Post-ischemic drug tuning of the immune response across the blood brain barrier affords neuroprotection in experimental stroke

The development of novel effective therapies aimed at reducing post-ischemic brain damage is currently a urgent challenge. Research should be focused on rescuing the peripheral territories of the penumbra, perfused by collateral blood vessels, where the evolution of tissue damage is strongly affected by inflammatory events that involve soluble mediators and specialised cells activated locally or recruited from the periphery (see Amantea et al., FEBS J 2009; 276:13-26; Iadecola & Anrather, Nat Medicine 2011; 17:796-808). Indeed, brain infiltration of dendritic cells, lymphocytes, macrophages and neutrophils does occur during the early stages after the insult (Gelderblom et al., Stroke 2009; 40:1849-57), and may significantly affect the evolution of brain damage as emphasised by the evidence that severe combined immunodeficient mice are protected from focal ischemic injury (Hurn et al., J Cereb Blood Flow Metab 2007; 27:1798-1805) and brain accumulation of neutrophils after stroke correlates with the severity of brain tissue damage and poor neurological outcome in humans (Akopov et al., Stroke 1996; 27:1739-43). More interestingly, the genes acutely induced in the blood of ischemic stroke patients are expressed mainly by polymorphonuclear leukocytes and to a lesser degree by monocytes and most of these genes regulate the inflammatory response (Tang et al J Cereb Blood Flow Metab 2006; 26:1089-1102; Oh et al J Neuroimmunol 2012; 249:60-65). Thus, immunity and inflammation are involved in all stages of the ischemic cascade, from the early damaging events triggered by arterial occlusion to the late regenerative processes underlying post-ischemic tissue repair. We have observed that a macrolide compound, azithromycin, administered upon reperfusion, improves the beneficial early activation of astrocytes, thus promoting glial scar formation, while reducing brain infiltration of pro-inflammatory M1-macrophages few hours after reperfusion in rodents subjected to transient middle cerebral artery occlusion (MCAo). At later stages after the insult, azithromycin promotes brain activation/infiltration of macrophages, likely resembling “anti-inflammatory” M2-phenotype cells (2.3 fold increase of F480+/Ym1+ cells after 24h of reperfusion) and reduces by 2-fold CD11b+/Gr1+ neutrophil infiltration in the ischemic hemisphere. These effects result in a significant reduction of brain infarct volume and neurological deficit assessed up to 7 days after reperfusion in rodents subjected to either mild (30 min) or robust (2h) MCAo. Thus, modulation of the immune response by a multitarget therapy aimed at improving the beneficial while reducing the detrimental components of the immune/inflammatory response may represent a promising approach for the development of novel drugs for stroke.