Fate mapping of peripherally-derived macrophages after traumatic brain injury in mice reveals a long-lasting population with a distinct transcriptomic signature

Traumatic brain injury (TBI) is an environmental risk factor for dementia and long-term neurological deficits, posing a significant public health challenge. TBI-induced neuroinflammation involves both brain-resident microglia and peripheral monocyte-derived macrophages (MDMs). Previous research has shown that MDMs contribute to the development of long-term memory deficits, yet their long-term behavior following brain infiltration remains unclear. To address this, our study uses two complementary fate-mapping mouse lines, CCR2-creERT2 and Ms4a3-cre, for precise and lasting tracking of MDMs in vivo. Here we show that MDMs persist in the brain for at least 8 months post-TBI in both male and female mice. MDMs retain phagocytic activity for at least 30 days post-TBI, remain transcriptionally distinct from microglia, and display a gene expression profile associated with aging and disease. Moreover, we identify a core transcriptomic signature of MDMs shared across various mouse models and brain perturbations, which is also enriched in the brain myeloid cells of male subjects with TBI and Alzheimer’s disease patients. These findings enhance our understanding of MDMs' dynamics after TBI and inform future targeted myeloid-based therapies.