Deduced amino acid sequence of the small hydrophobic protein of US avian pneumovirus has greater identity with that of human metapneumovirus than those of non-US avian pneumoviruses

Isolation and identification of a subtype C avian metapneumovirus in chickens in Jiangsu, China

Avian metapneumovirus (aMPV) is a viral pathogen that mainly causes respiratory signs and drops in egg production in turkeys, chickens, and ducks. Here, an aMPV subgroup C (aMPV/C) strain, designated GX22-01, was isolated and identified from severe respiratory disease in broiler breeder chickens in 2022 in Jiangsu, China, as evidenced by indirect immunofluorescence and western blotting using specific anti-viral protein antibodies and by sequence analysis of viral nucleoprotein (N) gene. N gene sequencing indicated that the GX22-01 strain shares a high identity (94.3%-99.8 %) with aMPV/C isolates, especially with Chinese aMPV/C isolates from ducks and chickens, which are divided into aMPV/C cluster through N gene-based phylogenetic analysis. The aMPV/C GX22-01 strain was continuously passaged in Vero cells and the viral titers approximately reached 106.0 TCID50/0.1 mL. Pathogenic analysis showed that aMPV/C GX22-01 strain inoculation caused respiratory signs in 2-week-old specific-pathogen-free (SPF) chickens and resulted in pathogenic damage in tracheae and lung tissues, accompanied by positive viral signals using indirect immunohistochemistry. These results provide epidemiological and pathogenic data for developing effective measures against aMPV/C infection in China.

Small hydrophobic (SH) proteins of Pneumoviridae and Paramyxoviridae: small but mighty

Small hydrophobic (SH) proteins are a class of viral accessory proteins expressed by many members of the negative-stranded RNA viral families Paramyxoviridae and Pneumoviridae. Identified SH proteins are type I or II transmembrane (TM) proteins with a single-pass TM domain. Little is known about the functions of SH proteins; however, several possess viroporin activity, enhancing membrane permeability of infected cells or those expressing SH protein. Moreover, several SH proteins inhibit apoptosis and immune signaling pathways within infected cells, including TNF and interferon signaling, or activate inflammasomes. SH proteins are generally nonessential for viral replication in vitro, but loss of SH is often associated with reduced replication in vivo, suggesting a role in enhancing viral replication or evading host immunity. Analogous proteins are expressed by a variety of pathogens of public health importance; thus, understanding the functional importance and mechanisms of SH proteins provides insight into the pathogenesis and replication of negative-sense RNA viruses.

SQSTM1 downregulates avian metapneumovirus subgroup C replication via mediating selective autophagic degradation of viral M2-2 protein

Avian metapneumovirus subgroup C (aMPV/C), an important pathogen causing acute respiratory infection in chickens and turkeys, contributes to substantial economic losses in the poultry industry worldwide. aMPV/C has been reported to induce autophagy, which is beneficial to virus replication. Sequestosome 1 (SQSTM1/P62), a selective autophagic receptor, plays a crucial role in viral replication by clearing ubiquitinated proteins. However, the relationship between SQSTM1-mediated selective autophagy and aMPV/C replication is unclear. In this study, we found that the expression of SQSTM1 negatively regulates aMPV/C replication by reducing viral protein expression and viral titers. Further studies revealed that the interaction between SQSTM1 and aMPV/C M2-2 protein is mediated via the Phox and Bem1 (PB1) domain of the former, which recognizes a ubiquitinated lysine at position 67 of the M2-2 protein, and finally degrades M2-2 via SQSTM1-mediated selective autophagy. Collectively, our results reveal that SQSTM1 degrades M2-2 via a process of selective autophagy to suppress aMPV/C replication, thereby providing novel insights for the prevention and control of aMPV/C infection.IMPORTANCEThe selective autophagy plays an important role in virus replication. As an emerging pathogen of avian respiratory virus, clarification of the effect of SQSTM1, a selective autophagic receptor, on aMPV/C replication in host cells enables us to better understand the viral pathogenesis. Previous study showed that aMPV/C infection reduced the SQSTM1 expression accompanied by virus proliferation, but the specific regulatory mechanism between them was still unclear. In this study, we demonstrated for the first time that SQSTM1 recognizes the 67th amino acid of M2-2 protein by the interaction between them, followed by M2-2 degradation via the SQSTM1-mediated selective autophagy, and finally inhibits aMPV/C replication. This information supplies the mechanism by which SQSTM1 negatively regulates viral replication, and provides new insights for preventing and controlling aMPV/C infection.

Avian Metapneumovirus Subgroup C Phosphoprotein Suppresses Type I Interferon Production by Blocking Interferon Regulatory Factor 3 Nuclear Translocation

Avian metapneumovirus subgroup C (aMPV/C) is an important pathogen that causes upper respiratory symptoms and egg production decline in turkeys and chickens. aMPV/C infection leads to inhibition of the host antiviral immune response. However, our understanding of the molecular mechanisms underlying host immune response antagonized by aMPV/C infection is limited. In this study, we demonstrated that the aMPV/C phosphoprotein (P) inhibits the IFN antiviral signaling pathway triggered by melanoma differentiation gene 5 (MDA5) and reduces interferon β (IFN-β) production and IFN-stimulated genes (ISGs) by targeting IFN regulatory factor 7 (IRF7) but not nuclear factor κB (NF-κB) in DF-1 cells. Moreover, we found that aMPV/C P protein only blocks the nuclear translocation of IRF3 by interacting with IRF3 in HEK-293T cells, instead of affecting IRF3 phosphorylation and inducing IRF3 degradation, which suppresses IRF3 signaling activation and results in a decrease in IFN-β production. Collectively, these results reveal a novel mechanism by which aMPV/C infection disrupts IFN-β production in the host. IMPORTANCE The innate immune response is the first defense line of host cells and organisms against viral infections. When RNA viruses infect cells, viral RNA induces activation of retinoic acid-induced gene I and melanoma differentiation gene 5, which initiates downstream molecules and finally produces type I interferon (IFN-I) to regulate antiviral immune responses. The mechanism for avian metapneumovirus (aMPV) modulating IFN-I production to benefit its replication remains unknown. Here, we demonstrate that phosphoprotein of aMPV subgroup C (aMPV/C) selectively inhibits the nuclear translocation of interferon regulatory 3 (IRF3), instead of affecting the expression and phosphorylation of IRF3, which finally downregulates IFN-I production. This study showed a novel mechanism for aMPV/C infection antagonizing the host IFN response.

First detection of Avian metapneumovirus subtype C Eurasian Lineage in a Eurasian wigeon (Mareca penelope) wintering in Northeastern Italy: an additional hint on the role of migrating birds in the viral epidemiology

Avian metapneumovirus (aMPV) economically affects the global poultry industry causing respiratory and reproductive disorders. Considering the paucity of data on the aMPV occurrence in European free-ranging avifauna, a molecular survey was conducted on wild birds of 23 species belonging to the orders Anseriformes, Charadriiformes or Passeriformes, captured alive and sampled in Northeast Italy as part of the national Avian influenza virus (AIV) surveillance activities. A total of 492 oropharyngeal swabs, collected from 2007 to 2010, all AIV negative, were screened from aMPV by subtype-specific qRT-PCR. An aMPV-C strain, named aMPV/C/IT/Wigeon/758/07, was found in a wintering young Eurasian wigeon (Mareca penelope) sampled in November 2007. The matrix, fusion, and attachment glycoprotein genes of the detected strain were subsequently amplified by specific independent RT-PCRs, then sequenced, and compared in a phylogenetic framework with known aMPV homologous sequences retrieved from GenBank. Close genetic relationships were found between the aMPV/C/IT/Wigeon/758/07 strain and subtype C Eurasian lineage strains isolated in the late 1990s in French domestic ducks, suggesting epidemiological links. Eurasian wigeons are indeed medium to long-range migrant dabbling ducks that move along the Black Sea/Mediterranean flyway, our finding might therefore be related to migratory bridges between countries. To our knowledge, this is the first molecular evidence of the occurrence of a subtype C in Italy and backdates the aMPV-C circulation to 2007. Moreover, results suggest the susceptibility of Eurasian wigeons to aMPV. Broader investigations are needed to assess the role of wild ducks and the significance of the wildfowl/poultry interface in the aMPV-C epidemiology.

Avian Metapneumovirus Subgroup C Induces Mitochondrial Antiviral Signaling Protein Degradation through the Ubiquitin-Proteasome Pathway

The mitochondrial antiviral signaling (MAVS) protein, a critical adapter, links the upstream recognition of viral RNA to downstream antiviral signal transduction. However, the interaction mechanism between avian metapneumovirus subgroup C (aMPV/C) infection and MAVS remains unclear. Here, we confirmed that aMPV/C infection induced a reduction in MAVS expression in Vero cells in a dose-dependent manner, and active aMPV/C replication was required for MAVS decrease. We also found that the reduction in MAVS occurred at the post-translational level rather than at the transcriptional level. Different inhibitors were used to examine the effect of proteasome or autophagy on the regulation of MAVS. Treatment with a proteasome inhibitor MG132 effectively blocked MAVS degradation. Moreover, we demonstrated that MAVS mainly underwent K48-linked ubiquitination in the presence of MG132 in aMPV/C-infected cells, with amino acids 363, 462, and 501 of MAVS being pivotal sites in the formation of polyubiquitin chains. Finally, E3 ubiquitin ligases for MAVS degradation were screened and identified and RNF5 targeting MAVS at Lysine 363 and 462 was shown to involve in MAVS degradation in aMPV/C-infected Vero cells. Overall, these results reveal the molecular mechanism underlying aMPV/C infection-induced MAVS degradation by the ubiquitin-proteasome pathway.

Proteome Analysis in a Mammalian Cell line Reveals that PLK2 is Involved in Avian Metapneumovirus Type C (aMPV/C)-Induced Apoptosis

Avian metapneumovirus subtype C (aMPV/C) causes an acute respiratory disease that has caused serious economic losses in the Chinese poultry industry. In the present study, we first explored the protein profile in aMPV/C-infected Vero cells using iTRAQ quantitative proteomics. A total of 921 of 7034 proteins were identified as significantly altered by aMPV/C infection. Three selected proteins were confirmed by Western blot analysis. Bioinformatics GO analysis revealed multiple signaling pathways involving cell cycle, endocytosis, and PI3K-Akt, mTOR, MAPK and p53 signaling pathways, which might participate in viral infection. In this analysis, we found that PLK2 expression was upregulated by aMPV/C infection and investigated whether it contributed to aMPV/C-mediated cellular dysfunction. Suppressing PLK2 attenuated aMPV/C-induced reactive oxygen species (ROS) production and p53-dependent apoptosis and reduced virus release. These results in a mammalian cell line suggest that high PLK2 expression correlates with aMPV/C-induced apoptosis and viral replication, providing new insight into the potential avian host cellular response to aMPV/C infection and antiviral targets.

Avian metapneumovirus subgroup C induces autophagy through the ATF6 UPR pathway

An increasing number of studies have demonstrated that macroautophagy/autophagy plays an important role in the infectious processes of diverse pathogens. However, it remains unknown whether autophagy is induced in avian metapneumovirus (aMPV)-infected host cells, and, if so, how this occurs. Here, we report that aMPV subgroup C (aMPV/C) induces autophagy in cultured cells. We demonstrated this relationship by detecting classical autophagic features, including the formation of autophagsomes, the presence of GFP-LC3 puncta and the conversation of LC3-I into LC3-II. Also, we used pharmacological regulators and siRNAs targeting ATG7 or LC3 to examine the role of autophagy in aMPV/C replication. The results showed that autophagy is required for efficient replication of aMPV/C. Moreover, infection with aMPV/C promotes autophagosome maturation and induces a complete autophagic process. Finally, the ATF6 pathway, of which one component is the unfolded protein response (UPR), becomes activated in aMPV/C-infected cells. Knockdown of ATF6 inhibited aMPV/C-induced autophagy and viral replication. Collectively, these results not only show that autophagy promotes aMPV/C replication in the cultured cells, but also reveal that the molecular mechanisms underlying aMPV/C-induced autophagy depends on regulation of the ER stress-related UPR pathway.

Human metapneumovirus small hydrophobic (SH) protein downregulates type I IFN pathway signaling by affecting STAT1 expression and phosphorylation

Type I interferon (IFN) is a key mediator of antiviral immunity. Human metapneumovirus (HMPV) inhibits IFN signaling, but does not encode homologues of known IFN antagonists. We tested the hypothesis that a specific viral protein prevents type I IFN signaling by targeting signal transducer and activator of transcription-1 (STAT1). We found that human airway epithelial cells (capable of expressing IFNs) became impaired for STAT1 phosphorylation even without direct infection due to intrinsic negative feedback. HMPV-infected Vero cells (incapable of expressing IFN) displayed lower STAT1 expression and impaired STAT1 phosphorylation in response to type I IFN treatment compared to mock-infected cells. Transient overexpression of HMPV small hydrophobic (SH) protein significantly inhibited STAT1 phosphorylation and signaling, and recombinant virus lacking SH protein was unable to inhibit STAT1 phosphorylation. Our results indicate a role for the SH protein of HMPV in the downregulation of type I IFN signaling through the targeting of STAT1.

Trypsin- and low pH-mediated fusogenicity of avian metapneumovirus fusion proteins is determined by residues at positions 100, 101 and 294

Avian metapneumovirus (aMPV) and human metapneumovirus (hMPV) are members of the genus Metapneumovirus in the subfamily Pneumovirinae. Metapneumovirus fusion (F) protein mediates the fusion of host cells with the virus membrane for infection. Trypsin- and/or low pH-induced membrane fusion is a strain-dependent phenomenon for hMPV. Here, we demonstrated that three subtypes of aMPV (aMPV/A, aMPV/B, and aMPV/C) F proteins promoted cell-cell fusion in the absence of trypsin. Indeed, in the presence of trypsin, only aMPV/C F protein fusogenicity was enhanced. Mutagenesis of the amino acids at position 100 and/or 101, located at a putative cleavage region in aMPV F proteins, revealed that the trypsin-mediated fusogenicity of aMPV F proteins is regulated by the residues at positions 100 and 101. Moreover, we demonstrated that aMPV/A and aMPV/B F proteins mediated cell-cell fusion independent of low pH, whereas the aMPV/C F protein did not. Mutagenesis of the residue at position 294 in the aMPV/A, aMPV/B, and aMPV/C F proteins showed that 294G played a critical role in F protein-mediated fusion under low pH conditions. These findings on aMPV F protein-induced cell-cell fusion provide new insights into the molecular mechanisms underlying membrane fusion and pathogenesis of aMPV.

Molecular comparisons of full length metapneumovirus (MPV) genomes, including newly determined French AMPV-C and -D isolates, further supports possible subclassification within the MPV Genus

Four avian metapneumovirus (AMPV) subgroups (A-D) have been reported previously based on genetic and antigenic differences. However, until now full length sequences of the only known isolates of European subgroup C and subgroup D viruses (duck and turkey origin, respectively) have been unavailable. These full length sequences were determined and compared with other full length AMPV and human metapneumoviruses (HMPV) sequences reported previously, using phylogenetics, comparisons of nucleic and amino acid sequences and study of codon usage bias. Results confirmed that subgroup C viruses were more closely related to HMPV than they were to the other AMPV subgroups in the study. This was consistent with previous findings using partial genome sequences. Closer relationships between AMPV-A, B and D were also evident throughout the majority of results. Three metapneumovirus "clusters" HMPV, AMPV-C and AMPV-A, B and D were further supported by codon bias and phylogenetics. The data presented here together with those of previous studies describing antigenic relationships also between AMPV-A, B and D and between AMPV-C and HMPV may call for a subclassification of metapneumoviruses similar to that used for avian paramyxoviruses, grouping AMPV-A, B and D as type I metapneumoviruses and AMPV-C and HMPV as type II.

Viral replication and lung lesions in BALB/c mice experimentally inoculated with avian metapneumovirus subgroup C isolated from chickens

Avian metapneumovirus (aMPV) emerged as an important respiratory pathogen causing acute respiratory tract infection in avian species. Here we used a chicken aMPV subgroup C (aMPV/C) isolate to inoculate experimentally BALB/c mice and found that the aMPV/C can efficiently replicate and persist in the lungs of mice for at least 21 days with a peak viral load at day 6 postinoculation. Lung pathological changes were characterized by increased inflammatory cells. Immunochemical assay showed the presence of viral antigens in the lungs and significant upregulation of pulmonary inflammatory cytokines and chemokines including MCP-1, MIP-1α, RANTES, IL-1β, IFN-γ, and TNF-α were detected following inoculation. These results indicate for the first time that chicken aMPV/C may replicate in the lung of mice. Whether aMPV/C has potential as zoonotic pathogen, further investigation will be required.

Small hydrophobic protein of human metapneumovirus does not affect virus replication and host gene expression in vitro

Human metapneumovirus (HMPV) encodes a small hydrophobic (SH) protein of unknown function. HMPV from which the SH open reading frame was deleted (HMPVΔSH) was viable and displayed similar replication kinetics, cytopathic effect and plaque size compared with wild type HMPV in several cell-lines. In addition, no differences were observed in infection efficiency or cell-to-cell spreading in human primary bronchial epithelial cells (HPBEC) cultured at an air-liquid interphase. Host gene expression was analyzed in A549 cells infected with HMPV or HMPVΔSH using microarrays and mass spectrometry (MS) based techniques at multiple time points post infection. Only minor differences were observed in mRNA or protein expression levels. A possible function of HMPV SH as apoptosis blocker, as proposed for several members of the family Paramyxoviridae, was rejected based on this analysis. So far, a clear phenotype of HMPV SH deletion mutants in vitro at the virus and host levels is absent.

Biochemical characterization of the small hydrophobic protein of avian metapneumovirus

Avian metapneumovirus (AMPV) is a paramyxovirus that has three membrane proteins (G, F, and SH). Among them, the SH protein is a small type II integral membrane protein that is incorporated into virions and is only present in certain paramyxoviruses. In the present study, we show that the AMPV SH protein is modified by N-linked glycans and can be released into the extracellular environment. Furthermore, we demonstrate that glycosylated AMPV SH proteins form homodimers through cysteine-mediated disulfide bonds, which has not been reported previously for SH proteins of paramyxoviruses.

Detection of and phylogenetic studies with avian metapneumovirus recovered from feral pigeons and wild birds in Brazil

The aim of the present study was to determine whether avian metapneumovirus (aMPV)-related viruses were present in wild and synanthropic birds in Brazil. Therefore, we analysed samples from wild birds, feral pigeons and domestic chickens in order to perform a phylogenetic comparison. To detect the presence of aMPV, a nested reverse transcriptase-polymerase chain reaction was performed with the aim of amplifying a fragment of 270 bases for subtype A and 330 bases for subtype B, comprising the gene coding the G glycoprotein. Positive samples for aMPV subtypes A and B were found in seven (13.2%) different asymptomatic wild birds and pigeons (50%) that had been received at the Bosque dos Jequitibás Zoo Triage Center, Brazil. Also analysed were positive samples from 15 (12.9%) domestic chickens with swollen head syndrome from several regions of Brazil. The positive samples from wild birds, pigeons and domestic chickens clustered in two major phylogenetic groups: some with aMPV subtype A and others with subtype B. The similarity of the G fragment nucleotide sequence of aMPV isolated from chickens and synanthropic and wild avian species ranged from 100 to 97.5% (from 100 to 92.5% for the amino acids). Some positive aMPV samples, which were obtained from wild birds classified in the Orders Psittaciformes, Anseriformes and Craciformes, clustered with subtype A, and others from the Anas and Dendrocygma genera (Anseriformes Order) with subtype B. The understanding of the epizootiology of aMPV is very important, especially if this involves the participation of non-domestic bird species, which would add complexity to their control on farms and to implementation of vaccination programmes for aMPV.

Serologic evidence of avian metapneumovirus infection among adults occupationally exposed to Turkeys

Genetically similar, the avian metapneumovirus (aMPV) and the human MPV (hMPV) are the only viruses in the Metapneumovirus genus. Previous research demonstrated the ability of hMPV to cause clinical disease in turkeys. In this controlled, cross-sectional, seroepidemiological study, we examined the hypothesis that aMPV might infect humans. We enrolled 95 adults occupationally exposed to turkeys and 82 nonexposed controls. Sera from study participants were examined for antibodies against aMPV and hMPV. Both in bivariate (OR=3.2; 95% CI: 1.1-9.2) and in multivariate modelling adjusting for antibody to hMPV (OR=4.1; 95% CI: 1.3-13.1), meat-processing workers were found to have an increased odds of previous infection with aMPV compared to controls. While hMPV antibody cross-reactivity is evident, these data suggest that occupational exposure to turkeys is a risk factor for human infection with aMPV. More studies are needed to validate these findings, to identify modes of aMPV transmission, and to determine risk factors associated with infection.

Evolutionary dynamics of human and avian metapneumoviruses

Human (HMPV) and avian (AMPV) metapneumoviruses are closely related viruses that cause respiratory tract illnesses in humans and birds, respectively. Although HMPV was first discovered in 2001, retrospective studies have shown that HMPV has been circulating in humans for at least 50 years. AMPV was first isolated in the 1970s, and can be classified into four subgroups, A-D. AMPV subgroup C is more closely related to HMPV than to any other AMPV subgroup, suggesting that HMPV has emerged from AMPV-C upon zoonosis. Presently, at least four genetic lineages of HMPV circulate in human populations - A1, A2, B1 and B2 - of which lineages A and B are antigenically distinct. We used a Bayesian Markov Chain Monte Carlo (MCMC) framework to determine the evolutionary and epidemiological dynamics of HMPV and AMPV-C. The rates of nucleotide substitution, relative genetic diversity and time to the most recent common ancestor (TMRCA) were estimated using large sets of sequences of the nucleoprotein, the fusion protein and attachment protein genes. The sampled genetic diversity of HMPV was found to have arisen within the past 119-133 years, with consistent results across all three genes, while the TMRCA for HMPV and AMPV-C was estimated to have existed around 200 years ago. The relative genetic diversity observed in the four HMPV lineages was low, most likely reflecting continual population bottlenecks, with only limited evidence for positive selection.

Evidence of avian metapneumovirus subtype C infection of wild birds in Georgia, South Carolina, Arkansas and Ohio, USA

Metapneumoviruses (MPVs) were first reported in avian species (aMPVs) in the late 1970s and in humans in 2001. Although aMPVs have been reported in Europe and Asia for over 20 years, the virus first appeared in the United States in 1996, leaving many to question the origin of the virus and why it proved to be a different subtype from those found elsewhere. To examine the potential role of migratory waterfowl and other wild birds in aMPV spread, our study focused on determining whether populations of wild birds have evidence of aMPV infection. Serum samples from multiple species were initially screened using a blocking enzyme-linked immunosorbent assay. Antibodies to aMPVs were identified in five of the 15 species tested: American coots, American crows, Canada geese, cattle egrets, and rock pigeons. The presence of aMPV-specific antibodies was confirmed with virus neutralization and western blot assays. Oral swabs were collected from wild bird species with the highest percentage of aMPV-seropositive serum samples: the American coots and Canada geese. From these swabs, 17 aMPV-positive samples were identified, 11 from coots and six from geese. Sequence analysis of the matrix, attachment gene and short hydrophobic genes revealed that these viruses belong to subtype C aMPV. The detection of aMPV antibodies and the presence of virus in wild birds in Georgia, South Carolina, Arkansas and Ohio demonstrates that wild birds can serve as a reservoir of subtype C aMPV, and may provide a potential mechanism to spread aMPVs to poultry in other regions of the United States and possibly to other countries in Central and South America.

Specificity and functional interaction of the polymerase complex proteins of human and avian metapneumoviruses

Human metapneumovirus (HMPV) and avian metapneumovirus (AMPV) have a similar genome organization and protein composition, but a different host range. AMPV subgroup C (AMPV-C) is more closely related to HMPV than other AMPVs. To investigate the specificity and functional interaction of the polymerase complex proteins of human and avian metapneumoviruses, a minireplicon system was generated for AMPV-C and used in combination with minireplicon systems for HMPV lineages A1 and B1. Viral RNA-like molecules representing HMPV-A1 and -B1, AMPV-A and -C and human respiratory syncytial virus were replicated efficiently by polymerase complexes of HMPV-A1 and -B1 and AMPV-C, but not by polymerase complexes of bovine parainfluenza virus 3. Upon exchange of HMPV and AMPV-C polymerase complex components, all chimeric polymerase complexes were functional; exchange between HMPVs did not result in altered polymerase activity, whereas exchange between HMPVs and AMPV-C did. Recombinant HMPV-B1 viruses in which polymerase genes were exchanged with those of HMPV-A1 replicated with normal kinetics in vitro, whilst replacement with AMPV-C genes resulted in moderate differences in virus replication. In hamsters, recombinant HMPV-B1 viruses in which individual polymerase genes were exchanged with those of AMPV-C were attenuated, irrespective of the results obtained with minireplicon systems or in vitro replication assays. This study provides insight into the specificity and functional interaction of polymerase complex proteins of human and avian metapneumoviruses, but neither minireplicon systems nor in vitro replication kinetics were found to be predictive for attenuation in permissive animals.

Deletion of the SH gene from avian metapneumovirus has a greater impact on virus production and immunogenicity in turkeys than deletion of the G gene or M2-2 open reading frame

Subgroup A avian metapneumoviruses lacking either the SH or G gene or the M2-2 open reading frame were generated by using a reverse-genetics approach. The growth properties of these viruses were studied in vitro and in vivo in their natural host. Deletion of the SH gene alone resulted in the generation of a syncytial-plaque phenotype and this was reversed by the introduction of the SH gene from a subgroup B, but not a subgroup C, virus. Infected turkeys were assessed for antibody production and the presence of viral genomic RNA in tracheal swabs. The virus with a deleted SH gene also showed the greatest impairment of replication both in cell culture and in infected turkeys. This contrasts with the situation with other pneumoviruses in culture and in model animals, where deletion of the SH gene results in little effect upon viral yield and a good antibody response. Replication of the G- and M2-2-deleted viruses was impaired more severely in turkeys than in cell culture, with only some animals showing evidence of virus growth and antibody production. There was no correlation between virus replication and antibody response, suggesting that replication sites other than the trachea may be important for induction of antibody responses.

Genetic characterization of avian metapneumovirus subtype C isolated from pheasants in a live bird market

Complete nucleotide sequences of two avian metapneumoviruses (aMPV), designated PL-1 and PL-2, were isolated from pheasants, revealing novel sequences of the first aMPV to be fully sequenced in Korea. The complete genome of both PL-1 and PL-2 was composed of 13,170 nucleotides. Phylogenetic analysis revealed that PL-1 belonged to aMPV subtype C, sharing higher homology in deduced amino acid sequence identities with hMPV, rather than with aMPV subtypes A and B. Replication of PL-1 in experimentally re-infected pheasants was confirmed by reverse transcription (RT)-polymerase chain reaction (PCR). Chickens and mice were experimentally inoculated with PL-1 to test the replication potential of PL-1 in other species. Although one specimen from the nasal turbinates of an inoculated chicken showed a slight trace of viral replication at 3 days post-infection (dpi), all of the infected mice were negative for aMPV by RT-PCR throughout the experiment, suggesting that PL-1 does not readily infect mammals. This is the first report of the isolation and complete genomic sequence of aMPV subtype C originating from pheasants.

Recovery of avian metapneumovirus subgroup C from cDNA: cross-recognition of avian and human metapneumovirus support proteins

Avian metapneumovirus (AMPV) causes an acute respiratory disease in turkeys and is associated with "swollen head syndrome" in chickens, contributing to significant economic losses for the U.S. poultry industry. With a long-term goal of developing a better vaccine for controlling AMPV in the United States, we established a reverse genetics system to produce infectious AMPV of subgroup C entirely from cDNA. A cDNA clone encoding the entire 14,150-nucleotide genome of AMPV subgroup C strain Colorado (AMPV/CO) was generated by assembling five cDNA fragments between the T7 RNA polymerase promoter and the autocatalytic hepatitis delta virus ribozyme of a transcription plasmid, pBR 322. Transfection of this plasmid, along with the expression plasmids encoding the N, P, M2-1, and L proteins of AMPV/CO, into cells stably expressing T7 RNA polymerase resulted in the recovery of infectious AMPV/CO. Characterization of the recombinant AMPV/CO showed that its growth properties in tissue culture were similar to those of the parental virus. The potential of AMPV/CO to serve as a viral vector was also assessed by generating another recombinant virus, rAMPV/CO-GFP, that expressed the enhanced green fluorescent protein (GFP) as a foreign protein. Interestingly, GFP-expressing AMPV and GFP-expressing human metapneumovirus (HMPV) could be recovered using the support plasmids of either virus, denoting that the genome promoters are conserved between the two metapneumoviruses and can be cross-recognized by the polymerase complex proteins of either virus. These results indicate a close functional relationship between AMPV/CO and HMPV.

Function of small hydrophobic proteins of paramyxovirus

Mumps virus (MuV), a rubulavirus of the paramyxovirus family, causes acute infections in humans. MuV has seven genes including a small hydrophobic (SH) gene, which encodes a type I membrane protein of 57 amino acid residues. The function of the SH protein is not clear, although its expression is not necessary for growth of MuV in tissue culture cells. It is speculated that MuV SH plays a role in viral pathogenesis. Simian virus 5 (SV5), a closely related rubulavirus, encodes a 44-amino-acid-residue SH protein. Recombinant SV5 lacking the SH gene (rSV5DeltaSH) is viable and has no growth defect in tissue culture cells. However, rSV5DeltaSH induces apoptosis in tissue culture cells and is attenuated in vivo. Neutralizing antibodies against tumor necrosis factor alpha (TNF-alpha) and TNF-alpha receptor 1 block rSV5DeltaSH-induced apoptosis, suggesting that SV5 SH plays an essential role in blocking the TNF-alpha-mediated apoptosis pathway. Because MuV is closely related to SV5, we hypothesize that the SH protein of MuV has a function similar to that of SV5, even though there is no sequence homology between them. To test this hypothesis and to study the function of MuV SH, we have replaced the open reading frame (ORF) of SV5 SH with the ORF of MuV SH in a SV5 genome background. The recombinant SV5 (rSV5DeltaSH+MuV-SH) was analyzed in comparison with SV5. It was found that rSV5DeltaSH+MuV-SH was viable and behaved like wild-type SV5, suggesting that MuV SH has a function similar to that of SV5 SH. Furthermore, both ectopically expressed SV5 SH and MuV SH blocked activation of NF-kappaB by TNF-alpha in a reporter gene assay, suggesting that both SH proteins can inhibit TNF-alpha signaling.

Expression of recombinant small hydrophobic protein for serospecific detection of avian pneumovirus subgroup C

The small hydrophobic (SH) gene of the avian pneumovirus (APV) Colorado isolate (CO), which belongs to subgroup C (APV/C), was expressed with a baculovirus vector. The recombinant SH protein was evaluated as a potential subgroup-specific diagnostic reagent in order to differentiate infections resulting from APV/C from those induced by APV/A, APV/B, and human metapneumovirus (hMPV). When the recombinant baculovirus was used to infect insect cells, a 31- to 38-kDa glycosylated form of the SH protein was produced and subsequently tested for reactivity with antibodies specific for APV/A, APV/B, APV/C, and hMPV. Western blot analysis showed that the expressed recombinant SH protein could only be recognized by APV/C-specific antibodies. This result was consistent with sequence analysis of the APV/C SH protein, which had very low (24%) amino acid identity with the corresponding protein of hMPV and no discernible identity with the SH protein of APV/A or APV/B. A recombinant SH protein-based enzyme-linked immunosorbent assay (ELISA) was developed, and it further confirmed the lack of reactivity of this protein with antisera raised to APV/A, APV/B, and hMPV and supported its designation as a subgroup-specific antigen. This finding indicated that the recombinant SH protein was a suitable antigen for ELISA-based detection of subgroup-specific antibodies in turkeys and could be used for serologically based differential diagnosis of APV and hMPV infections.

Comparison of the full-length genome sequence of avian metapneumovirus subtype C with other paramyxoviruses

We determined the nucleotide (nt) sequence of the small hydrophobic (SH), attachment glycoprotein (G), and RNA polymerase (L) genes, plus the leader and trailer regions of the Colorado strain of Avian metapneumovirus subtype C (aMPV/C) in order to complete the genome sequencing. The complete genome comprised of 13,134 nucleotides, with a 40 nt leader at its 3' end and a 45 nt trailer at its 5' end. The aMPV/C L gene was the largest with 6173 nt and consisting of a single open reading frame encoding a 2005 amino acids (aa) protein. Comparison of the aMPV/C SH, G, and L nt and predicted aa sequences with those of Human metapneumoviruses (hMPV) revealed higher nt and aa sequence identities than the sequence identities between the aMPV subtypes A, B, C, and D, supporting earlier finding that aMPV/C was closer evolutionary to hMPV than the other aMPV subtypes.

Sequence analysis of the large polymerase (L) protein of the US strain of avian metapneumovirus indicates a close resemblance to that of the human metapneumovirus

The complete nucleotide sequence of the large polymerase (L) protein of the avian metapneumovirus subgroup C strain Colorado was determined. The L protein gene of avian pneumovirus Colorado isolate (APV-C) was 6173 nucleotides in length from the gene-start to the gene-end and encoded a polypeptide of 2005 amino acids in length. The length of the L protein of APV-C was exactly the same as that of human metapneumovirus (hMPV) and one amino acid longer than the L protein of APV subgroup A. The L protein of APV-C showed 80% amino acid identity with the L protein of hMPV, but only 64% amino acid identity with the L protein of APV-A. The nucleotide and deduced amino acid sequences were compared with the corresponding sequences of eleven other paramyxoviruses. All six domains characteristic of paramyxovirus L proteins were also observed in the L protein of APV-C. All the polymerase core motifs in domain III were conserved to nearly 100% in the metapneumoviruses. Similarly, the putative ATP-binding motif in domain VI was completely conserved among the metapneumoviruses and differed in length, by one intermediate residue, from other paramyxoviruses. Phylogenetic analysis of the different L proteins also revealed a closer relationship between APV-C and hMPV.

Complete sequence of the G glycoprotein gene of avian metapneumovirus subgroup C and identification of a divergent domain in the predicted protein

The complete nucleotide sequences of the attachment glycoprotein (G) genes of three strains of avian metapneumovirus subgroup C (AMPV-C) were determined from the viral genomic and mRNAs. The G gene of AMPV-C was 1798 nt (1015 nt longer than previously reported) and the derived polypeptide had 585 aa. The deduced amino acid sequence of the predicted G protein of AMPV-C strain Colorado (AMPV-CO) showed 21-25 % amino acid identity to the G proteins of human metapneumoviruses, but only 14-16 % amino acid identity to those of other AMPV subgroups. The predicted G protein of AMPV-CO showed 98 and 81 % amino acid identity to those of AMPV-C strains Mn-1a and Mn-2a, respectively, indicating considerable sequence variation in the G proteins of AMPV-C isolates. Comparison of the G protein sequences of AMPV-CO and Mn-2a identified a highly divergent domain (48 % amino acid identity) at aa 300-450.

Human metapneumovirus: a new player among respiratory viruses

The human metapneumovirus (hMPV) is a newly described member of the Paramyxoviridae family belonging to the Metapneumovirus genus. Since its initial description in 2001, hMPV has been reported in most parts of the world and isolated from the respiratory tract of subjects from all age groups. Despite the fact that prospective and case-control studies have been limited, the epidemiology and clinical manifestations associated with hMPV have been found to be reminiscent of those of the human respiratory syncytial virus, with most severe respiratory tract infections occurring in infants, elderly subjects, and immunocompromised hosts. Additional research is needed to define the pathogenesis of this viral infection and the host's specific immune response.