Conditional expression of interferon-γ to enhance host responses to pulmonary bacterial infection

Strategies to augment host defense against pulmonary infection run the risk of inducing excess pulmonary inflammation and tissue injury. To address this problem, we investigated conditional expression in lung tissue of the murine interferon-γ (IFN-γ) transgene. A recombinant adenoviral vector (AdTetIFN) was constructed by placing a murine IFN-γ cDNA downstream of a tetracycline (Tet)-responsive promoter, inserted into a replication-defective adenoviral vector. Co-infection of target cells with AdTetIFN and a second vector encoding a reverse tetracycline controlled transactivator allowed doxycycline (Dox)-regulated IFN-γ production. We then administered 108 plaque-forming units (PFU) of AdTetIFN to mice by intratracheal injection. When the mice were provided with Dox in drinking water (0.5 mg/ml in 5% sucrose), there was significant release of IFN-γ in lavage fluid by ELISA in comparison to mice on water/sucrose alone (399±74 pg/ml vs undetectable, p<0.01). IFN-γ in lavage fluid was associated with upregulation of Class II Major histocompatibility complex markers on alveolar macrophages by flow cytometry, suggesting macrophage activation. We then injected AdTetIFN into mice three days prior to challenge with 104 CFU Klebsiella pneumoniae. Test mice were maintained on water+Dox and control mice on water+sucrose. Bacterial burden was assayed in lung tissue at serial intervals. At 24 h after challenge, mice on doxycycline had significantly lower infection burden in comparison to mice on water/sucrose (0.77±0.05 colony forming units/lung for 108 PFU AdTetIFN plus Dox compared to 1.4±0.11 colony-forming units/lung for AdTetIFN without Dox, p<0.05). Survival of the vector treated mice given doxycycline in drinking water was also enhanced. Microscopic examination of lavaged cells showed a significant increase in pulmonary neutrophils in the AdTetIFN+Dox mice in comparison to AdTetIFN+sucrose mice (16±1.0×105 vs 10+0.8 cells/lung, p<0.05). We conclude that local release of IFN-γ can be selectively activated to enhance neutrophil recruitment and host resistance to bacterial pneumonia.