CCR2 expression by brain microvascular endothelial cells is critical for macrophage transendothelial migration in response to CCL2

Monocyte Count and Systemic Immune-Inflammation Index Score as Predictors of Delayed Cerebral Ischemia after Aneurysmal Subarachnoid Hemorrhage

OBJECTIVE

Delayed cerebral ischemia (DCI) is a major cause of disability in patients who survive aneurysmal subarachnoid hemorrhage (aSAH). Systemic inflammatory markers, such as peripheral leukocyte count and systemic immune-inflammatory index (SII) score, have been considered predictors of DCI in previous studies. This study aims to investigate which systemic biomarkers are significant predictors of DCI.

METHODS

We conducted a retrospective, observational, single-center study of 170 patients with SAH admitted between May 2018 and March 2022. We analyzed the patients' clinical and laboratory parameters within 1 hour and 3-4 and 5-7 days after admission. The DCI and non-DCI groups were compared. Variables showing statistical significance in the univariate logistic analysis (p<0.05) were entered into a multivariate regression model.

RESULTS

Hunt-Hess grade "4-5" at admission, modified Fisher scale grade "3-4" at admission, hydrocephalus, intraventricular hemorrhage, and infection showed statistical significance (p<0.05) on a univariate logistic regression. Lymphocyte and monocyte count at admission, SII scores and C-reactive protein levels on days 3-4, and leukocyte and neutrophil counts on days 5-7 exhibited statistical significance on the univariate logistic regression. Multivariate logistic regression analysis revealed that monocyte count at admission (odds ratio [OR], 1.64; 95% confidence interval [CI], 1.04-2.65; p=0.036) and SII score at days 3-4 (OR, 1.55; 95% CI, 1.02-2.47; p=0.049) were independent predictors of DCI.

CONCLUSION

Monocyte count at admission and SII score 3-4 days after rupture are independent predictors of clinical deterioration caused by DCI after aSAH. Peripheral monocytosis may be the primer for the innate immune reaction, and the SII score at days 3-4 can promptly represent the propagated systemic immune reaction toward DCI.

Not just Glia-Dissecting brain macrophages in the mouse

Macrophages have emerged as critical cellular components of the central nervous system (CNS), promoting development, maintenance, and immune defense of the CNS. Here we will review recent advances in our understanding of brain macrophage heterogeneity, including microglia and border-associated macrophages, focusing on the mouse. Emphasis will be given to the discussion of strengths and limitations of the experimental approaches that have led to the recent insights and hold promise to further deepen our mechanistic understanding of brain macrophages that might eventually allow to harness their activities for the management of CNS pathologies.

Inherent Susceptibility to Acquired Epilepsy in Selectively Bred Rats Influences the Acute Response to Traumatic Brain Injury

Traumatic brain injury (TBI) often causes seizures associated with a neuroinflammatory response and neurodegeneration. TBI responses may be influenced by differences between individuals at a genetic level, yet this concept remains understudied. Here, we asked whether inherent differences in one's vulnerability to acquired epilepsy would determine acute physiological and neuroinflammatory responses acutely after experimental TBI, by comparing selectively bred "seizure-prone" (FAST) rats with "seizure-resistant" (SLOW) rats, as well as control parental strains (Long Evans and Wistar rats). Eleven-week-old male rats received a moderate-to-severe lateral fluid percussion injury (LFPI) or sham surgery. Rats were assessed for acute injury indicators and neuromotor performance, and blood was serially collected. At 7 days post-injury, brains were collected for quantification of tissue atrophy by cresyl violet (CV) histology, and immunofluorescent staining of activated inflammatory cells. FAST rats showed an exacerbated physiological response acutely post-injury, with a 100% seizure rate and mortality within 24 h. Conversely, SLOW rats showed no acute seizures and a more rapid neuromotor recovery compared with controls. Brains from SLOW rats also showed only modest immunoreactivity for microglia/macrophages and astrocytes in the injured hemisphere compared with controls. Further, group differences were apparent between the control strains, with greater neuromotor deficits observed in Long Evans rats compared with Wistars post-TBI. Brain-injured Long Evans rats also showed the most pronounced inflammatory response to TBI across multiple brain regions, whereas Wistar rats showed the greatest extent of regional brain atrophy. These findings indicate that differential genetic predisposition to develop acquired epilepsy (i.e., FAST vs. SLOW rat strains) determines acute responses after experimental TBI. Differences in the neuropathological response to TBI between commonly used control rat strains is also a novel finding, and an important consideration for future study design. Our results support further investigation into whether genetic predisposition to acute seizures predicts the chronic outcomes after TBI, including the development of post-traumatic epilepsy.

Quantitative Model Analysis and Simulation of Pharmacokinetics and Metastasis-Associated Lung Adenocarcinoma 1 RNA Knockdown Effect After Systemic Administration of Cholesterol-Conjugated DNA/RNA Heteroduplex Oligonucleotide Crossing Blood-Brain Barrier of Mice

The cholesterol-conjugated heteroduplex oligonucleotide (Chol-HDO) is a double-stranded complex; it comprises an antisense oligonucleotide (ASO) and its complementary strand with a cholesterol ligand. Chol-HDO is a powerful tool for achieving target RNA knockdown in the brains of mice after systemic injection. Here, a quantitative model analysis was conducted to characterize the relationship between the pharmacokinetics (PK) and pharmacodynamics (PD), non-coding RNA metastasis-associated lung adenocarcinoma 1 (Malat1) RNA, of Chol-HDO, in a time-dependent manner. The established PK model could describe regional differences in the observed brain concentration-time profiles. Incorporating the PD model enabled the unique knockdown profiles in the brain to be explained in terms of the time delay after single dosing and enhancement following repeated dosing. Moreover, sensitivity analysis of PK exposure/persistency, target RNA turnover, and knockdown potency identified key factors for the efficient and sustained target RNA knockdown in the brain. The simulation of an adequate dosing regimen quantitatively supported the benefit of Chol-HDO in terms of achieving a suitable dosing interval. This was achieved via sufficient and sustained brain exposure and subsequent strong and sustained target RNA knockdown in the brain, even after systemic injection. The present study provides new insights into drug discoveries and development strategies for HDO in patients with neurogenic disorders. SIGNIFICANCE STATEMENT: The quantitative model analysis presented here characterized the PK/PD relationship of Chol-HDO, enabled its simulation under various conditions or assumptions, and identified key factors for efficient and sustained RNA knockdown, such as PK exposure and persistency. Chol-HDO appears to be an efficient drug delivery system for the systemic administration of desired drugs to brain targets.

Transport of the Proinflammatory Chemokines C-C Motif Chemokine Ligand 2 (MCP-1) and C-C Motif Chemokine Ligand 5 (RANTES) across the Intact Mouse Blood-Brain Barrier Is Inhibited by Heparin and Eprodisate and Increased with Systemic Inflammation

One important function of the vascular blood-brain barrier (BBB) is to facilitate neuroimmune communication. The BBB fulfills this function, in part, through its ability to transport cytokines and chemokines. C-C motif chemokine receptor 2 (CCL2) (MCP-1) and C-C motif chemokine receptor 5 (CCL5) (RANTES) are proinflammatory chemokines that mediate neuroimmune responses to acute insults and aspects of brain injury and neurodegenerative diseases; however, a blood-to-brain transport system has not been evaluated for either chemokine in vivo. Therefore, we determined whether CCL2 and CCL5 in blood can cross the intact BBB and enter the brain. Using CD-1 mice, we found that ¹²⁵I-labeled CCL2 and CCL5 crossed the BBB and entered the brain parenchyma. We next aimed to identify the mechanisms of ¹²⁵I-CCL2 and ¹²⁵I-CCL5 transport in an in situ brain perfusion model. We found that both heparin and eprodisate inhibited brain uptake of ¹²⁵I-CCL2 and ¹²⁵I-CCL5 in situ, whereas antagonists of their receptors, CCR2 or CCR5, respectively, did not, suggesting that heparan sulfates at the endothelial surface mediate BBB transport. Finally, we showed that CCL2 and CCL5 transport across the BBB increased following a single injection of 0.3 mg/kg lipopolysaccharide. These data demonstrate that CCL2 and CCL5 in the brain can derive, in part, from the circulation, especially during systemic inflammation. Further, binding to the BBB-associated heparan sulfate is a mechanism by which both chemokines can cross the intact BBB, highlighting a novel therapeutic target for treating neuroinflammation. SIGNIFICANCE STATEMENT: Our work demonstrates that C-C motif chemokine ligand 2 (CCL2) and C-C motif chemokine ligand 5 (CCL5) can cross the intact blood-brain barrier and that transport is robustly increased during inflammation. These data suggest that circulating CCL2 and CCL5 can contribute to brain levels of each chemokine. We further show that the transport of both chemokines is inhibited by heparin and eprodisate, suggesting that CCL2/CCL5-heparan sulfate interactions could be therapeutically targeted to limit accumulation of these chemokines in the brain.

Microglia-derived CCL2 has a prime role in neocortex neuroinflammation

Background

In myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE), several areas of demyelination are detectable in mouse cerebral cortex, where neuroinflammation events are associated with scarce inflammatory infiltrates and blood–brain barrier (BBB) impairment. In this condition, the administration of mesenchymal stem cells (MSCs) controls neuroinflammation, attenuating astrogliosis and promoting the acquisition of stem cell traits by astrocytes. To contribute to the understanding of the mechanisms involved in the pathogenesis of EAE in gray matter and in the reverting effects of MSC treatment, the neocortex of EAE-affected mice was investigated by analyzing the cellular source(s) of chemokine CCL2, a molecule involved in immune cell recruitment and BBB-microvessel leakage.

Methods

The study was carried out by immunohistochemistry (IHC) and dual RNAscope IHC/in situ hybridization methods, using astrocyte, NG2-glia, macrophage/microglia, and microglia elective markers combined with CCL2.

Results

The results showed that in EAE-affected mice, hypertrophic microglia are the primary source of CCL2, surround the cortex neurons and the damaged BBB microvessels. In EAE-affected mice treated with MSCs, microgliosis appeared diminished very soon (6 h) after treatment, an observation that was long-lasting (tested after 10 days). This was associated with a reduced CCL2 expression and with apparently preserved/restored BBB features. In conclusion, the hallmark of EAE in the mouse neocortex is a condition of microgliosis characterized by high levels of CCL2 expression.

Conclusions

This finding supports relevant pathogenetic and clinical aspects of the human disease, while the demonstrated early control of neuroinflammation and BBB permeability exerted by treatment with MSCs may have important therapeutic implications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00365-5.

Unlocking the Untapped Potential of Endothelial Kinase and Phosphatase Involvement in Sepsis for Drug Treatment Design

Sepsis is a devastating clinical condition that can lead to multiple organ failure and death. Despite advancements in our understanding of molecular mechanisms underlying sepsis and sepsis-associated multiple organ failure, no effective therapeutic treatment to directly counteract it has yet been established. The endothelium is considered to play an important role in sepsis. This review highlights a number of signal transduction pathways involved in endothelial inflammatory activation and dysregulated endothelial barrier function in response to sepsis conditions. Within these pathways – NF-κB, Rac1/RhoA GTPases, AP-1, APC/S1P, Angpt/Tie2, and VEGF/VEGFR2 – we focus on the role of kinases and phosphatases as potential druggable targets for therapeutic intervention. Animal studies and clinical trials that have been conducted for this purpose are discussed, highlighting reasons why they might not have resulted in the expected outcomes, and which lessons can be learned from this. Lastly, opportunities and challenges that sepsis and sepsis-associated multiple organ failure research are currently facing are presented, including recommendations on improved experimental design to increase the translational power of preclinical research to the clinic.

Harnessing retinal phagocytes to combat pathological neovascularization in ischemic retinopathies?

Ischemic retinopathies (IR) are vision-threatening diseases that affect a substantial amount of people across all age groups worldwide. The current treatment options of photocoagulation and anti-VEGF therapy have side effects and are occasionally unable to prevent disease progression. It is therefore worthwhile to consider other molecular targets for the development of novel treatment strategies that could be safer and more efficient. During the manifestation of IR, the retina, normally an immune privileged tissue, encounters enhanced levels of cellular stress and inflammation that attract mononuclear phagocytes (MPs) from the blood stream and activate resident MPs (microglia). Activated MPs have a multitude of effects within the retinal tissue and have the potential to both counter and exacerbate the harmful tissue microenvironment. The present review discusses the current knowledge about the role of inflammation and activated retinal MPs in the major IRs: retinopathy of prematurity and diabetic retinopathy. We focus particularly on MPs and their secreted factors and cell–cell-based interactions between MPs and endothelial cells. We conclude that activated MPs play a major role in the manifestation and progression of IRs and could therefore become a promising new target for novel pharmacological intervention strategies in these diseases.

Boosting the peripheral immune response in the skeletal muscles improved motor function in ALS transgenic mice

Monocyte chemoattractant protein-1 (MCP1) is one of the most powerful pro-inflammatory chemokines. However, its signalling is pivotal in driving injured axon and muscle regeneration.

Activin A and CCR2 regulate macrophage function in testicular fibrosis caused by experimental autoimmune orchitis

Experimental autoimmune-orchitis (EAO), a rodent model of chronic testicular inflammation and fibrosis, replicates pathogenic changes seen in some cases of human spermatogenic disturbances. During EAO, increased levels of pro-inflammatory and pro-fibrotic mediators such as TNF, CCL2, and activin A are accompanied by infiltration of leukocytes into the testicular parenchyma. Activin A levels correlate with EAO severity, while elevated CCL2 acting through its receptor CCR2 mediates leukocyte trafficking and recruits macrophages. CCR2 + CXCR4 + macrophages producing extracellular matrix proteins contribute widely to fibrogenesis. Furthermore, testicular macrophages (TMs) play a critical role in organ homeostasis. Therefore, we aimed to investigate the role of the activin A/CCL2-CCR2/macrophage axis in the development of testicular fibrosis. Following EAO induction, we observed lower levels of organ damage, collagen deposition, and leukocyte infiltration (including fibronectin⁺, collagen I⁺ and CXCR4⁺ TMs) in Ccr2^-/- mice than in WT mice. Furthermore, levels of Il-10, Ccl2, and the activin A subunit Inhba mRNAs were lower in Ccr2^-/- EAO testes. Notably, fibronectin⁺ TMs were also present in biopsies from patients with impaired spermatogenesis and fibrotic alterations. Overexpression of the activin A antagonist follistatin reduced tissue damage and collagen I⁺ TM accumulation in WT EAO testes, while treating macrophages with activin A in vitro increased the expression of Ccr2, Fn1, Cxcr4, and Mmp2 and enhanced migration along a CCL2 gradient; these effects were abolished by follistatin. Taken together, our data indicate that CCR2 and activin A promote fibrosis during testicular inflammation by regulating macrophage function. Inhibition of CCR2 or activin A protects against damage progression, offering a promising avenue for therapeutic intervention.

Monocytes in central nervous system remyelination

Remyelination failure with aging and progression of neurodegenerative disorders contributes to axonal dysfunction, highlighting the importance of understanding the mechanisms underpinning this process to develop regenerative therapies. Central nervous system (CNS) macrophages, encompassing both resident microglia and blood monocyte-derived cells, play a crucial role in driving successful remyelination. Although there has been a focus on the critical roles of microglia in remyelination, the specific contribution of monocyte-derived macrophages is still not fully understood. Until recently, the lack of tools enabling distinction between CNS macrophage populations has hindered our understanding of monocyte influence on remyelination. Recent advances have allowed for identification and characterization of monocyte populations in health, aging and in neurodegenerative conditions like multiple sclerosis, indicating heterogeneity of monocyte subsets impacted by both intrinsic and extrinsic factors. Here, we discuss the new tools enabling distinction between macrophage populations and advancements in understanding the importance of monocytes in remyelination, and reflect on the potential for therapeutic targeting of monocytes to promote remyelination.

Cytokines and Chemokines in the Pathogenesis of Experimental Autoimmune Encephalomyelitis

Experimental autoimmune encephalomyelitis is a CD4⁺ T cell-mediated demyelinating disease of the CNS that serves as a model for multiple sclerosis. Cytokines and chemokines shape Th1 and Th17 effector responses as well as regulate migration of leukocytes to the CNS during disease. The CNS cellular infiltrate consists of Ag-specific and nonspecific CD4⁺ and CD8⁺ T cells, neutrophils, B cells, monocytes, macrophages, and dendritic cells. The mechanism of immune-mediated inflammation in experimental autoimmune encephalomyelitis has been extensively studied in an effort to develop therapeutic modalities for multiple sclerosis and, indeed, has provided insight in modern drug discovery. The present Brief Review highlights critical pathogenic aspects of cytokines and chemokines involved in generation of effector T cell responses and migration of inflammatory cells to the CNS. Select cytokines and chemokines are certainly important in the regulatory response, which involves T regulatory, B regulatory, and myeloid-derived suppressor cells. However, that discussion is beyond the scope of this brief review.

Peripheral Monocytosis at Admission to Predict Cerebral Infarct and Poor Functional Outcomes in Subarachnoid Hemorrhage Patients

OBJECTIVE

Increasing evidence points monocytes' role to be larger than thought in developing cerebral infarction (CI) after subarachnoid hemorrhage (SAH). However, there is no clinical evidence of the relationship between peripheral monocytes and CI and clinical outcomes. Therefore we determine whether an increase in monocytes in the acute phase is useful to predict CI and functional outcomes in SAH patients.

METHODS

We included 204 patients with an SAH diagnosis. We collected patient-related factors, comorbidities, Hunt-Hess grade, modified Fisher grade, treatment, delayed cerebral ischemia, CI, aneurysm characteristics, and peripheral monocytes from vein blood at admission. Poor outcomes were defined as modified Rankin Scale score ≥3.

RESULTS

Fifty (24.5%) patients had CI before discharge. In a multivariate model, increased monocytes at admission were significantly associated with CI after adjusting for IV-V Hunt-Hess grade and delayed cerebral ischemia (odds ratio: 3.193, 95% confidence interval: 1.069-9.532, P = 0.037). In receiver operating characteristic curve analysis, a monocyte count of 0.60 was identified as the best cutoff value to discriminate the development of CI (area under the curve = 0.622, P = 0.010; CI for monocytes <0.60 17.4% vs. CI for monocytes ≥0.60 29.1% P = 0.046). Admission monocytes ≥0.60 predicted poor functional outcomes at discharge (monocytes <0.60 52% vs. monocytes ≥0.60 64.7%) and at 12 months (monocytes <0.60 29.4% vs. monocytes ≥0.60 70.6%).

CONCLUSIONS

Increased peripheral monocytes at admission is a risk factor for developing CI after SAH. Moreover, short- and long-term poor clinical outcomes were associated with higher monocyte count. Therefore monocytes could be a convenient biomarker for prognosis unfavorable outcomes and a possible target for new therapeutic strategies.

Immune Characterization in Aneurysmal Subarachnoid Hemorrhage Reveals Distinct Monocytic Activation and Chemokine Patterns

The pathophysiology of delayed cerebral ischemia (DCI) after aneurysmal subarachnoid hemorrhage (aSAH) is incompletely understood. Intrathecal activation of inflammatory immune cells is suspected to play a major role for the induction of DCI. The aim of this study is to identify immune cell subsets and mediators involved in the pathogenesis of DCI. We prospectively collected blood and CSF from 25 patients with aSAH at early and late time points. We performed multicolor flow cytometry of peripheral blood and CSF, analyzing immune cell activation and pro-inflammatory cyto- and chemokines. In addition to the primary immune analysis, we retrospectively analyzed immune cell dynamics in the CSF of all our SAH patients. Our results show an increased monocyte infiltration secondary to aneurysm rupture in patients with DCI. Infiltrating monocytes are defined by a non-classical (CD14^dim CD16⁺) phenotype at early stages. The infiltration is most likely triggered by the intrathecal immune activation. Here, high levels of pro-inflammatory chemokines, such as CXCL1, CXCL9, CXCL10, and CXCL11, are detected. The intrathecal cellular activation profile of monocytes was defined by upregulation of CD163 and CD86 on monocytes and a presumable later differentiation into antigen-presenting plasmacytoid dendritic cells (pDCs) and hemosiderophages. Peripheral immune activation was reflected by CD69 upregulation on T cells. Analysis of DCI prevalence, Hunt and Hess grade, and clinical outcome correlated with the degree of immune activation. We demonstrate that monocytes and T cells are activated intrathecally after aSAH and mediate a local inflammatory response which is presumably driven by chemokines. Our data shows that the distinct pattern of immune activation correlates with the prevalence of DCI, indicating a pathophysiological connection to the incidence of vasospasm.

Neuroinflammation in Post-Traumatic Epilepsy: Pathophysiology and Tractable Therapeutic Targets

Epilepsy is a common chronic consequence of traumatic brain injury (TBI), contributing to increased morbidity and mortality for survivors. As post-traumatic epilepsy (PTE) is drug-resistant in at least one-third of patients, there is a clear need for novel therapeutic strategies to prevent epilepsy from developing after TBI, or to mitigate its severity. It has long been recognized that seizure activity is associated with a local immune response, characterized by the activation of microglia and astrocytes and the release of a plethora of pro-inflammatory cytokines and chemokines. More recently, increasing evidence also supports a causal role for neuroinflammation in seizure induction and propagation, acting both directly and indirectly on neurons to promote regional hyperexcitability. In this narrative review, we focus on key aspects of the neuroinflammatory response that have been implicated in epilepsy, with a particular focus on PTE. The contributions of glial cells, blood-derived leukocytes, and the blood–brain barrier will be explored, as well as pro- and anti-inflammatory mediators. While the neuroinflammatory response to TBI appears to be largely pro-epileptogenic, further research is needed to clearly demonstrate causal relationships. This research has the potential to unveil new drug targets for PTE, and identify immune-based biomarkers for improved epilepsy prediction.

Nef-induced CCL2 Expression Contributes to HIV/SIV Brain Invasion and Neuronal Dysfunction

C-C motif chemokine ligand 2 (CCL2) is a chemoattractant for leukocytes including monocytes, T cells, and natural killer cells and it plays an important role in maintaining the integrity and function of the brain. However, there is accumulating evidence that many neurological diseases are attributable to a dysregulation of CCL2 expression. Acquired immune deficiency syndrome (AIDS) encephalopathy is a severe and frequent complication in individuals infected with the human immunodeficiency virus (HIV) or the simian immunodeficiency virus (SIV). The HIV and SIV Nef protein, a progression factor in AIDS pathology, can be transferred by microvesicles including exosomes and tunneling nanotubes (TNT) within the host even to uninfected cells, and Nef can induce CCL2 expression. This review focuses on findings which collectively add new insights on how Nef-induced CCL2 expression contributes to neurotropism and neurovirulence of HIV and SIV and elucidates why adjuvant targeting of CCL2 could be a therapeutic option for HIV-infected persons.

The impact of modulating the blood-brain barrier on the electrophysiological and histological outcomes of intracortical electrodes

OBJECTIVE

Successful application of chronic intracortical electrodes remains highly variable. The biological mechanisms leading to electrode failure are still being explored. Recent work has shown a correlation between blood-brain barrier (BBB) integrity and long-term recordings. Here we proposed to modulate the BBB healing after intracortical electrode implantation, while evaluating the functional electrophysiology. The CCL2/CCR2 pathway was chosen based on previous work demonstrating the positive histological effects in an intracortical electrode model, as well as in other neurodegenerative models. By disrupting this pathway, recruitment of pro-inflammatory monocytes (a result of a breached BBB) is potentially reduced at the electrode interface.

APPROACH

Michigan electrodes were implanted for 2 and 12 weeks in rats, and a CCR2 antagonist (RS 102895) was administered daily to the treatment group. Functional electrodes were used for the 12 week cohort, and weekly electrophysiological recordings were taken. At 2 and 12 weeks, histology was analyzed.

MAIN RESULTS

At 12 weeks, the CCR2-antagonist group had significantly higher signal-to-noise ratios (SNRs) than control. CCR2-antagonism at 2 weeks significantly increased the neural population and decreased BBB breach. At 12 weeks, CCR2-antagonism significantly increased number of neurons and BBB  +  vasculature within 100 µm of the electrode interface.

SIGNIFICANCE

This work demonstrates that for intracortical electrodes, disruption of the CCL2/CCR2 pathway improves chronic outcomes in electrophysiology and histology.

Bone marrow dendritic cells regulate hematopoietic stem/progenitor cell trafficking

A resident population of dendritic cells (DCs) has been identified in murine bone marrow, but its contribution to the regulation of hematopoiesis and establishment of the stem cell niche is largely unknown. Here, we show that murine bone marrow DCs are perivascular and have a type 2 conventional DC (cDC2) immunophenotype. RNA expression analysis of sorted bone marrow DCs shows that expression of many chemokines and chemokine receptors is distinct from that observed in splenic cDC2s, suggesting that bone marrow DCs may represent a unique DC population. A similar population of DCs is present in human bone marrow. Ablation of conventional DCs (cDCs) results in hematopoietic stem/progenitor cell (HSPC) mobilization that is greater than that seen with ablation of bone marrow macrophages, and cDC ablation also synergizes with G-CSF to mobilize HSPCs. Ablation of cDCs is associated with an expansion of bone marrow endothelial cells and increased vascular permeability. CXCR2 expression in sinusoidal endothelial cells and the expression of two CXCR2 ligands, CXCL1 and CXCL2, in the bone marrow are markedly increased following cDC ablation. Treatment of endothelial cells in vitro with CXCL1 induces increased vascular permeability and HSPC transmigration. Finally, we show that HSPC mobilization after cDC ablation is attenuated in mice lacking CXCR2 expression. Collectively, these data suggest that bone marrow DCs play an important role in regulating HSPC trafficking, in part, through regulation of sinusoidal CXCR2 signaling and vascular permeability.

CCL2 Is a Vascular Permeability Factor Inducing CCR2-Dependent Endothelial Retraction during Lung Metastasis

Increased levels of the chemokine CCL2 in cancer patients are associated with poor prognosis. Experimental evidence suggests that CCL2 correlates with inflammatory monocyte recruitment and induction of vascular activation, but the functionality remains open. Here, we show that endothelial Ccr2 facilitates pulmonary metastasis using an endothelial-specific Ccr2-deficient mouse model (Ccr2^ecKO). Similar levels of circulating monocytes and equal leukocyte recruitment to metastatic lesions of Ccr2^ecKO and Ccr2^fl/fl littermates were observed. The absence of endothelial Ccr2 strongly reduced pulmonary metastasis, while the primary tumor growth was unaffected. Despite a comparable cytokine milieu in Ccr2^ecKO and Ccr2^fl/fl littermates the absence of vascular permeability induction was observed only in Ccr2^ecKO mice. CCL2 stimulation of pulmonary endothelial cells resulted in increased phosphorylation of MLC2, endothelial cell retraction, and vascular leakiness that was blocked by an addition of a CCR2 inhibitor. These data demonstrate that endothelial CCR2 expression is required for tumor cell extravasation and pulmonary metastasis. IMPLICATIONS: The findings provide mechanistic insight into how CCL2-CCR2 signaling in endothelial cells promotes their activation through myosin light chain phosphorylation, resulting in endothelial retraction and enhanced tumor cell migration and metastasis.

Inhibition of stromal cell-derived factor-1α/CXCR4 signaling restores the blood-retina barrier in pericyte-deficient mouse retinas

In diabetic retinopathy (DR), pericyte dropout from capillary walls is believed to cause the breakdown of the blood-retina barrier (BRB), which subsequently leads to vision-threatening retinal edema. While various proinflammatory cytokines and chemokines are upregulated in eyes with DR, their distinct contributions to disease progression remain elusive. Here, we evaluated roles of stromal cell-derived factor-1α (SDF-1α) and its receptor CXCR4 in the BRB breakdown initiated by pericyte deficiency. After inhibition of pericyte recruitment to developing retinal vessels in neonatal mice, endothelial cells (ECs) upregulated the expression of SDF-1α. Administration of CXCR4 antagonists, or EC-specific disruption of the CXCR4 gene, similarly restored the BRB integrity, even in the absence of pericyte coverage. Furthermore, CXCR4 inhibition significantly decreased both the expression levels of proinflammatory genes (P < 0.05) and the infiltration of macrophages (P < 0.05) into pericyte-deficient retinas. Taken together, EC-derived SDF-1α induced by pericyte deficiency exacerbated inflammation through CXCR4 in an autocrine or paracrine manner and thereby induced macrophage infiltration and BRB breakdown. These findings suggest that the SDF-1α/CXCR4 signaling pathway may be a potential therapeutic target in DR.

Reactive astrocytic S1P3 signaling modulates the blood-tumor barrier in brain metastases

Brain metastases are devastating complications of cancer. The blood-brain barrier (BBB), which protects the normal brain, morphs into an inadequately characterized blood-tumor barrier (BTB) when brain metastases form, and is surrounded by a neuroinflammatory response. These structures contribute to poor therapeutic efficacy by limiting drug uptake. Here, we report that experimental breast cancer brain metastases of low- and high permeability to a dextran dye exhibit distinct microenvironmental gene expression patterns. Astrocytic sphingosine-1 phosphate receptor 3 (S1P3) is upregulated in the neuroinflammatory response of the highly permeable lesions, and is expressed in patients' brain metastases. S1P3 inhibition functionally tightens the BTB in vitro and in vivo. S1P3 mediates its effects on BTB permeability through astrocytic secretion of IL-6 and CCL2, which relaxes endothelial cell adhesion. Tumor cell overexpression of S1P3 mimics this pathway, enhancing IL-6 and CCL-2 production and elevating BTB permeability. In conclusion, neuroinflammatory astrocytic S1P3 modulates BTB permeability.

Targeting of CCL2-CCR2-Glycosaminoglycan Axis Using a CCL2 Decoy Protein Attenuates Metastasis through Inhibition of Tumor Cell Seeding

The CCL2-CCR2 chemokine axis has an important role in cancer progression where it contributes to metastatic dissemination of several cancer types (e.g., colon, breast, prostate). Tumor cell–derived CCL2 was shown to promote the recruitment of CCR2⁺/Ly6C^hi monocytes and to induce vascular permeability of CCR2⁺ endothelial cells in the lungs. Here we describe a novel decoy protein consisting of a CCL2 mutant protein fused to human serum albumin (dnCCL2-HSA chimera) with enhanced binding affinity to glycosaminoglycans that was tested in vivo. The monocyte-mediated tumor cell transendothelial migration was strongly reduced upon unfused dnCCL2 mutant treatment in vitro. dnCCL2-HSA chimera had an extended serum half-life and thus a prolonged exposure in vivo compared with the dnCCL2 mutant. dnCCL2-HSA chimera bound to the lung vasculature but caused minimal alterations in the leukocyte recruitment to the lungs. However, dnCCL2-HSA chimera treatment strongly reduced both lung vascular permeability and tumor cell seeding. Metastasis of MC-38GFP, 3LL, and LLC1 cells was significantly attenuated upon dnCCL2-HSA chimera treatment. Tumor cell seeding to the lungs resulted in enhanced expression of a proteoglycan syndecan-4 by endothelial cells that correlated with accumulation of the dnCCL2-HSA chimera in the vicinity of tumor cells. These findings demonstrate that the CCL2-based decoy protein effectively binds to the activated endothelium in lungs and blocks tumor cell extravasation through inhibition of vascular permeability.

Dynamics of spinal microglia repopulation following an acute depletion

Our understanding on the function of microglia has been revolutionized in the recent 20 years. However, the process of maintaining microglia homeostasis has not been fully understood. In this study, we dissected the features of spinal microglia repopulation following an acute partial depletion. By injecting intrathecally Mac-1-saporin, a microglia selective immunotoxin, we ablated 50% microglia in the spinal cord of naive mice. Spinal microglia repopulated rapidly and local homeostasis was re-established within 14 days post-depletion. Mac-1-saporin treatment resulted in microglia cell proliferation and circulating monocyte infiltration. The latter is indeed part of an acute, transient inflammatory reaction that follows cell depletion, and was characterized by an increase in the expression of inflammatory molecules and by the breakdown of the blood spinal cord barrier. During this period, microglia formed cell clusters and exhibited a M1-like phenotype. MCP-1/CCR2 signaling was essential in promoting this depletion associated spinal inflammatory reaction. Interestingly, ruling out MCP-1-mediated secondary inflammation, including blocking recruitment of monocyte-derived microglia, did not affect depletion-triggered microglia repopulation. Our results also demonstrated that newly generated microglia kept their responsiveness to peripheral nerve injury and their contribution to injury-associated neuropathic pain was not significantly altered.

Sevoflurane and Isoflurane induce structural changes in brain vascular endothelial cells and increase blood-brain barrier permeability: Possible link to postoperative delirium and cognitive decline

A large percentage of patients subjected to general anesthesia at 65 years and older exhibit postoperative delirium (POD). Here, we test the hypothesis that inhaled anesthetics (IAs), such as Sevoflurane and Isoflurane, act directly on brain vascular endothelial cells (BVECs) to increase blood-brain barrier (BBB) permeability, thereby contributing to POD. Rats of young (3-5 months), middle (10-12 months) and old (17-19 months) ages were anesthetized with Sevoflurane or Isoflurane for 3h. After exposure, some were euthanized immediately; others were allowed to recover for 24h before sacrifice. Immunohistochemistry was employed to monitor the extent of BBB breach, and scanning electron microscopy (SEM) was used to examine changes in the luminal surfaces of BVECs. Quantitative immunohistochemistry revealed increased BBB permeability in older animals treated with Sevoflurane, but not Isoflurane. Extravasated immunoglobulin G showed selective affinity for pyramidal neurons. SEM demonstrated marked flattening of the luminal surfaces of BVECs in anesthetic-treated rats. Results suggest an aging-linked BBB compromise resulting from exposure to Sevoflurane. Changes in the luminal surface topology of BVECs indicate a direct effect on the plasma membrane, which may weaken or disrupt their BBB-associated tight junctions. Disruption of brain homeostasis due to plasma influx into the brain parenchyma and binding of plasma components (e.g., immunoglobulins) to neurons may contribute to POD. We propose that, in the elderly, exposure to some IAs can cause BBB compromise that disrupts brain homeostasis, perturbs neuronal function and thereby contributes to POD. If unresolved, this may progress to postoperative cognitive decline and later dementia.

BMP9 Crosstalk with the Hippo Pathway Regulates Endothelial Cell Matricellular and Chemokine Responses

Endoglin is a type III TGFβ auxiliary receptor that is upregulated in endothelial cells during angiogenesis and, when mutated in humans, results in the vascular disease hereditary hemorrhagic telangiectasia (HHT). Though endoglin has been implicated in cell adhesion, the underlying molecular mechanisms are still poorly understood. Here we show endoglin expression in endothelial cells regulates subcellular localization of zyxin in focal adhesions in response to BMP9. RNA knockdown of endoglin resulted in mislocalization of zyxin and altered formation of focal adhesions. The mechanotransduction role of focal adhesions and their ability to transmit regulatory signals through binding of the extracellular matrix are altered by endoglin deficiency. BMP/TGFβ transcription factors, SMADs, and zyxin have recently been implicated in a newly emerging signaling cascade, the Hippo pathway. The Hippo transcription coactivator, YAP1 (yes-associated protein 1), has been suggested to play a crucial role in mechanotransduction and cell-cell contact. Identification of BMP9-dependent nuclear localization of YAP1 in response to endoglin expression suggests a mechanism of crosstalk between the two pathways. Suppression of endoglin and YAP1 alters BMP9-dependent expression of YAP1 target genes CCN1 (cysteine-rich 61, CYR61) and CCN2 (connective tissue growth factor, CTGF) as well as the chemokine CCL2 (monocyte chemotactic protein 1, MCP-1). These results suggest a coordinate effect of endoglin deficiency on cell matrix remodeling and local inflammatory responses. Identification of a direct link between the Hippo pathway and endoglin may reveal novel mechanisms in the etiology of HHT.

Stimulatory role of the chemokine CCL2 in the migration and peptide expression of embryonic hypothalamic neurons

Neuroinflammation is a feedback mechanism against infection, with recent studies suggesting a neuromodulatory role. The chemokine, (C-C motif) ligand 2 (CCL2), and its receptor, (C-C motif) receptor type 2 (CCR2), affect neuromodulation and migration in response to damage. Although CCL2 co-localizes with neuropeptides in the hypothalamus that control voluntary behavior, the function of CCL2/CCR2 is unknown. This led us to consider the possibility that CCL2 acting through CCR2, under natural conditions, may affect the migration and peptide levels of hypothalamic neurons that control voluntary behavior. This study used primary embryonic hypothalamic neurons to examine the effect of CCL2 on migratory behavior and on levels of the peptides, enkephalin (ENK) and galanin. Treatment with CCL2 led to a significant, dose-dependent increase in the number of migrated neurons and an increase in the velocity and distance traveled. CCL2 also significantly increased the number of ENK-expressing and CCR2/ENK co-expressing neurons and the percentage of neurons that contain higher levels of ENK. Lastly, CCL2 produced a dose-dependent increase in expression of ENK and galanin. These results provide evidence for a stimulatory effect of CCL2 on embryonic hypothalamic neurons involving changes in migratory behavior, expression, and synthesis of neuropeptides that function in controlling behavior. Our results demonstrate that the chemokine, CCL2, functions through its receptor, CCR2, to stimulate the migration and expression of the orexigenic peptides, enkephalin (ENK) and galanin (GAL), in developing embryonic hypothalamic neurons that are important for controlling ingestive behavior. This evidence reveals broad effects of CCL2 in the developing hypothalamus, showing this chemokine system to be tightly linked to the hypothalamic peptide neurons.

P-glycoprotein regulates trafficking of CD8(+) T cells to the brain parenchyma

The trafficking of cytotoxic CD8(+) T lymphocytes across the lining of the cerebral vasculature is key to the onset of the chronic neuro-inflammatory disorder multiple sclerosis. However, the mechanisms controlling their final transmigration across the brain endothelium remain unknown. Here, we describe that CD8(+) T lymphocyte trafficking into the brain is dependent on the activity of the brain endothelial adenosine triphosphate-binding cassette transporter P-glycoprotein. Silencing P-glycoprotein activity selectively reduced the trafficking of CD8(+) T cells across the brain endothelium in vitro as well as in vivo. In response to formation of the T cell-endothelial synapse, P-glycoprotein was found to regulate secretion of endothelial (C-C motif) ligand 2 (CCL2), a chemokine that mediates CD8(+) T cell migration in vitro. Notably, CCL2 levels were significantly enhanced in microvessels isolated from human multiple sclerosis lesions in comparison with non-neurological controls. Endothelial cell-specific elimination of CCL2 in mice subjected to experimental autoimmune encephalomyelitis also significantly diminished the accumulation of CD8(+) T cells compared to wild-type animals. Collectively, these results highlight a novel (patho)physiological role for P-glycoprotein in CD8(+) T cell trafficking into the central nervous system during neuro-inflammation and illustrate CCL2 secretion as a potential link in this mechanism.

A systematic analysis of the peripheral and CNS effects of systemic LPS, IL-1β, [corrected] TNF-α and IL-6 challenges in C57BL/6 mice

It is increasingly clear that systemic inflammation has both adaptive and deleterious effects on the brain. However, detailed comparisons of brain effects of systemic challenges with different pro-inflammatory cytokines are lacking. In the present study, we challenged female C57BL/6 mice intraperitoneally with LPS (100 µg/kg), IL-1β (15 or 50 µg/kg), TNF-α (50 or 250 µg/kg) or IL-6 (50 or 125 µg/kg). We investigated effects on core body temperature, open field activity and plasma levels of inflammatory markers at 2 hours post injection. We also examined levels of hepatic, hypothalamic and hippocampal inflammatory cytokine transcripts. Hypothermia and locomotor hypoactivity were induced by LPS>IL-1β>TNF-α>>IL-6. Systemic LPS, IL-1β and TNF-α challenges induced robust and broadly similar systemic and central inflammation compared to IL-6, which showed limited effects, but did induce a hepatic acute phase response. Important exceptions included IFNβ, which could only be induced by LPS. Systemic IL-1β could not induce significant blood TNF-α, but induced CNS TNF-α mRNA, while systemic TNF-α could induce IL-1β in blood and brain. Differences between IL-1β and TNF-α-induced hippocampal profiles, specifically for IL-6 and CXCL1 prompted a temporal analysis of systemic and central responses at 1, 2, 4, 8 and 24 hours, which revealed that IL-1β and TNF-α both induced the chemokines CXCL1 and CCL2 but only IL-1β induced the pentraxin PTX3. Expression of COX-2, CXCL1 and CCL2, with nuclear localisation of the p65 subunit of NFκB, in the cerebrovasculature was demonstrated by immunohistochemistry. Furthermore, we used cFOS immunohistochemistry to show that LPS, IL-1β and to a lesser degree, TNF-α activated the central nucleus of the amygdala. Given the increasing attention in the clinical literautre on correlating specific systemic inflammatory mediators with neurological or neuropsychiatric conditions and complications, these data will provide a useful resource on the likely CNS inflammatory profiles resulting from systemic elevation of particular cytokines.

Effects of radiation combined injury on hippocampal function are modulated in mice deficient in chemokine receptor 2 (CCR2)

Chemokines and their receptors play a crucial role in normal brain function as well as in pathological conditions such as injury and disease-associated neuroinflammation. Chemokine receptor-2 (CCR2), which mediates the recruitment of infiltrating and resident microglia to sites of central nervous system (CNS) inflammation, is upregulated by ionizing irradiation and traumatic brain injury. Our objective was to determine if a deficiency in CCR2 and subsequent effects on brain microglia affect neurogenesis and cognitive function after radiation combined injury (RCI). CCR2 knock-out ⁻/⁻ and wild-type (WT) mice received 4 Gy of whole body ¹³⁷Cs irradiation. Immediately after irradiation, unilateral traumatic brain injury was induced using a controlled cortical impact system. Forty-four days postirradiation, animals were tested for hippocampus-dependent cognitive performance in the Morris water-maze. After cognitive testing, animals were euthanized and their brains snap frozen for immunohistochemical assessment of neuroinflammation (activated microglia) and neurogenesis in the hippocampal dentate gyrus. All animals were able to locate the visible and hidden platform locations in the water maze; however, treatment effects were seen when spatial memory retention was assessed in the probe trials (no platform). In WT animals that received combined injury, a significant impairment in spatial memory retention was observed in the probe trial after the first day of hidden platform training (first probe trial). This impairment was associated with increased neurogenesis in the ipsilateral hemisphere of the dentate gyrus. In contrast, CCR2⁻/⁻ mice, independent of insult showed significant memory retention in the first probe trial and there were no differences in the numbers of newly born neurons in the animals receiving irradiation, trauma or combined injury. Although the mechanisms involved are not clear, our data suggests that CCR2 deficiency can exert a protective effect preventing the impairment of cognitive function after combined injury.

Ccl2, Cx3cr1 and Ccl2/Cx3cr1 chemokine deficiencies are not sufficient to cause age-related retinal degeneration

Monocytes, macrophages, dendritic cells and microglia play critical roles in the local immune response to acute and chronic tissue injury and have been implicated in the pathogenesis of age-related macular degeneration. Defects in Ccl2-Ccr2 and Cx3cl1-Cx3cr1 chemokine signalling cause enhanced accumulation of bloated subretinal microglia/macrophages in senescent mice and this phenomenon is reported to result in the acceleration of age-related retinal degeneration. The purpose of this study was to determine whether defects in CCL2-CCR2 and CX3CL1-CX3CR1 signalling pathways, alone or in combination, cause age-dependent retinal degeneration. We tested whether three chemokine knockout mouse lines, Ccl2(-/-), Cx3cr1(-/-) and Ccl2(-/-)/Cx3cr1(-/-), in comparison to age-matched C57Bl/6 control mice show differences in subretinal macrophage accumulation and loss of adjacent photoreceptor cells at 12-14 months of age. All mouse lines are derived from common parental strains and do not carry the homozygous rd8 mutation in the Crb1 gene that has been a major confounding factor in previous reports. We quantified subretinal macrophages by counting autofluorescent lesions in fundus images obtained by scanning laser ophthalmoscopy (AF-SLO) and by immunohistochemistry for Iba1 positive cells. The accumulation of subretinal macrophages was enhanced in Ccl2(-/-), but not in Cx3cr1(-/-) or Ccl2(-/-)/Cx3cr1(-/-) mice. We identified no evidence of retinal degeneration in any of these mouse lines by TUNEL staining or semithin histology. In conclusion, CCL2-CCR2 and/or CX3CL1-CX3CR1 signalling defects may differentially affect the trafficking of microglia and macrophages in the retina during ageing, but do not appear to cause age-related retinal degeneration in mice.

Inflammation-induced dysfunction of the low-density lipoprotein receptor-related protein-1 at the blood-brain barrier: protection by the antioxidant N-acetylcysteine

Impairment in two blood-brain barrier (BBB) efflux transporters, p-glycoprotein (Pgp) and low-density lipoprotein receptor-related protein-1 (LRP-1) are thought to contribute to the progression of Alzheimer's disease (AD) by resulting in the brain accumulation of their substrate amyloid beta peptide (Aβ). The initial cause of impaired efflux, however, is unknown. We have shown that induction of systemic inflammation by intraperitoneal administration of lipopolysaccharide impairs the efflux of Aβ from the brain, suggesting that systemic inflammation could be one such initiator. In this study, we determined whether pre-administration of the antioxidant N-aceytlcysteine (Nac) has a protective effect against LPS-induced Aβ transporter dysfunction. Our findings were that Nac protected against LPS-induced Aβ transport dysfunction at the BBB through an LRP-1-dependent and Pgp-independent mechanism. This was associated with Nac exerting antioxidant effects in the periphery but not the brain, despite an increased rate of entry of Nac into the brain following LPS. We also found that Nac pre-administration resulted in lower blood levels of the cytokines and chemokines interferon-γ, interleukin-10, CCL2, CCL4, and CCL5, but only lowered CCL4 in the cerebral cortex and hippocampus. Finally, we observed that hippocampal cytokine responses to LPS were decreased compared to cortex. These findings demonstrate a novel mechanism by which antioxidants prevent Aβ accumulation in the brain caused by inflammation, and therefore protect against AD.

CCL2 disrupts the adherens junction: implications for neuroinflammation

Alterations to blood-brain barrier (BBB) adhesion molecules and junctional integrity during neuroinflammation can promote central nervous system (CNS) pathology. The chemokine CCL2 is elevated during CNS inflammation and is associated with endothelial dysfunction. The effects of CCL2 on endothelial adherens junctions (AJs) have not been defined. We demonstrate that CCL2 transiently induces Src-dependent disruption of human brain microvascular endothelial AJ. β-Catenin is phosphorylated and traffics from the AJ to PECAM-1 (platelet endothelial cell adhesion molecule-1), where it is sequestered at the membrane. PECAM-1 is also tyrosine-phosphorylated, an event associated with recruitment of the phosphatase SHP-2 (Src homology 2 domain-containing protein phosphatase) to PECAM-1, β-catenin release from PECAM-1, and reassociation of β-catenin with the AJ. Surface localization of PECAM-1 is increased in response to CCL2. This may enable the endothelium to sustain CCL2-induced alterations in AJ and facilitate recruitment of leukocytes into the CNS. Our novel findings provide a mechanism for CCL2-mediated disruption of endothelial junctions that may contribute to BBB dysfunction and increased leukocyte recruitment in neuroinflammatory diseases.

Neural inflammation and the microglial response in diabetic retinopathy

This chapter reviews the function of microglia and their potential roles in neural inflammation and pathological changes during diabetic retinopathy.

Differential modulation of retinal degeneration by Ccl2 and Cx3cr1 chemokine signalling

Microglia and macrophages are recruited to sites of retinal degeneration where local cytokines and chemokines determine protective or neurotoxic microglia responses. Defining the role of Ccl2-Ccr2 and Cx3cl1-Cx3cr1 signalling for retinal pathology is of particular interest because of its potential role in age-related macular degeneration (AMD). Ccl2, Ccr2, and Cx3cr1 signalling defects impair macrophage trafficking, but have, in several conflicting studies, been reported to show different degrees of age-related retinal degeneration. Ccl2/Cx3cr1 double knockout (CCDKO) mice show an early onset retinal degeneration and have been suggested as a model for AMD. In order to understand phenotypic discrepancies in different chemokine knockout lines and to study how defects in Ccl2 and/or Cx3cr1 signalling contribute to the described early onset retinal degeneration, we defined primary and secondary pathological events in CCDKO mice. To control for genetic background variability, we compared the original phenotype with that of single Ccl2, Cx3cr1 and Ccl2/Cx3cr1 double knockout mice obtained from backcrosses of CCDKO with C57Bl/6 mice. We found that the primary pathological event in CCDKO mice develops in the inferior outer nuclear layer independently of light around postnatal day P14. RPE and vascular lesions develop secondarily with increasing penetrance with age and are clinically similar to retinal telangiectasia not to choroidal neovascularisation. Furthermore, we provide evidence that a third autosomal recessive gene causes the degeneration in CCDKO mice and in all affected re-derived lines and subsequently demonstrated co-segregation of the naturally occurring RD8 mutation in the Crb1 gene. By comparing CCDKO mice with re-derived CCl2(-/-)/Crb1(Rd8/RD8), Cx3cr1(-/-)/Crb1(Rd8/RD8) and CCl2(-/-)/Cx3cr1(-/-)/Crb1(Rd8/RD8) mice, we observed a differential modulation of the retinal phenotype by genetic background and both chemokine signalling pathways. These findings indicate that CCDKO mice are not a model of AMD, but a model for an inherited retinal degeneration that is differentially modulated by Ccl2-Ccr2 and Cx3cl1-Cx3cr1 chemokine signalling.

Peripheral nerve injury alters blood-spinal cord barrier functional and molecular integrity through a selective inflammatory pathway

Peripheral nerve lesion triggers alterations in the spinal microenvironment that contribute to the pathogenesis of neuropathic pain. While neurons and glia have been implicated in these functional changes, it remains largely underexplored whether the blood-spinal cord barrier (BSCB) is also involved. The BSCB is an important component in the CNS homeostasis, and compromised BSCB has been associated with different pathologies affecting the spinal cord. Here, we demonstrated that a remote injury on the peripheral nerve in rats triggered a leakage of the BSCB, which was independent of spinal microglial activation. The increase of BSCB permeability to different size tracers, such as Evans Blue and sodium fluorescein, was restricted to the lumbar spinal cord and prominent for at least 4 weeks after injury. The spinal inflammatory reaction triggered by nerve injury was a key player in modulating BSCB permeability. We identified MCP-1 as an endogenous trigger for the BSCB leakage. BSCB permeability can also be impaired by circulating IL-1β. In contrast, antiinflammatory cytokines TGF-β1 and IL-10 were able to shut down the openings of the BSCB following nerve injury. Peripheral nerve injury caused a decrease in tight junction and caveolae-associated proteins. Interestingly, ZO-1 and occludin, but not caveolin-1, were rescued by TGF-β1. Furthermore, our data provide direct evidence that disrupted BSCB following nerve injury contributed to the influx of inflammatory mediators and the recruitment of spinal blood borne monocytes/macrophages, which played a major role in the development of neuropathic pain. These findings highlight the importance of inflammation in BSCB integrity and in spinal cord homeostasis.

COX-2 blockade suppresses gliomagenesis by inhibiting myeloid-derived suppressor cells

Epidemiologic studies have highlighted associations between the regular use of nonsteroidal anti-inflammatory drugs (NSAID) and reduced glioma risks in humans. Most NSAIDs function as COX-2 inhibitors that prevent production of prostaglandin E₂ (PGE₂). Because PGE₂ induces expansion of myeloid-derived suppressor cells (MDSC), we hypothesized that COX-2 blockade would suppress gliomagenesis by inhibiting MDSC development and accumulation in the tumor microenvironment (TME). In mouse models of glioma, treatment with the COX-2 inhibitors acetylsalicylic acid (ASA) or celecoxib inhibited systemic PGE₂ production and delayed glioma development. ASA treatment also reduced the MDSC-attracting chemokine CCL2 (C-C motif ligand 2) in the TME along with numbers of CD11b(+)Ly6G(hi)Ly6C(lo) granulocytic MDSCs in both the bone marrow and the TME. In support of this evidence that COX-2 blockade blocked systemic development of MDSCs and their CCL2-mediated accumulation in the TME, there were defects in these processes in glioma-bearing Cox2-deficient and Ccl2-deficient mice. Conversely, these mice or ASA-treated wild-type mice displayed enhanced expression of CXCL10 (C-X-C motif chemokine 10) and infiltration of cytotoxic T lymphocytes (CTL) in the TME, consistent with a relief of MDSC-mediated immunosuppression. Antibody-mediated depletion of MDSCs delayed glioma growth in association with an increase in CXCL10 and CTLs in the TME, underscoring a critical role for MDSCs in glioma development. Finally, Cxcl10-deficient mice exhibited reduced CTL infiltration of tumors, establishing that CXCL10 limited this pathway of immunosuppression. Taken together, our findings show that the COX-2 pathway promotes gliomagenesis by directly supporting systemic development of MDSCs and their accumulation in the TME, where they limit CTL infiltration.

The blood-brain barrier: geriatric relevance of a critical brain-body interface

The blood-brain barrier (BBB) represents the interface between the brain and other body tissues. Its ability to protect the brain from harmful compounds has attracted the attention of clinicians and investigators, but far from being a simple physical barrier, the BBB is a complex, heterogeneous, and dynamic tissue. The integrated function of the cerebral microvasculature, tight junction proteins, brain microvascular endothelial cells (BMECs), cellular transport pathways, and enzymatic machinery jointly contribute to normal BBB integrity. Aging, systemic diseases, and ischemic injury can disrupt these processes, resulting in a decline in overall BBB function and integrity. Based on the published literature, this study proposes that age- and disease-related BBB alterations play a key role in diminishing the ability of older patients to recover from acute ischemic stroke. Evidence linking deficits in the cerebral microvasculature and BBB integrity to dementia, medication-related cognitive decline, white matter disease (WMD or leukoaraiosis), and related geriatric syndromes including delirium, gait disorders, and urinary incontinence is also reviewed. Priority areas for a future research agenda include strategies to improve clinicians' ability to diagnose, prevent, and manage BBB abnormalities. In future years, in vivo measures such as functional and contrast-enhanced neuroimaging will be used to evaluate BBB integrity in older adults while also assessing the effectiveness of interventions, some targeting inflammatory pathways known to disrupt the BBB, for their ability to prevent or slow the progression of these complex multifactorial geriatric syndromes.

Tickets to the brain: role of CCR2 and CX3CR1 in myeloid cell entry in the CNS

Myeloid cells are mediators of central nervous system (CNS) damage and recovery in neuroinflammatory and neurodegenerative disorders. Besides endogenous myelomonocytic cell populations that reside in the brain already during development, newly migrated leukocytes are considered as important disease modulators in the adult brain. Thus, understanding of myeloid cell recruitment is pivotal for manipulating immune cell entry into the CNS and potentially reducing disease burden. Before myeloid cells engraft in the brain, they first tether to and roll on the activated brain endothelium, then they firmly adhere and eventually transmigrate into the damaged brain where they execute effector functions and differentiate into cells with microglia-like features. These steps are mainly regulated by adhesion molecules and by chemokines and their cognate receptors. Due to recent advances in our understanding of monocyte heterogeneity, the interest in chemokine receptors has significantly increased. Among others, the presence of the chemokine receptors CCR2 and CX(3)CR(1) is considered to be critical for both myeloid cell trafficking along inflamed vessels and subsequent accumulation in the brain. Therefore, these molecules present viable targets for therapeutic manipulations of myeloid cells destined for the CNS.

The role of the CCL2/CCR2 axis in mouse mast cell migration in vitro and in vivo

Tissue-resident mast cells (MCs) are important in allergic diseases. In a mouse model of allergic airways inflammation, an increase in peribronchiolar MCs was associated with increased concentrations of the chemokine CCL2 in lung lavage. MC progenitors (MCps) arising in bone marrow (BM) are recruited to tissues by transendothelial migration, and we found that CCL2 is chemotactic for MCps in freshly isolated BM in vitro. Immature, but not mature, BM-derived MCs migrated in response to CCL2 when cultured in IL-3+stem cell factor (SCF) but not when cultured in IL-3 alone. However, the cells under both culture conditions expressed mRNA for CCR2, the receptor for CCL2, and bound the radiolabeled chemokine with similar affinities, highlighting SCF as a key mediator in coupling CCR2 to downstream events, culminating in chemotaxis. Immature BM-derived MCs from IL-3 +SCF cultures, when administered i.v., accumulated at skin sites injected with CCL2 in vivo. MCp recruitment to the allergen-sensitized/challenged lung was significantly reduced in CCR2(-/-) and CCL2(-/-) mouse strains. However, reconstitution studies of sublethally irradiated and BM-reconstituted mice indicated that BM cells and stromal elements could provide CCL2, whereas the CCR2 function resided with stromal elements rather than BM cells. These experiments revealed a new function of SCF in chemokine receptor coupling, but they suggest a complex role of the CCL2/CCR2 axis in recruiting MCps during pulmonary inflammation.

Multiple roles of chemokine (C-C motif) ligand 2 in promoting prostate cancer growth

Prostate cancer continues to be the most common nonskin cancer diagnosed and the second leading cause of cancer death in men in the United States. Prostate cancer that has metastasized to bone remains incurable. The interactions between prostate cancer cells and the various cells of the host microenvironment result in enhanced growth of tumor cells and activation of host cells that together culminate in osteoblastic bone metastases. These dynamic tumor-host interactions are mediated by cancer and host-produced cytokines and chemokines. Among them, chemokine (C-C motif) ligand 2 (CCL2) has been identified as a prominent modulator of metastatic growth in the bone microenvironment. CCL2 is produced by bone marrow osteoblasts, endothelial cells, stromal cells, and prostate cancer cells. It has been demonstrated to modulate tumor-associated macrophage migration and promote osteoclast maturation. In addition, CCL2 functions through binding to its receptor CCR2 to induce prostate cell proliferation, migration, and invasion in both autocrine and paracrine manners. CCL2 protects prostate cancer cells from autophagic death by activating survivin through a PI3K/AKT (phosphatidylinositol 3-kinase/protein kinase B)-dependent mechanism. Inhibition of CCL2 substantially decreases macrophage infiltration, decreases osteoclast function, and inhibits prostate cancer growth in bone in preclinical animal models. The multiple roles of CCL2 in the tumor microenvironment make it an attractive therapeutic target in metastatic prostate cancer as well as in other cancers.

Role of chemokines in CNS health and pathology: a focus on the CCL2/CCR2 and CXCL8/CXCR2 networks

Chemokines and their receptors have crucial roles in the trafficking of leukocytes, and are of particular interest in the context of the unique immune responses elicited in the central nervous system (CNS). The chemokine system CC ligand 2 (CCL2) with its receptor CC receptor 2 (CCR2), as well as the receptor CXCR2 and its multiple ligands CXCL1, CXCL2 and CXCL8, have been implicated in a wide range of neuropathologies, including trauma, ischemic injury and multiple sclerosis. This review aims to overview the current understanding of chemokines as mediators of leukocyte migration into the CNS under neuroinflammatory conditions. We will specifically focus on the involvement of two chemokine networks, namely CCL2/CCR2 and CXCL8/CXCR2, in promoting macrophage and neutrophil infiltration, respectively, into the lesioned parenchyma after focal traumatic brain injury. The constitutive brain expression of these chemokines and their receptors, including their recently identified roles in the modulation of neuroprotection, neurogenesis, and neurotransmission, will be discussed. In conclusion, the value of evidence obtained from the use of Ccl2- and Cxcr2-deficient mice will be reported, in the context of potential therapeutics inhibiting chemokine activity which are currently in clinical trial for various inflammatory diseases.

CCR2+Ly-6Chi monocytes are crucial for the effector phase of autoimmunity in the central nervous system

The chemokine receptor CCR2 plays a vital role for the induction of autoimmunity in the central nervous system. However, it remains unclear how the pathogenic response is mediated by CCR2-bearing cells. By combining bone marrow chimerism with gene targeting we detected a mild disease-modulating role of CCR2 during experimental autoimmune encephalomyelitis, a model for central nervous system autoimmunity, on radio-resistant cells that was independent from targeted CCR2 expression on endothelia. Interestingly, absence of CCR2 on lymphocytes did not influence autoimmune demyelination. In contrast, engagement of CCR2 on accessory cells was required for experimental autoimmune encephalomyelitis induction. CCR2+Ly-6Chi monocytes were rapidly recruited to the inflamed central nervous system and were crucial for the effector phase of disease. Selective depletion of this specific monocyte subpopulation through engagement of CCR2 strongly reduced central nervous system autoimmunity. Collectively, these data indicate a disease-promoting role of CCR2+Ly-6Chi monocytes during autoimmune inflammation of the central nervous system.

Pertussis toxin induces angiogenesis in brain microvascular endothelial cells

Pertussis toxin (PTX) is an ancillary adjuvant used to elicit experimental allergic encephalomyelitis (EAE), the principal autoimmune model of multiple sclerosis. One mechanism whereby PTX potentiates EAE is to increase blood-brain barrier (BBB) permeability. To elucidate further the mechanism of action of PTX on the BBB, we investigated the genomic and proteomic responses of isolated mouse brain endothelial cells (MBEC) following intoxication. Among approximately 14,000 mouse genes tracked by cDNA microarray, 34 showed altered expression in response to PTX. More than one-third of these genes have roles in angiogenesis. Accordingly, we show that intoxication of MBEC induces tube formation in vitro and angiogenesis in vivo. The global effect of PTX on signaling protein levels and phosphorylation in MBEC was investigated by using Kinex antibody microarrays. In total, 113 of 372 pan-specific and 58 of 258 phospho-site-specific antibodies revealed changes >or=25% following intoxication. Increased STAT1 Tyr-701 and Ser-727 phosphorylation; reduced phosphorylation of the activating phospho-sites in Erk1, Erk2, and MAPKAPK2; and decreased phosphorylation of arrestin beta1 Ser-412 and Hsp27 Ser-82 were confirmed by Kinetworks multi-immunoblotting. The importance of signal transduction pathways on PTX-induced MBEC tube formation was evaluated pharmacologically. Inhibition of phospholipase C, MEK1, and p38 MAP kinase had little effect, whereas inhibition of cAMP-dependent protein kinase, protein kinase C, and phosphatidylinositol 3-kinase partially blocked tube formation. Taken together, these findings are consistent with the concept that PTX may lead to increased BBB permeability by altering endothelial plasticity and angiogenesis.

A destructive cascade mediated by CCL2 facilitates prostate cancer growth in bone

Monocyte chemoattractant protein 1 (CCL2) is a recently identified prominent regulator of prostate cancer growth and metastasis. The purpose of this study was to investigate the mechanistic role of CCL2 in prostate cancer growth in bone. The present study found that CCL2 was up-regulated in osteoblasts (3-fold by PC-3 and 2-fold by VCaP conditioned medium) and endothelial cells (2-fold by PC-3 and VCaP conditioned medium). Parathyroid hormone-related protein (PTHrP) treatment of osteoblastic cells up-regulated CCL2 and was blocked by a PTHrP antagonist, suggesting that prostate cancer-derived PTHrP plays an important role in elevation of osteoblast-derived CCL2. CCL2 indirectly increased blood vessel formation in endothelial cells through vascular endothelial growth factor-A, which was up-regulated 2-fold with administration of CCL2 in prostate cancer cells. In vivo, anti-CCL2 treatment suppressed tumor growth in bone. The decreased tumor burden was associated with decreased bone resorption (serum TRAP5b levels were decreased by 50-60% in anti-CCL2-treated animals from VCaP or PC-3 cell osseous lesions) and microvessel density was decreased by 70% in anti-CCL2-treated animals with bone lesions from VCaP cells. These data suggest that a destructive cascade is driven by tumor cell-derived, PTHrP-mediated induction of CCL2, which facilitates tumor growth via enhanced osteoclastic and endothelial cell activity in bone marrow. Taken together, CCL2 mediates the interaction between tumor-derived factors and host-derived chemokines acting in cooperation to promote skeletal metastasis.

Bone marrow-derived cell regulation of skeletal muscle regeneration

Limb regeneration requires the coordination of multiple stem cell populations to recapitulate the process of tissue formation. Therefore, bone marrow (BM) -derived cell regulation of skeletal muscle regeneration was examined in mice lacking the CC chemokine receptor 2 (CCR2). Myofiber size, numbers of myogenic progenitor cells (MPCs), and recruitment of BM-derived cells and macrophages were assessed after cardiotoxin-induced injury of chimeric mice produced by transplanting BM from wild-type (WT) or CCR2^−/− mice into irradiated WT or CCR2^−/− host mice. Regardless of the host genotype, muscle regeneration and recruitment of BM-derived cells and macrophages were similar in mice replenished with WT BM, whereas BM-derived cells and macrophage accumulation were decreased and muscle regeneration was impaired in all animals receiving CCR2^−/− BM. Furthermore, numbers of MPCs (CD34⁺/Sca-1⁻/CD45⁻ cells) were significantly increased in mice receiving CCR2^−/− BM despite the decreased size of regenerated myofibers. Thus, the expression of CCR2 on BM-derived cells regulated macrophage recruitment into injured muscle, numbers of MPC, and the extent of regenerated myofiber size, all of which were independent of CCR2 expression on host-derived cells. Future studies in regenerative medicine must include consideration of the role of BM-derived cells, possibly macrophages, in CCR2-dependent events that regulate effective skeletal muscle regeneration.—Sun, D., Martinez, C. O., Ochoa, O., Ruiz-Willhite, L., Bonilla, J. R., Centonze, V. E., Waite, L. L., Michalek, J. E., McManus, L. M., Shireman, P. K. Bone marrow-derived cell regulation of skeletal muscle regeneration.

Hemolysate-induced expression of intercellular adhesion molecule-1 and monocyte chemoattractant protein-1 expression in cultured brain microvascular endothelial cells via through ROS-dependent NF-kappaB pathways

In order to determine the possible effects of hemolysate on brain microvascular endothelial cells (BMECs), we examined the effects of hemolysate on the expression of intercellular adhesion molecule-1 (ICAM-1) and monocyte chemoattractant protein-1 (MCP-1), generation of reactive oxygen species (ROS), and NF-kappaB activation in rat BMECs. Hemolysate induced the expression of ICAM-1 and MCP-1 in endothelial cells. In addition, hemolysate stimulated nuclear translocation of the p65 subunit of NF-kappaB, and NF-kappaB DNA-binding activity in BMECs. Furthermore, hemolysate increased ROS generation, and hemolysate-induced ICAM-1and MCP-1 expression and NF-kappaB activation were abrogated in the presence of the direct scavenger of ROS. Taken together, our results indicate that hemolysate can induce inflammatory responses that increase expression of ICAM-1 and MCP-1, through ROS-dependent NF-kappaB activation in BMECs.

Transcellular transport of CCL2 across brain microvascular endothelial cells

The means by which the chemokine CCL2 produced in the brain parenchyma can recruit leukocytes lying behind the highly impervious endothelium of the blood-brain barrier (BBB) has remained a paradox. As other chemokines have been evidenced to stimulate their own synthesis and release by peripheral microvascular endothelial cells, and/or undergo transcytosis in the abluminal-to-luminal direction, we determined whether CCL2 experiences similar fates across brain microvascular endothelial cells (BMEC). Using cultured BMEC as a paradigm of the BBB, it was observed that exogenous unlabeled CCL2 actually depressed the release of endogenous CCL2, and further caused diminished CCL2 mRNA levels in these cells. On the other hand, exogenous (125)I-labeled CCL2 exhibited transport across BMEC in a manner that was sensitive to temperature, competition by excess unlabeled CCL2 but not unlabeled CCL3, knockdown of caveolin-1/caveolae, and elimination of the cognate CCL2 receptor CCR2. These results implied a facet of CCL2 transport by a transcellular mechanism partly involving binding of CCL2 to CCR2, and subsequent transfer to caveolae vesicles for transcytosis. This notion was supported by double-label immuno-electronmicroscopy, which revealed co-localization of caveolin-1 with exogenous CCL2, during this chemokine's transit across BMEC. Collectively, these findings provide a rationale by which CCL2, deposited on the abluminal side of the brain microvasculature during inflammatory episodes, can be relayed across the BBB to foster leukocyte recruitment.

Influence of proinflammatory cytokines and chemokines on the neuropathogenesis of oncornavirus and immunosuppressive lentivirus infections

Retroviral infection of the CNS can lead to severe debilitating neurological diseases in humans and other animals. Four general types of pathogenic effects with various retroviruses have been observed including: hemorrhage (TR1.3), spongiform encephalopathy (CasBrE, FrCasE, PVC211, NT40, Mol-ts1), demyelination with inflammatory lesions (HTLV-1, visna, CAEV), and encephalopathy with gliosis and proinflammatory chemokines and cytokines, usually with microglial giant cells and nodules [human immunodeficiencyvirus (HIV), feline immunodeficiencyvirus (FIV), simian immunodeficiency virus (SIV), Fr98]. This review focuses on this fourth group of retroviruses. In this latter group, proinflammatory cytokine and chemokine upregulation accompanies the disease process, and may influence pathogenesis by direct effects on resident CNS cells. The review first discusses the Fr98 murine polytropic virus system with particular reference to the roles of cytokines and chemokines in the pathogenic process. The Fr98 data are then compared and contrasted to the cytokine and chemokine data in the lentivirus systems, HIV, SIV, and FIV. Finally, various mechanisms are presented by which tumor necrosis factor (TNF) and several chemokines may alter the pathogenesis of retrovirus infection of the CNS.