Characterising negative regulation of CD8+ T cell function in tolerance and exhaustion

Abstract

CD8+ T cells play a vital role in the immune system by clearing pathogen-infected tissues, however self-reactive CD8+ T cells that escape thymic selection pose the danger of causing autoimmune disease. These self-reactive CD8+ T cells are controlled in the periphery by tolerance mechanisms, which inhibit their function (anergy) or induce apoptosis (deletion). However, the specific molecular pathways crucial for negatively regulating self-reactive CD8+ T cells are not well elucidated. A distinct form of negative regulation called "exhaustion" occurs within chronically stimulated effector CD8+ T cells during cancer and chronic infection. Due to the phenotypical similarities between CD8+ T cell tolerance and exhaustion, we aimed to understand if common underlying molecular pathways regulate these states.

The pro-apoptotic protein BIM is important in deletion of self-reactive CD8+ T cells, however the transcriptional control of Bim induction has been unclear. In Chapter 2, we assessed the contribution of the transcription factor FOXO3 in deletion of self-reactive CD8+ T cells given its role in Bim induction and cell death in effector and exhausted CD8+ T cells. While FOXO3 protein underwent activatory dephosphorylation during tolerance, FOXO3-deficient CD8+ T cells maintained the ability to induce BIM expression and undergo deletion. This result indicated that FOXO3 plays distinct roles in cell death of tolerant versus effector CD8+ T cells.

To further characterise CD8+ T cell tolerance pathways, in Chapter 3, we investigated whether the ubiquitin ligase adaptor NDFIP1, which is crucially required for CD4+ T cell anergy, influences CD8+ T cell tolerance. In a model of peptide-induced anergy, Ndfip1-deficient CD8+ T cells aberrantly expanded and differentiated into effector cells against high dose exogenous antigen, likely driven by increases in TCR signaling. In contrast, NDFIP1 was dispensable for peripheral deletion to low-dose exogenous antigen, and had little impact upon effector responses to acute infection. These results showed the importance of NDFIP1 in regulating CD8+ T cell tolerance and indicated that CD8+ T cell deletion and anergy are molecularly separable checkpoints.

While CD8+ T cell exhaustion appears distinct from tolerance, the transcriptional regulator EGR2 is commonly expressed between these states. In Chapter 4, we showed that exhausted CD8+ T cells in chronic LCMV infection expressed elevated levels of EGR2 compared to functional effectors. Loss of Egr2 severely disrupted terminal CD8+ T cell exhaustion in a cell intrinsic manner, with RNA-Seq results indicating a global enrichment of the exhaustion "stem cell" gene set in Egr2-deficient cells. Strikingly, the genes regulated by Egr2 during exhaustion appeared distinct from those controlled by Egr2 during T cell tolerance, suggesting that EGR2 is repurposed during T cell exhaustion.

These findings indicate that while there is molecular overlap between CD8+ T cell tolerance and exhaustion, shared proteins may differ in their mechanism of action. Such insight into the differences between tolerance and exhaustion checkpoints is important for refining future immunotherapies against cancer and autoimmune diseases. Show more