Schematic depiction of potential and demonstrated roles of indolamine 2,3-dioxygenase (IDO) activation and downstream l-kynurenine metabolites inflammation, with particular focus on EAE and MS. CTLA4: cytotoxic T-lymphocyte-associated protein 4, EAE: experimental autoimmune encephalomyelitis, DC: dendritic cell, IDO: indolamine 2,3-dioxygenase, IFN: interferon, IL: interleukin, LPS: lipopolysaccharide, MS, multiple sclerosis, PMN: polymorphonuclear cell, TGFβ: transforming growth factor β, Th: T helper lymphocyte, Treg: regulatory T lymphocyte, TNFα: tumor necrosis factor α, Trp: tryptophan. Up and down arrows represent increase and decrease, respectively.