Endogenous protection from ischemic brain injury by preconditioned monocytes

Abstract

Exposure to low dose lipopolysaccharide prior to cerebral ischemia is neuroprotective in stroke models, a phenomenon termed preconditioning. While it is well established that lipopolysaccharidepreconditioning 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 + Ly6C hi 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.Garcia-Bonilla, page 3

Dim light at night impairs recovery from global cerebral ischemia

“…For example, there are typically small variations in survival between experimental cohorts that may relate to differences in animals provided by the vendor, variations in reagents used for the procedure, noise levels in the colony/surgical suite, etc. Additionally, the cannula implantation may have induced inflammation in the CNS causing inflammatory pre-conditioning, which is a known protective factor against ischemic brain injury (for example, see (Garcia-Bonilla et al, 2018). Furthermore, this experiment may have included insufficient group sizes for detecting survival.…”

The meningeal and choroidal infiltration routes for leukocytes in stroke

“… 94 Interestingly, the meninges were associated with neuroprotection following an ischemic stroke via preconditioned monocytes. 95 Low doses of the proinflammatory mediator lipopolysaccharide (LPS) before an ischemic injury induced protection after transient occlusion of the middle cerebral artery, a phenomenon known as preconditioning. 95 , 96 This effect was recapitulated by adoptive transfer of monocytes isolated from LPS-preconditioned mice.…”

“… 95 Low doses of the proinflammatory mediator lipopolysaccharide (LPS) before an ischemic injury induced protection after transient occlusion of the middle cerebral artery, a phenomenon known as preconditioning. 95 , 96 This effect was recapitulated by adoptive transfer of monocytes isolated from LPS-preconditioned mice. Specifically, the neuroprotective monocytes were recruited to meningeal vessels, where they dampened the expression of proinflammatory genes involved in the activation of neutrophils and chemotactic factors such as Csf3, 97 indicating an essential role of the meningeal compartment as a key immunoregulator in ischemic brain injury.…”

CX3CR1-CCR2-dependent monocyte-microglial signaling modulates neurovascular leakage and acute injury in a mouse model of childhood stroke

“…73 Some recent data revealed that while harmful role of Ly6C high and CCR2 + monocytes in adult stroke is well defined, immune preconditioning by low dose LPS protects adult mice from MCAO via induction of Ly6C high in monocytes and reprograming of gene expression in monocytes, with CCR2 playing an essential role in neuroprotection. 74 These data raise a possibility that the magnitude, the timing and the phenotypes of infiltrated and differentiated monocytes can be tuned to support long-term recovery but it is unknown whether similar scenario exists in injured juvenile brain. An improved understanding of leukocyte/neurovascular/microglial axis would shed light on how to enhance vascular remapping and brain connectivity after CAIS.…”