Immunity and inflammation are key elements of the pathobiology of stroke, a devastating illness second only to cardiac ischemia as a cause of death worldwide. The immune system participates in the brain damage produced by ischemia, and the damaged brain, in turn, exerts an immunosuppressive effect that promotes fatal infections that threaten the survival of people after stroke. Inflammatory signaling is involved in all stages of the ischemic cascade, from the early damaging events triggered by arterial occlusion to the late regenerative processes underlying post-ischemic tissue repair. Recent developments have revealed that stroke engages both innate and adaptive immunity. But adaptive immunity triggered by newly exposed brain antigens does not have an impact on the acute phase of the damage. Nevertheless, modulation of adaptive immunity exerts a remarkable protective effect on the ischemic brain and offers the prospect of new stroke therapies. As immunomodulation is not devoid of deleterious side effects, a better understanding of the reciprocal interaction between the immune system and the ischemic brain is essential to harness the full therapeutic potential of the immunology of stroke.

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The immunology of stroke: from mechanisms to translation

Inflammation plays an important role in the pathogenesis of ischemic stroke and other forms of ischemic brain injury. Experimentally and clinically, the brain responds to ischemic injury with an acute and prolonged inflammatory process, characterized by rapid activation of resident cells (mainly microglia), production of proinflammatory mediators, and infiltration of various types of inflammatory cells (including neutrophils, different subtypes of T cells, monocyte/macrophages, and other cells) into the ischemic brain tissue. These cellular events collaboratively contribute to ischemic brain injury. Despite intense investigation, there are still numerous controversies concerning the time course of the recruitment of inflammatory cells in the brain and their pathogenic roles in ischemic brain injury. In this review, we provide an overview of the time-dependent recruitment of different inflammatory cells following focal cerebral I/R. We discuss how these cells contribute to ischemic brain injury and highlight certain recent findings and currently unanswered questions about inflammatory cells in the pathophysiology of ischemic stroke.

https://jlb.onlinelibrary.wiley.com/doi/epdf/10.1189/jlb.1109766

Inflammatory mechanisms in ischemic stroke: role of inflammatory cells.

Monocytes express various receptors, which monitor and sense environmental changes. Monocytes are highly plastic and heterogeneous, and change their functional phenotype in response to environmental stimulation. Evidence from murine and human studies has suggested that monocytosis can be an indicator of various inflammatory diseases. Monocytes can differentiate into inflammatory or anti-inflammatory subsets. Upon tissue damage or infection, monocytes are rapidly recruited to the tissue, where they can differentiate into tissue macrophages or dendritic cells. Given the rapid progress in monocyte research from broad spectrum of inflammatory diseases, there is a need to summarize our knowledge in monocyte heterogeneity and its impact in human disease. In this review, we describe the current understanding of heterogeneity of human and murine monocytes, the function of distinct subsets of monocytes, and a potential mechanism for monocyte differentiation. We emphasize that inflammatory monocyte subsets are valuable biomarkers for inflammatory diseases, including cardiovascular diseases.

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Monocyte and macrophage differentiation: circulation inflammatory monocyte as biomarker for inflammatory diseases

Summary Treatments for acute ischaemic stroke continue to evolve after the superior value of endovascular thrombectomy was confirmed over systemic thrombolysis. Unfortunately, numerous neuroprotective drugs have failed to show benefit in the treatment of acute ischaemic stroke, making the search for new treatments imperative. Increased awareness of the relevance of rigorous preclinical testing, and appropriate selection of study participants, might overcome the barriers to progress in stroke research. Relevant areas of interest include the search for safe and effective treatment strategies that combine neuroprotection reperfusion, better use of advanced brain imaging for patient selection, and wider implementation of prehospital conducted clinical trials. Randomised controlled trials of combination treatments completed within the past 5 years have included growth factors, hypothermia, minocycline, natalizumab, fingolimod, and uric acid; the latter two drugs with alteplase produced encouraging results. Blocking of excitotoxicity is also being reassessed in clinical trials with new approaches, such as the postsynaptic density-95 inhibitor NA-1, or peritoneal dialysis to remove excess glutamate. The findings of these randomised trials are anticipated to improve treatment options and clinical outcomes in of patients with acute stroke.

https://thelancet.com/pdfs/journals/laneur/PIIS1474-4422(16)00114-9.pdf

Neuroprotection in acute stroke: targeting excitotoxicity, oxidative and nitrosative stress, and inflammation

Stroke, the third leading cause of death and disability worldwide, is undergoing a change in perspective with the emergence of new ideas on neurodegeneration The concept that stroke is a disorder solely of blood vessels has been expanded to include the effects of a detrimental interaction between glia, neurons, vascular cells, and matrix components, which is collectively referred to as the neurovascular unit Following the acute stroke, the majority of which are ischemic, there is secondary neuroinflammation that both promotes further injury, resulting in cell death, but conversely plays a beneficial role, by promoting recovery The proinflammatory signals from immune mediators rapidly activate resident cells and influence infiltration of a wide range of inflammatory cells (neutrophils, monocytes/macrophages, different subtypes of T cells, and other inflammatory cells) into the ischemic region exacerbating brain damage In this review, we discuss how neuroinflammation has both beneficial as well as detrimental roles and recent therapeutic strategies to combat pathological responses Here, we also focus on time-dependent entry of immune cells to the ischemic area and the impact of other pathological mediators, including oxidative stress, excitotoxicity, matrix metalloproteinases (MMPs), high-mobility group box 1 (HMGB1), arachidonic acid metabolites, mitogen-activated protein kinase (MAPK), and post-translational modifications that could potentially perpetuate ischemic brain damage after the acute injury Understanding the time-dependent role of inflammatory factors could help in developing new diagnostic, prognostic, and therapeutic neuroprotective strategies for post-stroke inflammation

/pdf/neuroinflammation-friend-and-foe-for-ischemic-stroke-3ivhensbsu.pdf

Neuroinflammation: friend and foe for ischemic stroke

Background and Purpose— Ischemic stroke leads to significant morbidity and mortality in the Western world Early reperfusion strategies remain the treatment of choice but can initiate and augment an inflammatory response causing secondary brain damage The understanding of postischemic inflammation is very limited The objectives of this study were to define the temporal and spatial infiltration of immune cell populations and their activation patterns in a murine cerebral ischemia–reperfusion injury model Methods— Transient middle cerebral artery occlusion was induced for 1 hour followed by 12-hour to 7-day reperfusion in C57/BL6 mice Immunohistochemistry and flow cytometry were used to quantify the infiltrating immune cell subsets Results— Accumulation of microglia and infiltration of the ischemic hemisphere by macrophages, lymphocytes, and dendritic cells (DCs) preceded the neutrophilic influx DCs were found to increase 20-fold and constituted a substantial proportion of infiltrating cells DCs exhi

Doerthe Behrens

Papers

Temporal and Spatial Dynamics of Cerebral Immune Cell Accumulation in Stroke