ObjectiveTo analyse the role of the apoptosis-inducing Fas receptor in the depletion of CD4+ and CD8+ T cells in HIV-infected individuals.MethodsPeripheral blood lymphocytes (PBL) obtained from HIV-infected subjects of all 1993 Centers for Disease Control and Prevention (CDC) stages and from non-infected controls were examined. A two-colour cytofluorometry was employed using monoclonal antibodies against Fas receptor (CD95) in combination with the surface markers CD4, CD8, CD28, CD26 and CD45RO. CD4+ and CD8+ T-cell-enriched PBL were used as target cells to assess their susceptibility to lysis by CD4+ cytotoxic T lymphocytes (CTL) which kill via the Fas pathway.Results: Fas+ PBL are more elevated in HIV-infected individuals than in HIV-negative controls and increase significantly from CDC stages A to C. Whereas Fas+CD4+ and Fas-CD4+ T-cell populations decline in parallel with the progression of HIV infection, the Fas+CD8+, but not of the Fas-CD8+ fraction, significantly increases. The Fas+CD8+ lymphocytes are susceptible to Fas-mediated lysis as they are efficiently killed by Fas-ligand+CD4+ CTL.ConclusionThe Fas receptor may contribute, but not as a unique cause, to the decline of CD4+ T cells in HIV-infected individuals. This and the significant increase of the number of Fas+CD8+ T cells indicates that Fas-mediated immune regulation is disturbed.