Fingolimod Suppresses the Proinflammatory Status of Interferon-γ-Activated Cultured Rat Astrocytes

Astroglia, the primary homeostatic cells of the central nervous system, play an important role in neuroinflammation. They act as facultative immunocompetent antigen-presenting cells (APCs), expressing major histocompatibility complex (MHC) class II antigens upon activation with interferon (IFN)-γ and possibly other proinflammatory cytokines that are upregulated in disease states, including multiple sclerosis (MS). We characterized the anti-inflammatory effects of fingolimod (FTY720), an established drug for MS, and its phosphorylated metabolite (FTY720-P) in IFN-γ-activated cultured rat astrocytes. The expression of MHC class II compartments, β₂ adrenergic receptor (ADR-β₂), and nuclear factor kappa-light-chain enhancer of activated B cells subunit p65 (NF-κB p65) was quantified in immunofluorescence images acquired by laser scanning confocal microscopy. In addition, MHC class II-enriched endocytotic vesicles were labeled by fluorescent dextran and their mobility analyzed in astrocytes subjected to different treatments. FTY720 and FTY720-P treatment significantly reduced the number of IFN-γ-induced MHC class II compartments and substantially increased ADR-β₂ expression, which is otherwise small or absent in astrocytes in MS. These effects could be partially attributed to the observed decrease in NF-κB p65 expression, because the NF-κB signaling cascade is activated in inflammatory processes. We also found attenuated trafficking and secretion from dextran-labeled endo-/lysosomes that may hinder efficient delivery of MHC class II molecules to the plasma membrane. Our data suggest that FTY720 and FTY720-P at submicromolar concentrations mediate anti-inflammatory effects on astrocytes by suppressing their action as APCs, which may further downregulate the inflammatory process in the brain, constituting the therapeutic effect of fingolimod in MS.