| We developed a novel approach for targeting retroviral vectors to specific cell types. This technique relies on the use of a bridge protein composed of the extracellular domain of a retroviral receptor fused to a cellular ligand. The retroviral receptor-ligand bridge protein was predicted to be a bi-functional reagent that could bind both to cell surface cognate ligand receptors and to the viral envelope. Upon the binding of this bridge protein to cells, the extracellular domain of the retroviral receptor would then mediate viral entry into cells.;As a model system, we constructed a bridge protein composed of the extracellular domain of Tva, the cellular receptor for subgroup A avian leukosis virus (ALV-A), fused to the N-terminus of the mature form of human epidermal growth factor (EGF). A proline-rich linker was inserted between the two domains, to favor their independent folding and function. The Tva-EGF bridge protein bound specifically to cells expressing EGF receptors (EGFR) and mediated specific ALV-A entry, but cells that did not express EGFR were not infected. ALV-A entry into cells mediated by Tva-EGF was on average 23% relative to a control cell line that expressed Tvasyn, a transmembrane form of Tva.;To explore the versatility of this approach, we tested other cellular ligands and a single chain antibody for their ability to mediate ALV-A entry into cells expressing their cognate ligand receptors or antigens. Bridge proteins, Tva-herbeta1, Tva-VEGF110, and Tva-MR1, in which the EGF-like domain of heregulin beta1, the 110 N-terminal amino acids of vascular endothelial growth factor, or a single chain antibody, MR1, respectively, were fused at the C-terminus of the extracellular domain of Tva were generated. These bridge proteins bound to cells that expressed their cognate ligand receptors or antigens and mediated specific ALV-A entry into these cells. In the future, this method of viral targeting may prove to be a versatile means to deliver retroviral vectors to specific cell types in vivo. |
