Olfactory transmission of neurotropic viruses

Biological and Psychological Factors Determining Neuropsychiatric Outcomes in COVID-19

PURPOSE OF REVIEW

We present biological and psychological factors implicated in psychiatric manifestations of SARS-CoV-2, as well as its neuroinvasive capability and immune pathophysiology.

RECENT FINDINGS

Preexisting mental illness leads to worse clinical outcomes in COVID-19. The presence of the virus was reported in the cerebrospinal fluid (CSF) and brain tissue post-mortem. Most common psychiatric manifestations include delirium, mood disorders, anxiety disorders, and posttraumatic stress disorder. "Long-COVID" non-syndromal presentations include "brain-fogginess," autonomic instability, fatigue, and insomnia. SARS-CoV-2 infection can trigger prior vulnerabilities based on the priming of microglia and other cells, induced or perpetuated by aging and mental and physical illnesses. COVID-19 could further induce priming of neuroimmunological substrates leading to exacerbated immune response and autoimmunity targeting structures in the central nervous system (CNS), in response to minor immune activating environmental exposures, including stress, minor infections, allergens, pollutants, and traumatic brain injury.

Neurotropism of SARS-CoV-2 and its neuropathological alterations: Similarities with other coronaviruses

A novel coronavirus (SARS-CoV-2) emerged from Wuhan, China, and spread quickly around the world. In addition to fever, cough and shortness of breath, it was confirmed that the patients also have manifestations towards the central nervous system (CNS), especially those critically ill ones. In this review, we will discuss how SARS-CoV-2 gain access to the CNS and the possible consequences. Both SARS-CoV-2 and SARS-CoV-1 in 2002 share the same receptor angiotensin-converting enzyme 2 (ACE2), which can be found in the brain and mediate the disease process. Both direct attack of SARS-CoV-2 and the abnormal immune response in the CNS would contribute to the disease. Also, there is a relationship between SARS-CoV-2 and the occurrence of acute cerebrovascular diseases.

Anosmia: a missing link in the neuroimmunology of coronavirus disease 2019 (COVID-19)

Just before 2020 began, a novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), brought for humans a potentially fatal disease known as coronavirus disease 2019 (COVID-19). The world has thoroughly been affected by COVID-19, while there has been little progress towards understanding the pathogenesis of COVID-19. Patients with a severe phenotype of disease and those who died from the disease have shown hyperinflammation and were more likely to develop neurological manifestations, linking the clinical disease with neuroimmunological features. Anosmia frequently occurs early in the course of COVID-19. The prevalence of anosmia would be influenced by self-diagnosis as well as self-misdiagnosis in patients with COVID-19. Despite this, the association between anosmia and COVID-19 has been a hope for research, aiming to understand the pathogenesis of COVID-19. Studies have suggested differently probable mechanisms for the development of anosmia in COVID-19, including olfactory cleft syndrome, postviral anosmia syndrome, cytokine storm, direct damage of olfactory sensory neurons, and impairment of the olfactory perception center in the brain. Thus, the observation of anosmia would direct us to find the pathogenesis of COVID-19 in the central nervous system, and this is consistent with numerous neurological manifestations related to COVID-19. Like other neurotropic viruses, SARS-CoV-2 might be able to enter the central nervous system via the olfactory epithelium and induce innate immune responses at the site of entry. Viral replication in the nonneural olfactory cells indirectly causes damage to the olfactory receptor nerves, and as a consequence, anosmia occurs. Further studies are required to investigate the neuroimmunology of COVID-19 in relation to anosmia.

Neurological manifestations and comorbidity associated with COVID-19: an overview

First in 2002, severe acute respiratory syndrome coronavirus (SARS-CoV), second in 2012, Middle East respiratory syndrome coronavirus (MERS-CoV), and now the third in the December 2019, emergence of tremendously pathogenic and large-scale epidemic novel coronavirus (SARS-CoV-2) has brought the worst conditions into the human inhabitants of the twenty-first century. The SARS-CoV-2 uses the resembling receptor, angiotensin-converting enzyme 2 (ACE2) as that for SARS-CoV, and mainly feasts through the respiratory tract. The ACE2 receptor appearances have been also detected upon glial cells and neurons, which makes them a potential target of SARS-CoV-2 disease (COVID-19). Consequently, cells expressing ACE2, apart from lung and cardiovascular tissue, neurons and glial cells may act as targets and are thus vulnerable to SARS-CoV-2 systemic infection as well as its central nervous system (CNS) comorbidities. Investigation of the neurological manifestations of COVID-19 is a step towards better understanding the SARS-CoV-2 infections, inhibiting the additional spread and treating patients affected by this pandemic. In this concern, more clinical examinations for CNS involvement of SARS-CoV-2 are warranted. In this article, we have reviewed the neurological characteristic features of COVID-19 patients, latent neurotropic mechanisms of SARS-CoV-2 involvement in the comorbidity associated with CNS disorders, and neurological manifestations associated with COVID-19. Therefore, in the perspective of COVID-19 pandemic, clinicians and healthcare workers should be aware of a wide spectrum of neurological manifestations associated with COVID-19 along with their signs and symptoms for initial diagnosis and isolation of the patients.

Potential neuroinvasive pathways of SARS-CoV-2: Deciphering the spectrum of neurological deficit seen in coronavirus disease-2019 (COVID-19)

Coronavirus disease-2019 (COVID-19) was declared a global pandemic on 11 March 2020. Scientists and clinicians must acknowledge that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has the potential to attack the human body in multiple ways simultaneously and exploit any weaknesses of its host. A multipronged attack could potentially explain the severity and extensive variety of signs and symptoms observed in patients with COVID-19. Understanding the diverse tactics of this virus to infect the human body is both critical and incredibly complex. Although patients diagnosed with COVID-19 have primarily presented with pulmonary involvement, viral invasion, and injury to diverse end organs is also prevalent and well documented in these patients, but has been largely unheeded. Human organs known for angiotensin-converting enzyme 2 (ACE2) expression including the gastrointestinal tract, kidneys, heart, adrenals, brain, and testicles are examples of extra pulmonary tissues with confirmed invasion by SARS-CoV-2. Initial multiple organ involvement may present with vague signs and symptoms to alert health care professionals early in the course of COVID-19. Another example of an ongoing, yet neglected element of the syndromic features of COVID-19, are the reported findings of loss of smell, altered taste, ataxia, headache, dizziness, and loss of consciousness, which suggest a potential for neural involvement. In this review, we further deliberate on the neuroinvasive potential of SARS-CoV-2, the neurologic symptomology observed in COVID-19, the host-virus interaction, possible routes of SARS-CoV-2 to invade the central nervous system, other neurologic considerations for patients with COVID-19, and a collective call to action.

Is anosmia the price to pay in an immune-induced scorched-earth policy against COVID-19?

Since the outbreak of Coronavirus Disease 2019 (COVID-19), loss of smell has increasingly been reported as a frequent clinical sign. Understanding the underlying mechanism and the prognostic value of this symptom will help better manage patients. SARS-CoV-2, as SARS-CoV-1, may likely spread to the central nervous system (CNS) via the olfactory nerve, a known gateway for respiratory neurotropic viruses. We hypothesise that sudden loss of smell due to COVID-19 is the consequence of a protective host defence mechanism involving apoptosis of olfactory receptor neurons. Sacrificing smelling over neuroprotection is a logical strategy, even more so as olfaction is the only sense with the ability to regenerate in adults. Induced apoptosis of olfactory neurons has been shown in mice, successfully preventing neuroinvasion. On the other hand, adult olfactory neurogenesis has been shown to be regulated in part by the immune system, allowing to restore olfactory function. Understanding anosmia as part of a defence mechanism would support the concept of sudden anosmia as being a positive prognostic factor in the short term. Also, it may orient research to investigate the risk of future neurodegenerative disease linked to persisting coronavirus in neurons.

SARS-CoV-2, an Underestimated Pathogen of the Nervous System

Numerous clinical studies have reported neurological symptoms in COVID-19 patients since the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), apart from the atypical signs of pneumonia. Angiotensin-converting enzyme-2 (ACE-2), a potential receptor for SARS-CoV-2 entry, is expressed on various brain cells and cerebral parts, i.e., subfornical organ, paraventricular nucleus, nucleus of the tractus solitarius, and rostral ventrolateral medulla, as well as in non-cardiovascular areas such as the motor cortex and raphe. The resident CNS cells like astrocytes and microglia also express ACE-2, thus highlighting the vulnerability of the nervous system to SARS-CoV-2 infection. Additionally, transmembrane serine protease 2 (TMPRSS2) and furin facilitate virus entry into the host. Besides, the probable routes of virus entry into the nervous system include the hematogenic pathway, through the vagus, the olfactory nerve, or the enteric nervous system. However, the trajectory of SARS-CoV-2 to the brain needs investigation. Furthermore, a Th17-mediated cytokine storm is seen in COVID-19 cases with higher levels of IL-1β/2/7/8/9/10/17, GM-CSF, IFN-γ, TNF-α, CXCL-10, MCP1, and MIP1α/β. Some cytokines can cross the blood-brain barrier and activate the brain's immune cells to produce neural cytokines, leading to neuronal dysfunctions. Nonetheless, most of the neurological conditions developed due to viral infections may not have effective and registered treatments. Although, some antivirals may inhibit the virus-mediated pathogenesis and prove to be suitable in COVID-19 treatment. Therefore, clinicians' and researchers' collective expertise may unravel the potential of SARS-CoV-2 infection to prevent short-term and long-term CNS damage.

COVID-19-Related Anosmia: The Olfactory Pathway Hypothesis and Early Intervention

Anosmia is a well-described symptom of Corona Virus Disease 2019 (COVID-19). Several respiratory viruses are able to cause post-viral olfactory dysfunction, suggesting a sensorineural damage. Since the olfactory bulb is considered an immunological organ contributing to prevent the invasion of viruses, it could have a role in host defense. The inflammatory products locally released in COVID-19, leading to a local damage and causing olfactory loss, simultaneously may interfere with the viral spread into the central nervous system. In this context, olfactory receptors could play a role as an alternative way of SARS-CoV-2 entry into cells locally, in the central nervous system, and systemically. Differences in olfactory bulb due to sex and age may contribute to clarify the different susceptibility to infection and understand the role of age in transmission and disease severity. Finally, evaluation of the degree of functional impairment (grading), central/peripheral anosmia (localization), and the temporal course (evolution) may be useful tools to counteract COVID-19.

Neurological Damage by Coronaviruses: A Catastrophe in the Queue!

Neurological disorders caused by neuroviral infections are an obvious pathogenic manifestation. However, non-neurotropic viruses or peripheral viral infections pose a considerable challenge as their neuropathological manifestations do not emerge because of primary infection. Their secondary or bystander pathologies develop much later, like a syndrome, during and after the recovery of patients from the primary disease. Massive inflammation caused by peripheral viral infections can trigger multiple neurological anomalies. These neurological damages may range from a general cognitive and motor dysfunction up to a wide spectrum of CNS anomalies, such as Acute Necrotizing Hemorrhagic Encephalopathy, Guillain-Barré syndrome, Encephalitis, Meningitis, anxiety, and other audio-visual disabilities. Peripheral viruses like Measles virus, Enteroviruses, Influenza viruses (HIN1 series), SARS-CoV-1, MERS-CoV, and, recently, SARS-CoV-2 are reported to cause various neurological manifestations in patients and are proven to be neuropathogenic even in cellular and animal model systems. This review presents a comprehensive picture of CNS susceptibilities toward these peripheral viral infections and explains some common underlying themes of their neuropathology in the human brain.

How SARS-Cov-2 can involve the central nervous system. A systematic analysis of literature of the department of human neurosciences of Sapienza University, Italy

Italy is currently one of the countries most affected by the global emergency of COVID-19, a lethal disease of a novel coronavirus renamed as SARS-CoV-2. SARS-CoV-2 shares highly homological sequence with the most studied SARS-CoV, and causes acute, highly deadly pneumonia (COVID-19) with clinical symptoms similar to those reported for SARS-CoV and MERS-CoV. Increasing evidence shows that these coronaviruses are not always confined to the respiratory tract and that they may also neuroinvasive and neurotropic, with potential neuropathological consequences in vulnerable populations. The aim of this study is to predict a likely CNS involvement by SARS-CoV-2 by studying the pathogenic mechanisms in common with other better known and studied coronaviruses with which it shares the same characteristics. Understanding the mechanisms of neuroinvasion and interaction of HCoV (including SARS-Cov-2) with the CNS is essential to evaluate potentially pathological short- and long-term consequences. Autopsies of the COVID-19 patients, detailed neurological investigation, and attempts to isolate SARS-CoV-2 from the endothelium of cerebral microcirculation, cerebrospinal fluid, glial cells, and neuronal tissue can clarify the role played by COVID-19 in CNS-involvement and in the ongoing mortalities as has been in the recent outbreak.

Nasal lavage containing Angiotensin-Converting Enzyme-2 agonist can prevent and reduce viral load in COVID-19

COVID-19 has been the talk of the year 2020, taking many lives and leaving others in critical conditions. It has clearly and severally been reported that the SARSCoV-2 uses the Angiotensin Converting Enzyme-2 receptors to penetrate and infect cells. Reports have also stated that the nasal and olfactory mucosa are overloaded with these receptors. We emphasize that anosmia in COVID-19 is secondary to the binding of the SARSCoV-2 to Angiotensin Converting Enzyme-2 receptors on the olfactory mucosa. A hypotheses pertaining to the presentation, diagnosis, management and possible prevention of SARS-CoV-2 is proposed. Given the high false negative rates of the polymerase chain reaction (PCR) tests, we suggest that COVID-19 negative patients with anosmia without any other nasal symptom should raise a high index of suspicion and should be further evaluated. We propose the formulation and use of Angiotensin Converting Enzyme-2 receptors agonist or angiotensin receptor blockers (ARBs) as nasal lavage, to reduce the viral load of confirmed positive patients, and as a mode of prevention, especially in high risk patients, until a vaccine is developed. These medications are readily available and testing this theory involves determination of the correct dosage of angiotensin receptor blockers or ACE inhibitors (via dilution in water) that can be used as nasal lavage and performing efficacy trials. Potential side effects to be monitored for include low blood pressure or changes in heart rate. Administration of a medicated nasal lavage may be easier and rapidly disseminated on the nasal mucosa.

Independent and Correlated Role of Apolipoprotein E ɛ4 Genotype and Herpes Simplex Virus Type 1 in Alzheimer's Disease

The ɛ4 allele of the Apolipoprotein E (APOE) gene in individuals infected by Herpes simplex virus type 1 (HSV-1) has been demonstrated to be a risk factor in Alzheimer's disease (AD). APOE-ɛ4 reduces the levels of neuronal cholesterol, interferes with the transportation of cholesterol, impairs repair of synapses, decreases the clearance of neurotoxic peptide amyloid-β (Aβ), and promotes the deposition of amyloid plaque, and eventually may cause development of AD. HSV-1 enters host cells and can infect the olfactory system, trigeminal ganglia, entorhinal cortex, and hippocampus, and may cause AD-like pathological changes. The lifecycle of HSV-1 goes through a long latent phase. HSV-1 induces neurotropic cytokine expression with pro-inflammatory action and inhibits antiviral cytokine production in AD. It should be noted that interferons display antiviral activity in HSV-1-infected AD patients. Reactivated HSV-1 is associated with infectious burden in cognitive decline and AD. Finally, HSV-1 DNA has been confirmed as present in human brains and is associated with APOEɛ4 in AD. HSV-1 and APOEɛ4 increase the risk of AD and relate to abnormal autophagy, higher concentrations of HSV-1 DNA in AD, and formation of Aβ plaques and neurofibrillary tangles.

Targeting the cholinergic anti-inflammatory pathway with vagus nerve stimulation in patients with Covid-19?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), at the origin of the worldwide COVID-19 pandemic, is characterized by a dramatic cytokine storm in some critical patients with COVID-19. This storm is due to the release of high levels of pro-inflammatory cytokines such as interleukin (IL)-1 β, IL-6, tumor necrosis factor (TNF), and chemokines by respiratory epithelial and dendritic cells, and macrophages. We hypothesize that this cytokine storm and the worsening of patients’ health status can be dampened or even prevented by specifically targeting the vagal-driven cholinergic anti-inflammatory pathway (CAP). The CAP is a concept that involves an anti-inflammatory effect of vagal efferents by the release of acetylcholine (ACh). Nicotinic acetylcholine receptor alpha7 subunit (α7nAChRs) is required for ACh inhibition of macrophage-TNF release and cytokine modulation. Hence, targeting the α7nAChRs through vagus nerve stimulation (VNS) could be of interest in the management of patients with SARS-CoV-2 infection. Indeed, through the wide innervation of the organism by the vagus nerve, especially the lungs and gastrointestinal tract, VNS appears as a serious candidate for a few side effect treatment that could dampen or prevent the cytokine storm observed in COVID-19 patients with severe symptoms. Finally, a continuous vagal tone monitoring in patients with COVID-19 could be used as a predictive marker of COVID-19 illness course but also as a predictive marker of response to COVID-19 treatment such as VNS or others.

T cell engagement of cross-presenting microglia protects the brain from a nasal virus infection

The neuroepithelium is a nasal barrier surface populated by olfactory sensory neurons that detect odorants in the airway and convey this information directly to the brain via axon fibers. This barrier surface is especially vulnerable to infection, yet respiratory infections rarely cause fatal encephalitis, suggesting a highly evolved immunological defense. Here, using a mouse model, we sought to understand the mechanism by which innate and adaptive immune cells thwart neuroinvasion by vesicular stomatitis virus (VSV), a potentially lethal virus that uses olfactory sensory neurons to enter the brain after nasal infection. Fate-mapping studies demonstrated that infected central nervous system (CNS) neurons were cleared noncytolytically, yet specific deletion of major histocompatibility complex class I (MHC I) from these neurons unexpectedly had no effect on viral control. Intravital imaging studies of calcium signaling in virus-specific CD8+ T cells revealed instead that brain-resident microglia were the relevant source of viral peptide-MHC I complexes. Microglia were not infected by the virus but were found to cross-present antigen after acquisition from adjacent neurons. Microglia depletion interfered with T cell calcium signaling and antiviral control in the brain after nasal infection. Collectively, these data demonstrate that microglia provide a front-line defense against a neuroinvasive nasal infection by cross-presenting antigen to antiviral T cells that noncytolytically cleanse neurons. Disruptions in this innate defense likely render the brain susceptible to neurotropic viruses like VSV that attempt to enter the CNS via the nose.

Disentangling the Hypothesis of Host Dysosmia and SARS-CoV-2: The Bait Symptom That Hides Neglected Neurophysiological Routes

The respiratory condition COVID-19 arises in a human host upon the infection with SARS-CoV-2, a coronavirus that was first acknowledged in Wuhan, China, at the end of December 2019 after its outbreak of viral pneumonia. The full-blown COVID-19 can lead, in susceptible individuals, to premature death because of the massive viral proliferation, hypoxia, misdirected host immunoresponse, microthrombosis, and drug toxicities. Alike other coronaviruses, SARS-CoV-2 has a neuroinvasive potential, which may be associated with early neurological symptoms. In the past, the nervous tissue of patients infected with other coronaviruses was shown to be heavily infiltrated. Patients with SARS-CoV-2 commonly report dysosmia, which has been related to the viral access in the olfactory bulb. However, this early symptom may reflect the nasal proliferation that should not be confused with the viral access in the central nervous system of the host, which can instead be allowed by means of other routes for spreading in most of the neuroanatomical districts. Axonal, trans-synaptic, perineural, blood, lymphatic, or Trojan routes can gain the virus multiples accesses from peripheral neuronal networks, thus ultimately invading the brain and brainstem. The death upon respiratory failure may be also associated with the local inflammation- and thrombi-derived damages to the respiratory reflexes in both the lung neuronal network and brainstem center. Beyond the infection-associated neurological symptoms, long-term neuropsychiatric consequences that could occur months after the host recovery are not to be excluded. While our article does not attempt to fully comprehend all accesses for host neuroinvasion, we aim at stimulating researchers and clinicians to fully consider the neuroinvasive potential of SARS-CoV-2, which is likely to affect the peripheral nervous system targets first, such as the enteric and pulmonary nervous networks. This acknowledgment may shed some light on the disease understanding further guiding public health preventive efforts and medical therapies to fight the pandemic that directly or indirectly affects healthy isolated individuals, quarantined subjects, sick hospitalized, and healthcare workers.

[Alzheimer's disease, neuro-inflammation, and herpes viruses, a path that traces its way]

Infection of the brain with various types of pathogens, and the resulting inflammatory response, is becoming increasingly important in our understanding of the etiology of Alzheimer's disease (AD). The fact that several genes identified as risk factors are actually involved in the modulation of the immune response, as well as the very diversity of the infectious agents identified as possible actors in the evolution of this disease, argue in favor of the neuro-inflammatory hypothesis, as does the demonstration that the protein Aβ, one of the most important markers of AD, is an antimicrobial peptide. Among others, herpes viruses (mainly, but not only, HSV-1), which can establish latent infections in brain neurons, especially in the elder population, punctuated by episodes of reactivation following stress or immunosuppression, appear as very strong candidates to play an etiological role, if only as cofactors, of AD. Recent results show that, in human and rat neurons, infection with HSV-1 increases the formation of Aβ along the amyloidogenic pathway, as well as the phosphorylation of Tau proteins, another essential marker of AD. The growing evidence that chronic infections and defense mechanisms, including inflammatory processes, are at the heart of AD, warrants reviewing antiviral drugs such as acyclovir, and possibly vaccination, as potential avenues for AD control.

Potential neurological impact of coronaviruses: implications for the novel SARS-CoV-2

Coronaviruses (CoV) are viruses widely known to cause severe respiratory distress due to the prominent clinical symptoms presented. These symptoms, which include fever and dry cough, are frequently found in individuals with CoV infection. Neurological manifestations of CoV have often been neglected; however, recent studies have reported neurological consequences of CoV infection. Here, we review these literatures and discuss the neurologic impact of CoV while highlighting potential implications of the novel SARS-CoV-2 in the nervous system. We also discuss the possible routes by which these viruses invade the nervous system and the mechanism by which they may induce neurological damage.

Detection of meningococcus, pneumococcus, Haemophilus influenzae, and group A streptococcus DNA in pediatric adenoid bioptats

Meningococcal, pneumococcal, streptococcal A and Haemophilus influenzae infections are manifested in different clinical forms, ranging from bacterial carriage to generalized life-threatening conditions. However, a connection between bacterial carriage and disease development has not been fully explored. A PCR assay was performed with adenoid biopsy samples collected from 112 children after planned adenotomy to detect Neisseria meningitidis, Streptococcus pneumoniae, Streptococcus pyogenes, H. influenzae carriage. A DNA specific to at least one of the four studied microbial species was found in 104 samples (92.86%) so that: meningococcal DNA was detected in one sample (0.9%), pneumococcal — in 98 (87.5%), H. influenzae — in 19 (16.96%), and streptococcal A — in 42 (37.5%) samples. However, none of these species was found in 8 children (7.14%). A sole S. pneumoniae was detected in 54 samples (48.2%), whereas S. pyogenes — in 5 samples (4.5%). Moreover, two bacterial species were simultaneously as follows: N. meningitidis and S. pneumoniae — in 1 sample (0.9%), S. pneumoniae and H. influenzae — in 7 samples (6.3%); H. influenzae and S. pyogenes — in 1 sample (0.9%); S. pneumoniae and S. pyogenes — in 25 samples (22.3%). A triple combination consisting of S. pneumoniae, H. influenzae and S. pyogenes bacteria were detected together in 11 patients (9.8%). Meningococcal serogrouping revealed no connection with any of the 6 most common global serogroups responsible for epidemic incidence rise (A, B, C, W-135, X, Y). A clear tendency for prevalence of S. pyogenes DNA in adenoid pediatric biopsies in children diagnosed with “Adenoids and tonsils hypertrophy” vs. “Adenoids hypertrophy” was observed. It is noteworthy, a high relative prevalence of pneumococcal carriage (87.5%), found by us was of special importance. Pediatric carriers serving as a reservoir for virulent pneumococcal species pose a threat both for themselves and surrounding people. Thus, PCR-based data of adenoid biopsies may be a promising approach for future studies, as a potential to identify live viable but nonculturable bacteria in clinical specimens will contribute to a more accurate assessment of carriage rate of meningococci, pneumococci, H. influenzae and group A streptococci.

An improved animal model for herpesvirus encephalitis in humans

Herpesviral encephalitis caused by Herpes Simplex Virus 1 (HSV-1) is one of the most devastating diseases in humans. Patients present with fever, mental status changes or seizures and when untreated, sequelae can be fatal. Herpes Simplex Encephalitis (HSE) is characterized by mainly unilateral necrotizing inflammation effacing the frontal and mesiotemporal lobes with rare involvement of the brainstem. HSV-1 is hypothesized to invade the CNS via the trigeminal or olfactory nerve, but viral tropism and the exact route of infection remain unclear. Several mouse models for HSE have been developed, but they mimic natural infection only inadequately. The porcine alphaherpesvirus Pseudorabies virus (PrV) is closely related to HSV-1 and Varicella Zoster Virus (VZV). While pigs can control productive infection, it is lethal in other susceptible animals associated with severe pruritus leading to automutilation. Here, we describe the first mutant PrV establishing productive infection in mice that the animals are able to control. After intranasal inoculation with a PrV mutant lacking tegument protein pUL21 and pUS3 kinase activity (PrV-ΔUL21/US3Δkin), nearly all mice survived despite extensive infection of the central nervous system. Neuroinvasion mainly occurred along the trigeminal pathway. Whereas trigeminal first and second order neurons and autonomic ganglia were positive early after intranasal infection, PrV-specific antigen was mainly detectable in the frontal, mesiotemporal and parietal lobes at later times, accompanied by a long lasting lymphohistiocytic meningoencephalitis. Despite this extensive infection, mice showed only mild to moderate clinical signs, developed alopecic skin lesions, or remained asymptomatic. Interestingly, most mice exhibited abnormalities in behavior and activity levels including slow movements, akinesia and stargazing. In summary, clinical signs, distribution of viral antigen and inflammatory pattern show striking analogies to human encephalitis caused by HSV-1 or VZV not observed in other animal models of disease.

In developed countries, more than 50% of humans are seropositive for the neurotropic Herpes Simplex Virus 1 (HSV-1) and two to four million cases of Herpes simplex encephalitis (HSE) are reported per year worldwide. Primary infection with HSV-1 takes place via the skin or the oral mucosa followed by intraaxonal retrograde spread to sensory ganglia of the peripheral nervous system where HSV-1 usually establishes latency. Further spread to the central nervous system results in HSE, a necrotizing encephalitis effacing predominantly the temporal and frontal lobes of the brain. Mice infected with HSV-1 develop encephalitis, but do not show the typical lesions and exhibit high mortality rates. Here we demonstrate that mice infected with a mutant pseudorabies virus lacking the tegument protein pUL21 and an active viral kinase pUS3 were able to survive the productive infection but developed lymphohistiocytic encephalitis with viral antigen distribution, inflammation and associated behavioral changes comparable to HSE in humans. These striking analogies offer new perspectives to study herpesviral encephalitis in a suitable animal model.

Staphylococcus aureus α-toxin: small pore, large consequences

The small β-pore-forming α-toxin, also termed α-hemolysin or Hla is considered to be an important virulence factor of Staphylococcus aureus. Perforation of the plasma membrane (PM) by Hla leads to uncontrolled flux of ions and water. Already a small number of toxin pores seems to be sufficient to induce complex cellular responses, many of which depend on the efflux of potassium. In this article, we discuss the implications of secondary membrane lesions, for example, by endogenous channels, for Hla-mediated toxicity, for calcium-influx and membrane repair. Activation of purinergic receptors has been proposed to be a major contributor to the lytic effects of various pore forming proteins, but new findings raise doubts that this holds true for Hla. However, the recently discovered cellular pore forming proteins gasdermin D and Mixed lineage kinase domain-like pseudokinase (MLKL) which perforate the PM from the cytosolic side might contribute to both calcium-influx-dependent damage and membrane repair. Activation of endogenous pore forming proteins by Hla above a threshold concentration could explain the apparent dependence of pore characteristics on toxin concentrations. If secondary membrane damage in the aftermath of Hla-attack contributes significantly to overall PM permeability, it might be an interesting target for new therapeutic approaches.

Antiviral therapy: Valacyclovir Treatment of Alzheimer's Disease (VALAD) Trial: protocol for a randomised, double-blind,placebo-controlled, treatment trial

INTRODUCTION

After infection, herpes simplex virus-1 (HSV1) becomes latent in the trigeminal ganglion and can enter the brain via retrograde axonal transport. Recurrent reactivation of HSV1 may lead to neurodegeneration and Alzheimer's disease (AD) pathology. HSV1 (oral herpes) and HSV2 (genital herpes) can trigger amyloid beta-protein (Aβ) aggregation and HSV1 DNA is common in amyloid plaques. Anti-HSV drugs reduce Aβ and phosphorylated tau accumulation in cell-culture models. Cognitive impairment is greater in patients with HSV seropositive, and antiviral drugs show robust efficacy against peripheral HSV infection. Recent studies of electronic health records databases demonstrate that HSV infections increase dementia risk, and that antiviral medication treatment reduces this risk. The generic antiviral drug valacyclovir was superior to placebo in improving memory in a schizophrenia pilot trial but has not been tested in AD.

METHODS AND ANALYSIS

In patients with mild AD who test positive for HSV1 or HSV2 serum antibodies, valacyclovir, repurposed as an anti-AD drug, will be compared with placebo (lactose pills) in 130 patients (65 valacyclovir and 65 placebo) in a randomised, double-blind, 78-week phase II proof-of-concept trial. Patients on valacyclovir, dose-titrated from 2 g to a targeted oral dose of 4 g daily, compared with placebo, are hypothesised to show smaller cognitive and functional decline, and, using 18F-Florbetapir positron emission tomography (PET) and 18F-MK-6240 PET imaging, to show less amyloid and tau accumulation, respectively. In the lumbar puncture subsample, cerebrospinal fluid acyclovir will be assayed to assess central nervous system valacyclovir penetration.

ETHICS AND DISSEMINATION

The trial is being overseen by the New York State Psychiatric Institute Institutional Review Board (protocol 7537), the National Institute on Ageing, and the Data Safety Monitoring Board. Written informed consent is obtained for all subjects. Results will be disseminated via publication, clinicaltrials.gov, media and conferences.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov identifier (NCT03282916) Pre-results.

Human Coronaviruses and Other Respiratory Viruses: Underestimated Opportunistic Pathogens of the Central Nervous System?

Respiratory viruses infect the human upper respiratory tract, mostly causing mild diseases. However, in vulnerable populations, such as newborns, infants, the elderly and immune-compromised individuals, these opportunistic pathogens can also affect the lower respiratory tract, causing a more severe disease (e.g., pneumonia). Respiratory viruses can also exacerbate asthma and lead to various types of respiratory distress syndromes. Furthermore, as they can adapt fast and cross the species barrier, some of these pathogens, like influenza A and SARS-CoV, have occasionally caused epidemics or pandemics, and were associated with more serious clinical diseases and even mortality. For a few decades now, data reported in the scientific literature has also demonstrated that several respiratory viruses have neuroinvasive capacities, since they can spread from the respiratory tract to the central nervous system (CNS). Viruses infecting human CNS cells could then cause different types of encephalopathy, including encephalitis, and long-term neurological diseases. Like other well-recognized neuroinvasive human viruses, respiratory viruses may damage the CNS as a result of misdirected host immune responses that could be associated with autoimmunity in susceptible individuals (virus-induced neuro-immunopathology) and/or viral replication, which directly causes damage to CNS cells (virus-induced neuropathology). The etiological agent of several neurological disorders remains unidentified. Opportunistic human respiratory pathogens could be associated with the triggering or the exacerbation of these disorders whose etiology remains poorly understood. Herein, we present a global portrait of some of the most prevalent or emerging human respiratory viruses that have been associated with possible pathogenic processes in CNS infection, with a special emphasis on human coronaviruses.

Olfactory dysfunction in the pathophysiological continuum of dementia

Sensory capacities like smell, taste, hearing, vision decline with aging, but increasing evidence show that sensory dysfunctions are one of the early signs diagnosing the conversion from physiological to pathological brain state. Smell loss represents the best characterized sense in clinical practice and is considered as one of the first preclinical signs of Alzheimer's and Parkinson's disease, occurring a decade or more before the onset of cognitive and motor symptoms. Despite the numerous scientific reports and the adoption in clinical practice, the etiology of sensory damage as prodromal of dementia remains largely unexplored and more studies are needed to resolve the mechanisms underlying sensory network dysfunction. Although both cognitive and sensory domains are progressively affected, loss of sensory experience in early stages plays a major role in reducing the autonomy of demented people in their daily tasks or even possibly contributing to their cognitive decline. Interestingly, the chemosensory circuitry is devoid of a blood brain barrier, representing a vulnerable port of entry for neurotoxic species that can spread to the brain. Furthermore, the exposure of the olfactory system to the external environment make it more susceptible to mechanical injury and trauma, which can cause degenerative neuroinflammation. In this review, we will summarize several findings about chemosensory impairment signing the conversion from healthy to pathological brain aging and we will try to connect those observations to the promising research linking environmental influences to sporadic dementia. The scientific body of knowledge will support the use of chemosensory diagnostics in the presymptomatic stages of AD and other biomarkers with the scope of finding treatment strategies before the onset of the disease.

The role of olfactory transport in the penetration of manganese oxide nanoparticles from blood into the brain

There is no doubt that various nanoparticles (NPs) can enter the brain from the nasal cavity. It is assumed that NPs can penetrate from blood into the central nervous system (CNS) only by breaking the blood–brain barrier (BBB). The accumulation of NPs in CNS can provoke many neurological diseases; therefore, the understanding of its mechanisms is of both academic and practical interest. Although hitting from the surface of the lungs into the bloodstream, NPs can accumulate in various mucous membranes, including the nasal mucosa. Thus, we cannot rule out the ability of NPs to be transported from the bloodstream to the brain through the olfactory uptake. To test this hypothesis, we used paramagnetic NPs of manganese oxide (Mn3O4-NPs), whose accumulation patterns in the mouse brain were recorded using T1-weighted magnetic resonance imaging. The effect of intranasal application of endocytosis and axonal transport inhibitors on the brain accumulation patterns of intranasally or intravenously injected Mn3O4-NPs was evaluated. A comparative analysis of the results showed that the transport of Mn3O4-NPs from the nasal cavity to the brain is more efficient than their local permeation through BBB into CNS from the bloodstream, for example with the accumulation of Mn3O4NPs in the dentate gyrus of the hippocampus, and through the capture and transport of NPs from the blood by olfactory epithelium cells. Also, experiments with the administration of chlorpromazine, a specific inhibitor of clathrin-dependent endocytosis, and methyl-β-cyclodextrin, inhibitor of the lipid rafts involved in the capture of substances by endothelium cells, showed differences in the mechanisms of NP uptake from the nasal cavity and from the bloodstream. In this study, we show a significant contribution of axonal transport to NP accumulation patterns in the brain, both from the nasal cavity and from the vascular bed. This explains the accumulation of different sorts of submicron particles (neurotropic viruses, insoluble xenobiotics, etc.), unable to pass BBB, in the brain. The results will add to the understanding of the pathogenesis of various neurodegenerative diseases and help studying the side effects of therapeutics administered intravenously.

Olfactory sensory neurons mediate ultrarapid antiviral immune responses in a TrkA-dependent manner

The nervous system regulates host immunity in complex ways. Vertebrate olfactory sensory neurons (OSNs) are located in direct contact with pathogens; however, OSNs' ability to detect danger and initiate immune responses is unclear. We report that nasal delivery of rhabdoviruses induces apoptosis in crypt OSNs via the interaction of the OSN TrkA receptor with the viral glycoprotein in teleost fish. This signal results in electrical activation of neurons and very rapid proinflammatory responses in the olfactory organ (OO), but dampened inflammation in the olfactory bulb (OB). CD8α⁺ cells infiltrate the OO within minutes of nasal viral delivery, and TrkA blocking, but not caspase-3 blocking, abrogates this response. Infiltrating CD8α⁺ cells were TCRαβ T cells with a nonconventional phenotype that originated from the microvasculature surrounding the OB and not the periphery. Nasal delivery of viral glycoprotein (G protein) recapitulated the immune responses observed with the whole virus, and antibody blocking of viral G protein abrogated these responses. Ablation of crypt neurons in zebrafish resulted in increased susceptibility to rhabdoviruses. These results indicate a function for OSNs as a first layer of pathogen detection in vertebrates and as orchestrators of nasal-CNS antiviral immune responses.

Olfactory sensory neurons mediate ultrarapid antiviral immune responses in a TrkA-dependent manner

The nervous system regulates host immunity in complex ways. Vertebrate olfactory sensory neurons (OSNs) are located in direct contact with pathogens; however, OSNs' ability to detect danger and initiate immune responses is unclear. We report that nasal delivery of rhabdoviruses induces apoptosis in crypt OSNs via the interaction of the OSN TrkA receptor with the viral glycoprotein in teleost fish. This signal results in electrical activation of neurons and very rapid proinflammatory responses in the olfactory organ (OO), but dampened inflammation in the olfactory bulb (OB). CD8α⁺ cells infiltrate the OO within minutes of nasal viral delivery, and TrkA blocking, but not caspase-3 blocking, abrogates this response. Infiltrating CD8α⁺ cells were TCRαβ T cells with a nonconventional phenotype that originated from the microvasculature surrounding the OB and not the periphery. Nasal delivery of viral glycoprotein (G protein) recapitulated the immune responses observed with the whole virus, and antibody blocking of viral G protein abrogated these responses. Ablation of crypt neurons in zebrafish resulted in increased susceptibility to rhabdoviruses. These results indicate a function for OSNs as a first layer of pathogen detection in vertebrates and as orchestrators of nasal-CNS antiviral immune responses.

OR14I1 is a receptor for the human cytomegalovirus pentameric complex and defines viral epithelial cell tropism

A human cytomegalovirus (HCMV) pentameric glycoprotein complex (PC), gH-gL-UL128-UL130-UL131A, is necessary for viral infection of clinically relevant cell types, including epithelial cells, which are important for interhost transmission and disease. We performed genome-wide CRISPR/Cas9 screens of different cell types in parallel to identify host genes specifically required for HCMV infection of epithelial cells. This effort identified a multipass membrane protein, OR14I1, as a receptor for HCMV infection. This olfactory receptor family member is required for HCMV attachment, entry, and infection of epithelial cells and is dependent on the presence of viral PC. OR14I1 is required for AKT activation and mediates endocytosis entry of HCMV. We further found that HCMV infection of epithelial cells is blocked by a synthetic OR14I1 peptide and inhibitors of adenylate cyclase and protein kinase A (PKA) signaling. Identification of OR14I1 as a PC-dependent HCMV host receptor associated with epithelial tropism and the role of the adenylate cyclase/PKA/AKT-mediated signaling pathway in HCMV infection reveal previously unappreciated targets for the development of vaccines and antiviral therapies.

Intranasal Borna Disease Virus (BoDV-1) Infection: Insights into Initial Steps and Potential Contagiosity

Mammalian Bornavirus (BoDV-1) typically causes a fatal neurologic disorder in horses and sheep, and was recently shown to cause fatal encephalitis in humans with and without transplant reception. It has been suggested that BoDV-1 enters the central nervous system (CNS) via the olfactory pathway. However, (I) susceptible cell types that replicate the virus for successful spread, and (II) the role of olfactory ensheathing cells (OECs), remained unclear. To address this, we studied the intranasal infection of adult rats with BoDV-1 in vivo and in vitro, using olfactory mucosal (OM) cell cultures and the cultures of purified OECs. Strikingly, in vitro and in vivo, viral antigen and mRNA were present from four days post infection (dpi) onwards in the olfactory receptor neurons (ORNs), but also in all other cell types of the OM, and constantly in the OECs. In contrast, in vivo, BoDV-1 genomic RNA was only detectable in adult and juvenile ORNs, nerve fibers, and in OECs from 7 dpi on. In vitro, the rate of infection of OECs was significantly higher than that of the OM cells, pointing to a crucial role of OECs for infection via the olfactory pathway. Thus, this study provides important insights into the transmission of neurotropic viral infections with a zoonotic potential.

Viral Hypothesis and Antiviral Treatment in Alzheimer's Disease

PURPOSE OF REVIEW

Viruses, particularly herpes simplex virus (HSV), may be a cause of Alzheimer's disease (AD). The evidence supporting the viral hypothesis suggests that antiviral treatment trials, which have not been conducted, are warranted.

RECENT FINDINGS

HSV1 (oral herpes) and HSV2 (genital herpes) can trigger amyloid aggregation, and their DNA is common in amyloid plaques. HSV1 reactivation is associated with tau hyperphosphorylation and possibly tau propagation. Anti-HSV drugs reduce Aβ and p-tau accumulation in infected mouse brains. Clinically, after the initial oral infection, herpes simplex virus-1 (HSV1) becomes latent in the trigeminal ganglion and recurrent reactivation may produce neuronal damage and AD pathology. Clinical studies show cognitive impairment in HSV seropositive patients, and antiviral drugs show strong efficacy against HSV. An antiviral treatment trial in AD is clearly warranted. A phase II treatment trial with valacyclovir, an anti-HSV drug, recently began with evaluation of clinical and biomarker outcomes.

Molecular Mechanisms for Herpes Simplex Virus Type 1 Pathogenesis in Alzheimer's Disease

This review focuses on research in the areas of epidemiology, neuropathology, molecular biology and genetics that implicates herpes simplex virus type 1 (HSV-1) as a causative agent in the pathogenesis of sporadic Alzheimer’s disease (AD). Molecular mechanisms whereby HSV-1 induces AD-related pathophysiology and pathology, including neuronal production and accumulation of amyloid beta (Aβ), hyperphosphorylation of tau proteins, dysregulation of calcium homeostasis, and impaired autophagy, are discussed. HSV-1 causes additional AD pathologies through mechanisms that promote neuroinflammation, oxidative stress, mitochondrial damage, synaptic dysfunction, and neuronal apoptosis. The AD susceptibility genes apolipoprotein E (APOE), phosphatidylinositol binding clathrin assembly protein (PICALM), complement receptor 1 (CR1) and clusterin (CLU) are involved in the HSV lifecycle. Polymorphisms in these genes may affect brain susceptibility to HSV-1 infection. APOE, for example, influences susceptibility to certain viral infections, HSV-1 viral load in the brain, and the innate immune response. The AD susceptibility gene cholesterol 25-hydroxylase (CH25H) is upregulated in the AD brain and is involved in the antiviral immune response. HSV-1 interacts with additional genes to affect cognition-related pathways and key enzymes involved in Aβ production, Aβ clearance, and hyperphosphorylation of tau proteins. Aβ itself functions as an antimicrobial peptide (AMP) against various pathogens including HSV-1. Evidence is presented supporting the hypothesis that Aβ is produced as an AMP in response to HSV-1 and other brain infections, leading to Aβ deposition and plaque formation in AD. Epidemiologic studies associating HSV-1 infection with AD and cognitive impairment are discussed. Studies are reviewed supporting subclinical chronic reactivation of latent HSV-1 in the brain as significant in the pathogenesis of AD. Finally, the rationale for and importance of clinical trials treating HSV-1-infected MCI and AD patients with antiviral medication is discussed.

Acute and chronic viral infections

A large number of viruses belonging to various families are able to cause central nervous system (CNS) infections and contribute significantly to burden of disease in humans globally. Most viral CNS infections are benign and self-limiting, and most remain undiagnosed. However, some viruses can cause severe inflammation, leading to morbidity and mortality, and result in severe long-term residual damage and neurologic dysfunction in patients. The potential of viruses to cause CNS inflammation greatly varies depending on host factors, such as age, sex, and genetic background, as well as viral factors. Despite the need for protection against viral invasion and replication, the extent of the immune response in the CNS is carefully regulated to prevent excessive inflammation and tissue destruction leading to irretrievable loss of neurons. Direct cytopathology is for many virus infections a major cause of neurologic symptoms; however, the antiviral immune response can in some instances contribute substantially to pathology. This chapter highlights a selection of clinically important neurotropic viruses that infect the CNS and cause neurologic diseases such as meningitis, encephalitis, and myelitis in humans, with a focus on neuropathologic findings.

Contributions of neurotropic human herpesviruses herpes simplex virus 1 and human herpesvirus 6 to neurodegenerative disease pathology

Human herpesviruses (HVs) have developed ingenious mechanisms that enable them to traverse the defenses of the central nervous system (CNS). The ability of HVs to enter a state of latency, a defining characteristic of this viral family, allows them to persist in the human host indefinitely. As such, HVs represent the most frequently detected pathogens in the brain. Under constant immune pressure, these infections are largely asymptomatic in healthy hosts. However, many neurotropic HVs have been directly connected with CNS pathology in the context of other stressors and genetic risk factors. In this review, we discuss the potential mechanisms by which neurotropic HVs contribute to neurodegenerative disease (NDD) pathology by highlighting two prominent members of the HV family, herpes simplex virus 1 (HSV-1) and human herpesvirus 6 (HHV-6). We (i) introduce the infectious pathways and replicative cycles of HSV-1 and HHV-6 and then (ii) review the clinical evidence supporting associations between these viruses and the NDDs Alzheimer's disease (AD) and multiple sclerosis (MS), respectively. We then (iii) highlight and discuss potential mechanisms by which these viruses exert negative effects on neurons and glia. Finally, we (iv) discuss how these viruses could interact with other disease-modifying factors to contribute to the initiation and/or progression of NDDs.

Murine Olfactory Bulb Interneurons Survive Infection with a Neurotropic Coronavirus

Viral infection of the central nervous system (CNS) is complicated by the mostly irreplaceable nature of neurons, as the loss of neurons has the potential to result in permanent damage to brain function. However, whether neurons or other cells in the CNS sometimes survive infection and the effects of infection on neuronal function is largely unknown. To address this question, we used the rJHM strain (rJ) of mouse hepatitis virus (MHV), a neurotropic coronavirus that causes acute encephalitis in susceptible strains of mice. To determine whether neurons or other CNS cells survive acute infection with this virulent virus, we developed a recombinant JHMV that expresses Cre recombinase (rJ-Cre) and infected mice that universally expressed a silent (floxed) version of tdTomato. Infection of these mice with rJ-Cre resulted in expression of tdTomato in host cells. The results showed that some cells were able to survive the infection, as demonstrated by continued tdTomato expression after virus antigen could no longer be detected. Most notably, interneurons in the olfactory bulb, which are known to be inhibitory, represented a large fraction of the surviving cells. In conclusion, our results indicated that some neurons are resistant to virus-mediated cell death and provide a framework for studying the effects of prior coronavirus infection on neuron function.IMPORTANCE We developed a novel recombinant virus that allows the study of cells that survive an infection by a central nervous system-specific strain of murine coronavirus. Using this virus, we identified neurons and, to a lesser extent, nonneuronal cells in the brain that were infected during the acute phase of the infection and survived for approximately 2 weeks until the mice succumbed to the infection. We focused on neurons and glial cells within the olfactory bulb because the virus enters the brain at this site. Our results show that interneurons of the olfactory bulb were the primary cell type able to survive infection. Further, these results indicate that this system will be useful for functional and gene expression studies of cells in the brain that survive acute infection.

CK12a, a CCL19-like Chemokine That Orchestrates both Nasal and Systemic Antiviral Immune Responses in Rainbow Trout

Chemokines and chemokine receptors have rapidly diversified in teleost fish but their immune functions remain unclear. We report in this study that CCL19, a chemokine known to control lymphocyte migration and compartmentalization of lymphoid tissues in mammals, diversified in salmonids leading to the presence of six CCL19-like genes named CK10a, CK10b, CK12a, CK12b, CK13a, and CK13b. Salmonid CCL19-like genes all contain the DCCL-conserved motif but share low amino acid sequence identity. CK12 (but not CK10 or CK13) is constitutively expressed at high levels in all four trout MALT. Nasal vaccination with a live attenuated virus results in sustained upregulation of CK12 (but not CK10 or CK13) expression in trout nasopharynx-associated lymphoid tissue. Recombinant His-tagged trout CK12a (rCK12a) is not chemotactic in vitro but it increases the width of the nasal lamina propria when delivered intranasally. rCK12a delivered intranasally or i.p. stimulates the expression of CD8α, granulysin, and IFN-γ in mucosal and systemic compartments and increases nasal CD8α⁺ cell numbers. rCK12a is able to stimulate proliferation of head kidney leukocytes from Ag-experienced trout but not naive controls, yet it does not confer protection against viral challenge. These results show that local nasal production of CK12a contributes to antiviral immune protection both locally and systemically via stimulation of CD8 cellular immune responses and highlight a conserved role for CK12 in the orchestration of mucosal and systemic immune responses against viral pathogens in vertebrates.

Expression of miR-155 associated with Toll-like receptors 3, 7, and 9 transcription in the olfactory bulbs of cattle naturally infected with BHV5

Bovine herpesvirus 5 (BHV5) infection of young cattle is frequently associated with fatal neurological disease and, as such, represents an attractive model for studying the pathogenesis of viral-induced meningoencephalitis. Following replication in the nasal mucosa, BHV5 invades the central nervous system (CNS) mainly through the olfactory pathway. The innate immune response triggered by the host face to virus replication through the olfactory route is poorly understood. Recently, an upregulation of conserved pathogen-associated molecular pattern, as Toll-like receptors (TLRs), has been demonstrated in the CNS of BHV5 experimentally infected cows. A new perspective to understand host-pathogen interactions has emerged elucidating microRNAs (miRNAs) network that interact with innate immune response during neurotropic viral infections. In this study, we demonstrated a link between the expression of TLRs 3, 7, and 9 and miR-155 transcription in the olfactory bulbs (OB) of 16 cows suffering from acute BHV5-induced neurological disease. The OBs were analyzed for viral antigens and genome, miR-155 and TLR 3, 7, and 9 expression considering three major regions: olfactory receptor neurons (ORNs), glomerular layer (GL), and mitral cell layer (ML). BHV5 antigens and viral genomes, corresponding to glycol-C gene, were detected in all OBs regions by fluorescent antibody assay (FA) and PCR, respectively. TLR 3, 7, and 9 transcripts were upregulated in ORNs and ML, yet only ORN layers revealed a positive correlation between TLR3 and miR-155 transcription. In ML, miR-155 correlated positively with all TLRs studied. Herein, our results evidence miR-155 transcription in BHV5 infected OB tissue associated to TLRs expression specifically ORNs which may be a new window for further studies.

Herpes simplex virus-1 infects the olfactory bulb shortly following ocular infection and exhibits a long-term inflammatory profile in the form of effector and HSV-1-specific T cells

BACKGROUND

Herpes simplex virus 1 (HSV-1) infection can result in a life-threatening condition known as herpes simplex encephalitis (HSE). Trafficking patterns by which the virus reaches the central nervous system (CNS) following ocular infection are unresolved. We evaluated early viral dissemination pathways following ocular infection that involve trafficking to the olfactory bulb (OB). Additionally, we have characterized the capacity of HSV-1 to establish latency within OB tissue and profiled the local T lymphocyte response over the course of the acute infection into latency.

METHODS

Scarified corneas of C57BL/6 or reporter-inducible Rosa mice (RosaTd/Tm) were inoculated with HSV-1 and assessed for viral dissemination into the peripheral nervous system (PNS) and CNS by RT-PCR and confocal microscopy. T cells and the resident microglia activation signatures were analyzed by flow cytometry. T cell effector function in the form of IFN-γ secretion was measured by T cells isolated from OB in comparison to T cells from other nervous system sites known to harbor HSV-1-specific memory T cells.

RESULTS

Following ocular infection, HSV-1 viral titers from nasal secretions were detected as early as 48 h through 8 days post infection (8 DPI). HSV-1 gene expression was expressed as early as 2 days following ocular infection in the OB and was consistent with an enhanced expression in the ophthalmic, maxillary, and mandibular branch of the trigeminal nerve ganglia (TG). Rosa fluorescence protein expression (RFP+) representing HSV-1-infected cells from RosaTd/Tm mice was detected in the OB before other areas of the CNS (2 DPI). Additionally, during acute infection, most infected cells appeared to be anatomically distributed within the OB rather than other regions of the CNS. During latency (i.e., 30 DPI and beyond) despite no detectable infectious virus or lytic gene expression and low levels of latency associated transcripts, total effector (CD44+ CD62-) CD4+ T, CD8+ T, HSV-1-specific CD8+ T cells, and MHC class II positive resident microglia numbers continued to increase. CD4+ and CD8+ T cell populations isolated from the OB during latency were capable of responding to PMA/ionomycin in the production of IFN-γ similar to T cells from other tissue that possess latent virus including the TG and brain stem.

CONCLUSIONS

It is currently understood that HSV-1 traffics to the TG following ocular infection. We have identified a second conduit by which HSV-1 can directly access the CNS bypassing the brain stem. We have also recognized that the OB is unique in that during HSV-1 latency, latency-associated transcripts levels were marginally above uninfected controls. Despite these findings, the local immune response mimicked the phenotype of an active infection during latency.

Structural and Ultrastructural Alterations in Human Olfactory Pathways and Possible Associations with Herpesvirus 6 Infection

Structural and ultrastructural alterations in human olfactory pathways and putative associations with human herpesvirus 6 (HHV-6) infection were studied. The olfactory bulb/tract samples from 20 subjects with an unspecified encephalopathy determined by pathomorphological examination of the brain autopsy, 17 healthy age-matched and 16 younger controls were used. HHV-6 DNA was detected in 60, 29, and 19% of cases in these groups, respectively. In the whole encephalopathy group, significantly more HHV-6 positive neurons and oligodendrocytes were found in the gray matter, whereas, significantly more HHV-6 positive astrocytes, oligodendrocytes, microglia/macrophages and endothelial cells were found in the white matter. Additionally, significantly more HHV-6 positive astrocytes and, in particular, oligodendrocytes were found in the white matter when compared to the gray matter. Furthermore, when only HHV-6 PCR⁺ encephalopathy cases were studied, we observed similar but stronger associations between HHV-6 positive oligodendrocytes and CD68 positive cells in the white matter. Cellular alterations were additionally evidenced by anti-S100 immunostaining, demonstrating a significantly higher number of S100 positive cells in the gray matter of the whole encephalopathy group when compared to the young controls, and in the white matter when compared to both control groups. In spite the decreased S100 expression in the PCR⁺ encephalopathy group when compared to PCR^- cases and controls, groups demonstrated significantly higher number of S100 positive cells in the white compared to the gray matter. Ultrastructural changes confirming the damage of myelin included irregularity of membranes and ballooning of paranodal loops. This study shows that among the cellular targets of the nervous system, HHV-6 most severely affects oligodendrocytes and the myelin made by them.

Distribution of cellular HSV-1 receptor expression in human brain

Herpes simplex virus type 1 (HSV-1) is a neurotropic virus linked to a range of acute and chronic neurological disorders affecting distinct regions of the brain. Unusually, HSV-1 entry into cells requires the interaction of viral proteins glycoprotein D (gD) and glycoprotein B (gB) with distinct cellular receptor proteins. Several different gD and gB receptors have been identified, including TNFRSF14/HVEM and PVRL1/nectin 1 as gD receptors and PILRA, MAG, and MYH9 as gB receptors. We investigated the expression of these receptor molecules in different areas of the adult and developing human brain using online transcriptome databases. Whereas all HSV-1 receptors showed distinct expression patterns in different brain areas, the Allan Brain Atlas (ABA) reported increased expression of both gD and gB receptors in the hippocampus. Specifically, for PVRL1, TNFRFS14, and MYH9, the differential z scores for hippocampal expression, a measure of relative levels of increased expression, rose to 2.9, 2.9, and 2.5, respectively, comparable to the z score for the archetypical hippocampus-enriched mineralocorticoid receptor (NR3C2, z = 3.1). These data were confirmed at the Human Brain Transcriptome (HBT) database, but HBT data indicate that MAG expression is also enriched in hippocampus. The HBT database allowed the developmental pattern of expression to be investigated; we report that all HSV1 receptors markedly increase in expression levels between gestation and the postnatal/adult periods. These results suggest that differential receptor expression levels of several HSV-1 gD and gB receptors in the adult hippocampus are likely to underlie the susceptibility of this brain region to HSV-1 infection.

Intranasal Location and Immunohistochemical Characterization of the Equine Olfactory Epithelium

The olfactory epithelium (OE) is the only body site where neurons contact directly the environment and are therefore exposed to a broad variation of substances and insults. It can serve as portal of entry for neurotropic viruses which spread via the olfactory pathway to the central nervous system. For horses, it has been proposed and concluded mainly from rodent studies that different viruses, e.g., Borna disease virus, equine herpesvirus 1 (EHV-1), hendra virus, influenza virus, rabies virus, vesicular stomatitis virus can use this route. However, little is yet known about cytoarchitecture, protein expression and the intranasal location of the equine OE. Revealing differences in cytoarchitecture or protein expression pattern in comparison to rodents, canines, or humans might help to explain varying susceptibility to certain intranasal virus infections. On the other hand, disclosing similarities especially between rodents and other species, e.g., horses would help to underscore transferability of rodent models. Analysis of the complete noses of five adult horses revealed that in the equine OE two epithelial subtypes with distinct marker expression exist, designated as types a and b which resemble those previously described in dogs. Detailed statistical analysis was carried out to confirm the results obtained on the descriptive level. The equine OE was predominantly located in caudodorsal areas of the nasal turbinates with a significant decline in rostroventral direction, especially for type a. Immunohistochemically, olfactory marker protein and doublecortin (DCX) expression was found in more cells of OE type a, whereas expression of proliferating cell nuclear antigen and tropomyosin receptor kinase A was present in more cells of type b. Accordingly, type a resembles the mature epithelium, in contrast to the more juvenile type b. Protein expression profile was comparable to canine and rodent OE but equine types a and b were located differently within the nose and revealed differences in its cytoarchitecture when compared to canine OE. Equine OE type a closely resembles rat OE. Whether the observed differences contribute to species-specific susceptibility to intranasal insults such as virus infections has to be further investigated.

Herpes Simplex Virus-1 Encephalitis in Adults: Pathophysiology, Diagnosis, and Management

Herpetic infections have plagued humanity for thousands of years, but only recently have advances in antiviral medications and supportive treatments equipped physicians to combat the most severe manifestations of disease. Prompt recognition and treatment can be life-saving in the care of patients with herpes simplex-1 virus encephalitis, the most commonly identified cause of sporadic encephalitis worldwide. Clinicians should be able to recognize the clinical signs and symptoms of the infection and familiarize themselves with a rational diagnostic approach and therapeutic modalities, as early recognition and treatment are key to improving outcomes. Clinicians should also be vigilant for the development of acute complications, including cerebral edema and status epilepticus, as well as chronic complications, including the development of autoimmune encephalitis associated with antibodies to the N-methyl-D-aspartate receptor and other neuronal cell surface and synaptic epitopes. Herein, we review the pathophysiology, differential diagnosis, and clinical and radiological features of herpes simplex virus-1 encephalitis in adults, including a discussion of the most common complications and their treatment. While great progress has been made in the treatment of this life-threatening infection, a majority of patients will not return to their previous neurologic baseline, indicating the need for further research efforts aimed at improving the long-term sequelae.

Neuroinvasion and Inflammation in Viral Central Nervous System Infections

Neurotropic viruses can cause devastating central nervous system (CNS) infections, especially in young children and the elderly. The blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) have been described as relevant sites of entry for specific viruses as well as for leukocytes, which are recruited during the proinflammatory response in the course of CNS infection. In this review, we illustrate examples of established brain barrier models, in which the specific reaction patterns of different viral families can be analyzed. Furthermore, we highlight the pathogen specific array of cytokines and chemokines involved in immunological responses in viral CNS infections. We discuss in detail the link between specific cytokines and chemokines and leukocyte migration profiles. The thorough understanding of the complex and interrelated inflammatory mechanisms as well as identifying universal mediators promoting CNS inflammation is essential for the development of new diagnostic and treatment strategies.

The Olfactory Bulb: An Immunosensory Effector Organ during Neurotropic Viral Infections

In 1935, the olfactory route was hypothesized to be a portal for virus entry into the central nervous system (CNS). This hypothesis was based on experiments in which nasophayngeal infection with poliovirus in monkeys was prevented from spreading to their CNS via transection of olfactory tracts between the olfactory neuroepithelium (ONE) of the nasal cavity and the olfactory bulb (OB). Since then, numerous neurotropic viruses have been observed to enter the CNS via retrograde transport along axons of olfactory sensory neurons whose cell bodies reside in the ONE. Importantly, this route of infection can occur even after subcutaneous inoculation of arboviruses that can cause encephalitis in humans. While the olfactory route is now accepted as an important pathway for viral entry into the CNS, it is unclear whether it provides a way for infection to spread to other brain regions. More recently, studies of antiviral innate and adaptive immune responses within the olfactory bulb suggest it provides early virologic control. Here we will review the data demonstrating that neurotropic viruses gain access to the CNS initially via the olfactory route with emphasis on findings that suggest the OB is a critical immunosensory effector organ that effectively clears virus.

An attenuated virus vaccine appears safe to the central nervous system of rainbow trout (Oncorhynchus mykiss) after intranasal delivery

Nasal vaccines are very effective but the olfactory organ provides direct access of antigens to the brain. Infectious hematopoietic necrosis virus (IHNV) is known to cause high mortalities in salmonids. The purpose of this study is to evaluate the safety of a live attenuated IHNV nasal (I.N) vaccine in rainbow trout (Oncorhynchus mykiss). In the olfactory organ, the vaccine was detected 1 and 4 days after primary I.N vaccination but not in the intramuscular (i.m) or control groups. In the brain, IHNV was detected by RT-qPCR 4 and 21 days after i.m primary vaccination. One i.m and one I.N vaccinated trout were positive at days 4 and 28 days post-boost, respectively. Presence of IHNV in the brain of i.m vaccinated fish correlated with moderate increases in IL-1β and TNF-α expression in this tissue. These results demonstrate that IHNV vaccine lasts for 4 days in the local nasal environment and that nasal vaccination appears to be safe to the CNS of rainbow trout.

Henipaviruses

The first henipaviruses, Hendra virus (HeV), and Nipah virus (NiV) were pathogenic zoonoses that emerged in the mid to late 1990s causing serious disease outbreaks in livestock and humans. HeV was recognized in Australia 1994 in horses exhibiting respiratory disease along with a human case fatality, and then NiV was identified during a large outbreak of human cases of encephalitis with high mortality in Malaysia and Singapore in 1998–1999 along with respiratory disease in pigs which served as amplifying hosts. The recently identified third henipavirus isolate, Cedar virus (CedPV), is not pathogenic in animals susceptible to HeV and NiV disease. Molecular detection of additional henipavirus species has been reported but no additional isolates of virus have been reported. Central pathological features of both HeV and NiV infection in humans and several susceptible animal species is a severe systemic and often fatal neurologic and/or respiratory disease. In people, both viruses can also manifest relapsed encephalitis following recovery from an acute infection, particularly NiV. The recognized natural reservoir hosts of HeV, NiV, and CedPV are pteropid bats, which do not show clinical illness when infected. With spillovers of HeV continuing to occur in Australia and NiV in Bangladesh and India, these henipaviruses continue to be important transboundary biological threats. NiV in particular possesses several features that highlight a pandemic potential, such as its ability to infect humans directly from natural reservoirs or indirectly from other susceptible animals along with a capacity of limited human-to-human transmission. Several henipavirus animal challenge models have been developed which has aided in understanding HeV and NiV pathogenesis as well as how they invade the central nervous system, and successful active and passive immunization strategies against HeV and NiV have been reported which target the viral envelope glycoproteins.

Capsules, toxins and AtxA as virulence factors of emerging Bacillus cereus biovar anthracis

Emerging B. cereus strains that cause anthrax-like disease have been isolated in Cameroon (CA strain) and Côte d’Ivoire (CI strain). These strains are unusual, because their genomic characterisation shows that they belong to the B. cereus species, although they harbour two plasmids, pBCXO1 and pBCXO2, that are highly similar to the pXO1 and pXO2 plasmids of B. anthracis that encode the toxins and the polyglutamate capsule respectively. The virulence factors implicated in the pathogenicity of these B. cereus bv anthracis strains remain to be characterised. We tested their virulence by cutaneous and intranasal delivery in mice and guinea pigs; they were as virulent as wild-type B. anthracis. Unlike as described for pXO2-cured B. anthracis, the CA strain cured of the pBCXO2 plasmid was still highly virulent, showing the existence of other virulence factors. Indeed, these strains concomitantly expressed a hyaluronic acid (HA) capsule and the B. anthracis polyglutamate (PDGA) capsule. The HA capsule was encoded by the hasACB operon on pBCXO1, and its expression was regulated by the global transcription regulator AtxA, which controls anthrax toxins and PDGA capsule in B. anthracis. Thus, the HA and PDGA capsules and toxins were co-regulated by AtxA. We explored the respective effect of the virulence factors on colonisation and dissemination of CA within its host by constructing bioluminescent mutants. Expression of the HA capsule by itself led to local multiplication and, during intranasal infection, to local dissemination to the adjacent brain tissue. Co-expression of either toxins or PDGA capsule with HA capsule enabled systemic dissemination, thus providing a clear evolutionary advantage. Protection against infection by B. cereus bv anthracis required the same vaccination formulation as that used against B. anthracis. Thus, these strains, at the frontier between B. anthracis and B. cereus, provide insight into how the monomorphic B. anthracis may have emerged.

Anthrax is caused by the bacterium Bacillus anthracis that affects all mammals worldwide. It emerged more than 10,000 years ago from a Bacillus cereus precursor. In the past decade, B. cereus bacteria were isolated in the USA from anthrax-like pneumonia cases. They harbour one virulence plasmid very similar to the toxin–encoding plasmid of B. anthracis. Recently, an anthrax-like disease in great apes in Africa was caused by emerging B. cereus strains, named B. cereus biovar anthracis. These strains are atypical as they possess both plasmids coding for toxin and capsule similar to those so far found only in B. anthracis. These unusual pathogenic B. cereus are currently neglected. We explored the virulence of these pathogens and their colonisation and dissemination capacity within the murine host. We found that these toxinogenic strains harbour two capsules, the classical B. anthracis capsule and an additional polysaccharidic capsule. This latter capsule confers virulence alone or in combination with toxins. Both capsules are concomitantly expressed, under the control of a common global regulator and host signals. Our results show that acquisition of new genetic information by these B. cereus clearly gives them a selective advantage, favouring their dissemination within infected hosts and the environment.

The olfactory nerve: a shortcut for influenza and other viral diseases into the central nervous system

The olfactory nerve consists mainly of olfactory receptor neurons and directly connects the nasal cavity with the central nervous system (CNS). Each olfactory receptor neuron projects a dendrite into the nasal cavity on the apical side, and on the basal side extends its axon through the cribriform plate into the olfactory bulb of the brain. Viruses that can use the olfactory nerve as a shortcut into the CNS include influenza A virus, herpesviruses, poliovirus, paramyxoviruses, vesicular stomatitis virus, rabies virus, parainfluenza virus, adenoviruses, Japanese encephalitis virus, West Nile virus, chikungunya virus, La Crosse virus, mouse hepatitis virus, and bunyaviruses. However, mechanisms of transport via the olfactory nerve and subsequent spread through the CNS are poorly understood. Proposed mechanisms are either infection of olfactory receptor neurons themselves or diffusion through channels formed by olfactory ensheathing cells. Subsequent virus spread through the CNS could occur by multiple mechanisms, including trans-synaptic transport and microfusion. Viral infection of the CNS can lead to damage from infection of nerve cells per se, from the immune response, or from a combination of both. Clinical consequences range from nervous dysfunction in the absence of histopathological changes to severe meningoencephalitis and neurodegenerative disease.

Viral diseases of the central nervous system

Virus-induced diseases of the central nervous system (CNS) represent a significant burden to human health worldwide. The complexity of these diseases is influenced by the sheer number of different neurotropic viruses, the diverse routes of CNS entry, viral tropism, and the immune system. Using a combination of human pathological data and experimental animal models, we have begun to uncover many of the mechanisms that viruses use to enter the CNS and cause disease. This review highlights a selection of neurotropic viruses that infect the CNS and explores the means by which they induce neurological diseases such as meningitis, encephalitis, and myelitis.