Abstract WMP69: Monocyte-derived Macrophages Can Reduce Damage After Experimental Ischemic Stroke

Abstract

Background Post-ischemic inflammation plays an important role in stroke pathology. Understanding the details and mechanisms of post-stroke inflammation is essential to develop therapeutics that can reduce the damage and promote recovery. Macrophages are known to promote both injury and repair in various pathologies, and these divergent properties may be due to different macrophage subtypes. Here, we identify different macrophage subtypes in the brain after ischemic stroke and investigate their effects on stroke outcome. Methods C57BL/6 mice (male, 11-14 weeks old) were subjected to left transient middle cerebral artery occlusion (tMCAo). To deplete blood monocytes, clodronate-filled liposomes (CL) were used. Phosphate-buffered saline (PBS)-filled liposomes (PL) served as controls. Behavior was assessed using a neurological score. Infarct size was assessed by TTC at 2d and silver staining at 7d post-stroke. Immune cells in brain, blood, spleen and bone marrow were analyzed by flow cytometry. Results A time-course analysis of immune cells after stroke identified two macrophage subpopulations in the brain: Ly6Chigh and Ly6Clow. Ly6Chigh macrophages were the dominant cell population early after stroke, peaking by 3d and then decreasing in number by 5d. Conversely, the number of Ly6Clow macrophages were initially low and peaked at 5d. To investigate the effects of these macrophage subpopulations, we used clodronate depletion of monocytes with different injection paradigms. Injecting clodronate just after reperfusion, to target the Ly6Chigh macrophages, had no significant effect on infarct size at 2d and 7d post-stroke, or on functional deficit, compared to the PL group. However, when the injections were given every other day after stroke, from day 0 to day 6, CL-treated mice had significantly larger infarct sizes at 7d post-stroke (48.9±4.3 vs 35.6±5.8, % of contralateral hemisphere, n=10; p<0.01), and worse neurological scores (11.1±1.1 vs 6.2±1.1, n=10; p<0.01) than PL-treated mice. Moreover, the CL group had fewer brain macrophages at 5d. Temporal analysis of the macrophage subpopulations with this treatment paradigm is ongoing. Conclusion Macrophage can reduce brain damage and play a role in promoting recovery after stroke.