Glioblastoma (GBM) is the most common primary malignant tumor in the central nervous system. Our recent preclinical work has suggested that PD-1/PD-L1 plays an important immunoregulatory role to limit effective antitumor T-cell responses induced by active immunotherapy. However, little is known about the functional role that PD-1 plays on human T lymphocytes in patients with malignant glioma.

Experimental Design: In this study, we examined the immune landscape and function of PD-1 expression by T cells from tumor and peripheral blood in patients with malignant glioma.

We found several differences between PD-1⁺ tumor-infiltrating lymphocytes (TIL) and patient-matched PD-1⁺ peripheral blood T lymphocytes. Phenotypically, PD-1⁺ TILs exhibited higher expression of markers of activation and exhaustion than peripheral blood PD-1⁺ T cells, which instead had increased markers of memory. A comparison of the T-cell receptor variable chain populations revealed decreased diversity in T cells that expressed PD-1, regardless of the location obtained. Functionally, peripheral blood PD-1⁺ T cells had a significantly increased proliferative capacity upon activation compared with PD-1⁻ T cells.

Our evidence suggests that PD-1 expression in patients with glioma reflects chronically activated effector T cells that display hallmarks of memory and exhaustion depending on its anatomic location. The decreased diversity in PD-1⁺ T cells suggests that the PD-1–expressing population has a narrower range of cognate antigen targets compared with the PD-1 nonexpression population. This information can be used to inform how we interpret immune responses to PD-1–blocking therapies or other immunotherapies.