Adoptive transfer of immune subsets prior to MCAO does not exacerbate stroke outcome in splenectomized mice

Sex-Specific Immune Responses in Stroke

In both acute and chronic diseases, functional differences in host immune responses arise from a multitude of intrinsic and extrinsic factors. Two of the most important factors affecting the immune response are biological sex and aging. Ischemic stroke is a debilitating disease that predominately affects older individuals. Epidemiological studies have shown that older women have poorer functional outcomes compared with men, in part due to the older age at which they experience their first stroke and the increased comorbidities seen with aging. The immune response also differs in men and women, which could lead to altered inflammatory events that contribute to sex differences in poststroke recovery. Intrinsic factors including host genetics and chromosomal sex play a crucial role both in shaping the host immune system and in the neuroimmune response to brain injury. Ischemic stroke leads to altered intracellular communication between astrocytes, neurons, and resident immune cells in the central nervous system. Increased production of cytokines and chemokines orchestrate the infiltration of peripheral immune cells and promote neuroinflammation. To maintain immunosurveillance, the host immune and central nervous system are highly regulated by a diverse population of immune cells which are strategically distributed within the neurovascular unit and become activated with injury. In this review, we provide a comprehensive overview of sex-specific host immune responses in ischemic stroke.

Ischemic Preconditioning Modulates the Peripheral Innate Immune System to Promote Anti-Inflammatory and Protective Responses in Mice Subjected to Focal Cerebral Ischemia

The development of tolerance triggered by a sublethal ischemic episode (preconditioning, PC) involves a complex crosstalk between neurons, astrocytes and microglia, although the role of the peripheral immune system in this context is largely unexplored. Here, we report that severe cerebral ischemia caused by transient middle cerebral artery occlusion (MCAo) in adult male mice elevates blood counts of inflammatory neutrophils and monocytes, and plasma levels of miRNA-329-5p. These inflammatory responses are prevented by ischemic PC induced by 15 min MCAo, 72h before the severe insult (1h MCAo). As compared with sham-operated animals, mice subjected to either ischemic PC, MCAo or a combination of both (PC+MCAo) display spleen contraction. However, protein levels of Ym1 (a marker of polarization of myeloid cells towards M2/N2 protective phenotypes) are elevated only in spleen from the experimental groups PC and PC+MCAo, but not MCAo. Conversely, Ym1 protein levels only increase in circulating leukocytes from mice subjected to 1h MCAo, but not in preconditioned animals, which is coincident with a dramatic elevation of Ym1 expression in the ipsilateral cortex. By immunofluorescence analysis, we observe that expression of Ym1 occurs in amoeboid-shaped myeloid cells, mainly representing inflammatory monocytes/macrophages and neutrophils. As a result of its immune-regulatory functions, ischemic PC prevents elevation of mRNA levels of the pro-inflammatory cytokine interleukin (IL)-1β in the ipsilateral cortex, while not affecting IL-10 mRNA increase induced by MCAo. Overall, the elevated anti-inflammatory/pro-inflammatory ratio observed in the brain of mice pre-exposed to PC is associated with reduced brain infarct volume and ischemic edema, and with amelioration of functional outcome. These findings reaffirm the crucial and dualistic role of the innate immune system in ischemic stroke pathobiology, extending these concepts to the context of ischemic tolerance and underscoring their relevance for the identification of novel therapeutic targets for effective stroke treatment.

The "Dialogue" Between Central and Peripheral Immunity After Ischemic Stroke: Focus on Spleen

The immune response generated by the body after the incidence of ischemic stroke, runs through the comprehensive process of aftermath. During this process of ischemic stroke, the central neuroinflammation and peripheral immune response seriously affect the prognosis of patients, which has been the focus of research in recent years. As this research scenario progressed, the "dialogue" between central nervous inflammation and peripheral immune response after ischemic stroke has become more closely related. It's worth noting that the spleen, as an important peripheral immune organ, plays a pivotal role in this dialogue. Multiple mechanisms have previously been reported for brain-spleen crosstalk after ischemic stroke. Further, neuroinflammation in the brain can affect the peripheral immune state by activating/inhibiting spleen function. However, the activation of the peripheral immune inflammatory response can work reversibly in the spleen. It further affects intracerebral neuroinflammation through the injured blood-brain barrier. Therefore, paying close attention to the role of spleen as the pivot between central and peripheral immunity in ischemic stroke may help to provide a new target for immune intervention in the treatment of ischemic stroke. In the present review, we reviewed the important role of spleen in central neuroinflammation and peripheral immune response after ischemic stroke. We summarized the relevant studies and reports on spleen as the target of immune intervention which can provide new ideas for the clinical treatment of ischemic stroke.

CD147 as a key mediator of the spleen inflammatory response in mice after focal cerebral ischemia

Background

The splenic inflammatory response after cerebral ischemia has been implicated in secondary brain injury. We have recently reported that CD147 plays an important role in driving brain inflammation after ischemic stroke. In this study, we hypothesized that CD147 may play a role in the splenic inflammatory response after cerebral ischemia.

Methods

Transient (60 min) middle cerebral artery occlusion was induced in wild-type mice treated with an anti-CD147 antibody (αCD147) 1 h before ischemia onset. The splenic inflammatory response was evaluated at 4 and 24 h, representing the peak and early stage of splenic inflammatory activation in this model. Changes in mRNA and protein expression of CD147 and inflammatory markers were measured using RT-qPCR and western blot, respectively. Immune cells in the spleen and brain were measured using flow cytometry.

Results

CD147 expression was rapidly upregulated in the spleen at 4 and 24 h after ischemia onset. The splenic inflammatory response induced by cerebral ischemia was inhibited by αCD147 treatment as demonstrated by the reduced expression of cytokines (TNFα, IL-6, IL-1β) and monocyte chemoattractant protein-1 (MCP-1) in the spleen at 4 and 24 h after ischemia onset. Furthermore, reduced expression of Ly-6C and CCR2 coincided with a decrease in the number of Ly-6C^high MMs subset in the spleen at 4 h after ischemia onset. This suggests αCD147 treatment abrogates cerebral ischemia-induced inflammatory activation of splenic monocytes/macrophages (MMs). In addition, the experiment in splenectomized mice showed the spleen as the major source of infiltrated Ly-6C^high MMs subset in the ischemic brain and that brain infiltration of Ly-6C^high MMs was reduced by αCD147 treatment. These results reveal CD147 as a key mediator of the spleen’s inflammatory activation in response to cerebral ischemia.

Prothymosin alpha and its mimetic hexapeptide improve delayed tissue plasminogen activator-induced brain damage following cerebral ischemia

Tissue plasminogen activator (tPA) administration beyond 4.5 h of stroke symptoms is beneficial for patients but has an increased risk of cerebral hemorrhage. Thus, increasing the therapeutic window of tPA is important for stroke recovery. We previously showed that prothymosin alpha (ProTα) or its mimetic hexapeptide (P6Q) has anti-ischemic activity. Here, we examined the beneficial effects of ProTα or P6Q against delayed tPA-induced brain damage following middle cerebral artery occlusion (MCAO) or photochemically induced thrombosis in mice. Brain hemorrhage was observed by tPA administration during reperfusion at 4.5 and 6 h after MCAO. Co-administration of ProTα with tPA at 4.5 h inhibited hemorrhage and motor dysfunction 2-4 days, but not 7 days after MCAO. ProTα administration at 2 and 4.5 h after MCAO significantly inhibited tPA (4.5 h)-induced motor dysfunction and death more than 7 days. Administration of tPA caused the loss of tight junction proteins, zona occulden-1 and occludin, and up-regulation of matrix metalloproteinase-2/9, in a ProTα-reversible manner. P6Q administration abolished tPA (4.5 h)-induced hemorrhage and reversed tPA (6 h)-induced vascular damage and matrix metalloproteinase-2 and 9 up-regulation. Twice administrations of P6Q at 2 h alone and 6 h with tPA significantly improved motor dysfunction more than 7 days. In photochemically induced thrombosis ischemia, similar vascular leakage and neuronal damage (infarction and motor dysfunction) by late tPA (4.5 or 6 h) were also inhibited by P6Q. Thus, these studies suggest that co-administration with ProTα or P6Q would be beneficial to inhibit delayed tPA-induced hemorrhagic mechanisms in acute ischemic stroke.

The spleen may be an important target of stem cell therapy for stroke

Stroke is the most common cerebrovascular disease, the second leading cause of death behind heart disease and is a major cause of long-term disability worldwide. Currently, systemic immunomodulatory therapy based on intravenous cells is attracting attention. The immune response to acute stroke is a major factor in cerebral ischaemia (CI) pathobiology and outcomes. Over the past decade, the significant contribution of the spleen to ischaemic stroke has gained considerable attention in stroke research. The changes in the spleen after stroke are mainly reflected in morphology, immune cells and cytokines, and these changes are closely related to the stroke outcomes. Autonomic nervous system (ANS) activation, release of central nervous system (CNS) antigens and chemokine/chemokine receptor interactions have been documented to be essential for efficient brain-spleen cross-talk after stroke. In various experimental models, human umbilical cord blood cells (hUCBs), haematopoietic stem cells (HSCs), bone marrow stem cells (BMSCs), human amnion epithelial cells (hAECs), neural stem cells (NSCs) and multipotent adult progenitor cells (MAPCs) have been shown to reduce the neurological damage caused by stroke. The different effects of these cell types on the interleukin (IL)-10, interferon (IFN), and cholinergic anti-inflammatory pathways in the spleen after stroke may promote the development of new cell therapy targets and strategies. The spleen will become a potential target of various stem cell therapies for stroke represented by MAPC treatment.

G2A Protects Mice against Sepsis by Modulating Kupffer Cell Activation: Cooperativity with Adenosine Receptor 2b

G2A is a GPCR abundantly expressed in immune cells. G2A^-/- mice showed higher lethality, higher plasma cytokines, and an impaired bacterial clearance in response to a murine model of sepsis (cecal ligation and puncture), which were blocked by GdCl₃, an inhibitor of Kupffer cells. Anti-IL-10 Ab reversed the impaired bacterial clearance in G2A^-/- mice. Indomethacin effectively blocked both the increased i.p. IL-10 levels and the impaired bacterial clearance, indicating that disturbed PG system is the proximal cause of these phenomena. Stimulation with LPS/C5a induced an increase in Escherichia coli phagocytosis and intracellular cAMP levels in G2A^+/+ peritoneal macrophages but not G2A^-/- cells, which showed more PGE₂/nitrite release and intracellular reactive oxygen species levels. Heterologous coexpression of G2A and adenosine receptor type 2b (A2bAR) induced a synergistic increase in cAMP signaling in a ligand-independent manner, with the evidence of physical interaction of G2A with A2bAR. BAY 60-6583, a specific agonist for A2bAR, increased intracellular cAMP levels in Kupffer cells from G2A^+/+ but not from G2A^-/- mice. Both G2A and A2bAR were required for antiseptic action of lysophosphatidylcholine. These results show inappropriate activation of G2A^-/- Kupffer cells to septic insults due to an impaired cAMP signaling possibly by lack of interaction with A2bAR.

Amlexanox Inhibits Cerebral Ischemia-Induced Delayed Astrocytic High-Mobility Group Box 1 Release and Subsequent Brain Damage

High-mobility group box 1 (HMGB1) is increased in the cerebrospinal fluid (CSF) and serum during the early and late phases of brain ischemia and is known to contribute to brain damage. However, detailed characterization underlying cell type-specific HMGB1 release and pathophysiological roles of extracellularly released HMGB1 in ischemic brain remain unclear. Here, we examined cell type-specific HMGB1 release and the therapeutic potential of amlexanox, an inhibitor of nonclassical release, and of an anti-HMGB1 antibody against ischemic brain damage. HMGB1 depletion from neuronal nuclei was observed within 3 hours after transient middle cerebral artery occlusion (tMCAO), whereas the intracerebroventricular (i.c.v.) pretreatment with amlexanox blocked HMGB1 release from neurons, resulting in HMGB1 redistribution in the nuclei and cytoplasm. HMGB1 was selectively released from astrocytes 27 hours after tMCAO and this HMGB1 release was blocked by late treatment with amlexanox (i.c.v.) 24 hours after tMCAO. Proximity extension assay revealed that the HMGB1 level was elevated in the CSF at 3 and 27 hours after tMCAO. This late treatment with amlexanox significantly protected the brain from ischemic damage, but its pretreatment 30 minutes before tMCAO failed to show any protection. The late treatment (i.c.v.) with anti-HMGB1 antibody 24 hours after tMCAO also ameliorated ischemic brain damage 48 hours after tMCAO. Thus, the inhibition of brain damage by late treatment with amlexanox or anti-HMGB1 antibody indicates that late HMGB1 release plays a role in the maintenance of stroke-induced brain damage, and the inhibition of this release would be a novel therapeutic target for protection of ischemic brain damage.

Neuroprotective Properties of a Macrolide Antibiotic in a Mouse Model of Middle Cerebral Artery Occlusion: Characterization of the Immunomodulatory Effects and Validation of the Efficacy of Intravenous Administration

Repurposing the macrolide antibiotic azithromycin has recently been suggested as a promising neuroprotective strategy for the acute treatment of ischemic stroke. Here, we aim at further characterizing the immunomodulatory properties of intraperitoneal (i.p.) administration of this drug and, more importantly, at assessing whether neuroprotection can also be achieved by the more clinically relevant intravenous (i.v.) route of administration in a mouse model of focal cerebral ischemia induced by transient (30-min) middle cerebral artery occlusion (MCAo). A single i.p. injection of azithromycin (150 mg/kg) upon reperfusion prevented ischemia-induced spleen contraction and increased the number of MAC-1-immunopositive microglia/macrophages in the ischemic hemisphere 48 h after the insult. This was paralleled by an elevation of alternatively activated phenotypes (i.e., Ym1-immunopositive M2-polarized cells) and by a reduced expression of the pro-inflammatory marker myeloperoxidase. More importantly, i.v. administration of azithromycin upon reperfusion reduced MCAo-induced infarct volume and cerebral edema to an extent comparable to that obtained via the i.p. route. Although the i.p. route is often used for research purposes, it is impractical in the clinical setting; however, i.v. administration can easily be used in ischemic stroke patients who usually have i.v. access already established on hospital admission. The neuroprotective efficacy of the clinically relevant i.v. administration of azithromycin, together with its beneficial immunomodulatory properties reported in mice subjected to transient MCAo, suggests that this macrolide antibiotic can be effectively repurposed for the acute treatment of ischemic stroke. To this end, further work is needed to validate the efficacy of azithromycin in the clinical setting.

Neuroprotective DAMPs member prothymosin alpha has additional beneficial actions against cerebral ischemia-induced vascular damages

Prothymosin alpha (ProTα) suppresses stress-induced necrosis of cultured cortical neurons. As neuroprotection alone could not explain the long-lasting protective actions against cerebral ischemia by ProTα, we further examined whether ProTα, in addition to neuroprotective effects, has other anti-ischemic activities. When recombinant mouse ProTα (rmProTα) at 0.3 mg/kg was intravenously (i.v.) given 2 h after the start of tMCAO, all mice survived for more than 14 days. In evaluation of CD31- and tomato lectin-labeling as well as IgG and Evans blue leakage, rmProTα treatment (0.1 mg/kg) largely blocked ischemia-induced vascular damages. Therefore, rmProTα has novel beneficial effects against ischemia-induced brain damage through vascular mechanisms.

Splenectomy following MCAO inhibits the TLR4-NF-κB signaling pathway and protects the brain from neurodegeneration in rats

BACKGROUND AND PURPOSE

The Toll-like receptor 4(TLR4)/nuclear factor kappa B NF-κB inflammatory pathway contributes to secondary inflammation in many diseases including stroke. Moreover, the neuroprotective effect of splenectomy in stroke is supported by a vast body of experimental evidence. Nevertheless, the underlying mechanism(s) by which splenectomy enhance neuroprotection in stroke is still poorly understood. Our study aimed to investigate whether post-ischemic splenectomy modulate the TLR4/NF-κB inflammatory pathway in stroke.

METHODS

Immunohistochemistry was used to evaluate the levels of TLR4 and NF-κB expression in brain areas (parietal lobe, hippocampus and striatum) of rats that underwent: MCAO-splenectomy surgery (MS ); MCAO surgery without splenectomy (MCAO control or MC); Sham MCAO and splenectomy surgery (sham control group or SC group respectively. Apoptosis in these areas was assessed by TUNEL detection technique.

RESULTS

The levels of TLR4 and NF-κB expression were significantly reduced in splenectomized rats relative to the MS group (P<0.01). Additionally, the number of apoptotic cells in the ischemic hemisphere were significantly higher in both MCAO groups compared to the Sham group (P<0.05), between day 1 and day 7. An exception was observed in the striatum where the difference in apoptotic rate between the MS and sham group was not statistically significant at the same time points. Moreover, the variation apoptotic rate in different cerebral zone correlated to variation in TLR4 and NF-κB expression.

CONCLUSION

In summary, our study provides further evidence of neuroprotective effect of splenectomy in ischemic stroke. Our results suggest that such an effect might be due to the inhibition of theTLR4/NF-κB inflammatory pathway.