Targeted Gene Delivery to Alveolar Macrophages via Fc Receptor-Mediated Endocytosis

Alveolar macrophage (AM) plays important roles in lung homeostasis and pathogenesis of diseases. The study of macrophage gene function and regulation as well as its potential therapeutic intervention will require the development of vectors capable of safe and efficient transfer of DNA to the AM. In the present study, we report a new transfection system that utilizes Fc receptor-mediated endocytosis as a means to target DNA to the AM. This system employs molecular conjugates consisting of a cognate moiety, in this case IgG which recognizes the AM Fc receptor, covalently-linked to a DNA-binding moiety, such as a cationic polyamine. A Complex was formed between immunoglobulin G-polylysine conjugate (IgG-pL) and plasmid DNA carrying the LacZ reporter gene (pSVβ). The conjugate-DNA complex was added directly to the AMs in culture and incubated for 24 h, after which LacZ gene expression was analyzed for β-galactosidase activity by microfluorometry using a fluorogenic β-galactosidase substrate, 5-dodecanoylaminofluorescein di-β-D-galactopyranoside (C₁₂FDG). The AMs treated with the IgG-pL/DNA complex exhibited galactosidase activity significantly augmented over background levels. Effective gene transfer was shown to require both the DNA-binding moiety and cognate moiety for the cell surface receptor. Specific internalization of the complex by the Fc receptor pathway was verified by competitive inhibition using excess IgG. Under this condition, LacZ gene expression was inhibited, suggesting complex internalization through the Fc mediated endocytosis pathway. The requirement of Fc receptors for complex internalization was further demonstrated using cells that lack Fc receptors, e.g., alveolar epithelial cells. When exposed to the IgG-pL/pSVβ complex, these epithelial cells showed no susceptibility to gene transfer. Thus, the immune conjugate system may be used to accomplish targeted gene delivery to the AMs via the endocytosis pathway. Finally, the conjugate system was found to be nontoxic at concentrations effectively enhancing gene transfer, thereby, suggesting its potential safety in vivo.