CD8+T cells from naturally infected disease-resistant sooty mangabeys (Cercocebus atys) secrete a soluble factor which inhibits the in vitro replication of the simian immunodeficiency virus (SIV). To gain further insight on the mechanism(s) involved. CD8+effector T cells and target cells from sooty mangabeys were immortalized and cloned. The target cells were then stably transfected with an SIV-LTR-CAT construct or with the parental CAT plasmid as a control. A quantitative RT-PCR method, providing the necessary sensitivity, was developed to monitor the influence of the cloned CD8+T cells on the CATmRNA contained in the target cells. It could be demonstrated that a soluble factor was secreted by the cloned CD8+T cells from sooty mangabeys, which appeared to regulate CATmRNA activity in a dose-dependent and reversible manner. Kinetic experiments showed that the CATmRNA transcriptional activity was initially augmented at 30 min postcoculture and was followed by a marked decrease in transcriptional activity after a few hours. This immediate early response could be mitigated utilizing H7, Calmodulin, or PDTC (a pyrrolidone derivative of dithiocarbamate), suggesting that the pathway was protein kinase-dependent and that the NF-B site may be involved. The inhibitory effect could also be overcome using a protein synthesis inhibitor, suggesting that protein synthesis was needed to negatively regulate CATmRNA activity and hence SIV promoter activity.