Early Cytokine Release and Microglial Migration After Cranial Irradiation is Inhibited by FTY720, A Candidate Drug for Mitigating Radiation-Induced Neurocognitive Toxicity

Abstract

Recent investigations have revealed a link between radiation-induced cognitive dysfunction and neuroinflammation. Previously, we showed that FTY720, an FDA-approved oral agent for multiple sclerosis treatment, attenuated radiation-induced neurocognitive deficits in mice. Therapeutic effects of FTY720 have been postulated to involve mitigation of neuroinflammation but this has never been demonstrated. In this study we examined the effects of cranial irradiation and FTY720 treatment on microglial migration and cytokine release in the brain. Six week old C57/Bl/6J mice were randomized to receive 7 Gy single dose whole brain irradiation versus 0 Gy (sham). They were further randomized to receive FTY720 (0.5 mg/kg) or vehicle alone via intraperitoneal injection three times a week for one week prior to irradiation. At 24 hours, 48 hours, and 5 days after irradiation, the mice were sacrificed, and cytokine levels were measured in different areas of hippocampus and cortex using ELISA-based assays. To investigate the role of microglial activation, immunohistochemical studies were also performed from the harvested brain tissue at the same time points. In irradiated mice levels of TNF-alpha but not IL-6 were found to increase compared to non-irradiated controls. The elevated TNF-alpha levels were seen at 24 hours and persisted at 48 hours, returning to baseline by the five day mark. The observed increase in TNF-alpha was significantly higher in the hippocampus compared to cortex. Pre-treatment with FTY720 fully blocked the cortical and hippocampal increases in TNF-alpha at 48 hours. The immunohistochemistry revealed an increase in microglial activation preferentially in the hippocampus compared to cortical region, indicating a neuroinflammatoty reaction induced by radiation. Cranial irradiation resulted in microglial migration with early elevation of TNF-alpha, and this effect was particularly pronounced in the hippocampus. FTY720, which had been previously shown to mitigate radiation-induced memory and learning deficits in mice, was now found to suppress the TNF spike. Further studies are needed to elucidate the role of TNF production in post-radiation neuroinflammation and to evaluate it as a potential target for mitigating radiation-induced neurocognitive changes.