We investigated a natural mechanism of resistance to retroviruses that occurs in mice and other species with a view toward whether a similar resistance could be devised for HIV in man. Mice that inherit a gene (designated Fv4) related to a retroviral envelope are resistant to infection with retroviruses that use a particular (ecotropic) virus receptor. It is believed that the resistance is due to blockage of the receptor by endogenously produced envelope protein. Such blockage could occur because of interaction of receptor with envelope inside the cell, as occurs in cells co-expressing the CD4 receptor for HIV and HIV envelope, or because of interaction of secreted envelope protein with the receptor on the surface of other cells. Consistent with the latter model, we found that a soluble form of Fv4 envelope (gp70) is secreted in serum and in supernatants of tissue culture cells expressing Fv4 envelope, and that this secreted envelope can block infection in vitro. This result may explain why mice transplanted with a mixture of Fv4- resistant and control bone marrow are partially resistant to infection with murine retroviruses. In a related tissue culture system for HIV, we found that when HIV envelope and vpu genes were co-expressed with the HIV receptor CD4, cell surface CD4 decreased due to rapid degradation of CD4 inside the cell. However, cells that initially expressed low levels of surface CD4 gave rise to cells with high surface CD4 and vice versa. A possible clue to the mechanism of inconstant down-regulation of CD4 is that drugs such as brefeldin A that block intracellular transport of proteins had markedly different effects on CD4 in cells depending on whether they also expressed HIV envelope and vpu. We are investigating models of the kinetics of synthesis, transport and vpu/env-mediated degradation of CD4 that may provide insights into the bimodal nature of surface CD4. We believe that better understanding of this phenomenon will suggest ways to enhance down-regulation of CD4 in order to enhance envelope-mediated resistance for HIV. We also worked on a new expression system for retroviral genes using Semliki Forest virus (SFV). We constructed SFV vectors encoding functional murine retroviral gag and env genes and are investigating possible uses of this expression system for retroviral vaccination and gene therapy.