Macrophage migration inhibitory factor facilitates production of CCL5 in astrocytes following rat spinal cord injury

Abstract

Background: Astrocytes act as immune effector cells with the ability to produce a wide array of chemokines and cytokines in response to various stimuli. Macrophage migration inhibitory factor (MIF) is inducibly expressed in injured spinal cord contributing to excessive inflammation that affects motor functional recovery. Unknown is whether MIF can facilitate inflammatory responses through stimulating release of chemokines from astrocytes following spinal cord injury. Methods: Following the establishment of the contusion spinal cord injury rat model, the correlation of chemokine (C-C motif) ligand 5 (CCL5) expression with that of MIF was assayed by Western blot, ELISA, and immunohistochemistry. Immunoprecipitation was used to detect MIF interaction with membrane CD74 receptor. Intracellular signal transduction of MIF/CD74 axis was analyzed by transcriptome sequencing of primary astrocytes and further validated by treatment of various inhibitors. The effects of CCL5 released by astrocytes on macrophage migration were performed by transwell migration assay. The post-injury locomotor functions were assessed using the Basso, Beattie, and Bresnahan (BBB) locomotor scale. Results: The protein levels of chemokine CCL5/RANTES were remarkably increased in the astrocytes of rat injured spinal cord, in parallel with the expression of MIF. Treatment of MIF inhibitor 4-IPP in the lesion sites resulted in a significant decrease of CCL5 protein levels. In vitro study revealed MIF was capable of facilitating CCL5 production of astrocytes through interaction with CD74 membrane receptor, and knockdown of this receptor attenuated such effects. Production of CCL5 in astrocytes was significantly blocked by inhibitor of c-Jun N-terminal kinase, rather than by those of ERK and P38. Recombinant CCL5 protein was found to be more effective in promoting migration of M2- compared to M1-type macrophages. Conclusion: Collectively, these data reveal a novel function of MIF in regulation of CCL5 release from astrocytes, which in turn favors for recruitment of inflammatory cells to the injured site of the spinal cord, in association with activation of excessive inflammation.