Recombinant human Metapneumovirus lacking the small hydrophobic SH and/or attachment G glycoprotein: deletion of G yields a promising vaccine candidate

Intracellular processing, glycosylation, and cell surface expression of human metapneumovirus attachment glycoprotein

The biosynthesis and posttranslational processing of human metapneumovirus attachment G glycoprotein were investigated. After pulse-labeling, the G protein accumulated as three species with molecular weights of 45,000, 50,000, and 53,000 (45K, 50K, and 53K, respectively). N-Glycosidase digestion indicated that these forms represent the unglycosylated precursor and N-glycosylated intermediate products, respectively. After an appropriate chase, these three naive forms were further processed to a mature 97K form. The presence of O-linked sugars in mature G protein was confirmed by O-glycanase digestion and lectin-binding assay using Arachis hypogaea (peanut agglutinin), an O-glycan-specific lectin. In addition, in the O-glycosylation-deficient cell line (CHO ldlD cell), the G protein could not be processed to the mature form unless the exogenous Gal and GalNAc were supplemented, which provided added evidence supporting the O-linked glycosylation of G protein. The maturation of G was completely blocked by monensin but was partially sensitive to brefeldin A (BFA), suggesting the O-linked glycosylation of G initiated in the trans-Golgi compartment and terminated in the trans-Golgi network. Enzymatic deglycosylation analysis confirmed that the BFA-G was a partial mature form containing N-linked oligosaccharides and various amounts of O-linked carbohydrate side chains. The expression of G protein at the cell surface could be detected by indirect immunofluorescence staining assay. Furthermore, cell surface immunoprecipitation displayed an efficient intracellular transport of G protein.

Identification of a novel virulence factor in recombinant pneumonia virus of mice

Pneumonia virus of mice (PVM) is a murine relative of human respiratory syncytial virus (HRSV). Here we developed a reverse genetics system for PVM based on a consensus sequence for virulent strain 15. Recombinant PVM and a version engineered to express green fluorescent protein replicated as efficiently as the biological parent in vitro but were 4- and 12.5-fold attenuated in vivo, respectively. The G proteins of HRSV and PVM have been suggested to contribute to viral pathogenesis, but this had not been possible to study in a defined manner in a fully permissive host. As a first step, we evaluated recombinant mutants bearing a deletion of the entire G gene (Delta G) or expressing a G protein lacking its cytoplasmic tail (Gt). Both G mutants replicated as efficiently in vitro as their recombinant parent, but both were nonpathogenic in mice at doses that would otherwise be lethal. We could not detect replication of the Delta G mutant in mice, indicating that its attenuation is based on a severe reduction in the virus load. In contrast, the Gt mutant appeared to replicate as efficiently in mice as its recombinant parent. Thus, the reduction in virulence associated with the Gt mutant could not be accounted for by a reduction in viral replication. These results identified the cytoplasmic tail of G as a virulence factor whose effect is not mediated solely by the viral load. In addition to its intrinsic interest, a recombinant virus that replicates with wild-type-like efficiency but does not cause disease defines optimal properties for vaccine development.

Experimental infection of macaques with human metapneumovirus induces transient protective immunity

Human metapneumovirus (hMPV), a member of the family Paramyxoviridae, is a causative agent of acute respiratory-tract illness. Two main hMPV lineages circulate worldwide and reinfections occur frequently. It is unclear what level of protection is induced by natural hMPV infection, what the durability of this protection is and whether it differs for reinfection with homologous or heterologous viruses. Here, protective immunity in cynomolgus macaques at different time points after inoculation with molecularly cloned prototype viruses of the two main lineages of hMPV has been addressed. Animals received a homologous challenge at 4, 6 or 12 weeks after the primary infection. In addition, animals that had been inoculated three times within 10 weeks were challenged with homologous or heterologous virus 8 months later. Primary infection with 10(7) TCID(50) resulted in virus shedding and induction of virus-neutralizing antibody responses, with higher titres against the homologous than the heterologous virus. Infections associated with virus shedding and seroconversion protected completely from homologous reinfection within 6 weeks, and partly at 12 weeks, after primary infection. Eight months later, protection had waned to virtually undetectable levels. This study demonstrates that experimental hMPV infection induces transient protective immunity.

Frequent frameshift and point mutations in the SH gene of human metapneumovirus passaged in vitro

During the preparation of recombinant derivatives of the CAN97-83 clinical isolate of human metapneumovirus (HMPV), consensus nucleotide sequencing of the recovered RNA genomes provided evidence of frequent sequence heterogeneity at a number of genome positions. This heterogeneity was suggestive of sizable subpopulations containing mutations. An analysis of molecularly cloned cDNAs confirmed the presence of mixed populations. The biologically derived virus on which the recombinant system is based also contained sizeable mutant subpopulations, whose presence was confirmed by biological cloning and nucleotide sequencing. Most of the mutations occurred in the SH gene. For example, partial consensus sequencing of 40 independent preparations of recombinant HMPV (wild-type and various derivatives) showed that 31 of these preparations contained a total of 41 instances of small insertions in the SH gene and a total of five small insertions elsewhere. In each of these 31 preparations, there was at least one insert in SH that changed the reading frame and would yield a truncated protein. Nearly all of these insertions involved adding one or more A residues to various tracks of four or more A residues, with the most frequent site being a tract of seven A residues. There were also two instances of nucleotide deletions and numerous instances of nucleotide substitution point mutations, mostly in the SH gene. The occurrence of mutant subpopulations was greatly reduced by the replacement of the SH gene with a synthetic version in which these oligonucleotide tracts were eliminated by silent nucleotide changes. We suggest that we frequently detected subpopulations in which the expression of full-length SH protein was ablated because it provided a modest selective advantage to this clinical isolate in vitro. Adaptation involving the functional loss of a gene is unusual for an RNA virus.

Live vaccines for human metapneumovirus designed by reverse genetics

Human metapneumovirus (HMPV) was first described in 2001 and has quickly become recognized as an important cause of respiratory tract disease worldwide, especially in the pediatric population. A vaccine against HMPV is required to prevent severe disease associated with infection in infancy. The primary strategy is to develop a live-attenuated virus for intranasal immunization, which is particularly well suited against a respiratory virus. Reverse genetics provides a means of developing highly characterized 'designer' attenuated vaccine candidates. To date, several promising vaccine candidates have been developed, each using a different mode of attenuation. One candidate involves deletion of the G glycoprotein, providing attenuation that is probably based on reduced efficiency of attachment. A second candidate involves deletion of the M2-2 protein, which participates in regulating RNA synthesis and whose deletion has the advantageous property of upregulating transcription and increasing antigen synthesis. A third candidate involves replacing the P protein gene of HMPV with its counterpart from the related avian metapneumovirus, thereby introducing attenuation owing to its chimeric nature and host range restriction. Another live vaccine strategy involves using an attenuated parainfluenza virus as a vector to express HMPV protective antigens, providing a bivalent pediatric vaccine. Additional modifications to provide improved vaccines will also be discussed.

Human metapneumovirus infection

Initially, human metapneumovirus (hMPV) was isolated from children with clinical symptoms of respiratory syncytial virus (RSV) infection in whom RSV could not be detected. Since then, numerous reports have described the detection of hMPV in clinical specimens from children, adults and the elderly (both immunocompetent and immunocompromised patients), diagnosed with an acute respiratory illness all over the world. hMPV is associated with a substantial number of respiratory tract infections in otherwise healthy children, with clinical illnesses similar to those associated with other common respiratory viruses. Serological surveys have shown that hMPV is a ubiquitous virus that infects all children by the age of 5–10 years and has been circulating in humans for at least 50 years. hMPV is a member of the Metapneumovirus genus of the Paramyxoviridae family, a group of negative-stranded RNA viruses. Genetic studies on hMPV have demonstrated the presence of two distinct hMPV serotypes each divided in two subgroups. Diagnosis is made by RT-PCR assays on respiratory secretions. Rapid antigen detection tests are not yet available and its growth in cell cultures is fastidious. No vaccines, antibodies (monoclonal or polyclonal), or chemotherapeutic agents are currently licensed for use to prevent or treat hMPV infections. The contribution of hMPV to pediatric respiratory tract infections suggests that it will be important to develop a vaccine against this virus in combination with those being developed for RSV and parainfluenza viruses. Reverse genetics technology is currently used to develop multivalent vaccines against hMPV and a variety of other important respiratory viruses such as RSV. Additional research to define the pathogenesis of this viral infection and the host’ specific immune response will enhance our knowledge to guide the search for preventive and therapeutical strategies.

[Human metapneumovirus]

Human metapneumovirus (hMPV), first isolated in the Netherlands in 2001, is a member of the genus Metapneumovirus of the sub-family Pneumovirinae of the family Paramyxoviridae. The genomic organization of hMPV is 3'-N-P-M-F-M2-SH-G-L-5'. hMPV resembles the sole member of this genus, avian pneumovirus. hMPV is the most closely related human pathogen to respiratory syncytial virus. Phylogenetic analysis of the nucleotide sequences indicated that there were two genetic groups. Furthermore, each group could be subdivided into two subgroups. hMPV encodes three surface proteins, F, G and SH proteins. The majority of antibodies to hMPV in serum were antibody against F protein, which mediates cross-group neutralization and protection. The incidences of hMPV-associated respiratory infection estimate 5 to 10% in children and 2 to 4% in adults. hMPV generally causes upper respiratory tract infection and flu-like illness, the virus can be associated with lower tract infections, such as wheezy bronchitis, bronchitis, bronchiolitis and pneumonia, in very young children, elderly persons, and immunocompromised patients. hMPV has a seasonal peak during the spring in Japan. Reinfection with hMPV frequently occurs in children, implying that the host immune response induced by natural infection provides incomplete protection. The RT-PCR test is the most sensitive test for detection of hMPV.

Characterization of human metapneumovirus F protein-promoted membrane fusion: critical roles for proteolytic processing and low pH

Human metapneumovirus (HMPV) is a recently described human pathogen of the pneumovirus subfamily within the paramyxovirus family. HMPV infection is prevalent worldwide and is associated with severe respiratory disease, particularly in infants. The HMPV fusion protein (F) amino acid sequence contains features characteristic of other paramyxovirus F proteins, including a putative cleavage site and potential N-linked glycosylation sites. Propagation of HMPV in cell culture requires exogenous trypsin, which cleaves the F protein, and HMPV, like several other pneumoviruses, is infectious in the absence of its attachment protein (G). However, little is known about HMPV F-promoted fusion, since the HMPV glycoproteins have yet to be analyzed separately from the virus. Using syncytium and luciferase reporter gene fusion assays, we determined the basic requirements for HMPV F protein-promoted fusion in transiently transfected cells. Our data indicate that proteolytic cleavage of the F protein is a stringent requirement for fusion and that the HMPV G protein does not significantly enhance fusion. Unexpectedly, we also found that fusion can be detected only when transfected cells are treated with trypsin and exposed to low pH, indicating that this viral fusion protein may function in a manner unique among the paramyxoviruses. We also analyzed the F protein cleavage site and three potential N-linked glycosylation sites by mutagenesis. Mutations in the cleavage site designed to facilitate endogenous cleavage did so with low efficiency, and our data suggest that all three N-glycosylation sites are utilized and that each affects cleavage and fusion to various degrees.

Epidemiology of human metapneumovirus

Since the discovery of human metapneumovirus (hMPV) in 2001, the virus has been identified worldwide. hMPV is a common respiratory pathogen, particularly in infants and young children. The virus is associated with both upper and lower respiratory tract infections and may be a trigger for asthma. At least two major genotypes of hMPV circulate during community outbreaks. Whether these genotypes represent distinct serotypes remains controversial. The major challenges faced by the medical and scientific communities are the understanding of the pathogenesis of hMPV disease and the development of a safe and effective vaccine to protect against infection and disease caused by this newly recognized respiratory virus.

Modification of the trypsin-dependent cleavage activation site of the human metapneumovirus fusion protein to be trypsin independent does not increase replication or spread in rodents or nonhuman primates

The contribution of cleavage activation of the fusion F protein of human metapneumovirus (HMPV) to replication and pathogenicity in rodents and nonhuman primates was investigated. Recombinant HMPVs were generated in which the naturally occurring trypsin-dependent cleavage sequence (R-Q-S-R downward arrow) was replaced by each of three sequences whose cleavage in vitro does not depend upon added trypsin. Two of these were multibasic sequences derived from avian metapneumovirus type A (R-R-R-R) or type C (R-K-A-R), with the former containing the consensus furin protease cleavage motif (R-X-R/K-R downward arrow). The third one (R-Q-P-R) was derived from a recently described trypsin independent HMPV isolate (J. H. Schickli, J. Kaur, N. Ulbrandt, R. R. Spaete, and R. S. Tang, J. Virol. 79:10678-10689, 2005). To preclude the possibility of conferring even greater virulence to this significant human pathogen, the modifications were done in an HMPV variant that was attenuated by the deletion of two of the three envelope glycoproteins, SH and G. Each of the introduced cleavage sequences conferred trypsin independent F cleavage and growth to HMPV in vitro. However, they differed in the efficiency of trypsin independent growth and plaque formation in vitro: R-R-R-R > R-K-A-R > R-Q-P-R > R-Q-S-R. The R-R-R-R mutant was the only one whose growth in vitro was not augmented by added trypsin, indicative of highly efficient trypsin independent cleavage. When inoculated intranasally into hamsters, there was essentially no difference in the magnitude of replication in the upper or lower respiratory tract between the mutants, and virus was not detected in organs outside of the respiratory tract. Evaluation of the most cleavage-efficient mutant, R-R-R-R, in African green monkeys showed that there was no detectable change in the magnitude of replication in the upper and lower respiratory tract or in immunogenicity and protective efficacy against HMPV challenge. These results suggest that cleavage activation is not a major determinant of HMPV virulence.

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.

Human metapneumovirus in paediatric patients

Acute respiratory tract infections (ARTIs) are a leading cause of morbidity and mortality in children worldwide, but the aetiology of many ARTIs is still unknown. In 2001, researchers in The Netherlands reported the discovery of a previously unidentified pathogen called human metapneumovirus (hMPV). Since its initial description, hMPV has been associated with ARTI in Europe (Italy, France, Spain, the UK, Germany, Denmark, Finland and Norway), America (the USA, Canada, Argentina and Brazil), Asia (India, Japan, China and Singapore), Australia and South Africa in individuals of all ages. The incidence of infection varies from 1.5% to 25%, indicating that hMPV is a ubiquitous virus with a worldwide distribution. hMPV seems to play an important role as a cause of paediatric upper and lower respiratory tract infection, with similar, but not identical, epidemiological and clinical features to those of respiratory syncytial virus and influenza virus. Moreover, the socio‐economic impact of hMPV‐infected children on their families seems to be considerable, which suggests that, like influenza virus, hMPV infection may be a substantial public health problem for the community. It may be associated with significant morbidity and mortality in pre‐term infants and children with underlying clinical conditions, although more adequately controlled studies are needed to confirm its importance in such patients. Many fundamental questions concerning the pathogenesis of hMPV disease and the host's specific immune response remain to be answered. Further studies are also required to properly define hMPV diagnosis, treatment and prevention strategies.

New generation live vaccines against human respiratory syncytial virus designed by reverse genetics

Development of a live pediatric vaccine against human respiratory syncytial virus (RSV) is complicated by the need to immunize young infants and the difficulty in balancing attenuation and immunogenicity. The ability to introduce desired mutations into infectious virus by reverse genetics provides a method for identifying and designing highly defined attenuating mutations. These can be introduced in combinations as desired to achieve gradations of attenuation. Attenuation is based on several strategies: multiple independent temperature-sensitive point mutations in the polymerase, a temperature-sensitive point mutation in a transcription signal, a set of non-temperature-sensitive mutations involving several genes, deletion of a viral RNA synthesis regulatory protein, and deletion of viral IFN alpha/beta antagonists. The genetic stability of the live vaccine can be increased by judicious choice of mutations. The virus also can be engineered to increase the level of expression of the protective antigens. Protective antigens from antigenically distinct RSV strains can be added or swapped to increase the breadth of coverage. Alternatively, the major RSV protective antigens can be expressed from transcription units added to an attenuated parainfluenza vaccine virus, making a bivalent vaccine. This would obviate the difficulties inherent in the fragility and inefficient in vitro growth of RSV, simplifying vaccine design and use.

Human Metapneumovirus Elicits Weak IFN-γ Memory Responses Compared with Respiratory Syncytial Virus

Human metapneumovirus (MPV) is a recently discovered pathogen that causes repeated lower respiratory tract infections beginning in infancy. The prevalence, nature and control of human regulatory responses to MPV are unknown. In this study, we develop and optimize systems to evaluate MPV-driven cytokine responses. Using primary culture of human PBMC from previously exposed adults, MPV-stimulated responses were directly compared with those elicited by genetically and clinically similar respiratory syncytial virus (RSV). Intense IL-6 production was evident following culture with infectious or inactivated RSV. MPV elicited IL-6 responses averaging 3.5-fold more intense (p < 0.001). Virus-dependent expression of IL-11, IL-12, IFN-alpha, and other innate immunity cytokines differed little between MPV and RSV. When examining adaptive immunity, RSV infection elicited strong IFN-gamma responses by all 60 adults. In marked contrast, MPV elicited IFN-gamma in a lower frequency of adults (p < 0.002) and at levels averaging 6-fold weaker (p < 0.001). These Th1-dominated responses were CD4, CD8, CD86 dependent, and were closely paralleled by strong virus-driven IL-10 and CCL5 production. For MPV and RSV, Th2 (IL-5, IL-13) responses were sporadic, occurring in 10-40% of the population. Thus, MPV and RSV, although both ubiquitous and leading to very high levels of infection, seroconversion, and clinically similar presentation in the population, evoke distinct innate and adaptive T cell-dependent cytokine responses. Although both viruses yield Th1-dominated responses with strong IL-10 and CCL5 production, MPV restimulation results in markedly more robust IL-6 and significantly weaker adaptive cytokine responses, in both prevalence and intensity, than does RSV.

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.

Cytotoxic T-lymphocyte epitope vaccination protects against human metapneumovirus infection and disease in mice

Human metapneumovirus (hMPV) has emerged as an important human respiratory pathogen causing upper and lower respiratory tract infections in young children and older adults. In addition, hMPV infection is associated with asthma exacerbation in young children. Recent epidemiological evidence indicates that hMPV may cocirculate with human respiratory syncytial virus (hRSV) and mediate clinical disease similar to that seen with hRSV. Therefore, a vaccine for hMPV is highly desirable. In the present study, we used predictive bioinformatics, peptide immunization, and functional T-cell assays to define hMPV cytotoxic T-lymphocyte (CTL) epitopes recognized by mouse T cells restricted through several major histocompatibility complex class I alleles, including HLA-A*0201. We demonstrate that peptide immunization with hMPV CTL epitopes reduces viral load and immunopathology in the lungs of hMPV-challenged mice and enhances the expression of Th1-type cytokines (gamma interferon and interleukin-12 [IL-12]) in lungs and regional lymph nodes. In addition, we show that levels of Th2-type cytokines (IL-10 and IL-4) are significantly lower in hMPV CTL epitope-vaccinated mice challenged with hMPV. These results demonstrate for the first time the efficacy of an hMPV CTL epitope vaccine in the control of hMPV infection in a murine model.

Individual contributions of the human metapneumovirus F, G, and SH surface glycoproteins to the induction of neutralizing antibodies and protective immunity

We evaluated the individual contributions of the three surface glycoproteins of human metapneumovirus (HMPV), namely the fusion F, attachment G, and small hydrophobic SH proteins, to the induction of serum HMPV-binding antibodies, serum HMPV-neutralizing antibodies, and protective immunity. Using reverse genetics, each HMPV protein was expressed individually from an added gene in recombinant human parainfluenza virus type 1 (rHPIV1) and used to infect hamsters once or twice by the intranasal route. The F protein was highly immunogenic and protective, whereas G and SH were only weakly or negligibly immunogenic and protective, respectively. Thus, in contrast to other paramyxoviruses, the HMPV attachment G protein is not a major neutralization or protective antigen. Also, although the SH protein of HMPV is a virion protein that is much larger than its counterparts in previously studied paramyxoviruses, it does not appear to be a significant neutralization or protective antigen.

Human metapneumovirus: a ubiquitous and long-standing respiratory pathogen

BACKGROUND

Human metapneumovirus (hMPV) is a recently described human pathogen first identified in respiratory specimens of young children suffering from respiratory syndromes ranging from mild to severe.

METHODS AND RESULTS

Virological studies have reported the presence of hMPV infections in many countries from all continents. Seroprevalence studies have indicated that the virus has been circulating in humans for more than 50 years and that it infects virtually all children by the ages of 5-10 years. In young children, hMPV has been mainly associated with bronchiolitis but also with pneumonitis, otitis media and acute exacerbation of asthma. The contribution of hMPV in respiratory syndromes of adults has been studied considerably less; initial studies have indicated a role for this pathogen in flu-like syndromes and in significant percentages of chronic obstructive pulmonary disease exacerbations and cases of community-acquired pneumonia during the winter-spring period. Both primate and rodent experimental models have been used to characterize the pathogenesis of this respiratory virus. In some of these models, intranasal hMPV inoculation has elicited not only important viral replication but also significant pulmonary inflammation and clinical disease. Recently a few groups have developed reverse genetic systems for hMPV, allowing a better understanding of viral pathogenesis and generation of attenuated viral strains for immunization.

CONCLUSIONS

Recent studies on hMPV have provided a better understanding of the epidemiology and pathogenesis associated with this viral infection, and have enhanced the prospect of developing efficient therapeutic agents and vaccine candidates.

Infection of nonhuman primates with recombinant human metapneumovirus lacking the SH, G, or M2-2 protein categorizes each as a nonessential accessory protein and identifies vaccine candidates

Recombinant human metapneumovirus (HMPV) in which the SH, G, or M2 gene or open reading frame was deleted by reverse genetics was evaluated for replication and vaccine efficacy following topical administration to the respiratory tract of African green monkeys, a permissive primate host. Replication of the deltaSH virus was only marginally less efficient than that of wild-type HMPV, whereas the deltaG and deltaM2-2 viruses were reduced sixfold and 160-fold in the upper respiratory tract and 3,200-fold and 4,000-fold in the lower respiratory tract, respectively. Even with the highly attenuated mutants, there was unequivocal HMPV replication at each anatomical site in each animal. Thus, none of these three proteins is essential for HMPV replication in a primate host, although G and M2-2 increased the efficiency of replication. Each gene-deletion virus was highly immunogenic and protective against wild-type HMPV challenge. The deltaG and deltaM2-2 viruses are promising vaccine candidates that are based on independent mechanisms of attenuation and are appropriate for clinical evaluation.

Rapid human metapneumovirus microneutralization assay based on green fluorescent protein expression

We describe a simple and expedited microneutralization assay for human metapneumovirus virus (HMPV) based on a recombinant HMPV expressing the enhanced green fluorescent protein (rHMPV-GFP). Test serum dilutions were incubated with fixed amounts of rHMPV-GFP and inoculated onto Vero cells, and the growth of non-neutralized rHMPV-GFP was visualized by fluorescent microscopy of living cells. A preliminary titer could be determined following 3 days of incubation. GFP expression was sufficient to be read by an automated scanner after 4-5 days of incubation, which also provided a permanent record. In comparison, the conventional serum neutralization assay requires a longer incubation time plus the additional steps of fixation and staining or immunostaining. rHMPV-GFP-based titers could be determined by the 50% infectivity endpoint method of Reed and Muench [Reed, L.J., Muench, H., 1938. A simple method of estimating fifty per cent endpoint. Am. J. Hyg. 27, 493-497], or by automated scanning and non-linear regression to determine the 50% endpoint of GFP fluorescence. The latter method was two- to three-fold more sensitive. This assay also permits automation and up-scaling, making it suitable for broad HMPV seroepidemiology studies and experiments that require large scale serology, such as vaccine studies.

Deletion of M2 gene open reading frames 1 and 2 of human metapneumovirus: effects on RNA synthesis, attenuation, and immunogenicity

The M2 gene of human metapneumovirus (HMPV) contains two overlapping open reading frames (ORFs), M2-1 and M2-2. The expression of separate M2-1 and M2-2 proteins from these ORFs was confirmed, and recombinant HMPVs were recovered in which expression of M2-1 and M2-2 was ablated individually or together [rdeltaM2-1, rdeltaM2-2, and rdeltaM2(1+2)]. Each M2 mutant virus directed efficient multicycle growth in Vero cells. The ability to recover HMPV lacking M2-1 contrasts with human respiratory syncytial virus, for which M2-1 is an essential transcription factor. Expression of the downstream HMPV M2-2 ORF was not reduced when translation of the upstream M2-1 ORF was silenced, indicating that it is initiated separately. The rdeltaM2-2 mutants exhibited a two- to fivefold increase in the accumulation of mRNA, normalized to the genome template, suggesting that M2-2 has a role in regulating RNA synthesis. Replication and immunogenicity were tested in hamsters. Animals infected intranasally with rdeltaM2-1 or rdeltaM2(1+2) did not have recoverable virus in the lungs or nasal turbinates on days 3 or 5 postinfection and did not develop HMPV-neutralizing serum antibodies or resistance to HMPV challenge. Thus, M2-1 appears to be essential for significant virus replication in vivo. In animals infected with rdeltaM2-2, virus was recovered from only 1 of 12 animals and only in the nasal turbinates on a single day. However, all of the animals developed a high titer of HMPV-neutralizing serum antibodies and were highly protected against challenge with wild-type HMPV. The HMPV rdeltaM2-2 virus is a promising and highly attenuated HMPV vaccine candidate.

Complete sequence of the RNA genome of pneumonia virus of mice (PVM)

Pneumonia virus of mice (PVM) is an enveloped RNA-containing virus of Family Paramyxoviridae. Sequences had been determined previously for a number of PVM genes, although these represented cloned cDNAs rather than consensus sequences. Sequences were not available for the 3' -leader and 5' -trailer regions that constitute the genome termini or for the large polymerase L gene that accounts for 43% of the genome. Also, the available sequences were from an attenuated variant of strain 15, whereas the present study analyzed the version of strain 15 that is available from the American Type Culture Collection (ATCC) and is highly virulent in mice. Analysis of unclosed RT-PCR products yielded a complete consensus sequence of 14,886 nt (GenBank accession number AY729016). Of the regions for which sequences had been previously reported for the non-pathogenic strain, there were 13 nucleotide differences and 10 amino acid differences compared to the present consensus sequence for the virulent isolate. The various genes of PVM shared 29-62% nucleotide sequence identity and 10-60% amino acid sequence identity with human or bovine respiratory syncytial virus (HRSV and BRSV), its closest relatives.

Human Metapneumovirus: An Important Cause of Respiratory Disease in Children and Adults

Human metapneumovirus is a paramyxovirus that was discovered in 2001 in the Netherlands. Epidemiologic studies have shown it to be a major cause of acute respiratory tract disease in normal infants and children worldwide, with a seasonal occurrence and spectrum of clinical illness most similar to the closely related respiratory syncytial virus. The greatest prevalence of severe disease requiring hospitalization in otherwise healthy children appears to be in those aged between 6 and 12 months, older than the peak age of hospitalizations for respiratory syncytial virus. Human metapneumovirus is also a significant cause of acute respiratory disease in adults, particularly the elderly and those with comorbid conditions such as chronic obstructive pulmonary disease, asthma, and cancer. Because there is no rapid diagnostic assay, reverse transcriptase polymerase chain reaction is most widely used. Animal models have been developed, and candidate live-attenuated vaccines are in preclinical trials, offering the potential for future interventions in high-risk groups.