Improved Recovery and Delayed Cytokine Induction after Closed Head Injury in Mice with Central Overexpression of the Secreted Isoform of the Interleukin-1 Receptor Antagonist

Abstract

The acute inflammatory response following traumatic brain injury (TBI) has been shown to play an important role in the development of secondary tissue damage. The proinflammatory cytokines interleukin-1(IL-1) and tumor necrosis factor-α (TNFα), are induced early after brain injury and have been implicated in the delayed damage. The IL-1 receptor antagonist (IL-1ra) has been shown to modulate the proinflammatory cytokine cascade by blocking the binding of IL-1 to its signaling receptor. In this study, we investigated the effect of transgenic overexpression of IL-1ra on the cytokine expression and neurological damage in a closed head injury (CHI) model of TBI. The neurological recovery, as analyzed by neurological severity score (NSS), was significantly higher in transgenic mice overexpressing the human secreted form of IL-1ra in astrocytes, directed by the murine glial fibrillary acidic protein promoter, as compared to wild-type mice. Analysis of tissue levels of cytokines by ELISA showed increased levels of TNFα in the cerebral cortex from the wild type mice 1 h after injury. After 4 h significant increases in the levels of IL-1β and IL-6 were observed in the wild type mice. In the transgenic mice, on the other hand, no effect on TNFα levels was observed and no significant increases in IL-1β and IL-6 levels could be detected until 6 h after injury. Thus, it can be concluded that blockage of IL-1 signaling by elevated levels of IL-1ra has a neuroprotective effect, in agreement with previous reports, and that central overexpression of IL-1ra results in delayed proinflammatory cytokine induction and improved neurological recovery after traumatic brain injury.