Induction of inhibitory central nervous system-derived and stimulatory blood-derived dendritic cells suggests a dual role for granulocyte-macrophage colony-stimulating factor in central nervous system inflammation

The mononuclear phagocyte system, particularly dendritic cells, plays several pivotal roles in the development of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis. Here, we demonstrate that functionally distinct dendritic cell subpopulations are present in the central nervous system during experimental autoimmune encephalomyelitis. At peak experimental autoimmune encephalomyelitis, the majority of dendritic cells consisted of a CD11b(+)F4/80(+) inflammatory dendritic cell subtype. Both granulocyte-macrophage colony-stimulating factor and chemokine (C-C motif) ligand 2 were previously suggested to recruit 'inflammatory' monocyte-derived dendritic cells to the central nervous system during experimental autoimmune encephalomyelitis. We show that intra-cerebral production of granulocyte-macrophage colony-stimulating factor leading to chemokine (C-C motif) ligand 2 induction and attraction of chemokine (C-C motif) receptor 2-positive precursors suffices to recruit dendritic cell populations identical to those observed in experimental autoimmune encephalomyelitis into the central nervous system of healthy mice. This does not occur with fms-like tyrosine kinase-3-ligand treatment. Both during experimental autoimmune encephalomyelitis and upon intra-cerebral granulocyte-macrophage colony-stimulating factor production, all myeloid dendritic cells, lymphoid dendritic cells and periphery-derived inflammatory dendritic cells stimulated T cell proliferation, whereas inflammatory dendritic cells that differentiated from central nervous system precursors inhibited T cell activation and pro-inflammatory cytokine production. Despite the capacity of granulocyte-macrophage colony-stimulating factor to induce central nervous system-derived inhibitory inflammatory dendritic cells, the administration of granulocyte-macrophage colony-stimulating factor into mice with experimental autoimmune encephalomyelitis resulted in exacerbated disease. Granulocyte-macrophage colony-stimulating factor thus has a dual role in the central nervous system: it directs both central nervous system-derived dendritic cells towards an inhibitory phenotype and recruits peripheral dendritic cells exhibiting pro-inflammatory functions.