Recent advances in techniques for gene transfer and expression have raised the possibility that diseases might be treated at the genetic level. The most commonly advocated method for gene therapy is via retroviral gene delivery. However, this approach has several drawbacks: 1) retrovirus vectors are not tissue-specific and therefore the target cells must be removed from the subject, infected in vitro and then returned, 2) the genes inserted into the retrovirus vectors are often expressed at low levels, and 3) the inserted genes may be lost when the target cells undergo terminal differentiation. The aim of this proposal is to design strategies to address the first of these problems and thereby improve the delivery of genes by designing retroviral vectors which exhibit tropism for specific cell types; more specifically, it aims to develop a model system for the targeted delivery of genes erythroid cells via tissue-specific retroviral vectors which recognize and bind erythropoietin (Epo) receptors. Such tissue-tropic vectors will be created by the construction of modified packaging cells lines which produce recombinant virus with targeting ligands engineered into the viral coat, which would thereby deliver the virus specifically to cells expressing the Epo-receptor. Three different targeting ligands will be employed: (1) a hybrid viral coat protein consisting of the peptide hormone erythropoietin (Epo) inserted into the envelope (env) gene of ecotropic Moloney murine leukemia virus, (2) Epo itself, anchored to the virion surface by the virion coat, and (3) a naturally-occuring mutant retroviral envelope protein, the Friend spleen focus-forming virus envelope glycoprotein (gp55), which is known to bind the Epo-receptor. These targeting ligands will be expressed in the psi2 or GP101 packaging cell lines, which normally produce ecotropic (i.e., murine-specific) virus, and used to package retroviral vectors containing the neomycin resistance and beta- galactosidase genes. The resultant virions. will be assayed for specific infectivity and transduction of non-murine Epo receptor-positive target cells. Since the target cells are non-murine, any infection will be the result of a specific binding interaction between the targeting ligand on the viral surface and the Epo-receptor on the target cell surface. The value of using targeting ligands which recognize the Epo receptor lies in its high degree of tissue specificity. The development of retroviral vectors which target specifically to erythroid cells will have important implications for future implementation of gene therapy for red blood cell disorders, and may represent a delivery system which is potentially applicable in vivo. Furthermore, the ability to target retroviral vectors to specific cell types may serve as a useful tool in studies of regulation of tissue-specific and developmental stage-specific gene expression, and contribute to understanding the mechanisms and signal involved in viral infection.