Microglia and a functional type I IFN pathway are required to counter HSV-1-driven brain lateral ventricle enlargement and encephalitis

Unveiling the Involvement of Herpes Simplex Virus-1 in Alzheimer's Disease: Possible Mechanisms and Therapeutic Implications

Viruses pose a significant challenge and threat to human health, as demonstrated by the current COVID-19 pandemic. Neurodegeneration, particularly in the case of Alzheimer's disease (AD), is significantly influenced by viral infections. AD is a neurodegenerative disease that affects people of all ages and poses a significant threat to millions of individuals worldwide. The precise mechanism behind its development is not yet fully understood; however, the emergence and advancement of AD can be hastened by various environmental factors, such as bacterial and viral infections. There has been a longstanding suspicion that the herpes simplex virus-1 (HSV-1) may have a role to play in the development or advancement of AD. Reactivation of HSV-1 could potentially lead to damage to neurons, either by direct means or indirectly by triggering inflammation. This article provides an overview of the connection between HSV-1 infections and immune cells (astrocytes, microglia, and oligodendrocytes) in the progression of AD. It summarizes recent scientific research on how HSV-1 affects neurons, which could potentially shed light on the clinical features and treatment options for AD. In addition, the paper has explored the impact of HSV-1 on neurons and its role in various aspects of AD, such as Aβ secretion, tau hyperphosphorylation, metabolic dysregulation, oxidative damage, apoptosis, and autophagy. It is believed that the immune response triggered by HSV-1 reactivation plays a role in the development of neurodegeneration in AD. Despite the lack of a cure for AD, researchers have made significant efforts to study the clinical and pathological aspects of the disease, identify biomarkers, and gain insight into its underlying causes. The goal is to achieve early diagnosis and develop treatments that can modify the progression of the disease. The current article discusses the most promising therapy for combating the viral impacts, which provides additional evidence for the frequent reactivations of latent HSV-1 in the AD brain. However, further research is still required to establish the molecular and cellular mechanisms underlying the development of AD through the reactivation of HSV-1. This could potentially lead to new insights in drug development aimed at preventing HSV-1 reactivation and the subsequent development and progression of AD.

Neuroglial responses to bacterial, viral, and fungal neuroinfections

Evidence regarding the host's response to peripheral pathogens in humans abound, whereas studies on the pathogenesis of central nervous system-penetrating infections are relatively scarce. However, given the spate of epidemic and pandemic neuroinfections in the 21st century, the field has experienced a renewed interest lately. This chapter discusses a timely and exciting topic on the roles of glial cells, mainly microglia and astrocytes, in neuroinvasive infections. This chapter considered fungal, viral, and bacterial neuroinfections, X-raying their neuroinvasiveness, neurotropism, and neurovirulence before focusing on specific examples notable for each category, including Escherichia coli, Cryptococcus neoformans, and SARS-CoV-2. These infections are renowned worldwide for a high case-fatality rate, leaving many survivors with life-long morbidity and others with a bleak future neurologic prognosis. Importantly, the chapter discusses possible ways microglia and astrocytes are culpable in these infections and provides approaches by which they can be manipulated for therapeutic purposes, identifying viable research gaps in the process. Additionally, it offers a synopsis of ongoing works considering microglial selective targeting to attenuate the pathology, morbidity, and mortality associated with these neuroinfections. Considering that microglia and astrocytes are first responders in the central nervous system, targeting these glial cells could be the game changer in managing existing and emerging neuroinvasive infections.

A Better Understanding of the Clinical and Pathological Changes in Viral Retinitis: Steps to Improve Visual Outcomes

Infectious retinitis, though rare, poses a significant threat to vision, often leading to severe and irreversible damage. Various pathogens, including viruses, bacteria, tick-borne agents, parasites, and fungi, can cause this condition. Among these, necrotizing herpetic retinitis represents a critical spectrum of retinal infections primarily caused by herpes viruses such as varicella-zoster virus (VZV), herpes simplex virus (HSV), and cytomegalovirus (CMV). This review underscores the retina's susceptibility to viral infections, focusing on the molecular mechanisms through which herpetic viruses invade and damage retinal tissue, supported by clinical and preclinical evidence. We also identify existing knowledge gaps and propose future research directions to deepen our understanding and improve therapeutic outcomes.

Antisecretory Factor 16 (AF16): A Promising Avenue for the Treatment of Traumatic Brain Injury-An In Vitro Model Approach

Traumatic brain injury (TBI) is caused by an external mechanical force to the head, resulting in abnormal brain functioning and clinical manifestations. Antisecretory factor (AF16) is a potential therapeutic agent for TBI treatment due to its ability to inhibit fluid secretion and decrease inflammation, intracranial pressure, and interstitial fluid build-up, key hallmarks presented in TBI. Here, we investigated the effect of AF16 in an in vitro model of neuronal injury, as well as its impact on key components of the autophagy pathway and mitochondrial dynamics. N2Awt cells were treated with AF16, injured using a scratch assay, and analysed using confocal microscopy, correlative light and electron microscopy (CLEM), flow cytometry, and western blotting. Our results reveal that AF16 enhances autophagy activity, regulates mitochondrial dynamics, and provides protection as early as 6 h post-injury. Fluorescently labelled AF16 was observed to localise to lysosomes and the autophagy compartment, suggesting a role for autophagy and mitochondrial quality control in conferring AF16-associated neuronal protection. This study concludes that AF16 has potential as a therapeutic agent for TBI treatment through is regulation of autophagy and mitochondrial dynamics.

Ependymal cells: roles in central nervous system infections and therapeutic application

Ependymal cells are arranged along the inner surfaces of the ventricles and the central canal of the spinal cord, providing anatomical, physiological and immunological barriers that maintain cerebrospinal fluid (CSF) homeostasis. Based on this, studies have found that alterations in gene expression, cell junctions, cytokine secretion and metabolic disturbances can lead to dysfunction of ependymal cells, thereby participating in the onset and progression of central nervous system (CNS) infections. Additionally, ependymal cells can exhibit proliferative and regenerative potential as well as secretory functions during CNS injury, contributing to neuroprotection and post-injury recovery. Currently, studies on ependymal cell primarily focus on the basic investigations of their morphology, function and gene expression; however, there is a notable lack of clinical translational studies examining the molecular mechanisms by which ependymal cells are involved in disease onset and progression. This limits our understanding of ependymal cells in CNS infections and the development of therapeutic applications. Therefore, this review will discuss the molecular mechanism underlying the involvement of ependymal cells in CNS infections, and explore their potential for application in clinical treatment modalities.

α7 nicotinic receptor activation mitigates herpes simplex virus type 1 infection in microglia cells

Herpes simplex virus type 1 (HSV-1), a neurotropic DNA virus, establishes latency in neural tissues, with reactivation causing severe consequences like encephalitis. Emerging evidence links HSV-1 infection to chronic neuroinflammation and neurodegenerative diseases. Microglia, the central nervous system's (CNS) immune sentinels, express diverse receptors, including α7 nicotinic acetylcholine receptors (α7 nAChRs), critical for immune regulation. Recent studies suggest α7 nAChR activation protects against viral infections. Here, we show that α7 nAChR agonists, choline and PNU-282987, significantly inhibit HSV-1 replication in microglial BV2 cells. Notably, this inhibition is independent of the traditional ionotropic nAChR signaling pathway. mRNA profiling revealed that choline stimulates the expression of antiviral factors, IL-1β and Nos2, and down-regulates the apoptosis genes and type A Lamins in BV2 cells. These findings suggest a novel mechanism by which microglial α7 nAChRs restrict viral infections by regulating innate immune responses.

Small Animal Models to Study Herpes Simplex Virus Infections

Herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) are two of the most prevalent human viruses worldwide. They are known to cause a variety of diseases including genital herpes, meningitis, encephalitis, cold sores and herpes stromal keratitis. The seropositive rate for HSV-1 is around 90%, whereas for HSV-2 it remains around 20-25% for the general adult population. The infections caused by these viruses remain difficult to study because a large proportion of infected individuals are asymptomatic. Furthermore, given the neurotropic characteristics of the virus, studies aimed at understanding the complex pathogenesis in humans is difficult. As a result, animal models have been developed to understand several characteristics of HSV biology, pathogenesis, disease and host responses to infection. These models are also commonly used as the first evaluation of new drugs and vaccines. There are several well-established animal models to study infection with HSV, including mice, guinea pigs and rabbits. Variables within the animal models depend on the species of animal, route of infection, viral strain, dosage, etc. This review aims at summarizing the most commonly used animal models to study HSV pathogenesis and therapies.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Innate Lymphoid Cells and Interferons Limit Neurologic and Articular Complications of Brucellosis

Brucellosis is a globally significant zoonotic disease. Human patients with brucellosis develop recurrent fever and focal complications, including arthritis and neurobrucellosis. The current study investigated the role of innate lymphoid cells (ILCs) in the pathogenesis of focal brucellosis caused by Brucella melitensis. After footpad infection, natural killer cells and ILC1 cells both limited joint colonization by Brucella. Mice lacking natural killer cells, and in particular mice lacking all ILCs, also developed marked arthritis after footpad infection. Following pulmonary infection, mice lacking adaptive immune cells and ILCs developed arthritis, neurologic complications, and meningitis. Adaptive immune cells and ILCs both limited colonization of the brain by Brucella following pulmonary infection. Transcriptional analysis of Brucella-infected brains revealed marked up-regulation of genes associated with inflammation and interferon responses, as well as down-regulation of genes associated with neurologic function. Type II interferon deficiency resulted in colonization of the brain by Brucella, but mice lacking both type I and type II interferon signaling more rapidly developed clinical signs of neurobrucellosis, exhibited hippocampal neuronal loss, and had higher levels of Brucella in their brains than mice lacking type II interferon signaling alone. Collectively, these findings indicate ILCs and interferons play an important role in prevention of focal complications during Brucella infection, and that mice with deficiencies in ILCs or interferons can be used to study pathogenesis of neurobrucellosis.

The Host-Pathogen Interplay: A Tale of Two Stories within the Cornea and Posterior Segment

Ocular infectious diseases are an important cause of potentially preventable vision loss and blindness. In the following manuscript, we will review ocular immunology and the pathogenesis of herpesviruses and Pseudomonas aeruginosa infections of the cornea and posterior segment. We will highlight areas of future research and what is currently known to promote bench-to-bedside discoveries to improve clinical outcomes of these debilitating ocular diseases.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

Type I Interferon Signaling Is Critical During the Innate Immune Response to HSV-1 Retinal Infection

Purpose

Acute retinal necrosis (ARN) is a herpesvirus infection of the retina with blinding complications. In this study, we sought to create a reproducible mouse model of ARN that mimics human disease to better understand innate immunity within the retina during virus infection.

Methods

C57Bl/6J wild type (WT) and type I interferon receptor-deficient (IFNAR-/-) mice were infected with varying amounts of herpes simplex virus type 1 (HSV-1) via subretinal injection. Viral titers, optical coherence tomography (OCT) and fundus photography, the development of encephalitis, and ocular histopathology were scored and compared between groups of WT and IFNAR-/- mice.

Results

The retina of WT mice could be readily infected with HSV-1 via subretinal injection resulting in retinal whitening and full-thickness necrosis as determined by in vivo imaging and histopathology. In IFNAR-/- mice, HSV-1-induced retinal pathology was significantly worse when compared with WT mice, and viral titers were significantly elevated within two days after infection and persisted to day 5 after infection within the retina. These results were also observed in the brain where there were significantly higher viral titers and frequency of encephalitis in IFNAR-/- when compared to WT mice.

Conclusions

Collectively, these findings show that our new mouse model of ARN mimics human disease and can be used to study innate immunity within the retina. We conclude that type I interferons are critical in containing HSV-1 locally within retinal tissues and prohibiting spread into the brain.

Intratumoral administration of the antisecretory peptide AF16 cures murine gliomas and modulates macrophage functions

Glioblastoma has remained the deadliest primary brain tumor while its current therapy offers only modest survival prolongation. Immunotherapy has failed to record notable benefits in routine glioblastoma treatment. Conventionally, immunotherapy relies on T cells as tumor-killing agents; however, T cells are outnumbered by macrophages in glioblastoma microenvironment. In this study, we explore the effect of AF16, a peptide from the endogenous antisecretory factor protein, on the survival of glioma-bearing mice, the tumor size, and characteristics of the tumor microenvironment with specific focus on macrophages. We elucidate the effect of AF16 on the inflammation-related secretome of human and murine macrophages, as well as human glioblastoma cells. In our results, AF16 alone and in combination with temozolomide leads to cure in immunocompetent mice with orthotopic GL261 gliomas, as well as prolonged survival in immunocompromised mice. We recorded decreased tumor size and changes in infiltration of macrophages and T cells in the murine glioma microenvironment. Human and murine macrophages increased expression of proinflammatory markers in response to AF16 treatment and the same effect was seen in human primary glioblastoma cells. In summary, we present AF16 as an immunomodulatory factor stimulating pro-inflammatory macrophages with a potential to be implemented in glioblastoma treatment protocols.

Absence of the lectin-like domain of thrombomodulin reduces HSV-1 lethality of mice with increased microglia responses

Background

Herpes simplex virus 1 (HSV-1) can induce fatal encephalitis. Cellular factors regulate the host immunity to affect the severity of HSV-1 encephalitis. Recent reports focus on the significance of thrombomodulin (TM), especially the domain 1, lectin-like domain (TM-LeD), which modulates the immune responses to bacterial infections and toxins and various diseases in murine models. Few studies have investigated the importance of TM-LeD in viral infections, which are also regulated by the host immunity.

Methods

In vivo studies comparing wild-type and TM-LeD knockout mice were performed to determine the role of TM-LeD on HSV-1 lethality. In vitro studies using brain microglia cultured from mice or a human microglia cell line to investigate whether and how TM-LeD affects microglia to reduce HSV-1 replication in brain neurons cultured from mice or in a human neuronal cell line.

Results

Absence of TM-LeD decreased the mortality, tissue viral loads, and brain neuron apoptosis of HSV-1-infected mice with increases in the number, proliferation, and phagocytic activity of brain microglia. Moreover, TM-LeD deficiency enhanced the phagocytic activity of brain microglia cultured from mice or of a human microglia cell line. Co-culture of mouse primary brain microglia and neurons or human microglia and neuronal cell lines revealed that TM-LeD deficiency augmented the capacity of microglia to reduce HSV-1 replication in neurons.

Conclusions

Overall, TM-LeD suppresses microglia responses to enhance HSV-1 infection.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02426-w.

Decreased Brain Ventricular Volume in Psychiatric Inpatients with Serotonin Reuptake Inhibitor Treatment

Background

Brain ventricles have been reported to be enlarged in several neuropsychiatric disorders and in aging. Whether human cerebral ventricular volume can decrease over time with psychiatric treatment is not well-studied. The aim of this study was to examine whether inpatients taking serotonin reuptake inhibitors (SRI) exhibited reductions in cerebral ventricular volume.

Methods

Psychiatric inpatients, diagnosed mainly with depression, substance use, anxiety, and personality disorders, underwent two imaging sessions (Time 1 and Time 2, approximately 4 weeks apart). FreeSurfer was used to quantify volumetric features of the brain, and ANOVA was used to analyze ventricular volume differences between Time 1 and Time 2. Inpatients' brain ventricle volumes were normalized by dividing by estimated total intracranial volume (eTIV). Clinical features such as depression and anxiety levels were collected at Time 1, Time 1.5 (approximately 2 weeks apart), and Time 2.

Results

Inpatients consistently taking SRIs (SRI + , n = 44) showed statistically significant reductions of brain ventricular volumes particularly for their left and right lateral ventricular volumes. Reductions in their third ventricular volume were close to significance (p = .068). The inpatients that did not take SRIs (SRI-, n = 25) showed no statistically significant changes in brain ventricular volumes. The SRI + group also exhibited similar brain structural features to the healthy control group based on the 90% confidence interval comparsions on brain ventricular volume parameters, whereas the SRI- group still exhibited relatively enlarged brain ventricular volumes after treatment.

Conclusions

SRI treatment was associated with decreased brain ventricle volume over treatment.

The Durability of Vaccine Efficacy against Ocular HSV-1 Infection Using ICP0 Mutants 0∆NLS and 0∆RING Is Lost over Time

Vaccines to viral pathogens in experimental animal models are often deemed successful if immunization enhances resistance of the host to virus challenge as measured by cumulative survival, reduction in virus replication and spread and/or lessen or eliminate overt tissue pathology. Furthermore, the duration of the protective response against challenge is another important consideration that drives a vaccination regimen. In the current study, we assessed the durability of two related vaccines, 0∆NLS and 0∆RING, against ocular herpes simplex virus type 1 (HSV-1) challenge in mice thirty days (short-term) and one year (long-term) following the vaccine boost. The short-term vaccine efficacy study found the 0∆RING vaccine to be nearly equivalent to the 0∆NLS vaccine in comparison to vehicle-vaccinated mice in terms of controlling virus replication and preserving the visual axis. By comparison, the long-term assessment of the two vaccines found notable differences and less efficacy overall as noted below. Specifically, the results show that in comparison to vehicle-vaccinated mice, the 0∆NLS and 0∆RING vaccinated groups were more resistant in terms of survival and virus shedding following ocular challenge. Moreover, 0∆NLS vaccinated mice also possessed significantly less infectious virus in the peripheral and central nervous systems but not the cornea compared to mice vaccinated with vehicle or 0∆RING which had similar levels. However, all vaccinated groups showed similar levels of blood and lymphatic vessel genesis into the central cornea 30 days post infection. Likewise, corneal opacity was also similar among all groups of vaccinated mice following infection. Functionally, the blink response and visual acuity were 25-50% lower in vaccinated mice 30 days post infection compared to measurements taken prior to infection. The results demonstrate a dichotomy between resistance to infection and functional performance of the visual axis that collectively show an overall loss in vaccine efficacy long-term in comparison to short-term studies in a conventional prime-boost protocol.

The Structure of the Spinal Cord Ependymal Region in Adult Humans Is a Distinctive Trait among Mammals

In species that regenerate the injured spinal cord, the ependymal region is a source of new cells and a prominent coordinator of regeneration. In mammals, cells at the ependymal region proliferate in normal conditions and react after injury, but in humans, the central canal is lost in the majority of individuals from early childhood. It is replaced by a structure that does not proliferate after damage and is formed by large accumulations of ependymal cells, strong astrogliosis and perivascular pseudo-rosettes. We inform here of two additional mammals that lose the central canal during their lifetime: the Naked Mole-Rat (NMR, Heterocephalus glaber) and the mutant hyh (hydrocephalus with hop gait) mice. The morphological study of their spinal cords shows that the tissue substituting the central canal is not similar to that found in humans. In both NMR and hyh mice, the central canal is replaced by tissue reminiscent of normal lamina X and may include small groups of ependymal cells in the midline, partially resembling specific domains of the former canal. However, no features of the adult human ependymal remnant are found, suggesting that this structure is a specific human trait. In order to shed some more light on the mechanism of human central canal closure, we provide new data suggesting that canal patency is lost by delamination of the ependymal epithelium, in a process that includes apical polarity loss and the expression of signaling mediators involved in epithelial to mesenchymal transitions.

Cytokines and chemokines expression pattern in herpes simplex virus type-1 encephalitis

To explore the differently expressed cytokines and chemokines to understand the pathways that lead to herpes simplex encephalitis (HSE). Mice in the experimental group were inoculated intracranially with HSV-1. A high-throughput cytokine chip assay was employed to assess the expression of cytokines/chemokines in the mice brain. GO, KEGG, and PPIs analyses were used to investigate the biological process (BP), pathways and interaction network of the differently expressed proteins (DEPs) in HSE. 13 DEPs and various proteins-related signal pathways were identified in HSE, including three new factors (IL-1α, MIP-1γ, and sTNF RI). The proteins were mainly implicated in leukocyte activation and chemotaxis. Additionally, the DEPs constituted a pivotal protein interaction network where IL-6 might be a mediator. 13 DEPs and a series of related signal pathways were associated with the pathophysiological mechanisms responsible for HSE. IL-6 might be a key mediator in the inflammatory responses to the disease.

Construction and optimization of herpes simplex virus vectors for central nervous system gene delivery based on CRISPR/Cas9-mediated genome editing

AIMS

We aim to define parameters affecting the safety and long-term transgene expression of attenuated HSV-1 vectors and optimize the expression cassettes to achieve robust and sustained expression in CNS.

BACKGROUND

Engineered, attenuated Herpes simplex virus (HSV) vectors are promising vehicles for gene delivery to the peripheral and central nervous systems. The virus latent promoter (LAP) is commonly used to drive exogenous gene expression; however, parameters affecting the safety and long-term transgene expression of attenuated HSV-1 vectors have not been fully understood.

OBJECTIVE

This study aimed to construct attenuated HSV-1 vectors using the CRISPR-Cas9 system and examine the influence of transgene cassette construction and insertion site on transgene expression and vector safety.

METHOD

In this study, we used a CRISPR-Cas9 system to accurately and efficiently edit attenuated HSV-1 strain 1716, and constructed two series of recombinant virus LMR and LMRx with different sets of gene cassettes insertion in Exon1(LAP2) and 2.0 kb intron downstream of LAP, respectively. The transgene expression and viral gene transcriptional kinetics were compared in in-vitro cell lines. The reporter gene expression and safety profiles of each vector were further evaluated in the mouse hippocampus gene transduction model.

RESULT

The in-vitro cell line analysis indicated that the insertion of a gene expression cassette would disrupt virus gene transcription. Mouse hippocampus transducing analysis suggested that complete expression cassette insertion at 2.0 kb intron could achieve robust and longtime gene expression than the other constructs. Recombinants with gene expression cassettes lacked Poly (A), which induced significant neuronal inflammation due to persistent viral antigen expression and microglia activation.

CONCLUSION

Our results indicated that the integrity of LAT transcripts was not necessary for the establishment of long-term latent expression. Exogenous strong promoters (like cBh promoter) could remain active during latency when placed in Exon1 or 2.0 Kb Intron of LAT locus, although their transcriptional activity declined with time. Consistent with previous research, the foreign gene expression would last much longer when the gene cassette was located downstream of Exon1, which suggested a role of LAP2 in maintaining promoter activity during latency. Besides, over-transcription of the downstream part of LAT may induce continuous activation of the attenuated vectors, suggesting an important role of LAT in maintaining viral reactivation potential.

The old guard: Age-related changes in microglia and their consequences

Among all major organs, the brain is one of the most susceptible to the inexorable effects of aging. Throughout the last decades, several studies in human cohorts and animal models have revealed a plethora of age-related changes in the brain, including reduced neurogenesis, oxidative damage, mitochondrial dysfunction and cell senescence. As the main immune effectors and first responders of the nervous tissue, microglia are at the center of these events. These cells experience irrevocable changes as a result from cumulative exposure to environmental triggers, such as stress, infection and metabolic dysregulation. The age-related immunosenescent phenotype acquired by microglia is characterized by profound modifications in their transcriptomic profile, secretome, morphology and phagocytic activity, which compromise both their housekeeping and defensive functions. As a result, aged microglia are no longer capable of establishing effective immune responses and sustaining normal synaptic activity, directly contributing to age-associated cognitive decline and neurodegeneration. This review discusses how lifestyle and environmental factors drive microglia dysfunction at the molecular and functional level, also highlighting possible interventions to reverse aging-associated damage to the nervous and immune systems.

The Potential Role of Herpes Simplex Virus Type 1 and Neuroinflammation in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease affecting ~50 million people worldwide. To date, there is no cure and current therapies have not been effective in delaying disease progression. Therefore, there is an urgent need for better understanding of the pathogenesis of AD and to rethink possible therapies. Herpes simplex virus type 1 (HSV1) has recently received growing attention for its potential role in sporadic AD. The virus is a ubiquitous human pathogen that infects mucosal epithelia and invades the peripheral nervous system (PNS) of its host to establish a reactivable, latent infection. Upon reactivation, HSV1 spreads back to the epithelium and initiates a new infection, causing epithelial lesions. Occasionally, the virus spreads from the PNS to the brain after reactivation. In this review, we discuss current work on the pathogenesis of AD and summarize research results that support a potential role for HSV1 in the infectious hypothesis of AD. We also highlight recent findings on the neuroinflammatory response, which has been proposed to be the main driving force of AD, starting early in the course of the disease. Relevant rodent models to study neuroinflammation in AD and novel therapeutic approaches are also discussed. Throughout this review, we focus on several aspects of HSV1 pathogenesis, including its primary role as an invader of the PNS, that should be considered in the etiology of AD. We also point out some of the contradictory data and remaining knowledge gaps that require further research to finally fully understand the cause of AD in humans.

Lack of neonatal Fc receptor does not diminish the efficacy of the HSV-1 0ΔNLS vaccine against ocular HSV-1 challenge

The neonatal Fc receptor (FcRn) is constitutively expressed in the cornea and is up-regulated in response to herpes simplex virus type 1 (HSV-1). Previously, we found targeting cornea FcRn expression by small interfering RNA-mediated knockdown reduced the local efficacy of HSV-1 0ΔNLS vaccinated C57BL/6 mice against ocular challenge with HSV-1. The current study was undertaken to evaluate the HSV-1 0ΔNLS vaccine efficacy in FcRn deficient (FcRn KO) mice challenged with HSV-1. Whereas there was little neutralizing antibody detected in the serum of HSV-1 0ΔNLS vaccinated FcRn KO mice, these mice exhibited the same degree of protection against ocular challenge with HSV-1 as wild type (WT) C57BL/6 mice as measured by cumulative survival, infectious virus shed or retained in tissue, and corneal pathology including opacity and neovascularization. Mock-vaccinated FcRn KO mice were found to be more sensitive to ocular HSV-1 infection compared to mock-vaccinated (WT) mice in terms of cumulative survival and virus shedding. In addition, the FcRn KO mice generated significantly fewer effector (CD3⁺CD44⁺CD62L^-) and central (CD3⁺CD44⁺CD62L⁺) memory CD8⁺ T cells compared to the WT mice 7 days post infection. Collectively, mock-vaccinated FcRn KO mice are susceptible to ocular HSV-1 infection but HSV-1 0ΔNLS vaccinated FcRn KO mice are resistant suggesting that in addition to the FcRn, other pathways are involved in mediating the protective effect of the HSV-1 0ΔNLS vaccine against subsequent HSV-1 challenge.

Dementia and cognitive impairment in adults as sequels of HSV-1-related encephalitis: a review

Considering the variety of mechanisms of Herpes simplex virus (HSV-1) contamination and its broad invasive potential of the nervous system, a life-long latent infection is established. Infected adult individuals may be susceptible to viral reactivation when under the influence of multiple stressors, especially regarding immunocompromised patients. This guides a series of neuroinflammatory events on the cerebral cortex, culminating, rarely, in encephalitis and cytotoxic / vasogenic brain edema. A sum of studies of such processes provides an explanation, even though not yet completely clarified, on how the clinical evolution to cognitive impairment and dementia might be enabled. In addition, it is of extreme importance to recognize the current dementia and cognitive deficit worldwide panorama. The aim of this literature review is to elucidate the available data upon the pathophysiology of HSV-1 infection as well as to describe the clinical panorama of the referred afflictions.

Analysis of ALS-related proteins during herpes simplex virus-2 latent infection

BACKGROUND

Genetics have provided hints on potential molecular pathways involved in neurodegenerative diseases (NDD). However, the number of cases caused exclusively by genetic alterations is low, suggesting an important contribution of environmental factors to NDDs. Among these factors, viruses like herpes simplex viruses (HSV-2), capable of establishing lifelong infections within the nervous system (NS), are being proposed to have a role in NDDs. Despite promising data, there is a significant lack of knowledge on this and an urgent need for more research.

METHODS

We have set up a mouse model to study HSV latency and its associated neuroinflammation in the spinal cord. The goal of this model was to observe neuroinflammatory changes caused by HSV latent infections, and if those changes were similar to alterations observed in the spinal cord of amyotrophic lateral sclerosis (ALS) patients.

RESULTS

In infected spinal cords, we have observed a strong leukocyte infiltration and a severe alteration of microglia close to motor neurons. We have also analyzed ALS-related proteins: we have not found changes in TDP-43 and Fus in neurons, but interestingly, we have found decreased protein levels of C9orf72, of which coding gene is severely altered in some familial forms of ALS and is critical for microglia homeostasis.

CONCLUSIONS

Latent infection of HSV in the spinal cord showed altered microglia and leukocyte infiltration. These inflammatory features resembled to those observed in the spinal cord of ALS patients. No changes mimicking ALS neuropathology, such as TDP-43 cytoplasmic inclusions, were found in infected spinal cords, but a decrease in protein levels of C9orf72 was observed. Then, further studies should be required to determine whether HSV-2 has a role in ALS.

CNS Macrophages and Infant Infections

The central nervous system (CNS) harbors its own immune system composed of microglia in the parenchyma and CNS-associated macrophages (CAMs) in the perivascular space, leptomeninges, dura mater, and choroid plexus. Recent advances in understanding the CNS resident immune cells gave new insights into development, maturation and function of its immune guard. Microglia and CAMs undergo essential steps of differentiation and maturation triggered by environmental factors as well as intrinsic transcriptional programs throughout embryonic and postnatal development. These shaping steps allow the macrophages to adapt to their specific physiological function as first line of defense of the CNS and its interfaces. During infancy, the CNS might be targeted by a plethora of different pathogens which can cause severe tissue damage with potentially long reaching defects. Therefore, an efficient immune response of infant CNS macrophages is required even at these early stages to clear the infections but may also lead to detrimental consequences for the developing CNS. Here, we highlight the recent knowledge of the infant CNS immune system during embryonic and postnatal infections and the consequences for the developing CNS.

Cholecystokinin-8 attenuates methamphetamine-induced inflammatory activation of microglial cells through CCK2 receptor

Methamphetamine (METH) exposure reportedly promotes microglial activation and pro-inflammatory cytokines secretion. Sustained inflammation in abusers of psychostimulant drugs further induces neural damage. Cholecystokinin-8 (CCK-8) is a gut-brain peptide which exerts a wide range of biological activities in the gastrointestinal tract and central nervous system. We previously found that pre-treatment with CCK-8 inhibited behavioural and histologic changes typically induced by repeated exposure to METH. Here, we aimed to estimate the effects of CCK-8 on METH-induced neuro-inflammation, which is markedly characterized by microglia activation and increased pro-inflammatory cytokines production in vivo and in vitro. Moreover, we assessed the subtypes of the CCK receptor mediating the regulatory effects of CCK-8, and the changes in the NF-κB signalling pathway. We found that CCK-8 inhibited METH-induced microglial activation and IL-6 and TNF-α generation in vivo and in vitro in a dose-dependent manner. Furthermore, co-treatment of CCK-8 with METH significantly attenuated the activation of the NF-κB signalling pathway by activating the CCK2 receptor subtype in N9 cells. In conclusion, our findings indicated the inhibitory effect of CCK-8 on METH-induced neuro-inflammation in vivo and in vitro, and suggested the underlying mechanism may involve the activation of the CCK2 receptor, which downregulated the NF-κB signalling pathway induced by METH stimulation.

Patterns of Herpes Simplex Virus 1 Infection in Neural Progenitor Cells

This study employed human induced pluripotent stem cells (hiPSCs) to model the interaction of HSV-1 with NPCs, which reside in the neurogenic niches of the CNS and play a fundamental role in adult neurogenesis. Herein, we provide evidence that in NPCs infected at an MOI as low as 0.001, HSV-1 can establish a latent state, suggesting that (i) a variant of classical HSV-1 latency can be established during earlier stages of neuronal differentiation and (ii) neurogenic niches in the brain may constitute additional sites of viral reactivation. Lytic HSV-1 infections impaired NPC migration, which represents a critical step in neurogenesis. A difference in susceptibility to HSV-1 infection between two-dimensional (2D) and three-dimensional (3D) NPC cultures was observed, highlighting the potential value of 3D cultures for modeling host-pathogen interactions. Together, our results are relevant in light of observations relating HSV-1 infection to postencephalitic cognitive dysfunction.

Herpes simplex virus 1 (HSV-1) can induce damage in brain regions that include the hippocampus and associated limbic structures. These neurogenic niches are important because they are associated with memory formation and are highly enriched with neural progenitor cells (NPCs). The susceptibility and fate of HSV-1-infected NPCs are largely unexplored. We differentiated human induced pluripotent stem cells (hiPSCs) into NPCs to generate two-dimensional (2D) and three-dimensional (3D) culture models to examine the interaction of HSV-1 with NPCs. Here, we show that (i) NPCs can be efficiently infected by HSV-1, but infection does not result in cell death of most NPCs, even at high multiplicities of infection (MOIs); (ii) limited HSV-1 replication and gene expression can be detected in the infected NPCs; (iii) a viral silencing mechanism is established in NPCs exposed to the antivirals (E)-5-(2-bromovinyl)-2′-deoxyuridine (5BVdU) and alpha interferon (IFN-α) and when the antivirals are removed, spontaneous reactivation can occur at low frequency; (iv) HSV-1 impairs the ability of NPCs to migrate in a dose-dependent fashion in the presence of 5BVdU plus IFN-α; and (v) 3D cultures of NPCs are less susceptible to HSV-1 infection than 2D cultures. These results suggest that NPC pools could be sites of HSV-1 reactivation in the central nervous system (CNS). Finally, our results highlight the potential value of hiPSC-derived 3D cultures to model HSV-1–NPC interaction.

IMPORTANCE This study employed human induced pluripotent stem cells (hiPSCs) to model the interaction of HSV-1 with NPCs, which reside in the neurogenic niches of the CNS and play a fundamental role in adult neurogenesis. Herein, we provide evidence that in NPCs infected at an MOI as low as 0.001, HSV-1 can establish a latent state, suggesting that (i) a variant of classical HSV-1 latency can be established during earlier stages of neuronal differentiation and (ii) neurogenic niches in the brain may constitute additional sites of viral reactivation. Lytic HSV-1 infections impaired NPC migration, which represents a critical step in neurogenesis. A difference in susceptibility to HSV-1 infection between two-dimensional (2D) and three-dimensional (3D) NPC cultures was observed, highlighting the potential value of 3D cultures for modeling host-pathogen interactions. Together, our results are relevant in light of observations relating HSV-1 infection to postencephalitic cognitive dysfunction.

HSV1 VP1-2 deubiquitinates STING to block type I interferon expression and promote brain infection

Herpes simplex virus (HSV) is the main cause of viral encephalitis in the Western world, and the type I interferon (IFN) system is important for antiviral control in the brain. Here, we have compared Ifnb induction in mixed murine brain cell cultures by a panel of HSV1 mutants, each devoid of one mechanism to counteract the IFN-stimulating cGAS-STING pathway. We found that a mutant lacking the deubiquitinase (DUB) activity of the VP1-2 protein induced particularly strong expression of Ifnb and IFN-stimulated genes. HSV1 ΔDUB also induced elevated IFN expression in murine and human microglia and exhibited reduced viral replication in the brain. This was associated with increased ubiquitination of STING and elevated phosphorylation of STING, TBK1, and IRF3. VP1-2 associated directly with STING, leading to its deubiquitination. Recruitment of VP1-2 to STING was dependent on K150 of STING, which was ubiquitinated by TRIM32. Thus, the DUB activity of HSV1 VP1-2 is a major viral immune-evasion mechanism in the brain.

CD8 T Cells and STAT1 Signaling Are Essential Codeterminants in Protection from Polyomavirus Encephalopathy

JC polyomavirus (JCPyV), a human-specific virus, causes the aggressive brain-demyelinating disease progressive multifocal leukoencephalopathy (PML) in individuals with depressed immune status. The increasing incidence of PML in patients receiving immunotherapeutic and chemotherapeutic agents creates a pressing clinical need to define biomarkers to stratify PML risk and develop anti-JCPyV interventions. Mouse polyomavirus (MuPyV) CNS infection causes encephalopathology and may provide insight into JCPyV-PML pathogenesis. Type I, II, and III interferons (IFNs), which all signal via the STAT1 transcription factor, mediate innate and adaptive immune defense against a variety of viral infections. We previously reported that type I and II IFNs control MuPyV infection in non-central nervous system (CNS) organs, but their relative contributions to MuPyV control in the brain remain unknown. To this end, mice deficient in type I, II, or III IFN receptors or STAT1 were infected intracerebrally with MuPyV. We found that STAT1, but not type I, II, or III IFNs, mediated viral control during acute and persistent MuPyV encephalitis. Mice deficient in STAT1 also developed severe hydrocephalus, blood-brain barrier permeability, and increased brain infiltration by myeloid cells. CD8 T cell deficiency alone did not increase MuPyV infection and pathology in the brain. In the absence of STAT1 signaling, however, depletion of CD8 T cells resulted in lytic infection of the choroid plexus and ependymal lining, marked meningitis, and 100% mortality within 2 weeks postinfection. Collectively, these findings indicate that STAT1 signaling and CD8 T cells cocontribute to controlling MuPyV infection in the brain and CNS injury.IMPORTANCE A comprehensive understanding of JCPyV-induced PML pathogenesis is needed to define determinants that predispose patients to PML, a goal whose urgency is heightened by the lack of anti-JCPyV agents. A handicap to achieving this goal is the lack of a tractable animal model to study PML pathogenesis. Using intracerebral inoculation with MuPyV, we found that MuPyV encephalitis in wild-type mice causes an encephalopathy, which is markedly exacerbated in mice deficient in STAT1, a molecule involved in transducing signals from type I, II, and III IFN receptors. CD8 T cell deficiency compounded the severity of MuPyV neuropathology and resulted in dramatically elevated virus levels in the CNS. These findings demonstrate that STAT1 signaling and CD8 T cells concomitantly act to mitigate MuPyV-encephalopathy and control viral infection.

What Can Mimic Multiple Sclerosis?

This article discusses mimics of multiple sclerosis (MS). Excluded in this discussion are neuromyelitis optica and vasculitis, discussed in other articles in this journal. Covered entities include posterior reversible encephalopathy syndrome, reversible vasoconstriction syndrome, acute disseminated encephalomyelitis, Sussac's Syndrome, and chronic idiopathic demyelinating polyneuropathy. There are also multiple infectious entities that mimic MS including; progressive multi-focal leukoencephalopathy (PML), Toxoplasmosis, Tuberculosis, Herpes Simplex Virus, Cytomegalovirus, Varicella zoster virus, Epstein Barr virus, Cryptococcus and Human immunodeficiency virus. In addition, there are leukoencephalopathies that can present in adulthood including Adrenoleukodystrophy, Metachromatic leukodystrophy, Cerebral autosomal dominant idiopathic leukoencephalopathy, Leigh's and Alexanders disease that could be mistaken for MS.

Microglia Are Essential to Protective Antiviral Immunity: Lessons From Mouse Models of Viral Encephalitis

Viral encephalitis is a rare but clinically serious consequence of viral invasion of the brain and insight into its pathogenesis is urgently needed. Important research questions concern the involvement of the host innate immune response in pathogenesis, key to which is the role played by microglia, resident macrophages of the brain parenchyma. Do microglia have a protective function, by coordinating the innate immune response to viral infection, or do they drive pathogenic neuroinflammation? Here we synthesize recent data from mouse models of acute viral encephalitis, which reveal an unambiguously protective role for microglia. Depletion of microglia, via blockade of colony-stimulating factor 1 receptor (CSF1R) signaling, led to increased viral replication accompanied by more severe neurological disease and heightened mortality. Whilst the underlying mechanism(s) remain to be defined, microglial interactions with T cells and phagocytosis of infected neurones appear to play a role. Paradoxically, the production of inflammatory cytokines was increased in several instances following viral infection in microglia-depleted brains, suggesting that: (i) cells other than microglia mediate inflammatory responses and/or (ii) microglia may exert a regulatory function. Under certain circumstances the microglial antiviral response might contribute negatively to longer-term neurological sequelae, although fewer studies have focused on this aspect in encephalitis models. Understanding regulation of the microglial response, and how it contributes to disease is therefore a priority for future studies. Collectively, these findings demonstrate the central role of microglia in pathogenesis, suggesting the exciting possibility that defects of microglial function might contribute to encephalitis susceptibility and/or outcome in humans.

β-Arrestin 2 protects against neurological function defects in HSV-1-induced encephalitis mice

The pathogenesis of herpes simplex encephalitis (HSE) needs to be fully explored. β-Arrestin 2 (Arrb2) is highly expressed in brain tissues and plays a key role in the regulation of systemic immune reactions by modulating various signaling pathways. However, the expression of Arrb2 in microglial cells and its influence on HSE prognosis is still undefined. We explore the pathophysiological effect of Arrb2 in the brain using experimental HSE mice. The expression of Arrb2 in microglia was decreased significantly 48 hours following HSV-1 infection. Arrb2 overexpression transgenic (TG) mice had a significantly lower mortality and survival rate was improved by 40% compared to wild-type mice. Arrb2 suppressed the generation of proinflammatory cytokines TNF-α and IL-6 and increased anti-inflammatory cytokines IL-10 and IL-4 expression. Arrb2 also inhibited the activation of the transcription factor NF-κB in microglial cells. Arrb2 TG mice attenuated the blood-brain barrier breakdown and relieved cerebral edema, meanwhile, Arrb2 improved mice neurological function compared with wild-type mice. Overall, Arrb2 favored microglia of the M2 phenotype, attenuated brain proinflammatory responses, protected the blood vessel wall integrity, reduced HSV-1-induced neurological impairment, and improved the survival rate in HSE mice.

Modulation of Type-I Interferon Response by hsa-miR-374b-5p During Japanese Encephalitis Virus Infection in Human Microglial Cells

Japanese Encephalitis virus (JEV) is a neurotropic ssRNA virus, belonging to the Flaviviridae family. JEV is one of the leading causes of the viral encephalitis in Southeast-Asian countries. JEV primarily infects neurons however, the microglial activation has been reported to further enhance the neuroinflammation and promote neuronal death. The PI3K/AKT pathway has been reported to play an important role in type-I interferon response via IRF3. Phosphatase and tensin homolog (PTEN), a negative regulator of PI3K/AKT pathway, participates in microglial polarization and neuroinflammation. The microRNAs are small non-coding endogenously expressed RNAs, which regulate the gene expression by binding at 3′ UTR of target gene. The human microglial cells were infected with JEV (JaOArS982 strain) and up-regulation of microRNA; hsa-miR-374b-5p was confirmed by qRT-PCR. The genes in PI3K/AKT pathway, over-expression and knock-down studies of hsa-miR-374b-5p with and without JEV infection were analyzed through immuno blotting. The regulatory role of hsa-miR-374b-5p on the expression of type-I interferon was determined by luciferase assays. JEV infection modulated the expression of hsa-miR-374b-5p and PI3K/AKT pathway via PTEN. The over-expression of hsa-miR-374b-5p suppressed the PTEN while up-regulated the AKT and IRF3 proteins, whereas, the knockdown rescued the PTEN expression and suppressed the AKT and IRF3 proteins. The modulation of hsa-miR-374b-5p regulated the type-I interferon response during JEV infection. In present study, we have shown the modulation of PTEN by hsa-miR-374b-5p, which regulated the PI3K/AKT/IRF3 axis in JEV infected microglial cells.

A Novel Murine Model Expressing a Chimeric mSCARB2/hSCARB2 Receptor Is Highly Susceptible to Oral Infection with Clinical Isolates of Enterovirus 71

Enterovirus 71 (EV71) infection is generally associated with hand-foot-and-mouth disease (HFMD) and may cause severe neurological disorders and even death. An effective murine oral infection model for studying the pathogenesis of various clinical EV71 isolates is lacking. We developed a transgenic (Tg) mouse that expresses an EV71 receptor, that is, human scavenger receptor class B member 2 (hSCARB2), in a pattern highly similar to that of endogenous murine SCARB2 (mSCARB2) protein. A FLAG-tagged SCARB2 cDNA fragment composed of exons 3 to 12 was inserted into a murine Scarb2 gene-containing bacterial artificial chromosome (BAC) clone, and the resulting transgene was used for establishment of chimeric receptor-expressing Tg mice. Tg mice intragastrically (i.g.) infected with clinical isolates of EV71 showed neurological symptoms, such as ataxia and paralysis, and fatality. There was an age-dependent decrease in susceptibility to viral infection. Pathological characteristics of the infected Tg mice resembled those of encephalomyelitis in human patients. Viral infection was accompanied by microglial activation. Clodronate treatment of the brain slices from Tg mice enhanced viral replication, while lipopolysaccharide treatment significantly inhibited it, suggesting an antiviral role for microglia during EV71 infection. Taken together, this Tg mouse provides a model that closely mimics natural infection for studying EV71 pathogenesis and for evaluating the efficacy of vaccines or other antiviral drugs.IMPORTANCE The availability of a murine model of EV71 infection is beneficial for the understanding of pathogenic mechanisms and the development and assessment of vaccines and antiviral drugs. However, the lack of a murine oral infection model thwarted the study of pathogenesis induced by clinically relevant EV71 strains that are transmitted via the oral-oral or oral-fecal route. Our Tg mice could be intragastrically infected with clinically relevant EV71 strains in an efficient way and developed neurological symptoms and pathological changes strikingly resembling those of human infection. Moreover, these mice showed an age-dependent change in susceptibility that is similar to the human case. This Tg mouse, when combined with the use of other genetically modified mice, potentially contributes to studying the relationship between developmental changes in immunity and susceptibility to virus.

Herpes Simplex Virus Type 1 Infection of the Central Nervous System: Insights Into Proposed Interrelationships With Neurodegenerative Disorders

Herpes simplex virus type 1 (HSV-1) is highly prevalent in humans and can reach the brain without evident clinical symptoms. Once in the central nervous system (CNS), the virus can either reside in a quiescent latent state in this tissue, or eventually actively lead to severe acute necrotizing encephalitis, which is characterized by exacerbated neuroinflammation and prolonged neuroimmune activation producing a life-threatening disease. Although HSV-1 encephalitis can be treated with antivirals that limit virus replication, neurological sequelae are common and the virus will nevertheless remain for life in the neural tissue. Importantly, there is accumulating evidence that suggests that HSV-1 infection of the brain both, in symptomatic and asymptomatic individuals could lead to neuronal damage and eventually, neurodegenerative disorders. Here, we review and discuss acute and chronic infection of particular brain regions by HSV-1 and how this may affect neuron and cognitive functions in the host. We review potential cellular and molecular mechanisms leading to neurodegeneration, such as protein aggregation, dysregulation of autophagy, oxidative cell damage and apoptosis, among others. Furthermore, we discuss the impact of HSV-1 infection on brain inflammation and its potential relationship with neurodegenerative diseases.

Microglia control the spread of neurotropic virus infection via P2Y12 signalling and recruit monocytes through P2Y12-independent mechanisms

Neurotropic herpesviruses can establish lifelong infection in humans and contribute to severe diseases including encephalitis and neurodegeneration. However, the mechanisms through which the brain’s immune system recognizes and controls viral infections propagating across synaptically linked neuronal circuits have remained unclear. Using a well-established model of alphaherpesvirus infection that reaches the brain exclusively via retrograde transsynaptic spread from the periphery, and in vivo two-photon imaging combined with high resolution microscopy, we show that microglia are recruited to and isolate infected neurons within hours. Selective elimination of microglia results in a marked increase in the spread of infection and egress of viral particles into the brain parenchyma, which are associated with diverse neurological symptoms. Microglia recruitment and clearance of infected cells require cell-autonomous P2Y12 signalling in microglia, triggered by nucleotides released from affected neurons. In turn, we identify microglia as key contributors to monocyte recruitment into the inflamed brain, which process is largely independent of P2Y12. P2Y12-positive microglia are also recruited to infected neurons in the human brain during viral encephalitis and both microglial responses and leukocyte numbers correlate with the severity of infection. Thus, our data identify a key role for microglial P2Y12 in defence against neurotropic viruses, whilst P2Y12-independent actions of microglia may contribute to neuroinflammation by facilitating monocyte recruitment to the sites of infection.

Insights into the pathogenesis of herpes simplex encephalitis from mouse models

A majority of the world population is infected with herpes simplex viruses (HSV; human herpesvirus types 1 and 2). These viruses, perhaps best known for their manifestation in the genital or oral mucosa, can also cause herpes simplex encephalitis, a severe and often fatal disease of the central nervous system. Antiviral therapies for HSV are only partially effective since the virus can establish latent infections in neurons, and severe pathological sequelae in the brain are common. A better understanding of disease pathogenesis is required to develop new strategies against herpes simplex encephalitis, including the precise viral and host genetic determinants that promote virus invasion into the central nervous system and its associated immunopathology. Here we review the current understanding of herpes simplex encephalitis from the host genome perspective, which has been illuminated by groundbreaking work on rare herpes simplex encephalitis patients together with mechanistic insight from single-gene mouse models of disease. A complex picture has emerged, whereby innate type I interferon-mediated antiviral signaling is a central pathway to control viral replication, and the regulation of immunopathology and the balance between apoptosis and autophagy are critical to disease severity in the central nervous system. The lessons learned from mouse studies inform us on fundamental defense mechanisms at the interface of host–pathogen interactions within the central nervous system, as well as possible rationales for intervention against infections from severe neuropathogenic viruses.

Uncovering inherent cellular plasticity of multiciliated ependyma leading to ventricular wall transformation and hydrocephalus

Specialized, differentiated cells often perform unique tasks that require them to maintain a stable phenotype. Multiciliated ependymal cells (ECs) are unique glial cells lining the brain ventricles, important for cerebral spinal fluid circulation. While functional ECs are needed to prevent hydrocephalus, they have also been reported to generate new neurons: whether ECs represent a stable cellular population remains unclear. Via a chemical screen we found that mature ECs are inherently plastic, with their multiciliated state needing constant maintenance by the Foxj1 transcription factor, which paradoxically is rapidly turned over by the ubiquitin-proteasome system leading to cellular de-differentiation. Mechanistic analyses revealed a novel NF-κB-independent IKK2 activity stabilizing Foxj1 in mature ECs, and we found that known IKK2 inhibitors including viruses and growth factors robustly induced Foxj1 degradation, EC de-differentiation, and hydrocephalus. Although mature ECs upon de-differentiation can divide and regenerate multiciliated ECs, we did not detect evidence supporting EC’s neurogenic potential.

Multiciliated ependymal cells (ECs) in the mammalian brain are glial cells facilitating cerebral spinal fluid movement. This study describes an inherent cellular plasticity of ECs as maintained by Foxj1 and IKK2 signaling, and shows resulting hydrocephalus when EC de-differentiation is triggered.

New advances in CNS immunity against viral infection

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Advances in CNS immunity and anatomy bridge the CNS and the peripheral immune system.

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Region-specific antiviral responses alter BBB permeability during viral invasion.

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CNS barriers have anatomical specializations with tailored defenses against pathogens.

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Immunocytotherapies for persistent CNS infections can promote non-cytopathic viral clearance.

The central nervous system (CNS) is an immunologically specialized organ where restrictive barrier structures protect the parenchyma from inflammation and infection. This protection is important in preventing damage to non-renewable resident cell populations, such as neurons, responsible for functions ranging from executive to autonomic. Despite these barriers, the CNS can be infected through several entry portals, giving rise to meningitis and encephalitis. Following infection, resident cells recruit peripherally derived immune cells to sites of viral infection. In this review, we discuss recent advances in immune recruitment and entry at barrier structures as well as current immunotherapeutic strategies for the treatment of persistent viral infections.

Long-term consequences of topical dexamethasone treatment during acute corneal HSV-1 infection on the immune system

Herpes simplex virus type 1 (HSV-1) is a leading cause of neurotrophic keratitis (NTK). NTK is characterized by decreased corneal sensation from damage to the corneal sensory fibers. We have reported on the regression of corneal nerves and their function during acute HSV-1 infection. That nerve loss is followed by an aberrant process of nerve regeneration during the latent phase of infection that lacks functional recovery. We recently showed the elicited immune response in the infected cornea, and not viral replication itself, is part of the mechanism responsible for the nerve degeneration process after infection. Specifically, we showed infected corneas topically treated with dexamethasone (DEX) significantly retained both structure and sensitivity of the corneal nerve network in comparison to mice treated with control eye drops, consistent with decreased levels of proinflammatory cytokines and reduced influx of macrophages and CD8+ T cells into the cornea. This study was undertaken to analyze the long-term effect of such a localized, immunosuppressive paradigm (DEX drops on the cornea surface during the first 8 d of HSV-1 infection) on the immune system and on corneal pathology. We found the profound immunosuppressive effect of DEX on lymphoid tissue was sustained in surviving mice for up to 30 d postinfection (p.i.). DEX treatment had prolonged effects, preserving corneal innervation and its function and blunting neovascularization, as analyzed at 30 d p.i. Our data support previously reported observations of an association between the persistent presence of inflammatory components in the latently infected cornea and structural and functional nerve defects in NTK.

Tumor Necrosis Factor Alpha-Induced Recruitment of Inflammatory Mononuclear Cells Leads to Inflammation and Altered Brain Development in Murine Cytomegalovirus-Infected Newborn Mice

Congenital human cytomegalovirus (HCMV) infection is a significant cause of abnormal neurodevelopment and long-term neurological sequelae in infants and children. Resident cell populations of the developing brain have been suggested to be more susceptible to virus-induced cytopathology, a pathway thought to contribute to the clinical outcomes following intrauterine HCMV infection. However, recent findings in a newborn mouse model of the infection in the developing brain have indicated that elevated levels of proinflammatory mediators leading to mononuclear cell activation and recruitment could underlie the abnormal neurodevelopment. In this study, we demonstrate that treatment with tumor necrosis factor alpha (TNF-α)-neutralizing antibodies decreased the frequency of CD45⁺ Ly6C^hi CD11b⁺ CCR2⁺ activated myeloid mononuclear cells (MMCs) and the levels of proinflammatory cytokines in the blood and the brains of murine CMV-infected mice. This treatment also normalized neurodevelopment in infected mice without significantly impacting the level of virus replication. These results indicate that TNF-α is a major component of the inflammatory response associated with altered neurodevelopment that follows murine CMV infection of the developing brain and that a subset of peripheral blood myeloid mononuclear cells represent a key effector cell population in this model of virus-induced inflammatory disease of the developing brain.IMPORTANCE Congenital human cytomegalovirus (HCMV) infection is the most common viral infection of the developing human fetus and can result in neurodevelopmental sequelae. Mechanisms of disease leading to neurodevelopmental deficits in infected infants remain undefined, but postulated pathways include loss of neuronal progenitor cells, damage to the developing vascular system of the brain, and altered cellular positioning. Direct virus-mediated cytopathic effects cannot explain the phenotypes of brain damage in most infected infants. Using a mouse model that recapitulates characteristics of the brain infection described in human infants, we have shown that TNF-α plays a key role in brain inflammation, including recruitment of inflammatory mononuclear cells. Neutralization of TNF-α normalized neurodevelopmental abnormalities in infected mice, providing evidence that virus-induced inflammation is a major component of disease in the developing brain. These results suggest that interventions limiting inflammation associated with the infection could potentially improve the neurologic outcome of infants infected in utero with HCMV.

Protective role of CX3CR1 signalling in resident cells of the central nervous system during experimental herpes simplex virus encephalitis

CX3CR1 is an important chemokine receptor expressed on the surface of microglia and blood leukocytes, including monocytes. Signalling through this receptor influences the immune activity of microglia and monocyte trafficking into the central nervous system (CNS) in several neurological diseases. During experimental herpes simplex virus 1 (HSV-1) encephalitis (HSE), CX3CR1 deficiency has been reported to exacerbate the outcome of the disease. However, the precise contribution of CX3CR1 expressed in resident cells of the CNS or peripheral monocytes in protection against HSE remains unclear. To dissect the role of CX3CR1 during HSE, we reconstituted irradiated C57BL/6 WT and CX3CR1-/- mice with CX3CR1-/- (CX3CR1-/-→WT) and WT (WT→CX3CR1-/-) bone marrow cells, respectively. Our results showed that following intranasal infection with 1.2×106 p.f.u. of HSV-1, mortality rates were significantly higher in CX3CR1-/- (61.7 %) and WT→CX3CR1-/- (66.2 %) compared to WT (16.6 %; P=0.012 and P=0.016, respectively) and CX3CR1-/-→WT animals (20 %; P=0.013 and P=0.011, respectively). Higher mortality rates in CX3CR1-/- and WT→CX3CR1-/- mice were associated with increased infectious viral titres and wider HSV dissemination in brains, as well as an overproduction of inflammatory cytokines and chemokines including IL-1β, IL-6, IFN-γ, C-C motif ligand 2 and C-C motif ligand 5. Furthermore, CX3CR1 deficiency in resident cells of the CNS resulted in excessive and sustained Ly6Chi inflammatory monocyte and neutrophil infiltration into the brain. These data suggest that CX3CR1 deficiency in resident cells of the CNS affects mouse survival, HSV-1 replication control and cerebral inflammatory response whereas its deficiency in the haematopoietic system does not appear to influence the outcome of HSE.

Impact of Type I Interferon on the Safety and Immunogenicity of an Experimental Live-Attenuated Herpes Simplex Virus 1 Vaccine in Mice

Viral fitness dictates virulence and capacity to evade host immune defenses. Understanding the biological underpinnings of such features is essential for rational vaccine development. We have previously shown that the live-attenuated herpes simplex virus 1 (HSV-1) mutant lacking the nuclear localization signal (NLS) on the ICP0 gene (0ΔNLS) is sensitive to inhibition by interferon beta (IFN-β) in vitro and functions as a highly efficacious experimental vaccine. Here, we characterize the host immune response and in vivo pathogenesis of HSV-1 0ΔNLS relative to its fully virulent parental strain in C57BL/6 mice. Additionally, we explore the role of type 1 interferon (IFN-α/β) signaling on virulence and immunogenicity of HSV-1 0ΔNLS and uncover a probable sex bias in the induction of IFN-α/β in the cornea during HSV-1 infection. Our data show that HSV-1 0ΔNLS lacks neurovirulence even in highly immunocompromised mice lacking the IFN-α/β receptor. These studies support the translational viability of the HSV-1 0ΔNLS vaccine strain by demonstrating that, while it is comparable to a virulent parental strain in terms of immunogenicity, HSV-1 0ΔNLS does not induce significant tissue pathology.IMPORTANCE HSV-1 is a common human pathogen associated with a variety of clinical presentations ranging in severity from periodic "cold sores" to lethal encephalitis. Despite the consistent failures of HSV subunit vaccines in clinical trials spanning the past 28 years, opposition to live-attenuated HSV vaccines predicated on unfounded safety concerns currently limits their widespread acceptance. Here, we demonstrate that a live-attenuated HSV-1 vaccine has great translational potential.

Defining nervous system susceptibility during acute and latent herpes simplex virus-1 infection

Herpes simplex viruses are neurotropic human pathogens that infect and establish latency in peripheral sensory neurons of the host. Herpes Simplex Virus-1 (HSV-1) readily infects the facial mucosa that can result in the establishment of a latent infection in the sensory neurons of the trigeminal ganglia (TG). From latency, HSV-1 can reactivate and cause peripheral pathology following anterograde trafficking from sensory neurons. Under rare circumstances, HSV-1 can migrate into the central nervous system (CNS) and cause Herpes Simplex Encephalitis (HSE), a devastating disease of the CNS. It is unclear whether HSE is the result of viral reactivation within the TG, from direct primary infection of the olfactory mucosa, or from other infected CNS neurons. Areas of the brain that are susceptible to HSV-1 during acute infection are ill-defined. Furthermore, whether the CNS is a true reservoir of viral latency following clearance of virus during acute infection is unknown. In this context, this review will identify sites within the brain that are susceptible to acute infection and harbor latent virus. In addition, we will also address findings of HSV-1 lytic gene expression during latency and comment on the pathophysiological consequences HSV-1 infection may have on long-term neurologic performance in animal models and humans.

Both Cerebral and Hematopoietic Deficiencies in CCR2 Result in Uncontrolled Herpes Simplex Virus Infection of the Central Nervous System in Mice

CCR2 is a chemokine receptor expressed on the surface of blood leukocytes, particularly «Ly6Chi» inflammatory monocytes and microglia. Signaling through this receptor is thought to influence the immune activity of microglia as well as monocytes egress from the bone marrow (BM) and their trafficking into the central nervous system (CNS) in several neurological diseases. During experimental herpes simplex virus 1 (HSV-1) encephalitis (HSE), CCR2 deficiency has been reported to exacerbate the outcome of the disease. However, the precise contribution of CCR2 expressed in cells of the CNS or peripheral monocytes in the protection against HSE remains unclear. To dissect the differential role of CCR2 during HSE, chimeric mice with receptor deficiency in the brain or blood cells were generated by transplanting wild-type (WT) C57BL/6 or CCR2-/- BM-derived cells in CCR2-/- (WT→CCR2-/-) and WT (CCR2-/-→WT) mice, respectively. Our results indicate that following intranasal infection with 1.2x106 plaque forming units of HSV-1, CCR2 deficiency in hematopoietic cells and, to a lesser extent, in CNS exacerbates the outcome of HSE. Mortality rates of CCR2-/- (71.4%) and CCR2-/-→WT (57.1%) mice were significantly higher than that of WT (15.3%; P<0.01 and P<0.05, respectively) but the difference did not reach statistical significance for WT→CCR2-/- animals (42.8%; P = 0.16). Both peripheral and CNS deficiencies in CCR2 resulted in increased infectious viral titers and wider dissemination of HSV antigens in the brain as well as an overproduction of inflammatory cytokines and chemokines including IL-1β, IL-6, CCL2, CCL3 and CCL5. Furthermore, CCR2 deficiency in the hematopoietic system altered monocytes egress from the BM and their recruitment to the CNS, which may contribute to the failure in HSV-1 containment. Collectively, these data suggest that CCR2 expressed on cells of CNS and especially on peripheral monocytes is important for the control of HSV-1 replication and inflammatory environment during experimental HSE.

Sensing of HSV-1 by the cGAS-STING pathway in microglia orchestrates antiviral defence in the CNS

Herpes simplex encephalitis (HSE) is the most common form of acute viral encephalitis in industrialized countries. Type I interferon (IFN) is important for control of herpes simplex virus (HSV-1) in the central nervous system (CNS). Here we show that microglia are the main source of HSV-induced type I IFN expression in CNS cells and these cytokines are induced in a cGAS–STING-dependent manner. Consistently, mice defective in cGAS or STING are highly susceptible to acute HSE. Although STING is redundant for cell-autonomous antiviral resistance in astrocytes and neurons, viral replication is strongly increased in neurons in STING-deficient mice. Interestingly, HSV-infected microglia confer STING-dependent antiviral activities in neurons and prime type I IFN production in astrocytes through the TLR3 pathway. Thus, sensing of HSV-1 infection in the CNS by microglia through the cGAS–STING pathway orchestrates an antiviral program that includes type I IFNs and immune-priming of other cell types.

The cGAS–STING pathway is an important immune defence pathway against viral infection, including HSV-1. Here the authors use an HSV-1 encephalitis model and show microglia are the main producers of type 1 interferons that induce antiviral activity in neurons and prime the TLR3-interferon pathway in astrocytes.

Herpesvirus-Associated Lymphadenitis Distorts Fibroblastic Reticular Cell Microarchitecture and Attenuates CD8 T Cell Responses to Neurotropic Infection in Mice Lacking the STING-IFNα/β Defense Pathways

Type I IFN (IFN-α/β)-driven immune responses to acute viral infection are critical to counter replication and prevent dissemination. However, the mechanisms underlying host resistance to HSV type 1 (HSV-1) are incompletely understood. In this study, we show that mice with deficiencies in IFN-α/β signaling or stimulator of IFN genes (STING) exhibit exacerbated neurovirulence and atypical lymphotropic dissemination of HSV-1 following ocular infection. Synergy between IFN-α/β signaling and efficacy of early adaptive immune responses to HSV-1 were dissected using bone marrow chimeras and adoptive cell transfer approaches to profile clonal expansion, effector function, and recruitment of HSV-specific CD8(+) T cells. Lymphotropic viral dissemination was commensurate with abrogated CD8(+) T cell responses and pathological alterations of fibroblastic reticular cell networks in the draining lymph nodes. Our results show that resistance to HSV-1 in the trigeminal ganglia during acute infection is conferred in part by STING and IFN-α/β signaling in both bone marrow-derived and -resident cells, which coalesce to support a robust HSV-1-specific CD8(+) T cell response.

A STING-dependent innate-sensing pathway mediates resistance to corneal HSV-1 infection via upregulation of the antiviral effector tetherin

Type 1 interferons (IFNs; IFNα/β) mediate immunological host resistance to numerous viral infections, including herpes simplex virus type 1 (HSV-1). The pathways responsible for IFNα/β signaling during the innate immune response to acute HSV-1 infection in the cornea are incompletely understood. Using a murine ocular infection model, we hypothesized that the stimulator of IFN genes (STING) mediates resistance to HSV-1 infection at the ocular surface and preserves the structural integrity of this mucosal site. Viral pathogenesis, tissue pathology, and host immune responses during ocular HSV-1 infection were characterized by plaque assay, esthesiometry, pachymetry, immunohistochemistry, flow cytometry, and small interfering RNA transfection in wild-type C57BL/6 (WT), STING-deficient (STING(-/-)), and IFNα/β receptor-deficient (CD118(-/-)) mice at days 3-5 postinfection. The presence of STING was critical for sustained control of HSV-1 replication in the corneal epithelium and resistance to viral neuroinvasion, but loss of STING had a negligible impact with respect to gross tissue pathology. Auxiliary STING-independent IFNα/β signaling pathways were responsible for maintenance of corneal integrity. Lymphatic vessels, mast cells, and sensory innervation were compromised in CD118(-/-) mice concurrent with increased tissue edema. STING-dependent signaling led to the upregulation of tetherin, a viral restriction factor we identify is important in containing the spread of HSV-1 in vivo.