Novel trauma-associated microglia influence immune reactivity and pathology in the chronic phase after traumatic brain injury

Traumatic brain injury (TBI) affects millions of people annually, and over 50% of these individuals live with chronic symptoms, including cognitive dysfunction and neuropsychiatric complications. Inflammation after TBI leads to priming of microglia that may alter normal immune responses and increase the risk for neurodegenerative pathology. Thus, the aims of this study were to delineate microglia profiles chronically after TBI in the context of immune challenge (lipopolysaccharide, LPS) and microglia forced turnover (CSFR1 antagonist, PLX5622). Here, diffuse TBI in mice caused chronic cortical inflammation that was associated with primed microglia and increased neuronal injury (plasma NfL, neuronal PTEN, NeuN, and NSE). Transient immune activation with LPS at 30 dpi exacerbated the morphological restructuring of cortical astrocytes and microglia and augmented tissue injury. Single-cell RNA-sequencing analyses of cortical cells 30 dpi that ~20% microglia were Trauma Associated Microglia (TRAM) with higher expression of DEGS that overlapped with Primed (e.g. Il1b, Cd74, Cd14 ) and Disease-associated microglial profiles ( Apoe, Lyz2, Clec7a, Axl ). With LPS challenge at 30 dpi there was an amplified inflammatory response in microglia that was paralleled by a reduced heat shock regulatory response. Notably, this microglial single-cell RNA profile after LPS challenge in TBI was conserved within microglia from aged (24-month-old) mice that were challenged with LPS. With immune challenge, TRAMs were highly reactive and had augmented pro-inflammatory profiles. These Reactive-TRAMs at 30 dpi were reduced (~ 50%) by forced turnover of microglia after TBI (7–14 dpi). In the context of the overall response to LPS, however, forced turnover resulted in an overall intermediate activation profile of microglia. This partial restoration may be related to a subpopulation of inflammatory microglia ( Csf1r low / Lgals3 high /MAC2 + ) that were resistant to PLX5622-mediated depletion. Nonetheless, other cells benefited from forced microglia turnover after TBI including macrophages, astrocytes, and oligodendrocytes. In summary, Trauma associated microglia were evident chronically and contributed to enhanced immune reactivity and subsequent exacerbation of tissue pathology.