CBM-04PD-1 EXPRESSION IDENTIFIES EXHAUSTED TUMOR INFILTRATING REGULATORY T CELLS IN GLIOBLASTOMA

BACKGROUND: Foxp3+ regulatory T cells (Tregs) are central in ensuring protective immune responses while preventing immunopathology. While Tregs exhibit defective function in autoimmune disorders, they appear to be enriched in tumor immune infiltrates, suggesting a role in failure of immune-mediated tumor rejection. Tumor-infiltrating Tregs express higher levels of the co-inhibitory receptor PD-1 compared to circulating Tregs, especially in patients with glioblastoma. PD-1 is overexpressed by effector T cells in chronic immune responses and plays a pivotal role in driving T cell exhaustion in these conditions. METHODS: We examined the phenotype and function of CD4+CD25+CD127−Foxp3+ Tregs from peripheral blood and tumor infiltrates of patients with different grades of glioma, including glioblastoma (n = 20). Assessments included functional evaluation of Tregs in co-culture suppression assays with blood-derived T effector cells, and transcriptional profiling by RNA sequencing. RESULTS: By comparing the functionality of Tregs derived from blood and tumors of patients with glioblastoma, we established increased Treg expression of PD-1 is associated with impaired suppressive function and production of proinflammatory cytokines. Transcriptional profiling of PD-1+ Tregs revealed tumor-infiltrating PD-1+ Tregs are enriched for several exhaustion signatures, confirming PD-1 is a marker of exhaustion not only for effector T cells but also for Tregs. Finally, we investigated phenotype and function of PD-1+ Tregs in the circulation of healthy individuals (no known history of cancer, autoimmune disease, or other inflammatory conditions), and observed a similar loss of suppressive function and an exhausted transcriptional profile. CONCLUSIONS: Expression of PD-1 marks a population of dysfunctional Tregs that occurs in normal immunity and is enriched in cancer, specifically in the tumor. That Tregs from the tumor fail to suppress effector immune responses suggests that loss of effector T-cell function is more likely to result from chronic stimulation by the tumor than from active regulation by Tregs.