Neuroinflammation During RNA Viral Infections.

Neurotropic RNA viruses continue to emerge and are increasingly linked to diseases of the central nervous system (CNS) despite viral clearance. Indeed, the overall mortality of viral encephalitis in immunocompetent individuals is low, suggesting efficient mechanisms of virologic control within the CNS. Both immune and neural cells participate in this process, which requires extensive innate immune signaling between resident and infiltrating cells, including microglia and monocytes, that regulate the effector functions of antiviral T and B cells as they gain access to CNS compartments. While these interactions promote viral clearance via mainly neuroprotective mechanisms, they may also promote neuropathology and, in some cases, induce persistent alterations in CNS physiology and function that manifest as neurologic and psychiatric diseases. This review discusses mechanisms of RNA virus clearance and neurotoxicity during viral encephalitis with a focus on the cytokines essential for immune and neural cell inflammatory responses and interactions. Understanding neuroimmune communications in the setting of viral infections is essential for the development of treatments that augment neuroprotective processes while limiting ongoing immunopathological processes that cause ongoing CNS disease.

Vaccination reduces central nervous system IL-1ß and memory deficits after COVID-19 in mice.

Up to 25% of individuals infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) exhibit postacute cognitive sequelae. Although millions of cases of coronavirus disease 2019 (COVID-19)-mediated memory dysfunction are accumulating worldwide, the underlying mechanisms and how vaccination lowers risk are unknown. Interleukin-1 (IL-1), a key component of innate immune defense against SARS-CoV-2 infection, is elevated in the hippocampi of individuals with COVID-19. Here we show that intranasal infection of C57BL/6J mice with SARS-CoV-2 Beta variant leads to central nervous system infiltration of Ly6Chi monocytes and microglial activation. Accordingly, SARS-CoV-2, but not H1N1 influenza virus, increases levels of brain IL-1ß and induces persistent IL-1R1-mediated loss of hippocampal neurogenesis, which promotes postacute cognitive deficits. Vaccination with a low dose of adenoviral-vectored spike protein prevents hippocampal production of IL-1ß during breakthrough SARS-CoV-2 infection, loss of neurogenesis and subsequent memory deficits. Our study identifies IL-1ß as one potential mechanism driving SARS-CoV-2-induced cognitive impairment in a new mouse model that is prevented by vaccination.

Fragile-like regulatory T cells modulate opioid withdrawal in humans and mice.

Opioid use alters peripheral immune functions via unknown mechanisms. In a recent issue of Cell, Zhu et al. report increased fragile-like regulatory T cells in patients with opioid use disorder and in morphine-treated mice. In mice, Treg cell-derived interferon-γ within the brain promotes withdrawal-associated alterations in synapses.

Zika Virus Infection during Pregnancy in Mice Causes Placental Damage and Fetal Demise.

Zika virus (ZIKV) infection in pregnant women causes intrauterine growth restriction, spontaneous abortion, and microcephaly. Here, we describe two mouse models of placental and fetal disease associated with in utero transmission of ZIKV. Female mice lacking type I interferon signaling (Ifnar1(-/-)) crossed to wild-type (WT) males produced heterozygous fetuses resembling the immune status of human fetuses. Maternal inoculation at embryonic day 6.5 (E6.5) or E7.5 resulted in fetal demise that was associated with ZIKV infection of the placenta and fetal brain. We identified ZIKV within trophoblasts of the maternal and fetal placenta, consistent with a trans-placental infection route. Antibody blockade of Ifnar1 signaling in WT pregnant mice enhanced ZIKV trans-placental infection although it did not result in fetal death. These models will facilitate the study of ZIKV pathogenesis, in utero transmission, and testing of therapies and vaccines to prevent congenital malformations.

Astrocytes decrease adult neurogenesis during virus-induced memory dysfunction via IL-1.

Memory impairment following West Nile virus neuroinvasive disease (WNND) is associated with loss of hippocampal synapses with lack of recovery. Adult neurogenesis and synaptogenesis are fundamental features of hippocampal repair, which suggests that viruses affect these processes. Here, in an established model of WNND-induced cognitive dysfunction, transcriptional profiling revealed alterations in the expression of genes encoding molecules that limit adult neurogenesis, including interleukin 1 (IL-1). Mice that had recovered from WNND exhibited fewer neuroblasts and increased astrogenesis without recovery of hippocampal neurogenesis at 30 d. Analysis of cytokine production in microglia and astrocytes isolated ex vivo revealed that the latter were the predominant source of IL-1. Mice deficient in the IL-1 receptor IL-1R1 and that had recovered from WNND exhibited normal neurogenesis, recovery of presynaptic termini and resistance to spatial learning defects, the last of which likewise occurred after treatment with an IL-1R1 antagonist. Thus, 'preferential' generation of proinflammatory astrocytes impaired the homeostasis of neuronal progenitor cells via expression of IL-1; this might underlie the long-term cognitive consequences of WNND but also provides a therapeutic target.

Infectious immunity in the central nervous system and brain function.

Inflammation is emerging as a critical mechanism underlying neurological disorders of various etiologies, yet its role in altering brain function as a consequence of neuroinfectious disease remains unclear. Although acute alterations in mental status due to inflammation are a hallmark of central nervous system (CNS) infections with neurotropic pathogens, post-infectious neurologic dysfunction has traditionally been attributed to irreversible damage caused by the pathogens themselves. More recently, studies indicate that pathogen eradication within the CNS may require immune responses that interfere with neural cell function and communication without affecting their survival. In this Review we explore inflammatory processes underlying neurological impairments caused by CNS infection and discuss their potential links to established mechanisms of psychiatric and neurodegenerative diseases.

Multimodal neuro-nanotechnology: Challenging the existing paradigm in glioblastoma therapy.

Integrating multimodal neuro- and nanotechnology-enabled precision immunotherapies with extant systemic immunotherapies may finally provide a significant breakthrough for combatting glioblastoma (GBM). The potency of this approach lies in its ability to train the immune system to efficiently identify and eradicate cancer cells, thereby creating anti-tumor immune memory while minimizing multi-mechanistic immune suppression. A critical aspect of these therapies is the controlled, spatiotemporal delivery of structurally defined nanotherapeutics into the GBM tumor microenvironment (TME). Architectures such as spherical nucleic acids or poly(beta-amino ester)/dendrimer-based nanoparticles have shown promising results in preclinical models due to their multivalency and abilities to activate antigen-presenting cells and prime antigen-specific T cells. These nanostructures also permit systematic variation to optimize their distribution, TME accumulation, cellular uptake, and overall immunostimulatory effects. Delving deeper into the relationships between nanotherapeutic structures and their performance will accelerate nano-drug development and pave the way for the rapid clinical translation of advanced nanomedicines. In addition, the efficacy of nanotechnology-based immunotherapies may be enhanced when integrated with emerging precision surgical techniques, such as laser interstitial thermal therapy, and when combined with systemic immunotherapies, particularly inhibitors of immune-mediated checkpoints and immunosuppressive adenosine signaling. In this perspective, we highlight the potential of emerging treatment modalities, combining advances in biomedical engineering and neurotechnology development with existing immunotherapies to overcome treatment resistance and transform the management of GBM. We conclude with a call to action for researchers to leverage these technologies and accelerate their translation into the clinic.

Protective and Pathological Immunity during Central Nervous System Infections.

The concept of immune privilege of the central nervous system (CNS) has dominated the study of inflammatory processes in the brain. However, clinically relevant models have highlighted that innate pathways limit pathogen invasion of the CNS and adaptive immunity mediates control of many neural infections. As protective responses can result in bystander damage, there are regulatory mechanisms that balance protective and pathological inflammation, but these mechanisms might also allow microbial persistence. The focus of this review is to consider the host-pathogen interactions that influence neurotropic infections and to highlight advances in our understanding of innate and adaptive mechanisms of resistance as key determinants of the outcome of CNS infection. Advances in these areas have broadened our comprehension of how the immune system functions in the brain and can readily overcome immune privilege.

Histidine-rich protein II nanoparticle delivery of heme iron load drives endothelial inflammation in cerebral malaria.

Histidine-rich protein II (HRPII) is secreted by Plasmodium falciparum during the blood stage of malaria infection. High plasma levels of HRPII are associated with cerebral malaria, a severe and highly fatal complication of malaria. HRPII has been shown to induce vascular leakage, the hallmark of cerebral malaria, in blood-brain barrier (BBB) and animal models. We have discovered an important mechanism for BBB disruption that is driven by unique features of HRPII. By characterizing serum from infected patients and HRPII produced by P. falciparum parasites in culture, we found that HRPII exists in large multimeric particles of 14 polypeptides that are richly laden with up to 700 hemes per particle. Heme loading of HRPII is required for efficient binding and internalization via caveolin-mediated endocytosis in hCMEC/D3 cerebral microvascular endothelial cells. Upon acidification of endolysosomes, two-thirds of the hemes are released from acid-labile binding sites and metabolized by heme oxygenase 1, generating ferric iron and reactive oxygen species. Subsequent activation of the NLRP3 inflammasome and IL-1ß secretion resulted in endothelial leakage. Inhibition of these pathways with heme sequestration, iron chelation, or anti-inflammatory drugs protected the integrity of the BBB culture model from HRPII:heme. Increased cerebral vascular permeability was seen after injection of young mice with heme-loaded HRPII (HRPII:heme) but not with heme-depleted HRPII. We propose that during severe malaria infection, HRPII:heme nanoparticles in the bloodstream deliver an overwhelming iron load to endothelial cells to cause vascular inflammation and edema. Disrupting this process is an opportunity for targeted adjunctive therapies to reduce the morbidity and mortality of cerebral malaria.

Post-exposure intranasal IFNα suppresses replication and neuroinvasion of Venezuelan Equine Encephalitis virus within olfactory sensory neurons.

BACKGROUND: Venezuelan Equine Encephalitis virus (VEEV) may enter the central nervous system (CNS) within olfactory sensory neurons (OSN) that originate in the nasal cavity after intranasal exposure. While it is known that VEEV has evolved several mechanisms to inhibit type I interferon (IFN) signaling within infected cells, whether this inhibits virologic control during neuroinvasion along OSN has not been studied. METHODS: We utilized an established murine model of intranasal infection with VEEV and a repository of scRNAseq data from IFN-treated OSN to assess the cellular targets and IFN signaling responses after VEEV exposure. RESULTS: We found that immature OSN, which express higher levels of the VEEV receptor LDLRAD3 than mature OSN, are the first cells infected by VEEV. Despite rapid VEEV neuroinvasion after intranasal exposure, olfactory neuroepithelium (ONE) and olfactory bulb (OB) IFN responses, as assessed by evaluation of expression of interferon signaling genes (ISG), are delayed for up to 48 h during VEEV neuroinvasion, representing a potential therapeutic window. Indeed, a single intranasal dose of recombinant IFNα triggers early ISG expression in both the nasal cavity and OB. When administered at the time of or early after infection, IFNα treatment delayed onset of sequelae associated with encephalitis and extended survival by several days. VEEV replication after IFN treatment was also transiently suppressed in the ONE, which inhibited subsequent invasion into the CNS. CONCLUSIONS: Our results demonstrate a critical and promising first evaluation of intranasal IFNα for the treatment of human encephalitic alphavirus exposures.

Astrocyte interleukin-3 preps microglia.

Brain ß-amyloid (Aß) deposition is a biomarker for Alzheimer's disease (AD) and other dementias, in which Aß amounts correlate with disease burden. McAlpine et al. reveal that astrocyte expression or administration of interleukin (IL)-3 in the context of aggregated Aß endows microglia with enhanced capability to cluster and clear Aß oligomers.

Targeting IFN-λ Signaling Promotes Recovery from Central Nervous System Autoimmunity.

Type III IFNs (IFNLs) are newly discovered cytokines, acting at epithelial and other barriers, that exert immunomodulatory functions in addition to their primary roles in antiviral defense. In this study, we define a role for IFNLs in maintaining autoreactive T cell effector function and limiting recovery in a murine model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis. Genetic or Ab-based neutralization of the IFNL receptor (IFNLR) resulted in lack of disease maintenance during experimental autoimmune encephalomyelitis, with loss of CNS Th1 effector responses and limited axonal injury. Phenotypic effects of IFNLR signaling were traced to increased APC function, with associated increase in T cell production of IFN-γ and GM-CSF. Consistent with this, IFNL levels within lesions of CNS tissues derived from patients with MS were elevated compared with MS normal-appearing white matter. Furthermore, expression of IFNLR was selectively elevated in MS active lesions compared with inactive lesions or normal-appearing white matter. These findings suggest IFNL signaling as a potential therapeutic target to prevent chronic autoimmune neuroinflammation.

The multitaskers of the brain: Glial responses to viral infections and associated post-infectious neurologic sequelae.

Many viral infections cause acute and chronic neurologic diseases which can lead to degeneration of cortical functions. While neurotropic viruses that gain access to the central nervous system (CNS) may induce brain injury directly via infection of neurons or their supporting cells, they also alter brain function via indirect neuroimmune mechanisms that may disrupt the blood-brain barrier (BBB), eliminate synapses, and generate neurotoxic astrocytes and microglia that prevent recovery of neuronal circuits. Non-neuroinvasive, neurovirulent viruses may also trigger aberrant responses in glial cells, including those that interfere with motor and sensory behaviors, encoding of memories and executive function. Increasing evidence from human and animal studies indicate that neuroprotective antiviral responses that amplify levels of innate immune molecules dysregulate normal neuroimmune processes, even in the absence of neuroinvasion, which may persist after virus is cleared. In this review, we discuss how select emerging and re-emerging RNA viruses induce neuroimmunologic responses that lead to dysfunction of higher order processes including visuospatial recognition, learning and memory, and motor control. Identifying therapeutic targets that return the neuroimmune system to homeostasis is critical for preventing virus-induced neurodegenerative disorders.

Microglia at the scene of the crime: what their transcriptomics reveal about brain health.

PURPOSE OF REVIEW: Microglia, which arise from primitive myeloid precursors that enter the central nervous system (CNS) during early development, are the first responders to any perturbance of homeostasis. Although their activation has become synonymous with neurologic disease, it remains unclear whether microglial responses are the cause of or response to neuropathology. Here, we review new insights in the roles of microglia during CNS health and disease, including preclinical studies that transcriptionally profile microglia to define their functional states. RECENT FINDINGS: Converging evidence suggests that innate immune activation of microglia is associated with overlapping alterations in their gene expression profiles regardless of the trigger. Thus, recent studies examining neuroprotective microglial responses during infections and aging mirror those observed during chronic neurologic diseases, including neurodegeneration and stroke. Many of these insights derive from studies of microglial transcriptomes and function in preclinical models, some of which have been validated in human samples. During immune activation, microglia dismantle their homeostatic functions and transition into subsets capable of antigen presentation, phagocytosis of debris, and management of lipid homeostasis. These subsets can be identified during both normal and aberrant microglial responses, the latter of which may persist long-term. The loss of neuroprotective microglia, which maintain a variety of essential CNS functions, may therefore, in part, underlie the development of neurodegenerative diseases. SUMMARY: Microglia exhibit a high level of plasticity, transforming into numerous subsets as they respond to innate immune triggers. Chronic loss of microglial homeostatic functions may underlie the development of diseases with pathological forgetting.

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

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

Mechanisms of coronavirus infectious disease 2019-related neurologic diseases.

PURPOSE OF REVIEW: As of January 8, 2022, a global pandemic caused by infection with severe acute respiratory syndrome coronavirus (SARS-CoV)-2, a new RNA virus, has resulted in 304,896,785 cases in over 222 countries and regions, with over 5,500,683 deaths (www.worldometers.info/coronavirus/). Reports of neurological and psychiatric symptoms in the context of coronavirus infectious disease 2019 (COVID-19) range from headache, anosmia, and dysgeusia, to depression, fatigue, psychosis, seizures, delirium, suicide, meningitis, encephalitis, inflammatory demyelination, infarction, and acute hemorrhagic necrotizing encephalopathy. Moreover, 30-50% of COVID-19 survivors develop long-lasting neurologic symptoms, including a dysexecutive syndrome, with inattention and disorientation, and/or poor movement coordination. Detection of SARS-CoV-2 RNA within the central nervous system (CNS) of patients is rare, and mechanisms of neurological damage and ongoing neurologic diseases in COVID-19 patients are unknown. However, studies demonstrating viral glycoprotein effects on coagulation and cerebral vasculature, and hypoxia- and cytokine-mediated coagulopathy and CNS immunopathology suggest both virus-specific and neuroimmune responses may be involved. This review explores potential mechanistic insights that could contribute to COVID-19-related neurologic disease. RECENT FINDINGS: While the development of neurologic diseases during acute COVID-19 is rarely associated with evidence of viral neuroinvasion, new evidence suggests SARS-CoV-2 Spike (S) protein exhibits direct inflammatory and pro-coagulation effects. This, in conjunction with immune dysregulation resulting in cytokine release syndrome (CRS) may result in acute cerebrovascular or neuroinflammatory diseases. Additionally, CRS-mediated loss of blood-brain barrier integrity in specific brain regions may contribute to the expression of proinflammatory mediators by neural cells that may impact brain function long after resolution of acute infection. Importantly, host co-morbid diseases that affect vascular, pulmonary, or CNS function may contribute to the type of neurologic disease triggered by SARS-COV-2 infection. SUMMARY: Distinct effects of SARS-CoV-2 S protein and CNS compartment- and region-specific responses to CRS may underlie acute and chronic neuroinflammatory diseases associated with COVID-19.

Levels of Circulating NS1 Impact West Nile Virus Spread to the Brain.

Dengue virus (DENV) and West Nile virus (WNV) are arthropod-transmitted flaviviruses that cause systemic vascular leakage and encephalitis syndromes, respectively, in humans. However, the viral factors contributing to these specific clinical disorders are not completely understood. Flavivirus nonstructural protein 1 (NS1) is required for replication, expressed on the cell surface, and secreted as a soluble glycoprotein, reaching high levels in the blood of infected individuals. Extracellular DENV NS1 and WNV NS1 interact with host proteins and cells, have immune evasion functions, and promote endothelial dysfunction in a tissue-specific manner. To characterize how differences in DENV NS1 and WNV NS1 might function in pathogenesis, we generated WNV NS1 variants with substitutions corresponding to residues found in DENV NS1. We discovered that the substitution NS1-P101K led to reduced WNV infectivity in the brain and attenuated lethality in infected mice, although the virus replicated efficiently in cell culture and peripheral organs and bound at wild-type levels to brain endothelial cells and complement components. The P101K substitution resulted in reduced NS1 antigenemia in mice, and this was associated with reduced WNV spread to the brain. Because exogenous administration of NS1 protein rescued WNV brain infectivity in mice, we conclude that circulating WNV NS1 facilitates viral dissemination into the central nervous system and impacts disease outcomes. IMPORTANCE Flavivirus NS1 serves as an essential scaffolding molecule during virus replication but also is expressed on the cell surface and is secreted as a soluble glycoprotein that circulates in the blood of infected individuals. Although extracellular forms of NS1 are implicated in immune modulation and in promoting endothelial dysfunction at blood-tissue barriers, it has been challenging to study specific effects of NS1 on pathogenesis without disrupting its key role in virus replication. Here, we assessed WNV NS1 variants that do not affect virus replication and evaluated their effects on pathogenesis in mice. Our characterization of WNV NS1-P101K suggests that the levels of NS1 in the circulation facilitate WNV dissemination to the brain and affect disease outcomes. Our findings facilitate understanding of the role of NS1 during flavivirus infection and support antiviral strategies for targeting circulating forms of NS1.