Amplified Gliosis and Interferon-Associated Inflammation in the Aging Brain Following Diffuse Traumatic Brain Injury

Traumatic brain injury (TBI) is associated with chronic psychiatric complications and increased risk for development of neurodegenerative pathology. Aged individuals account for most TBI-related hospitalizations and deaths. Nonetheless, neurobiological mechanisms that underlie worsened functional outcomes after TBI in the elderly remain unclear. Therefore, this study aimed to identify pathways that govern differential responses to TBI with age. Here, adult (2 mo) and aged (16-18 mo) male C57BL/6 mice were subjected to diffuse brain injury (midline fluid percussion), and cognition, gliosis, and neuroinflammation were determined 7 or 30 days post injury (dpi). Cognitive impairment was evident 7 dpi, independent of age. There was enhanced morphological restructuring of microglia and astrocytes 7 dpi in the cortex and hippocampus of aged mice compared to adults. Transcriptional analysis revealed robust age-dependent amplification of cytokine/chemokine, complement, innate immune, and interferon-associated inflammatory gene expression in the cortex 7 dpi. Ingenuity Pathway Analysis of the transcriptional data showed that type I interferon (IFN) signaling was significantly enhanced in the aged brain after TBI compared to adults. Age prolonged inflammatory signaling and microgliosis 30 dpi with increased presence of rod microglia. Based on these results, a STING (stimulator of interferon genes) agonist DMXAA was used to determine if augmenting IFN signaling worsened cortical inflammation and gliosis after TBI. DMXAA-treated Adult-TBI mice showed comparable expression of myriad genes that were overexpressed in the cortex of Aged-TBI mice, including Irf7, Clec7a, Cxcl10, and Ccl5. Overall, diffuse TBI promoted amplified IFN signaling in aged mice, resulting in extended inflammation and gliosis. SIGNIFICANCE STATEMENT: Elderly individuals are at higher risk of complications following Traumatic Brain Injury (TBI). Individuals over 70 years old have the highest rates of TBI-related hospitalization, neurodegenerative pathology, and death. Although inflammation has been linked with poor outcomes in aging, the specific biological pathways driving worsened outcomes after TBI in aging remain undefined. In this study, we identify amplified interferon-associated inflammation and gliosis in aged mice following TBI that was associated with persistent inflammatory gene expression and microglial morphological diversity 30 dpi. Stimulator of Interferon Genes (STING) agonist DMXAA was used to demonstrate a causal link between augmented interferon signaling and worsened neuroinflammation after TBI. Therefore, interferon signaling may represent a therapeutic target to reduce inflammation-associated complications following TBI.