Astrocytes: Initiators of and Responders to Inflammation

We are in the midst of a glial renaissance; astrocytes, essential for brain homeostasis and neuroprotection, have experienced resurgence in focused analyses. New roles in synaptic plasticity, innate immunity and control of recruited immune cells have placed astrocytes at the center of central nervous system functions. Astrocytes have been shown to receive and convey information to all neural cell types in a coordinated effort to respond to injury and infection, initiating reparative mechanisms. Astrocytes detect injury and infection signals from neurons, microglia, oligodendrocytes and endothelial cells, responding by secreting cytokines, chemokines and growth factors, which may activate immune defenses. While regional heterogeneity in astrocyte form and function has been appreciated since the early 1990s, technologic advances have allowed scientists to show only that astrocytes may be as individualized as neurons. Adult astrocytes may undergo a morphological and functional transformation referred to as astrogliosis. Newly generated astrocytes exhibit heterogenous phenotypes; thus, some remove toxic molecules, restore blood-brain barrier function, and promote extracellular matrix components to support axonal growth and repair, while others inhibit neuronal repair and regeneration. This chapter will introduce some of the cellular and molecular components involved in astrocyte responses induced by inflammatory mediators or pathogens during neuroinflammation or neuroinfectious diseases

LSR/angulin-1 is a tricellular tight junction protein involved in blood-brain barrier formation.

The blood-brain barrier (BBB) is a term used to describe the unique properties of central nervous system (CNS) blood vessels. One important BBB property is the formation of a paracellular barrier made by tight junctions (TJs) between CNS endothelial cells (ECs). Here, we show that Lipolysis-stimulated lipoprotein receptor (LSR), a component of paracellular junctions at points in which three cell membranes meet, is greatly enriched in CNS ECs compared with ECs in other nonneural tissues. We demonstrate that LSR is specifically expressed at tricellular junctions and that its expression correlates with the onset of BBB formation during embryogenesis. We further demonstrate that the BBB does not seal during embryogenesis in Lsr knockout mice with a leakage to small molecules. Finally, in mouse models in which BBB was disrupted, including an experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis and a middle cerebral artery occlusion (MCAO) model of stroke, LSR was down-regulated, linking loss of LSR and pathological BBB leakage

COVID-19 induces CNS cytokine expression and loss of hippocampal neurogenesis

Infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is associated with acute and postacute cognitive and neuropsychiatric symptoms including impaired memory, concentration, attention, sleep and affect. Mechanisms underlying these brain symptoms remain understudied. Here we report that SARS-CoV-2-infected hamsters exhibit a lack of viral neuroinvasion despite aberrant blood-brain barrier permeability. Hamsters and patients deceased from coronavirus disease 2019 (COVID-19) also exhibit microglial activation and expression of interleukin (IL)-1β and IL-6, especially within the hippocampus and the medulla oblongata, when compared with non-COVID control hamsters and humans who died from other infections, cardiovascular disease, uraemia or trauma. In the hippocampal dentate gyrus of both COVID-19 hamsters and humans, we observed fewer neuroblasts and immature neurons. Protracted inflammation, blood-brain barrier disruption and microglia activation may result in altered neurotransmission, neurogenesis and neuronal damage, explaining neuropsychiatric presentations of COVID-19. The involvement of the hippocampus may explain learning, memory and executive dysfunctions in COVID-19 patients

Decreased antiviral immune response within the central nervous system of aged mice is associated with increased lethality of West Nile virus encephalitis

West Nile virus (WNV) is an emerging pathogen that causes disease syndromes ranging from a mild flu-like illness to encephalitis. While the incidence of WNV infection is fairly uniform across age groups, the risk of lethal encephalitis increases with advanced age. Prior studies have demonstrated age-related, functional immune deficits that limit systemic antiviral immunity and increase mortality; however, the effect of age on antiviral immune responses specifically within the central nervous system (CNS) is unknown. Here, we show that aged mice exhibit increased peripheral organ and CNS tissue viral burden, the latter of which is associated with alterations in activation of both myeloid and lymphoid cells compared with similarly infected younger animals. Aged mice exhibit lower MHCII expression by microglia, and higher levels of PD1 and lower levels of IFNγ expression by WNV-specific CD

Single-cell RNA transcriptome analysis of CNS immune cells reveals CXCL16/CXCR6 as maintenance factors for tissue-resident T cells that drive synapse elimination

BACKGROUND: Emerging RNA viruses that target the central nervous system (CNS) lead to cognitive sequelae in survivors. Studies in humans and mice infected with West Nile virus (WNV), a re-emerging RNA virus associated with learning and memory deficits, revealed microglial-mediated synapse elimination within the hippocampus. Moreover, CNS-resident memory T (T METHODS: Here, we examined immune cells within the murine WNV-recovered forebrain using single-cell RNA sequencing to identify putative ligand-receptor pairs involved in intercellular communication between T cells and microglia. Clustering and differential gene analyses were followed by protein validation and genetic and antibody-based approaches utilizing an established murine model of WNV recovery in which microglia and complement promote ongoing hippocampal synaptic loss. RESULTS: Profiling of host transcriptome immune cells at 25 days post-infection in mice revealed a shift in forebrain homeostatic microglia to activated subpopulations with transcriptional signatures that have previously been observed in studies of neurodegenerative diseases. Importantly, CXCL16/CXCR6, a chemokine signaling pathway involved in T CONCLUSIONS: We provide a comprehensive assessment of the role of CXCL16/CXCR6 as an interaction link between microglia and CD

Profiling the mouse brain endothelial transcriptome in health and disease models reveals a core blood-brain barrier dysfunction module.

Blood vessels in the CNS form a specialized and critical structure, the blood-brain barrier (BBB). We present a resource to understand the molecular mechanisms that regulate BBB function in health and dysfunction during disease. Using endothelial cell enrichment and RNA sequencing, we analyzed the gene expression of endothelial cells in mice, comparing brain endothelial cells with peripheral endothelial cells. We also assessed the regulation of CNS endothelial gene expression in models of stroke, multiple sclerosis, traumatic brain injury and seizure, each having profound BBB disruption. We found that although each is caused by a distinct trigger, they exhibit strikingly similar endothelial gene expression changes during BBB disruption, comprising a core BBB dysfunction module that shifts the CNS endothelial cells into a peripheral endothelial cell-like state. The identification of a common pathway for BBB dysfunction suggests that targeting therapeutic agents to limit it may be effective across multiple neurological disorders

A single intranasal or intramuscular immunization with chimpanzee adenovirus-vectored SARS-CoV-2 vaccine protects against pneumonia in hamsters

The development of an effective vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the etiologic agent of coronavirus disease 2019 (COVID-19), is a global priority. Here, we compare the protective capacity of intranasal and intramuscular delivery of a chimpanzee adenovirus-vectored vaccine encoding a prefusion stabilized spike protein (chimpanzee adenovirus [ChAd]-SARS-CoV-2-S) in Golden Syrian hamsters. Although immunization with ChAd-SARS-CoV-2-S induces robust spike-protein-specific antibodies capable of neutralizing the virus, antibody levels in serum are higher in hamsters vaccinated by an intranasal compared to intramuscular route. Accordingly, against challenge with SARS-CoV-2, ChAd-SARS-CoV-2-S-immunized hamsters are protected against less weight loss and have reduced viral infection in nasal swabs and lungs, and reduced pathology and inflammatory gene expression in the lungs, compared to ChAd-control immunized hamsters. Intranasal immunization with ChAd-SARS-CoV-2-S provides superior protection against SARS-CoV-2 infection and inflammation in the upper respiratory tract. These findings support intranasal administration of the ChAd-SARS-CoV-2-S candidate vaccine to prevent SARS-CoV-2 infection, disease, and possibly transmission

Glial inflammatory responses regulate neurocognitive recovery following viral encephalitis

The concept of the brain as an immune privileged organ has been slowly shifting as an in-creasing number of studies have demonstrated that even under homeostatic conditions, commu-nication between the nervous and immune system is essential for proper brain function. Given their roles in innate and adaptive immunity, glial cells and infiltrating immune cells have been placed at the center of this communication axis. Microglia, astrocytes, and T cells have all been shown to receive and convey information to all neural cell types in a coordinated effort to re-spond to injury and infection and initiate reparative mechanisms as well as influence cognitive processing through the section of cytokines. Considering the necessity of homeostatic neuroim-mune interactions for normal cognitive function, we sought to understand how alterations in the neuroimmune landscape, which occur in aging, viral encephalitis, neurodegeneration, and other neuroinflammatory conditions, may influence cognitive capability. Here, we use an in vivo model of viral encephalitis with high survival rates in adult mice, induced by a mutant West Nile viral strain (WNV-NS5-E218A), to study the impact of persistent glial activation and T cell infiltration, and subsequently, its influence on spatial learning capabili-ties in the post-infectious CNS. We first demonstrate that T cell-derived interferon (IFN)-γ sig-naling to microglia underlies spatial learning deficits associated with our recovery model of WNV and a newly established model of Zika encephalitis. Virally infected animals experience changes in cognitive function via the loss of presynaptic termini or neural apoptosis with elimina-tion of postsynaptic termini, respectively (Chapter 2). Additionally, we show that in our WNV recovery model, astrocytes are a source of the anti-neurogenic cytokine, interleukin (IL)-1β, that continuously signals via neural stem cells to inhibit neural correlates of memory. Such neural cor-related are measured by a decrease in proliferating neurons within the hippocampal denate gyrus, persistent synapse loss, alterations to neural activity, and a lack of spatial learning recovery (Chapter 3). The finding in these and subsequent chapters suggest that neuroimmune dysregula-tion can lead to defects in neurocognitive function, while also shedding light on targetable mech-anisms that may drive cognitive dysfunction in viral encephalitis as well as other neurodegenera-tive diseases

CCR2 signaling restricts SARS-CoV-2 infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a historic pandemic of respiratory disease (coronavirus disease 2019 [COVID-19]), and current evidence suggests that severe disease is associated with dysregulated immunity within the respiratory tract. However, the innate immune mechanisms that mediate protection during COVID-19 are not well defined. Here, we characterize a mouse model of SARS-CoV-2 infection and find that early CCR2 signaling restricts the viral burden in the lung. We find that a recently developed mouse-adapted SARS-CoV-2 (MA-SARS-CoV-2) strain as well as the emerging B.1.351 variant trigger an inflammatory response in the lung characterized by the expression of proinflammatory cytokines and interferon-stimulated genes. Using intravital antibody labeling, we demonstrate that MA-SARS-CoV-2 infection leads to increases in circulating monocytes and an influx of CD4

LSR/angulin-1 is a tricellular tight junction protein involved in blood-brain barrier formation.

The blood-brain barrier (BBB) is a term used to describe the unique properties of central nervous system (CNS) blood vessels. One important BBB property is the formation of a paracellular barrier made by tight junctions (TJs) between CNS endothelial cells (ECs). Here, we show that Lipolysis-stimulated lipoprotein receptor (LSR), a component of paracellular junctions at points in which three cell membranes meet, is greatly enriched in CNS ECs compared with ECs in other nonneural tissues. We demonstrate that LSR is specifically expressed at tricellular junctions and that its expression correlates with the onset of BBB formation during embryogenesis. We further demonstrate that the BBB does not seal during embryogenesis in Lsr knockout mice with a leakage to small molecules. Finally, in mouse models in which BBB was disrupted, including an experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis and a middle cerebral artery occlusion (MCAO) model of stroke, LSR was down-regulated, linking loss of LSR and pathological BBB leakage