Macrophage migration inhibitory factor facilitates prostaglandin E2 production of astrocytes to tune inflammatory milieu following spinal cord injury

Background: Astrocytes have been shown to produce several pro- and anti-inflammatory cytokines to maintain homeostasis of microenvironment in response to vast array of CNS insults. Some inflammation-related cytokines are responsible for regulating such cell events. Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine that can be inducibly expressed in the lesioned spinal cord. Unknown is whether MIF can facilitate the production of immunosuppressive factors from astrocytes to tune milieu following spinal cord injury. Methods: Following establishment of contusion SCI rat model, correlation of PGE(2) synthesis-related protein levels with that of MIF was assayed by Western blot. ELISA assay was used to detect production of PGE(2), TNF-alpha, IL-1 beta, and IL-6. Immunohistochemistry was performed to observe colocalization of COX2 with GFAP- and S100 beta-positive astrocytes. The primary astrocytes were treated by various inhibitors to validate relevant signal pathway. Results: The protein levels of MIF and COX2, but not of COX1, synchronously increased following spinal cord injury. Treatment of MIF inhibitor 4-IPP to the lesion sites significantly reduced the expression of COX2, mPGES-1, and as a consequence, the production of PGE(2). Astrocytes responded robustly to the MIF interference, by which regulated MAPK/COX2/PGE(2) signal pathway through coupling with the CD74 membrane receptor. MIF-induced production of PGE(2) from astrocytes was able to suppress production of TNF-alpha, but boosted production of IL-1 beta and IL-6 in LPS-activated macrophages. Conclusion: Collectively, these results reveal a novel function of MIF-mediated astrocytes, which fine-tune inflammatory microenvironment to maintain homeostasis. These suggest an alternative therapeutic strategy for CNS inflammation.