This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In contrast to pathogenic lentiviral infections, chronic SIV infection in its natural host is characterized by a lack of chronic immune activation. In recent studies we have shown that acute events following SIV infection in rhesus macaques differ from those occurring in the natural host sooty mangabeys with regards to the presence of CD4+ T lymphocyte apoptosis and induction of TNF-receptor associated apoptosis-inducing ligand (TRAIL). The goal of this project is to better define differences in the early host response to SIV in pathogenic and nonpathogenic SIV infection.