Efficient cell infection by Moloney murine leukemia virus-derived particles requires minimal amounts of envelope glycoprotein.

Retrovirus entry into cells is mediated by specific interactions between the retrovirally encoded Env envelope glycoprotein and a host cell surface receptor. Though a number of peptide motifs responsible for the structure as well as for the binding and fusion activities of Env have been identified, only a few quantitative data concerning the infection process are available. Using an inducible expression system, we have expressed various amounts of ecotropic and amphotropic Env at the surfaces of Moloney murine leukemia virus-derived vectors and assayed for the infectivity of viral particles. Contrary to the current view that numerous noncooperative Env-viral receptor interactions are required for cell infection, we report here that very small amounts of Env are sufficient for optimal infection. However, increasing Env density clearly accelerates the rate at which infectious attachment to cells occurs. Moreover, our data also show that a surprisingly small number of Env molecules are sufficient to drive infection, albeit at a reduced efficiency, and that, under conditions of low expression, Env molecules act cooperatively. These observations have important consequences for our understanding of natural retroviral infection as well as for the design of cell-targeted infection techniques involving retroviral vectors.

Cell targeting by murine retroviral vectors.

Gene therapy involves the transfer of new genetic material to cells of individuals with the aim of conferring a therapeutical benefit. Theoretically, gene therapy can be used to treat a variety of life-threatening disorders, such as inherited genetic diseases, cancer, chronic viral infections as well as a number of other severe diseases which are not treatable at present. To this aim, both efficient gene delivery techniques and controlled gene expression systems are required. Engineered murine retroviruses are the most widely used vectors for stable clinical gene transfer because of their ability to integrate--along with the transgenes they are engineered to carry--into the genome of infected cells. However, this technology still suffers from a number of drawbacks and limitations. Particularly, the ability to target cells of therapeutic interest together with the controlled expression of transferred genes would improve both the efficiency and the safety of retroviral vectors. Such improvements would additionally allow the development of new animal models of human diseases as well as enable more fundamental investigations in the fields of oncogenesis and developmental biology.