Cytokine Release by Microglia Exposed to Neurologic Injury Is Amplified by Lipopolysaccharide.

ABSTRACT

INTRODUCTION: Traumatic brain injury (TBI) is a leading cause of death and morbidity in the trauma population. Microglia drive the secondary neuroinflammatory response after TBI. We sought to determine if the microglial response to neurologic injury was exacerbated by a second stimulus after exposure to neurologic injury. METHODS: Sprague-Dawley rats (age 2-3 wk) were divided into injured and noninjured groups. Injured rats underwent a controlled cortical impact injury; noninjured rats remained naïve to any injury and served as the control group. Primary rat microglia were isolated and applied to in vitro cultures. After incubation for 24 h, the microglia were stimulated with lipopolysaccharide (LPS) or norepinephrine. Twenty-four hours after stimulation, cell culture supernatant was collected. Tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) production were measured by standard enzyme-linked immunosorbent assays. GraphPad Prism was used for statistical analysis. RESULTS: When compared to noninjured microglia, LPS induced a significantly greater production of TNF-α in microglia isolated from the injured ipsilateral (versus noninjured = 938.8 ± 155.1, P < 0.0001) and injured contralateral hemispheres (versus noninjured = 426.6 ± 155.1, P < 0.0001). When compared to microglia from noninjured cerebral tissue, IL-6 production was significantly greater after LPS stimulation in the injured ipsilateral hemisphere (mean difference versus noninjured = 9540 ± 3016, P = 0.0101) and the contralateral hemisphere (16,700 ± 3016, P < 0.0001). Norepinephrine did not have a significant effect on IL-6 or TNF-α production. CONCLUSIONS: LPS stimulation may amplify the release of proinflammatory cytokines from postinjury microglia. These data suggest that post-TBI complications, like sepsis, may propagate neuroinflammation by augmenting the proinflammatory response of microglia.