The contribution of neutrophils to lethal sensitivity and cytokine balance governing T1 and T2 host responses was assessed in a murine model of Legionella pneumophila pneumonia, Neutrophil depletion by administration of granulocyte-specific mAb RB6-8C5 at 1 day before infection rendered mice - 100-fold more susceptible to lethal pneumonia induced by L, pneumophila. However, this treatment did not alter early bacterial clearance, despite a substantial decrease in neutrophil influx at this time point. Cytokine profiles in the lungs of control mice demonstrated strong T1 responses, characterized by an increase of IFN-gamma and IL-12, In contrast, neutrophil-depleted mice exhibited significantly lower levels of IFN-gamma and IL-12, and elevation of T2 cytokines, IL-4 and IL-10, Immunohistochemistry of bronchoalveolar lavage cells demonstrated the presence of IL-12 in neutrophils, but not alveolar macrophages, Moreover, IL-12 was detected in lavage cell lysates by ELISA, which was paralleled to neutrophil number. However, intratracheal administration of recombinant murine IL-12 did not restore resistance, whereas reconstitution of IFN-gamma drastically improved bacterial clearance and survival in neutrophil-depleted mice. Taken together, these data demonstrated that neutrophils play crucial roles in primary L, pneumophila infection, not via direct killing but more immunomodulatory effects. Our results suggest that the early recruitment of neutrophils may contribute to T1 polarization in a murine model of L. pneumophila pneumonia.