Letter by Gauberti and Vivien Regarding Article, “Amplification of Regulatory T Cells Using a CD28 Superagonist Reduces Brain Damage After Ischemic Stroke in Mice”

Diverse functions and mechanisms of regulatory T cell in ischemic stroke

The inflammatory and immune processes are key pathophysiological processes in the ischemic stroke, including leukocyte infiltration and destruction of the blood-brain-barrier (BBB), which further lead to increased post-ischemic inflammation. Regulatory T cells (Tregs) are a specific subset of T lymphocytes that play a pivotal role in suppressing the activation of immune system, maintaining immune homeostasis, and regulating inflammation induced by pathogens and environmental toxins. We would like to discuss the paradox function of Tregs in ischemic stroke. The accumulating data indicate that Tregs are involved in the immune regulation and self-tolerance after ischemic stroke, contributing the outcome of ischemic stroke. Tregs could resist immune response overactivation, and were supposed to be the endogenous regulatory factors to control the immune response of ischemic brain. Although, there are still some controversies and unresolved issues about the functions and mechanisms of Tregs in ischemic stroke. More and more attention has been paid to Tregs in the pathogenesis of ischemic stroke and it might be a potential therapeutic target in the future. In this review, we will summarize the recent findings on the specific functions and mechanisms of Tregs and discuss its potential therapeutic role in ischemic stroke.

Anti-inflammatory treatments for stroke: from bench to bedside

So far, intravenous tissue-type plasminogen activator (tPA) and mechanical removal of arterial blood clot (thrombectomy) are the only available treatments for acute ischemic stroke. However, the short therapeutic window and the lack of specialized stroke unit care make the overall availability of both treatments limited. Additional agents to combine with tPA administration or thrombectomy to enhance efficacy and improve outcomes associated with stroke are needed. Stroke-induced inflammatory processes are a response to the tissue damage due to the absence of blood supply but have been proposed also as key contributors to all the stages of the ischemic stroke pathophysiology. Despite promising results in experimental studies, inflammation-modulating treatments have not yet been translated successfully into the clinical setting. This review will (a) describe the timing of the stroke immune pathophysiology; (b) detail the immune responses to stroke sift-through cell type; and (c) discuss the pitfalls on the translation from experimental studies to clinical trials testing the therapeutic pertinence of immune modulators.

Microbiota differences between commercial breeders impacts the post-stroke immune response

Experimental reproducibility between laboratories is a major translational obstacle worldwide, particularly in studies investigating immunomodulatory therapies in relation to brain disease. In recent years increasing attention has been drawn towards the gut microbiota as a key factor in immune cell polarization. Moreover, manipulation of the gut microbiota has been found effective in a diverse range of brain disorders. Within this study we aimed to test the impact of microbiota differences between mice from different sources on the post-stroke neuroinflammatory response. With this rationale, we have investigated the correlation between microbiota differences and the immune response in mice from three commercial breeders with the same genetic background (C57BL/6). While overall bacterial load was comparable, we detected substantial differences in species diversity and microbiota composition on lower taxonomic levels. Specifically, we investigated segmented filamentous bacteria (SFB)-which have been shown to promote T cell polarization-and found that they were absent in mice from one breeder but abundant in others. Our experiments revealed a breeder specific correlation between SFB presence and the ratio of Treg to Th17 cells. Moreover, recolonization of SFB-negative mice with SFB resulted in a T cell shift which mimicked the ratios found in SFB-positive mice. We then investigated the response to a known experimental immunotherapeutic approach, CD28 superagonist (CD28SA), which has been previously shown to expand the Treg population. CD28SA treatment had differing effects between mice from different breeders and was found to be ineffective at inducing Treg expansion in SFB-free mice. These changes directly corresponded to stroke outcome as mice lacking SFB had significantly larger brain infarcts. This study demonstrates the major impact of microbiota differences on T cell polarization in mice during ischemic stroke conditions, and following immunomodulatory therapies.

The "inflammatory penumbra" in ischemic stroke: From clinical data to experimental evidence

Purpose

The objective of the present review is to provide an overview of the available clinical and preclinical data supporting the existence of an "inflammatory penumbra" in ischemic stroke.

Findings

Recent data from clinical trials suggest the existence of an inflammatory area at risk, surrounding the initial ischemic lesion and secondarily infiltrated by lymphocytes, that is ultimately recruited by the ischemic core: called the "inflammatory penumbra." Experimental results support this concept. Lymphocytes, especially T-cells, enter the brain in the perilesional area in a vascular-cell adhesion molecule-1 dependent manner and participate in delayed neuronal cell death.

Methods

For writing this review, we used the more recent publications in the field, including the preclinical and clinical studies. We have also used our own experise in the field of in vivo imaging of inflammatory processes.

Discussion

Consequently, the intensity of the inflammatory reaction and the size of the inflammatory penumbra may vary considerably in patients, as it is the case in experimental stroke models in mice. By analogy with the ischemic penumbra of the acute phase of stroke, this secondary inflammatory penumbra represents a therapeutic opportunity during the subacute phase of stroke. Large clinical trials that target lymphocyte trafficking are currently taking place. However, to improve the benefit of such therapeutic strategies, adequate patient selection may be mandatory.

Conclusion

In this context, innovative imaging methods including magnetic resonance imaging of adhesion molecules may contribute to noninvasively detect this inflammatory penumbra and thus to select patients eligible for such therapy.

A Perspective on Roles Played by Innate and Adaptive Immunity in the Pathobiology of Neurodegenerative Disorders

Aberrant innate and adaptive immune responses are neurodegenerative disease effectors. Disease is heralded by a generalized, but subtle immune activation orchestrated by the release of extracellular prion-like aggregated and oxidized or otherwise modified proteins. These are responsible for an inflammatory neurotoxic cascade. The perpetrators of such events include effector T cells and activated microglia. What ensues are Alzheimer's and Parkinson's disease, amyotrophic lateral sclerosis and stroke with changed frequencies of effector T cell and reduced numbers or function of regulatory lymphocytes. The control of such immune responses could lead to new therapeutic strategies and the means to effectively combat a composite of diseases that have quite limited therapeutic options.

Dynamics of T cell responses after stroke

T cells are integral to the pathophysiology of stroke. The initial inflammatory cascade leads to T cell migration, which results in deleterious and protective effects mediated through CD4(+), CD(8)+, γδ T cells and regulatory T cells, respectively. Cytokines are central to the T cell responses, with key roles established for TNF-α, IFN-γ, IL-17, IL-21 and IL-10. Through communication with the systemic immune system via neural and hormonal pathways, there is also transient immunosuppression after severe strokes. With time, the inflammatory process eventually transforms to one more conducive of repair and recovery, though some evidence also suggests ongoing chronic inflammation. The role of antigen-specific T cell responses requires further investigation. As our understanding develops, there is increasing scope to modulate the T cell response after stroke.