Glial cells suppress postencephalitic CD8⁺ T lymphocytes through PD-L1

Engagement of the programmed death (PD)-1 receptor on activated cells by its ligand (PD-L1) is a mechanism for suppression of activated T-lymphocytes. Microglia, the resident inflammatory cells of the brain, are important for pathogen detection and initiation of innate immunity, however, a novel role for these cells as immune regulators has also emerged. PD-L1 on microglia has been shown to negatively regulate T-cell activation in models of multiple sclerosis and acute viral encephalitis. In this study, we investigated the role of glial cell PD-L1 in controlling encephalitogenic CD8⁺ T-lymphocytes, which infiltrate the brain to manage viral infection, but remain to produce chronic neuroinflammation. Using a model of chronic neuroinflammation following murine cytomegalovirus (MCMV)-induced encephalitis, we found that CD8⁺ T-cells persisting within the brain expressed PD-1. Conversely, activated microglia expressed PD-L1. In vitro, primary murine microglia, which express low basal levels of PD-L1, upregulated the co-inhibitory ligand on IFN-γ-treatment. Blockade of the PD-1: PD-L1 pathway in microglial: CD8⁺ T-cell co-cultures increased T-cell IFN-γ and interleukin (IL)-2 production. We observed a similar phenomenon following blockade of this co-inhibitory pathway in astrocyte: CD8⁺ T-cell co-cultures. Using ex vivo cultures of brain leukocytes, including microglia and CD8⁺ T-cells, obtained from mice with MCMV-induced chronic neuroinflammation, we found that neutralization of either PD-1 or PD-L1 increased IFN-γ production from virus-specific CD8⁺ T-cells stimulated with MCMV IE1_168-176 peptide. These data demonstrate that microglia and astrocytes control antiviral T-cell responses and suggest a therapeutic potential of PD1: PD-L1 modulation to manage the deleterious consequences of uncontrolled neuroinflammation.