Acute Clearance of Human Metapneumovirus Occurs Independently of Natural Killer Cells

Humanmetapneumovirus (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 thanmock-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 lowerrespiratory illness (1, 2). No vaccine is currently available, and HMPV causes recurrent infections, even in the immunocom-petent. 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 tha