TGFβ produced by IL-10 redirected astrocytes attenuates microglial activation

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 redirected astrocytes regulate the activation of microglia in a transforming growth factor (TGF)-beta 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 with microglia. Moreover, studies using primary cultures and an astrocyte-microglia coculture system revealed that astrocytes mediated the anti-inflammatory effects of IL-10 on microglia through the production of TGF beta. For instance, only when astrocytes were present did IL-10 stimulation reduce the expression of IL-1 beta and increase expression of anti-inflammatory mediators fractalkine receptor (CX(3)CR1) and interleukin 4 receptor-alpha (IL-4R alpha) in microglia. Importantly, these IL-10-astrocyte dependent effects on microglia were blocked by a TGF beta inhibitor. Furthermore, inhibition of TGF beta signaling in the brain resulted in prolonged sickness behavior and amplified pro-inflammatory cytokine expression in mice challenged with lipopolysaccharide. Taken together, IL-10 stimulated the production of TGF beta 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. GLIA 2014;62:881-895