Canine distemper virus (CDV)-specific immune response was measured in different dog populations. Three groups of vaccinated or wild-type virus exposed dogs were tested: dogs with a known vaccination history, dogs without a known vaccination history (shelter dogs), and dogs with potential exposure to wild-type CDV. The use of a T-cell proliferation assay demonstrated a detectable CDV-specific T-cell response from both spleen and blood lymphocytes of dogs. Qualitatively, antibody assays [enzyme-linked immunosorbent assay (ELISA) and neutralization assay] predicted the presence of a T-cell response well, although quantitatively neither antibody assays nor the T-cell assay correlated well with each other. An interesting finding from our study was that half of the dogs in shelters were not vaccinated (potentially posing a public veterinary health problem) and that antibody levels in dogs living in an environment with endemic CDV were lower than in vaccinated animals.
We have studied the immune responses to the two glycoproteins of the Morbillivirus canine distemper virus (CDV) after DNA vaccination of BALB/c mice. The plasmids coding for both CDV hemagglutinin (H) and fusion protein (F) induce high levels of antibodies which persist for more than 6 months. Intramuscular inoculation of the CDV DNA induces a predominantly immunoglobulin G2a (IgG2a) response (Th1 response), whereas gene gun immunization with CDV H evokes exclusively an IgG1 response (Th2 response). In contrast, the CDV F gene elicited a mixed, IgG1 and IgG2a response. Mice vaccinated (by gene gun) with either the CDV H or F DNA showed a class I-restricted cytotoxic lymphocyte response. Immunized mice challenged intracerebrally with a lethal dose of a neurovirulent strain of CDV were protected. However, approximately 30% of the mice vaccinated with the CDV F DNA became obese in the first 2 months following the challenge. This was not correlated with the serum antibody levels.
Canine distemper virus (CDV) is present worldwide and produces a lethal systemic infection of wild and domestic Canidae. Pre-existing antibodies acquired from vaccination or previous CDV infection might interfere the interpretation of a serologic diagnosis method. In addition, due to the high similarity of nucleic acid sequences between wild-type CDV and the new vaccine strain, current PCR derived methods cannot be applied for the definite confirmation of CD infection. Hence, it is worthy of developing a simple and rapid nucleotide-based assay for differentiation of wild-type CDV which is a cause of disease from attenuated CDVs after vaccination. High frequency variations have been found in the region spanning from the 3'-untranslated region (UTR) of the matrix (M) gene to the fusion (F) gene (designated M-F UTR) in a few CDV strains. To establish a differential diagnosis assay, an amplification refractory mutation analysis was established based on the highly variable region on M-F UTR and F regions.
Sequences of frequent polymorphisms were found scattered throughout the M-F UTR region; the identity of nucleic acid between local strains and vaccine strains ranged from 82.5% to 93.8%. A track of AAA residue located 35 nucleotides downstream from F gene start codon highly conserved in three vaccine strains were replaced with TGC in the local strains; that severed as target sequences for deign of discrimination primers. The method established in the present study successfully differentiated seven Taiwanese CDV field isolates, all belonging to the Asia-1 lineage, from vaccine strains.
The method described herein would be useful for several clinical applications, such as confirmation of nature CDV infection, evaluation of vaccination status and verification of the circulating viral genotypes.
Reverse transcription-PCR (RT-PCR) was used to detect canine distemper virus (CDV) nucleoprotein (NP) RNA in serum, whole blood, and cerebrospinal fluid (CSF) samples from 38 dogs with clinically suspected distemper. Results were correlated to clinical findings, anti-CDV neutralizing antibody titers, postmortem findings, and demonstration of CDV NP antigen by immunohistochemistry. The specificity of the RT-PCR was ensured by amplification of RNA from various laboratory CDV strains, restriction enzyme digestion, and Southern blot hybridization. In 29 of 38 dogs, CDV infection was confirmed by postmortem examination and immunohistochemistry. The animals displayed the catarrhal, systemic, and nervous forms of distemper. Seventeen samples (serum, whole blood, or CSF) from dogs with distemper were tested with three sets of primers targeted to different regions of the NP gene of the CDV Onderstepoort strain. Expected amplicons were observed in 82, 53, and 41% of the 17 samples, depending upon the primer pair used. With the most sensitive primer pair (primer pair I), CDV NP RNA was detected in 25 of 29 (86%) serum samples and 14 of 16 (88%) whole blood and CSF samples from dogs with distemper but not in body fluids from immunohistochemically negative dogs. Nucleotide sequence analysis of five RT-PCR amplicons from isolates from the field revealed few silent point mutations. These isolates exhibited greater homology to the Rockborn (97 to 99%) than to the Onderstepoort (95 to 96%) CDV strain. In summary, although the sensitivity of the RT-PCR for detection of CDV is strongly influenced by the location of the selected primers, this nucleic acid detection system represents a highly specific and sensitive method for the antemortem diagnosis of distemper in dogs, regardless of the form of distemper, humoral immune response, and viral antigen distribution.
Dogs that are persistently infected or that become moribund after exposure to canine distemper virus (CDV) have antibody that neutralized CDV when tested in dog lung macrophage cultures but failed to neutralize CDV when tested in epithelial, fibroblastic, or lymphatic cells. The antibody attached to protein A and was found in the immunoglobulin G fraction. The antibody bound complement and lysed CDV-infected target cells. The neutralizing activity in macrophages could be abolished (i) by pepsin digestion and removal of Fc portions from the antibody, (ii) by blocking the Fc receptors of macrophages with heat-treated normal dog serum, and (iii) by binding of protein A to Fc portions of the antibody. It was concluded that attachment of the CDV-antibody complex to Fc receptors of macrophages was essential for virus neutralization. If this attachment was hindered, the CDV-antibody complex became infectious for macrophages. In contrast, serum from recovering dogs neutralized CDV when tested in epithelial, fibroblastic, or lymphatic cells as well as in macrophages.
Theoretically, homogeneous environments favor the evolution of specialists whereas heterogeneous environments favor generalists. Canine distemper is a multi-host carnivore disease caused by canine distemper virus (CDV). The described cell receptor of CDV is SLAM (CD150). Attachment of CDV hemagglutinin protein (CDV-H) to this receptor facilitates fusion and virus entry in cooperation with the fusion protein (CDV-F). We investigated whether CDV strains co-evolved in the large, homogeneous domestic dog population exhibited specialist traits, and strains adapted to the heterogeneous environment of smaller populations of different carnivores exhibited generalist traits. Comparison of amino acid sequences of the SLAM binding region revealed higher similarity between sequences from Canidae species than to sequences from other carnivore families. Using an in vitro assay, we quantified syncytia formation mediated by CDV-H proteins from dog and non-dog CDV strains in cells expressing dog, lion or cat SLAM. CDV-H proteins from dog strains produced significantly higher values with cells expressing dog SLAM than with cells expressing lion or cat SLAM. CDV-H proteins from strains of non-dog species produced similar values in all three cell types, but lower values in cells expressing dog SLAM than the values obtained for CDV-H proteins from dog strains. By experimentally changing one amino acid (Y549H) in the CDV-H protein of one dog strain we decreased expression of specialist traits and increased expression of generalist traits, thereby confirming its functional importance. A virus titer assay demonstrated that dog strains produced higher titers in cells expressing dog SLAM than cells expressing SLAM of non-dog hosts, which suggested possible fitness benefits of specialization post-cell entry. We provide in vitro evidence for the expression of specialist and generalist traits by CDV strains, and fitness trade-offs across carnivore host environments caused by antagonistic pleiotropy. These findings extend knowledge on CDV molecular epidemiology of particular relevance to wild carnivores.
Canine distemper virus (CDV), a lymphotropic and neurotropic negative-stranded RNA virus of the Morbillivirus genus, causes a life-threatening disease in several carnivores, including domestic dogs. To identify the cellular receptor(s) involved in the uptake of CDV by susceptible cells, we isolated a monoclonal antibody (MAb K41) which binds to the cell surface and inhibits the CDV infection of several cell lines from various species. Pretreatment of cells with MAb K41 reduces the number of infectious centers and the size of the syncytia. Using affinity chromatography with MAb K41, we purified from HeLa and Vero cell extracts a 26-kDa protein which contained the amino acid sequence TKDEPQRETLK of human CD9, a member of the tetraspan transmembrane or transmembrane 4 superfamily of cell surface proteins. Transfection of NIH 3T3 or MDBK cells with a CD9 expression plasmid rendered these cells permissive for viral infection and raised virus production by a factor of 10 to 100. The mechanism involved is still unclear, since we were unable to detect direct binding of CDV to CD9 by using immunoprecipitation and a virus overlay protein binding assay. These findings indicate that human CD9 and its homologs in other species are necessary factors for the uptake of CDV by target cells, the formation of syncytia, and the production of progeny virus.
Canine distemper virus (CDV) and measles virus (MV) cause severe illnesses in their respective hosts. The viruses display a characteristic cytopathic effect by forming syncytia in susceptible cells. For CDV, the proficiency of syncytium formation varies among different strains and correlates with the degree of viral attenuation. In this study, we examined the determinants for the differential fusogenicity of the wild-type CDV isolate 5804Han89 (CDV5804), the small- and large-plaque-forming variants of the CDV vaccine strain Onderstepoort (CDVOS and CDVOL, respectively), and the MV vaccine strain Edmonston B (MVEdm). The cotransfection of different combinations of fusion (F) and hemagglutinin (H) genes in Vero cells indicated that the H protein is the main determinant of fusion efficiency. To verify the significance of this observation in the viral context, a reverse genetic system to generate recombinant CDVs was established. This system is based on a plasmid containing the full-length antigenomic sequence of CDVOS. The coding regions of the H proteins of all CDV strains and MVEdm were introduced into the CDV and MV genetic backgrounds, and recombinant viruses rCDV-H5804, rCDV-HOL, rCDV-HEdm, rMV-H5804, rMV-HOL, and rMV-HOS were recovered. Thus, the H proteins of the two morbilliviruses are interchangeable and fully functional in a heterologous complex. This is in contrast with the glycoproteins of other members of the family Paramyxoviridae, which do not function efficiently with heterologous partners. The fusogenicity, growth characteristics, and tropism of the recombinant viruses were examined and compared with those of the parental strains. All these characteristics were found to be predominantly mediated by the H protein regardless of the viral backbone used.
Natural and immune cytolysis of canine distemper virus (CDV)-infected target cells in vitro is described. Lymphocytes expressing natural cytotoxicity were found in specific-pathogen-free beagle dogs and in beagle-coonhound crosses before vaccination with CDV and indefinitely after vaccination, when the ephemeral immune lymphocyte-mediated cytotoxicity (ILMC) had declined. In contrast to the natural lymphocyte-mediated cytotoxicity, the ILMC was genetically restricted, could not be blocked by CDV-specific antibody, and was effective against measles virus-infected as well as CDV-infectd target cells. Lymphocyte populations were depleted of Fc receptor and surface immunoglobulin-bearing cells by rosetting techniques and tested in comparison. An antibody-dependent cell-mediated cytotoxicity was demostrated against CDV-infected target cells that were preincubated with CDV antibody when Fc receptor-bearing lymphocytes were not removed. The ILMC was measurable for approximately 10 days beginning at 6 days post-vaccination. In contrast, CDV antibody measured by virus neutralization and humoral cytotoxicity was detectable by 6 days postvaccination and persisted at peak levels for at least 5 months.
Canine distemper virus (CDV) causes a life-threatening disease in several carnivores including domestic dogs. Recently, we identified a molecule, CD9, a member of the tetraspan transmembrane protein family, which facilitates, and antibodies to which inhibit, the infection of tissue culture cells with CDV (strain Onderstepoort). Here we describe that an anti-CD9 monoclonal antibody (MAb K41) did not interfere with binding of CDV to cells and uptake of virus. In addition, in single-step growth experiments, MAb K41 did not induce differences in the levels of viral mRNA and proteins. However, the virus release of syncytium-forming strains of CDV, the virus-induced cell-cell fusion in lytically infected cultures, and the cell-cell fusion of uninfected with persistently CDV-infected HeLa cells were strongly inhibited by MAb K41. These data indicate that anti-CD9 antibodies selectively block virus-induced cell-cell fusion, whereas virus-cell fusion is not affected.
Morbilliviruses, including measles and canine distemper virus (CDV), are nonsegmented, negative-stranded RNA viruses that cause severe diseases in humans and animals. The transcriptional units in their genomes are separated by untranslated regions (UTRs), which contain essential transcription and translation signals. Due to its increased length, the region between the matrix (M) protein and fusion (F) protein open reading frames is of particular interest. In measles virus, the entire F 5′ region is untranslated, while several start codons are found in most other morbilliviruses, resulting in a long F protein signal peptide (Fsp). To characterize the role of this region in morbillivirus pathogenesis, we constructed recombinant CDVs, in which either the M-F UTR was replaced with that between the nucleocapsid (N) and phosphoprotein (P) genes, or 106 Fsp residues were deleted. The Fsp deletion alone had no effect in vitro and in vivo. In contrast, substitution of the UTR was associated with a slight increase in F gene and protein expression. Animals infected with this virus either recovered completely or experienced prolonged disease and death due to neuroinvasion. The combination of both changes resulted in a virus with strongly increased F gene and protein expression and complete attenuation. Taken together, our results provide evidence that the region between the morbillivirus M and F genes modulates virulence through transcriptional control of the F gene expression.
Canine distemper virus (CDV), a negative-strand RNA morbillivirus, causes a progressive demyelinating disease in which virus persistence plays an essential role. The antiviral immune response leads to virus clearance in the inflammatory lesions. However, CDV can replicate and persist outside these inflammatory lesions within the brain. How CDV is capable of persisting in the presence of an effective antiviral immune response is poorly understood. In the present investigation, we studied several aspects of virus replication in primary dog brain cell cultures (DBCC), comparing an attenuated CDV strain and a virulent CDV strain. Confluent DBCC were infected with either virulent A75/17-CDV or attenuated Onderstepoort-CDV and monitored for 60 days. Persistence was not associated with defective virus production, because all mRNAs and corresponding proteins were continuously expressed in the noncytolytic infection. Quantitative measurements did not detect a difference between the two types of infection in the rate of virus transcription and protein synthesis at the level of the single cell. However, electron microscopy and virus titration experiments showed that in the persistent CDV infection virus budding is strongly limited compared with that of the attenuated virus. Morphometry and immunocytochemistry showed profound differences in the way the two viruses spread in the culture. The attenuated CDV spread randomly to immediately adjacent cells, whereas persistent CDV spread selectively to more-distant cells by way of cell processes. In conclusion, the present study supports a mechanism of CDV persistence through selective spread by way of cell processes, enabling virulent CDV to invade the central nervous system without the need of releasing much virus into the extracellular space.
Canine distemper virus (CDV) has been rescued from a full-length cDNA clone. Besides Measles virus (MV) and Rinderpest virus, a third morbillivirus is now available for genetic analysis using reverse genetics. A plasmid p(+)CDV was constructed by sequential cloning using the Onderstepoort vaccine strain large-plaque-forming variant. The presence of a T7 promoter allowed transcription of full-length antigenomic RNA by a T7 RNA polymerase, which was provided by a host range mutant of vaccinia virus (MVA-T7). Plasmids expressing the nucleocapsid protein, the phosphoprotein, and the viral RNA-dependent RNA polymerase, also under control of a T7 promoter, have been generated. Infection of HeLa cells with MVA-T7 and subsequent transfection of p(+)CDV plus the helper plasmids led to syncytium formation and release of infectious recombinant (r) CDV. Comparison of the rescued virus with the parental virus revealed no major differences in the progression of infection or in the shape and size of syncytia. A genetic tag, consisting of two nucleotide changes within the coding region of the L protein, has been identified in the rCDV genome. Expression by rCDV of all the major viral structural proteins has been demonstrated by immunofluorescence.
Canine distemper virus (CDV) infects many carnivores, including ferrets and dogs, and is the member of the Morbillivirus genus most easily amenable to experimentation in a homologous small-animal system. To gain insights into the determinants of CDV pathogenesis, we isolated a strain highly virulent for ferrets by repeated passaging in these animals. Sequence comparison of the genome of this strain with that of its highly attenuated precursor revealed 19 mutations distributed almost evenly in the six genes. We then recovered a virus from a cDNA copy of the virulent CDV strain's consensus sequence by using a modified reverse genetics system based on B cells. We infected ferrets with this virus and showed that it fully retained virulence as measured by the timing of rash appearance, disease onset, and death. Body temperature, leukocyte number, lymphocyte proliferation activity, and cell-associated viremia also had similar kinetics. We then addressed the question of the relative importance of the envelope and other viral constituents for virulence. Viruses in which the envelope genes (matrix, fusion, and hemagglutinin) of the virulent strain were combined with the other genes of the attenuated strain caused severe rash and fever even if the disease onset was delayed. Viruses in which the nucleocapsid, polymerase, and phosphoprotein genes (coding also for the V and C proteins) of the virulent strain were combined with the envelope genes of the attenuated strain caused milder signs of disease. Thus, virulence-inducing mutations have accumulated throughout the genome.
Signaling lymphocytic activation molecule (SLAM, CD150) is the universal morbillivirus receptor. Based on the identification of measles virus (MV) hemagglutinin (H) amino acids supporting human SLAM-dependent cell entry, we mutated canine distemper virus (CDV) H and identified residues necessary for efficient canine SLAM-dependent membrane fusion. These residues are located in two nearby clusters in a new CDV H structural model. To completely abolish SLAM-dependent fusion, combinations of mutations were necessary. We rescued a SLAM-blind recombinant CDV with six mutations that did not infect ferret peripheral blood mononuclear cells while retaining full infectivity in epithelial cells.
To characterize the importance of infection of epithelial cells for morbillivirus pathogenesis, we took advantage of the severe disease caused by canine distemper virus (CDV) in ferrets. To obtain a CDV that was unable to enter epithelial cells but retained the ability to enter immune cells, we transferred to its attachment (H) protein two mutations shown to interfere with the interaction of measles virus H with its epithelial receptor, human nectin-4. As expected for an epithelial receptor (EpR)-blind CDV, this virus infected dog and ferret epithelial cells inefficiently and did not cause cell fusion or syncytium formation. On the other hand, the EpR-blind CDV replicated in cells expressing canine signaling lymphocyte activation molecule (SLAM), the morbillivirus immune cell receptor, with similar kinetics to those of wild-type CDV. While ferrets infected with wild-type CDV died within 12 days after infection, after developing severe rash and fever, animals infected with the EpR-blind virus showed no clinical signs of disease. Nevertheless, both viruses spread rapidly and efficiently in immune cells, causing similar levels of leukopenia and inhibition of lymphocyte proliferation activity, two indicators of morbillivirus immunosuppression. Infection was documented for airway epithelia of ferrets infected with wild-type CDV but not for those of animals infected with the EpR-blind virus, and only animals infected with wild-type CDV shed virus. Thus, epithelial cell infection is necessary for clinical disease and efficient virus shedding but not for immunosuppression.
The mechanism of viral persistence, the driving force behind the chronic progression of inflammatory demyelination in canine distemper virus (CDV) infection, is associated with non-cytolytic viral cell-to-cell spread. Here, we studied the molecular mechanisms of viral spread of a recombinant fluorescent protein-expressing virulent CDV in primary canine astrocyte cultures. Time-lapse video microscopy documented that CDV spread was very efficient using cell processes contacting remote target cells. Strikingly, CDV transmission to remote cells could occur in less than 6 h, suggesting that a complete viral cycle with production of extracellular free particles was not essential in enabling CDV to spread in glial cells. Titration experiments and electron microscopy confirmed a very low CDV particle production despite higher titers of membrane-associated viruses. Interestingly, confocal laser microscopy and lentivirus transduction indicated expression and functionality of the viral fusion machinery, consisting of the viral fusion (F) and attachment (H) glycoproteins, at the cell surface. Importantly, using a single-cycle infectious recombinant H-knockout, H-complemented virus, we demonstrated that H, and thus potentially the viral fusion complex, was necessary to enable CDV spread. Furthermore, since we could not detect CD150/SLAM expression in brain cells, the presence of a yet non-identified glial receptor for CDV was suggested. Altogether, our findings indicate that persistence in CDV infection results from intracellular cell-to-cell transmission requiring the CDV-H protein. Viral transfer, happening selectively at the tip of astrocytic processes, may help the virus to cover long distances in the astroglial network, “outrunning” the host’s immune response in demyelinating plaques, thus continuously eliciting new lesions.
Demyelinating encephalitis; CDV persistence; Primary brain cells; Swift cell-to-cell spread; Attachment protein
A competitive enzyme-linked immunosorbent assay (cELISA), using two monoclonal antibodies (MAbs), was developed and compared with the standard virus neutralization test (VNT) for detecting antibodies against canine distemper virus (CDV) and phocine distemper virus (PDV) in sera from dogs and various species of marine mammals. The test depends on the blocking of MAb binding to solid-phase antigen in the presence of positive serum. Test conditions were optimized by using control VNT-negative and -positive sera specific for CDV and PDV. A positive cutoff value of 30% inhibition, which represents the mean cutoff of a VNT-negative population (n = 623) plus 2 standard deviations, was adopted for the test. A total of 736 serum samples were tested by the new cELISA and by the VNT as the “gold standard.” An unexpected but useful finding was the ability of this CDV- and PDV-specific cELISA to also detect antibodies against the related pair dolphin morbillivirus and porpoise morbillivirus. Based on a subpopulation of 625 sera used in statistical analyses, the overall sensitivity and specificity of cELISA relative to those of the VNT were 94.9 and 97.7%, respectively. Because the cELISA proved to be nearly as sensitive and specific as the VNT while being simpler and more rapid, it would be an adequate screening test for suspect CDV or PDV cases and would also be useful for epidemiological surveillance of morbilliviral infections in marine mammal populations.
Virulence of canine distemper virus (CDV) adapted to in vitro growth in Vero or bovine cells was determined by inoculation into CDV-susceptible neonatal gnotobiotic dogs. When compared with dogs given virulent R252-CDV, Vero R252-CDV was attenuated at passage level 14. In contrast, dogs inoculated with bovine R252-CDV at the same passage level experienced rapid fatal neurological disease. Virulence was not linked to ability to infect or replicate in canine pulmonary macrophage cultures. Retention of virulence by bovine R252-CDV is unique and worthy of further study.
Canine distemper is a highly contagious viral disease caused by the canine distemper virus (CDV), which is a member of the Morbillivirus genus, Paramyxoviridae family. Animals that most commonly suffer from this disease belong to the Canidae family; however, the spectrum of natural hosts for CDV also includes several other families of the order Carnivora. The infectious disease presents worldwide distribution and maintains a high incidence and high levels of lethality, despite the availability of effective vaccines, and no specific treatment. CDV infection in dogs is characterized by the presentation of systemic and/or neurological courses, and viral persistence in some organs, including the central nervous system (CNS) and lymphoid tissues. An elucidation of the pathogenic mechanisms involved in canine distemper disease will lead to a better understanding of the injuries and clinical manifestations caused by CDV. Ultimately, further insight about this disease will enable the improvement of diagnostic methods as well as therapeutic studies.
This report describes the naturally occurring atypical neuropathological manifestation of systemic canine distemper virus (CDV) infection in two 16-day-old Pit Bull pups. CDV-induced changes affected the gray and white matter of the forebrain while sparing the hindbrain. Histologically, there was necrosis with destruction of the nervous parenchyma due to an influx of inflammatory and reactive cells associated with eosinophilic intranuclear inclusion bodies within glial cells. Positive immunoreactivity against CDV antigens was predominantly observed within astrocytes and neurons. RT-PCR was used to amplify CDV-specific amplicons from brain fragments. These findings suggest the participation of CDV in the etiopathogenesis of these lesions.
canine distemper virus; immunohistochemistry; neuropathology; RT-PCR
A simple and rapid (24-h) assay for peripheral blood lymphocyte-associated immunity to canine distemper virus (CDV) is described. The test is based upon leukocyte-associated inhibition of CDV-induced syncytia formation in Vero cells. The technique quantitates the response morphologically, thereby eliminating the requirement for release of radiolabeled compounds. Positive results were determined from specific-pathogen-free and gnotobiotic dogs exposed to CDV via hyperimmunization, vaccination with a modified live virus vaccine, and after virulent virus infection. Preinoculation lymphocytes and lymphocytes from non-immune dogs did not inhibit CDV-induced syncytia formation. Maximum responses were observed 7 to 21 days after initial exposure and declined thereafter. The method can be used to further investigate the role of immune lymphocytes in the recovery from CDV infection.
Paramyxovirus glycoproteins are posttranslationally modified by the addition of N-linked glycans, which are often necessary for correct folding, processing, and cell surface expression. To establish the contribution of N glycosylation to morbillivirus attachment (H) protein function and overall virulence, we first determined the use of the potential N-glycosylation sites in the canine distemper virus (CDV) H proteins. Biochemical characterization revealed that the three sites conserved in all strains were N glycosylated, whereas only two of the up to five additional sites present in wild-type strains are used. A wild-type virus with an H protein reproducing the vaccine strain N-glycosylation pattern remained lethal in ferrets but with a prolonged course of disease. In contrast, introduction of the vaccine H protein in the wild-type context resulted in complete attenuation. To further characterize the role of N glycosylation in CDV pathogenesis, the N-glycosylation sites of wild-type H proteins were successively deleted, including a nonstandard site, to ultimately generate a nonglycosylated H protein. Despite reduced expression levels, this protein remained fully functional. Recombinant viruses expressing N-glycan-deficient H proteins no longer caused disease, even though their immunosuppressive capacities were retained, indicating that reduced N glycosylation contributes to attenuation without affecting immunosuppression.
Canine distemper virus (CDV) uses signaling lymphocyte activation molecule (SLAM), expressed on immune cells, as a receptor. However, epithelial and neural cells are also affected by CDV in vivo. Wild-type CDV strains showed efficient replication with syncytia in Vero cells expressing dog nectin4, and the infection was blocked by an anti-nectin4 antibody. In dogs with distemper, CDV antigen was preferentially detected in nectin4-positive neurons and epithelial cells, suggesting that nectin4 is an epithelial cell receptor for CDV and also involved in its neurovirulence.
A seroepidemiological survey of canine distemper virus (CDV) infection in Asian felids revealed that the prevalence of antibodies varied depending on region and, in some cases, exposure to dogs. The serologic pattern in cats with antibodies indicated that they had likely been exposed to field strains rather than typical CDV vaccine strains.