L. Spetch, T. L. Bowlin and A. Casola associated with toxicity, as indicated by an increased loss of body weight, an important parameter of hMPV-induced clinical illness (see below), in the animals infected and treated with NMSO3 compared with the animals infected with virus alone. Mice infected with 16107 TCID50 hMPV lost 5–10 % of their original body weight, with a peak that occurred between days 1 and 2 p.i. They gradually returned to their original weight and then started to lose weight again for an additional 4–5 days before returning to baseline levels. Mock-infected mice did not loose weight (data not shown). HMPV-infected mice treated with NMSO3 showed a similar body weight loss in the first 3 days of infection; however, they did not loose additional weight after returning to baseline levels, as shown in Fig. 1(a). We then tested the efficacy of 50 mg NMSO3 kg21 given at 1 day p.i. NMSO3 treatment was still effective in reducing viral replication, although not as well as administration immediately after infection (reduction of ~0.6 log vs 1 log in viral titres). Mice infected with hMPV and treated with NMSO3 the day after infection showed a body weight loss similar to the untreated infected mice, but seemed to recover faster than the untreated mice, as shown in Fig. 1(b). As administering 50 mg NMSO3 kg21 at the time of infection seemed to be the most effective treatment in modulating hMPV-induced clinical disease, all subsequent experiments were performed using this treatment protocol. To determine whether NMSO3 altered hMPV-induced lung inflammation, total and differential cell counts in bronchoalveolar lavages (BALs) were measured. In mock- infected mice, total lung cell count was between 16104 and 1.56104, with macrophages representing the majority of the cell population. We observed a significant attenuation of total cellular influx following NMSO3 treatment in HMPV-infected mice on day 1 p.i. (P50.056), with a similar trend at day 4 p.i. (Fig. 2a). HMPV infection induces a significant recruitment of neutrophils to the lung within the first 3 days of infection, whilst mononuclear cells, including macrophages/monocytes and lymphocytes, start to increase at day 3 p.i., representing the majority of the lung inflammatory cells from day 5 p.i. (Kolli et al., 2008). NMSO3 administration did not cause significant changes in the distribution of the inflammatory cell population at day 1 p.i. between treated and untreated groups, whilst there was a significant increase in neutrophil recruitment in the airways by day 4 p.i. (Fig. 2b). HMPV infection of BALB/c mice results in significant production of cytokines and chemokines (Alvarez et al., 2004; Guerrero-Plata et al., 2005; Hamelin et al., 2005), which are likely to play a major role in pulmonary inflammation. Therefore, we determined whether NMSO3 Fig. 1. Effect of NMSO3 administration on hMPV-induced body weight loss. Mice were treated with 50 mg NMSO3 kg”1 at the time of hMPV infection (a) or at 24 h p.i. (b). The results show a representative diagram from three different experiments. All data points represent the means±SD of four to five animals. 2710 Fig. 2. Effect of NMSO3 on airway inflammation. (a) The total number of cells was measured in the BAL of hMPV-infected mice, either untreated or treated with NMSO3, at days 1 and 4 p.i. Data are expressed as means±SD and are representative of three different experiments. (b) Differential cell count was measured in BALs of hMPV-infected mice, untreated or treated with NMSO3, at days 1 and 4 p.i. *, P,0.01 relative to hMPV-infected mice. Journal of General Virology 89