Microglia and Infiltrating T-Cells Adopt Long-Term, Age-Specific, Transcriptional Changes After Traumatic Brain Injury in Mice

Abstract

Aged traumatic brain injury (TBI) patients suffer increased mortality and long-term neurocognitive and neuropsychiatric morbidity compared with younger patients. Microglia, the resident innate immune cells of the brain, are complicit in both. We hypothesized that aged microglia would fail to return to a homeostatic state after TBI and adopt a long-term injury-associated state within aged brains compared with young brains after TBI. Young and aged male C57BL/6 mice underwent TBI via controlled cortical impact versus sham injury and were sacrificed 4 months post-TBI. We used single-cell RNA sequencing to examine age-associated cellular responses after TBI. Brains were harvested, and CD45+ cells were isolated via fluorescence-activated cell sorting. cDNA libraries were prepared using the 10x Genomics Chromium Single Cell 3′ Reagent Kit, followed by sequencing on a HiSeq 4,000 instrument and computational analyses. Post-injury, aged mice demonstrated a disparate microglial gene signature and an increase in infiltrating T cells compared with young adult mice. Notably, aged mice post-injury had a subpopulation of age-specific, immune-inflammatory microglia resembling the gene profile of neurodegenerative disease-associated microglia with enriched pathways involved in leukocyte recruitment and brain-derived neurotrophic factor signaling. Meanwhile, post-injury, aged mice demonstrated heterogeneous T-cell infiltration with gene profiles corresponding to CD8 effector memory, CD8 naive-like, CD8 early active T cells, and Th1 cells with enriched pathways, such as macromolecule synthesis. Taken together, our data showed that the aged brain had an age-specific gene signature change in both T-cell infiltrates and microglia, which may contribute to its increased vulnerability to TBI and the long-term sequelae of TBI.

Progressive lifespan modifications in the corpus callosum following a single juvenile concussion in male mice monitored by diffusion MRI

“…Long-term inflammatory responses after TBI have been reported by our group and others (Chen, Islam et al 2023, Obenaus, Rodriguez-Grande et al 2023). Given the phenotypic changes in astrocytes we investigated if microglia (via IBA1 immunohistochemistry) within the ipsilateral CC exhibited an activated morphology characteristic of inflammatory states.…”

Anti-CD49d Antibody Treatment Improves Survival and Attenuates Neurocognitive Deficits after Traumatic Brain Injury in Aged Mice

“…Inhibiting CCR3 in T cells attenuates this T-cell migration into the brain and alleviates microgliosis in aged mouse brains (14). In line with current literature, previous work in our laboratory indicated that microglia from aged mouse brains demonstrate more significant levels of activation after TBI than microglia from young mouse brains after TBI (6). Single-cell RNA sequencing (scRNAseq) showed that in the chronic phase of TBI, microglia from aged mouse brains still significantly upregulate genes, including S100a9, S100a8, Ccl2, Ccl3, and Pdgfb, that are involved in T-cell migration and chemotaxis, inducing the accumulation of T cells within aged brains (6).…”

“…However, the underlying pathophysiologic mechanisms of these differential outcomes remain elusive. Multiple findings have indicated that the adaptive immune system plays a role in pathologic consequences within the CNS after brain injury in aged subjects (6,22,23). Understanding the role of the adaptive immune system after TBI is a critically unmet research need as it has the potential to pinpoint age-specific therapeutic targets.…”

“…Brain cells were isolated as previously published by our laboratory (6). In brief, brain tissue was injected with 3 mL of digestion buffer (2.5 mg/mL Liberase TL and 1 mg/mL DNase I in HBSS; Roche, Basel, Switzerland), morcellated, and transferred to C-tubes.…”

“…Our laboratory has previously published that most T cells accumulating within the aged mouse brain are antigen-experienced CD8 + T effector memory (EM) cells (6). Furthermore, we have shown that these CD8 + T cells are constitutively activated with upregulation of proinflammatory genes contributing to progressive neurocognitive and functional decline (6,7).…”