Astrocytes play important roles in the central nervous system (CNS) during health and disease. Through genome-wide analyses we detected a transcriptional response to type I interferons (IFN-I) in astrocytes during experimental CNS autoimmunity and also in CNS lesions from multiple sclerosis (MS) patients. IFN-I signaling in astrocytes reduces inflammation and experimental autoimmune encephalomyelitis (EAE) disease scores via the ligand-activated transcription factor aryl hydrocarbon receptor (AhR) and suppressor of cytokine signaling 2 (SOCS2). The anti-inflammatory effects of nasally administered IFN-β are partly mediated by AhR. Dietary tryptophan is metabolized by the gut microbiota into AhR agonists that act on astrocytes to limit CNS inflammation. EAE scores were increased following ampicillin treatment during the recovery phase, and CNS inflammation was reduced in antibiotic-treated mice by supplementation with the tryptophan metabolites indole, indoxyl-3-sulfate (I3S), indole-3-propionic acid (IPA) and indole-3-aldehyde (IAld), or the bacterial enzyme tryptophanase. In individuals with MS, the circulating levels of AhR agonists were decreased. These findings suggest that IFN-I produced in the CNS act in combination with metabolites derived from dietary tryptophan by the gut flora to activate AhR signaling in astrocytes and suppress CNS inflammation.

Microglia and astrocytes modulate inflammation and neurodegeneration in the central nervous system (CNS). Microglia modulate pro-inflammatory and neurotoxic activities in astrocytes, but the mechanisms involved are not completely understood. Here we report that TGFα and VEGF-B produced by microglia regulate the pathogenic activities of astrocytes in the experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis. Microglia-derived TGFα acts via the ErbB1 receptor in astrocytes to limit their pathogenic activities and EAE development. Conversely, microglial VEGF-B triggers FLT-1 signalling in astrocytes and worsens EAE. VEGF-B and TGFα also participate in the microglial control of human astrocytes. Furthermore, expression of TGFα and VEGF-B in CD14 cells correlates with the multiple sclerosis lesion stage. Finally, metabolites of dietary tryptophan produced by the commensal flora control microglial activation and TGFα and VEGF-B production, modulating the transcriptional program of astrocytes and CNS inflammation through a mechanism mediated by the aryl hydrocarbon receptor. In summary, we identified positive and negative regulators that mediate the microglial control of astrocytes. Moreover, these findings define a pathway through which microbial metabolites limit pathogenic activities of microglia and astrocytes, and suppress CNS inflammation. This pathway may guide new therapies for multiple sclerosis and other neurological disorders.

The specific signals mediating the activation of microglia and astrocytes as a prelude to, or consequence of, CNS inflammation continue to be defined. We investigated TLRs as novel receptors mediating innate immune responses in human glial cells. We find that microglia express mRNA for TLRs 1–9, whereas astrocytes express robust TLR3, low-level TLR 1, 4, 5, and 9, and rare-to-undetectable TLR 2, 6, 7, 8, and 10 mRNA (quantitative real-time PCR). We focused on TLRs 3 and 4, which can signal through both the MyD88-dependent and -independent pathways, and on the MyD88-restricted TLR2. By flow cytometry, we established that microglia strongly express cell surface TLR2; TLR3 is expressed at higher levels intracellularly. Astrocytes express both cell surface and intracellular TLR3. All three TLRs trigger microglial activation upon ligation. TLR3 signaling induces the strongest proinflammatory polarizing response, characterized by secretion of high levels of IL-12, TNF-α, IL-6, CXCL-10, and IL-10, and the expression of IFN-β. CXCL-10 and IL-10 secretion following TLR4 ligation are comparable to that of TLR3; however, other responses were lower or absent. TLR2-mediated responses are dominated by IL-6 and IL-10 secretion. Astrocytes respond to TLR3 ligation, producing IL-6, CXCL-10, and IFN-β, implicating these cells as contributors to proinflammatory responses. Initial TLR-mediated glial activation also regulates consequent TLR expression; while TLR2 and TLR3 are subject to positive feedback, TLR4 is down-regulated in microglia. Astrocytes up-regulate all three TLRs following TLR3 ligation. Our data indicate that activation of innate immune responses in the CNS is not homogeneous but rather tailored according to cell type and environmental signal.