MyD88 signaling by neurons induces chemokines that recruit protective leukocytes to the virus-infected CNS

Alpha-synuclein pathology enhances peripheral and CNS immune responses to bacterial endotoxins

Increasing evidence points to infectious diseases as contributor to the pathogenesis of neurodegeneration in Parkinson's disease (PD), probably driven by a peripheral and CNS inflammatory response together with alpha-synuclein (aSyn) pathology. Pro-inflammatory lipopolysaccharide (LPS) endotoxin is suggested as a risk factor, and LPS shedding gram-negative bacteria are more prevalent in the gut-microbiome of PD patients. Here, we investigated whether LPS could contribute to the neurodegenerative disease progression via neuroinflammation, especially under conditions of aSyn pathology. To investigate this, we created a double-hit model based on the Thy1-aSyn mouse line (line 61), an established aSyn-overexpression model of PD, exposed to a single intraperitoneal injection of LPS at a dose of 0.8 mg/kg (equivalent to approximately 1,200,000 EU/kg). Clinical parameters, flow cytometry of blood and immune cells in the brain, brain immunohistology and motor behavior were evaluated over time. As expected, the LPS dosage induced transient acute symptoms and mild weight loss in mice, with full recovery after 7 days. In aSyn over-expressing mice, this single low dose of LPS was sufficient to alter the expression of specific markers on blood and brain immune cells and induced brain region-specific microgliosis that were present at 7 days post LPS injection. At 14 days post injection of LPS, aSyn expression was reduced in wild-type mice, indicating a specific response of the endogenous protein to the endotoxin. At this early time point, motor behavior is not yet robustly impacted by the observed pathological alterations. In conclusion, aSyn pathology renders the peripheral and central immune response more sensitive to a single low dose of bacterial endotoxin, which mimics a transient dysbiosis or gut infection. Thus, this data suggests that such peripheral triggers should be monitored in PD patients for instance by blood immune cell response as biomarkers. Furthermore, results from this study lend further support to the development of treatments aiming to reduce the impact of bacterial dysbiosis as a promising strategy to mitigate PD progression.

MAVS signaling shapes microglia responses to neurotropic virus infection

Viral encephalitis is characterized by a series of immunological reactions that can control virus infection in the brain, but dysregulated responses may cause excessive inflammation and brain damage. Microglia are brain-resident myeloid cells that are specialized in surveilling the local CNS environment and in case of viral brain infection they contribute to the control of the infection and to restriction of viral dissemination. Here, we report that after exposure to neurotropic vesicular stomatitis virus (VSV), murine in vitro microglia cultures showed rapid upregulation of a broad range of pro-inflammatory and antiviral genes, which were stably expressed over the entire 8 h infection period. Additionally, a set of immunomodulatory genes was upregulated between 6 and 8 h post infection. In microglia cultures, the induction of several immune response pathways including cytokine responses was dependent on mitochondrial antiviral-signaling protein (MAVS). Consequently, in Mavs-deficient microglia the control of virus propagation failed as indicated by augmented virus titers and the accumulation of viral transcripts. Thus, in the analyzed in vitro system, MAVS signaling is critically required to achieve full microglia activation and to mediate profound antiviral effects. In Mavs-deficient mice, intranasal VSV instillation caused higher disease severity than in WT mice and virus dissemination was noticed beyond the olfactory bulb. Virus spread to inner regions of the olfactory bulb, i.e., the granular cell layer, correlated with the recruitment of highly inflammatory non-microglia myeloid cells into the olfactory bulb in Mavs-/- mice. Furthermore, increased cytokine levels were detected in the nasal cavity, the olfactory bulb and in other brain regions. Thus, microglial MAVS signaling is critically needed for virus sensing, full microglia activation, and for orchestration of protective immunity in the virus-infected CNS.

Orchestration of antiviral responses within the infected central nervous system

Many newly emerging and re-emerging viruses have neuroinvasive potential, underscoring viral encephalitis as a global research priority. Upon entry of the virus into the CNS, severe neurological life-threatening conditions may manifest that are associated with high morbidity and mortality. The currently available therapeutic arsenal against viral encephalitis is rather limited, emphasizing the need to better understand the conditions of local antiviral immunity within the infected CNS. In this review, we discuss new insights into the pathophysiology of viral encephalitis, with a focus on myeloid cells and CD8+ T cells, which critically contribute to protection against viral CNS infection. By illuminating the prerequisites of myeloid and T cell activation, discussing new discoveries regarding their transcriptional signatures, and dissecting the mechanisms of their recruitment to sites of viral replication within the CNS, we aim to further delineate the complexity of antiviral responses within the infected CNS. Moreover, we summarize the current knowledge in the field of virus infection and neurodegeneration and discuss the potential links of some neurotropic viruses with certain pathological hallmarks observed in neurodegeneration.

Transcriptomic analysis unveils bona fide molecular signatures of microglia under conditions of homeostasis and viral encephalitis

Microglia serve as a front-line defense against neuroinvasive viral infection, however, determination of their actual transcriptional profiles under conditions of health and disease is challenging. Here, we used various experimental approaches to delineate the transcriptional landscape of microglia during viral infection. Intriguingly, multiple activation genes were found to be artificially induced in sorted microglia and we demonstrated that shear stress encountered during cell sorting was one of the key inducers. Post-hoc analysis revealed that publicly available large-scale single-cell RNA sequencing datasets were significantly tainted by aberrant signatures that are associated with cell sorting. By exploiting the ribosomal tagging approach, we developed a strategy to enrich microglia-specific transcripts by comparing immunoprecipitated RNA with total RNA. Such enriched transcripts were instrumental in defining bona fide signatures of microglia under conditions of health and virus infection. These unified microglial signatures may serve as a benchmark to retrospectively assess ex vivo artefacts from available atlases. Leveraging the microglial translatome, we found enrichment of genes implicated in T-cell activation and cytokine production during the course of VSV infection. These data linked microglia with T-cell re-stimulation and further underscored that microglia are involved in shaping antiviral T-cell responses in the brain. Collectively, our study defines the transcriptional landscape of microglia under steady state and during viral encephalitis and highlights cellular interactions between microglia and T cells that contribute to the control of virus dissemination.

IRF3 inhibits inflammatory signaling pathways in macrophages to prevent viral pathogenesis

Viral inflammation contributes to pathogenesis and mortality during respiratory virus infections. IRF3, a critical component of innate antiviral immune responses, interacts with pro-inflammatory transcription factor NF-κB, and inhibits its activity. This mechanism helps suppress inflammatory gene expression in virus-infected cells and mice. We evaluated the cells responsible for IRF3-mediated suppression of viral inflammation using newly engineered conditional Irf3Δ/Δ mice. Irf3Δ/Δ mice, upon respiratory virus infection, showed increased susceptibility and mortality. Irf3 deficiency caused enhanced inflammatory gene expression, lung inflammation, immunopathology, and damage, accompanied by increased infiltration of pro-inflammatory macrophages. Deletion of Irf3 in macrophages (Irf3MKO) displayed, similar to Irf3Δ/Δ mice, increased inflammatory responses, macrophage infiltration, lung damage, and lethality, indicating that IRF3 in these cells suppressed lung inflammation. RNA-seq analyses revealed enhanced NF-κB-dependent gene expression along with activation of inflammatory signaling pathways in infected Irf3MKO lungs. Targeted analyses revealed activated MAPK signaling in Irf3MKO lungs. Therefore, IRF3 inhibited inflammatory signaling pathways in macrophages to prevent viral inflammation and pathogenesis.

Resiquimod Induces C-C Motif Chemokine Ligand 2 Via Nuclear Factor-Kappa B in SH-SY5Y Human Neuroblastoma Cells

Toll-like receptor (TLR) 7 plays an important role in recognizing virus-derived nucleic acids. TLR7 signaling in astrocytes and microglia is critical for activating immune responses against neurotrophic viruses. Neurons express TLR7, similar to glial cells; however, the role of neuronal TLR7 has not yet been fully elucidated. This study sought to determine whether resiquimod, the TLR7/8 agonist, induces the expression of inflammatory chemokines in SH-SY5Y human neuroblastoma cells. Immunofluorescence microscopy revealed that TLR7 was constitutively expressed in SH-SY5Y cells. Stimulation with resiquimod induced C-C motif chemokine ligand 2 (CCL2) expression, accompanied by the activation of nuclear factor-kappa B (NF-κB) in SH-SY5Y cells. Resiquimod increased mRNA levels of C-X-C motif chemokine ligand 8 (CXCL8) and CXCL10, while the increase was slight at the protein level. Knockdown of NF-κB p65 eliminated resiquimod-induced CCL2 production. This study provides novel evidence that resiquimod has promising therapeutic potential against central nervous system viral infections through its immunostimulatory effects on neurons.

Innate immune response to SARS-CoV-2 infection contributes to neuronal damage in human iPSC-derived peripheral neurons

Severe acute respiratory coronavirus 2 (SARS-CoV-2) causes neurological disease in the peripheral and central nervous system (PNS and CNS, respectively) of some patients. It is not clear whether SARS-CoV-2 infection or the subsequent immune response are the key factors that cause neurological disease. Here, we addressed this question by infecting human induced pluripotent stem cell-derived CNS and PNS neurons with SARS-CoV-2. SARS-CoV-2 infected a low number of CNS neurons and did not elicit a robust innate immune response. On the contrary, SARS-CoV-2 infected a higher number of PNS neurons. This resulted in expression of interferon (IFN) λ1, several IFN-stimulated genes and proinflammatory cytokines. The PNS neurons also displayed alterations characteristic of neuronal damage, as increased levels of sterile alpha and Toll/interleukin receptor motif-containing protein 1, amyloid precursor protein and α-synuclein, and lower levels of cytoskeletal proteins. Interestingly, blockade of the Janus kinase and signal transducer and activator of transcription pathway by Ruxolitinib did not increase SARS-CoV-2 infection, but reduced neuronal damage, suggesting that an exacerbated neuronal innate immune response contributes to pathogenesis in the PNS. Our results provide a basis to study coronavirus disease 2019 (COVID-19) related neuronal pathology and to test future preventive or therapeutic strategies.

Dynamics of reactive astrocytes fosters tissue regeneration after cuprizone-induced demyelination

Demyelination in the central nervous system (CNS) is a hallmark of many neurodegenerative diseases such as multiple sclerosis (MS) and others. Here, we studied astrocytes during de- and remyelination in the cuprizone mouse model. To this end, we exploited the ribosomal tagging (RiboTag) technology that is based on Cre-mediated cell type-selective HA-tagging of ribosomes. Analyses were performed in the corpus callosum of GFAP-Cre+/- Rpl22HA/wt mice 5 weeks after cuprizone feeding, at the peak of demyelination, and 0.5 and 2 weeks after cuprizone withdrawal, when remyelination and tissue repair is initiated. After 5 weeks of cuprizone feeding, reactive astrocytes showed inflammatory signatures with enhanced expression of genes that modulate leukocyte migration (Tlr2, Cd86, Parp14) and they produced the chemokine CXCL10, as verified by histology. Furthermore, demyelination-induced reactive astrocytes expressed numerous ligands including Cx3cl1, Csf1, Il34, and Gas6 that act on homeostatic as well as activated microglia and thus potentially mediate activation and recruitment of microglia and enhancement of their phagocytotic activity. During early remyelination, HA-tagged cells displayed reduced inflammatory response signatures, as indicated by shutdown of CXCL10 production, and enhanced expression of osteopontin (SPP1) as well as of factors that are relevant for tissue remodeling (Timp1), regeneration and axonal repair. During late remyelination, the signatures shifted towards resolving inflammation by active suppression of lymphocyte activation and differentiation and support of glia cell differentiation. In conclusion, we detected highly dynamic astroglial transcriptomic signatures in the cuprizone model, which reflects excessive communication among glia cells and highlights different astrocyte functions during neurodegeneration and regeneration.

ER stress induces caspase-2-tBID-GSDME-dependent cell death in neurons lytically infected with herpes simplex virus type 2

Neurotropic viruses, including herpes simplex virus (HSV) types 1 and 2, have the capacity to infect neurons and can cause severe diseases. This is associated with neuronal cell death, which may contribute to morbidity or even mortality if the infection is not controlled. However, the mechanistic details of HSV-induced neuronal cell death remain enigmatic. Here, we report that lytic HSV-2 infection of human neuron-like SH-SY5Y cells and primary human and murine brain cells leads to cell death mediated by gasdermin E (GSDME). HSV-2-induced GSDME-mediated cell death occurs downstream of replication-induced endoplasmic reticulum stress driven by inositol-requiring kinase 1α (IRE1α), leading to activation of caspase-2, cleavage of the pro-apoptotic protein BH3-interacting domain death agonist (BID), and mitochondria-dependent activation of caspase-3. Finally, necrotic neurons released alarmins, which activated inflammatory responses in human iPSC-derived microglia. In conclusion, lytic HSV infection in neurons activates an ER stress-driven pathway to execute GSDME-mediated cell death and promote inflammation.

Tinospora cordifolia Miers enhances the immune response in mice immunized with JEV-vaccine: A network pharmacology and experimental approach

BACKGROUND

Tinospora cordifolia Miers. (TC) (Giloya/Guduchi) is a native Indian herb, reported for its wide array of medicinal activities including immunomodulatory activity. However, the exact pharmacological mechanism of TC as an immunomodulatory agent remains unclear. Central to this, to the best of our knowledge, no study has explored the immunoadjuvant potential of TC in response to the Japanese encephalitis (JE) vaccines.

PURPOSE

The study aims to explore the immunoadjuvant potential of TC ethanolic extract in response to the JE vaccine and illustrates its potential mechanism of immunomodulation using an integrated approach of network pharmacology and in-vivo experimental study.

STUDY DESIGN AND METHODS

Initially, the extract was prepared and the components of TC were identified through high-resolution liquid chromatography mass spectrometry (HR-LC/MS). The compounds were then screened for network pharmacology analysis. Next, the drug and disease targets were identified and the network was constructed using Cytoscape 3.7.2 to obtain different signalling pathways of TC in JEV. We then evaluated the immunoadjuvant potential of TC ethanolic extract in mice immunized with inactivated JE vaccine (SA-14-14-2 strain). BALB/c mice were supplemented with TC extract (30 and 100 mg/kg, i.g.), daily for 56 days, marked with immunization on 28th day of the study, by JE vaccine. Blood was collected for flow cytometry and haematological analysis (total and differential cell counts). The surface expression of immune-cell markers (CD3+, CD4+, CD19+, CD11c+, CD40+) were evaluated on day 0 (pre-immunization), day 14 and 28 post-immunization. Additionally, inflammatory cytokines (IFN-γ+/IL-17A+) were evaluated post-14 and 28 days of immunization.

RESULTS

The HR-LC/MS analysis identified the presence of glycosides, terpenoids, steroids and alkaloids in the TC extract. Through network analysis, 09 components and 166 targets were obtained, including pathways that involve toll-like receptor signalling, pattern-recognition receptor signalling, cytokine receptor and cytokine mediated signalling, etc. The in-vivo results showed that preconditioning with TC ethanolic extract significantly elevated the haematological variables (leucocyte count) as well as the surface expression of CD markers (B and T cell subsets) on day 0 (pre-immunization), day 14 and 28 post-immunization. Furthermore, preconditioning of TC demonstrated a dose-dependant augmentation of immune cells (CD3+, CD4+, CD19+, CD11c+) and inflammatory cytokines (IFN-γ+/IL-17A+) on day 14 and 28 post-immunization when compared to vaccine alone group.

CONCLUSION

Results showed that preconditioning with TC extract before immunization might play a potential role in enhancing the cell-mediated as well as humoral immunity. Altogether, the combinatorial approach of network pharmacology and in-vivo animal experimentation demonstrated the immunoadjuvant potential of TC in response to JEV vaccine.

Expression and Role of Toll-like Receptors in Facial Nerve Regeneration after Facial Nerve Injury

Facial nerve palsy directly impacts the quality of life, with patients with facial nerve palsy showing increased rates of depression and limitations in social activities. Although facial nerve palsy is not life-threatening, it can devastate the emotional and social lives of affected individuals. Hence, improving the prognosis of patients with this condition is of vital importance. The prognosis of patients with facial nerve palsy is determined by the cause of the disease, the degree of damage, and the treatment provided. The facial nerve can be easily damaged by middle ear and temporal bone surgery, trauma or infection, and tumors of the peripheral facial nerve or tumors surrounding the nerve secondary to systemic disease. In addition, idiopathic, acquired immunodeficiency syndrome and autoimmune diseases may damage the facial nerve. The treatment used for facial paralysis depends on the cause. Treatment of facial nerve amputation injury varies depending on the degree of facial nerve damage, comorbidities, and duration of injury. Recently, interest has increased in Toll-like receptors (TLRs) related to innate immune responses, as these receptors are known to be related to nerve regeneration. In addition to innate immune cells, both neurons and glia of the central nervous system (CNS) and peripheral nervous system (PNS) express TLRs. A comprehensive literature review was conducted to assess the expression and role of TLRs in peripheral nerve injury and subsequent regeneration. Studies conducted on rats and mice have demonstrated the expression of TLR1-13. Among these, TLR2-5 and TLR7 have received the most research attention in relation to facial nerve degeneration and regeneration. TLR10, TLR11, and TLR13 increase during compression injury of the facial nerve, whereas during cutting injury, TLR1-5, TLR8, and TLR10-13 increase, indicating that these TLRs are involved in the degeneration and regeneration of the facial nerve following each type of injury. Inadequate TLR expression or absence of TLR responses can hinder regeneration after facial nerve damage. Animal studies suggest that TLRs play an important role in facial nerve degeneration and regeneration.

Olfactory detection of viruses shapes brain immunity and behavior in zebrafish

Olfactory sensory neurons (OSNs) are constantly exposed to pathogens, including viruses. However, serious brain infection via the olfactory route rarely occurs. When OSNs detect a virus, they coordinate local antiviral immune responses to stop virus progression to the brain. Despite effective immune control in the olfactory periphery, pathogen-triggered neuronal signals reach the CNS via the olfactory bulb (OB). We hypothesized that neuronal detection of a virus by OSNs initiates neuroimmune responses in the OB that prevent pathogen invasion. Using zebrafish ( Danio rerio ) as a model, we demonstrate viral-specific neuronal activation of OSNs projecting into the OB, indicating that OSNs are electrically activated by viruses. Further, behavioral changes are seen in both adult and larval zebrafish after viral exposure. By profiling the transcription of single cells in the OB after OSNs are exposed to virus, we found that both microglia and neurons enter a protective state. Microglia and macrophage populations in the OB respond within minutes of nasal viral delivery followed decreased expression of neuronal differentiation factors and enrichment of genes in the neuropeptide signaling pathway in neuronal clusters. Pituitary adenylate-cyclase-activating polypeptide ( pacap ), a known antimicrobial, was especially enriched in a neuronal cluster. We confirm that PACAP is antiviral in vitro and that PACAP expression increases in the OB 1 day post-viral treatment. Our work reveals how encounters with viruses in the olfactory periphery shape the vertebrate brain by inducing antimicrobial programs in neurons and by altering host behavior.

IFNAR signaling of neuroectodermal cells is essential for the survival of C57BL/6 mice infected with Theiler's murine encephalomyelitis virus

BACKGROUND

Theiler's murine encephalomyelitis virus (TMEV) is a single-stranded RNA virus that causes encephalitis followed by chronic demyelination in SJL mice and spontaneous seizures in C57BL/6 mice. Since earlier studies indicated a critical role of type I interferon (IFN-I) signaling in the control of viral replication in the central nervous system (CNS), mouse strain-specific differences in pathways induced by the IFN-I receptor (IFNAR) might determine the outcome of TMEV infection.

METHODS

Data of RNA-seq analysis and immunohistochemistry were used to compare the gene and protein expression of IFN-I signaling pathway members between mock- and TMEV-infected SJL and C57BL/6 mice at 4, 7 and 14 days post-infection (dpi). To address the impact of IFNAR signaling in selected brain-resident cell types, conditional knockout mice with an IFNAR deficiency in cells of the neuroectodermal lineage (NesCre±IFNARfl/fl), neurons (Syn1Cre±IFNARfl/fl), astrocytes (GFAPCre±IFNARfl/fl), and microglia (Sall1CreER±IFNARfl/fl) on a C57BL/6 background were tested. PCR and an immunoassay were used to quantify TMEV RNA and cytokine and chemokine expression in their brain at 4 dpi.

RESULTS

RNA-seq analysis revealed upregulation of most ISGs in SJL and C57BL/6 mice, but Ifi202b mRNA transcripts were only increased in SJL and Trim12a only in C57BL/6 mice. Immunohistochemistry showed minor differences in ISG expression (ISG15, OAS, PKR) between both mouse strains. While all immunocompetent Cre-negative control mice and the majority of mice with IFNAR deficiency in neurons or microglia survived until 14 dpi, lack of IFNAR expression in all cells (IFNAR-/-), neuroectodermal cells, or astrocytes induced lethal disease in most of the analyzed mice, which was associated with unrestricted viral replication. NesCre±IFNARfl/fl mice showed more Ifnb1, Tnfa, Il6, Il10, Il12b and Ifng mRNA transcripts than Cre-/-IFNARfl/fl mice. IFNAR-/- mice also demonstrated increased IFN-α, IFN-β, IL1-β, IL-6, and CXCL-1 protein levels, which highly correlated with viral load.

CONCLUSIONS

Ifi202b and Trim12a expression levels likely contribute to mouse strain-specific susceptibility to TMEV-induced CNS lesions. Restriction of viral replication is strongly dependent on IFNAR signaling of neuroectodermal cells, which also controls the expression of key pro- and anti-inflammatory cytokines during viral brain infection.

Roles of neuronal toll-like receptors in neuropathic pain and central nervous system injuries and diseases

Toll-like receptors (TLRs) are innate immune receptors that are expressed in immune cells as well as glia and neurons of the central and peripheral nervous systems. They are best known for their role in the host defense in response to pathogens and for the induction of inflammation in infectious and non-infectious diseases. In the central nervous system (CNS), TLRs modulate glial and neuronal functions as well as innate immunity and neuroinflammation under physiological or pathophysiological conditions. The majority of the studies on TLRs in CNS pathologies investigated their overall contribution without focusing on a particular cell type, or they analyzed TLRs in glia and infiltrating immune cells in the context of neuroinflammation and cellular activation. The role of neuronal TLRs in CNS diseases and injuries has received little attention and remains underappreciated. The primary goal of this review is to summarize findings demonstrating the pivotal and unique roles of neuronal TLRs in neuropathic pain, Alzheimer's disease, Parkinson's disease and CNS injuries. We discuss how the current findings warrant future investigations to better define the specific contributions of neuronal TLRs to these pathologies. We underline the paucity of information regarding the role of neuronal TLRs in other neurodegenerative, demyelinating, and psychiatric diseases. We draw attention to the importance of broadening research on neuronal TLRs in view of emerging evidence demonstrating their distinctive functional properties.

Imaging viral infection in vivo to gain unique perspectives on cellular antiviral immunity

The past decade has seen near continual global public health crises caused by emerging viral infections. Extraordinary increases in our knowledge of the mechanisms underlying successful antiviral immune responses in animal models and during human infection have accompanied these viral outbreaks. Keeping pace with the rapidly advancing field of viral immunology, innovations in microscopy have afforded a previously unseen view of viral infection occurring in real-time in living animals. Here, we review the contribution of intravital imaging to our understanding of cell-mediated immune responses to viral infections, with a particular focus on studies that visualize the antiviral effector cells responding to infection as well as virus-infected cells. We discuss methods to visualize viral infection in vivo using intravital microscopy (IVM) and significant findings arising through the application of IVM to viral infection. Collectively, these works underscore the importance of developing a comprehensive spatial understanding of the relationships between immune effectors and virus-infected cells and how this has enabled unique discoveries about virus/host interactions and antiviral effector cell biology.

Beneficial and detrimental functions of microglia during viral encephalitis

Microglia are resident immune cells of the central nervous system (CNS) with multiple functions in health and disease. Their response during encephalitis depends on whether inflammation is triggered in a sterile or infectious manner, and in the latter case on the type of the infecting pathogen. Even though recent technological innovations advanced the understanding of the broad spectrum of microglia responses during viral encephalitis (VE), it is not entirely clear which microglia gene expression profiles are associated with antiviral and detrimental activities. Here, we review novel approaches to study microglia and the latest concepts of their function in VE. Improved understanding of microglial functions will be essential for the development of new therapeutic interventions for VE.

Toll-like Receptors in Viral Encephalitis

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