTLA-4, coupled with marked reduction of IFN-g  production by intratumoral CD8⁺ T cells. To shed light on how Gfi1 maintains the chromatin  structure and transcriptome in exhausted CD8⁺ T cells, we conducted transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) as well as RNA-seq, which unveiled a prominent dysregulation in the absence of Gfi1. Mechanistic studies with in vivo deletion of CD4⁺ T cells and chimeric mice supported that Gfi1-mediated formation of effector-like CD8⁺ T cell is a cell-autonomous effect. Intriguingly, we also identified a previously underappreciated subset co-expressing in Ly108 and CX₃CR1 (double-positive (DP) cells). These DP cells had low Gfi1 expression and possessed distinct chromatin profiles, representing a transitory subset downstream of progenitors that develop to effector-like and terminally-exhausted cells, a process dependent on Gfi1. Future investigations should focus on identifying factors  that dictate the decision-making process in DP cells, that is, to become effector-like or terminally-exhausted cells. These efforts would be greatly facilitated by determining direct binding sites/partners of Gfi1, although our initial efforts using commonly-used Gfi1 antibodies were largely unsuccessful, likely because of the limited cell numbers.  Furthermore, given the essential role of Gfi1 in anti-CTLA-4 efficacy, fine-tuning Gfi1 activity in T cells appears to be an enticing approach to promote therapeutic effects of ICBs, as T cell exhaustion is a significant therapeutic barrier to ICBs.  

In conclusion, we show that Gfi1 in CD8⁺ T cells epigenetically and transcriptionally controls the generation of effector-like cells during chronic infections and tumor progression (see below figure)⁽⁸⁾.

References

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