Human iPSC-derived trigeminal neurons lack constitutive TLR3-dependent immunity that protects cortical neurons from HSV-1 infection

Human TMEFF1 is a restriction factor for herpes simplex virus in the brain

Most cases of herpes simplex virus 1 (HSV-1) encephalitis (HSE) remain unexplained1,2. Here, we report on two unrelated people who had HSE as children and are homozygous for rare deleterious variants of TMEFF1, which encodes a cell membrane protein that is preferentially expressed by brain cortical neurons. TMEFF1 interacts with the cell-surface HSV-1 receptor NECTIN-1, impairing HSV-1 glycoprotein D- and NECTIN-1-mediated fusion of the virus and the cell membrane, blocking viral entry. Genetic TMEFF1 deficiency allows HSV-1 to rapidly enter cortical neurons that are either patient specific or derived from CRISPR-Cas9-engineered human pluripotent stem cells, thereby enhancing HSV-1 translocation to the nucleus and subsequent replication. This cellular phenotype can be rescued by pretreatment with type I interferon (IFN) or the expression of exogenous wild-type TMEFF1. Moreover, ectopic expression of full-length TMEFF1 or its amino-terminal extracellular domain, but not its carboxy-terminal intracellular domain, impairs HSV-1 entry into NECTIN-1-expressing cells other than neurons, increasing their resistance to HSV-1 infection. Human TMEFF1 is therefore a host restriction factor for HSV-1 entry into cortical neurons. Its constitutively high abundance in cortical neurons protects these cells from HSV-1 infection, whereas inherited TMEFF1 deficiency renders them susceptible to this virus and can therefore underlie HSE.

Modulation of host immunity by sensory neurons

Recent studies have uncovered a new role for sensory neurons in influencing mammalian host immunity, challenging conventional notions of the nervous and immune systems as separate entities. In this review we delve into this groundbreaking paradigm of neuroimmunology and discuss recent scientific evidence for the impact of sensory neurons on host responses against a wide range of pathogens and diseases, encompassing microbial infections and cancers. These valuable insights enhance our understanding of the interactions between the nervous and immune systems, and also pave the way for developing candidate innovative therapeutic interventions in immune-mediated diseases highlighting the importance of this interdisciplinary research field.

Human assembloid model of the ascending neural sensory pathway

The ascending somatosensory pathways convey crucial information about pain, touch, itch, and body part movement from peripheral organs to the central nervous system. Despite a significant need for effective therapeutics modulating pain and other somatosensory modalities, clinical translation remains challenging, which is likely related to species-specific features and the lack of in vitro models to directly probe and manipulate this polysynaptic pathway. Here, we established human ascending somatosensory assembloids (hASA)- a four-part assembloid completely generated from human pluripotent stem cells that integrates somatosensory, spinal, diencephalic, and cortical organoids to model the human ascending spinothalamic pathway. Transcriptomic profiling confirmed the presence of key cell types in this circuit. Rabies tracing and calcium imaging showed that sensory neurons connected with dorsal spinal cord projection neurons, which ascending axons further connected to thalamic neurons. Following noxious chemical stimulation, single neuron calcium imaging of intact hASA demonstrated coordinated response, while four-part concomitant extracellular recordings and calcium imaging revealed synchronized activity across the assembloid. Loss of the sodium channel SCN9A, which causes pain insensitivity in humans, disrupted synchrony across the four-part hASA. Taken together, these experiments demonstrate the ability to functionally assemble the essential components of the human sensory pathway. These findings could both accelerate our understanding of human sensory circuits and facilitate therapeutic development.

Cell Intrinsic Determinants of Alpha Herpesvirus Latency and Pathogenesis in the Nervous System

Alpha herpesvirus infections (α-HVs) are widespread, affecting more than 70% of the adult human population. Typically, the infections start in the mucosal epithelia, from which the viral particles invade the axons of the peripheral nervous system. In the nuclei of the peripheral ganglia, α-HVs establish a lifelong latency and eventually undergo multiple reactivation cycles. Upon reactivation, viral progeny can move into the nerves, back out toward the periphery where they entered the organism, or they can move toward the central nervous system (CNS). This latency-reactivation cycle is remarkably well controlled by the intricate actions of the intrinsic and innate immune responses of the host, and finely counteracted by the viral proteins in an effort to co-exist in the population. If this yin-yang- or Nash-equilibrium-like balance state is broken due to immune suppression or genetic mutations in the host response factors particularly in the CNS, or the presence of other pathogenic stimuli, α-HV reactivations might lead to life-threatening pathologies. In this review, we will summarize the molecular virus-host interactions starting from mucosal epithelia infections leading to the establishment of latency in the PNS and to possible CNS invasion by α-HVs, highlighting the pathologies associated with uncontrolled virus replication in the NS.

Reconciling Mouse and Human Immunology at the Altar of Genetics

Immunity to infection has been extensively studied in humans and mice bearing naturally occurring or experimentally introduced germline mutations. Mouse studies are sometimes neglected by human immunologists, on the basis that mice are not humans and the infections studied are experimental and not natural. Conversely, human studies are sometimes neglected by mouse immunologists, on the basis of the uncontrolled conditions of study and small numbers of patients. However, both sides would agree that the infectious phenotypes of patients with inborn errors of immunity often differ from those of the corresponding mutant mice. Why is that? We argue that this important question is best addressed by revisiting and reinterpreting the findings of both mouse and human studies from a genetic perspective. Greater caution is required for reverse-genetics studies than for forward-genetics studies, but genetic analysis is sufficiently strong to define the studies likely to stand the test of time. Genetically robust mouse and human studies can provide invaluable complementary insights into the mechanisms of immunity to infection common and specific to these two species.

Variations in Aspects of Neural Precursor Cell Neurogenesis in a Human Model of HSV-1 Infection

Encephalitis, the most significant of the central nervous system (CNS) diseases caused by Herpes simplex virus 1 (HSV-1), may have long-term sequelae in survivors treated with acyclovir, the cause of which is unclear. HSV-1 exhibits a tropism toward neurogenic niches in CNS enriched with neural precursor cells (NPCs), which play a pivotal role in neurogenesis. NPCs are susceptible to HSV-1. There is a paucity of information regarding the influence of HSV-1 on neurogenesis in humans. We investigated HSV-1 infection of NPCs from two individuals. Our results show (i) HSV-1 impairs, to different extents, the proliferation, self-renewing, and, to an even greater extent, migration of NPCs from these two subjects; (ii) The protective effect of the gold-standard antiherpetic drug acyclovir (ACV) varies with viral dose and is incomplete. It is also subject to differences in terms of efficacy of the NPCs derived from these two individuals. These results suggest that the effects of HSV-1 may have on aspects of NPC neurogenesis may vary among individuals, even in the presence of acyclovir, and this may contribute to the heterogeneity of cognitive sequelae across encephalitis survivors. Further analysis of NPC cell lines from a larger number of individuals is warranted.

Anti-pruritic effect of isothiocyanates: Potential involvement of toll-like receptor 3 signaling

The innate immune system has an emerging role as a mediator of neuro-immune communication and a therapeutic target for itch. Toll-like receptor 3 (TLR3) plays an important role in itch, as shown in TLR3 knock-out mice. In this study, to evaluate effects of TLR3 inhibitors on histamine-independent itch, we used two kinds of isothiocyanate (ITC). Both phenethyl isothiocyanate (PEITC) and sulforaphane (SFN) inhibited Poly I:C (PIC)-induced signaling in the RAW264.7 cell line. We then investigated the anti-pruritic effect of these compounds on PIC- and chloroquine (CQ)-induced scratching behavior. PEITC and SFN both suppressed PIC-evoked scratching behavior in mice, and PEITC also inhibited CQ-induced acute itch. Finally, we examined the oxazolone-induced chronic itch model in mice. Surprisingly, oral dosing of both compounds suppressed scratching behaviors that were observed in mice. Our findings demonstrate that TLR3 is a critical mediator in acute and chronic itch transduction in mice and may be a promising therapeutic target for pruritus in human skin disorders. It is noteworthy that SFN has potential for use as an antipruritic as it is a phytochemical that is used as a supplement.

In vitro systems to study inborn errors of immunity using human induced pluripotent stem cells

In the last two decades, the exponential progress in the field of genetics could reveal the genetic impact on the onset and progression of several diseases affecting the immune system. This knowledge has led to the discovery of more than 400 monogenic germline mutations, also known as “inborn errors of immunity (IEI)”. Given the rarity of various IEI and the clinical diversity as well as the limited available patients’ material, the continuous development of novel cell-based in vitro models to elucidate the cellular and molecular mechanisms involved in the pathogenesis of these diseases is imperative. Focusing on stem cell technologies, this review aims to provide an overview of the current available in vitro models used to study IEI and which could lay the foundation for new therapeutic approaches. We elaborate in particular on the use of induced pluripotent stem cell-based systems and their broad application in studying IEI by establishing also novel infection culture models. The review will critically discuss the current limitations or gaps in the field of stem cell technology as well as the future perspectives from the use of these cell culture systems.

Expression and mechanisms of interferon-stimulated genes in viral infection of the central nervous system (CNS) and neurological diseases

Interferons (IFNs) bind to cell surface receptors and activate the expression of interferon-stimulated genes (ISGs) through intracellular signaling cascades. ISGs and their expression products have various biological functions, such as antiviral and immunomodulatory effects, and are essential effector molecules for IFN function. ISGs limit the invasion and replication of the virus in a cell-specific and region-specific manner in the central nervous system (CNS). In addition to participating in natural immunity against viral infections, studies have shown that ISGs are essential in the pathogenesis of CNS disorders such as neuroinflammation and neurodegenerative diseases. The aim of this review is to present a macroscopic overview of the characteristics of ISGs that restrict viral neural invasion and the expression of the ISGs underlying viral infection of CNS cells. Furthermore, we elucidate the characteristics of ISGs expression in neurological inflammation, neuropsychiatric disorders such as depression as well as neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). Finally, we summarize several ISGs (ISG15, IFIT2, IFITM3) that have been studied more in recent years for their antiviral infection in the CNS and their research progress in neurological diseases.

Human stem cell models to study placode development, function and pathology

Placodes are embryonic structures originating from the rostral ectoderm that give rise to highly diverse organs and tissues, comprising the anterior pituitary gland, paired sense organs and cranial sensory ganglia. Their development, including the underlying gene regulatory networks and signalling pathways, have been for the most part characterised in animal models. In this Review, we describe how placode development can be recapitulated by the differentiation of human pluripotent stem cells towards placode progenitors and their derivatives, highlighting the value of this highly scalable platform as an optimal in vitro tool to study the development of human placodes, and identify human-specific mechanisms in their development, function and pathology.

From rare disorders of immunity to common determinants of infection: Following the mechanistic thread

The immense interindividual clinical variability during any infection is a long-standing enigma. Inborn errors of IFN-γ and IFN-α/β immunity underlying rare infections with weakly virulent mycobacteria and seasonal influenza virus have inspired studies of two common infections: tuberculosis and COVID-19. A TYK2 genotype impairing IFN-γ production accounts for about 1% of tuberculosis cases, and autoantibodies neutralizing IFN-α/β account for about 15% of critical COVID-19 cases. The discovery of inborn errors and mechanisms underlying rare infections drove the identification of common monogenic or autoimmune determinants of related common infections. This "rare-to-common" genetic and mechanistic approach to infectious diseases may be of heuristic value.

Using 2D and 3D pluripotent stem cell models to study neurotropic viruses

Understanding the impact of viral pathogens on the human central nervous system (CNS) has been challenging due to the lack of viable human CNS models for controlled experiments to determine the causal factors underlying pathogenesis. Human embryonic stem cells (ESCs) and, more recently, cellular reprogramming of adult somatic cells to generate human induced pluripotent stem cells (iPSCs) provide opportunities for directed differentiation to neural cells that can be used to evaluate the impact of known and emerging viruses on neural cell types. Pluripotent stem cells (PSCs) can be induced to neural lineages in either two- (2D) or three-dimensional (3D) cultures, each bearing distinct advantages and limitations for modeling viral pathogenesis and evaluating effective therapeutics. Here we review the current state of technology in stem cell-based modeling of the CNS and how these models can be used to determine viral tropism and identify cellular phenotypes to investigate virus-host interactions and facilitate drug screening. We focus on several viruses (e.g., human immunodeficiency virus (HIV), herpes simplex virus (HSV), Zika virus (ZIKV), human cytomegalovirus (HCMV), SARS-CoV-2, West Nile virus (WNV)) to illustrate key advantages, as well as challenges, of PSC-based models. We also discuss how human PSC-based models can be used to evaluate the safety and efficacy of therapeutic drugs by generating data that are complementary to existing preclinical models. Ultimately, these efforts could facilitate the movement towards personalized medicine and provide patients and physicians with an additional source of information to consider when evaluating available treatment strategies.

Global analyses of mRNA expression in human sensory neurons reveal eIF5A as a conserved target for inflammatory pain

Nociceptors are a type of sensory neuron that are integral to most forms of pain. Targeted disruption of nociceptor sensitization affords unique opportunities to prevent pain. An emerging model for nociceptors are sensory neurons derived from human stem cells. Here, we subjected five groups to high-throughput sequencing: human induced pluripotent stem cells (hiPSCs) prior to differentiation, mature hiPSC-derived sensory neurons, mature co-cultures containing hiPSC-derived astrocytes and sensory neurons, mouse dorsal root ganglion (DRG) tissues, and mouse DRG cultures. Co-culture of nociceptors and astrocytes promotes expression of transcripts enriched in DRG tissues. Comparisons of the hiPSC models to tissue samples reveal that many key transcripts linked to pain are present. Markers indicative of a range of neuronal subtypes present in the DRG were detected in mature hiPSCs. Intriguingly, translation factors were maintained at consistently high expression levels across species and culture systems. As a proof of concept for the utility of this resource, we validated expression of eukaryotic initiation factor 5A (eIF5A) in DRG tissues and hiPSC samples. eIF5A is subject to a unique posttranslational hypusine modification required for its activity. Inhibition of hypusine biosynthesis prevented hyperalgesic priming by inflammatory mediators in vivo and diminished hiPSC activity in vitro. Collectively, our results illuminate the transcriptomes of hiPSC sensory neuron models. We provide a demonstration for this resource through our investigation of eIF5A. Our findings reveal hypusine as a potential target for inflammation associated pain in males.

Intrinsic antiviral immunity of barrier cells revealed by an iPSC-derived blood-brain barrier cellular model

Physiological blood-tissue barriers play a critical role in separating the circulation from immune-privileged sites and denying access to blood-borne viruses. The mechanism of virus restriction by these barriers is poorly understood. We utilize induced pluripotent stem cell (iPSC)-derived human brain microvascular endothelial cells (iBMECs) to study virus-blood-brain barrier (BBB) interactions. These iPSC-derived cells faithfully recapitulate a striking difference in in vivo neuroinvasion by two alphavirus isolates and are selectively permissive to neurotropic flaviviruses. A model of cocultured iBMECs and astrocytes exhibits high transendothelial electrical resistance and blocks non-neurotropic flaviviruses from getting across the barrier. We find that iBMECs constitutively express an interferon-induced gene, IFITM1, which preferentially restricts the replication of non-neurotropic flaviviruses. Barrier cells from blood-testis and blood-retinal barriers also constitutively express IFITMs that contribute to the viral resistance. Our application of a renewable human iPSC-based model for studying virus-BBB interactions reveals that intrinsic immunity at the barriers contributes to virus exclusion.

Using a stem cell-derived cellular model and a panel of human pathogenic viruses, Cheng et al. show a mechanism by which some viruses can penetrate the blood-brain barrier and cause diseases in the central nervous system.

The neuroinvasiveness, neurotropism, and neurovirulence of SARS-CoV-2

Severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) infection is associated with a diverse spectrum of neurological complications during the acute and postacute stages. The pathogenesis of these complications is complex and dependent on many factors. For accurate and consistent interpretation of experimental data in this fast-growing field of research, it is essential to use terminology consistently. In this article, we outline the distinctions between neuroinvasiveness, neurotropism, and neurovirulence. Additionally, we discuss current knowledge of these distinct features underlying the pathogenesis of SARS-CoV-2-associated neurological complications. Lastly, we briefly discuss the advantages and limitations of different experimental models, and how these approaches can further be leveraged to advance the field.

Varicella Zoster Virus Neuronal Latency and Reactivation Modeled in Vitro

Latency and reactivation in neurons are critical aspects of VZV pathogenesis that have historically been difficult to investigate. Viral genomes are retained in many human ganglia after the primary infection, varicella; and about one-third of the naturally infected VZV seropositive population reactivates latent virus, which most often clinically manifests as herpes zoster (HZ or Shingles). HZ is frequently complicated by acute and chronic debilitating pain for which there remains a need for more effective treatment options. Understanding of the latent state is likely to be essential in the design of strategies to reduce reactivation. Experimentally addressing VZV latency has been difficult because of the strict human species specificity of VZV and the fact that until recently, experimental reactivation had not been achieved. We do not yet know the neuron subtypes that harbor latent genomes, whether all can potentially reactivate, what the drivers of VZV reactivation are, and how immunity interplays with the latent state to control reactivation. However, recent advances have enabled a picture of VZV latency to start to emerge. The first is the ability to detect the latent viral genome and its expression in human ganglionic tissues with extraordinary sensitivity. The second, the subject of this chapter, is the development of in vitro human neuron systems permitting the modeling of latent states that can be experimentally reactivated. This review will summarize recent advances of in vitro models of neuronal VZV latency and reactivation, the limitations of the current systems, and discuss outstanding questions and future directions regarding these processes using these and yet to be developed models. Results obtained from the in vitro models to date will also be discussed in light of the recent data gleaned from studies of VZV latency and gene expression learned from human cadaver ganglia, especially the discovery of VZV latency transcripts that seem to parallel the long-studied latency-associated transcripts of other neurotropic alphaherpesviruses.

Inborn errors of TLR3- or MDA5-dependent type I IFN immunity in children with enterovirus rhombencephalitis

Enterovirus (EV) infection rarely results in life-threatening infection of the central nervous system. We report two unrelated children with EV30 and EV71 rhombencephalitis. One patient carries compound heterozygous TLR3 variants (loss-of-function F322fs2* and hypomorphic D280N), and the other is homozygous for an IFIH1 variant (loss-of-function c.1641+1G>C). Their fibroblasts respond poorly to extracellular (TLR3) or intracellular (MDA5) poly(I:C) stimulation. The baseline (TLR3) and EV-responsive (MDA5) levels of IFN-β in the patients’ fibroblasts are low. EV growth is enhanced at early and late time points of infection in TLR3- and MDA5-deficient fibroblasts, respectively. Treatment with exogenous IFN-α2b before infection renders both cell lines resistant to EV30 and EV71, whereas post-infection treatment with IFN-α2b rescues viral susceptibility fully only in MDA5-deficient fibroblasts. Finally, the poly(I:C) and viral phenotypes of fibroblasts are rescued by the expression of WT TLR3 or MDA5. Human TLR3 and MDA5 are critical for cell-intrinsic immunity to EV, via the control of baseline and virus-induced type I IFN production, respectively.

Mechanisms of viral inflammation and disease in humans

[Figure: see text].

Pathogenesis and virulence of herpes simplex virus

Two of the most prevalent human viruses worldwide, herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2, respectively), cause a variety of diseases, including cold sores, genital herpes, herpes stromal keratitis, meningitis and encephalitis. The intrinsic, innate and adaptive immune responses are key to control HSV, and the virus has developed mechanisms to evade them. The immune response can also contribute to pathogenesis, as observed in stromal keratitis and encephalitis. The fact that certain individuals are more prone than others to suffer severe disease upon HSV infection can be partially explained by the existence of genetic polymorphisms in humans. Like all herpesviruses, HSV has two replication cycles: lytic and latent. During lytic replication HSV produces infectious viral particles to infect other cells and organisms, while during latency there is limited gene expression and lack of infectious virus particles. HSV establishes latency in neurons and can cause disease both during primary infection and upon reactivation. The mechanisms leading to latency and reactivation and which are the viral and host factors controlling these processes are not completely understood. Here we review the HSV life cycle, the interaction of HSV with the immune system and three of the best-studied pathologies: Herpes stromal keratitis, herpes simplex encephalitis and genital herpes. We also discuss the potential association between HSV-1 infection and Alzheimer's disease.

Toll-like Receptors in Viral Encephalitis

Viral encephalitis is a rare but serious syndrome. In addition to DNA-encoded herpes viruses, such as herpes simplex virus and varicella zoster virus, RNA-encoded viruses from the families of Flaviviridae, Rhabdoviridae and Paramyxoviridae are important neurotropic viruses. Whereas in the periphery, the role of Toll-like receptors (TLR) during immune stimulation is well understood, TLR functions within the CNS are less clear. On one hand, TLRs can affect the physiology of neurons during neuronal progenitor cell differentiation and neurite outgrowth, whereas under conditions of infection, the complex interplay between TLR stimulated neurons, astrocytes and microglia is just on the verge of being understood. In this review, we summarize the current knowledge about which TLRs are expressed by cell subsets of the CNS. Furthermore, we specifically highlight functional implications of TLR stimulation in neurons, astrocytes and microglia. After briefly illuminating some examples of viral evasion strategies from TLR signaling, we report on the current knowledge of primary immunodeficiencies in TLR signaling and their consequences for viral encephalitis. Finally, we provide an outlook with examples of TLR agonist mediated intervention strategies and potentiation of vaccine responses against neurotropic virus infections.

Neuron-intrinsic immunity to viruses in mice and humans

Viral encephalitis is a major neglected medical problem. Host defense mechanisms against viral infection of the central nervous system (CNS) have long remained unclear. The few previous studies of CNS-specific immunity to viruses in mice in vivo and humans in vitro have focused on the contributions of circulating leukocytes, resident microglial cells and astrocytes, with neurons long considered passive victims of viral infection requiring protection from extrinsic antiviral mechanisms. The last decade has witnessed the gradual emergence of the notion that neurons also combat viruses through cell-intrinsic mechanisms. Forward genetic approaches in humans have shown that monogenic inborn errors of TLR3, IFN-α/β, or snoRNA31 immunity confer susceptibility to herpes simplex virus 1 (HSV-1) infection of the forebrain, whereas inborn errors of DBR1 underlie brainstem infections due to various viruses, including HSV-1. The study of human pluripotent stem cell (hPSC)-derived CNS-resident cells has unraveled known (i.e. TLR3-dependent IFN-α/β immunity) and new (i.e. snoRNA31-dependent or DBR1-dependent immunity) cell-intrinsic antiviral mechanisms operating in neurons. Reverse genetic approaches in mice have confirmed that some known antiviral mechanisms also operate in mouse neurons (e.g. TLR3 and IFN-α/β immunity). The search for human inborn errors of immunity (IEIs) underlying various forms of viral encephalitis, coupled with mouse models in vivo, and hPSC-based culture models of CNS and peripheral nervous system cells and organoids in vitro, should shed further light on the cell-specific and tissue-specific mechanisms of host defense against viruses in the human brain.

Local Immune Control of Latent Herpes Simplex Virus Type 1 in Ganglia of Mice and Man

Herpes simplex virus type 1 (HSV-1) is a prevalent human pathogen. HSV-1 genomes persist in trigeminal ganglia neuronal nuclei as chromatinized episomes, while epithelial cells are typically killed by lytic infection. Fluctuations in anti-viral responses, broadly defined, may underlay periodic reactivations. The ganglionic immune response to HSV-1 infection includes cell-intrinsic responses in neurons, innate sensing by several cell types, and the infiltration and persistence of antigen-specific T-cells. The mechanisms specifying the contrasting fates of HSV-1 in neurons and epithelial cells may include differential genome silencing and chromatinization, dictated by variation in access of immune modulating viral tegument proteins to the cell body, and protection of neurons by autophagy. Innate responses have the capacity of recruiting additional immune cells and paracrine activity on parenchymal cells, for example via chemokines and type I interferons. In both mice and humans, HSV-1-specific CD8 and CD4 T-cells are recruited to ganglia, with mechanistic studies suggesting active roles in immune surveillance and control of reactivation. In this review we focus mainly on HSV-1 and the TG, comparing and contrasting where possible observational, interventional, and in vitro studies between humans and animal hosts.

TLR3 controls constitutive IFN-β antiviral immunity in human fibroblasts and cortical neurons

Human herpes simplex virus 1 (HSV-1) encephalitis can be caused by inborn errors of the TLR3 pathway, resulting in impairment of CNS cell-intrinsic antiviral immunity. Deficiencies of the TLR3 pathway impair cell-intrinsic immunity to vesicular stomatitis virus (VSV) and HSV-1 in fibroblasts, and to HSV-1 in cortical but not trigeminal neurons. The underlying molecular mechanism is thought to involve impaired IFN-α/β induction by the TLR3 recognition of dsRNA viral intermediates or by-products. However, we show here that human TLR3 controls constitutive levels of IFNB mRNA and secreted bioactive IFN-β protein, and thereby also controls constitutive mRNA levels for IFN-stimulated genes (ISGs) in fibroblasts. Tlr3-/- mouse embryonic fibroblasts also have lower basal ISG levels. Moreover, human TLR3 controls basal levels of IFN-β secretion and ISG mRNA in induced pluripotent stem cell-derived cortical neurons. Consistently, TLR3-deficient human fibroblasts and cortical neurons are vulnerable not only to both VSV and HSV-1, but also to several other families of viruses. The mechanism by which TLR3 restricts viral growth in human fibroblasts and cortical neurons in vitro and, by inference, by which the human CNS prevents infection by HSV-1 in vivo, is therefore based on the control of early viral infection by basal IFN-β immunity.

Herpes simplex encephalitis in a patient with a distinctive form of inherited IFNAR1 deficiency

Inborn errors of TLR3-dependent IFN-α/β- and IFN-λ-mediated immunity in the CNS can underlie herpes simplex virus 1 (HSV-1) encephalitis (HSE). The respective contributions of IFN-α/β and IFN-λ are unknown. We report a child homozygous for a genomic deletion of the entire coding sequence and part of the 3'-UTR of the last exon of IFNAR1, who died of HSE at the age of 2 years. An older cousin died following vaccination against measles, mumps, and rubella at 12 months of age, and another 17-year-old cousin homozygous for the same variant has had other, less severe, viral illnesses. The encoded IFNAR1 protein is expressed on the cell surface but is truncated and cannot interact with the tyrosine kinase TYK2. The patient's fibroblasts and EBV-B cells did not respond to IFN-α2b or IFN-β, in terms of STAT1, STAT2, and STAT3 phosphorylation or the genome-wide induction of IFN-stimulated genes. The patient's fibroblasts were susceptible to viruses, including HSV-1, even in the presence of exogenous IFN-α2b or IFN-β. HSE is therefore a consequence of inherited complete IFNAR1 deficiency. This viral disease occurred in natural conditions, unlike those previously reported in other patients with IFNAR1 or IFNAR2 deficiency. This experiment of nature indicates that IFN-α/β are essential for anti-HSV-1 immunity in the CNS.

Next-Generation Human Cerebral Organoids as Powerful Tools To Advance NeuroHIV Research

Long-term effective use of antiretroviral therapy (ART) among people with HIV (PWH) has significantly reduced the burden of disease, yet a cure for HIV has not been universally achieved, likely due to the persistence of an HIV reservoir. The central nervous system (CNS) is an understudied HIV sanctuary. Importantly, due to viral persistence in the brain, cognitive disturbances persist to various degrees at high rates in PWH despite suppressive ART. Given the complexity and accessibility of the CNS compartment and that it is a physiologically and anatomically unique immune site, human studies to reveal molecular mechanisms of viral entry, reservoir establishment, and the cellular and structural interactions leading to viral persistence and brain injury to advance a cure and either prevent or limit cognitive impairments in PWH remain challenging. Recent advances in human brain organoids show that they can mimic the intercellular dynamics of the human brain and may recapitulate many of the events involved in HIV infection of the brain (neuroHIV). Human brain organoids can be produced, spontaneously or with addition of growth factors and at immature or mature states, and have become stronger models to study neurovirulent viral infections of the CNS. While organoids provide opportunities to study neuroHIV, obstacles such as the need to incorporate microglia need to be overcome to fully utilize this model. Here, we review the current achievements in brain organoid biology and their relevance to neuroHIV research efforts.

Molecular mechanisms of nonself nucleic acid recognition by the innate immune system

Nucleic acids (NAs) represent one of the most important classes of molecules recognized by the innate immune system. However, NAs are not limited to pathogens, but are also present within the host. As such, the immune system has evolved an elaborate set of pathogen recognition receptors (PRRs) that employ various strategies to recognize distinct types of NAs, while reliably distinguishing between self and nonself. The here-employed strategies encompass the positioning of NA-sensing PRRs in certain subcellular compartments that potentially come in contact with pathogens but not host NAs, the existence of counterregulatory measures that keep endogenous NAs below a certain threshold, and also the specific identification of certain nonself patterns. Here, we review recent advances in the molecular mechanisms of NA recognition by TLRs, RLRs, and the cGAS-STING axis. We highlight the differences in NA-PRR interfaces that confer specificity and selectivity toward an NA ligand, as well as the NA-dependent induced conformational changes required for signal transduction.

Human stem cell models to study host-virus interactions in the central nervous system

Advancements in human pluripotent stem cell technology offer a unique opportunity for the neuroimmunology field to study host-virus interactions directly in disease-relevant cells of the human central nervous system (CNS). Viral encephalitis is most commonly caused by herpesviruses, arboviruses and enteroviruses targeting distinct CNS cell types and often leading to severe neurological damage with poor clinical outcomes. Furthermore, different neurotropic viruses will affect the CNS at distinct developmental stages, from early prenatal brain development to the aged brain. With the unique flexibility and scalability of human pluripotent stem cell technology, it is now possible to examine the molecular mechanisms underlying acute infection and latency, determine which CNS subpopulations are specifically infected, study temporal aspects of viral susceptibility, perform high-throughput chemical or genetic screens for viral restriction factors and explore complex cell-non-autonomous disease mechanisms. Therefore, human pluripotent stem cell technology has the potential to address key unanswered questions about antiviral immunity in the CNS, including emerging questions on the potential CNS tropism of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Studying human nociceptors: from fundamentals to clinic

Chronic pain affects one in five of the general population and is the third most important cause of disability-adjusted life-years globally. Unfortunately, treatment remains inadequate due to poor efficacy and tolerability. There has been a failure in translating promising preclinical drug targets into clinic use. This reflects challenges across the whole drug development pathway, from preclinical models to trial design. Nociceptors remain an attractive therapeutic target: their sensitization makes an important contribution to many chronic pain states, they are located outside the blood-brain barrier, and they are relatively specific. The past decade has seen significant advances in the techniques available to study human nociceptors, including: the use of corneal confocal microscopy and biopsy samples to observe nociceptor morphology, the culture of human nociceptors (either from surgical or post-mortem tissue or using human induced pluripotent stem cell derived nociceptors), the application of high throughput technologies such as transcriptomics, the in vitro and in vivo electrophysiological characterization through microneurography, and the correlation with pain percepts provided by quantitative sensory testing. Genome editing in human induced pluripotent stem cell-derived nociceptors enables the interrogation of the causal role of genes in the regulation of nociceptor function. Both human and rodent nociceptors are more heterogeneous at a molecular level than previously appreciated, and while we find that there are broad similarities between human and rodent nociceptors there are also important differences involving ion channel function, expression, and cellular excitability. These technological advances have emphasized the maladaptive plastic changes occurring in human nociceptors following injury that contribute to chronic pain. Studying human nociceptors has revealed new therapeutic targets for the suppression of chronic pain and enhanced repair. Cellular models of human nociceptors have enabled the screening of small molecule and gene therapy approaches on nociceptor function, and in some cases have enabled correlation with clinical outcomes. Undoubtedly, challenges remain. Many of these techniques are difficult to implement at scale, current induced pluripotent stem cell differentiation protocols do not generate the full diversity of nociceptor populations, and we still have a relatively poor understanding of inter-individual variation in nociceptors due to factors such as age, sex, or ethnicity. We hope our ability to directly investigate human nociceptors will not only aid our understanding of the fundamental neurobiology underlying acute and chronic pain but also help bridge the translational gap.

Viral infection of human neurons triggers strain-specific differences in host neuronal and viral transcriptomes

Infection with herpes simplex virus 1 (HSV-1) occurs in over half the global population, causing recurrent orofacial and/or genital lesions. Individual strains of HSV-1 demonstrate differences in neurovirulence in vivo, suggesting that viral genetic differences may impact phenotype. Here differentiated SH-SY5Y human neuronal cells were infected with one of three HSV-1 strains known to differ in neurovirulence in vivo. Host and viral RNA were sequenced simultaneously, revealing strain-specific differences in both viral and host transcription in infected neurons. Neuronal morphology and immunofluorescence data highlight the pathological changes in neuronal cytoarchitecture induced by HSV-1 infection, which may reflect host transcriptional changes in pathways associated with adherens junctions, integrin signaling, and others. Comparison of viral protein levels in neurons and epithelial cells demonstrated that a number of differences were neuron-specific, suggesting that strain-to-strain variations in host and virus transcription are cell type-dependent. Together, these data demonstrate the importance of studying virus strain- and cell-type-specific factors that may contribute to neurovirulence in vivo, and highlight the specificity of HSV-1-host interactions.

Derivation of Peripheral Nociceptive, Mechanoreceptive, and Proprioceptive Sensory Neurons from the same Culture of Human Pluripotent Stem Cells

The three peripheral sensory neuron (SN) subtypes, nociceptors, mechanoreceptors, and proprioceptors, localize to dorsal root ganglia and convey sensations such as pain, temperature, pressure, and limb movement/position. Despite previous reports, to date no protocol is available allowing the generation of all three SN subtypes at high efficiency and purity from human pluripotent stem cells (hPSCs). We describe a chemically defined differentiation protocol that generates all three SN subtypes from the same starting population, as well as methods to enrich for each individual subtype. The protocol yields high efficiency and purity cultures that are electrically active and respond to specific stimuli. We describe their molecular character and maturity stage and provide evidence for their use as an axotomy model; we show disease phenotypes in hPSCs derived from patients with familial dysautonomia. Our protocol will allow the modeling of human disorders affecting SNs, the search for treatments, and the study of human development.

Lethal Infectious Diseases as Inborn Errors of Immunity: Toward a Synthesis of the Germ and Genetic Theories

It was first demonstrated in the late nineteenth century that human deaths from fever were typically due to infections. As the germ theory gained ground, it replaced the old, unproven theory that deaths from fever reflected a weak personal or even familial constitution. A new enigma emerged at the turn of the twentieth century, when it became apparent that only a small proportion of infected individuals die from primary infections with almost any given microbe. Classical genetics studies gradually revealed that severe infectious diseases could be driven by human genetic predisposition. This idea gained ground with the support of molecular genetics, in three successive, overlapping steps. First, many rare inborn errors of immunity were shown, from 1985 onward, to underlie multiple, recurrent infections with Mendelian inheritance. Second, a handful of rare and familial infections, also segregating as Mendelian traits but striking humans resistant to other infections, were deciphered molecularly beginning in 1996. Third, from 2007 onward, a growing number of rare or common sporadicinfections were shown to result from monogenic, but not Mendelian, inborn errors. A synthesis of the hitherto mutually exclusive germ and genetic theories is now in view.

Defects in LC3B2 and ATG4A underlie HSV2 meningitis and reveal a critical role for autophagy in antiviral defense in humans

Recurrent herpesvirus infections can manifest in different forms of disease, including cold sores, genital herpes, and encephalitis. There is an incomplete understanding of the genetic and immunological factors conferring susceptibility to recurrent herpes simplex virus 2 (HSV2) infection in the central nervous system (CNS). Here, we describe two adult patients with recurrent HSV2 lymphocytic Mollaret's meningitis that each carry a rare monoallelic variant in the autophagy proteins ATG4A or LC3B2. HSV2-activated autophagy was abrogated in patient primary fibroblasts, which also exhibited significantly increased viral replication and enhanced cell death. HSV2 antigen was captured in autophagosomes of infected cells, and genetic inhibition of autophagy by disruption of autophagy genes, including ATG4A and LC3B2, led to enhanced viral replication and cell death in primary fibroblasts and a neuroblastoma cell line. Activation of autophagy by HSV2 was sensitive to ultraviolet (UV) irradiation of the virus and inhibited in the presence of acyclovir, but HSV2-induced autophagy was independent of the DNA-activated STING pathway. Reconstitution of wild-type ATG4A and LC3B2 expression using lentiviral gene delivery or electroporation of in vitro transcribed mRNA into patient cells restored virus-induced autophagy and the ability to control HSV2 replication. This study describes a previously unknown link between defective autophagy and an inborn error of immunity that can lead to increased susceptibility to HSV2 infection, suggesting an important role for autophagy in antiviral immunity in the CNS.

The Anti-Inflammatory Effect of Taurine on Cardiovascular Disease

Taurine is a non-protein amino acid that is expressed in the majority of animal tissues. With its unique sulfonic acid makeup, taurine influences cellular functions, including osmoregulation, antioxidation, ion movement modulation, and conjugation of bile acids. Taurine exerts anti-inflammatory effects that improve diabetes and has shown benefits to the cardiovascular system, possibly by inhibition of the renin angiotensin system. The beneficial effects of taurine are reviewed.

Human Neural Stem Cell Systems to Explore Pathogen-Related Neurodevelopmental and Neurodegenerative Disorders

Building and functioning of the human brain requires the precise orchestration and execution of myriad molecular and cellular processes, across a multitude of cell types and over an extended period of time. Dysregulation of these processes affects structure and function of the brain and can lead to neurodevelopmental, neurological, or psychiatric disorders. Multiple environmental stimuli affect neural stem cells (NSCs) at several levels, thus impairing the normal human neurodevelopmental program. In this review article, we will delineate the main mechanisms of infection adopted by several neurotropic pathogens, and the selective NSC vulnerability. In particular, TORCH agents, i.e., Toxoplasma gondii, others (including Zika virus and Coxsackie virus), Rubella virus, Cytomegalovirus, and Herpes simplex virus, will be considered for their devastating effects on NSC self-renewal with the consequent neural progenitor depletion, the cellular substrate of microcephaly. Moreover, new evidence suggests that some of these agents may also affect the NSC progeny, producing long-term effects in the neuronal lineage. This is evident in the paradigmatic example of the neurodegeneration occurring in Alzheimer's disease.

Herpes Simplex Virus Type 1 Interactions with the Interferon System

The interferon (IFN) system is one of the first lines of defense activated against invading viral pathogens. Upon secretion, IFNs activate a signaling cascade resulting in the production of several interferon stimulated genes (ISGs), which work to limit viral replication and establish an overall anti-viral state. Herpes simplex virus type 1 is a ubiquitous human pathogen that has evolved to downregulate the IFN response and establish lifelong latent infection in sensory neurons of the host. This review will focus on the mechanisms by which the host innate immune system detects invading HSV-1 virions, the subsequent IFN response generated to limit viral infection, and the evasion strategies developed by HSV-1 to evade the immune system and establish latency in the host.

Human inborn errors of immunity: An expanding universe

Molecular, cellular, and clinical studies of human inborn errors of immunity have revolutionized our understanding of their pathogenesis, considerably broadened their spectrum of immunological and clinical phenotypes, and enabled successful targeted therapeutic interventions. These studies have also been of great scientific merit, challenging a number of immunological notions initially established in inbred mice while revealing previously unrecognized mechanisms of host defense by leukocytes and other cells and of both innate and adaptive tolerance to self.

Interferon-independent STING signaling promotes resistance to HSV-1 in vivo

The Stimulator of Interferon Genes (STING) pathway initiates potent immune responses upon recognition of DNA. To initiate signaling, serine 365 (S365) in the C-terminal tail (CTT) of STING is phosphorylated, leading to induction of type I interferons (IFNs). Additionally, evolutionary conserved responses such as autophagy also occur downstream of STING, but their relative importance during in vivo infections remains unclear. Here we report that mice harboring a serine 365-to-alanine (S365A) mutation in STING are unexpectedly resistant to Herpes Simplex Virus (HSV)-1, despite lacking STING-induced type I IFN responses. By contrast, resistance to HSV-1 is abolished in mice lacking the STING CTT, suggesting that the STING CTT initiates protective responses against HSV-1, independently of type I IFNs. Interestingly, we find that STING-induced autophagy is a CTT- and TBK1-dependent but IRF3-independent process that is conserved in the STING S365A mice. Thus, interferon-independent functions of STING mediate STING-dependent antiviral responses in vivo.

Protein Dynamics in Cytosolic DNA-Sensing Antiviral Innate Immune Signaling Pathways

Antiviral innate immunity works as the first line of host defense against viral infection. Pattern recognition receptors (PRRs) and adaptor proteins involved in the innate immune signaling pathways play critical roles in controlling viral infections via the induction of type I interferon and its downstream interferon-stimulated genes. Dynamic changes of adaptor proteins contribute to precise regulation of the activation and shut-off of signaling transduction, though numerous complex processes are involved in achieving dynamic changes to various proteins of the host and viruses. In this review, we will summarize recent progress on the trafficking patterns and conformational transitions of the adaptors that are involved in the antiviral innate immune signaling pathway during viral DNA sensing. Moreover, we aim to dissect the relationships between protein dynamics and DNA-sensing antiviral innate immune responses, which will reveal the underlying mechanisms controlling protein activity and maintaining cell homeostasis. By comprehensively revealing protein dynamics in cytosolic DNA-sensing antiviral innate immune signaling pathways, we will be able to identify potential new targets for the therapies of certain autoimmune diseases.

Herpes simplex virus encephalitis update

PURPOSE OF REVIEW

HSV is the most frequently identified cause of infectious encephalitis, in Western countries. This article is an update on the topic based on a review of recent studies from 2017 to 2018.

RECENT FINDINGS

Acyclovir is still the first line treatment, and no new drugs are currently available for clinical use. The major considerations for HSV encephalitis are as follows: point one, clinical evaluation remains the most important factor, as though CSF HSV PCR has a good sensitivity, in a small proportion of patients the initial testing might be negative. MRI brain is the first line imaging test, and mesial temporal lobe involvement and other typical findings are important for diagnosis; point 2, there should be emphasis on sequela, short-term, and long-term outcomes, and not just case fatality rated in future studies and clinical management. Auto-immune encephalitis can be triggered by HSV, and should be considered in patients who are not responding to treatment; point 3, future studies should be on better management of sequela, and better treatment regimens including those targeting the immune response.

SUMMARY

Autoimmune encephalitis is a clearly identified complication of HSV encephalitis. Inflammatory mechanisms are linked to the clinical presentation as well as severity and poor outcome. Initial corticosteroid therapy has to be evaluated in order to prevent complications.

Herpes Simplex Virus 1 ICP34.5 Alters Mitochondrial Dynamics in Neurons

Expression of viral genes and activation of innate antiviral responses during infection result in an increase in reactive oxygen species (ROS) and toxic by-products of energy metabolism which can lead to cell death. The mitochondrion and its associated proteins are crucial regulators of these responses and related pathways such as autophagy and apoptosis. Through a mass spectrometry approach, we have shown that the herpes simplex virus 1 (HSV-1) neurovirulence- and autophagy-modulating protein ICP34.5 interacts with numerous mitochondrion-associated factors. Specifically, we showed that amino acids 68 to 87 of ICP34.5, the domain that binds beclin1 and controls neurovirulence, are necessary for interactions with PGAM5, KEAP1, and other regulators of the antioxidant response, mitochondrial trafficking, and programmed cell death. We further show that while this domain interacts with multiple cellular stress response factors, it does not alter apoptosis or antioxidant gene expression. That said, the attenuated replication of a recombinant virus lacking residues 68 to 87 (termed Δ68-87) in primary human fibroblasts was restored by addition of ferric nitrate. Furthermore, in primary mouse neurons, the perinuclear localization of mitochondria that follows infection with HSV-1 was notably absent following Δ68-87 infection. Through this 20-amino-acid domain, ICP34.5 significantly reduces mitochondrial motility in axons of neurons. We propose the hypothesis that ICP34.5 promotes perinuclear mitochondrial localization by modulating transport of mitochondria through interaction with PGAM5. These data expand upon previous observations of altered mitochondrial dynamics following alphaherpesvirus infections and identify a key determinant of this activity during HSV-1 infections.IMPORTANCE Herpes simplex virus persists lifelong in neurons and can reactivate to cause recurrent lesions in mucosal tissues. A key determinant of virulence is the viral protein ICP34.5, of which residues 68 to 87 significantly contribute to neurovirulence through an unknown mechanism. Our report provides evidence that residues 68 to 87 of ICP34.5 are required for binding mitochondrion-associated factors. These interactions alter mitochondrial dynamics in neurons, thereby facilitating viral replication and pathogenesis.

The human genetic determinism of life-threatening infectious diseases: genetic heterogeneity and physiological homogeneity?

Multicellular eukaryotes emerged late in evolution from an ocean of viruses, bacteria, archaea, and unicellular eukaryotes. These macroorganisms are exposed to and infected by a tremendous diversity of microorganisms. Those that are large enough can even be infected by multicellular fungi and parasites. Each interaction is unique, if only because it operates between two unique living organisms, in an infinite diversity of circumstances. This is neatly illustrated by the extraordinarily high level of interindividual clinical variability in human infections, even for a given pathogen, ranging from a total absence of clinical manifestations to death. We discuss here the idea that the determinism of human life-threatening infectious diseases can be governed by single-gene inborn errors of immunity, which are rarely Mendelian and frequently display incomplete penetrance. We briefly review the evidence in support of this notion obtained over the last two decades, referring to a number of focused and thorough reviews published by eminent colleagues in this issue of Human Genetics. It seems that almost any life-threatening infectious disease can be driven by at least one, and, perhaps, a great many diverse monogenic inborn errors, which may nonetheless be immunologically related. While the proportions of monogenic cases remain unknown, a picture in which genetic heterogeneity is combined with physiological homogeneity is emerging from these studies. A preliminary sketch of the human genetic architecture of severe infectious diseases is perhaps in sight.

Herpes simplex virus encephalitis of childhood: inborn errors of central nervous system cell-intrinsic immunity

Herpes simplex virus 1 (HSV-1) encephalitis (HSE) is the most common sporadic viral encephalitis in Western countries. Over the last 15 years, human genetic and immunological studies have provided proof-of-principle that childhood HSE can result from inborn errors of central nervous system (CNS)-specific, cell-intrinsic immunity to HSV-1. HSE-causing mutations of eight genes disrupt known (TLR3-dependent IFN-α/β immunity) and novel (dependent on DBR1 or snoRNA31) antiviral mechanisms. Monogenic inborn errors confer susceptibility to forebrain (TLR3-IFN or snoRNA31) or brainstem (DBR1) HSE. Most of these disorders display incomplete clinical penetrance, with the possible exception of DBR1 deficiency. They account for a small, but non-negligible proportion of cases (about 7%). These findings pave the way for the gradual definition of the genetic and immunological architecture of childhood HSE, with both biological and clinical implications.

Human inborn errors of immunity to herpes viruses

Infections with any of the nine human herpes viruses (HHV) can be asymptomatic or life-threatening. The study of patients with severe diseases caused by HHVs, in the absence of overt acquired immunodeficiency, has led to the discovery or diagnosis of various inborn errors of immunity. The related inborn errors of adaptive immunity disrupt α/β T-cell rather than B-cell immunity. Affected patients typically develop HHV infections in the context of other infectious diseases. However, this is not always the case, as illustrated by inborn errors of SAP-dependent T-cell immunity to EBV-infected B cells. The related inborn errors of innate immunity disrupt leukocytes other than T and B cells, non-hematopoietic cells, or both. Patients typically develop only a single type of infection due to HHV, although, again, this is not always the case, as illustrated by inborn errors of TLR3 immunity resulting in HSV1 encephalitis in some patients and influenza pneumonitis in others. Most severe HHV infections in otherwise healthy patients remains unexplained. The forward human genetic dissection of isolated and syndromic HHV-driven illnesses will establish the molecular and cellular basis of protective immunity to HHVs, paving the way for novel diagnosis and management strategies.

Viral genetic diversity and its potential contributions to the development and progression of neonatal herpes simplex virus (HSV) disease

PURPOSE OF REVIEW

Neonatal infection by herpes simplex virus (HSV) 1 or 2 presents a devastating burden to new parents, due to the unpredictability of severe clinical outcomes, as well as the potential for lifelong reactivation. While just under half of neonatal HSV infections have mild clinical impacts akin to those observed in adults, the other half experience viral spread throughout the body (disseminated infection) and/or the brain (central nervous system infection).

SUMMARY

Here we summarize current data on clinical diagnostic measures, antiviral therapy, and known factors of human host biology that contribute to the distinct neonatal outcomes of HSV infection.

RECENT FINDINGS

We then explore recent new data on how viral genetic diversity between infections may impact clinical outcomes. Further research will be critical to build upon these early findings and to provide statistical power to our ability to discern and/or predict the potential clinical path of a given neonatal infection.

Cell Line Models for Human Cytomegalovirus Latency Faithfully Mimic Viral Entry by Macropinocytosis and Endocytosis

Human cytomegalovirus (HCMV) enters primary CD34+ hematopoietic progenitor cells by macropinocytosis, where it establishes latency in part because its tegument-transactivating protein, pp71, remains associated with endosomes and is therefore unable to initiate productive, lytic replication. Here we show that multiple HCMV strains also enter cell line models used to study latency by macropinocytosis and endocytosis. In all latency models tested, tegument-delivered pp71 was found to be colocalized with endosomal markers and was not associated with the seven other cytoplasmic localization markers tested. Like the capsid-associated pp150 tegument protein, we initially detected capsid proteins in association with endosomes but later detected them in the nucleus. Inhibitors of macropinocytosis and endocytosis reduced latent viral gene expression and precluded reactivation. Importantly, we utilized electron microscopy to observe entry by macropinocytosis and endocytosis, providing additional visual corroboration of the findings of our functional studies. Our demonstration that HCMV enters cell line models for latency in a manner indistinguishable from that of its entry into primary cells illustrates the utility of these cell lines for probing the mechanisms, host genetics, and small-molecule-mediated inhibition of HCMV entry into the cell types where it establishes latency.IMPORTANCE Primary cells cultured in vitro currently provide the highest available relevance for examining molecular and genetic requirements for the establishment, maintenance, and reactivation of HCMV latency. However, their expense, heterogeneity, and intransigence to both long-term culture and molecular or genetic modification create rigor and reproducibility challenges for HCMV latency studies. There are several cell line models for latency not obstructed by deficiencies inherent in primary cells. However, many researchers view cell line studies of latency to be physiologically irrelevant because of the perception that these models display numerous and significant differences from primary cells. Here, we show that the very first step in a latent HCMV infection, entry of the virus into cells, occurs in cell line models in a manner indistinguishable from that in which it occurs in primary CD34+ hematopoietic progenitor cells. Our data argue that experimental HCMV latency is much more similar than it is different in cell lines and primary cells.

Human inborn errors of immunity to infection affecting cells other than leukocytes: from the immune system to the whole organism

Studies of vertebrate immunity have traditionally focused on professional cells, including circulating and tissue-resident leukocytes. Evidence that non-professional cells are also intrinsically essential (i.e. not via their effect on leukocytes) for protective immunity in natural conditions of infection has emerged from three lines of research in human genetics. First, studies of Mendelian resistance to infection have revealed an essential role of DARC-expressing erythrocytes in protection against Plasmodium vivax infection, and an essential role of FUT2-expressing intestinal epithelial cells for protection against norovirus and rotavirus infections. Second, studies of inborn errors of non-hematopoietic cell-extrinsic immunity have shown that APOL1 and complement cascade components secreted by hepatocytes are essential for protective immunity to trypanosome and pyogenic bacteria, respectively. Third, studies of inborn errors of non-hematopoietic cell-intrinsic immunity have suggested that keratinocytes, pulmonary epithelial cells, and cortical neurons are essential for tissue-specific protective immunity to human papillomaviruses, influenza virus, and herpes simplex virus, respectively. Various other types of genetic resistance or predisposition to infection in human populations are not readily explained by inborn variants of genes operating in leukocytes and may, therefore, involve defects in other cells. The probing of this unchartered territory by human genetics is reshaping immunology, by scaling immunity to infection up from the immune system to the whole organism.

Sensory neuropathy and nociception in rodent models of Parkinson's disease

Parkinson's disease (PD) often manifests with prodromal pain and sensory losses whose etiologies are not well understood. Multiple genetic and toxicity-based rodent models of PD partly recapitulate the histopathology and motor function deficits. Although far less studied, there is some evidence that rodents, similar to humans, develop sensory manifestations of the disease, which may precede motor disturbances and help to elucidate the underlying mechanisms of PD-associated pain at the molecular and neuron circuit levels. The present Review summarizes nociception and other sensory functions in frequently used rodent PD models within the context of the complex phenotypes. In terms of mechanisms, it appears that the acute loss of dopaminergic neurons in systemic toxicity models (MPTP, rotenone) primarily causes nociceptive hyperexcitability, presumably owing to a loss of inhibitory control, whereas genetic models primarily result in a progressive loss of heat perception, reflecting sensory fiber neuropathies. At the molecular level, neither α-synuclein deposits alone nor failure of mitophagy alone appear to be strong enough to result in axonal or synaptic pathology of nociceptive neurons that manifest at the behavioral level, and peripheral sensory loss may mask central ‘pain’ in behavioral tests. Hence, allostatic combinations or additional challenges and novel behavioral assessments are needed to better evaluate PD-associated sensory neuropathies and pain in rodents.

Summary: Rodent models of Parkinson's disease partially develop prodromal somatosensory and olfactory dysfunctions reminiscent of sensory neuropathies in patients and reveal mechanistic insight, but data are incomplete and fragmented.

Severe influenza pneumonitis in children with inherited TLR3 deficiency

Autosomal recessive IRF7 and IRF9 deficiencies impair type I and III IFN immunity and underlie severe influenza pneumonitis. We report three unrelated children with influenza A virus (IAV) infection manifesting as acute respiratory distress syndrome (IAV-ARDS), heterozygous for rare TLR3 variants (P554S in two patients and P680L in the third) causing autosomal dominant (AD) TLR3 deficiency. AD TLR3 deficiency can underlie herpes simplex virus-1 (HSV-1) encephalitis (HSE) by impairing cortical neuron-intrinsic type I IFN immunity to HSV-1. TLR3-mutated leukocytes produce normal levels of IFNs in response to IAV. In contrast, TLR3-mutated fibroblasts produce lower levels of IFN-β and -λ, and display enhanced viral susceptibility, upon IAV infection. Moreover, the patients' iPSC-derived pulmonary epithelial cells (PECs) are susceptible to IAV. Treatment with IFN-α2b or IFN-λ1 rescues this phenotype. AD TLR3 deficiency may thus underlie IAV-ARDS by impairing TLR3-dependent, type I and/or III IFN-mediated, PEC-intrinsic immunity. Its clinical penetrance is incomplete for both IAV-ARDS and HSE, consistent with their typically sporadic nature.

Lund Human Mesencephalic (LUHMES) Neuronal Cell Line Supports Herpes Simplex Virus 1 Latency In Vitro

Lund human mesencephalic (LUHMES) cells are human embryonic neuronal precursor cells that can be maintained as proliferating cells due to the expression of a tetracycline-regulatable (Tet-off) v-myc transgene. They can be differentiated to postmitotic neurons by the addition of tetracycline, glial cell-derived neurotrophic factor (GDNF), and dibutyryl cAMP. We demonstrate that these cells can be infected with herpes simplex virus 1 (HSV-1) at a multiplicity of infection (MOI) of 3 with the majority of cells surviving. By 6 days postinfection, there is a loss of lytic gene transcription and an increase in the numbers of neurons that express the latency-associated transcripts (LATs). Importantly, the virus can then be reactivated by the addition of a phosphoinositide 3-kinase inhibitor, which has previously been shown to reactivate HSV-1 in rat neuron cultures. While rodent primary culture neuron systems have been described, these are limited by their lack of scalability, as it is difficult to obtain more than 500,000 neurons to employ for a given experiment. Several recent papers have described a human dorsal root ganglion (DRG) neuron culture model and human induced pleuripotent stem cell (iPSC) neuron culture models that are scalable, but they require that the presence of an antiviral suppression be maintained following HSV-1 infection. The human LUHMES cell model of HSV-1 infection described here may be especially useful for studying HSV-1 latency and reactivation on account of its scalability, its amenability to maintenance of latency without the continual use of antiviral inhibitors, and its latent gene expression profile which mirrors many properties observed in vivo, importantly, the heterogeneity of cells expressing the LATs.IMPORTANCE Herpes simplex virus (HSV) is responsible for significant morbidity in humans due to its ability to cause oral and genital lesions, ocular disease, and encephalitis. While antivirals can attenuate the severity and frequency of disease, there is no vaccine or cure. Understanding the molecular details of HSV latency and reactivation is key to the development of new therapies. One of the difficulties in studying HSV latency has been the need to rely on establishment of latent infections in animal models. While rodent primary neuron culture models have shown promise, they yield relatively small numbers of latently infected neurons for biochemical and molecular analyses. Here we present the use of a human central nervous system (CNS)-derived conditionally proliferating cell line that can be differentiated into mature neurons and latently infected with HSV-1. This model shows promise as a scalable tool to study molecular and biochemical aspects of HSV-1 latency and reactivation in human neurons.

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.