Microglia are considered to be potential antigen-presenting cells and have the ability to present antigen under pathological conditions. Nevertheless, whether and how microglia are involved in immune regulation are largely unknown. Here, we investigated the suppressive activity of microglia during experimental autoimmune encephalomyelitis(EAE) induced by myelin oligodendrocyte glycoprotein, with the goal of understanding their role in regulating the T cell reaction. Using flow cytometric analysis, we found that microglia were characterized by increased cell number and up-regulated programmed death ligand-1(PD-L1) at the peak phase of EAE. Meanwhile,both the CD~(4+)T cells and microglia that infiltrated the central nervous system expressed higher levels of PD1, the receptor for PD-L1, accompanied by a decline of Th1 cells.In an ex vivo co-culture system, microglia from EAE mice inhibited the proliferation of antigen-specific CD~(4+)T cells and the differentiation of Th1 cells, and this was significantly inhibited by PD-L1 blockade. Further,microglia suppressed Th1 cells via nitric oxide(NO), the production of which was dependent on PD-L1. Thus, these data suggest a scenario in which microglia are involved in the regulation of EAE by suppressing Th1-cell differentiation via the PD-L1-NO pathway. Nevertheless, how and how microglia are involved in immune response are vital unknown. Here, we investigated the suppressive activity of microglia during experimental autoimmune encephalomyelitis ( EAE) induced by myelin oligodendrocyte glycoprotein, with the goal of understanding their role in regulating the T cell reaction. Using flow cytometric analysis, we found that microglia were characterized by increased cell number and up-regulated programmed death ligand-1 (PD-L1 ) at the peak phase of EAE. Meanwhile, both of the CD4 ~ T cells and microglia that infiltrated the central nervous system are higher levels of PD1, the receptor for PD-L1, accompanied by a decline of Th1 cells. an ex vivo co-culture system, microglia from EAE mice inhibited the proliferation of antigen-specific CD ~ (4+) T cells and the differentiation of Th1 cells, and this was significantly inhibited by PD-L1 blockade. Further, microglia suppressed Th1 cells via nitric oxide (NO), the production of which was dependent on PD-L1. These data suggest a scenario in which microglia are involved in the regulation of EAE by draining Th1-cell differentiation via the PD-L1-NO pathway.