The key to the success of gene therapy is the development of efficient gene transfer vectors. One critical element of such vectors for in vivo delivery is the ability of the vectors to transduce genes in a cell-type-specific manner. In our laboratory, we have been working on cell-type-specific gene transfer vectors derived from Spleen Necrosis Virus (SNV), an amphotropic C-type avian retrovirus, for the last decade. SNV is a member of the reticuloendotheliosis virus family and does not infect human cells in tissue culture. Retroviruses infect cells through interactions between viral envelope proteins and their respective cell surface receptors. Hence, the tropism of a retrovirus is determined by its envelope protein. Our strategy to develop targeted SNV vectors was to graft target-binding ligands to the viral envelope proteins creating fusion proteins that would enable target-specific gene transfer. To date, scAs (single-chain antibodies) and HIV-1 envelope protein gp120 have been used as the targeting ligands by us. ScAs and gp120 were fused directly to the N-terminus of the SNV envelope transmembrane units (TM) and were expressed to the viral surface as the targeting ligand-TM fusion proteins. Viral particles displaying such chimeric envelope proteins were competent for infection of cells that expressed receptors for the targeting ligands. The specificity of such infections were demonstrated by infection interference assays. With the success of the in vitro experiments, we proceeded to in vivo cell-type-specific infection representing a significant step towards cell-type-specific gene transfer in gene therapy. Thus, the target-specific vectors were tested in vivo in a SCID mouse model. SNV vectors displaying scAs specific for the cell surface protein Her2/neu were able to infect Her2/neu-positive cells but not the Her2/neu-negative cells, both of which had been injected in a mixture intraperitoneally before infection. Results of our studies have demonstrated that SNV viruses are versatile for target-specific gene delivery of the targeting vectors in vitro and in vivo .
