Endogenous protection from ischemic brain injury by preconditioned monocytes

Exposure to low dose lipopolysaccharide prior to cerebral ischemia is neuroprotective in stroke models, a phenomenon termed preconditioning. While it is well established that lipopolysaccharide-preconditioning induces central and peripheral immune responses, the cellular mechanisms modulating ischemic injury remain unclear. Here, we investigated the role of immune cells in the brain protection afforded by preconditioning and we tested whether monocytes may be reprogrammed by ex vivo lipopolysaccharide exposure thus modulating the inflammatory injury after cerebral ischemia in male mice. We found that systemic injection of low-dose lipopolysaccharide induces a distinct subclass of CD115+Ly6Chi monocytes that protect the brain after transient middle cerebral artery occlusion in mice. Remarkably, adoptive transfer of monocytes isolated from preconditioned mice into naïve mice 7 hours after transient middle cerebral artery occlusion reduced brain injury. Gene expression and functional studies showed that IL-10, iNOS and CCR2 in monocytes are essential for the neuroprotection. This protective activity was elicited even if mouse or human monocytes were exposed ex vivo to lipopolysaccharide and then injected into male mice after stroke. Cell tracking studies showed that protective monocytes are mobilized from the spleen and reach brain and meninges, wherein they suppressed post-ischemic inflammation and neutrophils influx into the brain parenchyma. Our findings unveil a previously unrecognized subpopulation of splenic monocytes capable to protect the brain with an extended therapeutic window, and provide the rationale for cell therapies based on the delivery of autologous or allogeneic protective monocytes into patients with ischemic stroke.Significance StatementInflammation is a key component of the pathophysiology of the brain in stroke, a leading cause of death and disability with limited therapeutic options. Here, we investigate endogenous mechanisms of protection against cerebral ischemia. Using LPS preconditioning as an approach to induce ischemic tolerance in mice, we found the generation of neuroprotective monocytes within the spleen from where they traffic to the brain and meninges suppressing post-ischemic inflammation. Importantly, systemic LPS preconditioning can be mimicked by adoptive transfer of in vitro-preconditioned mouse or human monocytes at translational relevant time points after stroke. This model of neuroprotection may facilitate clinical efforts to increase the efficacy of bone marrow mononuclear cell treatments in acute neurological diseases such as cerebral ischemia.