Sex Dimorphism Influences Cortical Microglial Morphological and Phenotypic Marker Profile after Closed Head Mild Traumatic Brain Injury in Rats.

Neuroinflammation is a nearly ubiquitous secondary injury process after traumatic brain injury (TBI) involving microglia. The time course of microglial functional transition between pro-inflammatory and anti-inflammatory states after mild TBI (mTBI) and the potential influence of sex in microglial response is not well-understood. To investigate interactions between sex and microglial activation states in the subacute post-mTBI period, we performed a morphological and phenotypic marker analysis on cells from male and female rats following closed head single impact (smTBI), repetitive impacts (rmTBI), or sham conditions at 24 h, 72 h, or 1 week postinjury. There was a significant increase in microglia population 24 h post-smTBI and at all time points for rmTBI in both male and female cells. Single-cell morphological analysis (24 microglia per animal) revealed no clear sex differences in microglial activation state. However, Sholl analysis demonstrated an increase in branching complexity for smTBI female cells at 24 h (area under the curve [AUC] 154 ± 2.1, p = 0.03) and at 72 h for rmTBI (AUC 229 ± 6.6, p = 0.006), but no increase in branching was observed in male cells. Principal component analysis similarly demonstrated that female cells formed distinct clusters at 72 h and 1 week, suggesting a change in morphology. There was an increase in anti-inflammatory marker, CD206, at 72 h for female cells in both smTBI and rmTBI groups. However, for males, most cells were KV1.3-positive (pro-inflammatory) even at 1 week in smTBI and rmTBI groups. Altogether, these data demonstrate microglial cells are pro-inflammatory 24 h after mTBI, but there is a robust difference between sexes, with female cells transitioning earlier from the pro-inflammatory state to the anti-inflammatory state compared with male cells. These results contribute to our understanding of sexual dimorphism associated with microglial recovery following mTBI and warrant further study of associated cellular pathways.