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

Establishment and application of a quadruple real-time RT-PCR for detecting avian metapneumovirus

In order to develop an appropriate method for high-throughput detection of avian metapneumovirus, a quadruple real-time reverse-transcription polymerase chain reaction assay was established with four pairs of specific primers and four specific probes based on the G or M gene of aMPV-A, aMPV-B, aMPV-C and aMPV-D. Its specificity and sensitivity were evaluated, and clinical samples were tested by the method. The results showed that all the four subgroups of avian metapneumovirus can be detected in the quadruple real-time RT-PCR assay simultaneously, with a detection limit of 100-1000 cRNA copies/reaction. The other common poultry viruses were negative. In the avian clinical sample detection, 39 out of 1920 clinical samples collected from 8 provinces were positive. Compared with published RT-PCR assays, the κ value of the quadruple real-time RT-PCR assay in 1920 avian clinical samples was 1.000 (P < 0.001). The established method could be used for the rapid detection of the four subgroups of avian metapneumovirus with high specificity and high sensitivity.

Zoonotic Origins of Human Metapneumovirus: A Journey from Birds to Humans

Metapneumoviruses, members of the family Pneumoviridae, have been identified in birds (avian metapneumoviruses; AMPV’s) and humans (human metapneumoviruses; HMPV’s). AMPV and HMPV are closely related viruses with a similar genomic organization and cause respiratory tract illnesses in birds and humans, respectively. AMPV can be classified into four subgroups, A–D, and is the etiological agent of turkey rhinotracheitis and swollen head syndrome in chickens. Epidemiological studies have indicated that AMPV also circulates in wild bird species which may act as reservoir hosts for novel subtypes. HMPV was first discovered in 2001, but retrospective studies have shown that HMPV has been circulating in humans for at least 50 years. AMPV subgroup C is more closely related to HMPV than to any other AMPV subgroup, suggesting that HMPV has evolved from AMPV-C following zoonotic transfer. In this review, we present a historical perspective on the discovery of metapneumoviruses and discuss the host tropism, pathogenicity, and molecular characteristics of the different AMPV and HMPV subgroups to provide increased focus on the necessity to better understand the evolutionary pathways through which HMPV emerged as a seasonal endemic human respiratory virus.

First report of Avian metapneumovirus type B in Iraqi broiler flocks with swollen head syndrome

Background and Aim:

Swollen head syndrome (SHS) is a complex disease caused by various agents, including bacterial and viral pathogens, as well as environmental factors. Avian metapneumovirus (aMPV) is one of the most important causes of respiratory diseases and SHS in poultry and one of the most widespread viruses worldwide; however, it has not been recorded in Iraq. This study aimed at the molecular identification and subtyping of aMPV in poultry, with the objectives of investigating the prevalence of aMPV in infected broiler flocks with SHS and molecular typing using primers specific to the study of the prevalence of subtypes A, B, and C of aMPV.

Materials and Methods:

This study was performed on 67 broiler farms that reported typical SHS from September 2018 to August 2019. Swabs were collected from the trachea, infraorbital sinuses, and lung, then uploaded on FTA cards and subjected to an RNA extraction protocol.

Results:

aMPV was detected in 16 (23.8%) samples. Molecular typing using primers specific to the attachment glycoprotein (G) gene showed that all positive samples belonged to subtype B, as assessed using the real-time polymerase chain reaction technique.

Conclusion:

aMPV may be the main etiological factor causing SHS in poultry. Moreover, this was the first report of the prevalence of subtype B aMPV strains in broiler farms in Iraq.

Consensus and variations in cell line specificity among human metapneumovirus strains

Human metapneumovirus (HMPV) has been a notable etiological agent of acute respiratory infection in humans, but it was not discovered until 2001, because HMPV replicates only in a limited number of cell lines and the cytopathic effect (CPE) is often mild. To promote the study of HMPV, several groups have generated green fluorescent protein (GFP)-expressing recombinant HMPV strains (HMPV^GFP). However, the growing evidence has complicated the understanding of cell line specificity of HMPV, because it seems to vary notably among HMPV strains. In addition, unique A2b clade HMPV strains with a 180-nucleotide duplication in the G gene (HMPV A2b_180nt-dup strains) have recently been detected. In this study, we re-evaluated and compared the cell line specificity of clinical isolates of HMPV strains, including the novel HMPV A2b_180nt-dup strains, and six recombinant HMPV^GFP strains, including the newly generated recombinant HMPV A2b_180nt-dup strain, MG0256-EGFP. Our data demonstrate that VeroE6 and LLC-MK2 cells generally showed the highest infectivity with any clinical isolates and recombinant HMPV^GFP strains. Other human-derived cell lines (BEAS-2B, A549, HEK293, MNT-1, and HeLa cells) showed certain levels of infectivity with HMPV, but these were significantly lower than those of VeroE6 and LLC-MK2 cells. Also, the infectivity in these suboptimal cell lines varied greatly among HMPV strains. The variations were not directly related to HMPV genotypes, cell lines used for isolation and propagation, specific genome mutations, or nucleotide duplications in the G gene. Thus, these variations in suboptimal cell lines are likely intrinsic to particular HMPV strains.

Engineered Newcastle disease virus expressing the F and G proteins of AMPV-C confers protection against challenges in turkeys

Avian metapneumovirus (AMPV) infects the respiratory and reproductive tracts of domestic poultry, resulting in substantial economic losses for producers. Live attenuated vaccines appear to be the most effective in countries where the disease is prevalent. However, reversion to virulence has been demonstrated in several studies. Therefore, the development of a stable and safe next generation vaccine against the AMPV disease is needed. In the present study, we generated a recombinant Newcastle disease virus (NDV) vectoring the fusion (F) protein and glycoprotein (G) genes of AMPV subtype-C (AMPV-C) as a bivalent vaccine candidate using reverse genetics technology. The recombinant virus, rLS/AMPV-C F&G, was slightly attenuated in vivo, yet maintained similar characteristics in vitro when compared to the parental LaSota virus. Vaccination of turkeys with rLS/AMPV-C F&G induced both AMPV-C and NDV-specific antibody responses, and provided significant protection against pathogenic AMPV-C challenge and complete protection against velogenic NDV challenge. These results suggest that the rLS/AMPV-C F&G recombinant virus is a safe and effective bivalent vaccine candidate and that the expression of both F and G proteins of AMPV-C induces a protective response against the AMPV-C disease.

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.

The pathogenicity of avian metapneumovirus subtype C wild bird isolates in domestic turkeys

Background

Avian metapneumovirus subtype C (aMPV/C) causes severe upper respiratory disease in turkeys. Previous report revealed the presence of aMPV/C in wild birds in the southeast regions of the U.S.

Methods

In this study, aMPV/C positive oral swabs from American coots (AC) and Canada geese (CG) were passaged three times in the respiratory tract of specific pathogen free (SPF) turkeys and used as aMPV/C P3 virus isolates in subsequent studies.

Results

Wild bird P3 isolates showed similar growth characteristics when compared to virulent aMPV/C in chicken embryo fibroblast ( CEF) cell cultures and their glycoprotein G gene sequence was closely related to the G gene of aMPV/C Colorado reference virus. Three-day-old commercial or SPF turkeys were inoculated oculonasally with wild bird aMPV/C P3 isolates. At 5 and 7 days post-inoculation (DPI), severe clinical signs were observed in both of the AC and CG virus-exposed groups. Viral RNA was detected in tracheal swabs by reverse transcriptase polymerase chain reaction (RT-PCR). In addition, immunohistochemistry showed virus replication in the nasal turbinate and trachea. All virus-exposed turkeys developed positive antibody response by 14 DPI.

Conclusions

Our data demonstrate that aMPV/C wild bird isolates induced typical aMPV/C disease in the domestic turkeys.

Development and optimization of a direct plaque assay for human and avian metapneumoviruses

The genus Metapneumovirus within the subfamily Pneumovirinae and family Paramyxoviridae includes only two viruses, human metapneumovirus (hMPV) and avian metapneumovirus (aMPV), which cause respiratory disease in humans and birds, respectively. These two viruses grow poorly in cell culture and other quantitation methods, such as indirect immuno-staining and immuno-fluorescent assays, are expensive, time consuming, and do not allow for plaque purification of the virus. In order to enhance research efforts for studying these two viruses, a direct plaque assay for both hMPV and aMPV has been developed. By optimizing the chemical components of the agarose overlay, it was found that both hMPV with a trypsin-independent F cleavage site and aMPV formed clear and countable plaques in a number of mammalian cell lines (such as Vero-E6 and LLC-MK2 cells) after 5 days of incubation. The plaque forming assay has similar sensitivity and reliability as the currently used immunological methods for viral quantitation. The plaque assay is also a more simple, rapid, and economical method compared to immunological assays, and in addition allows for plaque purification of the viruses. The direct plaque assay will be a valuable method for the quantitation and evaluation of the biological properties of some metapneumoviruses.

Characterization of the biosynthesis and cell surface expression of avian metapneumovirus attachment glycoprotein

Biosynthesis, glycosylation and cell surface expression of the AMPV/C G protein were examined in eukaryotic cell lines (LLC-MK2, CHO-K1, CHO-1d1D). Immature G gene products with a molecular mass of 42, 45 and 58-90 kilodaltons (kDa) were identified by SDS-PAGE and represented glycosylated intermediates. Tunicamycin treatment of transfected cells confirmed the presence of N-linked carbohydrate moieties on these intermediate species and identified a 38 kDa unglycosylated precursor. A fully processed, mature form of the protein migrated around 110 kDa. The presence of O-linked sugars on the mature G protein was confirmed by using the O-glycosylation deficient CHO-ldlD cell line supplemented with exogenous Gal and GalNAc. Binding of the lectin Arachis hypogaea (peanut agglutinin) confirmed the presence of O-linked sugars on the mature protein and its intracellular transport to the cell surface was demonstrated by indirect immunofluorescent staining.

Fusion protein is the main determinant of metapneumovirus host tropism

Human metapneumovirus (HMPV) and avian metapneumovirus subgroup C (AMPV-C) infect humans and birds, respectively. This study confirmed the difference in host range in turkey poults, and analysed the contribution of the individual metapneumovirus genes to host range in an in vitro cell-culture model. Mammalian Vero-118 cells supported replication of both HMPV and AMPV-C in contrast to avian quail fibroblast (QT6) cells in which only AMPV-C replicated to high titres. Inoculation of Vero-118 and QT6 cells with recombinant HMPV in which genes were exchanged with those of AMPV-C revealed that the metapneumovirus fusion (F) protein is the main determinant for host tropism. Chimeric viruses in which polymerase complex proteins were exchanged between HMPV and AMPV-C replicated less efficiently compared with HMPV in QT6 cells. Using mini-genome systems, it was shown that exchanging these polymerase proteins resulted in reduced replication and transcription efficiency in QT6 cells. Examination of infected Vero-118 and QT6 cells revealed that viruses containing the F protein of AMPV-C yielded larger syncytia compared with viruses containing the HMPV F protein. Cell-content mixing assays revealed that the F protein of AMPV-C was more fusogenic compared with the F protein of HMPV, and that the F2 region is responsible for the difference observed between AMPV-C and HMPV F-promoted fusion in QT6 and Vero-118 cells. This study provides insight into the determinants of host tropism and membrane fusion of metapneumoviruses.

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.

Prevalence of human metapneumovirus among hospitalized children younger than 1 year in Catalonia, Spain

Human metapneumovirus was discovered recently respiratory virus implicated in both upper and lower respiratory tract infection. In children, the clinical symptoms of human metapneumovirus are similar to those produced by respiratory syncytial virus, ranging from mild to severe diseases such as bronchiolitis and pneumonia. The aim of the present study was to describe the prevalence of human metapneumovirus and other common respiratory viruses among admitted to hospital infants. From January 2006 to June 2006, 99 nasopharyngeal aspirates were collected from hospitalized children younger than 12 months in order to study respiratory viruses. Human metapneumovirus detection was performed by cell culture and two RT‐PCR targeting on polymerase and fusion genes. The latter gene was used for phylogenetic analysis. In 67/99 children (67%) at least one viral pathogen was identified, the viruses detected most frequently were respiratory syncytial virus (35%), human metapneumovirus (25%) and rhinovirus (19%). The results obtained in this study, show that: (1) human metapneumovirus is one of the most important viruses among children less than 12 months; (2) children infected with human metapneumovirus were significantly older than those infected by respiratory syncytial virus; (3) human metapneumovirus was associated more frequently with pneumonia whereas respiratory syncytial virus was only detected in patients with bronchiolitis; (4) there was a clear epidemiological succession pattern with only a small overlap among the viruses detected most frequently; (5) all human metapneumovirus samples were clustered within sublineage A2. J. Med. Virol. 80:1452–1460, 2008. © 2008 Wiley‐Liss, Inc.

Glycoprotein gene truncation in avian metapneumovirus subtype C isolates from the United States

The length of the published glycoprotein (G) gene sequences of avian metapneumovirus subtype-C (aMPV-C) isolated from domestic turkeys and wild birds in the United States (1996–2003) remains controversial. To explore the G gene size variation in aMPV-C by the year of isolation and cell culture passage levels, we examined 21 turkey isolates of aMPV-C at different cell culture passages. The early domestic turkey isolates of aMPV-C (aMPV/CO/1996, aMPV/MN/1a-b, and 2a-b/97) had a G gene of 1,798 nucleotides (nt) that coded for a predicted protein of 585 amino acids (aa) and showed >97% nt similarity with that of aMPV-C isolated from Canada geese. This large G gene got truncated upon serial passages in Vero cell cultures by deletion of 1,015 nt near the end of the open reading frame. The recent domestic turkey isolates of aMPV-C lacked the large G gene but instead had a small G gene of 783 nt, irrespective of cell culture passage levels. In some cultures, both large and small genes were detected, indicating the existence of a mixed population of the virus. Apparently, serial passage of aMPV-C in cell cultures and natural passage in turkeys in the field led to truncation of the G gene, which may be a mechanism of virus evolution for survival in a new host or environment.

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.

Species-specific deletion of the viral attachment glycoprotein of avian metapneumovirus

The avian metapneumovirus (AMPV) genome encodes the fusion (F), small hydrophobic (SH), and attachment glycoprotein (G) as envelope glycoproteins. The F and G proteins mainly function to allow viral entry into host cells during the early steps of the virus life cycle. The highly variable AMPV G protein is a major determinant for distinguishing virus subtypes. Sequence analysis was used to determine if any differences between avian or mammalian cell propagated subtype C AMPV could be detected for the 1.8kb G gene. As a result, the complete 1.8kb G gene was found to be present when AMPV was propagated in our immortal turkey turbinate (TT-1) cell line regardless of passage number. Surprisingly, AMPV propagated for 15 or more passages in mammalian Vero cells revealed an essentially deleted G gene in the viral genome, resulting in no G gene mRNA expression. Although the Vero cell propagated AMPV genome contained a small 122 nucleotide fragment of the G gene, no other mRNA variants were detected from either mammalian or avian propagated AMPV. The G gene truncation might be caused by cellular molecular mechanisms that are species-specific. The lack of viral gene deletions suggests that avian cell propagated AMPV will provide a better alternative host for live recombinant vaccine development based on a reverse genetics system.

Laboratory evaluation of a quantitative real-time reverse transcription PCR assay for the detection and identification of the four subgroups of avian metapneumovirus

Avian metapneumovirus (AMPV) is an important pathogen causing respiratory diseases and egg drops in several avian species. Four AMPV subgroups have been identified. The laboratory diagnosis of AMPV infections relies on serological methods, on labour-intensive virus isolation procedures, and on recently developed subgroup specific reverse transcription PCR (RT-PCR) protocols. In the present study, both the specificity and sensitivity of a commercial real-time reverse transcription PCR (RRT-PCR) for the detection and identification of the four AMPV subgroups were evaluated. Fifteen non-AMPV avian viruses belonging to 7 genera and 32 AMPV belonging to the 4 subgroups were tested. No non-AMPV virus was detected, whereas all AMPV viruses were identified in agreement with their previous molecular and antigenic subgroup assignment. The sensitivity and quantitating ability of the RRT-PCR assay were determined using serial dilutions of RNA derived either from AMPV virus stocks or from runoff transcripts. In all cases, linear dose/responses were observed. The detection limits of the different subgroups ranged from 500 to 5000 RNA copies and from 0.03 to 3.16TCID50/ml. The results were reproducible under laboratory conditions, thus showing that quantitative RRT-PCR is a new and powerful tool for the rapid and sensitive detection, identification and quantitation of AMPVs.

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.

Chimeric recombinant human metapneumoviruses with the nucleoprotein or phosphoprotein open reading frame replaced by that of avian metapneumovirus exhibit improved growth in vitro and attenuation in vivo

Chimeric versions of recombinant human metapneumovirus (HMPV) were generated by replacing the nucleoprotein (N) or phosphoprotein (P) open reading frame with its counterpart from the closely related avian metapneumovirus (AMPV) subgroup C. In Vero cells, AMPV replicated to an approximately 100-fold-higher titer than HMPV. Surprisingly, the N and P chimeric viruses replicated to a peak titer that was 11- and 25-fold higher, respectively, than that of parental HMPV. The basis for this effect is not known but was not due to obvious changes in the efficiency of gene expression. AMPV and the N and P chimeras were evaluated for replication, immunogenicity, and protective efficacy in hamsters. AMPV was attenuated compared to HMPV in this mammalian host on day 5 postinfection, but not on day 3, and only in the nasal turbinates. In contrast, the N and P chimeras were reduced approximately 100-fold in both the upper and lower respiratory tract on day 3 postinfection, although there was little difference by day 5. The N and P chimeras induced a high level of neutralizing serum antibodies and protective efficacy against HMPV; AMPV was only weakly immunogenic and protective against HMPV challenge, reflecting antigenic differences. In African green monkeys immunized intranasally and intratracheally, the mean peak titer of the P chimera was reduced 100- and 1,000-fold in the upper and lower respiratory tracts, whereas the N chimera was reduced only 10-fold in the lower respiratory tract. Both chimeras were comparable to wild-type HMPV in immunogenicity and protective efficacy. Thus, the P chimera is a promising live HMPV vaccine candidate that paradoxically combines improved growth in vitro with attenuation in vivo.

Comparison of avian cell substrates for propagating subtype C avian metapneumovirus

Avian metapneumovirus (AMPV) is a respiratory viral pathogen that causes turkey rhinotracheitis (TRT) or swollen head syndrome (SHS) in chickens. AMPV was first isolated in South Africa during the early 1970s and has subsequently spread worldwide during the 1980s to include Europe, Asia, and South America. In 1996, a genetically distinct AMPV subgroup C was isolated in the US following an outbreak of TRT. Vero cells are currently the best available substrate for AMPV propagation but are of non-avian origin. A number of different avian cell substrates have been compared to determine which is the most suitable for the propagation of AMPV to sufficiently high titers. Of the cell substrates tested, primary turkey turbinate and kidney and chicken kidney cells produced titers equal to or greater than Vero cells. Turkey turbinate and kidney epithelial cells that were life-span extended by the ectopic expression of human telomerase catalytic subunit (HTERT) initially displayed AMPV titers comparable to Vero cell controls, but declined in virus production with increased passage in culture. Interestingly, plaques emanating from Vero propagated virus were relatively small and dispersed, when analyzed by immunofluorescent assays (IFA), while both turkey turbinate and kidney cell propagated AMPV produced larger plaques. Even with these differences, there were no changes in the predicted amino acid sequences of the nucleocapsid (N) and phosphoprotein (P) genes of AMPV propagated in either turkey turbinate or Vero host cells. However, the fusion (F) gene showed 11 amino acid differences (98.7% identity) between the two host cell types. These results suggest that AMPV propagated in homologous avian cellular substrates may produce more infectious virus with possibly more effective fusion activity, compared to Vero cell propagation.

European and American Subgroup C Isolates of Avian Metapneumovirus belong to Different Genetic Lineages

The gene encoding the attachment glycoprotein (G) was sequenced in three French isolates of-subgroup C avian metapneumovirus (APV-C) from ducks. With 1771 nt, this gene proved as long as recently published for North-American APV-C isolates from turkeys. The nt sequences of the duck viruses shared 99% identity but proved only 75-83% identical with their North-American counterparts, viruses of both origins encoding 585 amino acid (aa)-long G proteins. Alignments revealed more homogeneity within the European and North-American groups (at least 98 and 79% aa identity, respectively) than between European and North-American viruses (at best 70% a identity), and confirmed the presence of an extracellular divergent domain (positions 302-484) in APV-C G. A phylogenetic analysis demonstrated that North-American and French isolates of APV-C belonged to significantly different genetic lineages, in agreement with the different geographical origin and host species of these viruses.