Abstract Body: Introduction: Children typically recover better after stroke than adults. In adults, neuroinflammation exacerbates infarct volume acutely and drives infarct-induced neurodegeneration chronically. The evolution of neuroinflammation after stroke in the juvenile brain is poorly understood compared to perinatal or adult stroke. Here, we quantify the initiation and resolution of inflammation after stroke in juvenile mice.

Methods: Wild-type male juvenile (p28) and adult (6 months) C57BL/6J mice were randomized to sham surgery or DH (distal hypoxic) stroke, an established model of cortical stroke that produces chronic secondary neurodegeneration in connected gray and white matter regions. We performed immunostaining at 3 days (acute) and 8 weeks (chronic) after stroke to quantify myeloid (CD68, Iba1), astroglial (GFAP), and lymphocytic (B220, CD3) inflammation within the infarct and sites of secondary neurodegeneration.

Results: Acutely, juvenile mice had 267% more CD68 (p = 0.0023) and 235% more Iba1 (p = 0.0086) staining in the thalamus. Juveniles also had greater myeloid cell activation in connected white matter tracts, with 322% more CD68 (p < 0.001) and 696% more Iba1 (p = 0.0002) staining in the corpus callosum, and 260% more Iba1 staining in the corticospinal tract (p = 0.0059) than adults (n=6-11/group). There was no difference in acute GFAP staining. In contrast, by 8 weeks after stroke, juveniles had less CD68 staining in the thalamus (19% of adult, p <0.0001), corpus callosum (68% of adult, p = 0.0428), and corticospinal tracts (55% of adult, p = 0.0009) (n = 6-10/group). Juveniles and adults had comparable lymphocytic neuroinflammation in the infarct.

Conclusions: Acutely, juveniles had widespread myeloid cell activation in gray and white matter sites that are connected to the infarct and undergo secondary neurodegeneration, which, unexpectedly, was markedly greater than adults. Strikingly, this relationship reversed by 8 weeks after stroke, with juveniles demonstrating significantly more efficient resolution of inflammation than adults. These results suggest that children's ability to terminate the acute inflammatory response after stroke may be protective and could contribute to the more favorable outcomes after pediatric stroke. Future studies will test hypothesis that early termination of the acute inflammatory response after stroke is protective and will compare the transcriptomic response to stroke in juvenile vs adult myeloid cells.