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Antimicrobial Agents and Chemotherapy, January 2008, p. 279-287, Vol. 52, No. 1
0066-4804/08/$08.00+0     doi:10.1128/AAC.00793-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Identification and Evaluation of a Highly Effective Fusion Inhibitor for Human Metapneumovirus

Céline Deffrasnes,¹ Marie-Ève Hamelin,¹ Gregory A. Prince,² and Guy Boivin^1*

Research Center in Infectious Diseases of the Centre Hospitalier Universitaire de Québec and Laval University, Quebec City, Quebec, Canada,¹ Virion Systems, Inc., Rockville, Maryland²

Received 19 June 2007/ Returned for modification 26 July 2007/ Accepted 22 October 2007

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Human metapneumovirus (hMPV) can cause acute upper and lower respiratory tract infections that are particularly severe in young children, elderly subjects, and immunocompromised patients. To date, no treatments or vaccines are available for hMPV infections. Our objective was to assess the inhibitory potential of several peptides derived from the heptad repeat A and B (HRA and HRB) domains of the hMPV fusion protein. Nine candidate peptides were expressed in Escherichia coli or obtained synthetically and tested in vitro and in an animal model. Excellent in vitro inhibition of an hMPV strain of the A1 subgroup was obtained with five peptides, with 50% inhibitory concentrations ranging from 1.4 nM to 3.3 µM. One peptide, HRA2, displayed very potent activity against all four hMPV subgroups. It was also moderately active against human respiratory syncytial virus (strain A2) but displayed no activity against human parainfluenza virus type 3. BALB/c mice that received the HRA2 peptide and a lethal hMPV intranasal challenge simultaneously were completely protected from clinical symptoms and mortality. On day 5 postinfection, HRA2-treated mice had undetectable lung viral loads which were significantly less than those of untreated mice (3 x 10⁴ 50% tissue culture infective doses/lung). Pulmonary inflammation, levels of proinflammatory cytokines/chemokines (RANTES, gamma interferon, and monocyte chemoattractant protein 1) and airway obstruction were also significantly decreased in HRA2-treated mice. The results of this study demonstrate that potent antivirals can be derived from the hMPV fusion protein HR domains. Moreover, hMPV, compared to other paramyxoviruses and to the human immunodeficiency virus, seems to be more susceptible to HRA- than HRB-derived peptides.

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Human metapneumovirus (hMPV) is a newly discovered paramyxovirus (41). It belongs to the Paramyxoviridae family, the Pneumovirinae subfamily, and the Metapneumovirus genus. Other well-known human members of this family include the human respiratory syncytial virus (hRSV) and human parainfluenza viruses (hPIV), as well as measles and mumps viruses. hMPV is most closely related to avian pneumovirus type C, which is responsible for laryngotracheitis and high mortality rates in poultry farms (31, 41). Serological studies from The Netherlands suggested that hMPV has been circulating in humans since at least 1958 and that virtually all children are infected by the age of 5 to 10 years (41). Infections caused by hMPV have been reported worldwide and are responsible for 5 to 10% of hospitalizations due to respiratory tract infections in young children (11). hMPV is associated with both upper and lower respiratory tract infections, mainly in young children, elderly subjects, and immunocompromised patients (2). Some infections can be life-threatening in infants with underlying diseases (2, 30) and in hematopoietic stem cell transplant recipients (7, 13). A recent study has also reported an outbreak of severe hMPV infections associated with many deaths in a long-term-care facility (3). Despite the development of specific vaccines and monoclonal antibodies (6, 9, 39, 47), no prophylactic or therapeutic modalities have been approved for hMPV infections.

hMPV isolates can be classified into two major groups (A and B) and at least four subgroups (A1, A2, B1, and B2) based mainly on sequence analysis of the P, F, and G genes (4, 26, 32, 42). The hMPV genome consists of a single strand of negative-sense RNA which codes for nine viral proteins. Three of the latter are glycoproteins potentially exposed at the surface of the virion and infected cells: the fusion (F), attachment, and small hydrophobic proteins (12, 40). The F protein, which is the major antigenic determinant (37), is a class I transmembrane protein mediating virus-cell fusion and infected cell-cell fusion as found in other paramyxoviruses (21, 28, 36). Membrane fusion mechanisms have been studied largely for fusogenic viruses such as human immunodeficiency virus (HIV), influenza virus, and other paramyxoviruses (17, 21, 23, 25, 29, 38, 44, 49). It is believed that hMPV follows the same pathway to penetrate target cells. The F protein first requires activation by a proteolytic cleavage event mediated by a furin-like enzyme of the host cell. Then, the fusion process involves insertion of the hydrophobic fusion peptide into the target cell membrane and a refolding of the F protein. This step requires the interaction of two specific domains: heptad repeats A and B (HRA and HRB). These alpha-helix-forming domains are sequence motifs composed of seven repeated amino acids labeled a-b-c-d-e-f-g, where a and d are usually hydrophobic amino acids and e and g are charged amino acids. The hydrophobic amino acids pack together in the interior of the helix and interact with other a and d residues from nearby helices so that they wrap around each other, forming a very stable coiled-coil structure (5). In hMPV, the F proteins assemble as trimers inside which three HRA (as well as three HRB) are creating a coiled coil. Then, the two pairs of coiled-coils (HRA and HRB) interact, creating a six-helix bundle structure that brings the viral and the cellular membranes close together, causing membrane merge and fusion pore dilatation. As seen with HIV, hRSV, and hPIV, exogenous HRA or HRB peptides can compete with their endogenous counterparts, inhibiting fusion and, consequently, infection (18-20, 22, 44, 46, 48).

Herein, we designed and evaluated several potential hMPV inhibitors based on the sequences of the HRA and HRB domains of the F protein.

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Cell lines and viruses. LLC-MK2 (rhesus monkey kidney) and HEp-2 (human laryngeal carcinoma) cells were grown in minimum essential medium (MEM) (GIBCO; Invitrogen, Burlington, Ontario, Canada) supplemented with 10% fetal bovine serum (FBS). hMPV clinical strains C-85473 (group A1), CAN97-83 (group A2), CAN97-82 (group B1), and CAN98-75 (group B2), as well as hPIV-3 strain ATCC VR-93 and hRSV strain ATCC VR-1540 (strain A2), were used in this study.

Virus propagation. hMPV was grown in LLC-MK2 cells using Opti-MEM I medium (Invitrogen) containing 2 µg/ml of trypsin (Sigma, Oakville, Ontario, Canada) (hMPV infection medium). The virus titers were reported as 50% tissue culture infectious doses (TCID₅₀) per ml using the Reed and Muench method. The hRSV A2 strain was propagated in HEp-2 cells using MEM with 2% FBS, and titers were reported as PFU per ml. The hPIV-3 strain was propagated in LLC-MK2 cells, and titers were reported as for hRSV.

hMPV immunostaining. Confluent LLC-MK2 cells were incubated with hMPV for 1.5 to 2 h, then the medium was removed, and an overlay containing 0.8% methylcellulose and hMPV infection medium was added. After 3 to 5 days, cell monolayers were fixed with formalin and blocked with phosphate-buffered saline (PBS)-5% milk. Cells were incubated for 1 h with the anti-hMPV F monoclonal antibody 1016 (a gift from MedImmune Inc., Gaithersburg, MD) and then incubated for 1 h with horseradish peroxidase-labeled goat anti-Armenian hamster antibodies (Jackson ImmunoResearch Laboratories, Inc., West Grove, PA). Plaques were stained with TrueBlue substrate (KPL, Gaithersburg, MD). Experiments were performed in triplicate, and the results were reported as the means ± standard errors of the means (SEM).

Peptide production. Nine fragments encompassing the HRA and HRB domains were amplified from hMPV strain C-85473 and cloned in the pET30a vector (Novagen, Mississauga, Ontario, Canada) by using EcoRI and HindIII restriction sites. Peptides were expressed in Escherichia coli BL21(Star) cells (Invitrogen) for 5 h at 25°C and purified using Ni-nitrilotriacetic acid agarose columns (QIAGEN, Mississauga, Ontario, Canada). Peptides were filtered on 0.22-µm membranes, quantified with a BCA protein kit (Pierce, Rockford, IL), and kept at –80°C until further use. The residual peptide produced by the empty pET30a vector was used as a negative control in the experiments. Two peptides were also synthesized at the Eastern Quebec Peptide Synthesis Facility (Quebec City, QC, Canada) using the peptide synthesizer AB433A. Quality control analysis was performed by using high-pressure liquid chromatography and matrix-assisted laser desorption ionization-time of flight mass spectrometry, and purity was estimated at >95%. The synthetic peptides were solubilized in PBS at concentrations varying between 100 µM and 500 µM.

Cytotoxicity assay. The cytotoxicities of the expressed peptides and the pET30a control peptide were evaluated with an MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay in LLC-MK2 cells. Serial dilutions of each peptide were made in hMPV infection medium and then added to the cells. After 3 days of incubation, 100 µl of a 1-mg/ml concentration of MTT was added to each well for 2 h at 37°C. The dye was solubilized with 100 µl/well of acidic isopropanol, and absorbance was read at a wavelength of 570 nM. Absorbance values that were lower than those of the control cells (optical density of 0.2 to 0.4) indicated a reduction in the rate of cell proliferation, i.e., cytotoxicity.

hMPV inhibition assays. Serial twofold dilutions of each peptide were mixed with 10⁷ TCID₅₀ of hMPV strain C-85473 in infection medium and then added to LLC-MK2 cells. After 2 h, the medium was removed and fresh medium containing serial twofold dilutions of peptides was added. On day 3, supernatants were collected and stored at –80°C until quantification by a real-time reverse transcriptase-PCR assay for the N gene using previously described primers (27) and a new TaqMan probe (5'-FAM-CTTGRTGCAATGATGAYGGTGTCACTGCXT-Tamra-PH-3').

Immunostaining was also used to assess the inhibitory activity of the HRA2 peptide against all four hMPV subgroups. Briefly, confluent LLC-MK2 cells were infected with 30 to 50 PFU of hMPV and serial twofold dilutions of the peptide for 1.5 to 2 h. The medium was then removed, and an overlay containing 0.8% methylcellulose and serial twofold dilutions of the peptide was added. After 4 days, the cell monolayers were fixed with formalin and immunostained as described earlier. All 50% inhibitory concentration (IC₅₀) values were calculated from experiments performed in triplicate.

Immunostaining was then used to determine the hMPV subgroup A1 fusion events inhibited by the HRA2 peptide. The experiment was done as described above, but the peptide was added during virus adsorption and/or in the methylcellulose overlay.

hRSV and hPIV-3 inhibition assays. The inhibitory activity of the HRA2 peptide against hRSV was determined by plaque assay. Briefly, confluent HEp-2 cells were infected with 30 to 50 PFU of hRSV and serial twofold dilutions of HRA2 in MEM with 2% FBS for 1.5 h. The medium was then replaced by an overlay medium containing 0.8% methylcellulose (Sigma) and serial dilutions of HRA2. Cells were incubated for 4 days before fixation with formalin and staining with crystal violet. Plaques and syncytia were visualized under light microscopy, and the IC₅₀ values were determined by visual count. A similar strategy was used to evaluate the activity of the HRA2 peptide against hPIV-3, except that the overlay medium contained 0.8% SeaPlaque agarose (Mandel Scientific Company, Ltd., Guelph, Ontario, Canada).

Animal experiments. A lethal BALB/c mouse model for hMPV infection was used to evaluate peptide efficacy. Ninety 4- to 6-week-old female BALB/c mice (Charles Rivers Laboratories, Ontario) were divided into five groups (18 mice/group) as follows: group A received the HRA2 peptide only; group B received hMPV only; group C received hMPV and the HRA2 peptide simultaneously; group D mice were first infected by hMPV and then received the HRA2 peptide 24 h later; and group E received hMPV and the HRB1 control peptide simultaneously. The mice were anesthetized with isoflurane [2-chloro-2-(difluoromethoxy)-1,1,1-trifluoro-ethane] and inoculated intranasally with PBS or 2 x 10⁶ TCID₅₀/ml of hMPV strain C-85473 (group A1) in 70 µl of infection medium and received, or did not receive, 7 µM of synthetic peptide by the same route. All mice were housed in groups of three in microisolator cages and monitored daily for mortality, weight loss, and clinical signs of disease. Mice were sacrificed when they had lost >25% of their initial weight or at the end of the experiment.

Lung viral titers, cytokine levels, and histopathological scores. On day 5 postinfection, 12 mice per group were sacrificed and their lungs were collected and quickly frozen in liquid nitrogen. Lungs from six mice per group were homogenized in one ml of hMPV infection medium and centrifuged for 10 min at 350 x g, and then 100-µl amounts of supernatant were used for viral titration studies. A second aliquot of 500 µl of supernatant was kept to quantify the cytokines/chemokines RANTES, monocyte chemoattractant protein 1 (MCP-1), and gamma interferon (IFN-) by enzyme-linked immunosorbent assay as described elsewhere (16). For histopathological studies, lungs from six mice per group were fixed with 10% buffered formalin, embedded in paraffin, sectioned in 5-µm slices, and stained with hematoxylin-eosin. Four types of pulmonary inflammation (peribronchiolitis, perivasculitis, interstitial inflammation, and alveolitis) were evaluated to determine histopathological scores based on a scale ranging from 0 to 4 as described elsewhere (16, 34).

Airway obstruction. On day 5 postinfection, the breathing patterns were characterized in six mice/group by using an unrestrained whole-body flowthrough plethysmograph system as previously reported (15). Mice were allowed to acclimate to the chamber for 30 min, and then respiratory parameters were recorded for 5 min and enhance pause (P_enh) values were calculated. The P_enh value is an indicator of lung function. This value increases when airway obstruction is observed, as the mice generate a larger inspiratory effort in order to fill their lungs (15, 24).

Statistical analyses. Statistical analyses were performed with GraphPad Prism 5 software using one-way analysis of variance and the Tukey multiple comparisons test.

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Peptide design. The peptides evaluated in this study encompassed the HRA and HRB domains present in the hMPV F protein (Table 1). Nine peptides of different lengths were selected based on previously reported inhibitory peptides active against other viruses, such as HIV, hRSV, and hPIV-3 (19, 22, 23, 46).

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TABLE 1. Amino acid sequences and IC50 values of HRA- and HRB-derived peptides from the hMPV F protein

Cytotoxicity and viral inhibition assays. There was no cytotoxicity associated with the different peptides at the highest concentrations tested (9 µM). Each peptide and the vector residual peptide used as a negative control were first tested for their inhibitory activities against an hMPV subgroup A1 strain by real-time PCR. Of the nine tested peptides (three encompassing HRA and six encompassing HRB), five showed good in vitro inhibition against hMPV, with IC₅₀ values of <3.3 µM (Table 1). Peptides derived from the HRA domain were generally more potent (three out of three) than those derived from the HRB region (two out of six). All peptides showed concentration-dependent inhibition against the challenge viral strain (data not shown).

One peptide in particular, the longest HRA-derived peptide, named HRA2, showed important inhibitory activity against the hMPV A1 strain, with an IC₅₀ value of 2.1 nM (Table 1). This peptide was thus selected for further evaluation against strains from all four hMPV subgroups. Results showed that the HRA2 peptide expressed in E. coli had significant and similar antiviral activities against all hMPV subgroups, with IC₅₀ values of <12 nM when tested by real-time PCR (Table 2). These low IC₅₀ values were also confirmed by performing plaque assays and immunostaining (Table 2). A synthetic version of peptide HRA2 lacking the extra amino acids from the pET30a vector was also evaluated for its antiviral activity against the four hMPV subgroups by plaque assay followed by immunostaining. This synthetic peptide had a slightly less important inhibitory activity than the E. coli-expressed peptide but still exhibited low IC₅₀ values against the four hMPV strains (IC₅₀ < 22 nM) (Table 2).

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TABLE 2. IC50 values of peptide HRA2 against strains from the four hMPV subgroups

We then sought to determine if the HRA2 peptide inhibits virus-cell or cell-cell fusion events. In a cell-based plaque assay experiment, the development of plaques is dependent on both initial infection and subsequent spread of infection to neighboring cells. By adding the HRA2 peptide during virus adsorption to the cells as well as in the methylcellulose overlay or only during virus adsorption or only in the overlay, it is possible to determine which fusion event is inhibited. As seen in Table 3, the addition of HRA2 peptide only in the overlay did not result in inhibition (IC₅₀ > 250 nM), which means that this peptide cannot inhibit cell-cell fusion. Moreover, the IC₅₀ values obtained when HRA2 was added only during adsorption or during both adsorption and in the overlay are not statistically different (IC_50s of 27.5 ± 7.0 nM and 20.1 ± 6.3 nM, respectively), which means that the HRA2 peptide is a strong inhibitor of virus-cell fusion but not of subsequent cell-cell fusion.

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TABLE 3. IC50 values of HRA2 peptide against hMPV group A1 when added at different time points in the plaque assay experiment

The inhibitory activity of the HRA2 peptide produced in E. coli was also evaluated by plaque assay against two other members of the Paramyxoviridae family, hRSV and hPIV-3. The peptide presented moderate activity against hRSV (IC₅₀ value of 1.6 ± 0.18 µM) and no activity against hPIV-3 at the highest concentration tested (9 µM).

In vivo activity of the HRA2 peptide. The simultaneous intranasal administration of hMPV and the synthetic HRA2 peptide (7 µM) protected all mice from death. In contrast, all hMPV-infected mice that received either the HRA2 peptide 24 h following viral infection, the synthetic inactive HRB1 peptide, or PBS died or were sacrificed based on weight loss of 25% beyond day 5. From day 0 to 5, simultaneously infected and HRA2-treated mice did not lose any weight and had no clinical signs of illness, whereas other groups of infected mice lost 20 to 23% of their initial weight, associated with ruffled fur and a decrease in activity (Fig. 1A). On day 5 postinfection, uninfected mice receiving the HRA2 peptide only, as well as simultaneously infected and HRA2-treated mice, had undetectable lung viral titers (<10² TCID₅₀/lung), whereas other groups of infected mice had viral titers of 3 x 10⁴ ± 1.3 x 10⁴ (hMPV only), 1.7 x 10⁴ ± 1 x 10⁴ (hMPV with HRA2 24 h later), and 4 x 10³ ± 2 x 10³ (hMPV and HRB1) TCID₅₀/lung (Fig. 1B).

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FIG. 1. (A) Body weight loss in groups of treated and untreated mice (6 mice/group). The weight of mice was evaluated on a daily basis until day 5 postinfection. (B) Mean lung viral titers on day 5 postinfection. *, Results are statistically different (P < 0.05) from those for infected, untreated mice (hMPV only). The error bars indicate SEM.

The degree of airway obstruction in HRA2-treated mice was similar to that of uninfected mice and was significantly decreased compared to that of untreated mice, with P_enh values of 0.32 ± 0.01, 0.35 ± 0.01, and 2.63 ± 0.13, respectively (Fig. 2). Sham-infected mice receiving PBS normally have a P_enh value of 0.5 (15). Delayed treatment with the HRA2 peptide 24 h postinfection or simultaneous treatment with the HRB1 peptide did not result in a significant decrease in the P_enh values compared to those of untreated mice.

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FIG. 2. Airway obstruction in groups of treated and untreated mice (6 mice/group). The degree of airway obstruction as measured by the Penh value was evaluated on day 5 postinfection. *, Results are statistically different (P < 0.05) from those for infected, untreated mice (hMPV only). The error bars indicate SEM.

The pulmonary levels of the cytokines and chemokines RANTES, IFN-, and MCP-1 were significantly decreased only in the group of infected mice that received early treatment with the HRA2 peptide (Fig. 3). Finally, groups of mice that received PBS (hMPV infection only, mock-treated group), the HRB1 peptide, or a delayed treatment with HRA2 presented important and similar levels of peribronchiolar, perivascular, interstitial, and alveolar inflammation, whereas the early administration of the HRA2 peptide at the time of infection led to a considerable reduction in all types of inflammation (Fig. 4A). Overall, pulmonary inflammation was decreased with early HRA2 treatment compared to inflammation in untreated mice, but this treatment could not completely resolve the inflammation caused by hMPV infection, as shown in Fig. 4B. Lung tissues from sham-infected but HRA2-treated mice showed no signs of inflammation and were visually like those from sham-infected mice (14-16).

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FIG. 3. Pulmonary levels of the cytokines and chemokines RANTES, MCP-1, and IFN- in mice (6 mice/group). The levels of the different cytokines/chemokines in lungs of treated and untreated mice were determined by enzyme-linked immunosorbent assay. *, Results are statistically different (P < 0.05) from those for infected, untreated mice (hMPV only). The error bars indicate SEM.

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FIG. 4. Lung inflammation on day 5 postinfection in groups of treated and untreated mice (6 mice/group). (A) Lungs were removed and fixed with 10% buffered formalin. Histopathological scores were determined based on peribronchiolar, perivascular, interstitial, and alveolar inflammation. (B) Five-micrometer sections of paraffin-embedded lung tissues stained with hematoxylin/eosin. A representative section (10x enlarged) is shown for each group.

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In this study, we showed that some peptides encompassing the HRA and HRB regions located at the N- and C-terminal portions of the hMPV F1 domain can significantly inhibit viral infection. In particular, a 48-amino-acid (aa) HRA peptide (HRA2) had very potent activity against all subgroups of hMPV, with IC₅₀ values of <25 nM. Furthermore, HRA2 completely prevented clinical signs and lung viral replication when administered to mice at the time of infection.

It has been proposed that peptides derived from the HR regions act by blocking the interaction between the endogenous HRA and HRB domains of the F protein, which is essential for the fusion of viral and cellular membranes. In contrast to results for other fusogenic viruses (22, 35, 44, 46), we found that peptides derived from the HRA sequence were generally more potent than those based on the HRB domain in inhibiting hMPV replication. The hMPV HRA peptides tested in this study were as potent as compounds T-118 (HRB peptide) against hRSV (22) and HR-1 (HRA peptide) against another hMPV strain (28). One of the peptides we designed from the HRA domain (HRA2) was found to be highly effective in inhibiting hMPV fusion, with a mean IC₅₀ of 16.8 ± 2.6 nM against four different hMPV strains (Table 2). The hMPV HR-1 peptide previously reported by Miller et al. (28) differed from our HRA2 peptide in its length (5 aa shorter at its N-terminal end) and in its activity (IC₅₀ value of 46 nM when tested against a single hMPV strain).

Our HRA2 peptide that was expressed in E. coli from the pET30a vector had a slightly better antiviral activity than the synthetic one (Table 2). This could be attributed to the extra peptide fused to HRA2 when produced from the vector. This extra peptide of 52 aa could stabilize the HRA2 -helix structure or simply help the peptide to fold correctly and gain an active conformation. This difference in IC₅₀ values could also be associated with impurities remaining in the E. coli-generated HRA2 peptide. Despite some differences in the amino acid sequences of the HRA and HRB domains among the four hMPV subgroups (Fig. 5), the HRA2 peptide had similar activities against all representative hMPV strains (Table 2). More unexpectedly, the HRA2 peptide showed some activity against hRSV, but to a lesser extent than against hMPV. This could be explained by the 48% amino acid identity between the HRA domains of hMPV and hRSV. The conserved helical structure of the HR domains between these two viruses could allow the interaction of the HRA2 peptide with the hRSV HRB domain, preventing its normal interaction with endogenous HRA. HR-derived peptides usually present sequence-specific inhibitory activity (35, 45, 46), but some groups have reported that cross-activity can be observed between viruses from different genera in the same family (hPIV-3 and Hendra virus [33] and hPIV-3 and hRSV [22]). On the other hand, the HRA2 peptide did not inhibit hPIV-3 replication, which could be attributed to the much lower level of amino acid sequence identity between hMPV and hPIV-3, i.e., 25% for both HRA and HRB (Fig. 5). A combined activity against hMPV and hRSV would be of benefit in clinical use, considering the similar mode of presentation for the two viruses, i.e., bronchiolitis. A more detailed evaluation of HRA2 activity against different strains of hRSV is needed both in vitro and in vivo.

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FIG. 5. Alignment of the HRA and HRB amino acid sequences from hMPV groups A1, A2, B1, and B2, as well as from hRSV (strain A2) and hPIV-3 fusion proteins. The sequences were aligned using ClustalW with BioEdit software (version 7.0.5.2). (A) Alignment of hMPV, hRSV, and hPIV-3 HRA sequences. Gray letters are amino acids added to the HRA sequence to form the HRA2 peptide. (B) Alignment of hMPV, hRSV, and hPIV-3 HRB sequences. (C) Amino acid identity for HRA, HRA2, and HRB sequences of hMPV, hRSV, and hPIV-3 fusion protein sequences. hMPV clinical strains C-85473 and NL-001 (GenBank accession no. AF371337) were used as representatives of the hMPV A1 group; hMPV Can97-83 (GenBank AY145296) was used as representative of the hMPV A2 group; hMPV Can97-82 (GenBank AY145295) was used as representative of the hMPV B1 group; hMPV Can98-75 (GenBank AY145289) was used as representative of the hMPV B2 group; hRSV strain A2 (GenBank M11486) and hPIV-3 (GenBank M14892) were also used in this alignment.

The HRA2 peptide presented a strong antiviral activity against the four known hMPV subgroups in vitro and was used in further animal studies. Using a lethal BALB/c mouse model of hMPV infection, we showed that all mice that received HRA2 at the time of infection survived and that none presented any weight loss or clinical signs of illness as reported previously (1, 10, 16). Moreover, when the HRA2 peptide was administered simultaneously with hMPV, no infectious viruses could be recovered from the lungs of mice on day 5 postinfection and, as a consequence of this potent inhibition of viral replication, the levels of proinflammatory cytokines (RANTES, MCP-1, and IFN-), airway obstruction, and pulmonary inflammation were all significantly decreased. These data suggest that early administration of the HRA2 peptide might be able to prevent the development of the long-term consequences associated with this virus, most notably, airway hyperresponsiveness (15, 43).

On the other hand, delayed treatment with HRA2, i.e., 24 h postinfection, did not protect mice from clinical signs (weight loss) and was associated with viral replication in the lungs similar to that found in untreated mice. Also, as expected from the in vitro assays, mice that received the HRB1 peptide were not protected from hMPV clinical manifestations, which confirms the fact that viral inhibition is sequence dependent.

The cell-based plaque assay first demonstrated that the HRA2 peptide activity is restricted to the fusion event taking place between viruses and cells and not the one occurring during the cell-cell spread of infection. Furthermore, the absence of benefits in mice receiving the 24-h-delayed treatment with the HRA2 peptide confirms that this peptide can inhibit only the virus-cell fusion event and not the cell-cell spread of hMPV infection. This suggests the use of the HRA2 peptide as a prophylactic agent instead of as a treatment once the disease is declared. Some explanations for the restricted fusion inhibition activity of the HRA2 peptide can be proposed. First, a larger amount of F proteins could be found on the surface of infected cells than on virions, which could improve the efficacy of cell-cell fusion, thus increasing the concentration of the HRA2 peptide required to inhibit this fusion. Also, the fusion mechanisms could be different in virus-cell fusion than in cell-cell fusion. Moreover, different proteins could be involved in the two fusion events, with the F protein being the most important protein in virus-cell fusion but, possibly, not in the cell-cell fusion event. More research is needed to ensure better understanding of these fusion events and their inhibition.

To pursue the development of the HRA2 compound, the most efficient delivery systems for human administration need to be identified. A carrier, such as a liposome, could protect HRA2 from potential degradation in the lungs and increase its half-life. The peptide could also be chemically modified for stronger resistance against degradation. In contrast to the approved anti-HIV-1 drug enfuvirtide (8), the HRA2 peptide would not need to be administered by subcutaneous injection but could be aerosolized or nebulized for direct penetration into the lungs.

In conclusion, we report for the first time the in vivo inhibition of hMPV infection by a fusion inhibitor analogous to the HRA domain. This peptide showed important antiviral activity when administered intranasally at the same time as viral challenge but no significant benefits when administration was delayed to 24 h postinfection. Our results indicate a potential for at least its prophylactic use, particularly in postexposure treatment of contacts of infected persons and possibly for early treatment in an epidemic context. More studies are needed to characterize the best delivery mode, dosage, and schedule of administration for this hMPV fusion inhibitor.

 
This work was supported by research grants from the Canadian Institutes of Health Research (MOP-62789) and Le Fonds de la Recherche en Santé du Québec (FRSQ) to G.B. G.B. is a national researcher scholar of the FRSQ and the holder of the Canada research chair on emerging viruses and antiviral resistance. C.D. has a Ph.D. scholarship from the FRSQ.

Sections of lung tissues were photographed with the help of the bioimaging platform from the Infectious Disease Research Center, CHUQ-CHUL, Quebec City, QC, Canada.

 
* Corresponding author. Mailing address: CHUQ-CHUL, Room RC-709, 2705 Blvd Laurier, Quebec City, Quebec, Canada G1V 4G2. Phone: (418) 654-2705. Fax: (418) 654-2715. E-mail: Guy.Boivin{at}crchul.ulaval.ca

Published ahead of print on 29 October 2007.

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Antimicrobial Agents and Chemotherapy, January 2008, p. 279-287, Vol. 52, No. 1
0066-4804/08/$08.00+0     doi:10.1128/AAC.00793-07
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