Activated human T lymphocytes exposed to apoptotic stimuli targeting mitochondria (i.e. staurosporine), enter an early, caspase-independent phase of commitment to apoptosis characterized by cell shrinkage and peripheral chromatin condensation. We show that during this phase, AIF is selectively released from the intermembrane space of mitochondria, and that Bax undergo conformational change, relocation to mitochondria, and insertion into the outer mitochondrial membrane, in a Bid-independent manner. We analyzed the subcellular distribution of cathepsins (Cat) B, D, and L, in a search for caspase-independent factors responsible for Bax activation and AIF release. All were translocated from lysosomes to the cytosol, in correlation with limited destabilization of the lysosomes and release of lysosomal molecules in a size selective manner. However, only inhibition of Cat D activity by pepstatin A inhibited the early apoptotic events and delayed cell death, even in the presence of bafilomycin A 1 , an inhibitor of vacuolar type H ؉ -ATPase, which inhibits acidification in lysosomes. Small interfering RNA-mediated gene silencing was used to inactivate Cat D, Bax, and AIF gene expression. This allowed us to define a novel sequence of events in which Cat D triggers Bax activation, Bax induces the selective release of mitochondrial AIF, and the latter is responsible for the early apoptotic phenotype.

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Primary CD4؉ T lymphocytes, supporting in vitro human immunodeficiency virus type 1 (HIV-1) replication, are destined to die by apoptosis. We explored the initial molecular events that act upstream from mitochondrial dysfunction in CD4 ؉ T lymphocytes exposed to the HIV-1 LAI strain. We tracked by immunofluorescence the cells expressing the p24 viral antigen and used Percoll density gradients to isolate a nonapoptotic CD4؉ T-cell subset with a high inner mitochondrial transmembrane potential (⌬⌿m) but no outer mitochondrial membrane (OMM) rupture. In most p24؉ (but not bystander p24 ؊ ) cells of this subset, the lysosomes were undergoing limited membrane permeabilization, allowing the lysosomal efflux of cathepsins (Cat) to the cytosol. This was also induced by HIV-1 isolates from infected patients. Using pepstatin A to inhibit Cat-D enzymatic activity and Cat-D small interfering RNA to silence the Cat-D gene, we demonstrate that once released into the cytosol, Cat-D induces the conformational change of Bax and its insertion into the OMM. Inhibition of Cat-D activity/expression also conferred a transient survival advantage upon productively HIV-1-infected cells, indicating that Cat-D is an early death factor. The transfection of activated CD4؉ T lymphocytes with a Nef expression vector rapidly induced the permeabilization of lysosomes and the release of Cat-D, with these two events preceding OMM rupture. These results reveal a previously undocumented mechanism in which Nef acts as an internal cytopathic factor and strongly suggest that this viral protein may behave similarly in the context of productive HIV-1 infection in CD4 ؉ T lymphocytes.