Role of apoptosis in rabies viral encephalitis: a comparative study in mice, canine, and human brain with a review of literature

Deoxynivalenol Mycotoxin Inhibits Rabies Virus Replication In Vitro

Rabies is a highly fatal disease, and it is vital to find effective ways to manage and control infection. There is a need for new effective antiviral drugs that are particularly effective treatments for rabies. Deoxynivalenol (DON) is known mainly for its toxicity, but at the molecular level, it can inhibit RNA and DNA replication, and there is increasing evidence that different doses of DON have a positive effect on inhibiting virus replication. Based on this, we evaluated the effect of DON on inhibiting the rabies virus in vitro. The inhibitory effect of DON on rabies virus activity was dose- and time-dependent, and 0.25 μg/mL of DON could inhibit 99% of rabies virus activity within 24 h. Furthermore, DON could inhibit the adsorption, entry, replication, and release of rabies virus but could not inactivate the virus. The inhibitory effect of DON on rabies virus may be achieved by promoting apoptosis. Our study provides a new perspective for the study of anti-rabies virus and expands the direction of action of mycotoxins.

The Amino Acid at Position 95 in the Matrix Protein of Rabies Virus Is Involved in Antiviral Stress Granule Formation in Infected Cells

Stress granules (SGs) are dynamic structures that store cytosolic messenger ribonucleoproteins. SGs have recently been shown to serve as a platform for activating antiviral innate immunity; however, several pathogenic viruses suppress SG formation to evade innate immunity. In this study, we investigated the relationship between rabies virus (RABV) virulence and SG formation, using viral strains with different levels of virulence. We found that the virulent Nishigahara strain did not induce SG formation, but its avirulent offshoot, the Ni-CE strain, strongly induced SG formation. Furthermore, we demonstrated that the amino acid at position 95 in the RABV matrix protein (M95), a pathogenic determinant for the Nishigahara strain, plays a key role in inhibiting SG formation, followed by protein kinase R (PKR)-dependent phosphorylation of the α subunit of eukaryotic initiation factor 2α (eIF2α). M95 was also implicated in the accumulation of RIG-I, a viral RNA sensor protein, in SGs and in the subsequent acceleration of interferon induction. Taken together, our findings strongly suggest that M95-related inhibition of SG formation contributes to the pathogenesis of RABV by allowing the virus to evade the innate immune responses of the host. IMPORTANCE Rabies virus (RABV) is a neglected zoonotic pathogen that causes lethal infections in almost all mammalian hosts, including humans. Recently, RABV has been reported to induce intracellular formation of stress granules (SGs), also known as platforms that activate innate immune responses. However, the relationship between SG formation capacity and pathogenicity of RABV has remained unclear. In this study, by comparing two RABV strains with completely different levels of virulence, we found that the amino acid mutation from valine to alanine at position 95 of matrix protein (M95), which is known to be one of the amino acid mutations that determine the difference in virulence between the strains, plays a major role in SG formation. Importantly, M95 was involved in the accumulation of RIG-I in SGs and in promoting interferon induction. These findings are the first report of the effect of a single amino acid substitution associated with SGs on viral virulence.

A Novel Rabies Vaccine Based on a Recombinant Bovine Herpes Virus Type 1 Expressing Rabies Virus Glycoprotein

Rabies is a highly prevalent zoonotic disease and a public health threat worldwide. Currently licensed rabies vaccines are effective but less is known which would protect cattle. This study describes the construction of a novel recombinant bovine herpes virus type I (BHV-1) expressing rabies virus glycoprotein (RABV G) instead of its gE glycoprotein (gE) by CRISPR-Cas9 and homologous recombination technology (BHV-1-ΔgE-G). Insertion of the RABV G gene is stable after 20 rounds of in vitro passaging and the recombinant virus replicates to high titers in MDBK cells. The RABV G expresses in the recombinant virus-infected cells and on the virion surface of BHV-1-ΔgE-G. One single immunization with BHV-1-ΔgE-G-activated dendritic cells (DCs) and B cells furthermore induced a protective immune response in mice against severe lethal challenge infection. A protective level of RABV-specific virus-neutralizing antibody (VNA) was detected in intramuscular immunized mice and cattle without any clinical symptoms. This research demonstrated that the BHV-1 vector-based RABV vaccine is a potential candidate for cattle.

Roles of apoptosis and autophagy in natural rabies infections

The aim of this study was to investigate the activity of apoptosis and autophagy in animals (cows, horses, donkeys, dogs and cats) naturally infected with rabies by using immunohistochemistry, immunofluorescence, and qPCR. The mRNA transcript levels of caspase-3, Bax, Bcl2 and LC3B were determined with qPCR. Caspase-3 and AIF immunopositivity were not observed in the immunohistochemical and immunofluorescence staining, whereas LC3B immunopositivity was determined intensively in the infected animals compared to the control groups. LC3B immunopositivity was detected in the cytoplasm of the Purkinje cells in the cerebellum of the cows, horses and donkeys, and also in the cytoplasm of the neurons in the cornu ammonis of the dogs and cats. While the expression levels of caspase-3 and Bax were downregulated, the Bcl2 expression was up-regulated in the infected animals compared to the uninfected animals. In addition, the LC3B levels were found to be significantly higher in the infected animals. To the best of our knowledge, this work represents the first report of neuronal death in the central nervous system by autophagy, rather than by caspase-dependent or AIF-containing caspase-independent apoptosis.

Lyssaviruses and the Fatal Encephalitic Disease Rabies

Lyssaviruses cause the disease rabies, which is a fatal encephalitic disease resulting in approximately 59,000 human deaths annually. The prototype species, rabies lyssavirus, is the most prevalent of all lyssaviruses and poses the greatest public health threat. In Africa, six confirmed and one putative species of lyssavirus have been identified. Rabies lyssavirus remains endemic throughout mainland Africa, where the domestic dog is the primary reservoir - resulting in the highest per capita death rate from rabies globally. Rabies is typically transmitted through the injection of virus-laden saliva through a bite or scratch from an infected animal. Due to the inhibition of specific immune responses by multifunctional viral proteins, the virus usually replicates at low levels in the muscle tissue and subsequently enters the peripheral nervous system at the neuromuscular junction. Pathogenic rabies lyssavirus strains inhibit innate immune signaling and induce cellular apoptosis as the virus progresses to the central nervous system and brain using viral protein facilitated retrograde axonal transport. Rabies manifests in two different forms - the encephalitic and the paralytic form - with differing clinical manifestations and survival times. Disease symptoms are thought to be due mitochondrial dysfunction, rather than neuronal apoptosis. While much is known about rabies, there remain many gaps in knowledge about the neuropathology of the disease. It should be emphasized however, that rabies is vaccine preventable and dog-mediated human rabies has been eliminated in various countries. The global elimination of dog-mediated human rabies in the foreseeable future is therefore an entirely feasible goal.

Innate Immune Signaling and Role of Glial Cells in Herpes Simplex Virus- and Rabies Virus-Induced Encephalitis

The environment of the central nervous system (CNS) represents a double-edged sword in the context of viral infections. On the one hand, the infectious route for viral pathogens is restricted via neuroprotective barriers; on the other hand, viruses benefit from the immunologically quiescent neural environment after CNS entry. Both the herpes simplex virus (HSV) and the rabies virus (RABV) bypass the neuroprotective blood-brain barrier (BBB) and successfully enter the CNS parenchyma via nerve endings. Despite the differences in the molecular nature of both viruses, each virus uses retrograde transport along peripheral nerves to reach the human CNS. Once inside the CNS parenchyma, HSV infection results in severe acute inflammation, necrosis, and hemorrhaging, while RABV preserves the intact neuronal network by inhibiting apoptosis and limiting inflammation. During RABV neuroinvasion, surveilling glial cells fail to generate a sufficient type I interferon (IFN) response, enabling RABV to replicate undetected, ultimately leading to its fatal outcome. To date, we do not fully understand the molecular mechanisms underlying the activation or suppression of the host inflammatory responses of surveilling glial cells, which present important pathways shaping viral pathogenesis and clinical outcome in viral encephalitis. Here, we compare the innate immune responses of glial cells in RABV- and HSV-infected CNS, highlighting different viral strategies of neuroprotection or Neuroinflamm. in the context of viral encephalitis.

Transcriptome Profile During Rabies Virus Infection: Identification of Human CXCL16 as a Potential New Viral Target

Rabies virus (RABV), the causative agent for rabies disease is still presenting a major public health concern causing approximately 60,000 deaths annually. This neurotropic virus (genus Lyssavirus, family Rhabdoviridae) induces an acute and almost always fatal form of encephalomyelitis in humans. Despite the lethal consequences associated with clinical symptoms of rabies, RABV limits neuro-inflammation without causing major histopathological lesions in humans. Nevertheless, information about the mechanisms of infection and cellular response in the central nervous system (CNS) remain scarce. Here, we investigated the expression of inflammatory genes involved in immune response to RABV (dog-adapted strain Tha) in mice, the most common animal model used to study rabies. To better elucidate the pathophysiological mechanisms during natural RABV infection, we compared the inflammatory transcriptome profile observed at the late stage of infection in the mouse brain (cortex and brain stem/cerebellum) with the ortholog gene expression in post-mortem brain biopsies of rabid patients. Our data indicate that the inflammatory response associated with rabies is more pronounced in the murine brain compared to the human brain. In contrast to murine transcription profiles, we identified CXC motif chemokine ligand 16 (CXCL16) as the only significant differentially expressed gene in post-mortem brains of rabid patients. This result was confirmed in vitro, in which Tha suppressed interferon alpha (IFN-α)-induced CXCL16 expression in human CNS cell lines but induced CXCL16 expression in IFN-α-stimulated murine astrocytes. We hypothesize that RABV-induced modulation of the CXCL16 pathway in the brain possibly affects neurotransmission, natural killer (NK) and T cell recruitment and activation. Overall, we show species-specific differences in the inflammatory response of the brain, highlighted the importance of understanding the potential limitations of extrapolating data from animal models to humans.

Analyses of cell death mechanisms related to amino acid substitution at position 95 in the rabies virus matrix protein

We previously reported that the avirulent fixed rabies virus strain Ni-CE induces a clear cytopathic effect in mouse neuroblastoma cells, whereas its virulent progenitor, the Nishigahara strain, does not. Infection with Nishigahara and Ni-CE mutants containing a single amino acid substitution in the matrix protein (M) demonstrated that the amino acid at position 95 of M (M95) is a cytopathic determinant. The characteristics of cell death induced by Ni-CE infection resemble those of apoptosis (rounded and shrunken cells, DNA fragmentation), but the intracellular signalling pathway for this process has not been fully investigated. In this study, we aimed to elucidate the mechanism by which M95 affects cell death induced by human neuroblastoma cell infection with the Nishigahara, Ni-CE and M95-mutated strains. We demonstrated that the Ni-CE strain induced DNA fragmentation, cell membrane disruption, exposure of phosphatidylserine (PS), activation of caspase-3/7 and anti-poly (ADP-ribose) polymerase 1 (PARP-1) cleavage, an early apoptosis indicator, whereas the Nishigahara strain did not induce DNA fragmentation, caspase-3/7 activation, cell membrane disruption, or PARP-1 cleavage, but did induce PS exposure. We also demonstrated that these characteristics were associated with M95 using M95-mutated strains. However, we found that Ni-CE induced cell death despite the presence of a caspase inhibitor, Z-VAD-FMK. In conclusion, our data suggest that M95 mutation-related cell death is caused by both the caspase-dependent and -independent pathways.

Enhancement of immune responses by co-stimulation of TLR3 - TLR7 agonists as a potential therapeutics against rabies in mouse model

Rabies is always fatal, when post-exposure prophylaxis is administered after the onset of clinical symptoms. To date, there is no effective treatment of rabies once clinical symptoms has initiated. Therefore, we aimed to provide evidences which indicate the promising effects of combination treatment with TLR agonists following rabies infection. Four groups of rabies infected-mice (10-mice/group) were treated with PolyI:C 50 μg (a TLR3 agonist), Imiquimod50 μg (a TLR7 agonist), (Poly + Imi)25 μg and (Poly + Imi)50 μg respectively. The immune responses in each experimental groups were investigated in the brain through evaluation of GFAP, MAP2, CD4, HSP70, TLR3, TLR7 and apoptotic cell expression as well as determination of IFN-γ, TNF-α and IL-4, levels. The treatment with combination of agonists (Poly + Imi)50 μg/mouse resulted a 75% decrease of mortality rate and better extended survival time following street rabies virus infection. Higher number of CD4⁺T cells, TLR3 and TLR7 expression in the brain parenchyma observed in the groups receiving both combined agonist therapies at the levels of 25 μg and 50 μg. In spite of decreased number of neuronal cell, significant higher number of astrocytes was shown in the group given (Poly + Imi)25 μg. The obtained results also pointed to the dramatic decrease of HSP70 expression in all groups of infected mice whereas higher number of apoptotic cells and Caspase 8 expression were recorded in (Poly + Imi)25 μg treated group. Furthermore, the cytokine profile consisting the increased levels of TNF-α, IFN-γ and IL-4 revealed that both humoral and cellular responses were highly modulated in combination therapy of 50 μg of Imiquimod and Poly I:C. Reduced viral load as quantified by real-time PCR of rabies N gene expression in the brain also correlated with the better survival of agonist-treated groups of mice. Based on obtained results, we have presented evidences of beneficial utilization of combined agonist therapy composed of TLR3/TLR7 ligands. This treatment regimen extended survival of infected mice and decreased significantly their mortality rate. We believe that the results of synergy-inducing protection of both TLR3/TLR7 agonists lead to the enhancement of innate immune responses cells residing in the CNS which warrant the studies to further understanding of crosstalk mechanisms in cellular immunity against rabies in the future.

Comparison of lncRNA and mRNA expression in mouse brains infected by a wild-type and a lab-attenuated Rabies lyssavirus

Rabies is a lethal disease caused by Rabies lyssavirus, commonly known as rabies virus (RABV), and results in nearly 100 % death once clinical symptoms occur in human and animals. Long non-coding RNAs (lncRNAs) have been reported to be associated with viral infection. But the role of lncRNAs involved in RABV infection is still elusive. In this study, we performed global transcriptome analysis of both of lncRNA and mRNA expression profiles in wild-type (WT) and lab-attenuated RABV-infected mouse brains by using next-generation sequencing. The differentially expressed lncRNAs and mRNAs were analysed by using the edgeR package. We identified 1422 differentially expressed lncRNAs and 4475 differentially expressed mRNAs by comparing WT and lab-attenuated RABV-infected brains. Then we predicted the enriched biological pathways by the Gene Ontology (GO) and Kyoto Encyclopaedia of Genes and Genomes (KEGG) database based on the differentially expressed lncRNAs and mRNAs. Our analysis revealed the relationships between lncRNAs and RABV-infection-associated immune response and ion transport-related pathways, which provide a fresh insight into the potential role of lncRNA in immune evasion and neuron injury induced by WT RABV.

Inflammatory response and MAPK and NF-κB pathway activation induced by natural street rabies virus infection in the brain tissues of dogs and humans

BACKGROUND

Street rabies virus (RABV) usually infects hosts at peripheral sites and migrates from motor or sensory nerves to the central nervous system. Several studies have found that inflammation is mild in a mouse model of street RABV infection. However, the pathogenetic mechanisms of street RABV in naturally infected dogs or humans are not well understood.

METHODS

Brain tissues collected from 3 dogs and 3 humans were used; these tissue samples were collected under the natural condition of rabies-induced death. The inflammatory response and pathway activation in the brain tissue samples of dogs and humans were evaluated by HE, IHC, ARY006, WB and ELISA. The clinical isolate street RABV strains CGS-17 and CXZ-15 from 30 six-week-old ICR mice were used to construct the mouse infection model presented here.

RESULTS

Neuronal degeneration and increased lymphocyte infiltration in the cerebral cortex, especially marked activation of microglia, formation of glial nodules, and neuronophagy, were observed in the dogs and humans infected with the street RABV strains. The various levels of proinflammatory chemokines, particularly CXCL1, CXCL12, CCL2, and CCL5, were increased significantly in the context of infection with street RABV strains in dogs and humans in relation to healthy controls, and the levels of MAPK and NF-κB phosphorylation were also increased in dogs and humans with natural infection. We also found that the degrees of pathological change, inflammatory response, MAPK and NF-κB signaling pathway activation were obviously increased during natural infection in dogs and humans compared with artificial model infection in mice.

CONCLUSION

The data obtained here provide direct evidence for the RABV-induced activation of the inflammatory response in a dog infection model, which is a relatively accurate reflection of the pathogenic mechanism of human street RABV infection. These observations provide insight into the precise roles of underlying mechanisms in fatal natural RABV infection.

Role of autophagy in nerve cell apoptosis in mice infected with street rabies virus

Rabies is an important zoonotic disease in Iran. Autophagy is a process that maintains homeostasis and can be used as an innate defense mechanism against viruses. Apoptosis is the process of programmed cell death induced by physiological and pathological conditions. The crosstalk of autophagy and apoptosis plays a key role in rabies virus infection. In the current study, NMRI mice intra-cranially received 3-Methyl Adenine (3-MA), rapamycin, street rabies virus (SRABV) and drugs plus SRABV. SRABV and Map1lc3, Beclin-1, Atg5 gene expression were assayed by real-time PCR. Immunohistochemistry was carried out via LC3 protein staining as an autophagy marker, and apoptotic cell death was measured using a TUNEL assay. Map1lc3, Beclin-1 and Atg5 genes expression was significantly increased in drug-plus-SRBV-treated tissues compared to control at 24 hpi. Map1lc3 and Atg5 gene expression showed a slight change in the drugs-plus-virus group compared with the control at 72 hpi. The presence of LC3 in the tissues of the group treated with rapamycin plus SRBV confirmed induction of autophagy, but it was not present in the tissues treated with 3-MA plus SRBV. Our data revealed that apoptosis was induced only in the groups receiving the SRBV or rapamycin or both at 24 hpi. Apoptosis was observed after 72 hours, when the drugs' effect had disappeared in all but the autophagy inhibitor group. Understanding the interaction of SRABV with autophagy pathway genes and its effect on host cell apoptosis may open a new horizon for human intervention and allow a deeper understanding of rabies infections.

Effect of Postmortem Degradation on the Preservation of Viral Particles and Rabies Antigens in Mice Brains. Light and Electron Microscopic Study

Rabies diagnosis is mainly made on fresh brain tissue postmortem by means of the direct immunofluorescence test. However, in some cases, it is not possible to use this technique, given that the affected nervous tissue goes through a postmortem degradation process, due to problems in the handling and transport of the samples. For this reason, the preservation in time of the rabies virus inclusions was assessed, as well as the immunoreactivity and the ultrastructure of viral particles in tissue with postmortem degradation. Brains of mice inoculated with rabies virus and control mice were processed for conventional histology, immunohistochemistry, electron microscopy, and immunoelectron microscopy in different postmortem times. In the processed tissues for hematoxylin and eosin, the presence of eosinophilic inclusions was not observed beyond 12 h postmortem. Surprisingly, the immunoreactivity of the viral antigens increased with time, at least until 30 h postmortem. It was possible to easily recognize the viral particles by means of conventional electron microscopy until 12 h postmortem. Immunoelectron microscopy allowed us to identify the presence of viral antigens disseminated in the neuronal cytoplasm until 30 h postmortem, but immunoreactive viral particles were not observed. The rabies infection did not cause histological or ultrastructural alterations different from those in the control group as a result of the postmortem degradation. In conclusion, the immunohistochemistry is a reliable test for rabies diagnosis in samples with postmortem degradation and that have been fixed with aldehydes.

Farr R, Green D, M. Haynes J, Bingham J, O’Brien CM, Dearnley M. Modelling Lyssavirus Infections in Human Stem Cell-Derived Neural Cultures

Rabies is a zoonotic neurological infection caused by lyssavirus that continues to result in devastating loss of human life. Many aspects of rabies pathogenesis in human neurons are not well understood. Lack of appropriate ex-vivo models for studying rabies infection in human neurons has contributed to this knowledge gap. In this study, we utilize advances in stem cell technology to characterize rabies infection in human stem cell-derived neurons. We show key cellular features of rabies infection in our human neural cultures, including upregulation of inflammatory chemokines, lack of neuronal apoptosis, and axonal transmission of viruses in neuronal networks. In addition, we highlight specific differences in cellular pathogenesis between laboratory-adapted and field strain lyssavirus. This study therefore defines the first stem cell-derived ex-vivo model system to study rabies pathogenesis in human neurons. This new model system demonstrates the potential for enabling an increased understanding of molecular mechanisms in human rabies, which could lead to improved control methods.

Astrocyte Infection during Rabies Encephalitis Depends on the Virus Strain and Infection Route as Demonstrated by Novel Quantitative 3D Analysis of Cell Tropism

Although conventional immunohistochemistry for neurotropic rabies virus (RABV) usually shows high preference for neurons, non-neuronal cells are also potential targets, and abortive astrocyte infection is considered a main trigger of innate immunity in the CNS. While in vitro studies indicated differences between field and less virulent lab-adapted RABVs, a systematic, quantitative comparison of astrocyte tropism in vivo is lacking. Here, solvent-based tissue clearing was used to measure RABV cell tropism in infected brains. Immunofluorescence analysis of 1 mm-thick tissue slices enabled 3D-segmentation and quantification of astrocyte and neuron infection frequencies. Comparison of three highly virulent field virus clones from fox, dog, and raccoon with three lab-adapted strains revealed remarkable differences in the ability to infect astrocytes in vivo. While all viruses and infection routes led to neuron infection frequencies between 7–19%, striking differences appeared for astrocytes. Whereas astrocyte infection by field viruses was detected independent of the inoculation route (8–27%), only one lab-adapted strain infected astrocytes route-dependently [0% after intramuscular (i.m.) and 13% after intracerebral (i.c.) inoculation]. Two lab-adapted vaccine viruses lacked astrocyte infection altogether (0%, i.c. and i.m.). This suggests a model in which the ability to establish productive astrocyte infection in vivo functionally distinguishes field and attenuated lab RABV strains.

Overexpression of MAP2 and NF-H Associated with Dendritic Pathology in the Spinal Cord of Mice Infected with Rabies Virus

Rabies is a viral infection that targets the nervous system, specifically neurons. The clinical manifestations of the disease are dramatic and their outcome fatal; paradoxically, conventional histopathological descriptions reveal only subtle changes in the affected nervous tissue. Some researchers have considered that the pathophysiology of rabies is based more on biochemical changes than on structural alterations, as is the case with some psychiatric diseases. However, we believe that it has been necessary to resort to other methods that allow us to analyze the effect of the infection on neurons. The Golgi technique is the gold standard for studying the morphology of all the components of a neuron and the cytoskeletal proteins are the structural support of dendrites and axons. We have previously shown, in the mouse cerebral cortex and now with this work in spinal cord, that rabies virus generates remarkable alterations in the morphological pattern of the neurons and that this effect is associated with the increase in the expression of two cytoskeletal proteins (MAP2 and NF-H). It is necessary to deepen the investigation of the pathogenesis of rabies in order to find therapeutic alternatives to a disease to which the World Health Organization classifies as a neglected disease.

Lab-Attenuated Rabies Virus Causes Abortive Infection and Induces Cytokine Expression in Astrocytes by Activating Mitochondrial Antiviral-Signaling Protein Signaling Pathway

Rabies is an ancient disease but remains endemic in most parts of the world and causes approximately 59,000 deaths annually. The mechanism through which the causative agent, rabies virus (RABV), evades the host immune response and infects the host central nervous system (CNS) has not been completely elucidated thus far. Our previous studies have shown that lab-attenuated, but not wild-type (wt), RABV activates the innate immune response in the mouse and dog models. In this present study, we demonstrate that lab-attenuated RABV causes abortive infection in astrocytes, the most abundant glial cells in the CNS. Furthermore, we found that lab-attenuated RABV produces more double-stranded RNA (dsRNA) than wt RABV, which is recognized by retinoic acid-inducible gene I (RIG-I) or melanoma differentiation-associated protein 5 (MDA5). Activation of mitochondrial antiviral-signaling protein (MAVS), the common adaptor molecule for RIG-I and MDA5, results in the production of type I interferon (IFN) and the expression of hundreds of IFN-stimulated genes, which suppress RABV replication and spread in astrocytes. Notably, lab-attenuated RABV replicates in a manner identical to that of wt RABV in MAVS−/− astrocytes. It was also found that lab-attenuated, but not wt, RABV induces the expression of inflammatory cytokines via the MAVS- p38/NF-κB signaling pathway. These inflammatory cytokines increase the blood–brain barrier permeability and thus enable immune cells and antibodies infiltrate the CNS parenchyma, resulting in RABV control and elimination. In contrast, wt RABV restricts dsRNA production and thus evades innate recognition by RIG-I/MDA5 in astrocytes, which could be one of the mechanisms by which wt RABV evades the host immune response in resident CNS cells. Our findings suggest that astrocytes play a critical role in limiting the replication of lab-attenuated RABV in the CNS.

Immunological aspects of rabies: a literature review

Rabies is a lethal disease caused by the neurotropic virus rabies virus (RABV), and it remains an important public health problem globally. It is known that the host immune response is important for control of viral infection and promoting viral clearance. In this context, it is well documented that, in addition to RABV neutralizing antibody, interferons and cell-mediated immunity also have an important role in preventing the establishment of disease. On the other hand, RABV suppresses host immunity through different mechanisms, for example, direct inhibition of host gene expression, sequestration of pathogen-associated molecular patterns, or modification of cytokine signalling pathways, which hinder the protective host immune responses to RABV infection. Here, we review the immunological aspects of rabies, highlighting innate and adaptive immunity, as well as the host evasion immune mechanisms used by the virus. Finally, we briefly discuss how this knowledge can direct new research and be harnessed for future therapeutic strategies.

Pathological lesions in the central nervous system and peripheral tissues of ddY mice with street rabies virus (1088 strain)

Most studies on rabies virus pathogenesis in animal models have employed fixed rabies viruses, and the results of those employing street rabies viruses have been inconsistent. Therefore, to clarify the pathogenesis of street rabies virus (1088 strain) in mice, 10⁶ focus forming units were inoculated into the right hindlimb of ddY mice (6 weeks, female). At 3 days postinoculation (DPI), mild inflammation was observed in the hindlimb muscle. At 5 DPI, ganglion cells in the right lumbosacral spinal dorsal root ganglia showed chromatolysis. Axonal degeneration and inflammatory cells increased with infection progress in the spinal dorsal horn and dorsal root ganglia. Right hindlimb paralysis was observed from 7 DPI, which progressed to quadriparalysis. However, no pathological changes were observed in the ventral horn and root fibers of the spinal cord. Viral antigen was first detected in the right hindlimb muscle at 3 DPI, followed by the right lumbosacral dorsal root ganglia, dorsal horn of spinal cord, left red nuclei, medulla oblongata and cerebral cortex (M1 area) at 5 DPI. These results suggested that the 1088 virus ascended the lumbosacral spinal cord via mainly afferent fibers at early stage of infection and moved to cerebral cortex (M1 area) using descending spinal tract. Additionally, we concluded that significant pathological changes in mice infected with 1088 strain occur in the sensory tract of the spinal cord; this selective susceptibility results in clinical features of the disease.

Pathobiological investigation of naturally infected canine rabies cases from Sri Lanka

Background

The recommended screening of rabies in ‘suspect’ animal cases involves testing fresh brain tissue. The preservation of fresh tissue however can be difficult under field conditions and formalin fixation provides a simple alternative that may allow a confirmatory diagnosis. The occurrence and location of histopathological changes and immunohistochemical (IHC) labelling for rabies in formalin fixed paraffin embedded (FFPE) canine brain is described in samples from 57 rabies suspect cases from Sri-Lanka. The presence of Negri bodies and immunohistochemical detection of rabies virus antigen were evaluated in the cortex, hippocampus, cerebellum and brainstem. The effect of autolysis and artefactual degeneration of the tissue was also assessed.

Results

Rabies was confirmed in 53 of 57 (93%) cases by IHC. IHC labelling was statistically more abundant in the brainstem. Negri bodies were observed in 32 of 53 (60.4%) of the positive cases. Although tissue degradation had no effect on IHC diagnosis, it was associated with an inability to detect Negri bodies. In 13 cases, a confirmatory Polymerase chain reaction (PCR) testing for rabies virus RNA was undertaken by extracting RNA from fresh frozen tissue, and also attempted using FFPE samples. PCR detection using fresh frozen samples was in agreement with the IHC results. The PCR method from FFPE tissues was suitable for control material but unsuccessful in our field cases.

Conclusions

Histopathological examination of the brain is essential to define the differential diagnoses of behaviour modifying conditions in rabies virus negative cases, but it is unreliable as the sole method for rabies diagnosis, particularly where artefactual change has occurred. Formalin fixation and paraffin embedding does not prevent detection of rabies virus via IHC labelling even where artefactual degeneration has occurred. This could represent a pragmatic secondary assay for rabies diagnosis in the field because formalin fixation can prevent sample degeneration. The brain stem was shown to be the site with most viral immunoreactivity; supporting recommended sampling protocols in favour of improved necropsy safety in the field. PCR testing of formalin fixed tissue may be successful in certain circumstances as an alternative test.

Impact of caspase-1/11, -3, -7, or IL-1β/IL-18 deficiency on rabies virus-induced macrophage cell death and onset of disease

Rabies virus is a highly neurovirulent RNA virus, which causes about 59000 deaths in humans each year. Previously, we described macrophage cytotoxicity upon infection with rabies virus. Here we examined the type of cell death and the role of specific caspases in cell death and disease development upon infection with two laboratory strains of rabies virus: Challenge Virus Standard strain-11 (CVS-11) is highly neurotropic and lethal for mice, while the attenuated Evelyn–Rotnycki–Abelseth (ERA) strain has a broader cell tropism, is non-lethal and has been used as an oral vaccine for animals. Infection of Mf4/4 macrophages with both strains led to caspase-1 activation and IL-1β and IL-18 production, as well as activation of caspases-3, -7, -8, and -9. Moreover, absence of caspase-3, but not of caspase-1 and -11 or -7, partially inhibited virus-induced cell death of bone marrow-derived macrophages. Intranasal inoculation with CVS-11 of mice deficient for either caspase-1 and -11 or -7 or both IL-1β and IL-18 led to general brain infection and lethal disease similar to wild-type mice. Deficiency of caspase-3, on the other hand, significantly delayed the onset of disease, but did not prevent final lethal outcome. Interestingly, deficiency of caspase-1/11, the key executioner of pyroptosis, aggravated disease severity caused by ERA virus, whereas wild-type mice or mice deficient for either caspase-3, -7, or both IL-1β and IL-18 presented the typical mild symptoms associated with ERA virus. In conclusion, rabies virus infection of macrophages induces caspase-1- and caspase-3-dependent cell death. In vivo caspase-1/11 and caspase-3 differently affect disease development in response to infection with the attenuated ERA strain or the virulent CVS-11 strain, respectively. Inflammatory caspases seem to control attenuated rabies virus infection, while caspase-3 aggravates virulent rabies virus infection.

Apoptosis in natural rabies virus infection in dogs

Introduction: In the present study apoptosis was investigated in the cornu ammonis and cerebellum of 10 dogs naturally infected with rabies virus. Diagnosis of rabies was based on the results of fluorescent antibody staining and experimental inoculation.

Material and Methods: The paraffin tissue sections were stained with haematoxylin and eosin, avidin-biotin complex peroxidase (ABC-P), and terminal deoxynucleotidyl transferase biotin-dUTP nick end-labelling (TUNEL) methods.

Results: Histopathological examination revealed encephalomyelitis of varying severity and the presence of Negri bodies. Dense rabies antigens were determined in the motor neurons with ABC-P method. On the other hand, Bcl-2 protein and Bax protein gave positive reaction in seven and five cases, respectively. TUNEL staining demonstrated very marked apoptotic changes in the nuclei of neurons localised deep in the substantia alba of the cerebellum. Similar changes were also determined in perivascular mononuclear cells and glia cells within the substantia alba. No apoptopic changes were found in the motor neurons of the cornu ammonis.

Conclusion: The absence of apoptotic changes in the neurons was considered to be the consequence of the necrotic changes that developed in these neurons.

Subversion of the Immune Response by Rabies Virus

Rabies has affected mankind for several centuries and is one of the oldest known zoonoses. It is peculiar how little is known regarding the means by which rabies virus (RABV) evades the immune response and kills its host. This review investigates the complex interplay between RABV and the immune system, including the various means by which RABV evades, or advantageously utilizes, the host immune response in order to ensure successful replication and spread to another host. Different factors that influence immune responses—including age, sex, cerebral lateralization and temperature—are discussed, with specific reference to RABV and the effects on host morbidity and mortality. We also investigate the role of apoptosis and discuss whether it is a detrimental or beneficial mechanism of the host’s response to infection. The various RABV proteins and their roles in immune evasion are examined in depth with reference to important domains and the downstream effects of these interactions. Lastly, an overview of the means by which RABV evades important immune responses is provided. The research discussed in this review will be important in determining the roles of the immune response during RABV infections as well as to highlight important therapeutic target regions and potential strategies for rabies treatment.

Perspectives in Diagnosis and Treatment of Rabies Viral Encephalitis: Insights from Pathogenesis

Rabies viral encephalitis, though one of the oldest recognized infectious disease of humans, remains an incurable, fatal encephalomyelitis, despite advances in understanding of its pathobiology. Advances in science have led us on the trail of the virus in the host, but the sanctuaries in which the virus remains hidden for its survival are unknown. Insights into host-pathogen interactions have facilitated evolving immunologic therapeutic strategies, though we are far from a cure. Most of the present-day knowledge has evolved from in vitro studies using fixed (attenuated) laboratory strains that may not be applicable in the clinical setting. Much remains to be unraveled about this elusive virus. This review attempts to re-examine the current advances in understanding of the pathobiology of the rabies virus that modulate the diagnosis, treatment, and prevention of this fatal disease.

Intracellular Spread of Rabies Virus Is Reduced in the Paralytic Form of Canine Rabies Compared to the Furious Form

Studies of the furious and paralytic forms of canine rabies at the early stage of disease have shown a more rapid viral colonization of the cerebral hemispheres in the furious form, as measured by viral antigen within neuronal cell bodies and viral RNA levels. Measurement of cellular processes separate from neuronal cell body provides a visual record of the spread of rabies virus which occurs across synapses. In this study, the amount of rabies viral antigen within cell processes was quantitatively assessed by image analysis in a cohort of naturally rabies infected non-vaccinated dogs (5 furious and 5 paralytic) that were sacrificed shortly after developing illness. Measurements were taken at different levels of the spinal cord, brain stem, and cerebrum. Results were compared to the amount of rabies viral antigen in neuronal cell bodies. Generally, the amount of rabies viral antigen in cell processes decreased in a rostral direction, following the pattern for the amount of rabies viral antigen in neuronal cell bodies and the percentage of involved cell bodies. However, there was a delay in cell process involvement following cell body involvement, consistent with replication occurring in the cell body region and subsequent transport out to cell processes. Greater amounts of antigen were seen in cell processes in dogs with the furious compared to paralytic form, at all anatomic levels examined. This difference was even evident when comparing (1) neurons with similar amounts of antigen, (2) similar percentages of involved neurons, and (3) anatomic levels that showed 100% positive neurons. These findings suggest that intracellular transport of the virus may be slower in the paralytic form, resulting in slower viral propagation. Possible mechanisms might involve host-specific differences in intracellular virus transport. The latter could be cytokine-mediated, since previous studies have documented greater inflammation in the paralytic form.

Dogs with rabies can show the furious or paralytic form. The virus spreads from nerve cell to nerve cell via connections in cell processes. There are greater amounts of virus in the nerve cell bodies in the furious form. Studying cell processes separate from cell body provides a visual record of the spread of rabies virus. The amount of rabies viral protein within cell processes was measured in dogs with rabies (5 furious and 5 paralytic) sacrificed shortly after developing illness. The amount of viral protein in cell processes decreased from spinal cord to brain, as did the amount of viral protein in cell bodies and the percentage of involved cell bodies. However, there was a delay in cell process involvement following cell body involvement, consistent with the virus replicating in the cell body region and later moving out to cell processes. Greater amounts of viral protein were seen in cell processes in dogs with the furious compared to paralytic form, by comparing nerve cells with similar amounts of antigen, or similar percentages of involved nerve cells. These findings suggest that intracellular transport of the virus may be slower in paralytic rabies, resulting in slower viral spread in the brain.

Enhancement of blood-brain barrier permeability and reduction of tight junction protein expression are modulated by chemokines/cytokines induced by rabies virus infection

Infection with laboratory-attenuated rabies virus (RABV) enhances blood-brain barrier (BBB) permeability, which has been demonstrated to be an important factor for host survival, since it allows immune effectors to enter the central nervous system (CNS) and clear RABV. To probe the mechanism by which RABV infection enhances BBB permeability, the expression of tight junction (TJ) proteins in the CNS was investigated following intracranial inoculation with laboratory-attenuated or wild-type (wt) RABV. BBB permeability was significantly enhanced in mice infected with laboratory-attenuated, but not wt, RABV. The expression levels of TJ proteins (claudin-5, occludin, and zonula occludens-1) were decreased in mice infected with laboratory-attenuated, but not wt, RABV, suggesting that enhancement of BBB permeability is associated with the reduction of TJ protein expression in RABV infection. RABV neither infects the brain microvascular endothelial cells (BMECs) nor modulates the expression of TJ proteins in BMECs. However, brain extracts prepared from mice infected with laboratory-attenuated, but not wt, RABV reduced TJ protein expression in BMECs. It was found that brain extracts from mice infected with laboratory-attenuated RABV contained significantly higher levels of inflammatory chemokines/cytokines than those from mice infected with wt RABV. Pathway analysis indicates that gamma interferon (IFN-γ) is located in the center of the cytokine network in the RABV-infected mouse brain, and neutralization of IFN-γ reduced both the disruption of BBB permeability in vivo and the downregulation of TJ protein expression in vitro. These findings indicate that the enhancement of BBB permeability and the reduction of TJ protein expression are due not to RABV infection per se but to virus-induced inflammatory chemokines/cytokines.

IMPORTANCE

Previous studies have shown that infection with only laboratory-attenuated, not wild-type, rabies virus (RABV) enhances blood-brain barrier (BBB) permeability, allowing immune effectors to enter the central nervous system (CNS) and clear RABV from the CNS. This study investigated the mechanism by which RABV infection enhances BBB permeability. It was found that RABV infection enhances BBB permeability by downregulation of tight junction (TJ) protein expression in the brain microvasculature. It was further found that it is not RABV infection per se but the chemokines/cytokines induced by RABV infection that downregulate the expression of TJ proteins and enhance BBB permeability. Blocking some of these cytokines, such as IFN-γ, ameliorated both the disruption of BBB permeability and the downregulation of TJ protein expression. These studies may provide a foundation for developing therapeutics for clinical rabies, such as medication that could be used to enhance BBB permeability.

Survival of rabid rabbits after intrathecal immunization

Rabies is a fatal zoonotic disease for which no effective treatment measures are currently available. Rabies virus (RABV) has anti-apoptotic and anti-inflammatory properties that suppress nerve cell damage and inflammation in the CNS. These features imply that the elimination of RABV from the CNS by appropriate treatment could lead to complete recovery from rabies. Ten rabbits showing neuromuscular symptoms of rabies after subcutaneous (SC) immunization using commercially available vaccine containing inactivated whole RABV particles and subsequent fixed RABV (CVS strain) inoculation into hind limb muscles were allocated into three groups. Three rabbits received no further treatment (the SC group), three rabbits received three additional SC immunizations using the same vaccine, and four rabbits received three intrathecal (IT) immunizations, in which the vaccine was inoculated directly into the cerebrospinal fluid (the SC/IT group). An additional three naïve rabbits were inoculated intramuscularly with RABV and not vaccinated. The rabbits exhibited neuromuscular symptoms of rabies within 4-8 days post-inoculation (dpi) of RABV. All of the rabbits died within 8-12 dpi with the exception of one rabbit in the SC group and all four rabbits in SC/IT group, which recovered and started to respond to external stimuli at 11-18 dpi and survived until the end of the experimental period. RABV was eliminated from the CNS of the surviving rabbits. We report here a possible, although still incomplete, therapy for rabies using IT immunization. Our protocol may rescue the life of rabid patients and prompt the future development of novel therapies against rabies.

Rabies virus infection in Eptesicus fuscus bats born in captivity (naïve bats)

The study of rabies virus infection in bats can be challenging due to quarantine requirements, husbandry concerns, genetic differences among animals, and lack of medical history. To date, all rabies virus (RABV) studies in bats have been performed in wild caught animals. Determining the RABV exposure history of a wild caught bat based on the presence or absence of viral neutralizing antibodies (VNA) may be misleading. Previous studies have demonstrated that the presence of VNA following natural or experimental inoculation is often ephemeral. With this knowledge, it is difficult to determine if a seronegative, wild caught bat has been previously exposed to RABV. The influence of prior rabies exposure in healthy, wild caught bats is unknown. To investigate the pathogenesis of RABV infection in bats born in captivity (naïve bats), naïve bats were inoculated intramuscularly with one of two Eptesicus fuscus rabies virus variants, EfV1 or EfV2. To determine the host response to a heterologous RABV, a separate group of naïve bats were inoculated with a Lasionycteris noctivagans RABV (LnV1). Six months following the first inoculation, all bats were challenged with EfV2. Our results indicate that naïve bats may have some level of innate resistance to intramuscular RABV inoculation. Additionally, naïve bats inoculated with the LnV demonstrated the lowest clinical infection rate of all groups. However, primary inoculation with EfV1 or LnV did not appear to be protective against a challenge with the more pathogenic EfV2.

Quantitative proteomics for identifying biomarkers for Rabies

INTRODUCTION

Rabies is a fatal acute viral disease of the central nervous system, which is a serious public health problem in Asian and African countries. Based on the clinical presentation, rabies can be classified into encephalitic (furious) or paralytic (numb) rabies. Early diagnosis of this disease is particularly important as rabies is invariably fatal if adequate post exposure prophylaxis is not administered immediately following the bite.

METHODS

In this study, we carried out a quantitative proteomic analysis of the human brain tissue from cases of encephalitic and paralytic rabies along with normal human brain tissues using an 8-plex isobaric tags for relative and absolute quantification (iTRAQ) strategy.

RESULTS AND CONCLUSION

We identified 402 proteins, of which a number of proteins were differentially expressed between encephalitic and paralytic rabies, including several novel proteins. The differentially expressed molecules included karyopherin alpha 4 (KPNA4), which was overexpressed only in paralytic rabies, calcium calmodulin dependent kinase 2 alpha (CAMK2A), which was upregulated in paralytic rabies group and glutamate ammonia ligase (GLUL), which was overexpressed in paralytic as well as encephalitic rabies. We validated two of the upregulated molecules, GLUL and CAMK2A, by dot blot assays and further validated CAMK2A by immunohistochemistry. These molecules need to be further investigated in body fluids such as cerebrospinal fluid in a larger cohort of rabies cases to determine their potential use as antemortem diagnostic biomarkers in rabies. This is the first study to systematically profile clinical subtypes of human rabies using an iTRAQ quantitative proteomics approach.

Reduced viral burden in paralytic compared to furious canine rabies is associated with prominent inflammation at the brainstem level

Background

The mechanisms that differentiate rabies infections into furious and paralytic forms remain undetermined. There are no neuropathological features in human brains that distinguish furious and paralytic rabies. This could be due to methodology and/or examination of specimens late in the disease course.

In this study, postmortem examination of brain (5 furious and 5 paralytic) and spinal cord (3 furious and 3 paralytic) specimens was performed in 10 rabies-infected dogs, sacrificed shortly after developing the illness. Rabies virus (RABV) antigen (percentage of positive neurons, average antigen area in positive neurons and average antigen area per neuron) and RNA were quantified at 15 different central nervous system (CNS) regions. The distribution and degree of inflammation were also studied.

Results

More RABV antigen was detected in furious rabies than paralytic in many of the CNS regions studied. Caudal-rostral polarity of viral antigen distribution was found in both clinical forms in order from greatest to least: spinal cord, brainstem, cerebellum, midline structures (caudate, thalamus), hippocampus, and cerebrum. In contrast, RABV RNA was most abundant in the cerebral midline structures. Viral RNA was found at significantly higher levels in the cerebral cortex, thalamus, midbrain and medulla of dogs with the furious subtype. The RNA levels in the spinal cord were comparable in both clinical forms. A striking inflammatory response was found in paralytic rabies in the brainstem.

Conclusions

These observations provide preliminary evidence that RABV antigen and RNA levels are higher in the cerebrum in furious rabies compared to the paralytic form. In addition, brainstem inflammation, more pronounced in paralytic rabies, may impede viral propagation towards the cerebral hemispheres.

Immune clearance of attenuated rabies virus results in neuronal survival with altered gene expression

Rabies virus (RABV) is a highly neurotropic pathogen that typically leads to mortality of infected animals and humans. The precise etiology of rabies neuropathogenesis is unknown, though it is hypothesized to be due either to neuronal death or dysfunction. Analysis of human brains post-mortem reveals surprisingly little tissue damage and neuropathology considering the dramatic clinical symptomology, supporting the neuronal dysfunction model. However, whether or not neurons survive infection and clearance and, provided they do, whether they are functionally restored to their pre-infection phenotype has not been determined in vivo for RABV, or any neurotropic virus. This is due, in part, to the absence of a permanent “mark” on once-infected cells that allow their identification long after viral clearance. Our approach to study the survival and integrity of RABV-infected neurons was to infect Cre reporter mice with recombinant RABV expressing Cre-recombinase (RABV-Cre) to switch neurons constitutively expressing tdTomato (red) to expression of a Cre-inducible EGFP (green), permanently marking neurons that had been infected in vivo. We used fluorescence microscopy and quantitative real-time PCR to measure the survival of neurons after viral clearance; we found that the vast majority of RABV-infected neurons survive both infection and immunological clearance. We were able to isolate these previously infected neurons by flow cytometry and assay their gene expression profiles compared to uninfected cells. We observed transcriptional changes in these “cured” neurons, predictive of decreased neurite growth and dysregulated microtubule dynamics. This suggests that viral clearance, though allowing for survival of neurons, may not restore them to their pre-infection functionality. Our data provide a proof-of-principle foundation to re-evaluate the etiology of human central nervous system diseases of unknown etiology: viruses may trigger permanent neuronal damage that can persist or progress in the absence of sustained viral antigen.

Rabies is an ancient and fatal neurological disease of animals and humans, caused by infection of the central nervous system (CNS) with Rabies virus (RABV). It is estimated that nearly 55,000 human RABV fatalities occur each year, though this number is likely much higher due to unreported exposures or failure of diagnosis. No treatment has been identified to cure disease after onset of symptoms. Neurovirologists still do not know the cause of rabies' dramatic symptoms and fatality, though it is thought to be due to neuronal loss or dysfunction. Here, we use a novel approach to permanently and genetically tag infected cells so that they can be identified after the infection has been cleared. This allowed us to define neuronal survival time following infection, and to assess neuronal function through gene expression analysis. We found that RABV infection does not lead to loss of neurons, but rather induces a permanent change in gene expression that may be related to the ability of RABV to cause permanent CNS disease. Our study provides evidence that viral infection of the brain can initiate long-term changes that may have consequences for nervous system health, even after the virus has been cleared from the CNS.