Studies of human immunodeficiency virus (HIV) and nonhuman primate models of pathogenic and nonpathogenic simian immunodeficiency virus (SIV) infections have suggested that enhanced ex vivo CD4 T-cell death is a feature of pathogenic infection in vivo. However, the relative contributions of the extrinsic and intrinsic pathways to programmed T-cell death in SIV infection have not been studied. We report here that the spontaneous death rate of CD4 þ T cells from pathogenic SIVmac251-infected rhesus macaques ex vivo is correlated with CD4 T-cell depletion and plasma viral load in vivo. CD4 þ T cells from SIVmac251-infected macaques showed upregulation of the death ligand (CD95L) and of the proapoptotic proteins Bim and Bak, but not of Bax. Both CD4 þ and CD8 þ T cells from SIVmac251-infected macaques underwent caspase-dependent death following CD95 ligation. The spontaneous death of CD4 þ and CD8 þ T cells was not prevented by a decoy CD95 receptor or by a broad-spectrum caspase inhibitor (zVADfmk), suggesting that this form of cell death is independent of CD95/CD95L interaction and caspase activation. IL-2 and IL-15 prevented the spontaneous death of CD4 þ and CD8 þ T cells, whereas IL-10 prevented only CD8 T-cell death and IL-7 had no effect on T-cell death. Our results indicate that caspasedependent and caspase-independent pathways are involved in the death of T cells in pathogenic SIVmac251-infected primates.

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