Human Metapneumovirus: A Ubiquitous and Long-Standing... : The Pediatric Infectious Disease Journal (original) (raw)

VIROLOGY

The human metapneumovirus (hMPV) is a newly described member of the Metapneumovirus genus within the Pneumovirinae subfamily of the Paramyxoviridae family.1 Its genome consists of a single negative strand of RNA of approximately 13 Kb containing 8 genes coding for 9 different proteins: the nucleoprotein (N), phosphoprotein (P), matrix protein (M), fusion protein (F), transcription elongation factor (M2.1), RNA synthesis regulatory factor (M2.2), small hydrophobic protein (SH), major attachment glycoprotein (G), and major polymerase subunit (L), in the order 3′-N-P-M-F-M2.1-M2.2-SH-G-L-5′.2,3 hMPV isolates can be classified into 2 major groups (A and B) and at least 4 subgroups based mainly on sequence analysis of the fusion, attachment and phosphoprotein genes.4–9 However, further investigations are still required to determine whether these genotypes represent different antigenic groups based on in vitro neutralization studies.9,10 Nucleotide and amino acid sequence identities between members of the 2 hMPV groups are 80% and 90%, respectively, with the highest variability occurring in the G and SH genes.3 This viral genome heterogeneity may be the cause of incomplete immunity and repeated infections.8,11,12

EPIDEMIOLOGY

Since its initial report by Dutch researchers in 2001, hMPV has been found in most parts of the world, with reports from the Americas, Europe, Asia and Australia.13 The virus has also been identified in respiratory tracts of HIV-infected and nonimmunocompromised children from South Africa.14 In temperate countries, hMPV seems to have a seasonal distribution overlapping with human respiratory syncytial virus (hRSV), with most cases reported during the Winter/early spring months. Although most studies have limited their surveillance to the typical respiratory virus period, most (86.7%) of hMPV recovered in cell culture in a Canadian center were isolated between December and May.5 In subtropical regions (such as Hong Kong), viral activity may differ with peak activity during the spring and early summer months.15 Several seroprevalence surveys have indicated that virtually all children are infected by the ages of 5–10 years,1,16–18 although the seroprevalence rate dropped to approximately 85% in Canadian individuals aged 40 years or older.19,20 In addition, studies have shown that hMPV is not a new pathogen, with serological evidence of human infection dating from 1958 in the Netherlands,1 and viral isolation for the past 10–20 years in Europe and Canada.1,5 Cases of severe hMPV infections in adults as well as reinfections suggest that, despite universal infection in childhood, new infections can occur throughout life due to incompletely protective immune responses and/or acquisition of new genotypes.5,11,12 Cocirculation of different hMPV genotypes has been reported in the same geographical area, although a small preliminary study has not found an association between viral type and severity of disease, such as the development of pneumonitis.6 The incubation period has been estimated to be 4–6 days.21 Although the shedding period and mode(s) of transmission of hMPV are not known, they are expected to be similar to those of hRSV, another member of the Pneumovirinae subfamily.

CLINICAL MANIFESTATION

Several reports have associated hMPV with acute respiratory tract infections (ARTI) in all age groups, with more severe diseases occurring in young children, elderly individuals and immunocompromised hosts.5,11,13,22–26 hMPV had initially been reported as a major respiratory tract pathogen in children. Many studies have indicated that after hRSV, it is one of the leading causes of bronchiolitis and it could account for 5% to 10% of hospitalizations for ARTI in young children.22,27,28 hMPV has also been associated with 12% to 15% of consultations for lower and upper respiratory tract infections in outpatient children.12 Within this population, the clinical features associated with hMPV infections are very similar to those induced by hRSV.22,24,28,29 The clinical symptoms include high fever, severe cough, difficulty breathing or abnormally rapid breathing and wheezing. These symptoms are indistinguishable from those in an hRSV-infected individual.22

The most commonly reported diagnosis associated with hMPV infection is bronchiolitis, with or without pneumonitis,22 but associations with acute wheezing and asthma exacerbation have also been reported.15,29,30 The virus has also been frequently detected in the nasopharynx of children with acute otitis media.22,31 Compared with hRSV infections, hospitalizations for severe hMPV infections tend to occur after the first months of life, peaking between 3 to 6 months of age, compared with <2 months.22,25 Also, the clinical outcome after hMPV infections tends to be generally less severe than that of hRSV when assessing the proportion of patients with hypoxemia or pneumonia and those admitted to intensive care units as shown in studies from Canada22 and Europe.24,25 Researchers suggest that hMPV may stimulate asthmatic episodes in children with a medical history of asthma; in a study, as many as 14% of hMPV cases had an asthma exacerbation after the infection.12 Children with underlying medical conditions may have more severe hMPV disease leading to hospitalization; in some studies from North America, 25% to 33% of cases requiring hospitalization occurred in children with underlying conditions such as prematurity, cardiopulmonary problems and immunosuppression.5,27 However, risk factors for severe hMPV infections have not been thoroughly investigated. Studies led by a group from England have indicated that coinfection by hMPV and hRSV was associated with more severe bronchiolitis, introducing the possibility that hMPV might be a determinant of hRSV disease severity.32,33 Study results detected coinfection with hMPV in 70% of infants with hRSV bronchiolitis sufficiently severe to require admission to the pediatric intensive care unit.32 However, such synergistic association has not been found in other studies.22,27,35 The possible synergistic interaction between hMPV and the severe acute respiratory syndrome coronavirus has been recently postulated during the 2003 severe acute respiratory syndrome outbreak in Canada and Hong Kong.36,37 On the other hand, such synergistic effects of hMPV on severe acute respiratory syndrome coronavirus-induced respiratory disease was not confirmed in experimental macaque models.38 Finally data from a case report demonstrated that fatal encephalitis of an infant could be correlated with hMPV because of postmortem hMPV RNA detected in brain and lung tissue.34 hMPV has also been reported in association with fatal lower respiratory tract disease in a hematopoietic stem-cell transplant recipient26 and in an immunocompromised child who was infected by both hMPV genotypes during 2 consecutive winter seasons.11

The contribution of hMPV in respiratory syndromes of adults has been studied considerably less than in children. hMPV has been associated with flu-like illnesses and colds in healthy adults.5,23,39 Falsey et al23 observed a higher rate of flu-like illnesses among young adults, although older adults experienced more dyspnea and wheezing, and those with cardiopulmonary conditions were ill for nearly twice as long as younger adults. Within the adult population, hMPV was found in 2% of individuals in England with influenza-like illnesses and no other viruses39 and in 4.5% of adults in Rochester, NY, with ARTI.23 Acute hMPV infections were also identified in 6 of 145 (4.1%) adult patients who consulted the emergency room for community-acquired pneumonia and chronic obstructive pulmonary disease exacerbations during 2 winter/spring seasons in Quebec, Canada.19 Four of the 6 hMPV-infected patients developed pneumonia and 2 had chronic obstructive pulmonary disease exacerbations, with a mean hospital stay of 10 days. hMPV-infected elderly patients with underlying illnesses may develop severe pneumonias, some of which can be fatal. In a retrospective study of 10 infected patients aged 65 years or older and hospitalized in Canada, 4 had pneumonitis and 2 died.5

LABORATORY DIAGNOSIS

hMPV growth in cell culture is fastidious, which may be one reason for its late identification. Most studies have reported reliable cytopathic effects only in tertiary monkey kidney or LLC-MK2 cells with variable growth in Vero cells.5,40 The cytopathic effect is variable, ranging from hRSV-like syncytium formation to focal rounding and cell destruction (Fig. 1). Typically cytopathic effects are displayed more than 10–14 days after inoculation. Detection of hMPV antigens by immunofluorescent antibody test is a rapid diagnostic modality although its sensitivity was found to be much lower than that of reverse transcription polymerase chain reaction.41 Reverse transcription polymerase chain reaction has become the method of choice for the diagnosis of hMPV infections. Rapid and sensitive reverse transcription polymerase chain reaction assays based on the real-time polymerase chain reaction methodology have been recently described, allowing amplification of different viral genes (more frequently the L, N or F genes) and their detection in ≤2 hours.42,43 The N gene seems to be a particularly attractive target, allowing sensitive detection of all 4 hMPV subgroups.44 Serologic testing only permits a retrospective diagnosis. Because infection is almost universal in childhood, a seroconversion or a ≥4-fold rise in antibody titers must be demonstrated to confirm recent infection. hMPV serological tests have consisted of immunofluorescence assays and enzyme-linked immunoabsorbent assay tests using hMPV-infected cells1,5,17,18,21,23 or recombinant viral proteins.16,20

FIGURE 1.:

Microscopic studies of uninfected (A) and hMPV- infected LLC-MK2 cells. Note the various cytopathic effects induced by different hMPV strains consisting of small round cells (B), and large, respiratory syncytial virus-like syncytia (C). Reproduced with permission from Clin Infect Dis. 2004;38:983–990.13

PATHOGENESIS

Recent experimental work using primates (chimpanzees, cynomolgus and rhesus macaques, African green monkeys) and small animals (hamsters, cotton rats, mice and ferrets) has been performed to characterize the pathogenesis associated with this viral infection. hMPV replicates to a various extent in the upper and lower respiratory tracts of many of the aforementioned animal models, although clinical symptoms after intranasal challenge have only been observed in chimpanzees, cynomolgus macaques (rhinitis) and BALB/c mice thus far.1,10,45–47 Using relatively high intranasal inoculum, hMPV has been shown to replicate efficiently in the lungs of BALB/c mice and to induce transient weight loss and dyspnea.45,46 Cotton rats were also described as good experimental models, supporting efficient hMPV lung replication with no apparent clinical symptoms.46,48 Similarly high titers were found in the lungs of Syrian golden hamsters with no clinical symptoms.10,49,50 In BALB/c mice and cotton rats, time course studies have indicated that peak viral titers are found around day 4–5 after infection and decrease thereafter.46,48 Alvarez et al45 have demonstrated that hMPV may present an initial biphasic replication pattern in lungs of BALB/c mice, with hMPV RNA still detectable more than 180 days after infection. Such persistence could be explained by an aberrant T helper cell type 2-like immune response, with impaired virus clearance after primary hMPV infection.51

Significant pulmonary inflammatory changes have been found in BALB/c mice45,46 and cotton rats46,48 (Fig. 2). In those 2 models, hMPV infection induced important pulmonary inflammation characterized by initial alveolitis and interstitial inflammation in mice45,46 and increased peribronchiolitis in cotton rats.46 Airway remodeling and increased mucus production was also observed in lungs of BALB/c mice.45 Interestingly significant inflammatory changes were still present in the latter animal model more than 21 days after viral challenge.46 Increase in many cytokines and chemokines such as interleukin (IL) 2, IL-8, IL-4, INF-γ, macrophage inflammatory protein 1α and monocyte chemotactic protein has been observed in the lungs or bronchoalveolar lavage of both mice and cotton rats in response to hMPV challenge.46,51 In humans, hMPV infection has also been associated with an increase of IL-8 in upper respiratory tract secretions and with chronic inflammatory changes of the airways, that is, intraalveolar foamy and hemosiderin-laden macrophages.52,53 When compared with hRSV, hMPV infections in humans seem to induce lower levels of inflammatory cytokines such as IL-12, tumor necrosis factor α, IL-6 and IL-1β.52 Thus far, no experimental human volunteer studies have been reported.

FIGURE 2.:

Hematoxylin and eosin stain of representative sections (magnification, ×10) from lungs of noninfected (A) and hMPV-infected (B) BALB/c mice on day 5 after intranasal infection challenge. Note the important alveolitis and interstitial inflammation present in lungs of the hMPV-infected mouse. Reproduced with permession from J Virol. 2005;79:8894–8903.46

ANTIVIRAL THERAPY

Ribavirin (1-β-d-ribofuranosyl-1,2,4-triazol-3-carboxamide), a nucleoside analog of guanosine, has similar antiviral activity (ie, similar 50% inhibitory concentration values) against hMPV and hRSV when evaluated in vitro.54 Studies show that it can be beneficial in the treatment of pneumonitis in infants and young children.54 NMSO3, a sulfated sialyl lipid, and heparin, another sulfated molecule, also significantly inhibit the replication of different hMPV strains.55 The latter 2 compounds are more effective in vitro when added before viral inoculation, suggesting they may prevent attachment or penetration of the virus into the cells. Regular intravenous immunoglobulins may be of some clinical value because they have good inhibitory activity against hMPV in vitro.54 However, the humanized monoclonal antibody against the hRSV fusion protein (palivizumab) has no in vitro activity against hMPV.54 Thus far, no drugs or compounds have been evaluated for their inhibitory activity against hMPV in vivo.

CURRENT RESEARCH AND VACCINE DEVELOPMENT

A major advance in the field of hMPV research has been the development of reverse genetic systems allowing manipulation of the viral genome.10,50,56–58 Such a tool is ideal to help further understand the pathogenesis of hMPV infection and to develop efficient vaccine candidates. For instance, the reporter gene for the green fluorescence protein was inserted into the hMPV genome, which may facilitate the initial recovery of this slow-growing virus.56 Also, foreign genes can be added to create chimeric viruses or viral genes can be mutated or deleted with the objective of generating attenuated vaccine strains.

Previous animal studies in hamsters and monkeys have indicated that the intranasal administration of one hMPV strain protects completely against subsequent challenge with homologous and heterologous strains.10,49 Live-attenuated vaccines, generated by reverse genetics, have been evaluated in small animals and primates. In one case, a chimeric vaccine expressing the F proteins of human parainfluenza virus (hPIV) 3 and hMPV in the background of a bovine PIV-3 strain was administered to hamsters and African green monkeys and was shown to induce production of neutralizing antibodies and to protect against challenges by wild-type hPIV-3 and hMPV.50,57 Another group designed a recombinant hPIV-1 virus expressing the F protein of hMPV (group A), which also induced production of neutralizing antibodies and protection against hPIV-1 and hMPV in hamsters.10 Notably this recombinant virus also induced protection against both hMPV groups. Finally recombinant hMPV viruses lacking the SH and/or G proteins were administrated intranasally to hamsters and were also shown to induce protection upon challenge with wild-type hMPV, further indicating that the F protein is the major protective antigen.58 Thus far, no clinical work on inactivated or subunit hMPV vaccines has been reported.

CONCLUSION

A tremendous amount of research has been accomplished since the first report on hMPV in 2001. hMPV is now considered a major respiratory pathogen in young children and recent reports indicate that it may also play a role in ARTI of elderly adults as well. Recent work, such as the development of suitable animal models and reverse genetic systems, has set the stage for a better understanding of hMPV pathogenesis and evaluation of antiviral agents and vaccine candidates.

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Keywords:

bronchiolitis; pneumonia; asthma; human metapneumovirus; chronic obstructive pulmonary disease exacerbations