Frequency of IFN-γ–producing cells responding to HIV-infected targets declines in more advanced disease stages. (a) Representative flow cytometric analysis of surface IFN-γ and CD8 staining on gated CD8 T cells in PBMCs from subject 307 after overnight stimulation with uninfected CD4 T cells (left) or HIV_IIIB (middle) and autologous (right) virus–infected CD4 T cell targets and capture of secreted IFN-γ on the cell surface. The numbers shown are the percentages of CD8 T cells producing IFN-γ. (b) The percentage of CD8 T cells producing IFN-γ in response to IIIB (black bars) and autologous (white bars) virus–infected targets are shown for subjects at various disease stages after subtracting background values for uninfected targets. In subjects 606 and CW15, the response was measured only against IIIB virus because no autologous virus could be generated.