Sex differences in aging with injury: the use of remote ischemic conditioning as an anti-inflammatory treatment for brain injury induced peripheral inflammation Maha Saber1,2, Katie Giordano1, Rachel Rowe1,2 and Jonathan Lifshitz1,2,3 1University of Arizona College of Medicine-Phoenix; 2Barrow Neurological Institute at Phoenix Children's Hospital; 3Phoenix VA Healthcare System
Introduction: Brain injury is one of the leading environmental risk factor for age related dementias (e.g. Alzheimer's disease) and neurodegeneration. Remote ischemic conditioning (RIC) is a brief reversible episode of ischemia applied to a limb or non-vital organ. This therapeutic strategy is used to protect major organs from ischemia-reperfusion injury associated with a stroke or myocardial infarction. Further, RIC reduced cognitive impairments in models of vascular dementia and halted the increase of acute biomarkers of brain injury. Though the mechanism of RIC is unknown, recent studies have suggested that RIC can modulate inflammatory signaling in health and disease, where sex differences exist in the inflammatory response after brain injury. Objectives: This study quantified the extent to which RIC modulated the peripheral and central immune response to experimental brain injury and sex differences. We hypothesized a reduction in the population of the peripheral macrophage response after brain injury and expected to see this most strongly seen in the male population. Methods: Diffuse brain injury by midline fluid percussion was performed on adult (8-10 weeks of age) male and female C57BL/6 (B6) mice. After 1 hour, half of the mice in both the injured and uninjured sham group received 4x5 minute sessions of RIC (elastic band on one thigh) with 5-minute reperfusion between each session. To examine the acute effects of RIC on the immune response, blood, spleen, and brains were collected at 3 and 7 days post injury (DPI), and processed for flow cytometry to quantify inflammatory monocytes in the spleen and blood (Ly6chighCd115+) and peripheral macrophages in the brain (Cd11b+ CD45high). A separate group of mice were used to determine chronic cognitive and anxiety related outcomes outcomes of RIC treatment. Tests included rotarod to test motor function, open-field and elevated plus for anxiety-like behavior, and Y-maze and Novel object recognition for spatial memory and cognition. Results: Female mice showed a more robust change in both peripheral inflammatory monocyte populations in the blood and spleen after TBI at 3DPI, and an increased peripheral monocyte population in the brain compared to sham controls. RIC treatment reduced brain injury induced peripheral monocyte populations down to sham levels in these females. Males showed a similar trend but were not significantly different from each other. Peripheral inflammatory monocyte and macrophage population changes were resolved by 7DPI. Conclusions: These data suggest that RIC can modulate the peripheral macrophage and monocytes response. Behavioral studies will be able to correlate whether this can be beneficial in reducing pathological age related deficits of brain injury. Further studies are needed to refine the mechanism and determine the therapeutic efficacy of immune modulation by RIC.