CBM-05: FUNCTIONAL DIFFERENCES BETWEEN PD-1⁺ AND PD-1^– CD4⁺ T EFFECTOR CELLS IN HEALTHY DONORS AND PATIENTS WITH GLIOBLASTOMA

BACKGROUND: The PD-1/PD-L1 immunomodulatory pathway plays a central role in regulating T-cell effector function and has been implicated in immune evasion in glioblastoma. It is also known that populations of tumor-infiltrating lymphocytes may be responsible for disease progression and poor prognosis. A substantial number of these lymphocytes are T effector cells (CD4⁺CD25⁻CD127⁺), however little is known about their function in the tumor microenvironment in glioblastoma or their expression of PD-1. METHODS: We characterized functional differences between T effector cells derived from blood and tumors of patients with glioblastoma (n = 20), with the hypothesis that PD-1 would mark a dysfunctional T effector population. T effector cell proliferative capacity was assessed by stimulation with anti-CD3/anti-CD28/anti-CD2 in thymidine incorporation assays; cytokine production profiles were assessed ex vivo by stimulation with PMA and ionomycin; transcriptional profiling was determined by RNA sequencing. We also performed transcriptional profiling on PD-1⁺ and PD-1⁻ T effector subsets isolated from the peripheral blood and tumors of patients with glioblastoma and from the peripheral blood of healthy donors. RESULTS: The frequency of PD-1⁺TIM-3⁺ T effectors in the peripheral blood of patients with glioblastoma was significantly increased compared to healthy controls. Tumor-derived PD-1⁺ T effectors displayed significantly less proliferation but retained the ability to produce inflammatory cytokines, including IFNγ. PD-1⁺ T effectors derived from healthy donors displayed impaired proliferation, which could not be rescued by PD-1/PD-L1 blockade or the addition of IL-2. PD-1⁺ T effector cells were enriched in several gene sets, indicating that PD-1⁺ T effector cells may be clonally exhausted. We also identified several genes that differentiated tumor-derived and peripheral blood-derived PD-1⁺ T effectors in glioblastoma patients. CONCLUSIONS: These data indicate that PD-1 expression marks a dysfunctional population of clonally exhausted T effector cells that may be important in the context of glioblastoma.