Variable susceptibility to infection in mouse and rat strains

Inbred and outbred mouse and rat strains have very different susceptibility to Sendai virus infection. Visualization of SeV infection in live animals demonstrates this difference.^[27] The 129/J mice tested were approximately 25,000-fold more sensitive than SJL/J mice.^[28] C57BL/6 mice are highly resistant to the virus, while DBA/2J mice are sensitive.^[29] C57BL/6 mice showed slight loss of body weight after SeV administration, which returned to normal later. Only 10% mortality rate was observed in C57BL/6 mice after the administration of very high virulent dose of 1*10⁵ TCID50.^[30] It was shown that resistance to the lethal effects of Sendai virus in mice is genetically controlled and expressed through control of viral replication within the first 72 hours of infection.^[29] Treatment of both strains with exogenous IFN before and during viral infection led to an increase in survival time in C57BL/6 mice, but all animals of both strains ultimately succumb to SeV caused disease.^[31] If a mouse survives a SeV infection, it develops a lifelong immunity to subsequent viral infections.^[32]

There are SeV-resistant F344 rats and susceptible BN rats.^[33]

Course of infection

In the host airways the virus titer reaches a peak after 5–6 days post infection initiation that decreases to undetectable levels by day 14.^[34] The virus promotes a descending respiratory infection, which begins in the nasal passages, passes through the trachea into the lungs and causes necrosis of the respiratory epithelium. The necrosis is mild in the first few days of infection, but later became severe by peaking around day 5. By day 9, the cells of the surface of the airways regenerate. Focal interstitial pneumonia can developed accompanied by inflammation and lesions of various degrees on the lungs. Usually, the respiratory system shows signs of healing within 3 weeks of infection, however, residual lesions, inflammation, or permanent scarring can occur. 6–8 days after the infection initiation serum antibodies appear. They remain detectable for about 1 year.

Symptoms in animals

• Sneezing
• Hunched posture
• Respiratory distress
• Porphyrin discharge from eyes and/or nose
• Lethargy
• Failure to thrive in surviving babies and young rats
• Anorexia

Diagnosis and prophylaxis

SeV induces lesions within the respiratory tract, usually associated with bacterial inflammation of the trachea and lung (tracheitis and bronchopneumonia, respectively). However, the lesions are limited, and aren't indicative solely of SeV infection. Detection, therefore, makes use of SeV-specific antigens in several serological methods, including ELISA, immunofluorescence, and hemagglutination assays, with particular emphasis on use of the ELISA for its high sensitivity (unlike the hemagglutination assay) and its fairly early detection (unlike the immunofluorescence assay).^[35]

In a natural setting, the respiratory infection of Sendai virus in mice is acute. From the extrapolation of the infection of laboratory mice, the presence of the virus may first be detected in the lungs 48 to 72 hours following exposure. As the virus replicates in the respiratory tract of an infected mouse, the concentration of the virus grows most quickly during the third day of infection. After that, the growth of the virus is slower but consistent. Typically, the peak concentration of the virus is on the sixth or seventh day, and rapid decline follows that by the ninth day. A fairly vigorous immune response mounted against the virus is the cause of this decline. The longest period of detected presence of the virus in a mouse lung is fourteen days after infection.^[36]

Eaton et al. advises that, when controlling an outbreak of SeV, disinfecting the laboratory environment and vaccinating the breeders, as well as eliminating infected animals and screening incoming animals, should clear the problem very quickly. Imported animals should be vaccinated with SeV and placed in quarantine, while, in the laboratory environment, breeding programs should be discontinued, and the non-breeding adults isolated for two months.^[37]

Apoptosis inhibition

Sendai virus P gene encodes a nested set of proteins (C', C, Y1 and Y2), which are named to collectively as the C proteins (see the section "genome structure" below). C proteins of SeV are able to suppress apoptosis.^[38] The antiapoptotic activity of the C proteins supports SeV infection in the host cells.

Interferon production and signal transduction inhibition

The virus prevents the stimulation of type 1 IFN production and subsequent cell apoptosis in response to virus infection by inhibiting the activation of IRF-3.^[39][40][41] Two virus proteins: C and V are mainly involved in this process. SeV can attenuate cell defense mechanisms and allow itself to escape from host innate immunity by inhibiting the interferon response pathway in addition to inhibiting the interferon production. The table below demonstrates the inhibition mechanism.

Anti-IFN activity of C protein is shared across the family Paramyxoviridae, and therefore appears to play an important role in paramyxovirus immune evasion.^[42] Human Parainfluenza Virus type 1 (HPIV1), which is a close relative of SeV and is (in contrast to SeV) a successful human pathogen, does not express V proteins, only C proteins. So, all needed functions provided by V in SeV can be provided by C in HPIV1. Thus, C and V have these "overlapping functions" because of the multi-faceted nature of host defense that can be countered at so many places, and exactly how well and where will in part explain host restriction.^[56]

Historic safety concerns

In 1952, Kuroya and his colleagues attempted to identify an infectious agent in human tissue samples at Tohoku University Hospital, Sendai, Japan. The samples were taken from the lung of a newborn child that was affected by fatal pneumonia. The primary isolate from the samples was passaged in mice and subsequently in embryonated eggs.^[68][69] The isolated infectious agent was later called Sendai virus, which was used interchangeably with the name “Hemagglutinating Virus of Japan”. Kuroya and his colleagues were convinced that they isolated the virus, which is a new etiological agent for human respiratory infections. Later in 1954, Fukumi and his colleagues at the Japan National Institute of Health put forward an alternative explanation for the origin of the virus. It was suggested that the mice used to passage the virus were infected with the mouse virus. Thus, mouse virus was later transferred to embryonated eggs, isolated and finally named the Sendai virus.^[70] This explanation of Fukumi, pointing to the mouse rather than the human origin of the virus, has been supported by numerous scientific data later. The historical aspects of the Sendai virus isolation and controversy behind it are well described in the review.^[4] Thus, for some time, it was erroneously assumed that Sendai virus is human disease causing pathogen.^[71] The incorrect assumption that the virus was isolated from human infectious material is still reported by Encyclopædia Britannica^[72] and by ATCC in the description of the history of the viral isolate Sendai/52.^[73] It was also believed that the virus could cause disease not only in humans but also in pigs, because antibodies to the virus were often found in their organisms during the swine epidemic in Japan in 1953–1956. High incidence of seropositivity to the virus was observed in pigs in 15 districts of Japan.^[71] An explanation was later found for this widespread detection of antibodies (see the section below). Yet, despite overwhelming evidence that indicate that SeV is host restrictive rodent pathogen, in some veterinary manuals.^[74] and safety leaflets,^[75][76] SeV is still listed as a virus that can cause disease in pigs. Similar information is provided by Encyclopædia Britannica.^[72] In reality, the multiple isolates of paramyxoviruses in pigs, using modern nucleic acid sequencing methods, have never been identified as SeV.^{[57][58][59][60][61][62]}

Antigenic stability and cross-reactive antibodies

All viruses in the family Paramyxoviridae are antigenically stable; therefore the family representatives that are close relatives and belong to the same genus, most likely, share common antigenic determinants. Thus, porcine parainfluenza 1,^[59][60] which has high sequence homology with SeV^[59] and also belongs to the same genus Respirovirus as SeV, probably, has cross-reactive antibodies with SeV. Perhaps the porcine parainfluenza 1 was responsible for pigs disease in Japan in 1953–1956.^[71] However, the antigenic cross-reactivity among these two representatives within the genus Respirovirus may explain why SeV antibodies were found in sick pigs, and why it was thought that SeV was the etiological causative agent of pigs disease. Human parainfluenza virus type 1, also shares common antigenic determinants with SeV and triggers the generation of cross-reactive neutralizing antibodies.^[63] This fact can explain wide spread detection of SeV antibodies in humans in the 1950s-1960s.^[71] Recently published study also showed this wide spread detection. The study that was published in 2011 demonstrated that SeV neutralizing antibodies (which were formed due to human parainfluenza virus type 1 past infection) can be detected in 92.5% of human subjects worldwide with a median EC₅₀ titer of 60.6 and values ranging from 5.9 to 11,324.^[66] Low anti-SeV antibodies background does not block the ability of SeV-base vaccine to promote antigen-specific T cell immunity.^[67]

SeV stimulates interferon production and transduction pathway

The main component of innate antiviral response is type I interferons (IFNs) production and most cells can produce type I IFNs, including IFN-α and -β.^[79] The recognition by cellular molecules that are called pattern recognition receptors (PRR) of triggering viral elements, such as the virus genomic RNA, the replication intermediary double-stranded RNA, or the viral ribonucleoproteins, promotes IFN production and response pathways. Viral genomic and protein components can bind variable PRRs and stimulate a signaling pathway that results in the activation of the transcription factors, which relocate to nucleus and trigger type I IFNs transcription.

Viral stimulation of RIG-1 and MDA-5 mediated IFN production

Long-term antiviral immunity

After viral infection in rodents, type I IFNs promote SeV clearance and speed up the migration and maturation of dendritic cells. However, soon after viral infection, animals efficiently generate cytotoxic T cells independently of type I IFN signaling and clear the virus from their lungs. Moreover, even the animals that are unresponsive to type I IFN develop long-term anti-SeV immunity in a form of memory response that includes generation of CD8⁺ T cells and neutralizing antibodies. This memory response can protect animals against further challenge with a lethal dose of virus.^[25]

Direct cancer cells killing. Malignant cells are vulnerable to SeV infection.

Sendai virus can infect and kill variable cancer cells (see section Sensitive cell lines and virus strains). However, some malignant cells are resistant to SeV infection. There are multiple explanations for such resistance. Not all cancer cells have cell entry receptors for the virus and not all cancer cells express virus processing serine proteases. There are also other mechanisms that can make a cancer cell resistant to an oncolytic virus. For example, some cancer cells maintain interferon response system that completely or partially protects a host cells from a virus infection.^[148] Therefore, biomarkers needed to be developed to identify tumors that might succumb to SeV mediated oncolysis.

Killing of malignant cells by virus triggered anti-tumor immunity

The virus triggers indirect immunomodulated death of malignant cells using a number of mechanisms, which are described in a published review.^[8] The viral enzyme neuraminidase (NA), which has sialidase activity, can make cancer cells more visible to the immune system by removing sialic acid residues from the surface of malignant cells.^[8] SeV activates natural killer cells (NK), cytotoxic T lymphocytes (CTL) and dendritic cells (DC). The secretion of interleukin-6, that is triggered by the virus, also inhibits regulatory T cells.

Stimulation of the secretion of cytokines

Intrinsic anti-tumor and anti-angiogenic functions of type I interferons

Interferons

Type I and type II interferons have anticancer activity (see the "Function" section in the "Interferon" article). Interferons can promote expression of major histocompatibility complex molecules, MHC I and MHC II, and stimulate immunoproteasome activity. All interferons drastically increase the presentation of MHC I dependent antigens. Interferon gamma (IFN-gamma) also strongly promotes the MHC II-dependent presentation of antigens.^[231] Higher MHC I expression leads to higher presentation of viral and abnormal peptides from cancer cells to cytotoxic T cells, while the immunoproteasome more efficiently processes these peptides for loading onto the MHC I molecule. Therefore, the recognition and killing of infected or malignant cells increases. Higher MHC II expression enhances presentation of viral and cancer peptides to helper T cells; which are releasing cytokines (such as more interferons, interleukins and other cytokines) that stimulate and co-ordinate the activity of other immune cells.^{[232][233][234]}

By down regulation of angiogenic stimuli produced by tumor cells interferon can also suppress angiogenesis^[235] In addition, they suppress the proliferation of endothelial cells. Such suppression causes a decrease in tumor vascularization and subsequent growth inhibition. Interferons can directly activate immune cells including macrophages and natural killer cells.^[232] INF-1 and interferon gamma (IFN-γ) production are triggered by SeV molecular components in many cells (See "Virus induced antiviral immunity" section above).^{[90][91][92][109]} It has been demonstrated that SeV can also induce the production of IFN type III (IFN-lambda)^[105] by human plasmacytoid dendritic cells.^[106]

Non interferons

Sendai virus can induce the production of many cytokines that enhance cellular immune responses against cancer cells. SeV stimulates the production of macrophage inflammatory protein-1α (MIB-1α) and –β (MIB-1β), RANTES (CCL5), tumor necrosis factor-alpha (TNF-alpha), tumor necrosis factor-beta (TNF-beta), interleukin-6 (IL-6 ), interleukin-8 (IL-8), interleukin-1 alpha (IL1A), interleukin-1 beta (IL1B), platelet-derived growth factor (PDGF-AB) and small concentrations of interleukin-2 (IL2) and GM-CSF.^[92][91][90] Even plasmids that deliver the F-coding gene of SeV to tumor cells in model animals trigger the production of RANTES (CCL5) in tumor-infiltrated T-lymphocytes.^[107]

SeV induces the production of B cell-activating factor by monocytes and by some other cells.^[116]

Heat-inactivated SeV virus induces the production of IL-10 and IL-6 cytokines by dendritic cells (DC).^[117] Most likely, F protein is responsible for this induction because reconstituted liposomes containing F protein can stimulate IL-6 production by DC. The production of IL-6 in response to SeV infection is restricted to conventional dendritic cells (DCs) subsets, such as CD4⁺ and double negative (dnDC).^[103]

The UV-inactivated SeV (and likely the alive virus as well) can stimulate dendritic cells to secrete chemokines and cytokines such as interleukin-6, interferon-beta, chemokine (C-C motif) ligand 5, and chemokine (C-X-C motif) ligand 10. These molecules activate both CD8⁺ T cells as well as natural killer cells and attract them to the tumor. It has been shown that in cancer cell lines, UV-inactivated SeV triggers the production of an intercellular adhesion molecule -1 (ICAM-1, CD54), which is a glycoprotein that serves as a ligand for macrophage-1 antigen (Mac-1) and lymphocyte function-associated antigen 1 (LFA-1 (integrin)). Mac-1 and LFA-1 are receptors found on leukocytes. This induced production happens through the activation of nuclear factor-κB downstream of the mitochondrial antiviral signaling pathway and the retinoic acid-inducible gene I. The increased concentration of ICAM-1 on the surface of cancer cells, which is triggered by SeV, increases the vulnerability of these cells to natural killer cells.^[118]

SeV suppression of regulatory T cells

Experiments with animal models have shown that, even after UV inactivation, SeV can block T-cell-mediated regulatory immunosuppression in tumors. The blocking mechanism is associated with the stimulation of SeV inactivated virions of interleukin 6 (IL-6) secretion by mature DCs. These effects lead to the eradication of most model tumors and inhibit the growth of the rest.^[137] It has been shown that F protein alone can trigger IL-6 production in DC in a fusion-independent manner.^[107]

Schematic diagram of Sendai Virus genome with Green Fluorescent Protein (GFP). (A) The recombinant virus genome expresses eGFP (239 residues) fused to the C-terminus of the L protein (2,228 residues). (B) Western blot analysis of GFP expressed in infected cells. Lysates of mock- (lane 1), recombinant virus genomes (lane 2 and lane 3) infected cells were reacted with anti-GFP antibody. (C) Analysis of construct localization in live cells. HeLa cells were infected with recombinant genomes and images of the cells were captured at the indicated hours post infection.

Human parainfluenza virus 1 (HPV1)

Wild type, attenuated SeV has been used in clinical trials involving both adults^[63] and children^[65] to immunize against HPIV-1.The virus administration in the form of nasal drops in doses ranging from 5 × 10⁵ 50% embryo infectious dose (EID50) to 5 × 10⁷ induced the production of neutralizing antibodies to the human virus without any measurable side effects. The results of these trials represent an evidence of safety for humans of replication competent Sendai virus administration. SeV antibodies that cross-reactive with HPIV-1 antibodies are present in most people, however, majority of people do not have high titer of these antibodies. The study that was published in 2011 demonstrated that SeV neutralizing antibodies (which were formed due to HPIV-1 past infection) can be detected in 92.5% subjects worldwide with a median EC₅₀ titer of 60.6 and values ranging from 5.9–11,324.^[66] Low anti-SeV antibodies background does not block the ability of SeV-base vaccine to promote antigen-specific T cell immunity.^[67]

Human immunodeficiency virus type 1 (HIV)

The development of T cell-based AIDS vaccines using Sendai virus vectors is taking place reached phase II clinical trial. Evaluation of the safety and immunogenicity of an intranasally administered replication-competent Sendai Virus–vectored HIV Type 1 gag vaccine demonstrated: induction of potent T-Cell and antibody responses in prime-boost regimens.^[18][17]

Respiratory syncytial virus (Human orthopneumovirus)

Sendai virus was also used as a backbone for vaccine against respiratory syncytial virus (HRSV).^[13][299] This virus (HRSV), is a major cause of lower respiratory tract infections and hospital visits during infancy and childhood. It was shown that administration of SeV-based RSV vaccine protects cotton rats^[300] and African green monkeys from this viral infection.^[299] The HRSV phase I clinical trial was completed in adults. It demonstrated high safety of the SeV-based construct that expressed HRSV envelope F glycoprotein.^[16]

Mycobacterium tuberculosis

SeV is currently used in preclinical studies as a backbone vector for vaccine against tuberculosis. Mucosal vaccination with SeV construct generates memory CD8 T cell immunity and promotes protection against Mycobacterium tuberculosis in mice.^{[301][15][302]}

Structure

Genome structure
The positions of translation initiation sites for products of the alternative reading frame of the P-coding mRNA

The SeV genome is non-segmented, negative-sense RNA, of about 15.384 n. in length, and contains the noncoding 3’ leader and 5’ trailer regions, which are about 50 nucleotides in length.^[4][266] As in other respiroviruses from family Paramyxoviridae, in SeV they work as cis-acting elements essential for replication. A 3’ leader sequence acts as a transcriptional promoter. Between these non-coding regions are located six genes, which encode the nucleocapsid (NP) protein, phosphoprotein (P), matrix protein (M), fusion protein (F), hemagglutinin-neuraminidase (HN) and large (L) protein in this order from the 3’ terminus.^[4][266] The RNA-dependent RNA polymerase of the SeV consists of the large protein (L) and the phosphoprotein (P). The structural gene sequence of SeV is as follows: 3′-NP-P-M-F-HN-L-5′. Intergenomic regions between these genes are three nucleotides long as in other respiroviruses. Additional proteins, which are frequently called non structural or accessory proteins can be produced from the P gene, using alternative reading frames.^[4][319] The Sendai virus P/C mRNA contains five ribosomal initiation sites between positions 81 and 201 from the 5' end. One of these sites initiates in the P open reading frame, whereas four others initiate a nested set of C proteins (C', C, Y1, Y2).^{[320][319][321]} These C proteins are initiated in the + 1 reading frame to that of P at different translation starting sites. Sendai virus uses ribosome shunting to express Y1 and Y2 proteins that initiate at the fourth and fifth start sites on the P/C mRNA (respectively).^[321] Three additional SeV proteins are also encoded by P/C mRNA. Two of these proteins V and W are products of RNA editing, at codon 317 of the mRNA - G residues are added co-transcriptionally, (+one G residue for V and +two G for W).^[318] The third - X protein is represented by 95 amino acids of the C terminal of the P protein and independently initiated by ribosomes.^[322] All these non-structural proteins have several functions, including the organization of viral RNA synthesis and helping the virus to infect rodent cells by escaping host innate immunity (see "The mechanism of viral immunosuppression in natural hosts" section above).^[318] It has also been found that C protein facilitates budding of virus-like particles^[323] and small amounts of C protein are associated with a viral capsid.^[317]

Evolution Stability

The genomes of non segmented negative-stranded RNA viruses (including paramyxoviruses) have a low rate of homologous recombination and evolve comparatively slowly.^[267][268] Multiple reasons for this genomic stability likely exist: (1) the genomes of these viruses are nonsegmented, therefore cannot undergo genetic reassortment, (2) each protein and each amino acid has an important function. Therefore, any new genetic insertion, substitution or deletion would lead to a decrease or total loss of function that would in turn cause the new virus variant to be less viable. (3) Sendai virus belong to viruses that are governed by the “rule of six”. SeV genome as genomes of other paramyxoviruses mainly include six genes, which encode for six major proteins.^[269] Low rate of homologous RNA recombination in paramyxoviruses probably results from this unusual genomic requirement for polyhexameric length (6n+0). Natural high genomic stability of SeV is a positive feature for it potential use as a vaccine vector or as an oncolytic agent. For any clinical or industrial applications, it is important that SeV genomic and inserted foreign trans genes would be expressed in a stable way. Genetic stability enables performance of many serial passages in cell cultures or embryonated chicken eggs without viral genomic change.

Adsorbtion and fusion

Sendai virus initiates infection process by host cell adsorbtion mediated by the recognition of specific receptor molecules.^[337] Hemagglutinin neuraminidase (HN) serves as a virus cell attachment protein that interacts with a specific cell entry receptor. NH has sialidase activity, and it is capable of cleaving sialic acid residues from the cell receptor. This cleavage triggers the fusion process of viral envelope and cell membrane, which promotes by cooperation of NH with the viral fusion protein (F).^[355] The SeV F-protein as other Paramyxovirus structural fusion proteins is a trimeric molecule that belongs to class I viral membrane fusion proteins.^[312] To perform the fusion function F protein must be proteolytically activated from it precursor inactive form F₀.^[356] This activation requires F₀ cleavage by host serine protease before the virus adsorbtion (see the section “proteolytic cleavage by cellular proteases”). F₀ must be cleaved by the host protease into F1 and F2 subunits that remained connected through a disulfide covalent bond. The cleavage site in the F₀-protein is located N-terminal to the fusion peptide which has N-terminal Hepta-Repeat 1 (HR1) and C-terminal Hepta-Repeat 2 (HR2) domains. The illustration below shows 5 stages of the fusion of the virus envelope and cellular host membrane.^[312] 1) The pre-fusion, F protein (highlighted in red) is protruding from the lipid bilayer of the viral envelope and is in a close proximity to the cellular membrane. 2) Receptor-HN binding, during the SeV host cell attachment process, triggers the release of the fusion peptide from the F-protein. The peptide inserts itself into the host cell membrane. This insertion is accompanied by the transformation of the HR1 domain from a helical structure to an extended helical trimeric coil-coil structure. 3) Transformed HR1 domain attaches viral F-protein to the host-cell membrane. 4) Two lipid bilayers (viral and cellular) fuse with each other. 5) The fusion of the HR2 and HR1 domains of the F-protein promotes the establishment of a stable six-helix bundle structure (6HB). The formation of the 6HB structure leads to the establishment of the pore and the completion of the fusion process. Viral genomic material enters the host cell through this formed pore.^[312]

Uncoating

After a merging of the host membrane and the viral envelope, the SeV according to one model is “uncoating” with diffusion of the viral envelope proteins into the host plasma membrane.^[357] According to another model the virus did not release its envelope proteins into the host membrane. The viral and host membranes are fused and a connecting structure is made. This connecting structure serves as a transportation "highway" for the viral ribonucleoprotein (RNP). Thus, RNP travels through the connecting structure to reach the cell interior^[357] allowing SeV genetic material to enter the host cell cytoplasm.^[355][358]

Cytoplasmic transcription and replication

Once in the cytoplasm, the SeV genomic RNA is getting involved, as a template, in two different RNA synthetic processes performed by RNA-dependent RNA polymerase, which consists of L and P proteins: (1) transcription to generate mRNAs and (2) replication to produce a positive-sense antigenome RNA that in turn acts as a template for production of progeny negative-strand genomes.^[359][360] RNA-dependent RNA polymerase promotes the generation of mRNAs methylated cap structures.^[361]

The NP protein is thought to have both structural and functional roles^[362] This protein concentration is believed to regulate the switch from RNA transcription to RNA replication. The genomic RNA functions as the template for the viral RNA transcription until the NP protein concentration increases. As the NP protein accumulates, the transition from the transcription to the replication occurs.^[363] The NP protein encapsidates the genomic RNA, forming a helical nucleocapsid which is the template for RNA synthesis by the viral RNA polymerase. The protein is a crucial component of the following complexes NP-P (P, phosphoprotein), NP-NP, nucleocapsid-polymerase, and RNA-NP. All these complexes are needed for the viral RNA replication.^[362]

Translation

Two different sets of proteins are translated from viral mRNAs.^[4] The first set is represented by six structural proteins that include nucleocapsid protein (NP), phosphoprotein (P), matrix protein (M), fusion protein (F), neuraminidase (NA) and large protein (L).^[4] All these proteins have variable functions and are incorporated into the viral capsid (see the section “virion structure” above). The second set is represented by seven non structural or accessory proteins.^[4] These proteins are translated from the polycistronic mRNA of P gene.^{[320][319][321]} This mRNA encodes eight translation products, and P-protein is only one of them. Alternative variants of translation are represented by V,W, C, C’,Y,Y’ and X proteins. The proteins C’, C, Y1, Y2 are products of mRNA alternative reading frame, they collectively referred as C-proteins or C-nested proteins and they share common C-terminal end.^[4][364] The X protein also shares the same C-terminal end and its translation also independently initiated by ribosomes.^[322] The proteins V and W are products of cotranscriptional mRNA editing. All these non-structural proteins have multiple functions, including the organization of viral RNA synthesis and helping the virus to infect host cells by escaping host innate immunity^[318] (see "The mechanism of viral immunosuppression in natural hosts" section above).

Possible model depicting formation of the viral assembly complex

Transportation of RNP and viral proteins to cellular membrane

After translation, SeV nucleocapsids (RNP complex) assemble, and move using microtubules network through intracellular vesicular trafficking pathway.^[280] In preparation for the budding process, three viral lipophilic proteins HN, F and M migrate through the secretory pathway to a host cell membrane.^[280][365] It is assumed that the interaction of these three proteins with each other is needed for their migration to cellular budding sites.^[280] The binding of lipophilic protein complex to the host membrane facilitates the interaction of this three protein complex with the SeV nucleocapsid.^[365] It has been shown that for efficient virion production SeV induces the β-cytoplasmic actin remodeling in its host cell.^[366]

Syncytium formation and direct cell-to-cell infection transmission

Two of SeV proteins: HA and F, after their binding directly to a cellular membrane, promote a cell-cell fusion, which leads to a large multinuclear cell formation (syncytium). This formation involves the fusion of infected cells with adjacent target cells and remains an important mechanism of direct cell-to-cell spread of viral components. Thus, a SeV infection in a form of genetic material in partially assembled virions can spread without any exposure to host neutralizing antibodies (see the section "Directed cells fusion (syncytium formation)" for details and references).

Budding

Sendai virus, as all other envelope viruses, uses host cellular membrane lipid bilayer for viral capsid membrane formation. Binding to a host cell membrane of viral proteins (M, HN and F) promotes their interaction with RNP complex, which is composed of the viral genomic RNA bound to SeV proteins (NP, P and L).^[365] Thus, all viral structural components, including viral glycoproteins and genomic RNP complex, are getting assembled together. After such assembling the infectious viral particles are budding out from individually or collectively infected cells (syncitia). It has been suggested that recirculating endosomes are involved in viral RNP complex translocation.^[280] C-protein facilitates budding by interacting with AIP1/Alix, which is a host protein that is involved in apoptosis and endosomal membrane trafficking.^[324] The infectious virus particles usually released by 24 hours post infection (hpi), and peak titers appeared between 48 and 72 hpi.^[285]

Persistent infection

Sendai virus can establish persistent infection in its host cells. Multiple rounds of virus subculturing result in a creation of new virus variants with high ability to establish persistent infection. These SeV variants develop certain genotypic changes.^[367] The persistent infection can also be established instantly in interferon regulatory factor 3 (IRF-3)-knockdown cells. IRF-3 is a key proapoptotic protein that after activation by SeV triggers apoptosis. IRF-3-knockdown cells express viral protein and produces low levels of infectious virions.^[368][369] IRF-3 controls the fate of the SeV-infected cells by triggering apoptosis and preventing persistence establishment; therefore its knock down allows persistence to occur.^[115] It was also reported that during SeV infection replication defective viral genomes (DVG) are forming^[370] and selectively protect a subpopulation of host cells from death, therefore promoting the establishment of persistent infections.^[371][372] In nature enzootic disease patterns suggest that the virus can be latent and can be cleared over the course of a year.

History

All Sendai virus strains belong to the same serotype. The origin of many strains of SeV was described in 1978.^[71] Some strains such as Ohita^[386] and Hamamatsu^[389] were described later. Ohita and Hamanatsu strains were isolated from separate epidemics in laboratory mice.^[390][391] According to the personal memory of Alisa G. Bukrinskaya, who has co-authored numerous publications related to SeV along with Prof. Viktor M. Zhdanov, starting in 1961,^[392] the Moscow strain of SeV^[144] was obtained by Prof. Viktor M. Zhdanov of the Ivanovsky Institute of Virology from Japan in the late 1950s or early 1960s,^[392] It is reported^[307] that the BB1 strain^[306] derived from the Moscow virus strain.^[144] The strain BB1 was given to the researchers of Institute of Viral Disease Control and Prevention, Beijing, China by researchers of Ivanovsky Institute of Virology, Moscow, Russia in 1960s.^[307]

Virulence

A field SeV isolate, that is attenuated through egg-passages, is less virulent for mouse respiratory cells.^[393] Therefore, the strains that were isolated from animals a few decades ago and went through multiple passages in eggs are less virulent for mice in comparison with the strains that are fresh field isolates.

Defective interfering genomes

Defective interfering (DI) genomes or defective viral genomes (DVGs) are replication defective viral RNA products generated during viral infections by many types of viruses, including SeV.^{[394][370][372]} It has been experimentally established that DI genomes can be readily produced by viral infection at high multiplicity.^[395][396] A single amino acid substitution in a nucleoprotein (NP) causes an increased production rate of DI genomes in the SeV Cantell strain, which is known for its particularly strong induction of interferon beta (IFN-β) during viral infection.^[397] It has been shown that DI are responsible for this strong IFN-β induction.^[398]

Strains origin and sequence ID

*The sequence of Enders strain is available from the US patent Modified Sendai virus vaccine and imaging vector

Strains sequence similarity