Human Metapneumovirus Persists in BALB/c Mice despite the Presence of Neutralizing Antibodies

Abstract

Human metapneumovirus (HMPV) has emerged as an important human respiratory pathogen causing upper and lower respiratory tract infections in young children and older adults. Recent epidemiological evidence indicates that HMPV may cocirculate with respiratory syncytial virus, and HMPV infection has been associated with other respiratory diseases. In this study, we show that BALB/c mice are susceptible to HMPV infection, the virus replicates in the lungs with biphasic growth kinetics in which peak titers occur at days 7 and 14 postinfection (p.i.), and infectious HMPV can be recovered from lungs up to day 60 p.i. In addition, we show that genomic HMPV RNA can be detected in the lungs for >180 days p.i. by reverse transcription-PCR; however, neither HMPV RNA nor infectious virus can be detected in serum, spleen, kidneys, heart, trachea, and brain tissue. Lung histopathology revealed prevalent mononuclear cell infiltration in the interstitium beginning at day 2 p.i. and peaking at day 4 p.i. which decreased by day 14 p.i. and was associated with airway remodeling. Increased mucus production evident at day 2 p.i. was concordant with increased bronchial and bronchiolar inflammation. HMPV-specific antibodies were detected by day 14 p.i., neutralizing antibody titers reached >6.46 log 2 end-point titers by day 28 p. i., and depletion of T cells or NK cells resulted in increased HMPV titers in the lungs, suggesting some immune control of viral persistence. This study shows that BALB/c mice are amenable for HMPV studies and indicates that HMPV persists as infectious virus in the lungs of normal mice for several weeks postinfection.Human metapneumovirus (HMPV) is tentatively a member of the Metapneumovirus genus based on genetic sequence similarity to the type species avian metapneumovirus (46). HMPV was first identified in respiratory specimens from young children hospitalized with mild to severe lower respiratory tract illness (46), and recent studies indicate that HMPV may cause upper and lower respiratory tract illness in patients between the ages of 2 months and 87 years (5,6,8,23,49,50). The disease burden associated with HMPV infection is not well known and may be complicated by the ability of HMPV to cocirculate with respiratory syncytial virus (RSV) in the community (28,37,50). A recent prospective study of young and older adults hospitalized for respiratory infections during the RSV season showed that HMPV was associated with approx-

Animal models for human Metapneumovirus (HMPV) infections

“…The findings showing that depletion of T cells or NK cells in infected BALB/c mice result in increased virus titers in the lungs are consistent with this hypothesis [1]. These latter data describing the inflammatory response in HMPV-infected mice [2] are consistent with the findings of a recent prospective study that demonstrated much lower levels of respiratory inflammatory cytokines in HMPV-infected infants than in those infected with RSV [11].…”

“…This unusual biphasic growth curve has not been observed for HMPV (or for that matter, RSV) in any other animal model by these or other investigators, and thus the biological significance is unclear. In addition to Alvarez et al [1,2], Hamelin et al [6] observed that the lung virus peaks somewhat earlier at day 5 p.i. in both BALB/c mice and cotton rats, an observation that is supported by Darniot et al [4].…”

“…In a recent report [1], HMPV infection of BALB/c mice led to an appearance of signs of illness, including body weight loss, ruffled coat, huddling, and heavy breathing from day 0 to day 7 post infection (p.i.). A decrease in body weight loss coincided with virus replication in lung tissues with a biphasic growth kinetics in which peak titers occurred at days 7 (10 8 PFU/g of lung tissue) and 14 p.i.…”

“…According to Alvarez et al [1,2], infectious HMPV could be recovered from the lung of the infected animals up to day 60, and viral RNA was present in lung tissues for ≥ 180 days (following depletion of T cells in the animals). Viable virus or viral RNA was not detected in the serum, spleen, kidneys, heart, trachea, and brain tissue.…”

“…Several animal species have been determined to be permissive for HMPV infection [1,4,6,9,10,12,18,23,27]. These models, which are described in more detail below, include small animals such as mice, cotton rats, hamsters, guinea pigs, ferrets, and primates, including chimpanzees, rhesus macaques and African green monkeys.…”