Anti-programmed cell death protein 1 (PD-1) therapy has demonstrated inconsistent therapeutic results in patients with glioblastoma (GBM) including those with profound impairments in CD8 T-cell effector responses.

We ablated the CD8α gene in BL6 mice and intercrossed them with Ntv-a mice to determine how CD8 T cells affect malignant progression in forming endogenous gliomas. Tumor-bearing mice were treated with PD-1 to determine the efficacy of this treatment in the absence of T cells. The tumor microenvironment of treated and control mice was analyzed by IHC and FACS.

We observed a survival benefit in immunocompetent mice with endogenously arising intracranial glioblastomas after intravenous administration of anti–PD-1. The therapeutic effect of PD-1 administration persisted in mice even after genetic ablation of the CD8 gene (CD8^−/−). CD11b⁺ and Iba1⁺ monocytes and macrophages were enriched in the glioma microenvironment of the CD8^−/− mice. The macrophages and microglia assumed a proinflammatory M1 response signature in the setting of anti–PD-1 blockade through the elimination of PD-1–expressing macrophages and microglia in the tumor microenvironment. Anti–PD-1 can inhibit the proliferation of and induce apoptosis of microglia through antibody-dependent cellular cytotoxicity, as fluorescently labeled anti–PD-1 was shown to gain direct access to the glioma microenvironment.

Our results show that the therapeutic effect of anti–PD-1 blockade in GBM may be mediated by the innate immune system, rather than by CD8 T cells. Anti–PD-1 immunologically modulates innate immunity in the glioma microenvironment—likely a key mode of activity.