Neutrophil elastase (NE) has been recognized as a double-edge sword as it can be both host-defensive and pro-inflammatory depending on its tissue level and microenvironment. The present study was designed to examine whether inhibition of NE activity by sivelestat, a specific NE inhibitor with low cellular permeability, is beneficial for acute lung injury induced by Streptococcus pneumoniae in hamsters. Intratracheal inoculation of live S. pneumoniae (1.5 x 10(7) CFU/Lung) into hamsters caused acute lung injury characterized by an increase in lung alveolar permeability and leukocytes accumulation in the lungs. These pathophysiological changes, which were associated with elevation of NE activity in the bronchoalveolar lavage fluid (BALF), were transient but remained high 4-22 h post-bacterial inoculation. Intravenous infusion of sivelestat at 3mg/kg/h 0-22 h after bacterial inoculation reduced the increase in NE activity and lung alveolar permeability, as indicated by leakage of pre-injected human serum albumin into BALF, without affecting the number of leukocytes in BALF and lung interstitial tissue. Interestingly, the number of bacteria in BALF and lung interstitial tissue was also reduced by sivelestat to less than 10% of the control values. Furthermore, S. pneumoniae-induced decrease in the level of surfactant protein D (SP-D), a protein that protects against bacterial infection, was preserved by sivelestat treatment with a significant negative correlation between NE activity and SP-D content in BALF. These results suggest that sivelestat is beneficial in acute lung injury associated with lung bacterial infection and that this inhibitor rather decreases infection by preserving host defense system.