Acute Clearance of Human Metapneumovirus Occurs Independently of Natural Killer Cells

Abstract

Human metapneumovirus (HMPV) is a major cause of respiratory disease. The role of NK cells in protection against HMPV is unclear. We show that while HMPV-infected C57BL/6 mice had higher numbers of functional lung NK cells than mock-treated mice, comparing NK cell-depleted and control mice did not reveal differences in lung viral titers, histopathology, cytokine levels, or T cell numbers or function. These data indicate that NK cells are not required for host control of HMPV. Human metapneumovirus (HMPV) is a major cause of lower respiratory illness (1, 2). No vaccine is currently available, and HMPV causes recurrent infections, even in the immunocompetent. The importance of natural killer (NK) cells for immunity against large DNA viruses is established (3-7); however, whether NK cells contribute to RNA virus immunity is controversial. While one study reported that NK cells decreased the influenza virus titer in mice (8), others found that NK cells increased lung inflammation associated with respiratory syncytial virus (RSV) and influenza (9, 10). The influence of NK cells on the adaptive immune response to respiratory viruses is also unclear and has not been addressed for HMPV. We sought to test the hypothesis that NK cells are required to clear HMPV infection.Lung NK cell numbers increase during HMPV infection and are functionally competent. We examined NK cells by flow cytometry in C57BL/6 (B6) mice (Jackson Laboratory) infected intranasally with 6 ϫ 10 5 PFU HMPV (A2 clinical strain TN/94-49) (11) ( Fig. 1A and B). Lung NK cell numbers in HMPV-infected animals increased significantly by day 1 postinfection (p.i.) and peaked on day 3; however, there was no increase of NK cells in mock-treated mice (inoculated with cell lysate) or in mice inoculated with UV-inactivated HMPV (Fig. 1C). To measure NK cell functionality, we performed intracellular cytokine staining for gamma interferon (IFN-␥) and surface staining for CD107a, a marker for cytotoxic-granule release (12, 13). There was a significantly higher number of reactive NK cells in HMPV-infected mice than in the mock-treated and UV-inactivated-HMPV groups, indicating that NK cells in infected animals were functional (Fig. 1D). Surface expression of CD69, an inducible activation marker (14), significantly increased on NK cells in infected mice on days 1 and 3 p.i. but not in mice in the mock-treated and UV-inactivated-HMPV groups (Fig. 1E) (data not shown). There was also a higher number of CD3 ϩ T cells in the HMPV-infected group than in the mock-treated and UV-inactivated-HMPV groups (data not shown). Together, these results suggest that HMPV replication results in increases in both NK cell number and functionality.Lung viral titers and cytokines remain unchanged with NK cell depletion. To test whether NK cells are required to clear HMPV, we intraperitoneally injected B6 mice with either anti-NK1.1 or an isotype control antibody (BioXCell) 5 days before infection, on the day of infection, and on day 5 p.i. for longer experiments ( Fig. 2A). This protocol de...

Human metapneumovirus - what we know now

“…Several studies show that cotton rats are the most permissive small animal model and that peak virus titers occurred at day four post infection 76 – 78 . Viral lung replication and disease vary between different inbred mouse strains; most work has been published in the BALB/c model, which exhibits substantial disease symptoms 79 – 88 . BALB/c and C57BL/6 mice may exhibit clinical symptoms such as difficulty breathing, weight loss, and ruffled fur, partly depending on the virus strain and inoculum 80 , 83 , 89 .…”

“…The role of natural killer (NK) cells is unclear. One study depleted NK cells with anti-CD49 antibody and reported prolonged viral replication 99 , whereas another depleted NK cells with the more specific antibody anti-NK1.1 and found no effect 88 .…”

Immune Response to Human Metapneumovirus Infection: What We Have Learned from the Mouse Model

“…Depletion of NK cells with anti-CD49b/Pan-NK cell monoclonal antibody in BALB/c mice resulted in increased lung viral titers on days 7, 28 and 60 after infection compared to NK cell competent mice. In contrast, work reported by Wen S. et al in C57BL/6 mice have demonstrated that NK cells do not contribute to hMPV clearance [ 51 ]. Lung NK cell numbers in infected mice were, however, increased as early as day 1 after hMPV infection and peaked on day 3 compared to mock infected mice.…”

Cell-Mediated Responses to Human Metapneumovirus Infection

“…The role of NK cells in the antiviral response against hMPV infection appears to be mouse strain dependent. In both BALB/c and C57BL/6 mouse strains, NK cells are recruited to the lungs upon hMPV infection [91,92,100,142]. Moreover, the recruited NKs are activated, as indicated by the increased release of cytotoxic granules [100,142].…”