Contribution to the histology of tick-borne encephalitis

Interferon Beta Contributes to Astrocyte Activation in the Brain following Reovirus Infection

Reovirus encephalitis in mice was used as a model system to investigate astrocyte activation (astrogliosis) following viral infection of the brain. Reovirus infection resulted in astrogliosis, as evidenced by increased expression of glial fibrillary acidic protein (GFAP), and the upregulation of genes that have been previously associated with astrocyte activation. Astrocyte activation occurred in regions of the brain that are targeted by reovirus but extended beyond areas of active infection. Astrogliosis also occurred following reovirus infection of ex vivo brain slice cultures (BSCs), demonstrating that factors intrinsic to the brain are sufficient to activate astrocytes and that this process can occur in the absence of any contribution from the peripheral immune response. In agreement with previous reports, reovirus antigen did not colocalize with GFAP in infected brains, suggesting that reovirus does not infect astrocytes. Reovirus-infected neurons produce interferon beta (IFN-β). IFN-β treatment of primary astrocytes resulted in both the upregulation of GFAP and cytokines that are associated with astrocyte activation. In addition, the ability of media from reovirus-infected BSCs to activate primary astrocytes was blocked by anti-IFN-β antibodies. These results suggest that IFN-β, likely released from reovirus-infected neurons, results in the activation of astrocytes during reovirus encephalitis. In areas where infection and injury were pronounced, an absence of GFAP staining was consistent with activation-induced cell death as a mechanism of inflammation control. In support of this, activated Bak and cleaved caspase 3 were detected in astrocytes within reovirus-infected brains, indicating that activated astrocytes undergo apoptosis.IMPORTANCE Viral encephalitis is a significant cause of worldwide morbidity and mortality, and specific treatments are extremely limited. Virus infection of the brain triggers neuroinflammation; however, the role of neuroinflammation in the pathogenesis of viral encephalitis is unclear. Initial neuroinflammatory responses likely contribute to viral clearance, but prolonged exposure to proinflammatory cytokines released during neuroinflammation may be deleterious and contribute to neuronal death and tissue injury. Activation of astrocytes is a hallmark of neuroinflammation. Here, we show that reovirus infection of the brain results in the activation of astrocytes via an IFN-β-mediated process and that these astrocytes later die by Bak-mediated apoptosis. A better understanding of neuroinflammatory responses during viral encephalitis may facilitate the development of new treatment strategies for these diseases.

Tick-Borne Flaviviruses and the Type I Interferon Response

Flaviviruses are globally distributed pathogens causing millions of human infections every year. Flaviviruses are arthropod-borne viruses and are mainly transmitted by either ticks or mosquitoes. Mosquito-borne flaviviruses and their interactions with the innate immune response have been well-studied and reviewed extensively, thus this review will discuss tick-borne flaviviruses and their interactions with the host innate immune response.

Fast type I interferon response protects astrocytes from flavivirus infection and virus-induced cytopathic effects

Background

Neurotropic flaviviruses such as tick-borne encephalitis virus (TBEV), Japanese encephalitis virus (JEV), West Nile virus (WNV), and Zika virus (ZIKV) are causative agents of severe brain-related diseases including meningitis, encephalitis, and microcephaly. We have previously shown that local type I interferon response within the central nervous system (CNS) is involved in the protection of mice against tick-borne flavivirus infection. However, the cells responsible for mounting this protective response are not defined.

Methods

Primary astrocytes were isolated from wild-type (WT) and interferon alpha receptor knock out (IFNAR^−/−) mice and infected with neurotropic flaviviruses. Viral replication and spread, IFN induction and response, and cellular viability were analyzed. Transcriptional levels in primary astrocytes treated with interferon or supernatant from virus-infected cells were analyzed by RNA sequencing and evaluated by different bioinformatics tools.

Results

Here, we show that astrocytes control viral replication of different TBEV strains, JEV, WNV, and ZIKV. In contrast to fibroblast, astrocytes mount a rapid interferon response and restrict viral spread. Furthermore, basal expression levels of key interferon-stimulated genes are high in astrocytes compared to mouse embryonic fibroblasts. Bioinformatic analysis of RNA-sequencing data reveals that astrocytes have established a basal antiviral state which contributes to the rapid viral recognition and upregulation of interferons. The most highly upregulated pathways in neighboring cells were linked to type I interferon response and innate immunity. The restriction in viral growth was dependent on interferon signaling, since loss of the interferon receptor, or its blockade in wild-type cells, resulted in high viral replication and virus-induced cytopathic effects. Astrocyte supernatant from TBEV-infected cells can restrict TBEV growth in astrocytes already 6 h post infection, the effect on neurons is highly reinforced, and astrocyte supernatant from 3 h post infection is already protective.

Conclusions

These findings suggest that the combination of an intrinsic constitutive antiviral response and the fast induction of type I IFN production by astrocytes play an important role in self-protection of astrocytes and suppression of flavivirus replication in the CNS.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0748-7) contains supplementary material, which is available to authorized users.

Subclinical Tick-Borne Encephalitis Virus in Experimentally Infected Apodemus agrarius

OBJECTIVES

In this study, we investigated the dose dependence of tick-borne encephalitis virus (TBEV) infection in one of the reservoirs, i.e. Apodemus agrarius, a small rodent species.

METHODS

The animals were challenged with TBEV per os and intramuscularly with infectious doses ranging from 1 to 1,500 plaque-forming units (pfu). Clinical signs were recorded and clinical and pathological features were evaluated by histological, immunohistochemical, and serological methods.

RESULTS

High perorally administered infectious doses resulted in virus replication in the brain, which is the first sign of subclinical viral encephalitis in the Apodemus genus. The animals seroconverted at infectious doses greater than 100 pfu, and all animals remained asymptomatic.

CONCLUSION

Our work shows the first evidence that subclinical TBEV encephalitis may occur in Apodemus species, depending on the virus load of the inoculum. The antiviral response of the local innate immune system may influence the resistance of Apodemus individuals to lower infectious doses. Per oral/nasal infection seems to be more dangerous for the host than other routes of infection.

Infection and injury of human astrocytes by tick-borne encephalitis virus

Tick-borne encephalitis (TBE), a disease caused by tick-borne encephalitis virus (TBEV), represents the most important flaviviral neural infection in Europe and north-eastern Asia. In the central nervous system (CNS), neurons are the primary target for TBEV infection; however, infection of non-neuronal CNS cells, such as astrocytes, is not well understood. In this study, we investigated the interaction between TBEV and primary human astrocytes. We report for the first time, to the best of our knowledge, that primary human astrocytes are sensitive to TBEV infection, although the infection did not affect their viability. The infection induced a marked increase in the expression of glial fibrillary acidic protein, a marker of astrocyte activation. In addition, expression of matrix metalloproteinase 9 and several key pro-inflammatory cytokines/chemokines (e.g. tumour necrosis factor α, interferon α, interleukin (IL)-1β, IL-6, IL-8, interferon γ-induced protein 10, macrophage inflammatory protein, but not monocyte chemotactic protein 1) was upregulated. Moreover, we present a detailed description of morphological changes in TBEV-infected cells, as investigated using three-dimensional electron tomography. Several novel ultrastructural changes were observed, including the formation of unique tubule-like structures of 17.9 ±0.15 nm diameter with associated viral particles and/or virus-induced vesicles and located in the rough endoplasmic reticulum of the TBEV-infected cells. This is the first demonstration that TBEV infection activates primary human astrocytes. The infected astrocytes might be a potential source of pro-inflammatory cytokines in the TBEV-infected brain, and might contribute to the TBEV-induced neurotoxicity and blood-brain barrier breakdown that occurs during TBE. The neuropathological significance of our observations is also discussed.

Tick-borne encephalitis: pathogenesis and clinical implications

Tick-borne encephalitis (TBE) is an important and severe neurological illness occurring in large areas of Europe and northern Asia. Only a small proportion of those infected develop clinical symptoms. The symptomatic cases are, however, characterized with fevers and debilitating encephalitis that might progress into chronic disease or fatal infections. This review summarizes data on clinical presentation, pathogenesis and pathology of TBE in humans, and of experimental TBE in animal models with the purpose to explain why is TBE such a severe disease clinically.

Rac1 and Scribble are targets for the arrest of neurite outgrowth by TBE virus NS5

Tick-borne encephalitis virus (TBEV) causes extensive CNS disease in humans known as TBE, however, relatively little is known of the molecular mechanisms for its progress. Here, we now show that TBEV produces defects in neuronal development of PC12 cells through a function of the viral NS5 protein. The methyltransferase domain of NS5 is critical and sufficient for restriction of nerve growth factor induced neurite outgrowth. This effect is reversed by expression of NS5 mutants unable to bind Scribble and unexpectedly, in Scribble depleted cells with binding-competent NS5. Furthermore, we also demonstrate that the Rho GTPase Rac1 and the guanine nucleotide-exchange factor, betaPIX are outcompeted by NS5 for binding to Scribble, linking to effects on neurite outgrowth by TBEV. Together, these findings provide the first experimental evidence that Rac1 and betaPIX are indirect targets of NS5 acting through the multifunctional polarity protein Scribble to oppose neuronal differentiation. In conclusion, our results offer a potential mechanism by which TBEV alters neuronal circuitry and opens new avenues for therapeutic interventions.

Morphological changes in human neural cells following tick-borne encephalitis virus infection

Tick-borne encephalitis (TBE) is one of the leading and most dangerous human viral neuroinfections in Europe and north-eastern Asia. The clinical manifestations include asymptomatic infections, fevers and debilitating encephalitis that might progress into chronic disease or fatal infection. To understand TBE pathology further in host nervous systems, three human neural cell lines, neuroblastoma, medulloblastoma and glioblastoma, were infected with TBE virus (TBEV). The susceptibility and virus-mediated cytopathic effect, including ultrastructural and apoptotic changes of the cells, were examined. All the neural cell lines tested were susceptible to TBEV infection. Interestingly, the neural cells produced about 100- to 10 000-fold higher virus titres than the conventional cell lines of extraneural origin, indicating the highly susceptible nature of neural cells to TBEV infection. The infection of medulloblastoma and glioblastoma cells was associated with a number of major morphological changes, including proliferation of membranes of the rough endoplasmic reticulum and extensive rearrangement of cytoskeletal structures. The TBEV-infected cells exhibited either necrotic or apoptotic morphological features. We observed ultrastructural apoptotic signs (condensation, margination and fragmentation of chromatin) and other alterations, such as vacuolation of the cytoplasm, dilatation of the endoplasmic reticulum cisternae and shrinkage of cells, accompanied by a high density of the cytoplasm. On the other hand, infected neuroblastoma cells did not exhibit proliferation of membranous structures. The virions were present in both the endoplasmic reticulum and the cytoplasm. Cells were dying preferentially by necrotic mechanisms rather than apoptosis. The neuropathological significance of these observations is discussed.

Inflammatory response in human tick-borne encephalitis: analysis of postmortem brain tissue

In Central European tick-borne encephalitis (TBE) mechanisms of tissue destruction are poorly understood. To evaluate the contribution of immunological mechanisms to tissue injury, the authors immunohistochemically analyzed paraffin-embedded autoptic brain tissue of 26 human TBE cases. In the parenchymal compartment, there was a predominance of macrophages/microglia and cytotoxic T cells. In addition, it was found that granzyme B-expressing lymphocytes were in close contact with TBE-expressing neurons up-regulating caspase-3. These findings indicate that cellular and humoral pathways of the immune system, especially granzyme B-releasing cytotoxic T cells and macrophages/microglia, mainly contribute to tissue destruction in TBE.

Visualization of Central European tick-borne encephalitis infection in fatal human cases

Central European tick-borne encephalitis (TBE) is caused by a flavivirus vectored by the Ixodes ricinus tick. In severe infections, TBE presents as (myelo)meningoencephalitis with considerable mortality. Characteristic neuropathologic changes feature a multinodular to patchy polioencephalomyelitis accentuated in spinal cord, brainstem, and cerebellum. Visualization of viral infection by immunohistochemistry has not yet been achieved. We analyzed immunohistochemically the distribution of viral antigens and its correlation with neuropathologic changes, serological data, and disease duration in 28 brains of cases with a clinical diagnosis of TBE and neuropathologically confirmed (meningo)encephalomyelitis. In 20 brains (including 10 seropositives), viral antigens were detectable. These cases were characterized by relatively short clinical duration ranging from 4 to 35 days. Immunoreactivity was most prominent in perikarya and processes of Purkinje cells and large neurons of dentate nucleus, inferior olives, and anterior horns. In addition, immunoreactivity was detected in neurons of other brainstem nuclei, isocortex, and basal ganglia. There was an inverse topographical association of severe inflammatory changes with presence of viral antigens. Some cytotoxic T cells were in direct contact with tick-borne encephalitis virus (TBEV)-infected neurons. We conclude that 1) TBE viral antigens are immunohistochemically detectable in brains of fatal cases with relatively short natural clinical course; 2) TBE virus neurotropism preferentially targets large neurons of anterior horns, medulla oblongata, pons, dentate nucleus, Purkinje cells, and striatum; 3) topographical correlation between inflammatory changes and distribution of viral antigens is poor; and 4) immunologic mechanisms may contribute to nerve cell destruction in human TBE.

Impact of direct virus-induced neuronal dysfunction and immunological damage on the progression of flavivirus (Modoc) encephalitis in a murine model

Flavivirus encephalitis is believed to be the result of two main mechanisms: (i) direct damage to and dysfunction of neurons as a result of viral replication and (ii) destruction of the brain tissue by an inflammatory response. The differential impact of both mechanisms on the progression of flavivirus encephalitis has not been clearly determined. We have now studied the encephalitis caused by Modoc virus (MODV) infection in (i) severe combined immunodeficiency (SCID) mice, (ii) immunocompetent NMRI mice, and (iii) NMRI mice under varying immunosuppressive treatment regimens. In SCID mice, Modoc virus infection proved to be uniformly lethal (100%). The virus replicated extensively in neurons and no signs of inflammation of the brain were observed. In immunocompetent NMRI mice, intranasal (but not intraperitoneal) inoculation with MODV caused severe encephalitis accompanied by a fulminate inflammatory response. When NMRI mice, infected with MODV via the intraperitoneal route, were submitted to a brief immunosuppressive treatment, they also developed encephalitis with an obvious inflammatory component. These animals died significantly earlier than NMRI mice, which received immunosuppressive treatment for a longer period of time. In the latter group, no signs of inflammation of the brain were noted. These models thus allow us to distinguish between the impact of direct viral replication and that of immunological factors in the development of MODV encephalitis, and let us to conclude that (i) replication of the virus in neurons is sufficient to cause fatal encephalitis and (ii) immunological factors contribute significantly to disease progression.

Tick-borne encephalitis: possibly a fatal disease in its acute stage. PCR amplification of TBE RNA from postmortem brain tissue

Tick-borne encephalitis has occurred regularly in Europe since it was first diagnosed in 1931 by Schneider. The mortality rate of patients with this disease is 1-2%. Death usually occurs in the acute stage of illness. A case report of a 28-year-old patient from Slovenia, who died shortly after the onset of tick-borne encephalitis, is described. The clinical course of disease, results of serological tests, neuropathological findings and polymerase chain reaction amplification of parts of viral genome from postmortem brain tissues are presented.

Severe tick borne encephalomyelitis after tick bite and passive immunisation

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Spectral EEG parameters in patients with tick-borne encephalitis: a follow-up study

Follow-up of spectral EEG parameters were reported for five patients with serologically verified tick-borne encephalitis. Comparisons were made for clinical data and findings of EEG analysis (power probability mapping, peak-power frequencies, and alpha/theta power ratios) were compared. A prominent feature of the EEG abnormality was the marked attenuation of background alpha activity. The peak-power frequency and absolute power analysis of three patients showed significant hemispheric asymmetry in the alpha and theta bands respectively. EEG topograms demonstrated persistent theta power fields in one patient. Alpha/theta power ratio was a reliable indicator for the recovery of rhythmic EEG activity during the convalescence. Reorganization of alpha activity appeared to lag behind the clinical improvement, its course was different from patient to patient. Because of the discrepancy of bioelectric and clinical findings the authors recommend long-term EEG monitoring in patients with tick-borne encephalitis.

Morphological demonstration of the virus of tick-borne encephalitis in the human brain

A case is presented in which the fourfold increase of the HI titer during the progression of the disease, and an increase in IgM content found at the beginning of the second week of the disease confirmed the diagnosis of tick-borne encephalitis. The light microscopic changes correspond to the findings accepted as characteristic to tick-borne encephalitis. Viruses, morphologically belonging to the Flavivirus genus were found by electron microscopy in the thalamus, substantia nigra, and cerebellum of the dissected brain. This paper presents the first demonstration of the virus in a case of a human tick-borne encephalitis.