New Immune Checkpoint PTGIR Found to Drive CD8 T Cell Exhaustion via NRF2
Scientists identify prostacyclin receptor PTGIR as a NRF2-regulated immune checkpoint that accelerates T cell exhaustion in cancer and chronic infection.
Summary
Researchers at Van Andel Institute discovered that the prostacyclin receptor PTGIR acts as a novel immune checkpoint controlling CD8+ T cell exhaustion. Using mouse models of chronic viral infection and cancer, they showed that NRF2—the master antioxidant regulator—paradoxically accelerates terminal T cell exhaustion despite reducing oxidative stress, doing so by upregulating PTGIR. When PTGIR was silenced, CD8+ T cells maintained stronger effector function, produced more IFN-γ and granzymes, and resisted terminal exhaustion. PTGIR signaling impairs T cell metabolism and cytokine production while inducing transcriptional features of exhaustion. These findings reveal a new druggable axis—NRF2→PTGIR—that could complement existing immune checkpoint inhibitor therapies.
Detailed Summary
CD8+ T cell exhaustion is a central mechanism by which tumors evade immune destruction, and while therapies targeting PD-1 and CTLA-4 have revolutionized oncology, many patients still fail to respond. Understanding the molecular programs that drive T cells from an effector to an exhausted state is therefore a high priority in cancer immunology and immunotherapy research.
The research team began with a meta-analysis of published RNA-seq datasets from murine CD8+ T cells isolated across models of acute infection (Listeria monocytogenes), chronic infection (LCMV clone 13), and autochthonous liver tumors. Gene set enrichment analysis (GSEA) revealed that the NRF2 oxidative stress response pathway was the most enriched oncogenic signature in terminally exhausted T cells compared to effector T cells. This unexpected finding prompted a mechanistic investigation using a conditional knockout mouse model in which Keap1—the cytosolic inhibitor of NRF2—was deleted specifically in T cells (Cd4-Cre; Keap1fl/fl).
Conditional Keap1 deletion constitutively activated NRF2 in CD8+ T cells, boosting glutathione synthesis and reducing reactive oxygen species (ROS). Despite this improved antioxidant status, Keap1-knockout mice showed accelerated terminal T cell exhaustion during chronic LCMV infection, with increased PD-1hi TIM-3hi terminally exhausted cells, reduced progenitor exhausted (Tpex) cells, and impaired viral control. Competitive co-transfer experiments confirmed that Keap1-deficient P14 CD8+ T cells had a cell-intrinsic disadvantage in chronic infection. Transcriptomic and chromatin accessibility (ATAC-seq) analyses revealed that NRF2 activation drove exhaustion-associated gene programs and opened chromatin at loci controlled by exhaustion-associated transcription factors.
Critically, the team identified the prostaglandin I2 (prostacyclin) receptor PTGIR as a transcriptional target of NRF2 in CD8+ T cells. PTGIR was strongly upregulated in terminally exhausted T cells across murine and human datasets and was enriched in CD8+ tumor-infiltrating lymphocytes (TILs). Using CRISPR-based silencing or genetic knockout approaches, the researchers showed that loss of PTGIR enhanced IFN-γ and granzyme B production, reduced expression of inhibitory receptors, and limited terminal exhaustion in both chronic LCMV infection and tumor models (B16 melanoma, MC38 colorectal cancer). Pharmacological activation of PTGIR with the prostacyclin analog iloprost suppressed effector function and promoted exhaustion-like transcriptional states, confirming receptor-mediated effects. Mechanistically, PTGIR signaling impaired mitochondrial metabolism and suppressed cytokine production through cAMP-dependent pathways, while inducing canonical exhaustion transcription factors such as TOX.
These findings establish PTGIR as a NRF2-dependent immune checkpoint and propose a novel immunosuppressive axis in which oxidative stress within the tumor microenvironment activates NRF2 in CD8+ T cells, upregulates PTGIR, and accelerates terminal exhaustion—providing a potential therapeutic target to enhance cancer immunotherapy.
Key Findings
- NRF2 is the top transcriptional signature enriched in terminally exhausted versus effector CD8+ T cells.
- Constitutive NRF2 activation (Keap1 deletion) paradoxically accelerates terminal T cell exhaustion despite reducing ROS.
- NRF2 directly upregulates prostacyclin receptor PTGIR in CD8+ T cells during chronic infection and cancer.
- PTGIR silencing enhances IFN-γ and granzyme B production and limits terminal exhaustion in vivo.
- PTGIR signals via cAMP to impair T cell metabolism and induce exhaustion transcription factors like TOX.
Methodology
The study used transcriptomic meta-analysis (RNA-seq, GSEA), conditional T cell-specific Keap1 knockout mice, LCMV clone 13 chronic infection, and syngeneic tumor models (B16, MC38). Functional assays included CRISPR-mediated PTGIR silencing, competitive co-transfers, flow cytometry, ATAC-seq, metabolomics, and pharmacological PTGIR agonism with iloprost.
Study Limitations
Findings are primarily from mouse models; translation to human tumors requires further validation in clinical samples. The study does not fully resolve whether PTGIR blockade is safe given prostacyclin's cardioprotective roles in vasculature. The relative contribution of PTGIR versus other NRF2 targets to exhaustion in diverse tumor types remains to be determined.
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