CD9-dependent regulation of Canine distemper virus-induced cell-cell fusion segregates with the extracellular domain of the haemagglutinin

Multiple Receptors Involved in Invasion and Neuropathogenicity of Canine Distemper Virus: A Review

The canine distemper virus (CDV) is a morbillivirus that infects a broad range of terrestrial carnivores, predominantly canines, and is associated with high mortality. Similar to another morbillivirus, measles virus, which infects humans and nonhuman primates, CDV transmission from an infected host to a naïve host depends on two cellular receptors, namely, the signaling lymphocyte activation molecule (SLAM or CD150) and the adherens junction protein nectin-4 (also known as PVRL4). CDV can also invade the central nervous system by anterograde spread through olfactory nerves or in infected lymphocytes through the circulation, thus causing chronic progressive or relapsing demyelination of the brain. However, the absence of the two receptors in the white matter, primary cultured astrocytes, and neurons in the brain was recently demonstrated. Furthermore, a SLAM/nectin-4-blind recombinant CDV exhibits full cell-to-cell transmission in primary astrocytes. This strongly suggests the existence of a third CDV receptor expressed in neural cells, possibly glial cells. In this review, we summarize the recent progress in the study of CDV receptors, highlighting the unidentified glial receptor and its contribution to pathogenicity in the host nervous system. The reviewed studies focus on CDV neuropathogenesis, and neural receptors may provide promising directions for the treatment of neurological diseases caused by CDV. We also present an overview of other neurotropic viruses to promote further research and identification of CDV neural receptors.

Expression of CD9 on porcine lymphocytes and its relation to T cell differentiation and cytokine production

In this work, we report on two novel monoclonal antibodies, specific for porcine CD9. CD9 is a tetraspanin that is expressed on a wide variety of cells. We phenotyped porcine immune cell subsets and found that CD9 was expressed on all monocytes as well as a subset of B cells. CD9 was variably expressed on T cells, with CD4 T cells containing the highest frequency of CD9⁺ cells. CD9 expression positively correlated with the frequency of central memory CD4 T cells in ex vivo PBMC. Therefore, we proceeded to explore CD9 as a marker of T cell function. Here we observed that CD9 was expressed on the vast majority of long-lived influenza A virus-specific effector cells that retained the capacity for cytokine production in response to in vitro recall antigen. Therefore, the new antibodies enable the detection of a cell surface molecule with functional relevance to T cells. Considering the importance of CD9 in membrane remodelling across many cell types, they will also benefit the wider field of swine biomedical research.

Host Cellular Receptors for the Peste des Petits Ruminant Virus

Peste des Petits Ruminant (PPR) is an important transboundary, OIE-listed contagious viral disease of primarily sheep and goats caused by the PPR virus (PPRV), which belongs to the genus Morbillivirus of the family Paramyxoviridae. The mortality rate is 90–100%, and the morbidity rate may reach up to 100%. PPR is considered economically important as it decreases the production and productivity of livestock. In many endemic poor countries, it has remained an obstacle to the development of sustainable agriculture. Hence, proper control measures have become a necessity to prevent its rapid spread across the world. For this, detailed information on the pathogenesis of the virus and the virus host interaction through cellular receptors needs to be understood clearly. Presently, two cellular receptors; signaling lymphocyte activation molecule (SLAM) and Nectin-4 are known for PPRV. However, extensive information on virus interactions with these receptors and their impact on host immune response is still required. Hence, a thorough understanding of PPRV receptors and the mechanism involved in the induction of immunosuppression is crucial for controlling PPR. In this review, we discuss PPRV cellular receptors, viral host interaction with cellular receptors, and immunosuppression induced by the virus with reference to other Morbilliviruses.

Probing Morbillivirus Antisera Neutralization Using Functional Chimerism between Measles Virus and Canine Distemper Virus Envelope Glycoproteins

Measles virus (MeV) is monotypic. Live virus challenge provokes a broadly protective humoral immune response that neutralizes all known measles genotypes. The two surface glycoproteins, H and F, mediate virus attachment and entry, respectively, and neutralizing antibodies to H are considered the main correlate of protection. Herein, we made improvements to the MeV reverse genetics system and generated a panel of recombinant MeVs in which the globular head domain or stalk region of the H glycoprotein or the entire F protein, or both, were substituted with the corresponding protein domains from canine distemper virus (CDV), a closely related morbillivirus that resists neutralization by measles-immune sera. The viruses were tested for sensitivity to human or guinea pig neutralizing anti-MeV antisera and to ferret anti-CDV antisera. Virus neutralization was mediated by antibodies to both H and F proteins, with H being immunodominant in the case of MeV and F being so in the case of CDV. Additionally, the globular head domains of both MeV and CDV H proteins were immunodominant over their stalk regions. These data shed further light on the factors constraining the evolution of new morbillivirus serotypes.

Fusion and hemagglutinin proteins of canine distemper virus promote osteoclast formation through NF-κB dependent and independent mechanisms

Paget's disease (PD) features abnormal osteoclasts (OC) which sharply increase in number and size and then intensely induce bone resorption. The purpose of this study was to determine the direct effects of canine distemper virus (CDV) and its fusion protein and hemagglutinin protein (F + H) on receptor activator of nuclear factor kappa-B ligand (RANKL) induced OC formation in vitro. Immunofluorescence assay, OC morphological and functional detection, intracellular signaling pathway detection, Real-time PCR analysis and ELISA were applied in this study. Immunofluorescence assay provided the conclusive proof that CDV can infect and replicate in RAW264.7 mouse monocyte cell line, primary human peripheral blood mononuclear cells (PBMC) and their further fused OC. Both CDV and F + H significantly promoted OC formation and bone resorption ability induced by RANKL. Meanwhile, intracellular signaling transduction analysis revealed CDV and F + H specifically upregulated the phosphorylation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK) induced by RANKL, respectively. Furthermore, without RANKL stimulation, both CDV and F + H slightly induced OC-like cells formation in RAW264.7 cell line even in the presence of NF-κB inhibitor. F + H upregulate OC differentiation and activity through modulation of NF-κB signaling pathway, and induce OC precursor cells merging dependent on the function of glycoproteins themselves. These results meant that F and H proteins play a pivotal role in CDV supporting OC formation. Moreover, this work further provide a new research direction that F and H proteins in CDV should be considered as a trigger during the pathogenesis of PD.

Tetraspanin Assemblies in Virus Infection

Tetraspanins (Tspans) are a family of four-span transmembrane proteins, known as plasma membrane “master organizers.” They form Tspan-enriched microdomains (TEMs or TERMs) through lateral association with one another and other membrane proteins. If multiple microdomains associate with each other, larger platforms can form. For infection, viruses interact with multiple cell surface components, including receptors, activating proteases, and signaling molecules. It appears that Tspans, such as CD151, CD82, CD81, CD63, CD9, Tspan9, and Tspan7, coordinate these associations by concentrating the interacting partners into Tspan platforms. In addition to mediating viral attachment and entry, these platforms may also be involved in intracellular trafficking of internalized viruses and assist in defining virus assembly and exit sites. In conclusion, Tspans play a role in viral infection at different stages of the virus replication cycle. The present review highlights recently published data on this topic, with a focus on events at the plasma membrane. In light of these findings, we propose a model for how Tspan interactions may organize cofactors for viral infection into distinct molecular platforms.

Measles vaccination: Threat from related veterinary viruses and need for continued vaccination post measles eradication

Measles virus (MV) is the only human virus within the morbillivirus genus of the Paramyxoviridae. The veterinary members are canine distemper virus (CDV), peste des petits ruminants virus (PPRV), Rinderpest Virus (RPV) as well as the marine morbilliviruses phocine distemper virus (PDV), dolphin morbillivirus (DMV) and porpoise morbillivirus (PMV). Morbilliviruses have a severe impact on humans and animal species. They confer diseases which have contributed to morbidity and mortality of the population on a global scale. There is substantial evidence from both natural and experimental infections that morbilliviruses can readily cross species barriers. Of most concern with regard to zoonosis is the more recently reported fatal infection of primates in Japan and China with strains of CDV which have adapted to this host. The close genetic relationship, shared cell entry receptors and similar pathogenesis between the morbilliviruses highlights the potential consequences of complete withdrawal of MV vaccination after eradication. Therefore, it would be prudent to continue the current MV vaccination. Ultimately development of novel, safe vaccines which have higher efficacy against the veterinary morbilliviruses is a priority. These would to protect the human population long term against the threat of zoonosis by these veterinary viruses.

Expression of canine distemper virus receptor nectin-4 in the central nervous system of dogs

Canine distemper virus (CDV) exhibits lymphotropic, epitheliotropic, and neurotropic nature, and causes a severe systemic infection in susceptible animals. Initially, signaling lymphocyte activation molecule (SLAM) expressed on immune cells has been identified as a crucial cellular receptor for CDV. Currently, nectin-4 expressed in epithelia has been shown to be another receptor for CDV. Our previous study demonstrated that neurons express nectin-4 and are infected with CDV. In this study, we investigated the distribution pattern of nectin-4 in various cell types in the canine central nervous system and showed its relation to CDV infection to further clarify the pathology of disease. Histopathological, immunohistochemical and immunofluorescent analyses were done using formalin-fixed paraffin-embedded tissues of CDV-infected dogs. Dual staining of nectin-4 and CDV antigen or nectin-4 and brain cell markers was performed. Nectin-4 was detected in ependymal cells, epithelia of choroid plexus, meningeal cells, neurons, granular cells, and Purkinje's cells. CDV antigens were detected in these nectin-4-positive cells, further suggesting contribution of nectin-4 for the CDV neurovirulence. On the other hand, astrocytes did not express nectin-4, although they were frequently infected with CDV. Since astrocytes are negative for SLAM expression, they must express an unidentified CDV receptor, which also contributes to CDV neurovirulence.

Use of SLAM and PVRL4 and identification of pro-HB-EGF as cell entry receptors for wild type phocine distemper virus

Signalling lymphocyte activation molecule (SLAM) has been identified as an immune cell receptor for the morbilliviruses, measles (MV), canine distemper (CDV), rinderpest and peste des petits ruminants (PPRV) viruses, while CD46 is a receptor for vaccine strains of MV. More recently poliovirus like receptor 4 (PVRL4), also known as nectin 4, has been identified as a receptor for MV, CDV and PPRV on the basolateral surface of polarised epithelial cells. PVRL4 is also up-regulated by MV in human brain endothelial cells. Utilisation of PVRL4 as a receptor by phocine distemper virus (PDV) remains to be demonstrated as well as confirmation of use of SLAM. We have observed that unlike wild type (wt) MV or wtCDV, wtPDV strains replicate in African green monkey kidney Vero cells without prior adaptation, suggesting the use of a further receptor. We therefore examined candidate molecules, glycosaminoglycans (GAG) and the tetraspan proteins, integrin β and the membrane bound form of heparin binding epithelial growth factor (proHB-EGF),for receptor usage by wtPDV in Vero cells. We show that wtPDV replicates in Chinese hamster ovary (CHO) cells expressing SLAM and PVRL4. Similar wtPDV titres are produced in Vero and VeroSLAM cells but more limited fusion occurs in the latter. Infection of Vero cells was not inhibited by anti-CD46 antibody. Removal/disruption of GAG decreased fusion but not the titre of virus. Treatment with anti-integrin β antibody increased rather than decreased infection of Vero cells by wtPDV. However, infection was inhibited by antibody to HB-EGF and the virus replicated in CHO-proHB-EGF cells, indicating use of this molecule as a receptor. Common use of SLAM and PVRL4 by morbilliviruses increases the possibility of cross-species infection. Lack of a requirement for wtPDV adaptation to Vero cells raises the possibility of usage of proHB-EGF as a receptor in vivo but requires further investigation.

Nectin4 is an epithelial cell receptor for canine distemper virus and involved in neurovirulence

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.

Tetraspanins: Small transmembrane proteins with big impact on membrane microdomain structures

Members of the tetraspanin family of transmembrane proteins including CD9, CD37, CD53, CD63, CD81, CD82, CD151, etc., contribute to the structural organization of the plasma membrane by forming microdomain structures, influencing cell fusion and regulating cell motility. Interestingly, K41, a CD9-specific monoclonal antibody (mAb), inhibits the release of human immunodeficiency virus (HIV-1), and the canine distemper virus (CDV)-, but not measles virus (MV)-induced cell-cell fusion. This mAb, which recognizes a conformational epitope on the large extracellular loop (LEL) of CD9, induced rapid relocation and clustering of CD9 in net-like structures at cell-cell contact areas.1 High-resolution analyses revealed that CD9 clustering is accompanied by the formation of microvilli that protrude from either side of adjacent cell surfaces, thus forming structures like microvilli zippers. While the cellular CD9-associated proteins beta1-integrin and EWI-F were co-clustered with CD9 at cell-cell interfaces, viral proteins in infected cells were differentially affected. MV envelope proteins were detected within, whereas CDV proteins were excluded from CD9 clusters, and thus, the tetraspanin CD9 can regulate cell-cell fusion by controlling the access of the viral fusion machinery to cell contact areas.

Canine distemper virus utilizes different receptors to infect chicken embryo fibroblasts and vero cells

Inducing animal viruses to adapt to chicken embryos or chicken embryo fibroblasts (CEF) is a common method to develop attenuated live vaccines with full security. Canine distemper virus (CDV) also does this, but the mechanisms and particular receptors remain unclear. Virus overlay protein blot assays were carried out on CEF membrane proteins, which were extracted respectively with a Mem-PER™ kit, a radioimmunoprecipitation assay buffer or a modified co-immunoprecipitation method, and revealed a common 57 kDa positive band that differed from the 42-kDa positive band in Vero cells and also from those receptors reported in lymphocytes and 293 cells, indicating a receptor diversity of CDV and the possibility of the 57-kDa protein acting as a receptor that is involved in adaptive infection of CDV Kunming strain to CEF.

Strategies for targeting tetraspanin proteins: potential therapeutic applications in microbial infections

The identification of novel targets and strategies for therapy of microbial infections is an area of intensive research due to the failure of conventional vaccines or antibiotics to combat both newly emerging diseases (e.g. viruses such as severe acute respiratory syndrome (SARS) and new influenza strains, and antibiotic-resistant bacteria) and entrenched, pandemic diseases exemplified by HIV. One clear approach to this problem is to target processes of the host organism rather than the microbe. Recent data have indicated that members of the tetraspanin superfamily, proteins with a widespread distribution in eukaryotic organisms and 33 members in humans, may provide such an approach.

Tetraspanins traverse the membrane four times, but are distinguished from other four-pass membrane proteins by the presence of conserved charged residues in the transmembrane domains and a defining ‘signature’ motif in the larger of the two extracellular domains (the EC2). They characteristically form promiscuous associations with one another and with other membrane proteins and lipids to generate a specialized type of microdomain: the tetraspanin-enriched microdomain (TEM). TEMs are integral to the main role of tetraspanins as ‘molecular organizers’ involved in functions such as membrane trafficking, cell-cell fusion, motility, and signaling. Increasing evidence demonstrates that tetraspanins are used by intracellular pathogens as a means of entering and replicating within human cells. Although previous investigations focused mainly on viruses such as hepatitis C and HIV, it is now becoming clear that other microbes associate with tetraspanins, using TEMs as a ‘gateway’ to infection.

In this article we review the properties and functions of tetraspanins/TEMs that are relevant to infective processes and discuss the accumulating evidence that shows how different pathogens exploit these properties in infection and in the pathogenesis of disease. We then investigate the novel and exciting possibilities of targeting tetraspanins for the treatment of infectious disease, using specific antibodies, recombinant EC2 domains, small-molecule mimetics, and small interfering RNA. Such therapies, directed at host-cell molecules, may provide alternative options for combating fast-mutating or newly emerging pathogens, where conventional approaches face difficulties.

Targeting of tetraspanin proteins--potential benefits and strategies

The tetraspanin transmembrane proteins have emerged as key players in malignancy, the immune system, during fertilization and infectious disease processes. Tetraspanins engage in a wide range of specific molecular interactions, occurring through the formation of tetraspanin-enriched microdomains (TEMs). TEMs therefore serve as a starting point for understanding how tetraspanins affect cell signalling, adhesion, morphology, motility, fusion and virus infection. An abundance of recent evidence suggests that targeting tetraspanins, for example, by monoclonal antibodies, soluble large-loop proteins or RNAi technology, should be therapeutically beneficial.

Pathogenesis and immunopathology of systemic and nervous canine distemper

Canine distemper is a worldwide occurring infectious disease of dogs, caused by a morbillivirus, closely related to measles and rinderpest virus. The natural host range comprises predominantly carnivores. Canine distemper virus (CDV), an enveloped, negative-sense RNA virus, infects different cell types, including epithelial, mesenchymal, neuroendocrine and hematopoietic cells of various organs and tissues. CDV infection of dogs is characterized by a systemic and/or nervous clinical course and viral persistence in selected organs including the central nervous system (CNS) and lymphoid tissue. Main manifestations include respiratory and gastrointestinal signs, immunosuppression and demyelinating leukoencephalomyelitis (DL). Impaired immune function, associated with depletion of lymphoid organs, consists of a viremia-associated loss of lymphocytes, especially of CD4+ T cells, due to lymphoid cell apoptosis in the early phase. After clearance of the virus from the peripheral blood an assumed diminished antigen presentation and altered lymphocyte maturation cause an ongoing immunosuppression despite repopulation of lymphoid organs. The early phase of DL is a sequel of a direct virus-mediated damage and infiltrating CD8+ cytotoxic T cells associated with an up-regulation of pro-inflammatory cytokines such as interleukin (IL)-6, IL-8, tumor necrosis factor (TNF)-alpha and IL-12 and a lacking response of immunomodulatory cytokines such as IL-10 and transforming growth factor (TGF)-beta. A CD4+-mediated delayed type hypersensitivity and cytotoxic CD8+ T cells contribute to myelin loss in the chronic phase. Additionally, up-regulation of interferon-gamma and IL-1 may occur in advanced lesions. Moreover, an altered balance between matrix metalloproteinases and their inhibitors seems to play a pivotal role for the pathogenesis of DL. Summarized, DL represents a biphasic disease process consisting of an initial direct virus-mediated process and immune-mediated plaque progression. Immunosuppression is due to early virus-mediated lymphocytolysis followed by still poorly understood mechanisms affecting antigen presentation and lymphocyte maturation.

CD9 clustering and formation of microvilli zippers between contacting cells regulates virus-induced cell fusion

Members of the tetraspanin family including CD9 contribute to the structural organization and plasticity of the plasma membrane. K41, a CD9-specific monoclonal antibody, inhibits the release of HIV-1 and canine distemper virus (CDV)- but not measles virus (MV)-induced cell-cell fusion. We now report that K41, which recognizes a conformational epitope on the large extracellular loop of CD9, induces rapid relocation and clustering of CD9 in net-like structures at cell-cell contact areas. High-resolution analyses revealed that CD9 clustering is accompanied by the formation of microvilli that protrude from either side of adjacent cell surfaces, thus forming structures like microvilli zippers. While the cellular CD9-associated proteins beta(1)-integrin and EWI-F were co-clustered with CD9 at cell-cell interfaces, viral proteins in infected cells were differentially affected. MV envelope proteins were detected within CD9 clusters, whereas CDV proteins were excluded from CD9 clusters. Thus, the tetraspanin CD9 can regulate cell-cell fusion by controlling the access of the fusion machinery to cell contact areas.