Microglia and infiltrating T-cells adopt long-term, age-specific, transcriptional changes after traumatic brain injury in mice.

Aged traumatic brain injury (TBI) patients suffer increased mortality and long-term neurocognitive and neuropsychiatric morbidity compared to 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 to young brains after TBI. Young and aged male C57BL/6 mice underwent TBI via controlled cortical impact (CCI) vs. sham injury and were sacrificed four months post-TBI. We utilized 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 4000 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 BDNF signaling. Meanwhile, post-injury, aged mice demonstrated heterogeneous T-cell infiltration with gene profiles corresponding to CD8 effector memory, CD8 naïve-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.