Modulation by the PPARγ Agonist Rosiglitazone

Early after stroke onset, ischemic injury is exacerbated by a robust inflammatory response that involves a local reaction as well as an influx of blood-borne cells recruited by cytokines, adhesion molecules, and chemokines. Among these cells, neutrophils are rapidly mobilized from the bone marrow to provide an effective innate immune response; they rapidly infiltrate into the ischemic brain (a few hours after occlusion), reach maximal levels at early time points (days 1–3), and then decrease over time. Because of several inflammatory mechanisms (adhesion to endothelium, reactive oxygen species generation, protease secretion, etc), neutrophil infiltration into the ischemic brain has been associated with increased tissue injury but its exact role in stroke pathogenesis remains under debate. Neutrophils have functional heterogeneity in vivo and a capacity to change their phenotype after in vitro cytokine exposure, a function that makes them plastic cells, capable of responding to extracellular stimuli in a context-dependent manner. Such plasticity has been largely studied in macrophages, and similar to M2 macrophages, neutrophils have been proposed to polarize to an N2 phenotype. In this context, peroxisome proliferator-activated receptor-γ (PPARγ), a ligand-activated transcription factor belonging to Background and Purpose—Neutrophils have been traditionally recognized as major mediators of a deleterious inflammatory response in acute ischemic stroke, but their potential as a therapeutic target remains unexplored. Recent evidence indicates that neutrophils may acquire different phenotypes and contribute to resolution of inflammation through the release of anti-inflammatory mediators. Thus, similar to M2 macrophages, neutrophils have been proposed to shift toward an N2 phenotype, a polarization that is peroxisome proliferator-activated receptor-γ dependent in macrophages. We hypothesize that peroxisome proliferator-activated receptor-γ activation with rosiglitazone induces changes in neutrophilic mobilization and phenotype that might influence stroke outcome. Methods—Brain sections and cell suspensions were prepared from mice exposed to permanent distal middle cerebral artery occlusion. Double immunostaining with stereological counting of brain sections and flow-cytometry analysis of brain cell suspensions were performed. Results—Rosiglitazone accelerated neutrophil infiltration to the ischemic core, concomitantly to neuroprotection. Some neutrophils (≈31%) expressed M2 markers, namely Ym1 and CD206 (mannose receptor). After treatment with the peroxisome proliferator-activated receptor-γ agonist rosiglitazone, most neutrophils (≈77%) acquired an N2 phenotype. Interestingly, rosiglitazone increased neutrophil engulfment by microglia/macrophages, a clearance that preferentially affected the N2 subset. Conclusions—We present the first evidence of neutrophil reprogramming toward an N2 phenotype in brain inflammation, which can be modulated by activation of the peroxisome proliferator-activated receptor-γ nuclear receptor. We also show that N2 polarization is associated with an increased neutrophil clearance, thus suggesting that this switch is a crucial event for resolution of inflammation that may participate in neuroprotection. (Stroke. 2013;44:3498-3508.)