Host factors and measles virus replication

Available online at www.sciencedirect.com

Host factors and measles virus replication Sebastien Delpeut1,2,*, Ryan S Noyce1,2,*, Ricky WC Siu1,2,* and Christopher D Richardson1,2,3 This review takes a general approach to describing host cell factors that facilitate measles virus (MeV) infection and replication. It relates our current understanding of MeV entry receptors, with emphasis on how these host cell surface proteins contribute to pathogenesis within its host. The roles of SLAM/CD150 lymphocyte receptor and the newly discovered epithelial receptor PVRL4/nectin-4 are highlighted. Host cell factors such as HSP72, Prdx1, tubulin, casein kinase, and actin, which are known to impact viral RNA synthesis and virion assembly, are also discussed. Finally the review describes strategies used by measles virus to circumvent innate immunity and confound the effects of interferon within the host cell. Proteomic studies and genome wide RNAi screens will undoubtedly advance our knowledge in the future.

Current Opinion in Virology 2012, 2:767–777

The MeV genome encodes a membrane-associated matrix (M) protein, the hemagglutinin (H) and fusion (F) envelope glycoproteins, the RNA polymeraseassociated phosphoprotein (P) and large polymerase (L) protein, and a nucleocapsid (N) protein that surrounds the viral genome (Figure 1a). The area encoding the P gene includes two overlapping open reading frames that code for the non-structural viral proteins, V and C (Figure 1b). Although these proteins, which are involved in suppression of the host immune response, are dispensable for virus replication in vitro, they are required during natural infections within the host [1]. This review will take a general approach to MeV infection, replication, and interaction of the virus with host cell factors. It will describe our current understanding of MeV attachment factors and entry receptors, with emphasis on how these receptors contribute to virus dissemination and pathogenesis within its host. Host factors involved in RNA synthesis and virus assembly will also be discussed. Finally, the review will highlight virus strategies to circumvent the immune response within the host.

This review comes from a themed issue on Virus replication in animals and plants

Host receptors involved in measles virus entry

Addresses 1 The Department of Microbiology and Immunology, Dalhousie University, Halifax, Nova Scotia B3H 1X5, Canada 2 IWK Health Centre, Canadian Center for Vaccinology, Goldbloom Pavilion, Halifax, Nova Scotia B3H 1X5, Canada 3 The Department of Pediatrics, Dalhousie University, Halifax, Nova Scotia, Canada Corresponding author: Richardson, Christopher D ([email protected], [email protected]) *

suppression and subsequent susceptibility to secondary infections that could last for weeks to months following resolution of the initial infection.

These authors contributed equally to this work.

Edited by Peter Nagy and Christopher Richardson For a complete overview see the Issue Available online 10th November 2012 1879-6257/$ – see front matter, # 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coviro.2012.10.008

Introduction Measles virus (MeV), a member of the genus Moribillivirus from the Paramyxoviridae family, is a highly pathogenic non-segmented negative-strand RNA virus that causes respiratory distress and immune suppression upon infection of its human host. The disease is characterized by a latent period of 10–14 days followed by early symptoms, which include cough, fever, Koplik’s spots, and the appearance of a maculopapular rash, a hallmark of MeV. The onset of rash coincides with the initiation of virus clearance via the immune response in an immunocompetent individual. Complications arise in young children and immunocompromised individuals owing to immune www.sciencedirect.com

Three known entry receptors have been identified to date for both laboratory-adapted strains and clinical isolates of MeV. CD46 (membrane cofactor protein, MCP) was initially shown to be an entry receptor for vaccine and laboratory-adapted strains of the virus [2,3]. This cell surface marker is expressed on nearly all nucleated human cells and protects the host cell from damage by complement [4]. Clinically relevant isolates of MeV enter the cells via two known receptors, CD150/signaling lymphocyte-activation molecule (SLAM) [5,6,7] and poliovirus receptor-like protein 4 (PVRL4), also known as nectin-4 [8,9]. Their restricted expression to either activated lymphocytes or the basolateral surface of epithelial cells, respectively, has contributed to our understanding of MeV tropism within the human host and will be discussed in further detail. Additional MeV attachment receptors have been identified and will also be reviewed, including dendritic cell-specific ICAM-3 grabbing non-integrin (DC-SIGN) and Neurokinin-1, although their physiological importance in MeV pathogenesis is still being elucidated. Current Opinion in Virology 2012, 2:767–777

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Figure 1

MeV-H to CD150/SLAM is postulated to either shift one of the H dimers relative to the other in a tetramer [14], or result in a twist in the heads of the dimers relative to each other [15], both resulting in the refolding of the metastable MeV-F and subsequent triggering of membrane fusion.

(a)

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Schematic diagram of the structure and viral proteins of measles virus (MeV). (a) The structure and organization of the MeV virion. MeV is a member of the morbillivirus genus, and contains a lipid envelope with the two viral glycoproteins, MeV-H and MeV-F, embedded throughout. The matrix protein is bound to the cytoplasmic domain of the glycoproteins and provides structural support for the newly formed virion. The negative stranded RNA genome is covered with the viral nucleoprotein MeV-N, with MeV-P and MeV-L forming the RdRp replication complex. (b) The organization of the MeV genome. The MeV genome contains 6 viral genes that are transcribed as oneThe RdRp may detach from the ribonucleoprotein (RNP) complex within the intergenic regions that separate viral genes, creating a transcription gradient whereby genes that are downstream of the 30 leader sequence have progressively less mRNA transcribed. The area encoding the MeV-P gene includes two overlapping open reading frames that code for non-structural proteins, V and C.

CD150/SLAM

The primary receptor for wild type strains of MeV, CD150/SLAM, was initially identified by Tatsuo et al. [7] and confirmed by others [5,6]. CD150/SLAM is expressed on activated T and B cells, macrophages and mature dendritic cells (DC) [10], accounting for its highly lymphotropic nature during in vivo infections. This 70 kDa type I transmembrane glycoprotein has two extracellular immunoglobulin superfamily domains, the variable (V) and constant (C2) regions, and an endodomain with an SH2 binding region [11]. CD150/SLAM normally serves as a costimulatory molecule to initiate distinct signal transduction networks in T-cells, natural killer (NK) cells and antigen presenting cells [12]. Three residues within the V domain of CD150/SLAM (I60, H61, and V63) interact with the MeV-H attachment glycoprotein to mediate virus-cell fusion [13]. Binding of Current Opinion in Virology 2012, 2:767–777

Evidence for the existence of an epithelial cell receptor has been accumulating over the past 10 years [16–19]. Recently, our laboratory [9,20] and subsequently Muhlebach et al. [8] independently identified the adherens junction protein, nectin-4/PVRL4 as the epithelial cell receptor responsible for measles virus infection using a comparative microarray approach to identify genes that were preferentially upregulated on MeV-susceptible tumor cell lines compared to non-susceptible cells. Expression of a subset of candidate cellular receptors in the non-susceptible cell line COS-1 clearly identified nectin-4/PVRL4 as being capable of inducing wtMeV infection and subsequent formation. Similar to CD150/ SLAM, the V domain is involved in MeV-H attachment [8], although the residues responsible for this interaction on nectin-4/PVRL4 still need to be identified. Nectin-4/PVRL4 is involved in the formation of adherens junctions during the polarization of epithelial cells. In general, nectins regulate several cellular activities, including cellular movement, polarization, differentiation, and entry of viruses (reviewed in [21]). In particular, nectin-4/PVRL4 functions first by forming homo cis dimers on the cell surface and then trans dimers on adjacent cells in a homophilic or heterophillic manner with nectin 1 (reviewed in [21]). Calcium-dependent ecadherin interactions between these cells begin to form, followed by the recruitment of tight junction proteins, including ZO-1 and occludin, culminating in the formation of a barrier that prevents incoming pathogens from migrating across the epithelial cell layer. Pathogens have evolved very sophisticated mechanisms to usurp this barrier and gain entry into the host. Virus entry receptors dictate the tropism of MeV within its host

MeV uses its two surface glycoproteins, H and F, to enter its target cell (reviewed in [22]). During the course of infection of its host, resident DCs and alveolar macrophages of the respiratory tract are the initial cells infected (Figure 2a) [23,24,25]. MeV-H initially attaches to these cells via DC-SIGN, a C-type lectin found on the surface of both macrophages and DCs [23,26]. Until recently, it was unclear how these cells were infected, given that CD150/SLAM was not present on their cell surface. Attachment of the H protein to DC-SIGN results in localization of an intracellular pool of CD150/SLAM to the cell surface through the activation of acid sphingomyelinase (Figure 2a) [10]. These infected DCs carry www.sciencedirect.com

Host factors and measles virus replication Delpeut et al. 769

Figure 2

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(b) Immune cell infection respiratory tract

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Major routes of MeV infection and transmission within a human host. (a) Initial infection of MeV via dendritic cells (DC) and alveolar macrophages (AM) occurs within the respiratory tract. The MeV-H attachment protein interacts with the C-type lectin, DC-SIGN, leading to a signal transduction cascade that results in the localization of CD150/SLAM to the cell surface via the activation of acid sphingomyleinase (SMase). MeV subsequently gains entry into the cell via the entry receptor CD150/SLAM. (b) Infected DCs and AM travel to the local draining lymph node where they interact with and infect activated T-cells and B-cells via CD150/SLAM that is expressed on their cell surface. MeV-infected T-cells and B-cells disseminate to secondary lymphoid organs, leading to secondary viremia and MeV spread. (c) Spread of MeV-infected lymphocytes to distal sites, including the respiratory tract. MeV-infected lymphocytes interact with the epithelial cell receptor, nectin-4/PVRL4, which is located in the adherens junctions on the basolateral surface of the epithelial cell (EC). MeV infection in the airway epithelium results in the virus assembly and release of virions into the airway lumen of the infected lung. (d) MeV can infect the central nervous system (CNS) in rare instances. Although the mechanism of neuronal infection still remains unclear, there is evidence to suggest that the substance P receptor, Neurokinin-1 may play a role in trans-synaptic spread of MeV RNPs. The role of nectin-4/PVRL4 in both neuronal and endothelial cell infections with MeV will also need to be carefully studied.

the virus to the draining lymph node where they subsequently infect resident activated T-cells and B-cells in a CD150/SLAM-dependent manner, resulting in virus amplification and the establishment of primary viremia (Figure 2b). From here, MeV disseminates to secondary www.sciencedirect.com

lymphoid organs, including the spleen, thymus, and tonsils, leading to secondary viremia that results in acute immunosuppression. Spread of the virus to distal sites, including the liver, skin, gastrointestinal tract, genital and respiratory mucosal surfaces, results in virus shedding and Current Opinion in Virology 2012, 2:767–777

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subsequent transmission to uninfected individuals (Figure 2c). Within the respiratory tract, infection is believed to occur via the basolateral side of the airway epithelium via migration of MeV-infected T, B, and DCs from the circulation. Here, nectin-4/PVRL4 is perfectly situated within the adherens junctions to interact with MeV virions that have decorated the surface of these infected lymphocytes, allowing entry into the epithelium. MeV exits the epithelium via the apical surface. Studies with nectin-4/PVRL4 receptor-blind MeV have shown that the virus remains lymphotropic, and can produce primary and secondary viremia. However, the epithelial receptor blind virus is incapable of being shed from the respiratory route, suggesting that nectin-4/PVRL4 plays a role in virus egress late in infection, rather than uptake during the initial stages of infection [27]. Although CD150/SLAM and nectin-4/PVRL4 establish the lymphocyte and epithelial cell tropism of wild type MeV strains, other types of cells within the host are susceptible to infection. Measles can be complicated by the onset of post infectious encephalomyelitis (PIE), and in rare instances subacute sclerosing panencephalitis (SSPE), owing to infection of neurons within the central nervous system (CNS) [28]. How MeV gets into the brain remains unclear. It has been suggested that free MeV may spread to the brain by infecting endothelial cells at the blood–brain barrier during secondary viremia [29,30]. Alternatively, the infiltration of MeV-infected lymphocytes into the CNS may contribute to neuropathology. The presence of nucleocapsids in the axons and at the synaptic gap suggests a contact-dependent, trans-synaptic spread of MeV. Interestingly, extracellular MeV was not detected in brain tissue from patients with SSPE, suggesting that infectious particles are not required for neuron-to-neuron spread [31]. The trans-synaptic spread of MeV in primary neurons may be mediated by an interaction between MeV and the substance P receptor, Neurokinin-1 (Figure 2d) [32]. In this case, the precise role of this receptor in MeV spread warrants further investigation, since MeV-F, rather than MeV-H is responsible for both attachment and membrane fusion. A role for nectin-4/PVRL4 in the infectivity of endothelial cells and neurons should not be ruled out. Recent evidence also suggests that nectin-4 may play a role in neurovirulence following infection with a closely related virus, canine distemper virus (CDV) [33]. Further studies will be required to investigate the utilization of nectin-4/PVRL4 at other sites during MeV pathogenesis. Overall, the identification of MeV entry receptors has contributed greatly to our understanding of the tropism and replication of this highly pathogenic virus.

Host factors involved in MeV RNA synthesis and assembly Like other Paramyxoviridae, transcription of the MeV genome starts immediately following entry into the cytoplasm of a cell using the RNA-dependent RNA Current Opinion in Virology 2012, 2:767–777

polymerase (RdRp) associated with the genome (Figure 1b) [34]. The 30 leader sequence contains recognition sites for the RdRp that sequentially transcribes the viral genes. The RdRp may detach from the ribonucleoprotein (RNP) complex within the intergenic regions that separate viral genes, creating a transcription gradient where genes that are downstream of the 30 leader sequence have progressively less mRNA transcribed. During MeV genome synthesis, the RdRp transcribes the entire genome and produces a full-length anti-genome, which is used as a template for synthesizing full-length viral RNA. Newly synthesized genomic RNA may serve as a template for viral replication, viral transcription, or is packaged into new virus particles. Knowledge of the involvement of host factors in the synthesis of MeV RNA is limited, and awaits further proteomic and RNAi genomic screens. However, comparative analysis of other paramyxoviruses and rhabdoviruses has implicated a number of host proteins in the process that are summarized below. A number of host factors, which bind and influence MeV replication, have been identified, and are summarized in Figure 3. Many of these factors have been identified by co-immune precipitation, yeast 2 hybrid, and proteomic approaches. These host factors include heat-shock protein 72 (HSP72), casein kinase II and peroxiredoxin 1 (Prdx1) [35,36,37,38]. HSP72 was shown to bind two conserved sequence motifs in the C-terminus of MeV-N, resulting in an increase in MeV genome replication, transcription, and viral gene expression. In mouse models of MeV infection, overexpression of HSP72 was associated with higher viral RNA levels and a high mortality rate [39,40]. The host factor casein kinase II has been shown to phosphorylate MeV-P in vitro [35]. Although there are many existing reports that attempt to characterize the phosphorylation of paramyxovirus P protein by host factors, the exact role of P protein phosphorylation appears to vary between different paramyxoviruses [41,42]. Much of this research was performed using non-MeV systems [43,44]. The host factor Prdx1 is an antioxidant enzyme that is normally responsible for eliminating cellular peroxide. It also binds to the C-terminal region of MeV-N. Prdx1 competes with MeV-P for binding to MeV-N early on in replication, suggesting that it functions as a driving force for RdRp by modulating the formation of the MeV-N–RdRp complex to regulate mRNA/genome synthesis. Silencing Prdx1 gene expression reduced the level of both viral mRNA and genomic RNA in infected cells [36]. Other host factors that interact with MeV proteins include a series of unidentified kinases, which phosphorylate amino acid residues in MeV-N and MeV-P [45,46–48]. Mass spectrometry analysis showed that one of these kinases phosphorylates serine residues 479 and 510 in MeV-N, promoting viral transcription and replication in a www.sciencedirect.com

Host factors and measles virus replication Delpeut et al. 771

Figure 3

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Host cell proteins influencing MeV RNA synthesis and virion assembly. Negative strand RNA genome associates with heat-shock protein 72 (HSP72), casein kinase II (CKII), tubulin, and peroxiredoxin 1 (Prdx1), which interact with the N protein of MeV. The genome of MeV is transcribed into viral RNA, which specify N, P, V, C, M, F, H, and L proteins. Alternatively, the negative strand RNA genome is transcribed to produce a full-length positive strand template for replication and production of more negative strand RNA genome. In association with actin filaments and M protein the mature virus assembles and buds from the plasma membrane.

mini-genome-based expression system [49]. The cytoskeletal component tubulin has also been shown to act as a positive transcription factor for RNA transcription and replication of MeV in vitro [50]. Moyer observed an interaction between MeV-L and b-tubulin using antib-tubulin antibodies, which in turn completely inhibited MeV RNA synthesis in vitro, suggesting that tubulin may play a role in the formation of the RdRp complex, leading www.sciencedirect.com

to a subsequent increase in RNA synthesis during MeV infection [50]. This is also true for Sendai virus [51,52]. Several Paramyxoviridae, including MeV, target epithelial cells of the respiratory tract for infection. They spread from cell-to-cell to form multinucleated syncytia using viral glycoproteins expressed on the basolateral cell surface. Alternatively, virions can assemble and bud from the Current Opinion in Virology 2012, 2:767–777

772 Virus replication in animals and plants

apical surfaces of these cells (described in the previous section). MeV assembly involves the co-ordinated effort of both viral and cellular proteins to successfully spread. Both MeV-H and MeV-F contain recognized Tyr sorting motifs in their cytoplasmic tails (H-12Y; F-549Y), which mediate their expression on the basolateral surface of the infected cell [53–55]. Cell sorting through the Tyr motifs is controlled through interaction with unidentified proteins and migration of H or F to the basolateral membranes, which is not inter-dependent [55]. Interestingly, in wild type strains of MeV, only the cytoplasmic tyrosine of MeV-H was shown to be important for virus spread between lymphocytes, whereas the tyrosine signal in both wtMeV-H and wtMeV-F are required for efficient cell-to-cell spread within epithelial cells [55]. The cytoskeletal protein actin is packaged within the infectious particles of both MeV and Sendai virus [56]. Actin filaments interact directly with the M protein of Newcastle disease virus and Sendai virus [57]. It has been suggested that actin filaments may be required for MeV assembly since blocking actin polymerization inhibits the maturation of the virus and results in the accumulation of RNP in infected cells [58]. Electron microscopy also revealed that actin filaments were involved in MeV assembly and budding, where the barbed ends of heavy meromyosin were observed to protrude into virus particles. This suggests that MeV may utilize the polymerization of actin filaments for assembly and budding [59]. Disruption of actin filaments or microtubules with chemical inhibitors was also found to inhibit the production of infectious MeV particles. Immunofluorescence studies did not show exact co-localization between actin or tubulin and individual MeV proteins, suggesting that additional cellular factors may be required to mediate the interaction between these proteins [60]. The details of interaction of host factors with MeV proteins during the processes of RNA synthesis and virus assembly are not well understood. It is also unclear what benefit MeV might gain from packaging actin filaments into the newly formed virion. Further research involving proteomic analysis and host genome wide screens are clearly required. Research efforts into understanding the specific roles of host factors in MeV replication may lead to the development of new inhibitors to control viral infections of paramyxoviruses, in general.

Measles virus overcomes host factors involved in innate immunity During infections by MeV, manipulation of the host immune response plays a key role in pathogenesis [61]. The interplay between host innate immunity and counteracting measures by the virus influences the outcome of infection. In turn, virus mediated effects on innate immunity have profound consequences upon the development of adaptive immunity within the host [62–64]. Immune suppression is a key consequence of MeV owing to the Current Opinion in Virology 2012, 2:767–777

infection of activated B cells, T cells, dendritic cells, and macrophages, which express SLAM/CD150. The innate immune system detects measles virus through the interaction of pathogen recognition receptors and pathogen-associated molecular patterns (PAMPs), which normally leads to an antiviral response within the cell [65,66]. The host recognizes both viral RNA and protein PAMPs from MeV, including the incoming viral genome, MeV-H, MeV-V, and MeV-C proteins. In macrophages and DCs, interaction of the H protein of wtMeV with tolllike receptor 2 (TLR 2) triggers interleukin-6 (IL-6) production and surface expression of SLAM. This leads to immune activation and probably contributes to effective spread of MeV throughout the host [67]. Another laboratory reported that IL-12 production is inhibited following the binding of vaccine strains of MeV to CD46 [68]. Similarly, IL-8 release by human pulmonary epithelial cells occurs after measles virus infection following PVRL4/nectin-4-dependent infections [69]. A major pathway involved in antiviral defense is the interferon (IFN) response, which involves the induction of type I interferon (IFN-a/b). This leads to the establishment of an antiviral state [70]. Following MeV entry into the host cell, viral RNA is recognized by innate immune sensors including cytosolic RNA helicases like RIG-I (retinoic acidinducible gene I), MDA5 (melanoma differentiation-associated gene 5) [71], and LGP2 (Laboratory of Genetics and Physiology 2). LPG2 is believed to serve as a positive regulator of RIG-I and MDA5-mediated antiviral responses [72]. To subvert viral RNA sensing, MeV has developed many strategies. The P protein of MeV binds to MDA5 and counteracts MDA5-mediated IFN responses [71]. RIG-I, the major MeV RNA sensor, is inhibited in an LGP2-dependent manner mediated through the interaction of LGP2 with the V protein [73,74]. Finally, the measles virus P protein suppresses toll-like receptor 4 (TLR4) signaling via by stimulating an upregulation of the ubiquitin-modifying enzyme, A20, in infected monocytes. P protein interacted indirectly with a negative regulatory motif in the A20 gene promoter, and released the suppression of A20 transcription [75]. These interactions, which occur during the sensing of MeV, are summarized in Figure 4. Sensing of virus RNA initiates the activation of nuclear transcription factors that include interferon regulatory factors 3 and 7 (IRF3, IRF7), and nuclear factor of the kappa light chain enhancer of B cells (NF-kB). This leads to transcription of the IFN-b gene, as well as inflammatory cytokines [71,76,77]. The incoming virion is the initial structure recognized by the host during MeV infection. At low MOIs, MeV replication is required to activate both TANK binding kinase (TBK)-1/IKKe and IRF3 [78]. At higher MOIs, however, the amount of nucleocapsid-like structures (assembled N protein) introduced into the cell is sufficient to activate IRF3 directly to induce IFN www.sciencedirect.com

Host factors and measles virus replication Delpeut et al. 773

Figure 4

(a) Interferon induction

(b) Interferon transduction

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Effects of MeV proteins on host factors involved in innate immunity. (a) Effects of MeV proteins on virus sensors and activators of IFN induction and cytokine synthesis. The C and V proteins of MeV negatively affect the viral RNA sensors RIG-I, MDA5, and LGP2. N protein of MeV can positively activate the phosphorylation of IRF-3 and stimulate IFN-b production. On the contrary, the V protein of MeV represses the activation of IRF-3. The V, P, and C proteins can bind subunits of NF-kB and block its effects on IFN-b and cytokine production. Finally, the C protein of MeV can inhibit transcription of IFN-b. A red stoplight represents repression of the signal while a green stoplight represents activation of the pathway. (b) Effects of MeV proteins on IFN signaling/transduction. The P and V proteins can repress phosphorylation of STAT-1/2 by JAK-1/TYK-2 kinases. P, N, and V proteins block the migration of STAT-1/2 transcription activators into the nucleus. The V protein of MeV can also block the activation of IFN-a synthesis by IRF-3/7. Finally, MeV can stimulate the effects of the interferon inducible gene ADAR1 and blocks the effects of the interferon inducible gene products OAS, PKR, and Mx. A red stoplight represents repression of the signal while a green stoplight represents activation of the pathway.

production [78]. MeV can counteract this effect and has evolved many countermeasures to antagonize the induction of IFN. In human plasmacytoid DCs, induction of IFN through the TLR7/9/MyD88-dependent pathway is abolished through the binding of MeV V protein to the adapter IKKa (inhibitor of nuclear factor kappa-B kinase subunit alpha), which inhibits IRF7 phosphorylation [79,80]. The MeV V protein also interacts with IRF3 and IRF7 to inhibit their transcriptional activities [79,81]. MeV-V was also shown to interact with and inhibit the NOD-like receptor pyrin domain containing (NLRP)3 inflammasome-mediated secretion of IL-1b [82]. Downstream of the IKKa/b complex, MeV P, V, and C proteins individually bind NF-kB subunits and prevent signaling. The strongest inhibitory effect is mediated through the sequestration of NF-kB subunit p65 bound to MeV-P in the cytosol [83]. The C protein of wild type MeV also accumulates in the nucleus to down regulate IFN-b transcription from its promoter, independent of IRF-3, suggesting the existence of a host nuclear target [84]. www.sciencedirect.com

However, the C protein of MeV vaccine strains contains a mutated nuclear localization signal (NLS), disrupting efficient import of the protein into the nucleus, which probably contributes to attenuation of the vaccine virus [84]. MeV can inhibit the amount of IFN synthesis within the infected cell, but it can also block its release and prevent the establishment of an antiviral state in neighboring uninfected cells. Effector pathways that are downstream of MeV sensors are shown in Figure 4a. Tagged versions of MeV-V protein have also been used in a proteomic analysis of virus-host interactions and further molecular partners may be validated in the near future [85]. Blocking the synthesis of IFN alone is not sufficient to allow MeV to fully escape the IFN response. MeV also interferes with the actual IFN signal transduction pathway (Figure 4b). Briefly, type I IFN signals through a receptor that leads to the phosphorylation and activation of signal transducers and activators of transcription 1and 2 (STAT1, STAT2) via Janus kinase 1 (Jak1) and tyrosine kinase 2 Current Opinion in Virology 2012, 2:767–777

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(Tyk2) [86]. Phosphorylated STAT1 and STAT2 heterodimers then translocate to the nucleus to form IFN-stimulated gene factor 3 (ISGF3), which binds IFN-stimulated response elements (ISREs) to drive the expression of IFNinducible proteins. These include MxA, the doublestranded RNA protein kinase (PKR), and the adenosine deaminase acting on RNA (ADAR1) [87,88,89–91]. In order to block the establishment of an antiviral state, MeV can inhibit signaling through the JAK/STAT pathway. The N and V proteins of MeV can prevent the nuclear import of activated STAT [87,88,92,93]. P protein of MeV can also induce STAT1 degradation [45]. In addition, the V and P proteins can also block Jak1 phosphorylation [75]. ADAR1 (adenosine deaminase acting on RNA) was originally thought to limit measles virus cytotoxicity and promote persistent infections in the brain by causing A to I(G) transitions in the M protein gene [94]. Surprisingly, through another mechanism that is still being worked out, host ADAR1 acts as a proviral and anti-apoptotic factor in MeV infections. It appears to act in opposition to PKR and helps the virus to prevent the activation of PKR and IRF3. It appears to act in opposition to PKR and helps the virus to prevent the activation of PKR and IRF3 and subsequent induction of IFN-b [91,95,96]. This suppression of MeV-mediated IFN-b production would ultimately enhance virus replication in ADAR1-expressing cells [96]. However, the antiviral activity of PKR is also disrupted by the C protein of MeV [92,97]. Further studies are required to improve our understanding of how MeV fights other antiviral IFN effectors.

Research Foundation (Grant #1200). C.D.R. is a Canada Research Chair (Tier I) in Vaccinology and Viral Therapeutics. R.S.N. is supported by a CIHR Banting Postdoctoral Fellowship and held a trainee award from the Beatrice Hunter Cancer Research Institute with funds provided by the Canadian Breast Cancer Foundation – Atlantic Region as part of The Terry Fox Strategic Health Research Training Program in Cancer Research in CIHR.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

2. 

Dorig RE, Marcil A, Chopra A, Richardson CD: The human CD46 molecule is a receptor for measles virus (Edmonston strain). Cell 1993, 75:295-305. Original report of a primate-specific receptor for vaccine strains of measles virus published by 2 different groups simultaneously.

3. 

Naniche D, Varior-Krishnan G, Cervoni F, Wild TF, Rossi B, Rabourdin-Combe C, Gerlier D: Human membrane cofactor protein (CD46) acts as a cellular receptor for measles virus. Journal of Virology 1993, 67:6025-6032. See Ref. [2]

4.

Riley-Vargas RC, Gill DB, Kemper C, Liszewski MK, Atkinson JP: CD46: expanding beyond complement regulation. Trends in Immunology 2004, 25:496-503.

5.

Erlenhoefer C, Wurzer WJ, Loffler S, Schneider-Schaulies S, ter Meulen V, Schneider-Schaulies J: CD150 (SLAM) is a receptor for measles virus but is not involved in viral contact-mediated proliferation inhibition. Journal of Virology 2001, 75:4499-4505.

6.

Hsu EC, Iorio C, Sarangi F, Khine AA, Richardson CD: CDw150(SLAM) is a receptor for a lymphotropic strain of measles virus and may account for the immunosuppressive properties of this virus. Virology 2001, 279:9-21.

Conclusions Host cell factors impact all stages of virus infection, viral nucleic acid synthesis, virion assembly, and evasion of the host innate immune system. Our understanding of MeV tropism and pathogenesis has greatly improved owing to the discovery of the host cellular attachment factors and entry receptors described here. The role of nectin-4/ PVRL4, along with other host factors in MeV infection of the CNS, still requires further investigation. A number of host factors have also been implicated in MeV transcription, replication, and assembly. However, the relevance of these interactions during MeV replication still remains unclear. Because of the immunosuppressive properties of MeV, interactions of this virus with components of the innate and adaptive immune systems are of paramount interest. Further research involving proteomic analysis and host genome wide screens is currently being conducted in a number of different laboratories and will undoubtedly identify other host factors involved in the replication of MeV. A knowledge of these interactions with host proteins promise to offer new targets for antiviral therapy against MeV and other viruses in general.

Acknowledgements This work was supported by grants from the Canadian Institute for Health Research (CIHR MOP 10638; CIHR MOP 114949) and Nova Scotia Health Current Opinion in Virology 2012, 2:767–777

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