Spread of a neurotropic murine coronavirus into the CNS via the trigeminal and olfactory nerves

Cognitive deficits and cortical volume loss in COVID-19-related hyposmia

BACKGROUND AND PURPOSE

Studies have found that up to 73% of COVID-19 patients experience hyposmia. It is unclear if the loss of smell in COVID-19 is due to damage to the peripheral or central mechanisms. This study aimed to explore the impacts of COVID-19-induced hyposmia on brain structure and cognitive functions.

METHODS

The study included 36 hyposmic (h-COV) and 21 normosmic (n-COV) participants who had recovered from mild COVID-19 infection, as well as 25 healthy controls (HCs). All participants underwent neurological examination, neuropsychiatric assessment and Sniffin' Sticks tests. High-resolution anatomical images were collected; olfactory bulb (OB) volume and cortical thickness were measured.

RESULTS

Addenbrooke's Cognitive Examination-Revised total and language sub-scores were slightly but significantly lower in the h-COV group compared to the HC group (p = 0.04 and p = 0.037). The h-COV group exhibited poorer performance in the Sniffin' Sticks test terms of discrimination score, identification score and the composite score compared to the n-COV and HC groups (p < 0.001, p = 0.001 and p = 0.002 respectively). A decrease in left and right OB volumes was observed in the h-COV group compared to the n-COV and HC groups (p = 0.003 and p = 0.006 respectively). The cortical thickness analysis revealed atrophy in the left lateral orbitofrontal cortex in the h-COV group compared to HCs. A significant low positive correlation of varying degrees was detected between discrimination and identification scores and both OB and left orbital sulci.

CONCLUSION

Temporary or permanent hyposmia after COVID-19 infection leads to atrophy in the OB and olfactory-related cortical structures and subtle cognitive problems in the long term.

Post-COVID-19 persistent olfactory, gustatory, and trigeminal chemosensory disorders: Definitions, mechanisms, and potential treatments

The nose and the oral cavities are the main sites for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry into the body. Smell and taste deficits are the most common acute viral manifestations. Persistent smell disorders are the most common and bothersome complications after SARS-CoV-2 infection, lasting for months to years. The mechanisms and treatment of persistent post-coronavirus disease 2019 (COVID-19) smell and taste disorders are still challenges. Information sources for the review are PubMed, Centers for Disease Control and Prevention, Ovid Medline, Embase, Scopus, Web of Science, International Prospective Register of Systematic Reviews, Cumulative Index to Nursing and Allied Health Literature, Elton Bryson Stephens Company, Cochrane Effective Practice and Organization of Care, Cooperation in Science and Technology, International Clinical Trials Registry Platform, World Health Organization, Randomized Controlled Trial Number Registry, and MediFind. This review summarizes the up-to-date information about the prevalence, patterns at onset, and prognoses of post-COVID-19 smell and taste disorders, evidence for the neurotropism of SARS-CoV-2 and the overlap between SARS-CoV-1, Middle East respiratory syndrome coronavirus, and SARS-CoV-2 in structure, molecular biology, mode of replication, and host pathogenicity, the suggested cellular and molecular mechanisms for these post-COVID19 chemosensory disorders, and the applied pharmacotherapies and interventions as trials to treat these disorders, and the recommendations for future research to improve understanding of predictors and mechanisms of these disorders. These are crucial for hopeful proper treatment strategies.

[COVID-19-associated anosmia]

The article is a systematic review of the literature data summarizes to date on the issue of COVID-19-associated anosmia. We mainly used full-text and abstract electronic databases (PubMed, Scopus and Web of Science). The paper discusses hypothetical mechanisms of development, clinical features, as well as methods of diagnosis and treatment of COVID-19-associated anosmia.

Electrophysiological evidence of subclinical trigeminal dysfunction in patients with COVID-19 and smell impairment: A pilot study

Background

Smell and taste disturbances are among the most frequent neurological symptoms in patients with COVID-19. A concomitant impairment of the trigeminal nerve has been suggested in subjects with olfactory dysfunction, although it has not been confirmed with objective measurement techniques. In this study, we explored the trigeminal function and its correlations with clinical features in COVID-19 patients with impaired smell perception using electrophysiological testing.

Methods

We enrolled 16 consecutive patients with mild COVID-19 and smell impairment and 14 healthy controls (HCs). Olfactory and gustatory symptoms were assessed with self-reported questionnaires. Electrophysiological evaluation of the masseter inhibitory reflex (MIR) and blink reflex (BR) was carried out to test the trigeminal function and its connections within the brainstem.

Results

Masseter inhibitory reflex (MIR) analysis revealed higher latency of ipsilateral and contralateral early silent period in patients when compared with HCs. No significant differences between groups were detected as regards the duration of the early and late silent period. However, several patients showed a prolonged duration of the early silent period. BR evaluation disclosed only an increased amplitude of early components in patients.

Conclusions

Patients with COVID-19 and smell impairment show a subclinical trigeminal nerve impairment. Trigeminal alterations mainly involve the oligosynaptic pathway, as a result of either direct viral damage or secondary neuroinflammation of the peripheral trigeminal fibers, whereas the polysynaptic ponto-medullary circuits seem to be spared. The prolonged duration of the early silent period and the increased amplitude of early BR response might reflect a compensatory upregulation of the trigeminal function as a consequence of the olfactory dysfunction.

Olfactory and gustatory disorders in COVID-19

Loss of olfaction is one of the symptoms most commonly reported by patients with coronavirus disease 2019 (COVID-19). Although the spontaneous recovery rate is high, recent studies have shown that up to 7% of patients remain anosmic for more than 12 months after the onset of infection, leaving millions of people worldwide suffering from severe olfactory impairment. Olfactory training remains the first recommended treatment. With the continued lack of approved drug treatments, new therapeutic options are being explored. This article reviews the current state of science on COVID-19-related olfactory disorders, focusing on epidemiology, pathophysiology, cure rates, currently available treatment options, and research on new treatments.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O).

METHODS

Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus.

RESULTS

The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies.

CONCLUSION

This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Microglia in antiviral immunity of the brain and spinal cord

Viral infections of the central nervous system (CNS) are a significant cause of neurological impairment and mortality worldwide. As tissue resident macrophages, microglia are critical initial responders to CNS viral infection. Microglia seem to coordinate brain-wide antiviral responses of both brain resident cells and infiltrating immune cells. This review discusses how microglia may promote this antiviral response at a molecular level, from potential mechanisms of virus recognition to downstream cytokine responses and interaction with antiviral T cells. Recent advancements in genetic tools to specifically target microglia in vivo promise to further our understanding about the precise mechanistic role of microglia in CNS infection.

Chlamydia pneumoniae can infect the central nervous system via the olfactory and trigeminal nerves and contributes to Alzheimer's disease risk

Chlamydia pneumoniae is a respiratory tract pathogen but can also infect the central nervous system (CNS). Recently, the link between C. pneumoniae CNS infection and late-onset dementia has become increasingly evident. In mice, CNS infection has been shown to occur weeks to months after intranasal inoculation. By isolating live C. pneumoniae from tissues and using immunohistochemistry, we show that C. pneumoniae can infect the olfactory and trigeminal nerves, olfactory bulb and brain within 72 h in mice. C. pneumoniae infection also resulted in dysregulation of key pathways involved in Alzheimer’s disease pathogenesis at 7 and 28 days after inoculation. Interestingly, amyloid beta accumulations were also detected adjacent to the C. pneumoniae inclusions in the olfactory system. Furthermore, injury to the nasal epithelium resulted in increased peripheral nerve and olfactory bulb infection, but did not alter general CNS infection. In vitro, C. pneumoniae was able to infect peripheral nerve and CNS glia. In summary, the nerves extending between the nasal cavity and the brain constitute invasion paths by which C. pneumoniae can rapidly invade the CNS likely by surviving in glia and leading to Aβ deposition.

COVID-19 related olfactory dysfunction

PURPOSE OF REVIEW

This article reviews the literature on COVID-19 related anosmia, focusing on the epidemiology, pathophysiology recovery rates, current available treatment options, and research regarding novel treatments.

RECENT FINDINGS

Loss of sense of smell is one of the most prevalent symptoms reported by patients after COVID-19 infection. Even though there is a high self-reported recovery rate, recent studies have demonstrated that up to 7% of the patients remain anosmic more than 12 months after onset, leaving millions worldwide with severe olfactory dysfunction. Olfactory training remains the first line recommended treatment. Given the paucity of effective medical treatments options researchers are exploring novel therapeutic options.

SUMMARY

Olfactory dysfunction remains a significant and persistent legacy of the COVID-19 pandemic, but heightened awareness may stimulate research that leads to the development of much-needed treatment options.

Störungen des Riech- und Schmeckvermögens bei COVID-19

Der Verlust des Riechvermögens ist eines der Symptome, die von Patienten mit COVID-19 mit am häufigsten angegeben werden. Obwohl die Spontanheilungsrate hoch ist, haben neuere Studien gezeigt, dass bis zu 7 % der Patienten mehr als zwölf Monate nach Beginn der Infektion anosmisch bleiben, sodass weltweit Millionen von Menschen unter schweren Riechstörungen leiden. Riechtraining ist nach wie vor die erste empfohlene Behandlungsform. Angesichts weiterhin fehlender zugelassener medikamentöser Behandlungsmöglichkeiten werden neue therapeutische Optionen erforscht.

Dieser Artikel gibt einen Überblick über den aktuellen Stand der Wissenschaft zu COVID-19-bedingten Riechstörungen, wobei der Schwerpunkt auf der Epidemiologie, der Pathophysiologie, den Heilungsraten, den derzeit verfügbaren Behandlungsmöglichkeiten und der Forschung zu neuen Behandlungsmethoden liegt.

Zitierweise: Klimek L, Hagemann J, Döge J, Freudelsperger L, Cuevas M, Klimek F, Hummel T. Olfactory and gustatory disorders in COVID-19. Allergo J Int 2022;31:243-50

https://doi.org/10.1007/s40629-022-00216-7

The Microvillar and Solitary Chemosensory Cells as the Novel Targets of Infection of SARS-CoV-2 in Syrian Golden Hamsters

Patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019, suffer from respiratory and non-respiratory symptoms. Among these symptoms, the loss of smell has attracted considerable attention. The objectives of this study were to determine which cells are infected, what happens in the olfactory system after viral infection, and how these pathologic changes contribute to olfactory loss. For this purpose, Syrian golden hamsters were used. First, we verified the olfactory structures in the nasal cavity of Syrian golden hamsters, namely the main olfactory epithelium, the vomeronasal organ, and their cellular components. Second, we found angiotensin-converting enzyme 2 expression, a receptor protein of SARS-CoV-2, in both structures and infections of supporting, microvillar, and solitary chemosensory cells. Third, we observed pathological changes in the infected epithelium, including reduced thickness of the mucus layer, detached epithelia, indistinct layers of epithelia, infiltration of inflammatory cells, and apoptotic cells in the overall layers. We concluded that a structurally and functionally altered microenvironment influences olfactory function. We observed the regeneration of the damaged epithelium, and found multilayers of basal cells, indicating that they were activated and proliferating to reconstitute the injured epithelium.

Patients with COVID-19-associated olfactory impairment also show impaired trigeminal function

Objective

Next to olfactory function, the nose can also perceive chemestetic sensations mediated by the trigeminal nerve. While olfactory dysfunction as a symptom of COVID-19 is well described, there has been little research on the limitation of other nasal sensory inputs due to SARS-CoV-2 infection. The aim of this study was to determine possible limitations of nasal chemesthesis after COVID-19 infection by a psychophysiological diagnostic tool.

Methods

In 65 patients with a PCR-confirmed, former COVID-19 disease, olfaction was tested by means of a sniffin' sticks test, tasting by taste sprays and chemesthesis with a menthol dilution series. The subjective self-assessment of the patients was recorded via a questionnaire.

Results

We found a restriction of nasal chemesthesis and the extent correlated with the loss of smell, as well as with the values of the taste score, but not with subjective self-assessment.

Conclusion

Not only the ability to smell and taste, but also nasal chemesthesis is affected by COVID-19.

A Review of Neurological Involvement in Patients with SARS-CoV-2 Infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative pathogen of the recent pandemic of coronavirus disease 19 (COVID-19). As the infection spreads, there is increasing evidence of neurological and psychiatric involvement in COVID-19. Headache, impaired consciousness, and olfactory and gustatory dysfunctions are common neurological manifestations described in the literature. Studies demonstrating more specific and more severe neurological involvement such as cerebrovascular insults, encephalitis and Guillain-Barre syndrome are also emerging. Respiratory failure, a significant condition that leads to mortality in COVID-19, is hypothesized to be partly due to brainstem impairment. Notably, some of these neurological complications seem to persist long after infection. This review aims to provide an update on what is currently known about neurological involvement in patients with COVID-19 due to SARS-CoV-2 infection. In this review, we demonstrate invasion routes of SARS-CoV-2, provide evidence to support the neurotropism hypothesis of the virus, and investigate the pathological mechanisms that underlie neurological complications associated with SARS-CoV-2.

Climate change, environment pollution, COVID-19 pandemic and mental health

Converging data would indicate the existence of possible relationships between climate change, environmental pollution and epidemics/pandemics, such as the current one due to SARS-CoV-2 virus. Each of these phenomena has been supposed to provoke detrimental effects on mental health. Therefore, the purpose of this paper was to review the available scientific literature on these variables in order to suggest and comment on their eventual synergistic effects on mental health. The available literature report that climate change, air pollution and COVID-19 pandemic might influence mental health, with disturbances ranging from mild negative emotional responses to full-blown psychiatric conditions, specifically, anxiety and depression, stress/trauma-related disorders, and substance abuse. The most vulnerable groups include elderly, children, women, people with pre-existing health problems especially mental illnesses, subjects taking some types of medication including psychotropic drugs, individuals with low socio-economic status, and immigrants. It is evident that COVID-19 pandemic uncovers all the fragility and weakness of our ecosystem, and inability to protect ourselves from pollutants. Again, it underlines our faults and neglect towards disasters deriving from climate change or pollution, or the consequences of human activities irrespective of natural habitats and constantly increasing the probability of spillover of viruses from animals to humans. In conclusion, the psychological/psychiatric consequences of COVID-19 pandemic, that currently seem unavoidable, represent a sharp cue of our misconception and indifference towards the links between our behaviour and their influence on the "health" of our planet and of ourselves. It is time to move towards a deeper understanding of these relationships, not only for our survival, but for the maintenance of that balance among man, animals and environment at the basis of life in earth, otherwise there will be no future.

Focal seizures with impaired awareness as long-term neurological complication of COVID-19: a case report

We report here the first case of a young individual otherwise healthy, who presented with frequent focal seizures with impaired awareness as a possible long-term complication of severe acute respiratory syndrome coronavirus-2 infection. Seizures were documented by electroencephalography and responded clinically and neuro-physiologically to antiseizure therapy. The patient underwent an extensive investigation including cerebrospinal fluid examination, conventional and quantitative brain magnetic resonance imaging, and 18-FDG positron emission tomography. Beyond the clinical interest, this case contributes to clarify the possible pathways by which SARS-CoV-2 may enter the central nervous system and cause long-term neurological complications.

Stroke in SARS-CoV-2 Infection: A Pictorial Overview of the Pathoetiology

Since the early days of the pandemic, there have been several reports of cerebrovascular complications during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Numerous studies proposed a role for SARS-CoV-2 in igniting stroke. In this review, we focused on the pathoetiology of stroke among the infected patients. We pictured the results of the SARS-CoV-2 invasion to the central nervous system (CNS) via neuronal and hematogenous routes, in addition to viral infection in peripheral tissues with extensive crosstalk with the CNS. SARS-CoV-2 infection results in pro-inflammatory cytokine and chemokine release and activation of the immune system, COVID-19-associated coagulopathy, endotheliitis and vasculitis, hypoxia, imbalance in the renin-angiotensin system, and cardiovascular complications that all may lead to the incidence of stroke. Critically ill patients, those with pre-existing comorbidities and patients taking certain medications, such as drugs with elevated risk for arrhythmia or thrombophilia, are more susceptible to a stroke after SARS-CoV-2 infection. By providing a pictorial narrative review, we illustrated these associations in detail to broaden the scope of our understanding of stroke in SARS-CoV-2-infected patients. We also discussed the role of antiplatelets and anticoagulants for stroke prevention and the need for a personalized approach among patients with SARS-CoV-2 infection.

Viral mouse models used to study multiple sclerosis: past and present

Multiple sclerosis (MS) is a common inflammatory demyelinating disease of the central nervous system. Although the etiology of MS is unknown, genetics and environmental factors, such as infections, play a role. Viral infections of mice have been used as model systems to study this demyelinating disease of humans. Three viruses that have long been studied in this capacity are Theiler’s murine encephalomyelitis virus, mouse hepatitis virus, and Semliki Forest virus. This review describes the viruses themselves, the infection process, the disease caused by infection and its accompanying pathology, and the model systems and their usefulness in studying MS.

Murine-β-coronavirus-induced neuropathogenesis sheds light on CNS pathobiology of SARS-CoV2

The pandemic caused by SARS-CoV-2 has caused widespread infection and significant mortality across the globe. Combined virology perspective of SARS-CoV-2 with a deep-rooted understanding of pathophysiological and immunological processes underlying the clinical manifestations of COVID-19 is of prime importance. The characteristic symptom of COVID-19 is respiratory distress with diffused alveolar damage, but emerging evidence suggests COVID-19 might also have neurologic consequences. Dysregulated homeostasis in the lungs has proven to be fatal, but one cannot ignore that the inability to breathe might be due to defects in the respiratory control center of the brainstem. While the mechanism of pulmonary distress has been documented in the literature, awareness of neurological features and their pathophysiology is still in the nascent state. This review makes references to the neuro-immune axis and neuro-invasive potential of SARS-CoV and SARS-CoV2, as well as the prototypic H-CoV strains in human brains. Simultaneously, considerable discussion on relevant experimental evidence of mild to severe neurological manifestations of fellow neurotropic murine-β-CoVs (m-CoVs) in the mouse model will help understand the underpinning mechanisms of Neuro-COVID. In this review, we have highlighted the neuroimmunopathological processes in murine CoVs. While MHV infection in mice and SARS-CoV-2 infection in humans share numerous parallels, there are critical differences in viral recognition and viral entry. These similarities are highlighted in this review, while differences have also been emphasized. Though CoV-2 Spike does not favorably interact with murine ACE2 receptor, modification of murine SARS-CoV2 binding domain or development of transgenic ACE-2 knock-in mice might help in mediating consequential infection and understanding human CoV2 pathogenesis in murine models. While a global animal model that can replicate all aspects of the human disease remains elusive, prior insights and further experiments with fellow m-β-CoV-induced cause-effect experimental models and current human COVID-19 patients data may help to mitigate the SARS-CoV-2-induced multifactorial multi-organ failure.

More that ACE2? NRP1 may play a central role in the underlying pathophysiological mechanism of olfactory dysfunction in COVID-19 and its association with enhanced survival

Three mechanisms have been proposed to account for COVID-19 associated olfactory dysfunction; obstruction of the olfactory cleft; epithelial injury and infection of the sustentacular supporting cells, which are known to express ACE2, or injury to the olfactory bulb due to axonal transport through olfactory sensory neurones. The absence of ACE2 expression by olfactory sensory neurones has led to the neurotropic potential of COVID-19 to be discounted. While an accumulating body of evidence supports olfactory epithelial injury as an important mechanism, this does not account for all the features of olfactory dysfunction seen in COVID-19; for example the duration of loss in some patients, evidence of changes within the olfactory bulb on MRI imaging, identification of viral particles within the olfactory bulb in post-mortem specimens and the inverse association between severity of COVID-19 and the prevalence of olfactory loss. The recent identification of a second route of viral entry mediated by NRP1 addresses many of these inconsistencies. Expression by the olfactory sensory neurones and their progenitor cells may facilitate direct injury and axonal transport to the olfactory bulb as well as a mechanism for delayed or absent recovery. Expression by regulatory T cells may play a central role in the cytokine storm. Variability in expression by age, race or gender may explain differing morbidity of infection and inverse association between anosmia and severity; in the case of higher expression there may be a higher risk of olfactory function but greater activation of regulatory T cells that may suppress the cytokine storm.

Neuropsychiatric manifestations of COVID-19 and possible pathogenic mechanisms: Insights from other coronaviruses

Coronavirus disease 2019 (COVID-19) pandemic caused by SARS-CoV-2 has emerged as a global public health threat. Though the fear, anxiety, and stress related to COVID-19 have been studied in depth, the direct effects of SARS-CoV-2 on the central nervous system (CNS) remain elusive. Research related to the earlier coronavirus (CoV) outbreaks (like Severe Acute Respiratory Syndrome, SARS and Middle East Respiratory Syndrome, MERS) shows the neurotropic nature of CoV and the plethora of neuropsychiatric effects that it can cause. Though the current health priorities in managing COVID-19 remain restricted to containment and targeting pulmonary symptoms, the potential acute and long-term neuropsychiatric sequelae of the infection can increase morbidity and worsen the quality of life. Emerging evidence shows neural spread of the novel coronavirus. Delirium, encephalopathy, olfactory disturbances, acute behavioral changes, headache and cerebrovascular accidents are its common neuropsychiatric complications. These are directly related to increase in peripheral immunological markers, severity of infection and case fatality rate. This narrative review synthesizes available evidence related to the neuropsychiatric manifestations of COVID-19. Also, as SARS-CoV-2 shares structural and functional similarities with its earlier congeners, this article proposes possible long-term neuropsychological sequelae and pathogenic mechanisms for the same, based on research in the other coronavirus outbreaks.

Global prevalence and pathogenesis of headache in COVID-19: A systematic review and meta-analysis

Background: This study was conducted to determine the prevalence of headache in coronavirus disease 2019 (COVID-19) and to assess its association as a predictor for COVID-19. This study also aimed to discuss the possible pathogenesis of headache in COVID-19. Methods: Available articles from PubMed, Scopus, and Web of Science were searched as of September 2 nd, 2020. Data on characteristics of the study, headache and COVID-19 were extracted following the PRISMA guidelines. Biases were assessed using the Newcastle-Ottawa scale. The cumulative prevalence of headache was calculated for the general population (i.e. adults and children). The pooled odd ratio (OR) with 95% confidence intervals (95%CI) was calculated using the Z test to assess the association between headache and the presence of COVID-19 cases. Results: We included 104,751 COVID-19 cases from 78 eligible studies to calculate the global prevalence of headache in COVID-19 and 17 studies were included to calculate the association of headache and COVID-19. The cumulative prevalence of headache in COVID-19 was 25.2% (26,464 out of 104,751 cases). Headache was found to be more prevalent, approximately by two-fold, in COVID-19 patients than in non-COVID-19 patients with symptoms of other respiratory viral infections, OR: 1.73; 95% CI: 1.94, 2.5 with p=0.04. Conclusion: Headache is common among COVID-19 patients and seems to be more common in COVID-19 patients compared to those with the non-COVID-19 viral infection. No definitive mechanisms on how headache  emerges in COVID-19 patients but several possible hypotheses have been proposed. However, extensive studies are warranted to elucidate the mechanisms. PROSPERO registration: CRD42020210332 (28/09/2020).

Global prevalence and pathogenesis of headache in COVID-19: A systematic review and meta-analysis

Background: This study was conducted to determine the prevalence of headache in coronavirus disease 2019 (COVID-19) and to assess its association as a predictor for COVID-19. This study also aimed to discuss the possible pathogenesis of headache in COVID-19. Methods: Available articles from PubMed, Scopus, and Web of Science were searched as of September 2 nd, 2020. Data on characteristics of the study, headache and COVID-19 were extracted following the PRISMA guidelines. Biases were assessed using the Newcastle-Ottawa scale. The cumulative prevalence of headache was calculated for the general population (i.e. adults and children). The pooled odd ratio (OR) with 95% confidence intervals (95%CI) was calculated using the Z test to assess the association between headache and the presence of COVID-19 cases. Results: We included 104,751 COVID-19 cases from 78 eligible studies to calculate the global prevalence of headache in COVID-19 and 17 studies were included to calculate the association of headache and COVID-19. The cumulative prevalence of headache in COVID-19 was 25.2% (26,464 out of 104,751 cases). Headache was found to be more prevalent, approximately by two-fold, in COVID-19 patients than in non-COVID-19 patients (other respiratory viral infections), OR: 1.73; 95% CI: 1.94, 2.5 with p=0.04. Conclusion: Headache is common among COVID-19 patients and seems to be more common in COVID-19 patients compared to those with the non-COVID-19 viral infection. No definitive mechanisms on how headache  emerges in COVID-19 patients but several possible hypotheses have been proposed. However, extensive studies are warranted to elucidate the mechanisms. PROSPERO registration: CRD42020210332 (28/09/2020).

Role of Systemic and Nasal Glucocorticoid Treatment in the Regulation of the Inflammatory Response in Patients with SARS-Cov-2 Infection

The Chinese outbreak of SARS-CoV-2 during 2019 has become pandemic and the most important concerns are the acute respiratory distress syndrome (ARDS) and hyperinflammation developed by the population at risk (elderly and/or having obesity, diabetes, and hypertension) in whom clinical evolution quickly progresses to multi-organ dysfunction and fatal outcome. Immune dysregulation is linked to uncontrolled proinflammatory response characterized by the release of cytokines (cytokines storm). A proper control of this response is mandatory to improve clinical prognosis. In this context, glucocorticoids are able to change the expression of several genes involved in the inflammatory response leading to an improvement in acute respiratory distress. Although there are contradictory data in the literature, in this report we highlight the potential benefits of glucocorticoids as adjuvant therapy for hyperinflammation control; emphasizing that adequate dosage, timing, and delivery are crucial to reduce the dysregulated peripheral-and neuro-inflammatory response with minimal adverse effects. We propose the use of the intranasal route for glucocorticoid administration, which has been shown to effectively control the neuro-and peripheral-inflammatory response using low doses without generating unwanted side effects.

Coronavirus Infection of the Central Nervous System: Animal Models in the Time of COVID-19

Naturally occurring coronaviral infections have been studied for several decades in the context of companion and production animals, and central nervous system involvement is a common finding, particularly in cats with feline infectious peritonitis (FIP). These companion and production animal coronaviruses have many similarities to recent human pandemic-associated coronaviruses such as SARS-CoV, MERS-CoV, and SARS-CoV2 (COVID-19). Neurological involvement is being increasingly recognized as an important clinical presentation in human COVID-19 patients, often associated with para-infectious processes, and potentially with direct infection within the CNS. Recent breakthroughs in the treatment of coronaviral infections in cats, including neurological FIP, have utilized antiviral drugs similar to those currently in human COVID-19 clinical trials. Differences in specific coronavirus and host factors are reflected in major variations in incidence and mechanisms of CNS coronaviral infection and pathology between species; however, broad lessons relating to treatment of coronavirus infection present within the CNS may be informative across species.

Viral infection and smell loss: The case of COVID-19

Olfactory disorders have been increasingly reported in individuals infected with SARS‐CoV‐2, the virus causing the coronavirus disease 2019 (COVID‐19). Losing the sense of smell has a strong impact on the quality of life, since it may lead to malnutrition, weight loss, food poisoning, depression, and exposure to dangerous chemicals. Individuals who suffer from anosmia (inability to smell) also cannot sense the flavor of food, which is a combination of taste and smell. Interestingly, infected individuals have reported sudden loss of smell with no congested nose, as is frequently observed in common colds or other upper respiratory tract infections. These observations suggest that SARS‐CoV‐2 infection leads to olfactory loss through a distinct mechanism, which is still unclear. This article provides an overview of olfactory loss and the recent findings relating to COVID‐19. Possible mechanisms of SARS‐CoV‐2‐induced olfactory loss are also discussed.

Unraveling the Possible Routes of SARS-COV-2 Invasion into the Central Nervous System

PURPOSE OF REVIEW

To describe the possible neuroinvasion pathways of Severe Acute Respiratory Syndrome-related Coronavirus-2 (SARS-CoV-2), the virus responsible for the Coronavirus disease-19 (Covid-19) pandemic.

RECENT FINDINGS

We present data regarding the family of Coronaviruses (CoVs) and the central nervous system (CNS), and describe parallels between SARS-CoV-2 and other members of the family, which have been investigated in more depth and combine these findings with the recent advancements regarding SARS-CoV-2.

SUMMARY

SARS-CoV-2 like other CoVs is neuroinvasive, neurotropic and neurovirulent. Two main pathways of CNS penetration seem to be the strongest candidates, the hematogenous and the neuronal. Τhe olfactory route in particular appears to play a significant role in neuroinvasion of coronaviruses and SARS-CoV-2, as well. However, existing data suggest that other routes, involving the nasal epithelium in general, lymphatic tissue and the CSF may also play roles in SARS-CoV-2 invasion into the CNS.

Neurobiology of coronaviruses: Potential relevance for COVID-19

In the first two decades of the 21st century, there have been three outbreaks of severe respiratory infections caused by highly pathogenic coronaviruses (CoVs) around the world: the severe acute respiratory syndrome (SARS) by the SARS-CoV in 2002-2003, the Middle East respiratory syndrome (MERS) by the MERS-CoV in June 2012, and Coronavirus Disease 2019 (COVID-19) by the SARS-CoV-2 presently affecting most countries In all of these, fatalities are a consequence of a multiorgan dysregulation caused by pulmonary, renal, cardiac, and circulatory damage; however, COVID patients may show significant neurological signs and symptoms such as headache, nausea, vomiting, and sensory disturbances, the most prominent being anosmia and ageusia. The neuroinvasive potential of CoVs might be responsible for at least part of these symptoms and may contribute to the respiratory failure observed in affected patients. Therefore, in the present manuscript, we have reviewed the available preclinical evidence on the mechanisms and consequences of CoVs-induced CNS damage, and highlighted the potential role of CoVs in determining or aggravating acute and long-term neurological diseases in infected individuals. We consider that a widespread awareness of the significant neurotropism of CoVs might contribute to an earlier recognition of the signs and symptoms of viral-induced CNS damage. Moreover, a better understanding of the cellular and molecular mechanisms by which CoVs affect CNS function and cause CNS damage could help in planning new strategies for prognostic evaluation and targeted therapeutic intervention.

Severe acute respiratory syndrome coronavirus 2 may be an underappreciated pathogen of the central nervous system

Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) causes a highly contagious respiratory disease referred to as COVID‐19. However, emerging evidence indicates that a small but growing number of COVID‐19 patients also manifest neurological symptoms, suggesting that SARS‐CoV‐2 may infect the nervous system under some circumstances. SARS‐CoV‐2 primarily enters the body through the epithelial lining of the respiratory and gastrointestinal tracts, but under certain conditions this pleiotropic virus may also infect peripheral nerves and gain entry into the central nervous system (CNS). The brain is shielded by various anatomical and physiological barriers, most notably the blood–brain barrier (BBB) which functions to prevent harmful substances, including pathogens and pro‐inflammatory mediators, from entering the brain. The BBB is composed of highly specialized endothelial cells, pericytes, mast cells and astrocytes that form the neurovascular unit, which regulates BBB permeability and maintains the integrity of the CNS. In this review, potential routes of viral entry and the possible mechanisms utilized by SARS‐CoV‐2 to penetrate the CNS, either by disrupting the BBB or infecting the peripheral nerves and using the neuronal network to initiate neuroinflammation, are briefly discussed. Furthermore, the long‐term effects of SARS‐CoV‐2 infection on the brain and in the progression of neurodegenerative diseases known to be associated with other human coronaviruses are considered. Although the mechanisms of SARS‐CoV‐2 entry into the CNS and neurovirulence are currently unknown, the potential pathways described here might pave the way for future research in this area and enable the development of better therapeutic strategies.

COVID-19 and the Chemical Senses: Supporting Players Take Center Stage

The main neurological manifestation of COVID-19 is loss of smell or taste. The high incidence of smell loss without significant rhinorrhea or nasal congestion suggests that SARS-CoV-2 targets the chemical senses through mechanisms distinct from those used by endemic coronaviruses or other common cold-causing agents. Here we review recently developed hypotheses about how SARS-CoV-2 might alter the cells and circuits involved in chemosensory processing and thereby change perception. Given our limited understanding of SARS-CoV-2 pathogenesis, we propose future experiments to elucidate disease mechanisms and highlight the relevance of this ongoing work to understanding how the virus might alter brain function more broadly.

Expression of ACE2 in Human Neurons Supports the Neuro-Invasive Potential of COVID-19 Virus

The recent outbreak of 2019 coronavirus disease (COVID-19), caused by a novel coronavirus, has now spread quickly worldwide. Like the severe acute respiratory syndrome coronavirus (SARS-CoV), this novel type of coronavirus, SARS-CoV-2, has been demonstrated to utilize angiotensin-converting enzyme 2 (ACE2) as an entry point to the cells. There is a growing body of reports indicating that COVID-19 patients, especially those in severe condition, exhibit neurological symptoms, thus supporting the possibility that SARS-CoV-2 could infect and damage neurons within the central nervous system in humans. Using human pluripotent stem cells-derived neurons, here we show the expression of ACE2 in human neurons via immunocytochemistry. From this perspective, we elaborate on the idea that the neuro-invasive potential of SARS-CoV-2 should be considered as a possible contributory factor, as well as a therapeutic target, for the severe respiratory symptoms in critical COVID-19 cases.

Acute necrotizing encephalopathy with SARS-CoV-2 RNA confirmed in cerebrospinal fluid

Here, we report a case of COVID-19-related acute necrotizing encephalopathy where SARS-CoV-2 RNA was found in CSF 19 days after symptom onset after testing negative twice. Although monocytes and protein levels in CSF were only marginally increased, and our patient never experienced a hyperinflammatory state, her neurologic function deteriorated into coma. MRI of the brain showed pathologic signal symmetrically in central thalami, subinsular regions, medial temporal lobes, and brain stem. Extremely high concentrations of the neuronal injury markers neurofilament light and tau, as well as an astrocytic activation marker, glial fibrillary acidic protein, were measured in CSF. Neuronal rescue proteins and other pathways were elevated in the in-depth proteomics analysis. The patient received IV immunoglobulins and plasma exchange. Her neurologic status improved, and she was extubated 4 weeks after symptom onset. This case report highlights the neurotropism of SARS-CoV-2 in selected patients and emphasizes the importance of repeated lumbar punctures and CSF analyses in patients with suspected COVID-19 and neurologic symptoms.

Anosmia in COVID-19: Mechanisms and Significance

The global pandemic of coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 remains a challenge for prevention due to asymptomatic or paucisymptomatic patients. Anecdotal and preliminary evidence from multiple institutions shows that these patients present with a sudden onset of anosmia without rhinitis. We aim to review the pathophysiology of anosmia related to viral upper respiratory infections and the prognostic implications. Current evidence suggests that SARS-CoV-2-related anosmia may be a new viral syndrome specific to COVID-19 and can be mediated by intranasal inoculation of SARS-CoV-2 into the olfactory neural circuitry. The clinical course of neuroinvasion of SARS-CoV-2 is yet unclear, however an extended follow up of these patients to assess for neurological sequelae including encephalitis, cerebrovascular accidents and long-term neurodegenerative risk may be indicated.

Olfactory Dysfunction in Coronavirus Disease 2019 Patients: Observational Cohort Study and Systematic Review

BACKGROUND

Olfactory dysfunction (OD) has been reported in coronavirus disease 2019 (COVID-19). However, there are knowledge gaps about the severity, prevalence, etiology, and duration of OD in COVID-19 patients.

METHODS

Olfactory function was assessed in all participants using questionnaires and the butanol threshold test (BTT). Patients with COVID-19 and abnormal olfaction were further evaluated using the smell identification test (SIT), sinus imaging, and nasoendoscopy. Selected patients received nasal biopsies. Systematic review was performed according to PRISMA guidelines. PubMed items from January 1, 2020 to April 23, 2020 were searched. Studies that reported clinical data on olfactory disturbances in COVID-19 patients were analyzed.

RESULTS

We included 18 COVID-19 patients and 18 controls. Among COVID-19 patients, 12 of 18 (67%) reported olfactory symptoms and OD was confirmed in 6 patients by BTT and SIT. Olfactory dysfunction was the only symptom in 2 patients. Mean BTT score of patients was worse than controls (P = .004, difference in means = 1.8; 95% confidence interval, 0.6-2.9). Sinusitis and olfactory cleft obstruction were absent in most patients. Immunohistochemical analysis of nasal biopsy revealed the presence of infiltrative CD68+ macrophages harboring severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigen in the stroma. Olfactory dysfunction persisted in 2 patients despite clinical recovery. Systematic review showed that the prevalence of olfactory disturbances in COVID-19 ranged from 5% to 98%. Most studies did not assess olfaction quantitatively.

CONCLUSIONS

Olfactory dysfunction is common in COVID-19 and may be the only symptom. Coronavirus disease 2019-related OD can be severe and prolonged. Mucosal infiltration by CD68+ macrophages expressing SARS-CoV-2 viral antigen may contribute to COVID-19-related OD.

Parkinsonism as a Third Wave of the COVID-19 Pandemic?

Since the initial reports of COVID-19 in December 2019, the world has been gripped by the disastrous acute respiratory disease caused by the SARS-CoV-2 virus. There are an ever-increasing number of reports of neurological symptoms in patients, from severe (encephalitis), to mild (hyposmia), suggesting the potential for neurotropism of SARS-CoV-2. This Perspective investigates the hypothesis that the reliance on self-reporting of hyposmia has resulted in an underestimation of neurological symptoms in COVID-19 patients. While the acute effect of the virus on the nervous system function is vastly overshadowed by the respiratory effects, we propose that it will be important to monitor convalescent individuals for potential long-term implications that may include neurodegenerative sequelae such as viral-associated parkinsonism. As it is possible to identify premorbid harbingers of Parkinson's disease, we propose long-term screening of SARS-CoV-2 cases post-recovery for these expressions of neurodegenerative disease. An accurate understanding of the incidence of neurological complications in COVID-19 requires long-term monitoring for sequelae after remission and a strategized health policy to ensure healthcare systems all over the world are prepared for a third wave of the virus in the form of parkinsonism.

Axonal Transport Enables Neuron-to-Neuron Propagation of Human Coronavirus OC43

Human coronaviruses (HCoVs) are recognized respiratory pathogens for which accumulating evidence indicates that in vulnerable patients the infection can cause more severe pathologies. HCoVs are not always confined to the upper respiratory tract and can invade the central nervous system (CNS) under still unclear circumstances. HCoV-induced neuropathologies in humans are difficult to diagnose early enough to allow therapeutic interventions. Making use of our already described animal model of HCoV neuropathogenesis, we describe the route of neuropropagation from the nasal cavity to the olfactory bulb and piriform cortex and then the brain stem. We identified neuron-to-neuron propagation as one underlying mode of virus spreading in cell culture. Our data demonstrate that both passive diffusion of released viral particles and axonal transport are valid propagation strategies used by the virus. We describe for the first time the presence along axons of viral platforms whose static dynamism is reminiscent of viral assembly sites. We further reveal that HCoV OC43 modes of propagation can be modulated by selected HCoV OC43 proteins and axonal transport. Our work, therefore, identifies processes that may govern the severity and nature of HCoV OC43 neuropathogenesis and will make possible the development of therapeutic strategies to prevent occurrences.IMPORTANCE Coronaviruses may invade the CNS, disseminate, and participate in the induction of neurological diseases. Their neuropathogenicity is being increasingly recognized in humans, and the presence and persistence of human coronaviruses (HCoV) in human brains have been proposed to cause long-term sequelae. Using our mouse model relying on natural susceptibility to HCoV OC43 and neuronal cell cultures, we have defined the most relevant path taken by HCoV OC43 to access and spread to and within the CNS toward the brain stem and spinal cord and studied in cell culture the underlying modes of intercellular propagation to better understand its neuropathogenesis. Our data suggest that axonal transport governs HCoV OC43 egress in the CNS, leading to the exacerbation of neuropathogenesis. Exploiting knowledge on neuroinvasion and dissemination will enhance our ability to control viral infection within the CNS, as it will shed light on underlying mechanisms of neuropathogenesis and uncover potential druggable molecular virus-host interfaces.

Highly activated cytotoxic CD8 T cells express protective IL-10 at the peak of coronavirus-induced encephalitis

Acute viral encephalitis requires rapid pathogen elimination without significant bystander tissue damage. In this article, we show that IL-10, a potent anti-inflammatory cytokine, is produced transiently at the peak of infection by CD8 T cells in the brains of coronavirus-infected mice. IL-10(+)CD8 and IL-10(-)CD8 T cells interconvert during acute disease, possibly based on recent Ag exposure. Strikingly, IL-10(+)CD8 T cells were more highly activated and cytolytic than IL-10(-)CD8 T cells, expressing greater levels of proinflammatory cytokines and chemokines, as well as cytotoxic proteins. Even though these cells are highly proinflammatory, IL-10 expressed by these cells was functional. Furthermore, IL-10 produced by CD8 T cells diminished disease severity in mice with coronavirus-induced acute encephalitis, suggesting a self-regulatory mechanism that minimizes immunopathological changes.

Murine coronavirus neuropathogenesis: determinants of virulence

Murine coronavirus, mouse hepatitis virus (MHV), causes various diseases depending on the strain and route of inoculation. Both the JHM and A59 strains, when inoculated intracranially or intranasally, are neurovirulent. Comparison of the highly virulent JHM isolate, JHM.SD, with less virulent JHM isolates and with A59 has been used to determine the mechanisms and genes responsible for high neuropathogenicity of MHV. The focus of this review is on the contributions of viral spread, replication, and innate and adaptive immunity to MHV neuropathogenesis. JHM.SD spreads more quickly among neurons than less neurovirulent MHVs, and is able to spread in the absence of the canonical MHV receptor, CEACAM1a. The observation that JHM.SD infects more cells and expresses more antigen, but produces less infectious virus per cell than A59, implies that efficient replication is not always a correlate of high neurovirulence. This is likely due to the unstable nature of the JHM.SD spike protein (S). JHM.SD induces a generally protective innate immune response; however, the strong neutrophil response may be more pathogenic than protective. In addition, JHM.SD induces only a minimal T-cell response, whereas the strong T-cell response and the concomitant interferon-γ (IFN-γ) induced by the less neurovirulent A59 is protective. Differences in the S and nucleocapsid (N) proteins between A59 and JHM.SD contribute to JHM.SD neuropathogenicity. The hemmagglutinin-esterase (HE) protein may enhance neuropathogenicity of some MHV isolates, but is unlikely a major contributor to the high neuroviruence of JHM.SD. Further data suggest that neither the internal (I) protein nor nonstructural proteins ns4, and ns2 are significant contributors to neurovirulence.

Murine coronavirus neuropathogenesis: determinants of virulence

Murine coronavirus, mouse hepatitis virus (MHV), causes various diseases depending on the strain and route of inoculation. Both the JHM and A59 strains, when inoculated intracranially or intranasally, are neurovirulent. Comparison of the highly virulent JHM isolate, JHM.SD, with less virulent JHM isolates and with A59 has been used to determine the mechanisms and genes responsible for high neuropathogenicity of MHV. The focus of this review is on the contributions of viral spread, replication, and innate and adaptive immunity to MHV neuropathogenesis. JHM.SD spreads more quickly among neurons than less neurovirulent MHVs, and is able to spread in the absence of the canonical MHV receptor, CEACAM1a. The observation that JHM.SD infects more cells and expresses more antigen, but produces less infectious virus per cell than A59, implies that efficient replication is not always a correlate of high neurovirulence. This is likely due to the unstable nature of the JHM.SD spike protein (S). JHM.SD induces a generally protective innate immune response; however, the strong neutrophil response may be more pathogenic than protective. In addition, JHM.SD induces only a minimal T-cell response, whereas the strong T-cell response and the concomitant interferon-γ (IFN-γ) induced by the less neurovirulent A59 is protective. Differences in the S and nucleocapsid (N) proteins between A59 and JHM.SD contribute to JHM.SD neuropathogenicity. The hemmagglutinin-esterase (HE) protein may enhance neuropathogenicity of some MHV isolates, but is unlikely a major contributor to the high neuroviruence of JHM.SD. Further data suggest that neither the internal (I) protein nor nonstructural proteins ns4, and ns2 are significant contributors to neurovirulence.

La Crosse virus infectivity, pathogenesis, and immunogenicity in mice and monkeys

BACKGROUND

La Crosse virus (LACV), family Bunyaviridae, was first identified as a human pathogen in 1960 after its isolation from a 4 year-old girl with fatal encephalitis in La Crosse, Wisconsin. LACV is a major cause of pediatric encephalitis in North America and infects up to 300,000 persons each year of which 70-130 result in severe disease of the central nervous system (CNS). As an initial step in the establishment of useful animal models to support vaccine development, we examined LACV infectivity, pathogenesis, and immunogenicity in both weanling mice and rhesus monkeys.

RESULTS

Following intraperitoneal inoculation of mice, LACV replicated in various organs before reaching the CNS where it replicates to high titer causing death from neurological disease. The peripheral site where LACV replicates to highest titer is the nasal turbinates, and, presumably, LACV can enter the CNS via the olfactory neurons from nasal olfactory epithelium. The mouse infectious dose50 and lethal dose50 was similar for LACV administered either intranasally or intraperitoneally. LACV was highly infectious for rhesus monkeys and infected 100% of the animals at 10 PFU. However, the infection was asymptomatic, and the monkeys developed a strong neutralizing antibody response.

CONCLUSION

In mice, LACV likely gains access to the CNS via the blood stream or via olfactory neurons. The ability to efficiently infect mice intranasally raises the possibility that LACV might use this route to infect its natural hosts. Rhesus monkeys are susceptible to LACV infection and develop strong neutralizing antibody responses after inoculation with as little as 10 PFU. Mice and rhesus monkeys are useful animal models for LACV vaccine immunologic testing although the rhesus monkey model is not optimal.

Murine encephalitis caused by HCoV-OC43, a human coronavirus with broad species specificity, is partly immune-mediated

The human coronavirus HCoV-OC43 causes a significant fraction of upper respiratory tract infections. Most coronaviruses show a strong species specificity, although the SARS-Coronavirus crossed species from palm civet cats to infect humans. Similarly, HCoV-OC43, likely a member of the same coronavirus group as SARS-CoV, readily crossed the species barrier as evidenced by its rapid adaptation to the murine brain [McIntosh, K., Becker, W.B., Chanock, R.M., 1967. Growth in suckling-mouse brain of "IBV-like" viruses from patients with upper respiratory tract disease. Proc Natl Acad Sci U.S.A. 58, 2268-73]. Herein, we investigated two consequences of this plasticity in species tropism. First, we showed that HCoV-OC43 was able to infect cells from a large number of mammalian species. Second, we showed that virus that was passed exclusively in suckling mouse brains was highly virulent and caused a uniformly fatal encephalitis in adult mice. The surface glycoprotein is a major virulence factor in most coronavirus infections. We identified three changes in the HCoV-OC43 surface glycoprotein that correlated with enhanced neurovirulence in mice; these were located in the domain of the protein responsible for binding to host cells. These data suggest that some coronaviruses, including HCoV-OC43 and SARS-CoV, readily adapt to growth in cells from heterologous species. This adaptability has facilitated the isolation of HCoV-OC43 viral variants with markedly differing abilities to infect animals and tissue culture cells.

Transcriptome profile within the mouse central nervous system and activation of myelin-reactive T cells following murine coronavirus infection

Multiple sclerosis (MS) is an autoimmune disease associated with environmental factors, possibly including several viruses such as the coronaviruses. Indeed, murine coronavirus (MHV) infection provides a well-known experimental model for MS studies. Intracerebral infection of C57BL/6 mice with MHV-A59 revealed that viral replication was efficient and that clearance of infectious virus occurred as soon as 7 days post-infection. Using cDNA arrays, analysis of gene expression profile in the brain revealed a modulation of 80 different genes following infection, with at least 27 of these genes having previously been directly related to innate or acquired immune responses. Concordingly, an important activation of auto-reactive T cells specific to myelin basic protein was demonstrated. Altogether, these results indicate that an MHV infection of the central nervous system (CNS) leads to an important host genomic response implicating immunity-related genes and to the activation of myelin-reactive autoimmune T cells.

Increased epitope-specific CD8+ T cells prevent murine coronavirus spread to the spinal cord and subsequent demyelination

CD8+ T cells are important for clearance of neurotropic mouse hepatitis virus (MHV) strain A59, although their possible role in A59-induced demyelination is not well understood. We developed an adoptive-transfer model to more clearly elucidate the role of virus-specific CD8+ T cells during the acute and chronic phases of infection with A59 that is described as follows. C57BL/6 mice were infected with a recombinant A59 virus expressing the gp33 epitope, an H-2Db-restricted CD8+ T-cell epitope encoded in the glycoprotein of lymphocytic choriomeningitis virus, as a fusion with the enhanced green fluorescent protein (RA59-gfp/gp33). P14 splenocytes (transgenic for a T-cell receptor specific for the gp33 epitope) were transferred at different times pre- and postinfection (p.i.). Adoptive transfer of P14 splenocytes 1 day prior to infection with RA59-gfp/gp33, but not control virus lacking the gp33 epitope, RA59-gfp, reduced weight loss and viral replication and spread in the brain and to the spinal cord. Furthermore, demyelination was significantly reduced compared to that in nonrecipients. However, when P14 cells were transferred on day 3 or 5 p.i., no difference in acute or chronic disease was observed compared to that in nonrecipients. Protection in mice receiving P14 splenocytes prior to infection correlated with a robust gp33-specific immune response that was not observed in mice receiving the later transfers. Thus, an early robust CD8+ T-cell response was necessary to reduce virus replication and spread, specifically to the spinal cord, which protected against demyelination in the chronic phase of the disease.

Virus demyelination

A number of viruses can initiate central nervous system (CNS) diseases that include demyelination as a major feature of neuropathology. In humans, the most prominent demyelinating diseases are progressive multifocal leukoencephalopathy, caused by JC papovirus destruction of oligodendrocytes, and subacute sclerosing panencephalitis, an invariably fatal childhood disease caused by persistent measles virus. The most common neurological disease of young adults in the developed world, multiple sclerosis, is also characterized by lesions of inflammatory demyelination; however, the etiology of this disease remains an enigma. A viral etiology is possible, because most demyelinating diseases of known etiology in both man and animals are viral. Understanding of the pathogenesis of virus-induced demyelination derives for the most part from the study of animal models. Studies with neurotropic strains of mouse hepatitis virus, Theiler's virus, and Semliki Forest virus have been at the forefront of this research. These models demonstrate how viruses enter the brain, spread, persist, and interact with immune responses. Common features are an ability to infect and persist in glial cells, generation of predominantly CD8(+) responses, which control and clear the early phase of virus replication but which fail to eradicate the infection, and lesions of inflammatory demyelination. In most cases demyelination is to a limited extent the result of direct virus destruction of oligodendrocytes, but for the most part is the consequence of immune and inflammatory responses. These models illustrate the roles of age and genetic susceptibility and establish the concept that persistent CNS infection can lead to the generation of CNS autoimmune responses.

Murine hepatitis virus--a model for virus-induced CNS demyelination

Most murine hepatitis virus (MHV) strains, as their name suggests, infect the liver. However, several murine strains are tropic for the central nervous system (CNS) and cause encephalitis with subsequent CNS demyelination. The CNS demyelination shares pathological similarities with human CNS demyelinating diseases such as multiple sclerosis (MS). These viruses are, therefore, used to study the role of the immune system in viral clearance from the CNS, in CNS demyelination, and in remyelination. Nevertheless, it is still unclear exactly how MHV induces demyelination and to what extent the immune system plays a role in this pathology. Here we review this field in the context of the immune response to MHV in the liver and the CNS focusing on studies that have been published in the past 5 years.

Functional consequences following infection of the olfactory system by intranasal infusion of the olfactory bulb line variant (OBLV) of mouse hepatitis strain JHM

The present study assessed the functional consequences of viral infection with a neurotropic coronavirus, designated MHV OBLV, that specifically targets central olfactory structures. Using standard operant techniques and a 'go, no-go' successive discrimination paradigm, six BALB/c mice were trained to discriminate between the presentation of an air or odor stimulus (three mice for each of the odorants propanol and propyl acetate). Two additional BALB/c mice were trained to discriminate between the presentation of air and the presentation of either vanillin or propionic acid. Following criterion performance, each mouse received an additional 2000 trials of overtraining. At completion of overtraining one mouse from the propanol and propyl acetate groups were allocated as untreated. The remaining six mice were inoculated with 300 microl of the OBLV stock per nostril for a total of 1.5 x 10(6) p.f.u. in 600 microl. Following a 1 month rest, untreated and inoculated animals were again tested on their respective air versus odor discrimination task. Untreated animals immediately performed at criterion levels. In contrast, inoculated animals varied in their capacity to discriminate between air and odorant. Five of the six inoculated mice showed massive disruption of the olfactory bulb, including death of mitral cells; the other was more modestly affected. In addition, the density of innervation of the olfactory mucosa by substance P-containing trigeminal fibers is also affected by inoculation. Those mice that remained anosmic to the training odorants had the most severe reduction in mitral cell number and substance P fiber density among the inoculated animals.

Acute and persistent infection of human neural cell lines by human coronavirus OC43

Human coronaviruses (HuCV) are recognized respiratory pathogens. Data accumulated by different laboratories suggest their neurotropic potential. For example, primary cultures of human astrocytes and microglia were shown to be susceptible to an infection by the OC43 strain of HuCV (A. Bonavia, N. Arbour, V. W. Yong, and P. J. Talbot, J. Virol. 71:800-806, 1997). We speculate that the neurotropism of HuCV will lead to persistence within the central nervous system, as was observed for murine coronaviruses. As a first step in the verification of our hypothesis, we have characterized the susceptibility of various human neural cell lines to infection by HuCV-OC43. Viral antigen, infectious virus progeny, and viral RNA were monitored during both acute and persistent infections. The astrocytoma cell lines U-87 MG, U-373 MG, and GL-15, as well as neuroblastoma SK-N-SH, neuroglioma H4, oligodendrocytic MO3.13, and the CHME-5 immortalized fetal microglial cell lines, were all susceptible to an acute infection by HuCV-OC43. Viral antigen and RNA and release of infectious virions were observed during persistent HuCV-OC43 infections ( approximately 130 days of culture) of U-87 MG, U-373 MG, MO3.13, and H4 cell lines. Nucleotide sequences of RNA encoding the putatively hypervariable viral S1 gene fragment obtained after 130 days of culture were compared to that of initial virus input. Point mutations leading to amino acid changes were observed in all persistently infected cell lines. Moreover, an in-frame deletion was also observed in persistently infected H4 cells. Some point mutations were observed in some molecular clones but not all, suggesting evolution of the viral population and the emergence of viral quasispecies during persistent infection of H4, U-87 MG, and MO3.13 cell lines. These results are consistent with the potential persistence of HuCV-OC43 in cells of the human nervous system, accompanied by the production of infectious virions and molecular variation of viral genomic RNA.

Transneuronal spread of Semliki Forest virus in the developing mouse olfactory system is determined by neuronal maturity

Many neurotropic virus infections have been shown to be virulent in neonatal and suckling mice but avirulent in weaned mice. The neurotropic alphavirus Semliki Forest virus is a well-studied example of this and importantly the age-related change in neurovirulence of this virus has been shown to be independent of specific immune responses. During the first two postnatal weeks many major physiological changes including axonogenesis, synaptogenesis and myelination occur within the rodent CNS. To investigate whether these changes affect virus replication, spread and virulence we have studied the course of infection in the mouse olfactory system. The olfactory system is well-characterized with regard to its development and neuroanatomy and represents an important route of entry of many neurotropic viruses. Following Semliki Forest virus infection, mice younger than 14 days-of-age died from a fulminant panencephalitis, whilst those 15 days and older survived and cleared the infection. Microscopic examination of brains from mice inoculated intranasally either bilaterally or unilaterally and stained by in situ hybridization to detect viral RNA revealed spread of infection along neurites in a circuit-specific manner. Spread in the main olfactory bulb and to primary, secondary and tertiary olfactory connections was observed. In neonatal mice virus rapidly spread throughout the olfactory system and the temporal progress of the infection correlated with the known connectivity patterns of this system. Both anterograde and retrograde axonal spread were observed. During the first three postnatal weeks the rate and extent of virus spread decreased with increasing age. Spread of infection between specific structures was closely related to neuronal maturation. As olfactory system connections matured transmission of virus was curtailed. In mice inoculated at six weeks or six months-of-age infection was minimal in and rarely observed beyond the continually renewed olfactory nerve layer. The ability of this virus to replicate and, or spread in the CNS is clearly linked to neuronal maturation.

Cytotoxic T-cell-resistant variants arise at early times after infection in C57BL/6 but not in SCID mice infected with a neurotropic coronavirus

Under certain conditions, C57BL/6 mice persistently infected with mouse hepatitis virus strain JHM (MHV-JHM) develop clinical disease and histological evidence of demyelination several weeks after inoculation with virus. In a previous report, we showed that mutations in the RNA encoding an immunodominant CD8 T-cell epitope within the surface glycoprotein (epitope S-510-518) were present in all persistently infected animals and that these mutations abrogated recognition by virus-specific cytotoxic T cells (CTLs) in direct ex vivo cytotoxicity assays. To obtain further evidence that these mutations were necessary for the development of clinical disease, the temporal course of their appearance was determined. Mutations in the epitope were identified by 10 to 12 days after inoculation, and in some mice, virus containing mutated epitope was the dominant species detected by 15 days. In addition, most mice that remain asymptomatic at 80 days after inoculation, a time after which clinical disease almost never develops, were infected with only wild-type virus. Finally, analysis of virus isolated from mice with severe combined immunodeficiency (SCID) revealed the presence only of wild-type epitope S-510-518. These results, by showing that mutations are not selected in SCID mice and occur at early times after inoculation in C57BL/6 mice, support the view that they result from immune pressure and contribute to virus persistence and demyelination in mice infected persistently with MHV-JHM.

Mouse hepatitis virus A59-induced demyelination can occur in the absence of CD8+ T cells

Mouse hepatitis virus causes a chronic demyelinating disease in C57BL/6 mice. While early studies suggested demyelination is due to direct cytolytic effects of virus on oligodendrocytes, there is increasing evidence for the involvement of the immune system in the mechanism of demyelination. In this study we have asked whether demyelination can occur in the absence of functional MHC class I expression and CD8+ T cells. We infected transgenic mice lacking expression of beta 2 microglobulin (beta 2 M -/- mice) with MHV-A59. In beta 2M-/- mice, virus was much more lethal than in either of the parental strains used to produce the mice; furthermore, while clearance from the CNS did occur in beta 2M-/- mice, it was slower than in C57BL/6 mice. This is consistent with the importance of CD8+ cells in viral clearance. Because of the increased sensitivity of the beta 2M-/- mice to infection, only low levels of virus could be used to evaluate chronic disease. Even at these low levels, demyelination did occur in some animals. To compare infection in beta 2M-/- and C57BL/6 mice we used a higher dose of an attenuated variant of MHV-A59, C12. The attenuated variant induced less demyelination in C57BL/6 mice compared to wild type A59, but the levels observed were not significantly different from those seen in beta 2M-/- mice. Thus, MHV-induced demyelination can occur in some animals in the absence of MHC class I and CD8+ cells.