Mechanisms of escape phenomenon of spinal cord and brainstem in human rabies

Detection of Apparent Early Rabies Infection by LN34 Pan-Lyssavirus Real-Time RT-PCR Assay in Pennsylvania

The Pennsylvania Department of Health Bureau of Laboratories (PABOL) tested 6855 animal samples for rabies using both the direct fluorescent antibody test (DFA) and LN34 pan-lyssavirus reverse transcriptase quantitative PCR (RT-qPCR) during 2017-2019. Only two samples (0.03%) were initially DFA negative but positive by LN34 RT-qPCR. Both cases were confirmed positive upon re-testing at PABOL and confirmatory testing at the Centers for Disease Control and Prevention by LN34 RT-qPCR and DFA. Rabies virus sequences from one sample were distinct from all positive samples processed at PABOL within two weeks, ruling out cross-contamination. Levels of rabies virus antigen and RNA were low in all brain structures tested, but were higher in brain stem and rostral spinal cord than in cerebellum, hippocampus or cortex. Taken together, the low level of rabies virus combined with higher abundance in more caudal brain structures suggest early infection. These cases highlight the increased sensitivity and ease of interpretation of LN34 RT-qPCR for low positive cases.

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.

Neuroglia infection by rabies virus after anterograde virus spread in peripheral neurons

The highly neurotropic rabies virus (RABV) enters peripheral neurons at axon termini and requires long distance axonal transport and trans-synaptic spread between neurons for the infection of the central nervous system (CNS). Recent 3D imaging of field RABV-infected brains revealed a remarkably high proportion of infected astroglia, indicating that highly virulent field viruses are able to suppress astrocyte-mediated innate immune responses and virus elimination pathways. While fundamental for CNS invasion, in vivo field RABV spread and tropism in peripheral tissues is understudied. Here, we used three-dimensional light sheet and confocal laser scanning microscopy to investigate the in vivo distribution patterns of a field RABV clone in cleared high-volume tissue samples after infection via a natural (intramuscular; hind leg) and an artificial (intracranial) inoculation route. Immunostaining of virus and host markers provided a comprehensive overview of RABV infection in the CNS and peripheral nerves after centripetal and centrifugal virus spread. Importantly, we identified non-neuronal, axon-ensheathing neuroglia (Schwann cells, SCs) in peripheral nerves of the hind leg and facial regions as a target cell population of field RABV. This suggests that virus release from axons and infected SCs is part of the RABV in vivo cycle and may affect RABV-related demyelination of peripheral neurons and local innate immune responses. Detection of RABV in axon-surrounding myelinating SCs after i.c. infection further provided evidence for anterograde spread of RABV, highlighting that RABV axonal transport and spread of infectious virus in peripheral nerves is not exclusively retrograde. Our data support a new model in which, comparable to CNS neuroglia, SC infection in peripheral nerves suppresses glia-mediated innate immunity and delays antiviral host responses required for successful transport from the peripheral infection sites to the brain.

Neuroanatomical evidence of the transport of the rabies virus through the propriospinal tract in the spinal cord of mice

Introduction: Information about the neuroanatomical details of the ascendant transport of the rabies virus through the spinal cord is scarce. Objective: To identify the neuroanatomical route of dissemination of the rabies virus at each of the levels of the spinal cord of mice after being inoculated intramuscularly. Materials and methods: Mice were inoculated with the rabies virus in the hamstrings. After 24 hours post-inoculation, every eight hours, five animals were sacrificed by perfusion with paraformaldehyde. Then, the spinal cord was removed, and transverse cuts were made at the lumbosacral, thoracic, and cervical levels. These were processed by immunohistochemistry for the detection of viral antigens. Results: The first antigens of rabies were observed as aggregated particles in the lumbar spinal cord at 24 hours post-inoculation, within the ventral horn in the same side of the inoculated limb. At 32 hours post inoculation the first motoneurons immunoreactive to the virus became visible. At 40 hours postinoculation the first immunoreactive neurons were revealed in the thoracic level, located on lamina 8 and at 48 hours post-inoculation in the cervical cord, also on lamina 8. At 56 hours post-inoculation the virus had spread throughout the spinal cord, but the animals still did not show signs of the disease. Conclusion: In the mouse model we used, the rabies virus entered the spinal cord through the motoneurons and probably used the descending propriospinal pathway for its retrograde axonal transport to the encephalus.

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.

Rabies virus as a transneuronal tracer of neuronal connections

Powerful transneuronal tracing technologies exploit the ability of some neurotropic viruses to travel across neuronal pathways and to function as self-amplifying markers. Rabies virus is the only viral tracer that is entirely specific, as it propagates exclusively between connected neurons by strictly unidirectional (retrograde) transneuronal transfer, allowing for the stepwise identification of neuronal connections of progressively higher order. Transneuronal tracing studies in primates and rodent models prior to the development of clinical disease have provided valuable information on rabies pathogenesis. We have shown that rabies virus propagation occurs at chemical synapses but not via gap junctions or cell-to-cell spread. Infected neurons remain viable, as they can express their neurotransmitters and cotransport other tracers. Axonal transport occurs at high speed, and all populations of the same synaptic order are infected simultaneously regardless of their neurotransmitters, synaptic strength, and distance, showing that rabies virus receptors are ubiquitously distributed within the CNS. Conversely, in the peripheral nervous system, rabies virus receptors are present only on motor endplates and motor axons, since uptake and transneuronal transmission to the CNS occur exclusively via the motor route, while sensory and autonomic endings are not infected. Infection of sensory and autonomic ganglia requires longer incubation times, as it reflects centrifugal propagation from the CNS to the periphery, via polysynaptic connections from sensory and autonomic neurons to the initially infected motoneurons. Virus is recovered from end organs only after the development of rabies because anterograde spread to end organs is likely mediated by passive diffusion, rather than active transport mechanisms.

Evasive strategies in rabies virus infection

Rabies virus (RABV) is a strictly neurotropic virus that slowly propagates in the nervous system (NS) of the infected host from the site of entry (usually due to a bite) up to the site of exit (salivary glands). Successful achievement of the virus cycle relies on the preservation of the neuronal network. Once RABV has entered the NS, its progression is not interrupted either by destruction of the infected neurons or by the immune response, which are major host mechanisms for combating viral infection. RABV has developed two main mechanisms to escape the host defenses: (1) its ability to kill protective migrating T cells and (2) its ability to sneak into the NS without triggering apoptosis of the infected neurons and preserving the integrity of neurites.

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

To evaluate the role of apoptosis in rabies encephalitis in humans and canines infected with wild-type street virus, in comparison with rodent model infected with street and laboratory passaged CVS strain, we studied postmortem brain tissue from nine humans, six canines infected with street rabies virus, and Swiss albino mice inoculated intramuscularly (IM) and intracerebrally (IC) with street and CVS strains. Encephalitis and high rabies antigen load were prominent in canine and human brains compared to rodents inoculated with street virus. Neuronal apoptosis was detectable only in sucking mice inoculated with CVS strain and minimal in street virus inoculated mice. In a time point study in suckling mice, DNA laddering was noted only terminally (7 days p.i.) following IC inoculation with CVS strain but not with street virus. In weanling and adult mice, apoptosis was restricted to inflammatory cells and absent in neurons similar to human and canine rabies-infected brains. Absence of neuronal apoptosis in wild-type rabies may facilitate intraneuronal survival and replication while apoptosis in inflammatory cells prevents elimination of the virus by abrogation of host inflammatory response.

Update on rabies

Human rabies is almost invariably fatal, and globally it remains an important public health problem. Our knowledge of rabies pathogenesis has been learned mainly from studies performed in experimental animal models, and a number of unresolved issues remain. In contrast with the neural pathway of spread, there is still no credible evidence that hematogenous spread of rabies virus to the central nervous system plays a significant role in rabies pathogenesis. Although neuronal dysfunction has been thought to explain the neurological disease in rabies, recent evidence indicates that structural changes involving neuronal processes may explain the severe clinical disease and fatal outcome. Endemic dog rabies results in an ongoing risk to humans in many resource-limited and resource-poor countries, whereas rabies in wildlife is important in North America and Europe. In human cases in North America, transmission from bats is most common, but there is usually no history of a bat bite and there may be no history of contact with bats. Physicians may not recognize typical features of rabies in North America and Europe. Laboratory diagnostic evaluation for rabies includes rabies serology plus skin biopsy, cerebrospinal fluid, and saliva specimens for rabies virus antigen and/or RNA detection. Methods of postexposure rabies prophylaxis, including wound cleansing and administration of rabies vaccine and human rabies immune globulin, are highly effective after recognized exposure. Although there have been rare survivors of human rabies, no effective therapy is presently available. Therapeutic coma (midazolam and phenobarbital), ketamine, and antiviral therapies (known as the "Milwaukee protocol") were given to a rabies survivor, but this therapy was likely not directly responsible for the favorable outcome. New therapeutic approaches for human rabies need to be developed. A better understanding of basic mechanisms involved in rabies pathogenesis may be helpful in the development of potential new therapies for the future.

RIG-I mediates nonsegmented negative-sense RNA virus-induced inflammatory immune responses of primary human astrocytes

While astrocytes produce key inflammatory mediators following exposure to neurotropic nonsegmented negative-sense RNA viruses such as rabies virus and measles virus, the mechanisms by which resident central nervous system (CNS) cells perceive such viral challenges have not been defined. Recently, several cytosolic DExD/H box RNA helicases including retinoic acid-inducible gene I (RIG-I) have been described that function as intracellular sensors of replicative RNA viruses. Here, we demonstrate that primary human astrocytes constitutively express RIG-I and show that such expression is elevated following exposure to a model neurotropic RNA virus, vesicular stomatitis virus (VSV). Evidence for the functional nature of RIG-I expression in these cells comes from the observation that this molecule associates with its downstream effector molecule, interferon promoter stimulator-1, following VSV infection and from the finding that a specific ligand for RIG-I elicits astrocyte immune responses. Importantly, RIG-I knockdown significantly reduces inflammatory cytokine production by VSV-infected astrocytes and inhibits the production of soluble neurotoxic mediators by these cells. These findings directly implicate RIG-I in the initiation of inflammatory immune responses by human glial cells and provide a potential mechanism underlying the neuronal cell death associated with acute viral CNS infections.

Proteomics analysis of BHK-21 cells infected with a fixed strain of rabies virus

Rabies is a neurotropic virus that causes a life threatening acute viral encephalitis. The complex relationship of rabies virus (RV) with the host leads to its replication and spreading toward the neural network, where viral pathogenic effects appeared as neuronal dysfunction. In order to better understand the molecular basis of this relationship, a proteomics study on baby hamster kidney cells infected with challenge virus standard strain of RV was performed. This cell line is an in vitro model for rabies infection and is commonly used for viral seed preparation. The direct effect of the virus on cellular protein machinery was investigated by 2-DE proteome mapping of infected versus control cells followed by LC-MS/MS identification. This analysis revealed significant changes in expression of 14 proteins, seven of these proteins were viral and the remaining were host proteins with different known functions: cytoskeletal (capping protein, vimentin), anti-oxidative stress (superoxide dismutase), regulatory (Stathmin), and protein synthesis (P0). Despite of limited changes appeared upon rabies infection, they present a set of interesting biochemical pathways for further investigation on viral-host interaction.

Neuronal apoptosis does not play an important role in human rabies encephalitis

It is generally accepted that there are not prominent features of neuronal cell death in rabies encephalitis. However, Hemachudha and coworkers recently reported widespread apoptosis in the central nervous system of several human rabies cases (BMC Infect Dis 5: 104, 2005). In this study we have evaluated morphological features and markers of neuronal apoptosis in postmortem brain tissue from 12 cases of human rabies who died in four different countries. Histopathological analysis, TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling) staining, and immunostaining for cleaved (activated) caspase-3 were performed on paraffin-embedded tissues from the cerebral cortex, hippocampus, and brainstem, and additional regional areas from one of the cases. We did not find morphological evidence of neuronal apoptosis or TUNEL staining in any of the cases of rabies encephalitis. Similarly, immunostained cleaved caspase-3 was not seen in neurons, but prominent staining was observed in microglial processes. We conclude that neuronal apoptosis does not play an important pathogenetic role in human rabies encephalitis.

Furious and paralytic rabies of canine origin: neuroimaging with virological and cytokine studies

Furious and paralytic rabies differ in clinical manifestations and survival periods. The authors studied magnetic resonance imaging (MRI) and cytokine and virus distribution in rabies-infected dogs of both clinical types. MRI examination of the brain and upper spinal cord was performed in two furious and two paralytic dogs during the early clinical stage. Rabies viral nucleoprotein RNA and 18 cytokine mRNAs at 12 different brain regions were studied. Rabies viral RNA was examined in four furious and four paralytic dogs during the early stage, and in one each during the late stage. Cytokine mRNAs were examined in two furious and two paralytic dogs during the early stage and in one each during the late stage. Larger quantities of rabies viral RNA were found in the brains of furious than in paralytic dogs. Interleukin-1beta and interferon-gamma mRNAs were found exclusively in the brains of paralytic dogs during the early stage. Abnormal hypersignal T2 changes were found at hippocampus, hypothalamus, brainstem, and spinal cord of paralytic dogs. More widespread changes of less intensity were seen in furious dog brains. During the late stage of infection, brains from furious and paralytic rabid dogs were similarly infected and there were less detectable cytokine mRNAs. These results suggest that the early stage of furious dog rabies is characterized by a moderate inflammation (as indicated by MRI lesions and brain cytokine detection) and a severe virus neuroinvasiveness. Paralytic rabies is characterized by delayed viral neuroinvasion and a more intense inflammation than furious rabies. Dogs may be a good model for study of the host inflammatory responses that may modulate rabies virus neuroinvasiveness.

Proteomic profiling reveals that rabies virus infection results in differential expression of host proteins involved in ion homeostasis and synaptic physiology in the central nervous system

To understand how rabies virus (RV) infection results in neuronal dysfunction, the authors employed proteomics technology to profile host responses to RV infection. In mice infected with wild-type (wt) RV, the expression of proteins involved in ion homeostasis was altered. H(+) ATPase and Na(+)/K(+) ATPase were up-regulated whereas Ca(2+) ATPase was down-regulated, which resulted in reduction of the intracellular Na(+) and Ca(2+) concentrations. Furthermore, infection with wt RV resulted in down-regulation of soluble NSF attachment receptor proteins (SNAREs) such as alpha-synaptosome-associated protein (SNAP), tripartite motif-containing 9 (TRIM9), syntaxin, and pallidin, all of which are involved in docking and fusion of synaptic vesicles to and with presynaptic membrane. As a consequence, accumulation of synaptic vesicles was observed in the presynapses of mice infected with wt RV. These data demonstrate that infection with wt RV results in alteration of host protein expression, particularly those involved in ion homeostasis and docking and fusion of synaptic vesicles to presynaptic membrane, which may lead to neuronal dysfunction. On the other hand, attenuated RV up-regulated the expression of proteins involved in the induction of apoptosis, explaining why apoptosis is observed only in cells or animals infected with attenuated RV in previous studies.

Rabies

Despite increases in our understanding of rabies pathogenesis, it remains an inevitably fatal disease. Lack of awareness, low level of political commitment to rabies control, and failure to recognize and correlate clinical, laboratory, and neuroimaging features contribute to continuing deaths. Clinical symptomatology, once believed to be unique, may be variable, even in patients associated with lyssaviruses of the same genotype. This article discusses virus transport, the role of virus and host response mechanisms in relation to protean clinical manifestations, and mechanisms responsible for relative intactness of consciousness in human rabies. Differential involvement of the anterior horn cell in furious rabies and the peripheral nerve in paralytic rabies is summarized. Escape mechanisms from host defenses explain why a fatal outcome is unavoidable regardless of therapy. Neuroprotective treatment, using a coma-induction regimen, proves not to be beneficial. Survival of patients with excellent recovery relies on early innate and adaptive immunity plus adequate intensive care support.