Arthur Liesz

Regulatory T cells are key cerebroprotective immunomodulators in acute experimental stroke

Systemic and local inflammatory processes have a key, mainly detrimental role in the pathophysiology of ischemic stroke. Currently, little is known about endogenous counterregulatory immune mechanisms. We examined the role of the key immunomodulators CD4+CD25+ forkhead box P3 (Foxp3)+ regulatory T lymphocytes (Treg cells), after experimental brain ischemia. Depletion of Treg cells profoundly increased delayed brain damage and deteriorated functional outcome. Absence of Treg cells augmented postischemic activation of resident and invading inflammatory cells including microglia and T cells, the main sources of deleterious cerebral tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), respectively. Early antagonization of TNF-α and delayed neutralization of IFN-γ prevented infarct growth in Treg cell–depleted mice. Intracerebral interleukin-10 (IL-10) substitution abrogated the cytokine overexpression after Treg cell depletion and prevented secondary infarct growth, whereas transfer of IL-10–deficient Treg cells in an adoptive transfer model was ineffective. In conclusion, Treg cells are major cerebroprotective modulators of postischemic inflammatory brain damage targeting multiple inflammatory pathways. IL-10 signaling is essential for their immunomodulatory effect.

Hemostatic Therapy in Experimental Intracerebral Hemorrhage Associated With the Direct Thrombin Inhibitor Dabigatran

Background and Purpose— Dabigatran-etexilate (DE) recently has been approved for stroke prevention in atrial fibrillation. However, lack of effective antagonists represents a major concern in the event of intracerebral hemorrhage (ICH). The aims of the present study were to establish a murine model of ICH associated with dabigatran, and to test the efficacy of different hemostatic factors in preventing hematoma growth. Methods— In C57BL/6 mice receiving DE (4.5 or 9.0 mg/kg), in vivo and in vitro coagulation assays and dabigatran plasma levels were measured repeatedly. Thirty minutes after inducing ICH by striatal collagenase injection, mice received an intravenous injection of saline, prothrombin complex concentrate (PCC; 100 U/kg), murine fresh-frozen plasma (200 &mgr;L), or recombinant human factor VIIa (8.0 mg/kg). ICH volume was quantified on brain cryosections 24 hours later. Results— DE substantially prolonged tail vein bleeding time and ecarin clotting time for 4 hours corresponding to dabigatran plasma levels. Intracerebral hematoma expansion was observed mainly during the first 3 hours on serial T2* MRI. Anticoagulation with high doses of DE increased the hematoma volume significantly. PCC and, less consistently, fresh-frozen plasma prevented excess hematoma expansion caused by DE, whereas recombinant human factor VIIa was ineffective. Prevention of hematoma growth and reversal of tail vein bleeding time by PCC were dose-dependent. Conclusions— The study provides strong evidence that PCC and, less consistently, fresh-frozen plasma prevent excess intracerebral hematoma expansion in a murine ICH model associated with dabigatran. The efficacy and safety of this strategy must be further evaluated in clinical studies.

Inhibition of lymphocyte trafficking shields the brain against deleterious neuroinflammation after stroke

T lymphocytes are increasingly recognized as key modulators of detrimental inflammatory cascades in acute ischaemic stroke, but the potential of T cell-targeted therapy in brain ischaemia is largely unexplored. Here, we characterize the effect of inhibiting leukocyte very late antigen-4 and endothelial vascular cell adhesion molecule-1-mediated brain invasion-currently the most effective strategy in primary neuroinflammatory brain disease in murine ischaemic stroke models. Very late antigen-4 blockade by monoclonal antibodies improved outcome in models of moderate stroke lesions by inhibiting cerebral leukocyte invasion and neurotoxic cytokine production without increasing the susceptibility to bacterial infections. Gene silencing of the endothelial very late antigen-4 counterpart vascular cell adhesion molecule-1 by in vivo small interfering RNA injection resulted in an equally potent reduction of infarct volume and post-ischaemic neuroinflammation. Furthermore, very late antigen-4-inhibition effectively reduced the post-ischaemic vascular cell adhesion molecule-1 upregulation, suggesting an additional cross-signalling between invading leukocytes and the cerebral endothelium. Dissecting the specific impact of leukocyte subpopulations showed that invading T cells, via their humoral secretion (interferon-γ) and immediate cytotoxic mechanisms (perforin), were the principal pathways for delayed post-ischaemic tissue injury. Thus, targeting T lymphocyte-migration represents a promising therapeutic approach for ischaemic stroke.

Infarct Volume is a Major Determiner of Post-Stroke Immune Cell Function and Susceptibility to Infection

Background and Purpose— Acute ischemic stroke in humans is associated with profound alterations in the immune system. Hallmarks of this stroke-induced immunodepression syndrome are: lymphocytopenia, impairment of T helper cell and monocyte function. We studied which stroke-specific factors predict these immunologic alterations and subsequent infections. Methods— Leukocyte/lymphocyte subsets were assessed serially by white blood cell count and fluorescence-activated cell sorter analysis in ischemic stroke patients (n=50) at baseline, day 1, and day 4 after stroke onset and compared to an age-matched control group (n=40). Concomitantly, monocytic human leukocyte antigen-DR expression and the in vitro function of blood monocytes measured by the production of tumor necrosis factor-&agr; upon stimulation with lipopolysaccharide were assessed. Associations of these immunologic parameters with stroke specific factors (National Institutes of Health Stroke Scale, infarct size) were explored. Multivariable logistic regression analysis was applied to identify early predictors for poststroke respiratory and urinary tract infections. Results— Infarct volume was the main factor associated with lymphocytopenia on day 1 and day 4 poststroke. Particularly, blood natural killer cell counts were reduced after stroke. Monocyte counts increased after ischemia paralleled by a profound deactivation predominantly after extensive infarcts. Reduced T helper cell counts, monocytic human leukocyte antigen-DR expression, and monocytic in vitro production of tumor necrosis factor-&agr; were associated with infections in univariate analyses. However, only stroke volume prevailed as independent early predictor for respiratory infections (OR 1.03; CI 1.01 to 1.04). Conclusions— Infarct volume determines the extent of lymphocytopenia, monocyte dysfunction, and is a main predictor for subsequent infections.

The spectrum of systemic immune alterations after murine focal ischemia: immunodepression versus immunomodulation.

Background and Purpose— Therapeutic modification of the postischemic immune processes is a key target of current experimental stroke research. For successful translation into the clinical setting, experimental studies must account for the impact of different strokes on the immune system including susceptibility to infection. Herein, we characterize the impact of 3 ischemia models on systemic immunological and microbiological parameters. Methods— In C57Bl/6 mice (n=235), the middle cerebral artery was occluded (MCAO) either permanently by distal coagulation or transiently by an intraluminal filament for 30 minutes or 90 minutes. Differential leukocyte counts were performed in blood and lymphatic organs. Lymphocyte subpopulations and apoptotic cells were characterized by flow cytometry. Blood cytokine concentrations were measured by ELISA. Microbiological cultures were grown from blood and lung samples. Results— Only extensive infarcts induced leukopenia 24 hours, 3 days and 7 days after MCAO and decreased lymphocyte counts in spleen, lymph nodes and thymus. In contrast, small infarcts led to no significant changes in differential blood count or reduction of overall cell counts in lymphatic organs. Splenic lymphocyte apoptosis and blood cytokine production was significantly increased after extensive lesions compared to mild ischemia. Hypothermia and weight loss occurred only in mice with large infarcts which also suffered from pneumonia and sepsis. In contrast to infarct size, location and side of the infarct did not affect physiological parameters and immune cell alterations. Conclusions— Postischemic systemic immunomodulation and infectious complications differ substantially among stroke models. Translational studies of immunomodulatory therapies for stroke must account for this heterogeneity.

Results of a preclinical randomized controlled multicenter trial (pRCT): Anti-CD49d treatment for acute brain ischemia

A multicenter preclinical, randomized, controlled trial (pRCT) of a potential stroke treatment in mice demonstrates the feasibility of performing clinical trial–like investigations in animal models. Tested, just like a human Over the last few decades, clinical trial design and analysis have become increasingly stringent. These refinements—designed to ensure valid conclusions for formal drug approval—have improved clinical trial reliability. Now, Llovera et al. have applied the principles of the gold-standard randomized, controlled clinical trial to a preclinical investigation. They tested an antibody to CD49d, which inhibits leukocyte migration into the brain, in two mouse models of stroke. Data from their six-center, preclinical, randomized controlled trial in mice show that the antibody significantly reduced both leukocyte invasion and infarct volume after a small cortical stroke but that it did not have any effect in the other model, in which the animal suffered a larger injury. The authors outline the many lessons learned from their experience for further application of preclinical randomized controlled trials to translational research. Numerous treatments have been reported to provide a beneficial outcome in experimental animal stroke models; however, these treatments (with the exception of tissue plasminogen activator) have failed in clinical trials. To improve the translation of treatment efficacy from bench to bedside, we have performed a preclinical randomized controlled multicenter trial (pRCT) to test a potential stroke therapy under circumstances closer to the design and rigor of a clinical randomized control trial. Anti-CD49d antibodies, which inhibit the migration of leukocytes into the brain, were previously investigated in experimental stroke models by individual laboratories. Despite the conflicting results from four positive and one inconclusive preclinical studies, a clinical trial was initiated. To confirm the preclinical results and to test the feasibility of conducting a pRCT, six independent European research centers investigated the efficacy of anti-CD49d antibodies in two distinct mouse models of stroke in a centrally coordinated, randomized, and blinded approach. The results pooled from all research centers revealed that treatment with CD49d-specific antibodies significantly reduced both leukocyte invasion and infarct volume after the permanent distal occlusion of the middle cerebral artery, which causes a small cortical infarction. In contrast, anti-CD49d treatment did not reduce lesion size or affect leukocyte invasion after transient proximal occlusion of the middle cerebral artery, which induces large lesions. These results suggest that the benefits of immune-targeted approaches may depend on infarct severity and localization. This study supports the feasibility of performing pRCTs.

Postischemic brain infiltration of leukocyte subpopulations differs among murine permanent and transient focal cerebral ischemia models.

Cellular and humoral inflammations play important roles in ischemic brain injury. The effectiveness of immunomodulatory therapies may critically depend on the chosen experimental model. Our purpose was to compare the post‐ischemic neuroinflammation among murine permanent and transient middle cerebral artery occlusion (MCAO) models. Permanent MCAO was induced by transtemporal electrocoagulation and 30 minutes or 90 minutes transient MCAO was induced by intraluminal filament in C57BL/6 mice. Infiltration of leukocyte subpopulations was quantified by immunohistochemistry and fluorescence‐activated cell sorting. Cerebral cytokine and adhesion molecule expression was measured by real‐time polymerase chain reaction (RT‐PCR). Neutrophil infiltration was noted at 24 h after transient MCAO, but did not further increase until 5 days in the permanent MCAO model. Few T cells were observed in both MCAO models at 24 h, but permanent MCAO demonstrated much more infiltrating T cells at 5 days. Pronounced microglial activation was evident at 24 h and 5 days after permanent but not after transient MCAO. The number of invading NK cells and expression of MHCII on CD11b+ cells did not differ among the three groups. Five days after MCAO, the expression of IL‐1, TNF‐α and IFN‐γ and of the adhesion molecules ICAM‐1 and VCAM‐1 was significantly higher in the permanent than in the transient MCAO groups. Cellular and humoral inflammation differs substantially among commonly used MCAO models. Neuroinflammation is more pronounced after permanent electrocoagulatory MCAO compared with 30 minutes and 90 minutes filament‐MCAO.

Boosting Regulatory T Cells Limits Neuroinflammation in Permanent Cortical Stroke

Inflammatory mechanisms contribute substantially to secondary tissue injury after brain ischemia. Regulatory T cells (Tregs) are key endogenous modulators of postischemic neuroinflammation. We investigated the potential of histone deacetylase inhibition (HDACi) to enhance Treg potency for experimental stroke in mice. HDACi using trichostatin A increased the number of Tregs and boosted their immunosuppressive capacity and interleukin (IL)-10 expression. In vivo treatment reduced infarct volumes and behavioral deficits after cortical brain ischemia, attenuated cerebral proinflammatory cytokine expression, and increased numbers of brain-invading Tregs. A similar effect was obtained using tubastatin, a specific inhibitor of HDAC6 and a key HDAC in Foxp3 regulation. The neuroprotective effect of HDACi depended on the presence of Foxp3+ Tregs, and in vivo and in vitro studies showed that the anti-inflammatory cytokine IL-10 was their main mediator. In summary, modulation of Treg function by HDACi is a novel and potent target to intervene at the center of neuroinflammation. Furthermore, this novel concept of modulating endogenous immune mechanisms might be translated to a broad spectrum of diseases, including primary neuroinflammatory and neurodegenerative disorders.

FTY720 Reduces Post-Ischemic Brain Lymphocyte Influx but Does Not Improve Outcome in Permanent Murine Cerebral Ischemia

Background The contribution of neuroinflammation and specifically brain lymphocyte invasion is increasingly recognised as a substantial pathophysiological mechanism after stroke. FTY720 is a potent treatment for primary neuroinflammatory diseases by inhibiting lymphocyte circulation and brain immigration. Previous studies using transient focal ischemia models showed a protective effect of FTY720 but did only partially characterize the involved pathways. We tested the neuroprotective properties of FTY720 in permanent and transient cortical ischemia and analyzed the underlying neuroimmunological mechanisms. Methodology/Principal Findings FTY720 treatment resulted in substantial reduction of circulating lymphocytes while blood monocyte counts were significantly increased. The number of histologically and flow cytometrically analyzed brain invading T- and B lymphocytes was significantly reduced in FTY720 treated mice. However, despite testing a variety of treatment protocols, infarct volume and behavioural dysfunction were not reduced 7d after permanent occlusion of the distal middle cerebral artery (MCAO). Additionally, we did not measure a significant reduction in infarct volume at 24h after 60 min filament-induced MCAO, and did not see differences in brain edema between PBS and FTY720 treatment. Analysis of brain cytokine expression revealed complex effects of FTY720 on postischemic neuroinflammation comprising a substantial reduction of delayed proinflammatory cytokine expression at 3d but an early increase of IL-1β and IFN-γ at 24 h after MCAO. Also, serum cytokine levels of IL-6 and TNF-α were increased in FTY720 treated animals compared to controls. Conclusions/Significance In the present study we were able to detect a reduction of lymphocyte brain invasion by FTY720 but could not achieve a significant reduction of infarct volumes and behavioural dysfunction. This lack of neuroprotection despite effective lymphopenia might be attributed to a divergent impact of FTY720 on cytokine expression and possible activation of innate immune cells after brain ischemia.

Microbiota Dysbiosis Controls the Neuroinflammatory Response after Stroke

Acute brain ischemia induces a local neuroinflammatory reaction and alters peripheral immune homeostasis at the same time. Recent evidence has suggested a key role of the gut microbiota in autoimmune diseases by modulating immune homeostasis. Therefore, we investigated the mechanistic link among acute brain ischemia, microbiota alterations, and the immune response after brain injury. Using two distinct models of acute middle cerebral artery occlusion, we show by next-generation sequencing that large stroke lesions cause gut microbiota dysbiosis, which in turn affects stroke outcome via immune-mediated mechanisms. Reduced species diversity and bacterial overgrowth of bacteroidetes were identified as hallmarks of poststroke dysbiosis, which was associated with intestinal barrier dysfunction and reduced intestinal motility as determined by in vivo intestinal bolus tracking. Recolonizing germ-free mice with dysbiotic poststroke microbiota exacerbates lesion volume and functional deficits after experimental stroke compared with the recolonization with a normal control microbiota. In addition, recolonization of mice with a dysbiotic microbiome induces a proinflammatory T-cell polarization in the intestinal immune compartment and in the ischemic brain. Using in vivo cell-tracking studies, we demonstrate the migration of intestinal lymphocytes to the ischemic brain. Therapeutic transplantation of fecal microbiota normalizes brain lesion-induced dysbiosis and improves stroke outcome. These results support a novel mechanism in which the gut microbiome is a target of stroke-induced systemic alterations and an effector with substantial impact on stroke outcome. SIGNIFICANCE STATEMENT We have identified a bidirectional communication along the brain–gut microbiota–immune axis and show that the gut microbiota is a central regulator of immune homeostasis. Acute brain lesions induced dysbiosis of the microbiome and, in turn, changes in the gut microbiota affected neuroinflammatory and functional outcome after brain injury. The microbiota impact on immunity and stroke outcome was transmissible by microbiota transplantation. Our findings support an emerging concept in which the gut microbiota is a key regulator in priming the neuroinflammatory response to brain injury. These findings highlight the key role of microbiota as a potential therapeutic target to protect brain function after injury.