Lipopolysaccharide-Induced Inflammation Aggravates Irradiation-Induced Injury to the Young Mouse Brain

Abstract

Radiotherapy is an effective treatment strategy in the treatment of brain tumors, but it is also a major cause of long-term complications, especially in survivors of pediatric brain tumors. Cognitive decline caused by cranial radiotherapy is thought, at least partly, to depend on injury to stem and progenitor cells in the dentate gyrus of the hippocampus. This study investigated the effects of lipopolysaccharide (LPS)-induced inflammation at the time of irradiation (IR) in the growing mouse brain. A single injection of LPS (0.3 mg/kg) was administered 24 h prior to cranial IR of 14-day-old male mice. LPS pretreatment increased the levels of the chemokine CCL2 and the cytokine IL-1β in the brain by 440 and 560%, respectively, compared to IR alone. IR disrupted hippocampal neurogenesis and the growth of the dentate gyrus, and the mice pretreated with LPS displayed an even more pronounced lack of growth than the vehicle-treated group 2 months after IR. The density of microglia was not affected, but LPS-pretreated mice displayed 48% fewer bromodeoxyuridine-positive cells and 43% fewer doublecortin-positive cells in the granule cell layer 2 months after IR compared with the vehicle-treated group. In conclusion, an ongoing inflammation in the brain at the time of IR further enhanced the IR-induced loss of neurogenesis, and may aggravate future cognitive deficits in patients treated with cranial radiotherapy.

Microglial and macrophage polarization—new prospects for brain repair

“…9, 30 Pro-inflammatory microglia/macrophages that secrete destructive factors have also been shown to hinder neurogenesis and aggravate long-term neurological deficits after injury. 31 In contrast, M2-activated microglia promote basal neurogenesis. 30, 32 Following brain injuries, microglia/macrophages that produce specific trophic factors can enhance neural precursor cell (NPC) proliferation, neuroblast migration, and functional integration into existing neural circuitry.…”

Anti-inflammatory effect of stem cells against spinal cord injury via regulating macrophage polarization

“…Proinflammatory M1 macrophages hinder neurogenesis by secreting destructive factors including IL-1b, IFN-γ, and TNF-α and aggravate long-term neurological deficits after injury. 48,62 In contrast, M2 microglia/macrophages promote basal neurogenesis. 63,64 Therefore, M1 and M2 macrophages have distinct effects on neurogenesis.…”

Cranial irradiation induces transient microglia accumulation, followed by long-lasting inflammation and loss of microglia

“…Accordingly, IR-induced neurogenesis impairment was exacerbated in animals with systemic inflammation [20], and anti-inflammatory treatment could partially restore neurogenesis and mitigate IR-induced cognitive deficits [16, 21]. Neuroinflammation represents the inflammatory response originating in the CNS as a consequence of brain injury, characterized by the activation of glial cells with the consequent production of pro-inflammatory agents, and may also involve infiltrating blood-borne immune cells.…”