BACKGROUND: Sepsis-associated encephalopathy (SAE) is a brain dysfunction caused by systemic inflammation, involving mitochondrial dysfunction, glial damage, and dysregulated inflammatory responses. Recent studies emphasize the role of the skull bone marrow (SBM) and dura mater (DM) in regulating neuroimmune responses through the Skull Bone Marrow-Dura Mater-Brain Axis (SBM-DM-B Axis), which could serve as a potential therapeutic target for neuroinflammatory disorders.
METHODS: In this study, infrared (IR) at different power levels (10, 30, 50, 100, 300, 500 mW) was tested to assess the photonic, thermal, and biological effects, aiming to determine the optimal power for confining IR effects to the SBM. The identified optimal power was subsequently applied in all experiments. To establish an SAE mouse model, LPS was administered intraperitoneally, and daily 10-minute IR exposures were applied for 4 consecutive days. The effects of SBM-confined IR on neuroimmune regulation and neuronal protection were assessed by quantitatively analyzing microglial morphology, area, density, connectivity, and neuronal morphology and density, as well as by motor behavioral tests (four-limb grip strength, 45° pole, and rotarod tests) and cognitive behavioral tests (Y-maze spontaneous alternation and novel object recognition). All analyses were conducted separately for the cortex and hippocampus to investigate spatial effects. Additionally, the role of the SBM-DM-B Axis was explored by evaluating the diameter of meningeal lymphatic vessels (mLVs), evaluating dural neutrophils, and quantifying dural macrophage number and morphology to assess the impact of IR on DM immune regulation. To further explore SBM-restricted IR effects and underlying mechanisms in the skull and cortex, reactive oxygen species (ROS), cytochrome c oxidase (CCO), ATP, and nitric oxide (NO) levels were measured, and paired transcriptomic profiling was conducted on skull and cortex tissues from the same mice.
RESULTS: Ex vivo and in vivo penetration assays confirmed that 50 mW IR photostimulation was confined to the SBM with minimal penetration into deeper brain regions, while temperature monitoring and HSP70/HSP90 expression demonstrated that scalp temperature remained within safe limits and no thermal damage or stress responses occurred, in contrast to higher power levels (500 mW), which induced significant inflammation and neuronal loss. In SAE mice, daily 10-min SBM-targeted IR for four days promoted macrophage polarization in the SBM and facilitated body weight recovery, remodeled cortical microglia, mitigating neuronal loss and structural disruption in the cortex and hippocampus, and improved motor function. At the DM level, 50 mW IR dilated mLVs, reduced scattered and aggregated neutrophils, and restored macrophage density, area, and morphology, reversing the LPS-induced enlargement and sparse distribution of macrophages. Paired skull-cortex transcriptomic analyses revealed opposite baseline responses: IR enhanced metabolic and proliferative programs in the skull while suppressing biosynthetic and mitochondrial pathways in the cortex. In SAE mice, both tissues showed concordant downregulation of scavenger receptor uptake and IL6-JAK-STAT3 signaling, indicating reduced phagocytosis and inflammatory signaling.
CONCLUSIONS: We demonstrate that 50 mW IR can be safely confined to the SBM, effectively modulating skull, meningeal, and brain immunity to protect neurons in SAE mice. This study establishes SBM-restricted IR photobiomodulation as a safe and effective strategy to reprogram neuroimmune responses, highlighting the SBM-DM-B Axis as a novel pathway for neuroimmune regulation and a promising target for non-invasive interventions in SAE and other acute brain injuries.
