Differential regulation of astrocyte TNF‐α expression by the cytokines TGF‐β, IL‐6 and IL‐10

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Abstract
In this study, we demonstrate that transforming growth factor-beta (TGF-β), interleukin-10 (IL-10) and interleukin-6 (IL-6) inhibit tumor necrosis factor-alpha expression by primary rat astrocytes. Treatment of astrocytes with TGF-β alone had no effect on TNF-α expression, however, TGF-β suppressed induction on TNF-α expression at both the protein and mRNA level. In contrast, IL-10 and IL-6 both inhibited TNF-α protein expression by astrocytes, but had no effect on mRNA levels. The extent of IL-6 mediated inhibition was greatest when astrocytes were pretreated with IL-6 for 12–24 hr, then exposed to the inducing stimuli, while IL-10 was an effective inhibitor even when added simultaneously with the inducing stimuli. Collectively, these data indicate that TGF-β, IL-6 and IL-10 are all capable of inhibiting TNF-α expression by astrocytes, although these immunosuppressive cytokines act at different levels of gene expression; i.e. TGF-β at the transcriptional level and IL-10/IL-6 at the translational level. These results indicate that TGF-β, IL-6 and IL-10 are important regulators of cytokine production by astrocytes under inflammatory conditions in the brain, and can contribute to controlling the production of detrimental cytokines such as TNF-α.