TGFβ produced by IL‐10 redirected astrocytes attenuates microglial activation

Abstract

While there clearly is an intimate relationship between astrocytes and microglia, few studies have examined these potentially dynamic interactions. In this study, cytokine-mediated communication between microglia and astrocytes under inflammatory conditions was investigated. We have previously shown that activated microglia produce Interleukin (IL)-10, a regulatory cytokine that plays an important role in resolving neuroinflammation. Nonetheless, the mechanism by which IL-10 attenuates pro-inflammatory cytokine expression in the brain is unclear. Here we show that IL-10 re-directed astrocytes regulate the activation of microglia in a Transforming growth factor (TGF)-β dependent manner. In support of this concept, astrocytes in the brain maintained higher IL-10 receptor (IL-10R1) expression and primary astrocytes in culture were markedly more sensitive to the anti-inflammatory effects of IL-10 compared to microglia. Moreover, studies using primary cultures and an astrocyte-microglia co-culture system revealed that astrocytes mediated the anti-inflammatory effects of IL-10 on microglia through the production of TGFβ. For instance, only when astrocytes were present did IL-10 stimulation reduce the expression of IL-1β and increase expression of anti-inflammatory mediators fractalkine receptor (CX3CR1) and interleukin 4 receptor-α (IL-4Rα) in microglia. Importantly, these IL-10-astrocyte dependent effects on microglia were blocked by a TGFβ inhibitor. Furthermore, inhibition of TGFβ signaling in the brain resulted in prolonged sickness behavior and amplified pro-inflammatory cytokine expression in mice challenged with lipopolysaccharide (LPS). Taken together, IL-10 stimulated the production of TGFβ by astrocytes, which in turn, attenuated microglial activation. Overall, these findings provide novel insight into the mechanisms by which astrocytes modulate microglia under inflammatory conditions.

Neuroimmune Response in Ischemic Preconditioning

“…Other studies have demonstrated that after IPC, astrocytes upregulate 14-3-3γ [102,103], a multifunctional scaffolding protein, and alleviate energy depletion through different pathways [102]. Ischemia induces a cellular redistribution of astrocytic gap junction proteins, including Cx34 [104,105]. Cx34-expressing astrocytes have been found in the peri-infarct region of the poststroke brain [104], where they may secrete growth factors that promote neuronal survival and stimulate angiogenesis in infarcted tissue [105][106][107].…”

“…Ischemia induces a cellular redistribution of astrocytic gap junction proteins, including Cx34 [104,105]. Cx34-expressing astrocytes have been found in the peri-infarct region of the poststroke brain [104], where they may secrete growth factors that promote neuronal survival and stimulate angiogenesis in infarcted tissue [105][106][107].…”

“…Also similar to microglia, astrocytes are capable of performing destructively by releasing high levels of nitric oxide, glutamate, TNF-α, and/or other neurotoxic cytokines [21], or may confer neuroprotection through trophic support [21,105]. Reactive astrocytes express glial fibrillary acidic protein (GFAP) and vimentin (Vim) [103].…”

Correction to: Neuroimmune Response in Ischemic Preconditioning

“…In a time course study of responses to peripheral LPS challenge, Norden et al [20] found that microglia are activated first, followed by astrocytes. Furthermore, microglial IL-10 stimulates astrocytic TGF-β in response to LPS challenge [27]. The large overlap between molecular pathways activated in response to LPS and ischemia is, in part, responsible for the phenomenon of crosstolerance (discussed previously), suggesting that IPC may modulate microglia/astrocyte interplay, with both cells playing important roles in conferring IPC-mediated neuroprotection.…”

“…The shared modulation of neuroimmune components appears to be responsible for the overlap in neuroprotection offered against ischemia by multiple preconditioning stimuli. [19,20], microglia [20], peripheral immune cells [19][20][21] HIF-1 Regulates metabolic genes to protect against hypoxia Multiple CNS cells [17,22], peripheral immune cells [23] Type TLRs Reprograms cellular genomic response to ischemia Circulating leukocytes [16], endothelial cells [16], microglia [17,18] TGF-β Promotes repair of neurovascular unit, regulates immune system function Astrocytes [20,27], microglia [27], T cells [28] TLR = Toll-like receptor; TNF = tumor necrosis factor; NFkB = nuclear factor kB; CNS = central nervous system; HIF = hypoxia-inducible factor; IFN = interferon; IRF = IFN regulatory factor; TGF = transforming growth factor…”