Alveolar macrophages inhibit retrovirus-mediated gene transfer to airway epithelia

Gene transfer with integrating vectors such as recombinant retrovirus has the potential to correct inherited lung diseases permanently. As a gene therapy target, the pulmonary epithelium presents several challenges to vector delivery in vivo. Many of the host defenses that have evolved to prevent infection from inhaled bacteria or viruses represent potential barriers to gene transfer to the lung. We performed in vitro studies to determine whether two components of the innate immune system of the lung, airway surface fluid and alveolar macrophages, inhibit retroviral gene transfer to airway epithelia. Human alveolar macrophages obtained by bronchoalveolar lavage from normal subjects were left untreated or activated with lipopolysaccharide (LPS) for 3 hr in the presence of subconfluent human bronchial epithelial cells (HBE); than 4 x 10(5) cfu DA-luciferase retrovirus was added. Three days after infection, luciferase activity was measured in cell lysates. When the epithelial cells were co-cultured with LPS-activated macrophages, retroviral gene transfer to HBE cells was reduced by approximately 60%. Nonactivated macrophages decreased the transfection to approximately 55% of control values. In control experiments with either activated or inactivated macrophages but without epithelia, no luciferase activity was detected, suggesting that terminally differentiated alveolar macrophages are not infected by the recombinant retrovirus. Pretreatment of alveolar macrophages with dexamethasone restored gene transfer to approximately 60% of control values. In contrast, incubation of retrovirus with airway surface fluid had no inhibitory effect on gene transfer. These experiments document that AM inhibit retrovirus-mediated gene transfer to airway epithelia in vitro, and may represent a barrier to retroviral gene transfer in vivo. These barriers may be overcome, at least partially, with pharmacological agents.