Anti-PD-1 Induces M1 Polarization in the Glioma Microenvironment and Exerts Therapeutic Efficacy in the Absence of CD8 Cytotoxic T Cells.

Abstract

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 <i>CD8α</i> 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.