My long-term career goal is to study scientific questions related to human genetic susceptibility to infection as a laboratory based clinician scientist. My current fellowship training, which has added Genetics to my previous training in Infectious Diseases, was specifically pursued in order to allow me to study mechanisms of host susceptibility to infection. This current research proposal seeks to study inhibition of HIV and SIV in hematopoietic cells by altering the expression of cellular receptors, an approach that is in line with my long-term interests in altering host susceptibility to infection. This K08 application is designed to take advantage of the expertise within three different local laboratories. If funded, this award will allow me to continue to work primarily in the laboratory of Phillip A. Sharp at MIT and to begin a transition in the second half of this award to an independent research position at Brigham and Women's Hospital. RNA interference (RNAi) is a mechanism of gene silencing that has been used to study gene function in vitro, and is believed to have therapeutic potential in human diseases. RNAi occurs in cells via complex endogenous machinery that recognizes double stranded RNA, cleaves it into small fragments (19-21 nucleotides), and then uses those fragments as guides to specifically degrade RNA species displaying complementary sequence. The small fragments, called short interfering RNA (siRNA), can be introduced directly into mammalian cells to enter this pathway at the post-cleavage level to induce the specific degradation of intracellular RNA. We have used the RNAi pathway in cultured human cells to block HIV entry via specific down regulation of CD4, CXCR4, and CCR5. While transfection of siRNA oligonucleotides has been used in these proof-of-principle experiments, new methods of delivery are now being employed. RNAi-inducing RNA hairpins (hpRNA) can be delivered to mammalian cells by retroviral transduction. This proposal seeks to investigate whether hematopoietic stem cells (HSC) from human peripheral blood and from rhesus macaque bone marrow can be transduced with retroviral vectors that will continue to deliver hpRNA following differentiation. The central hypothesis underlying this work is that retroviral-mediated delivery of hpRNA to HSCs will result in a sustained protection of derived cells from either HIV or SIV as a result of RNAi induced down-regulation of cell surface receptors.