Lyssavirus infection of muscle spindles and motor end plates in striated muscle of hamsters

Isolation of Rabies Virus from the Salivary Glands of Wild and Domestic Carnivores during a Skunk Rabies Epizootic

Rabies is a fatal zoonotic disease of global importance. Rabies virus is shed in the saliva of infected hosts and is primarily transmitted through bite contact. Canine rabies has been eliminated from the US, but wildlife constitutes more than 90% of the reported cases of animal rabies in the US each year. In the US, several wild carnivore species are reservoirs of distinct variants of rabies virus (RV). After decades of apparent absence, the south-central skunk (SCSK) RV variant was detected in Colorado in 2007 and resulted in a large-scale epizootic in striped skunk ( Mephitis mephitis) populations in northern Colorado starting in 2012. We attempted isolation of RV from salivary gland tissues from confirmed rabid carnivores, comprising 51 striped skunks and seven other wild and domestic carnivores collected during 2013 through 2015 in northern Colorado. We isolated RV from 84.0% (158/188; 95% confidence interval=78.1-88.6%) of striped skunk and 71% (17/24; 95% confidence interval =51-85%) of other carnivore salivary glands. These data suggested that infected reservoir and vector species were equally likely to shed the SCSK RV variant and posed a secondary transmission risk to humans and other animals.

Comparative pathogenesis of rabies in bats and carnivores, and implications for spillover to humans

Bat-acquired rabies is becoming increasingly common, and its diagnosis could be missed partly because its clinical presentation differs from that of dog-acquired rabies. We reviewed the scientific literature to compare the pathogenesis of rabies in bats and carnivores-including dogs-and related this pathogenesis to differences in the clinical presentation of bat-acquired and dog-acquired rabies in human beings. For bat-acquired rabies, we found that the histological site of exposure is usually limited to the skin, the anatomical site of exposure is more commonly the face, and the virus might be more adapted for entry via the skin than for dog-acquired rabies. These factors could help to explain several differences in clinical presentation between individuals with bat-acquired and those with dog-acquired rabies. A better understanding of these differences should improve the recording of a patient's history, enable drawing up of a more sophisticated list of clinical characteristics, and therefore obtain an earlier diagnosis of rabies after contact with a bat or carnivore that has rabies.

Genetic diseases conferring resistance to infectious diseases

This review considers available evidence for mechanisms of conferred adaptive advantages in the face of specific infectious diseases. In short, we explore a number of genetic conditions, which carry some benefits in adverse circumstances including exposure to infectious agents. The examples discussed are conditions known to result in resistance to a specific infectious disease, or have been proposed as being associated with resistance to various infectious diseases. These infectious disease—genetic disorder pairings include malaria and hemoglobinopathies, cholera and cystic fibrosis, tuberculosis and Tay-Sachs disease, mycotic abortions and phenylketonuria, infection by enveloped viruses and disorders of glycosylation, infection by filoviruses and Niemann–Pick C1 disease, as well as rabies and myasthenia gravis. We also discuss two genetic conditions that lead to infectious disease hypersusceptibility, although we did not cover the large number of immunologic defects leading to infectious disease hypersusceptibilities. Four of the resistance-associated pairings (malaria/hemogloginopathies, cholera/cystic fibrosis, tuberculosis/Tay-Sachs, and mycotic abortions/phenylketonuria) appear to be a result of selection pressures in geographic regions in which the specific infectious agent is endemic. The other pairings do not appear to be based on selection pressure and instead may be serendipitous. Nonetheless, research investigating these relationships may lead to treatment options for the aforementioned diseases by exploiting established mechanisms between genetically affected cells and infectious organisms. This may prove invaluable as a starting point for research in the case of diseases that currently have no reliably curative treatments, e.g., HIV, rabies, and Ebola.

Intravenous inoculation of a bat-associated rabies virus causes lethal encephalopathy in mice through invasion of the brain via neurosecretory hypothalamic fibers

The majority of rabies virus (RV) infections are caused by bites or scratches from rabid carnivores or bats. Usually, RV utilizes the retrograde transport within the neuronal network to spread from the infection site to the central nervous system (CNS) where it replicates in neuronal somata and infects other neurons via trans-synaptic spread. We speculate that in addition to the neuronal transport of the virus, hematogenous spread from the site of infection directly to the brain after accidental spill over into the vascular system might represent an alternative way for RV to invade the CNS. So far, it is unknown whether hematogenous spread has any relevance in RV pathogenesis. To determine whether certain RV variants might have the capacity to invade the CNS from the periphery via hematogenous spread, we infected mice either intramuscularly (i.m.) or intravenously (i.v.) with the dog-associated RV DOG4 or the silver-haired bat-associated RV SB. In addition to monitoring the progression of clinical signs of rabies we used immunohistochemistry and quantitative reverse transcription polymerase chain reaction (qRT-PCR) to follow the spread of the virus from the infection site to the brain. In contrast to i.m. infection where both variants caused a lethal encephalopathy, only i.v. infection with SB resulted in the development of a lethal infection. While qRT-PCR did not reveal major differences in virus loads in spinal cord or brain at different times after i.m. or i.v. infection of SB, immunohistochemical analysis showed that only i.v. administered SB directly infected the forebrain. The earliest affected regions were those hypothalamic nuclei, which are connected by neurosecretory fibers to the circumventricular organs neurohypophysis and median eminence. Our data suggest that hematogenous spread of SB can lead to a fatal encephalopathy through direct retrograde invasion of the CNS at the neurovascular interface of the hypothalamus-hypophysis system. This alternative mode of virus spread has implications for the post exposure prophylaxis of rabies, particularly with silver-haired bat-associated RV.

Rabies virus (RV) infects mammalian neurons and cycles in regionally distinct animal populations such as the red fox in Europe, domestic canines in Asia, or raccoons, skunks and bats in Northern America. Although human rabies can be prevented by pre- and post-exposure prophylaxis, more than 50,000 people die annually from the severe encephalopathy caused by RV. Recently, two cases of RV transmission by organ transplantation were reported. In our study, using intravenous inoculation of mice, we evaluated the pathogenetic relevance of virions that reach the bloodstream. Mice inoculated intravenously with a canine-derived RV survived the infection in contrast to intramuscularly injected mice, while mice infected with a silver-haired bat-related RV succumbed to the disease regardless of the route of inoculation. We found that the silver-haired bat-related RV was able to transit from the blood to the brain by invading neurosecretory fibers of the hypothalamus, which form neurohemal synapses lacking a blood-brain-barrier. This newly described route of brain invasion might reflect how RV reached the central nervous system from transplanted organs, since it takes longer to establish the neural connections between host and grafted tissue necessary for classical RV migration than the time until the infection became symptomatic in the two reported cases.

The histopathogenesis of paralytic rabies in six-week-old C57BL/6J mice following inoculation of the CVS-11 strain into the right triceps surae muscle

A fatal encephalomyelitis was developed after intracerebral and hind limb inoculation of in 6-week-old C57BL/6J mice by the inoculation of fixed rabies virus (CVS-11 strain), intracerebrally and into hind. After the intracerebral inoculation, virus antigens were detected in the cerebral cortex and hippocampus at 2 days postinoculation (PI), and later spread centrifugally to thalamus, brain stem, cerebellum, spinal cord and spinal ganglia. At 4 days PI, severe apoptosis and DNA fragmentation were observed in the hippocampus and cerebral cortex. All mice infected intracerebrally were dead without limb paralysis at from 10 to 11 days PI. In contrast, mice infected with virus intramuscularly were persistently observed virus antigens in the myocytes at the site of inoculation from 2 days PI. At 4 days PI, the antigens were demonstrated in the spinal dorsal root ganglia, spinal cord and muscle spindles without their detection in the cerebrum and hippocampus. There were no apoptosis in the spinal cord and dorsal root ganglia, however hind limb paralysis was found in all infected mice. Hind limb paralysis was progressed to quadriparalysis, and mice were dead from 11 to 13 days PI. From 4 days PI, necrosis of neuron was observed in the the spinal and dorsal ganglia with infiltration of lymphocyte. This study suggested that the necrosis of spinal neurons was more important to cause the paralysis of hind limb rather than the severe cerebral infection and apoptosis in C57BL/6J mice infected with CVS-11 strain. The virus primarily replicated in the muscles was ascended the spinal cord via afferent fibers and retrogradely invaded the cerebrum, and with subsequent spread to muscle spindles.

Molecular mechanisms of virus spread and virion components as tools of virulence. A review

Despite of differences in replication strategy among virus families, some basic principles have remained similar. Analogous mechanisms govern virus entry into cells and the use of enzymes which direct the replication of the virus genome. The function of many cell surface receptors (such as glycosoaminoglycans, glycoproteins, proteins) which interact with viral capsid proteins or envelope glycoproteins has recently been elucidated. The list of cellular receptors (Table I) is still far from being final. The capsid components, similarly as the envelope glycoproteins, may form specific pocket like sites, which interact with the cell surface receptors. Neutralizing antibodies usually react with antigenic domains adjacent to the receptor binding site(s) and hamper the close contact inevitable for virion attachment. In the case of more complex viruses, such as herpes simplex virus, different viral glycoproteins interact with several cellular receptors. At progressed phase of adsorption the virions are engulfed into endocytic vesicles and the virion fusion domain(s) become(s) activated. The outer capsid components of reoviruses which participate in adsorption and fusion may get activated already in the lumen of digestive tract, i.e. before their engulfment by resorptive epithelium cells. Activation of the hydrophobic fusion domain(s) is a further important step allowing to pass through the lipid bilayer when penetrating the cell membrane in order to reach the cytosol. Activation of the virion fusion domain is accomplished by a conformation change, which occurs at acid pH (influenza virus hemagglutinin, sigma 1 protein of the reovirus particle) and/or after protease treatment. The herpes simplex virus fusion factors (gD and gH) undergo conformation changes by a pH-independent mechanism triggered due to interaction with the cell surface receptor(s) and mediated by mutual interactions with the viral envelope glycoproteins. The virion capsid or envelope components participating in the entry and membrane fusion are not the only tools of virulence. The correct function of virus coded proteins, which participate in replication of the viral genome, and/or in the supply of necessary nucleotides, may be very essential. In the case of enteroviruses, which RNA interacts with ribosomes directly, the correct configuration of the non-coding viral RNA sequence is crucial for initiation of translation occurring in the absence of the classical "cap" structure.

Rabies virus entry at the neuromuscular junction in nerve-muscle cocultures

Early events in rabies virus entry into neurons were investigated in chick spinal cord-muscle cocultures. Rabies virus (CVS strain) was adsorbed to the surface of cells in the cold. At times up to 10 min of warming to 37 degrees C, virus was most intensely localized to dense swellings on the myotube surface. Texas Red-labeled alpha-bungarotoxin, which binds to nicotinic acetylcholine receptors, colocalized precisely with virus at the densities identifying these regions as neuromuscular junctions. Rabies virus also colocalized in the junctions with synapsin I, a marker for synaptic vesicles. The endosome tracers Lucifer Yellow, Texan Red-dextran, and rhodamine-wheat germ agglutinin were added to the cultures at the end of the virus adsorption period and the cultures were warmed. At 10 min, rabies virus and tracers colocalized at neuromuscular junctions and nerve terminals. At 30 min, rabies virus and tracers showed more intense fluorescence over nerve fibers and nerve cell bodies. At 60 min, nerve terminals, nerve fibers, and nerve cell bodies showed intense fluorescence and colocalization for rabies virus and tracers. LysoTracker Red, a marker for acidic compartments, colocalized with rabies virus at nerve-muscle contacts. These findings show that in nerve-muscle cocultures, the neuromuscular junction is the major site of entry into neurons. Colocalization of virus and endosome tracers within nerve terminals indicates that virus resides in an early endosome compartment, some of which are acidified. The progressive increase of virus and tracers in nerve fibers and nerve cell bodies over time is consistent with retrograde transport of endocytosed virus from the motor nerve terminal.

Low-affinity nerve-growth factor receptor (P75NTR) can serve as a receptor for rabies virus

A random-primed cDNA expression library constructed from the mRNA of neuroblastoma cells (NG108) was used to clone a specific rabies virus (RV) receptor. A soluble form of the RV glycoprotein (Gs) was utilized as a ligand to detect positive cells. We identified the murine low-affinity nerve-growth factor receptor, p75NTR. BSR cells stably expressing p75NTR were able to bind Gs and G-expressing lepidopteran cells. The ability of the RV glycoprotein to bind p75NTR was dependent on the presence of a lysine and arginine in positions 330 and 333 respectively of antigenic site III, which is known to control virus penetration into motor and sensory neurons of adult mice. P75NTR-expressing BSR cells were permissive for a non-adapted fox RV isolate (street virus) and nerve growth factor (NGF) decreased this infection. In infected cells, p75NTR associates with the RV glycoprotein and could be precipitated with anti-G monoclonal antibodies. Therefore, p75NTR is a receptor for street RV.

Characterization of protein involvement in rabies virus binding to BHK-21 cells

Prior studies established the specificity of rabies virus receptors on BHK-21 cells based on the saturability of the receptors and on competitive binding. In the present study, we used protease-treated cells to identify the involvement of protein in the specific binding of rabies virus to these cells. In addition, biochemical characterization of n-octylglucoside solubilized BHK-21 plasma membranes demonstrated the involvement of a protease sensitive, heat insensitive, integral membrane protein or protein complex in rabies virus binding to these cells. The membrane component that binds rabies virus is associated with a high molecular weight fraction of the n-octylglucoside-plasma membrane extract isolated by gel filtration. This high molecular weight fraction (approximately 450 KDa) is enriched with a cell surface integral membrane component that comigrates with denatured bovine serum fibronectin (220 KDa). This cellular component did not bind polyclonal antisera to fibronectin in Western blot (native or denatured) or immunoprecipitation experiments. Direct and specific virus binding to high molecular weight plasma membrane protein(s) separated on Western blots further confirmed the role of a protein receptor in rabies virus binding to these cells.

Evidence from the anti-idiotypic network that the acetylcholine receptor is a rabies virus receptor

We have developed idiotype-anti-idiotype monoclonal antibodies that provide evidence for rabies virus binding to the acetylcholine receptor (AChR). Hybridoma cell lines 7.12 and 7.25 resulted after fusion of NS-1 myeloma cells with spleen cells from a BALB/c mouse immunized with rabies virus strain CVS. Antibody 7.12 reacted with viral glycoprotein and neutralized virus infectivity in vivo. It also neutralized infectivity in vitro when PC12 cells, which express neuronal AChR, but not CER cells or neuroblastoma cells (clone N18), which have no AChR, were used. Antibody 7.25 reacted with nucleocapsid protein. Anti-idiotypic monoclonal antibody B9 was produced from fusion of NS-1 cells with spleen cells from a mouse immunized with 7.12 Fab. In an enzyme-linked immunosorbent assay and immunoprecipitation, B9 reacted with 7.12, polyclonal rabies virus immune dog serum, and purified AChR. The binding of B9 to 7.12 and immune dog serum was inhibited by AChR. B9 also inhibited the binding of 7.12 to rabies virus both in vitro and in vivo. Indirect immunofluorescence revealed that B9 reacted at neuromuscular junctions of mouse tissue. B9 also reacted in indirect immunofluorescence with distinct neurons in mouse and monkey brain tissue as well as with PC12 cells. B9 staining of neuronal elements in brain tissue of rabies virus-infected mice was greatly reduced. Rabies virus inhibited the binding of B9 to PC12 cells. Mice immunized with B9 developed low-titer rabies virus-neutralizing antibody. These mice were protected from lethal intramuscular rabies virus challenge. In contrast, anti-idiotypic antibody raised against nucleocapsid antibody 7.25 did not react with AChR.

Human immune response to rabies nucleocapsid and glycoprotein antigens

Antibodies to two components of rabies virus, nucleocapsid (N) and glycoprotein (G), were compared in 11 rabies patients with those in nine recipients of Vero cell rabies vaccine. All rabies vaccinees had antibodies to N and G components by day 10 after the first vaccine injection. A similar but not identical response was observed in three out of 11 rabies patients. Serum antibodies appeared in rabies patients as early as 3 days after onset of the first symptoms of the disease. In these antibody-positive rabies patients, levels of both antibodies, but particularly of anti-N antibody, were lower than in the vaccinated group. Our results suggest that the process of immune recognition and of antibody development in human rabies is more likely to occur early in the pre-clinical phase, and that reactivity to N protein may be crucial for elicitation of neutralizing antibody.

Rabies virus infection of cultured rat sensory neurons

The axonal transport of rabies virus (challenge virus strain of fixed virus) was studied in differentiated rat embryonic dorsal root ganglion cells. In addition, we observed the attachment of rabies virus to neuronal extensions and virus production by infected neurons. A compartmentalized cell culture system was used, allowing infection and manipulation of neuronal extensions without exposing the neural soma to the virus. The cultures consisted of 60% large neuronal cells whose extensions exhibited neurofilament structures. Rabies virus demonstrated high binding affinity to unmyelinated neurites, as suggested by assays of virus adsorption and immunofluorescence studies. The rate of axoplasmic transport of virus was 12 to 24 mm/day, including the time required for internalization of the virus into neurites. The virus transport could be blocked by cytochalasin B, vinblastine, and colchicine, none of which negatively affected the production of virus in cells once the infection was established. It was concluded that, for the retrograde transfer of rabies virus by neurites from the periphery to the neuronal soma, the integrity of tubulin- and actin-containing structures is essential. The rat sensory neurons were characterized as permissive, moderately susceptible, but low producers of rabies virus. These neurons were capable of harboring rabies virus for long periods of time and able to release virus into the culture medium without showing any morphological alterations. The involvement of sensory neurons in rabies virus pathogenesis, both in viral transport and as a site for persistent viral infection, is discussed.

Rabies virus binding to cellular membranes measured by enzyme immunoassay

The binding of rabies virus to cellular membranes was measured using an enzyme-linked immunosorbent assay (ELISA). Virus binding to membranes adsorbed to the wells of microtiter plates was detected with rabies virus antibody and alkaline phosphatase-linked second antibody. The greatest degree of binding was to myotube, neuroblastoma, and salivary gland membranes; intermediate levels occurred in striated muscle and nerve membranes; and low levels of binding were found in other membranes, including those of most parenchymal organs. Binding of rabies virus to myotube membranes was saturable, dependent on pH (with an optimum of pH 6.0), facilitated by the divalent cations Ca++, Mn++, and Mg++, and was temperature dependent. Binding was greatly reduced by inactivation of virus with beta-propiolactone or treatment of virus with trypsin. In embryonic chick myotubes, total acetylcholine receptor content and acetylcholinesterase activity undergo marked changes during development, first increasing and then decreasing at the time of hatching. Binding of rabies virus followed a similar pattern, indicating that the virus may interact with the acetylcholine receptor or other surface molecules undergoing similar developmental changes.

Rabies virus binding at neuromuscular junctions

Morphological, immunocytochemical, biochemical, and immunological techniques have been used to describe rabies virus binding to a sub-cellular unit and molecular complex at the neuromuscular junction (NMJ). Early after infection in vivo, virus antigen and virus particles were found by immunofluorescence, electron microscopy and immunoelectron microscopy in regions of high density acetylcholine receptors (AChR) at NMJs. One monoclonal antibody (alpha-Mab) to the alpha subunit of the AChR blocked attachment of radio-labeled rabies virus to cultured muscle cells bearing high density patches of AChR. A sub-cellular structure, resembling an array of AChR monomers, bound both rabies virus antigens and alpha-Mab. By immunoblotting with electrophoretically transferred motor endplate proteins, rabies virus proteins and alpha-Mab bound to two proteins of 43 000 and 110 000 daltons. A rabies virus glycoprotein antibody detected virus antigen bound to the 110 000 dalton protein. An auto-immune (anti-idiotypic) response followed immunization of mice with rabies virus glycoprotein antigen; the antibody was directed to the 110 000 dalton protein. This auto-antibody altered the kinetics of neutralization by rabies virus antibody and induced the formation of rabies virus antibody after inoculation of mice. These results define, at the neuromuscular junction, a rabies virus receptor which may be part of the acetylcholine receptor complex.

Is the acetylcholine receptor a rabies virus receptor?

Rabies virus was found on mouse diaphragms and on cultured chick myotubes in a distribution coinciding with that of the acetylcholine receptor. Treatment of the myotubes with alpha-bungarotoxin and d-tubocurarine before the addition of the virus reduced the number of myotubes that became infected with rabies virus. These findings together suggest that acetylcholine receptors may serve as receptors for rabies virus. The binding of virus to acetylcholine receptors, which are present in high density at the neuromuscular junction, would provide a mechanism whereby the virus could be locally concentrated at sites in proximity to peripheral nerves facilitating subsequent uptake and transfer to the central nervous system.