Spatio-temporal profile, phenotypic diversity, and fate of recruited monocytes into the post-ischemic brain

Acute brain injuries trigger microglia as an additional source of the proteoglycan NG2

NG2 is a type I transmembrane glycoprotein known as chondroitin sulfate proteoglycan 4 (CSPG4). In the healthy central nervous system, NG2 is exclusively expressed by oligodendrocyte progenitor cells and by vasculature pericytes. A large body of immunohistochemical studies showed that under pathological conditions such as acute brain injuries and experimental autoimmune encephalomyelitis (EAE), a number of activated microglia were NG2 immuno-positive, suggesting NG2 expression in these cells. Alternative explanations for the microglial NG2 labeling consider the biochemical properties of NG2 or the phagocytic activity of activated microglia. Reportedly, the transmembrane NG2 proteoglycan can be cleaved by a variety of proteases to deposit the NG2 ectodomain into the extracellular matrix. The ectodomain, however, could also stick to the microglial surface. Since microglia are phagocytic cells engulfing debris of dying cells, it is difficult to identify a genuine expression of NG2. Recent studies showing (1) pericytes giving rise to microglial after stroke, and (2) immune cells of NG2-EYFP knock-in mice lacking NG2 expression in an EAE model generated doubts for the de novo expression of NG2 in microglia after acute brain injuries. In the current study, we took advantage of three knock-in mouse lines (NG2-CreERT2, CX₃CR₁-EGFP and NG2-EYFP) to study NG2 expression indicated by transgenic fluorescent proteins in microglia after tMCAO (transient middle cerebral artery occlusion) or cortical stab wound injury (SWI). We provide strong evidence that NG2-expressing cells, including OPCs and pericytes, did not differentiate into microglia after acute brain injuries, whereas activated microglia did express NG2 in a disease-dependent manner. A subset of microglia continuously activated the NG2 gene at least within the first week after tMCAO, whereas within 3 days after SWI a limited number of microglia at the lesion site transiently expressed NG2. Immunohistochemical studies demonstrated that these microglia with NG2 gene activity also synthesized the NG2 protein, suggesting activated microglia as an additional source of the NG2 proteoglycan after acute brain injuries.

Sustained Increases in Immune Transcripts and Immune Cell Trafficking During the Recovery of Experimental Brain Ischemia

BACKGROUND AND PURPOSE

Stroke is a major cause of chronic neurological disability. There is considerable interest in understanding how acute transcriptome changes evolve into subacute and chronic patterns that facilitate or limit spontaneous recovery. Here we mapped longitudinal changes in gene expression at multiple time points after stroke in mice out to 6 months.

METHODS

Adult C57BL/6 mice were subjected to transient middle cerebral artery occlusion. Longitudinal transcriptome levels were measured at 10 time points after stroke from acute to recovery phases of ischemic stroke. Localization and the number of mononuclear phagocytes were determined in the postischemic brain. Whole-mount brain imaging was performed in asplenic mice receiving GFP+ (green fluorescent protein)-tagged splenocytes.

RESULTS

Sustained stroke-induced mRNA abundance changes were observed in both hemispheres with 2989 ipsilateral and 822 contralateral genes significantly perturbed. In the hemisphere ipsilateral to the infarct, genes associated with immune functions were strongly affected, including temporally overlapping innate and adaptive immunity and macrophage M1 and M2 phenotype-related genes. The strong immune gene activation was accompanied by the sustained infiltration of peripheral immune cells at acute, subacute, and recovery stages of stroke. The infiltrated immune cells were found in the infarcted area but also in remote regions at 2 months after stroke.

CONCLUSIONS

The study identifies that immune components are the predominant molecular signatures and they may propagate or continuously respond to brain injury in the subacute to chronic phase after central nervous system injury. The study suggests a potential immune-based strategy to modify injury progression and tissue remodeling in ischemic stroke, even months after the initiating event.

Neuroprotective modulation of microglia effector functions following priming with interleukin 4 and 13: current limitations in understanding their mode-of-action

In recent years the long-standing theory of microglia's properties for dual polarization towards a pro- or anti-inflammatory phenotype has been deeply challenged. Furthermore, the elucidation of microglia ontogenesis exposed intrinsic differences between microglia and peripheral myeloid cells, thereby further underscoring the need to re-evaluate microglia-specific activation behavior, especially within an inflamed central nervous system (CNS) environment. This review critically summarizes recent literature on the in vitro and in vivo response of murine microglia to the immune-modulatory cytokines interleukin 4 (IL4) and interleukin 13 (IL13), i.e. those driving the so-called anti-inflammatory phenotype. Here we highlight several pivotal factors that may influence experimental outcome and/or interpretation of in vitro and in vivo studies evaluating microglia's phenotypical and functional properties upon IL4/IL13 treatment. Finally, the current therapeutic relevance of IL4/IL13-induced microglia activation in both acute and chronic CNS disorders is discussed.

Neuroimmune interaction in seizures and epilepsy: focusing on monocyte infiltration

Epilepsy is a major neurological condition that affects millions of people globally. While a number of interventions have been developed to mitigate this condition, a significant number of patients are refractory to these treatments. Consequently, other avenues of research are needed. One such avenue is modulation of the immune system response to this condition, which has mostly focused on microglia, the resident immune cells of the central nervous system (CNS). However, other immune cells can impact neurological conditions, principally blood-borne monocytes that can infiltrate into brain parenchyma after seizures. As such, this review will first discuss how monocytes can be recruited to the CNS and how they can be distinguished from there immunological cousins, microglia. Then, we will explore what is known about the role monocytes have within seizure pathogenesis and epilepsy. Considering how little is known about monocyte function in seizure- and epilepsy-related pathologies, further studies are warranted that investigate infiltrated blood-borne monocytes as a potential therapeutic target for epilepsy treatment.

The Roles of Monocyte and Monocyte-Derived Macrophages in Common Brain Disorders

The brain is the most important and complex organ in most living creatures which serves as the center of the nervous system. The function of human brain includes controlling of the motion of the body and different organs and maintaining basic homeostasis. The disorders of the brain caused by a variety of reasons often severely impact the patients' normal life or lead to death in extreme cases. Monocyte is an important immune cell which is often recruited to the brain in a number of brain disorders. However, the role of monocytes may not be simply described as beneficial or detrimental. It significantly depends on the disease models and the stages of disease progression. In this review, we summarized the current knowledge about the role of monocytes and monocyte-derived macrophages during several common brain disorders. Major focuses include ischemic stroke, Alzheimer's disease, multiple sclerosis, intracerebral hemorrhage, and insomnia. The recruitment, differentiation, and function of monocyte in these diseases are reviewed.

Immune responses to stroke: mechanisms, modulation, and therapeutic potential

Stroke is the second leading cause of death worldwide and a leading cause of disability. Most strokes are caused by occlusion of a major cerebral artery, and substantial advances have been made in elucidating how ischemia damages the brain. In particular, increasing evidence points to a double-edged role of the immune system in stroke pathophysiology. In the acute phase, innate immune cells invade brain and meninges and contribute to ischemic damage, but may also be protective. At the same time, danger signals released into the circulation by damaged brain cells lead to activation of systemic immunity, followed by profound immunodepression that promotes life-threatening infections. In the chronic phase, antigen presentation initiates an adaptive immune response targeted to the brain, which may underlie neuropsychiatric sequelae, a considerable cause of poststroke morbidity. Here, we briefly review these pathogenic processes and assess the potential therapeutic value of targeting immunity in human stroke.

Neonatal Stroke and TLR1/2 Ligand Recruit Myeloid Cells through the Choroid Plexus in a CX3CR1-CCR2- and Context-Specific Manner

Neonatal stroke is as frequent as stroke in the elderly, but many pathophysiological injury aspects are distinct in neonates, including immune signaling. While myeloid cells can traffic into the brain via multiple routes, the choroid plexus (CP) has been identified as a uniquely educated gate for immune cell traffic during health and disease. To understand the mechanisms of myeloid cell trafficking via the CP and their influence on neonatal stroke, we characterized the phenotypes of CP-infiltrating myeloid cells after transient middle cerebral artery occlusion (tMCAO) in neonatal mice of both sexes in relation to blood-brain barrier permeability, injury, microglial activation, and CX3CR1-CCR2 signaling, focusing on the dynamics early after reperfusion. We demonstrate rapid recruitment of multiple myeloid phenotypes in the CP ipsilateral to the injury, including inflammatory CD45⁺CD11b⁺Ly6c^highCD86⁺, beneficial CD45⁺CD11b⁺Ly6c^lowCD206⁺, and CD45⁺CD11b⁺Ly6c^lowLy6g^high cells, but only minor leukocyte infiltration into acutely ischemic-reperfused cortex and negligible vascular albumin leakage. We report that CX3CR1-CCR2-mediated myeloid cell recruitment contributes to stroke injury. Considering the complexity of inflammatory cascades triggered by stroke and a role for TLR2 in injury, we also used direct TLR2 stimulation as an independent injury model. TLR2 agonist rapidly recruited myeloid cells to the CP, increased leukocytosis in the CSF and blood, but infiltration into the cortex remained low over time. While the magnitude and the phenotypes of myeloid cells diverged between tMCAO and TLR2 stimulation, in both models, disruption of CX3CR1-CCR2 signaling attenuated both monocyte and neutrophil trafficking to the CP and cortex.SIGNIFICANCE STATEMENT Stroke during the neonatal period leads to long-term disabilities. The mechanisms of ischemic injury and inflammatory response differ greatly between the immature and adult brain. We examined leukocyte trafficking via the choroid plexus (CP) following neonatal stroke in relation to blood-brain barrier integrity, injury, microglial activation, and signaling via CX3CR1 and CCR2 receptors, or following direct TLR2 stimulation. Ischemia-reperfusion triggered marked unilateral CX3CR1-CCR2 dependent accumulation of diverse leukocyte subpopulations in the CP without inducing extravascular albumin leakage or major leukocyte infiltration into the brain. Disrupted CX3CR1-CCR2 signaling was neuroprotective in part by attenuating monocyte and neutrophil trafficking. Understanding the migratory patterns of CP-infiltrating myeloid cells with intact and disrupted CX3CR1-CCR2 signaling could identify novel therapeutic targets to protect the neonatal brain.

Immunoreactive Cells After Cerebral Ischemia

The immune system is rapidly activated after ischemic stroke. As immune cells migrate and infiltrate across the blood-brain barrier into the ischemic region, a cascade of cellular and molecular biological reactions occur, involving migrated immune cells, resident glial cells, and the vascular endothelium. These events regulate infarction evolution and thus influence the outcome of ischemic stroke. Most immune cells exert dual effects on cerebral ischemia, and some crucial cells may become central targets in ischemic stroke treatment and rehabilitation.

Mind the Map: Technology Shapes the Myeloid Cell Space

The myeloid cell system shows very high plasticity, which is crucial to quickly adapt to changes during an immune response. From the beginning, this high plasticity has made cell type classification within the myeloid cell system difficult. Not surprising, naming schemes have been frequently changed. Recent advancements in multidimensional technologies, including mass cytometry and single-cell RNA sequencing, are challenging our current understanding of cell types, cell subsets, and functional states of cells. Despite the power of these technologies to create new reference maps for the myeloid cell system, it is essential to put these new results into context with previous knowledge that was established over decades. Here we report on earlier attempts of cell type classification in the myeloid cell system, discuss current approaches and their pros and cons, and propose future strategies for cell type classification within the myeloid cell system that can be easily extended to other cell types.

CX3CR1-CCR2-dependent monocyte-microglial signaling modulates neurovascular leakage and acute injury in a mouse model of childhood stroke

Stroke is among the top 10 causes of death in children. The developmental stage of the brain is central to stroke pathophysiology. The incidence of childhood arterial ischemic stroke (CAIS) is lower than of perinatal arterial ischemic stroke but the rate of recurrence is strikingly high. Vascular inflammation is seen as major contributor to CAIS but the mechanisms that govern structural-functional basis of vascular abnormalities remain poorly understood. To identify the contribution of immune-neurovascular interactions to CAIS, we established stroke model in postnatal day 21 (P21) mice. We demonstrate acute functional deficits and histological injury and chronic MRI-identifiable injury, brain atrophy and marked derangements in the vascular network. In contrast to negligible albumin leakage and neutrophil infiltration following acute perinatal stroke, CAIS leads to significantly increased albumin leakage and neutrophil infiltration in injured regions of wild type mice and mice with functional CX3CR1-CCR2 receptors. In mice with dysfunctional CX3CR1-CCR2 signaling, extravascular albumin leakage is significantly attenuated, infiltration of injurious Ccr2⁺-monocytes essentially aborted, accumulation of Ly6G+ neutrophils reduced and acute injury attenuated. Unique identifiers of microglia and monocytes revealed phenotypic changes in each cell subtype of the monocyte lineage after CAIS. Taken together, CX3CR1-CCR2-dependent microglia-monocyte signaling contributes to cerebrovascular leakage, inflammation and CAIS injury.

Inflammation within the neurovascular unit: Focus on microglia for stroke injury and recovery

Neuroinflammation underlies the etiology of multiple neurodegenerative diseases and stroke. Our understanding of neuroinflammation has evolved in the last few years and major players have been identified. Microglia, the brain resident macrophages, are considered sentinels at the forefront of the neuroinflammatory response to different brain insults. Interestingly, microglia perform other physiological functions in addition to their role in neuroinflammation. Therefore, an updated approach in which modulation, rather than complete elimination of microglia is necessary. In this review, the emerging roles of microglia and their interaction with different components of the neurovascular unit are discussed. In addition, recent data on sex differences in microglial physiology and in the context of stroke will be presented. Finally, the multiplicity of roles assumed by microglia in the pathophysiology of ischemic stroke, and in the presence of co-morbidities such as hypertension and diabetes are summarized.

Corticosterone-Mediated Body Weight Loss Is an Important Catabolic Process for Poststroke Immunity and Survival

Background and Purpose- Stroke-induced acute severe body weight (BW) loss is associated with a high rate of mortality during a critical poststroke period. Several interventions to reduce weight loss, however, have not been successful. Currently, the biological significance of this extraordinary catabolic process is not well understood. Spleen-derived monocytes/macrophages (MMs) are the major immune cells recruited to the injured brain. The trafficking of MMs has been shown to be important for tissue repair and recovery. The purpose of the study is to investigate whether the BW reduction is essential for MM-mediated immune response for mice to survive and whether a corticosterone-mediated catabolic event underlies the processes. Methods- C57BL/6 male mice (12-week-old) were subjected to transient middle cerebral artery occlusion. BW, total MMs, and their Ly-6Chigh and Ly-6Clow subsets were determined in the spleen, blood, and the brain in poststroke mice. Poststroke survival rate and MM subsets were determined in mice with adrenalectomy, sham-adrenalectomy, and adrenalectomy mice supplemented with corticosterone. Results- Stroke reduced BW with a maximum reduction at day 3 poststroke (17.2±5.2%). The reduction at day 3 was positively linked to injury severity and selective depletion of MMs, but no other types of immune cells, in the spleen. Notably, the splenic MM depletion was significantly greater in mice with severe BW reduction (≥18% at day 3). In the blood, stroke depleted circulating MMs to a similar degree in animals with moderate and severe BW loss. Ly-6C+ monocyte infiltration in the poststroke brain was greater in mice with severe BW loss. Blocking the catabolic process by adrenalectomy significantly increased poststroke mortality, but the mortality was partially rescued by corticosterone supplement in adrenalectomy mice. Conclusions- Stroke-induced BW loss facilitates MM-mediated immune response, and the adrenal corticosterone-mediated catabolic process is necessary for poststroke survival. Visual Overview- An online visual overview is available for this article.

The changes in systemic monocytes in humans undergoing surgical decompression for degenerative cervical myelopathy may influence clinical neurological recovery

BACKGROUND

Degenerative cervical myelopathy (DCM) is the most common cause of non-traumatic spinal cord injury worldwide. Surgical decompression is recommended as the preferred treatment strategy for DCM as it halts disease progression and improves neurologic symptoms. We previously demonstrated that neuroinflammation, including monocytes, plays a critical role in the pathobiology of DCM and in ischemic-reperfusion injury (IRI) following surgical decompression. Monocytes are able to enter the spinal cord and brain tissues due to damage to the blood spinal cord and blood brain barrier following injury. Studies have demonstrated that stroke patients and individuals undergoing hip replacement surgery have increased systemic levels of monocytes. Additionally, changes in the signalling responses of monocytes are associated with post-surgical recovery or with ischemic neural tissue damage. Herein, we investigated the role of systemic monocytes as a predictive biomarker for clinical recovery following decompressive surgery for DCM.

FINDINGS

There was a 2-fold increase in the number of monocytes in DCM patients at 24 h following decompression as compared to baseline levels, which was associated with a significant improvement in the modified Japanese Orthopedic Association scale (mJOA) at 6-months after surgery (p < .0001). In a mouse model of DCM, depleting acute monocytes reduced the non-classical (Ly6C^low) subset from circulation (p < .05) and resulted in a 1.8-fold increase in CD11b expression in the spinal cord at 5 weeks following decompression. Acute monocyte depletion was accompanied by a modest decline in long-term overground locomotion, as evidenced by significantly reduced hindlimb swing speed.

CONCLUSIONS

This work demonstrated that decompressive surgery leads to an acute increase in peripheral monocytes in human DCM patients, which is modestly associated with clinical recovery. We anticipate that this work could contribute to the implementation of routine measurements of blood monocyte subsets, their activation state, and production of cytokines following decompressive surgery. This information could help to select perioperative anti-inflammatory treatments that can enhance the beneficial effects of decompressive surgery and reduce the incidence of post-operative complications, while avoiding a reduction in systemic monocytes.

Potential Immunotherapeutic Targets on Myeloid Cells for Neurovascular Repair After Ischemic Stroke

Neurological deficits and cognitive dysfunctions caused by acute ischemic stroke pose enormous burden to the stroke families and the communities. Restoration of the normal function of the neurovascular unit following ischemic stroke is critical for improving neurological recovery and cognitive functions after stroke. Recent evidence suggests that the myeloid cells including both the resident microglia and infiltrating monocytes/macrophages and neutrophils are highly plastic in response to the environmental cues. They intimately interact with multiple components of the neurovascular unit in response to the alarmins, danger associated pattern molecules (DAMPs) and other signals released from the ischemic brain. The aim of this review is to discuss the reciprocal interactions between the myeloid cells and the ischemic neurovascular unit during the late repair phase of cerebral ischemic stroke. We also summarize potential immunotherapeutic targets on myeloid cells and new therapeutic approaches targeting myeloid cells, such as cell transplantation, mitochondrial dynamic and extracellular vesicles-based therapy et al to enhance neurovascular repair for better stroke recovery.

Peripheral TREM1 responses to brain and intestinal immunogens amplify stroke severity

Stroke is a multiphasic process in which initial cerebral ischemia is followed by secondary injury from immune responses to ischemic brain components. Here we demonstrate that peripheral CD11b+CD45+ myeloid cells magnify stroke injury via activation of triggering receptor expressed on myeloid cells 1 (TREM1), an amplifier of proinflammatory innate immune responses. TREM1 was induced within hours after stroke peripherally in CD11b+CD45+ cells trafficking to ischemic brain. TREM1 inhibition genetically or pharmacologically improved outcome via protective antioxidant and anti-inflammatory mechanisms. Positron electron tomography imaging using radiolabeled antibody recognizing TREM1 revealed elevated TREM1 expression in spleen and, unexpectedly, in intestine. In the lamina propria, noradrenergic-dependent increases in gut permeability induced TREM1 on inflammatory Ly6C+MHCII+ macrophages, further increasing epithelial permeability and facilitating bacterial translocation across the gut barrier. Thus, following stroke, peripheral TREM1 induction amplifies proinflammatory responses to both brain-derived and intestinal-derived immunogenic components. Critically, targeting this specific innate immune pathway reduces cerebral injury.

Old Dog New Tricks; Revisiting How Stroke Modulates the Systemic Immune Landscape

Infections in the post-acute phase of cerebral ischaemia impede optimal recovery by exacerbating morbidity and mortality. Our review aims to reconcile the increased infection susceptibility of patients post-stroke by consolidating our understanding of compartmentalised alterations to systemic immunity. Mounting evidence has catalogued alterations to numerous immune cell populations but an understanding of the mechanisms of long-range communication between the immune system, nervous system and other organs beyond the involvement of autonomic signalling is lacking. By taking our cues from established and emerging concepts of neuro-immune interactions, immune-mediated inter-organ cross-talk, innate immune training and the role of microbiota-derived signals in central nervous system (CNS) function we will explore mechanisms of how cerebral ischaemia could shape systemic immune function. In this context, we will also discuss a key question: how are immune requirements critical for mediating repair of the ischaemic insult balanced by the need for anti-microbial immunity post-stroke, given that they are mediated by mutually exclusive immune networks? Our reformed understanding of the immune landscape post-stroke and novel mechanisms at play could guide targeted therapeutic interventions and initiate a step-change in the clinical management of these infectious complications post-stroke.

Phagocytosis in the Brain: Homeostasis and Disease

Microglia are resident macrophages of the central nervous system and significantly contribute to overall brain function by participating in phagocytosis during development, homeostasis, and diseased states. Phagocytosis is a highly complex process that is specialized for the uptake and removal of opsonized and non-opsonized targets, such as pathogens, apoptotic cells, and cellular debris. While the role of phagocytosis in mediating classical innate and adaptive immune responses has been known for decades, it is now appreciated that phagocytosis is also critical throughout early neural development, homeostasis, and initiating repair mechanisms. As such, modulating phagocytic processes has provided unexplored avenues with the intent of developing novel therapeutics that promote repair and regeneration in the CNS. Here, we review the functional consequences that phagocytosis plays in both the healthy and diseased CNS, and summarize how phagocytosis contributes to overall pathophysiological mechanisms involved in brain injury and repair.

Meningeal Mast Cells as Key Effectors of Stroke Pathology

Stroke is the leading cause of adult disability in the United States. Because post-stroke inflammation is a critical determinant of damage and recovery after stroke, understanding the interplay between the immune system and the brain after stroke holds much promise for therapeutic intervention. An understudied, but important aspect of this interplay is the role of meninges that surround the brain. All blood vessels travel through the meningeal space before entering the brain parenchyma, making the meninges ideally located to act as an immune gatekeeper for the underlying parenchyma. Emerging evidence suggests that the actions of immune cells resident in the meninges are essential for executing this gatekeeper function. Mast cells (MCs), best known as proinflammatory effector cells, are one of the long-term resident immune cells in the meninges. Here, we discuss recent findings in the literature regarding the role of MCs located in the meningeal space and stroke pathology. We review the latest advances in mouse models to investigate the roles of MCs and MC-derived products in vivo, and the importance of using these mouse models. We examine the concept of the meninges playing a critical role in brain and immune interactions, reevaluate the perspectives on the key effectors of stroke pathology, and discuss the opportunities and challenges for therapeutic development.

Experimental severe malaria is resolved by targeting newly-identified monocyte subsets using immune-modifying particles combined with artesunate

Current treatment of severe malaria and associated cerebral malaria (CM) and respiratory distress syndromes are directed primarily at the parasite. Targeting the parasite has only partial efficacy in advanced infection, as neurological damage and respiratory distress are due to accumulation of host blood cells in the brain microvasculature and lung interstitium. Here, computational analysis identifies Ly6C^lo monocytes as a major component of the immune infiltrate in both organs in a preclinical mouse model. Specifically targeting Ly6C^lo monocyte precursors, identified by adoptive transfer, with immune-modifying particles (IMP) prevents experimental CM (ECM) in 50% of Plasmodium berghei ANKA-infected mice in early treatment protocols. Furthermore, treatment at onset of clinical ECM with 2 doses of a novel combination of IMP and anti-malarial drug artesunate results in 88% survival. This combination confers protection against ECM and mortality in late stage severe experimental malaria and provides a viable advance on current treatment regimens.

Myeloid cells as therapeutic targets in neuroinflammation after stroke: Specific roles of neutrophils and neutrophil-platelet interactions

Ischemic brain injury causes a local inflammatory response, involving the activation of resident brain cells such as microglia and the recruitment of infiltrating immune cells. Increasing evidence supports that plasticity of the myeloid cell lineage is determinant for the specific role of these cells on stroke outcome, from initiation and maintenance to resolution of post-ischemic inflammation. The aim of this review is to summarize some of the key characteristics of these cells and the mechanisms for their recruitment into the injured brain through interactions with platelets, endothelial cells and other leukocytes. Also, we discuss the existence of different leukocyte subsets in the ischemic tissue and, specifically, the impact of different myeloid phenotypes on stroke outcome, with special emphasis on neutrophils and their interplay with platelets. Knowledge of these cellular phenotypes and interactions may pave the way to new therapies able to promote protective immune responses and tissue repair after cerebral ischemia.

Screening of biopolymeric materials for cardiovascular surgery toxicity-Evaluation of their surface relief with assessment of morphological aspects of monocyte/macrophage polarization in atherosclerosis patients

The morphotypes of human macrophages (MPh) were studied in the culture on nano-structured biopolymer substrates, made from polyhydroxyalcanoates (PHAs) of five various monomer compositions, followed by the solvent evaporation. Its surface relief, which was further in direct contact with human cells in vitro, was analyzed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). It was shown, that the features of the micro/nano relief depend on the monomeric composition of the polymer substrates. Monocytes (MN) of patients with atherosclerosis and cardiac ischemia, undergoing stenting and conventional anti-atherosclerotic therapy, were harvested prior and after stenting. MN were isolated and cultured, with the transformation into MPh in direct contact with biopolymer culture substrates with different monomer composition and nano-reliefs, and transformed into MPh, in comparison with the same process on standard culture plastic. Sub-populations of cells with characteristic morphology in each phenotypic class were described, and their quantitative ratios for each sample of polymers were counted as an intermediate result in the development of "smart" material for cardiovascular devices. The results obtained allow us to assume, that the processes of MPh differentiation and polarization in vitro depend not only on the features of the micro/nano relief of biopolymer substrates, but also on the initial state of MN in vivo and general response of patients.

Intravenous Transplantation of Mesenchymal Stem Cells Reduces the Number of Infiltrated Ly6C+ Cells but Enhances the Proportions Positive for BDNF, TNF-1α, and IL-1β in the Infarct Cortices of dMCAO Rats

The resident microglial and infiltrating cells from peripheral circulation are involved in the pathological processes of ischemia stroke and may be regulated by mesenchymal stem/stromal cell (MSC) transplantation. The present study is aimed at differentiating the neurotrophic and inflammatory roles played by microglial vs. infiltrating circulation-derived cells in the acute phase in rat ischemic brains and explore the influences of intravenously infused allogeneic MSCs. The ischemic brain injury was induced by distal middle cerebral artery occlusion (dMCAO) in SD rats, with or without MSC infusion in the same day following dMCAO. Circulation-derived infiltrating cells in the brain were identified by Ly6C, a majority of which were monocytes/macrophages. Without MSC transplantation, among the infiltrated Ly6C⁺ cells, some were positive for BDNF, IL-1β, or TNF-α. Following MSC infusion, the overall number of Ly6C⁺ infiltrated cells was reduced by 50%. In contrast, the proportions of infiltrated Ly6C⁺ cells coexpressing BDNF, IL-1β, or TNF-α were significantly enhanced. Interestingly, Ly6C⁺ cells in the infarct area could produce either neurotrophic factor BDNF or inflammatory cytokines (IL-1β or TNF-α), but not both. This suggests that the Ly6C⁺ cells may constitute heterogeneous populations which react differentially to the microenvironments in the infarct area. The changes in cellular composition in the infarct area may have contributed to the beneficial effect of MSC transplantation.

Pivotal role of innate myeloid cells in cerebral post-ischemic sterile inflammation

Inflammatory responses play a multifaceted role in regulating both disability and recovery after ischemic brain injury. In the acute phase of ischemic stroke, resident microglia elicit rapid inflammatory responses by the ischemic milieu. After disruption of the blood-brain barrier, peripheral-derived neutrophils and mononuclear phagocytes infiltrate into the ischemic brain. These infiltrating myeloid cells are activated by the endogenous alarming molecules released from dying brain cells. Inflammation after ischemic stroke thus typically consists of sterile inflammation triggered by innate immunity, which exacerbates the pathologies of ischemic stroke and worsens neurological prognosis. Infiltrating immune cells sustain the post-ischemic inflammation for several days; after this period, however, these cells take on a repairing function, phagocytosing inflammatory mediators and cellular debris. This time-specific polarization of immune cells in the ischemic brain is a potential novel therapeutic target. In this review, we summarize the current understanding of the phase-dependent role of innate myeloid cells in ischemic stroke and discuss the cellular and molecular mechanisms of their inflammatory or repairing polarization from a therapeutic perspective.

Ischemia-reperfusion injury in stroke: impact of the brain barriers and brain immune privilege on neutrophil function

Reperfusion injury following ischemic stroke is a complex pathophysiological process involving numerous mechanisms ranging from the release of excitatory amino acids and ion disequilibrium to the induction of apoptosis and necrosis, to oxidative stress and inflammation. The migration of neutrophils into the brain parenchyma and release of their abundant proteases are generally considered the main cause of neuronal cell death and acute reperfusion injury following ischemic stroke. Recent findings in experimental and human stroke have challenged this view, as the majority of neutrophils were rather found to accumulate within the neurovascular unit (NVU) and the subarachnoid space (SAS) where they remain separated from the brain parenchyma by the glia limitans. The brain parenchyma is an immune-privileged site that is not readily accessible to immune cells and does not elicit stereotypic adaptive or innate immune responses. Understanding brain immune privilege requires intimate knowledge of its unique anatomy in which the brain barriers, that include the glia limitans, establish compartments that differ remarkably with regard to their accessibility to the immune system. We here propose that the brain immune privilege also extends to an ischemic insult, where the brain parenchyma does not evoke a rapid infiltration of neutrophils as observed in ischemic events in peripheral organs. Rather, neutrophil accumulation in the NVU and SAS could have a potential impact on cerebrospinal fluid (CSF) drainage from the central nervous system (CNS) and thus on edema formation and reperfusion injury after ischemic stroke. Integrating the anatomical and functional implications of the brain immune privilege with the unquestionable role of neutrophils in reperfusion injury is a prerequisite to exploit appropriate strategies for therapeutic interventions aiming to reduce neuronal cell death after ischemic stroke.

Endogenous Protection from Ischemic Brain Injury by Preconditioned Monocytes

Exposure to low-dose lipopolysaccharide (LPS) before cerebral ischemia is neuroprotective in stroke models, a phenomenon termed preconditioning (PC). Although it is well established that LPS-PC induces central and peripheral immune responses, the cellular mechanisms modulating ischemic injury remain unclear. Here, we investigated the role of immune cells in the brain protection afforded by PC and tested whether monocytes may be reprogrammed by ex vivo LPS exposure, thus modulating inflammatory injury after cerebral ischemia in male mice. We found that systemic injection of low-dose LPS induces a Ly6C^hi monocyte response that protects the brain after transient middle cerebral artery occlusion (MCAO) in mice. Remarkably, adoptive transfer of monocytes isolated from preconditioned mice into naive mice 7 h after transient MCAO reduced brain injury. Gene expression and functional studies showed that IL-10, inducible nitric oxide synthase, and CCR2 in monocytes are essential for neuroprotection. This protective activity was elicited even if mouse or human monocytes were exposed ex vivo to LPS and then injected into male mice after stroke. Cell-tracking studies showed that protective monocytes are mobilized from the spleen and reach the brain and meninges, where they suppress postischemic inflammation and neutrophil influx into the brain parenchyma. Our findings unveil a previously unrecognized subpopulation of splenic monocytes capable of protecting the brain with an extended therapeutic window and provide the rationale for cell therapies based on the delivery of autologous or allogeneic protective monocytes in patients after ischemic stroke.SIGNIFICANCE STATEMENT Inflammation is a key component of the pathophysiology of the brain in stroke, a leading cause of death and disability with limited therapeutic options. Here, we investigate endogenous mechanisms of protection against cerebral ischemia. Using lipopolysaccharide (LPS) preconditioning (PC) as an approach to induce ischemic tolerance in mice, we found generation of neuroprotective monocytes within the spleen, from which they traffic to the brain and meninges, suppressing postischemic inflammation. Importantly, systemic LPS-PC can be mimicked by adoptive transfer of in vitro-preconditioned mouse or human monocytes at translational relevant time points after stroke. This model of neuroprotection may facilitate clinical efforts to increase the efficacy of BM mononuclear cell treatments in acute neurological diseases such as cerebral ischemia.

Targeted intracerebral delivery of the anti-inflammatory cytokine IL13 promotes alternative activation of both microglia and macrophages after stroke

Background

Subtle adjustment of the activation status of CNS resident microglia and peripheral macrophages, to promote their neuroprotective and neuroregenerative functions, may facilitate research towards curing neurodegenerative disorders. In the present study, we investigated whether targeted intracerebral delivery of the anti-inflammatory cytokine interleukin (IL)13, by means of transplanting IL13-expressing mesenchymal stem cells (IL13-MSCs), can promote a phenotypic switch in both microglia and macrophages during the pro-inflammatory phase in a mouse model of ischemic stroke.

Methods

We used the CX₃CR1^eGFP/+ CCR2^RFP/+ transgenic mouse model to separately recognize brain-resident microglia from infiltrated macrophages. Quantitative immunohistochemical analyses were applied to characterize polarization phenotypes of both cell types.

Results

Distinct behaviors of both cell populations were noted dependent on the anatomical site of the lesion. Immunohistochemistry revealed that mice grafted with IL13-MSCs, in contrast to non-grafted and MSC-grafted control mice, were able to drive recruited microglia and macrophages into an alternative activation state, as visualized by a significant increase of Arg-1 and a noticeable decrease of MHC-II expression at day 14 after ischemic stroke. Interestingly, both Arg-1 and MHC-II were expressed more abundantly in macrophages than in microglia, further confirming the distinct behavior of both cell populations.

Conclusions

The current data highlight the importance of controlled and localized delivery of the anti-inflammatory cytokine IL13 for modulation of both microglia and macrophage responses after ischemic stroke, thereby providing pre-clinical rationale for the application of L13-MSCs in future investigations of neurodegenerative disorders.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1212-7) contains supplementary material, which is available to authorized users.

Potential immunotherapies for traumatic brain and spinal cord injury

Traumatic injury of the central nervous system (CNS) including brain and spinal cord remains a leading cause of morbidity and disability in the world. Delineating the mechanisms underlying the secondary and persistent injury versus the primary and transient injury has been drawing extensive attention for study during the past few decades. The sterile neuroinflammation during the secondary phase of injury has been frequently identified substrate underlying CNS injury, but as of now, no conclusive studies have determined whether this is a beneficial or detrimental role in the context of repair. Recent pioneering studies have demonstrated the key roles for the innate and adaptive immune responses in regulating sterile neuroinflammation and CNS repair. Some promising immunotherapeutic strategies have been recently developed for the treatment of CNS injury. This review updates the recent progress on elucidating the roles of the innate and adaptive immune responses in the context of CNS injury, the development and characterization of potential immunotherapeutics, as well as outstanding questions in this field.

Diverse Inflammatory Response After Cerebral Microbleeds Includes Coordinated Microglial Migration and Proliferation

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Background and Purpose—

Cerebral microbleeds are linked to cognitive decline, but it remains unclear how they impair neuronal function. Infarction is not typically observed near microbleeds, suggesting more subtle mechanisms, such as inflammation, may play a role. Because of their small size and largely asymptomatic nature, real-time detection and study of spontaneous cerebral microbleeds in humans and animal models are difficult.

Methods—

We used in vivo 2-photon microscopy through a chronic cranial window in adult mice to follow the inflammatory response after a cortical microhemorrhage of ≈100 µm diameter, induced by rupturing a targeted cortical arteriole with a laser.

Results—

The inflammatory response included the invasion of blood-borne leukocytes, the migration and proliferation of brain-resident microglia, and the activation of astrocytes. Nearly all inflammatory cells responding to the microhemorrhage were brain-resident microglia, but a small number of CX3CR1⁺ and CCR2⁺ macrophages, ultimately originating from the invasion of blood-borne monocytes, were also found near the lesion. We found a coordinated pattern of microglia migration and proliferation, where microglia within 200 µm of the microhemorrhage migrated toward the lesion over hours to days. In contrast, microglia proliferation was not observed until ≈40 hours after the lesion and occurred primarily in a shell-shaped region where the migration of microglia decreased their local density. These data suggest that local microglia density changes may trigger proliferation. Astrocytes activated in a similar region as microglia but delayed by a few days. By 2 weeks, this inflammatory response had largely resolved.

Conclusions—

Although microhemorrhages are small in size, the brain responds to a single bleed with an inflammatory response that involves brain-resident and blood-derived cells, persists for weeks, and may impact the adjacent brain microenvironment.

Immune responses in stroke: how the immune system contributes to damage and healing after stroke and how this knowledge could be translated to better cures?

Stroke is one of the leading causes of death and disability worldwide. The long-standing dogma that stroke is exclusively a vascular disease has been questioned by extensive clinical findings of immune factors that are associated mostly with inflammation after stroke. These have been confirmed in preclinical studies using experimental animal models. It is now accepted that inflammation and immune mediators are critical in acute and long-term neuronal tissue damage and healing following thrombotic and ischaemic stroke. Despite mounting information delineating the role of the immune system in stroke, the mechanisms of how inflammatory cells and their mediators are involved in stroke-induced neuroinflammation are still not fully understood. Currently, there is no available treatment for targeting the acute immune response that develops in the brain during cerebral ischaemia. No new treatment has been introduced to stroke therapy since the discovery of tissue plasminogen activator therapy in 1996. Here, we review current knowledge of the immunity of stroke and identify critical gaps that hinder current therapies. We will discuss advances in the understanding of the complex innate and adaptive immune responses in stroke; mechanisms of immune cell-mediated and factor-mediated vascular and tissue injury; immunity-induced tissue repair; and the importance of modulating immunity in stroke.

Generation of Hutat2:Fc Knockin Primary Human Monocytes Using CRISPR/Cas9

The ability of monocytes to travel through the bloodstream, traverse tissue barriers, and aggregate at disease sites endows these cells with the attractive potential to carry therapeutic genes into the nervous system. However, gene editing in primary human monocytes has long been a challenge. Here, we applied the CRISPR/Cas9 system to deliver the large functional Hutat2:Fc DNA fragment into the genome of primary monocytes to neutralize HIV-1 transactivator of transcription (Tat), an essential neurotoxic factor that causes HIV-associated neurocognitive disorder (HAND) in the nervous system. Following homology-directed repair (HDR), ∼10% of the primary human monocytes exhibited knockin of the Hutat2:Fc gene in the AAVS1 locus, the "safe harbor" locus of the human genome, without selection. Importantly, the release of Hutat2:Fc by these modified monocytes protected neurons from Tat-induced neurotoxicity, reduced HIV replication, and restored T cell homeostasis. Moreover, compared with lentiviral transfection, CRISPR-mediated knockin had the advantage of maintaining the migrating function of monocytes. These results establish CRISPR/Cas9-mediated Hutat2:Fc knockin monocytes and provide a potential method to cross the blood-brain barrier for HAND therapy.

Usage of Multiparameter Flow Cytometry to Study Microglia and Macrophage Heterogeneity in the Central Nervous System During Neuroinflammation and Neurodegeneration

The resident macrophages of the central nervous system (CNS), also known as microglia, and blood-derived macrophages play an important role in the functional activity of the normal CNS, as well as in the development of neuroinflammation during various neurodegenerative disorders. Microglia and macrophages represent heterogeneous populations, which can modulate CNS environment and have different effects on neuronal regeneration. In this chapter, the main features of microglial and macrophage subsets and current methods for investigation of their heterogeneity will be discussed.

Early treatment with Resolvin E1 facilitates myocardial recovery from ischaemia in mice

BACKGROUND AND PURPOSE

An appropriate inflammatory response is necessary for cardiac healing after acute myocardial infarction (MI). Resolvin E1 (RvE1) is an anti-inflammatory and pro-resolution lipid mediator derived from eicosapentaenoic acid. Here we have investigated the effects of RvE1 on the recovery of cardiac function after MI in mice.

EXPERIMENTAL APPROACH

Acute MI was induced by surgical ligation of the left anterior descending artery in male C57BL/6 mice. RvE1 (5 ng·g^-1 ·day^-1 ; i.p.) was given to mice at different times following MI. Cardiac function was monitored by transthoracic echocardiography at days 3, 7 and 14 after MI. Effects of RvE1 on the migration of subpopulations of monocytes/macrophages (Mos/Mps, Ly6C^hi and Ly6C^low ) were examined by flow cytometry and transwell assay.

KEY RESULTS

RvE1 administration from days 1 to 7 post-MI improved cardiac function, whereas treatment from days 7 to 14 markedly inhibited recovery of cardiac function. Early treatment with RvE1 post-MI suppressed the infiltration of dominant Ly6C^hi Mos/Mps and secretion of pro-inflammatory cytokines in injured hearts, which protected cardiomyocytes against apoptosis in the peri-infarct zones. Contrastingly, treatment with RvE1 1 week after MI decreased infiltration of Ly6C^low Mos/Mps and expression of pro-angiogenic factors in cardiac tissue, consequently reducing neovascularization in the peri-infarct zones. Additionally, RvE1 inhibited Mp migration by activating ChemR23 receptors.

CONCLUSION AND IMPLICATIONS

Treatment with RvE1 during the initial 7 days after MI facilitated cardiac healing by suppressing pro-inflammatory cytokine secretion, indicating that RvE1 may serve as an early therapeutic agent for acute MI.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

Myeloid but not epithelial tissue factor exerts protective anti-inflammatory effects in acid aspiration-induced acute lung injury

Essentials Tissue factor (TF) represents a central link between hemostasis and inflammation. We studied the roles of myeloid and airway epithelial TF in acid-caused acute lung injury (ALI). TF on myeloid cells displays a non-coagulatory role regulating the inflammatory response in ALI. Airway epithelial TF contributes to hemostatic functions, but is dispensable in ALI pathogenesis.

SUMMARY

Introduction Acute lung injury (ALI) is a life-threatening condition characterized by damaged alveolar-capillary structures and activation of inflammatory and hemostatic processes. Tissue factor (TF) represents a crucial link between inflammation and coagulation, as inflammatory mediators induce myeloid TF expression, and TF initiates extrinsic coagulation. Objective As pulmonary inflammation stimulates TF expression and TF modulates immune responses, we aimed to elucidate its impact on ALI. In particular, we wanted to distinguish the contributions of TF expressed on airway epithelial cells and TF expressed on myeloid cells. Methods Mice with different cell type-specific TF deficiency and wild-type littermates were intratracheally treated with hydrochloric acid, and leukocyte recruitment, cytokine levels, thrombin-antithrombin (TAT) complexes and pulmonary protein-rich infiltrates were analyzed. Results Our data demonstrate that a lack of epithelial TF did not influence acute responses, as bronchoalveolar neutrophil accumulation 8 h after ALI induction was unaltered. However, it led to mild, prolonged inflammation, as pulmonary leukocyte and erythrocyte numbers were still increased after 24 h, whereas those in wild-type mice had returned to basal levels. In contrast, myeloid TF was primarily involved in regulating the acute phase of ALI without affecting local coagulation, as indicated by increased bronchoalveolar neutrophil infiltration, pulmonary interleukin-6 levels, and edema formation, but equal TAT complex formation, 8 h after ALI induction. This augmented inflammatory response associated with myeloid TF deficiency was confirmed in vitro, as lipopolysaccharide-stimulated TF-deficient alveolar macrophages released increased levels of chemokine (C-X-C motif) ligand 1 and tumor necrosis factor-α as compared with wild-type macrophages. Conclusion We conclude that myeloid TF dampens inflammation in acid-induced ALI.

Homology analysis detects topological changes of Iba1 localization accompanied by microglial activation

The state of microglial activation provides important information about the central nervous system. However, a reliable index of microglial activation in histological samples has yet to be established. Here, we show that microglial activation induces topological changes of Iba1 localization that can be detected by analysis based on homology theory. Analysis of homology was applied to images of Iba1-stained tissue sections, and the 0-dimentional Betti number (b0: the number of solid components) and the 1-dimentional Betti number (b1: the number of windows surrounded by solid components) were obtained. We defined b1/b0 as the Homology Value (HV), and investigated its validity as an index of microglial activation using cerebral ischemia model mice. Microglial activation was accompanied by changes to Iba1 localization and morphology of microglial processes. In single microglial cells, the change of Iba1 localization increased b1. Conversely, thickening or retraction of microglial processes decreased b0. Consequently, microglial activation increased the HV. The HV of a tissue area increased with proximity to the ischemic core and showed a high degree of concordance with the number of microglia expressing activation makers. Furthermore, the HV of human metastatic brain tumor tissue also increased with proximity to the tumor. These results suggest that our index, based on homology theory, can be used to correctly evaluate microglial activation in various tissue images.