Simian immunodeficiency virus strain PBj14, SIV_smmPBj14, is unique among primate lentiviruses in its ability to trigger the proliferation of resting simian lymphocytes and to cause the rapid death of experimentally inoculated pigtailed macaques. Severe enteropathy, immune activation, and extensive apoptosis, particularly within gut-associated lymphoid tissue, characterize the acute disease syndrome associated with SIV_smmPBj14infection. In the present study, we examined whether the ability of this virus to cause widespread apoptosis might be linked to the up-regulation of Fas ligand (CD95L) expression in virally infected cells.In vitrostudies revealed that expression of the viral Nef protein, in the absence of any other viral gene product, was sufficient to up-regulate the transcriptional activity of the CD95L promoter and to cause cell surface expression of Fas ligand. This up-regulation was NFAT dependent (inhibited by cyclosporin A) and did not occur in cells that expressed a mutated derivative of the viral Nef protein, lacking a previously defined immunoreceptor tyrosine-based activation motif. These findings were corroborated by analysis of tissue sections from virally infected macaques. Immunohistochemical staining revealed that Fas ligand expression was efficiently up-regulated in the GALT of animals that had been experimentally infected with wild-type SIV_smmPBj14but not in animals that were infected with a nonacutely pathogenic viral mutant lacking the Nef ITAM. Taken together, these results suggest that the ability of SIV_smmPBj14to cause acutely lethal disease and to up-regulate FasL expression may be linked. Additional studies will be required to determine whether the induction of FasL expression is in itself important for acute disease pathogenesis.