Hilary A. Seifert

The spleen contributes to stroke induced neurodegeneration through interferon gamma signaling

Delayed neuronal death associated with stroke has been increasingly linked to the immune response to the injury. Splenectomy prior to middle cerebral artery occlusion (MCAO) is neuroprotective and significantly reduces neuroinflammation. The present study investigated whether splenic signaling occurs through interferon gamma (IFNγ). IFNγ was elevated early in spleens but later in the brains of rats following MCAO. Splenectomy decreased the amount of IFNγ in the infarct post-MCAO. Systemic administration of recombinant IFNγ abolished the protective effects of splenectomy with a concurrent increase in INFγ expression in the brain. These results suggest a role for spleen-derived IFNγ in stroke pathology.

Molecular and cellular immune responses to ischemic brain injury.

Despite extensive research into stroke pathology, there have not been any major recent advancements in stroke therapeutics. Animal models of cerebral ischemia and clinical data have been used to investigate the progressive neural injury that occurs after an initial ischemic insult. This has lead researchers to focus more on the peripheral immune response that is generated as a result of cerebral ischemia. The therapies that have been developed as a result of this research thus far have proven ineffective in clinical trials. The failure of these therapeutics in clinical trials is thought to be due to the broad immunosuppression elicited as a result of the treatments and the cerebral ischemia itself. Emerging evidence indicates a more selective modulation of the immune system following stroke could be beneficial. The spleen has been shown to exacerbate neural injury following experimental stroke and would provide a strong therapeutic target. Selecting facets of the immune system to target would allow the protective and regenerative properties of the immune response to remain intact while blunting the pro-inflammatory response generated towards the injured brain.

Pro-Inflammatory Interferon Gamma Signaling is Directly Associated with Stroke Induced Neurodegeneration

The delayed immune response to stroke is responsible for the increased neural injury that continues to occur after the initial ischemic event. This delayed immune response has been linked to the spleen, as splenectomy prior to middle cerebral artery occlusion (MCAO) is neuroprotective. Interferon gamma (IFNγ) is linked to the splenic response, which enhances neural injury following MCAO. IFNγ activates the expression of the inflammatory chemokine interferon-inducible protein 10 (IP-10). This study was designed to determine the role of IFNγ signaling in the inflammatory response following MCAO. Expression of IP-10 increased in the brain and the spleen following MCAO. Splenectomy inhibited the increase of IP-10 in the brain post-MCAO, while recombinant IFNγ administration to splenectomized rats returned IP-10 levels in the brain to levels found in rats after MCAO only. Systemic administration of an IFNγ neutralizing antibody to MCAO-treated rats reduced infarct volume and IP-10 levels in the brain. T cell infiltration was reduced in the MCAO-damaged brains of IFNγ antibody-treated animals relative to those that received isotype control antibodies. Additionally, inhibiting IFNγ signaling with splenectomy or an IFNγ neutralizing antibody blocked the induction of IP-10 expression and decreased neurodegeneration following MCAO. Targeting this pro-inflammatory pathway following stroke could be a promising stroke therapeutic.

Leukemia inhibitor factor promotes functional recovery and oligodendrocyte survival in rat models of focal ischemia

Human umbilical cord blood (HUCB) cells have shown efficacy in rodent models of focal ischemia and in vitro systems that recapitulate stroke conditions. One potential mechanism of protection is through secretion of soluble factors that protect neurons and oligodendrocytes (OLs) from oxidative stress. To overcome practical issues with cellular therapies, identification of soluble factors released by HUCB and other stem cells may pave the way for treatment modalities that are safer for a larger percentage of stroke patients. Among these soluble factors is leukemia inhibitory factor (LIF), a cytokine that exerts pleiotropic effects on cell survival. Here, data show that LIF effectively reduced infarct volume, reduced white matter injury and improved functional outcomes when administered to rats following permanent middle cerebral artery occlusion. To further explore downstream signaling, primary oligodendrocyte cultures were exposed to oxygen–glucose deprivation to mimic stroke conditions. LIF significantly reduced lactate dehydrogenase release from OLs, reduced superoxide dismutase activity and induced peroxiredoxin 4 (Prdx4) transcript. Additionally, the protective and antioxidant capacity of LIF was negated by both Akt inhibition and co‐incubation with Prdx4‐neutralising antibodies, establishing a role for the Akt signaling pathway and Prdx4‐mediated antioxidation in LIF protection.

The Effects of Clinically Relevant Hypertonic Saline and Conivaptan Administration on Ischemic Stroke

Cerebral edema after stroke is associated with poor neurological outcomes. Current therapies are limited to osmotic agents, such as hypertonic saline (HS), which reduce intracranial pressure. Although studies have demonstrated edema reductions following HS, tissue survival has not been thoroughly examined. Additionally, the efficacy of promising pharmacological agents has not been evaluated for synergy with osmotic agents. Conivaptan is an FDA-approved vasopressin receptor antagonist that may exert both osmotic and anti-inflammatory effects. In this study, rats were subjected to middle cerebral artery occlusion prior to treatment with 5 % HS bolus +5 % HS maintenance (HS), conivaptan alone (Con), conivaptan +5 % HS maintenance (Con + HS), or conivaptan +5 % HS bolus +5 % maintenance (Con + HSb). Treatments were initiated at six (Early) or 24 h (Late) following stroke and rats were euthanized at 48 h to evaluate infarct volume, brain edema, and microglia/macrophage activation. Infarct volume and brain edema in the Early HS, Early Con, and Late HS groups were significantly reduced compared with controls. Interestingly, only the Early Con group demonstrated reduced microglia/macrophage activation. These data suggest an anti-inflammatory mechanism for conivaptan and provide support for a multipronged approach combining osmotic agents with compounds that inhibit the neuroinflammatory response to stroke.

The Splenic Response to Ischemic Stroke: Neuroinflammation, Immune Cell Migration, and Experimental Approaches to Defining Cellular Mechanisms

The neural sequelae resulting from ischemic stroke involve multiple physiological systems resulting in acute injury followed by delayed cellular degeneration. Clinical studies targeting early neural injury uncovered in animal models of stroke have resulted in poor outcomes. The limited success of these approaches has broadened investigation into the delayed phase of infarct expansion, whereby the penumbral tissue surrounding the initial core infarct can be rescued prior to irreversible injury. Of particular relevance are the neuroimmune mechanisms that are activated from several hours to days following ischemic stroke and eventually lead to permanent infarction that cannot be rescued. New findings from animal studies suggest that the therapeutic window is wider than previously thought, thus allowing for selective targeting of inflammatory processes prior to irreversible injury.