Microglia are prominent producers of inflammatory cytokines during the hyperacute phase of ischemic stroke.

Current treatments for ischemic stroke focus on removing neurovascular-occluding clots but overlook the resulting neuroinflammation. Clinical trials targeting brain-infiltrating peripheral immune cells have failed to improved outcomes, leaving mechanisms of neuroinflammation poorly understood. Whilst many studies have examined the inflammatory cells and cytokine profile of the post-stroke brain at days and weeks following injury onset, we propose that interventions at these timepoints are too late to limit brain damage. In this study, we examined brain immune cell composition, cytokine levels, and neurological dysfunction at hyperacute (3 h) and acute (24 h) stages following a preclinical mouse model of ischemic stroke. Interestingly, we detected elevated inflammatory cytokines in brain tissue as early as 3 h, notably before infiltrating neutrophil and monocyte arrival at 24 h. Depletion of peripheral immune cells by antibodies or genetic alteration did not dampen neuroinflammation, nor improve sensorimotor function. Intravital imaging of Cx3cr1(gfp/+) mice showed that microglia, the brain-resident immune cell, display rapidly altered morphology, and swiftly upregulated cytokine production hyperacutely post-stroke. Altogether, our study provides evidence that microglia are key drivers of early neuroinflammation and modulating their function at clinically-relevant timepoints will improve stroke recovery.