Human immunodeficiency virus-type 1 (HIV-1) infection is characterized by increased immune cell apoptosis. Apoptosis can be triggered by signals that arise from within the cell, or by signals that are elicited by binding of extracellular "death ligands" to their "death receptors," most of which belong to the tumor necrosis factor (TNF)-receptor family, such as CD95 (Fas/Apo-1). In immune cells the oligomerization of CD95, induced by its ligand CD95L, and the recruitment of different intracytoplasmic molecules that in turn activate FLICE/caspase 8 are crucial. To study the role of CD95/CD95L interactions during HIV-1 infection, we developed an original method based upon quantitative-competitive (QC) RT-PCR that allowed us to quantify the amounts of mRNA coding for the total (tCD95) and membrane (mCD95) forms of CD95. We first studied the expression of different forms of CD95 mRNA in a classical model of chronic HIV infection using two infected cell lines of different origin--lymphocytic (ACH-2) or monocytic (U1). We have shown that infected cells of monocytic origin preferentially produce the "protective" (soluble) form of CD95, and no detectable CD95L mRNA, while lymphoid cells produce more mRNA for the membrane form of CD95 (which triggers apoptosis) along with low but detectable amounts of CD95L mRNA. One can hypothesize that a complex balance exists between pro-apoptotic events, perhaps triggered by the host to limit viral production, and anti-apoptotic events likely triggered by the virus to increase its production and survival. In cells of monocytic origin, which act as a reservoir for the virus, the anti-apoptotic molecules are favored; in cells of lymphocytic origin, molecules with an apoptotic meaning are prevalent.