Azithromycin protects mice against ischemic stroke injury by promoting macrophage transition towards M2 phenotype

• Azithromycin dose-dependently reduces brain damage caused by MCAo in mice.
• Azithromycin reduces brain infiltration of neutrophils and inflammatory macrophages.
• Neuroprotection is associated with elevation of brain and peritoneal M2 macrophages.
• Inhibition of arginase activity counteracted azithromycin-induced neuroprotection.

To develop novel and effective treatments for ischemic stroke, we investigated the neuroprotective effects of the macrolide antibiotic azithromycin in a mouse model system of transient middle cerebral artery occlusion. Intraperitoneal administration of azithromycin significantly reduced blood–brain barrier damage and cerebral infiltration of myeloid cells, including neutrophils and inflammatory macrophages. These effects resulted in a dose-dependent reduction of cerebral ischemic damage, and in a remarkable amelioration of neurological deficits up to 7 days after the insult. Neuroprotection was associated with increased arginase activity in peritoneal exudate cells, which was followed by the detection of Ym1- and arginase I-immunopositive M2 macrophages in the ischemic area at 24–48 h of reperfusion. Pharmacological inhibition of peritoneal arginase activity counteracted azithromycin-induced neuroprotection, pointing to a major role for drug-induced polarization of migratory macrophages towards a protective, non-inflammatory M2 phenotype.