Effector CD8⁺ T cells contain the highest levels of T-bet, which is associated with perforin and Grz B expression in resting cells. (A) Three populations of T-bet are typically observed in human CD8⁺ T cells. Shown is a representative flow cytometric plot of the T-bet staining profile within CD3⁺CD8⁺ T cells from an HIV-negative subject. (B) Representative flow cytometric plots of the memory distribution, as determined by CD27 and CD45RO, of the 3 populations of T-bet: negative (blue), dull (yellow), and bright (red). These various T-bet populations were overlaid onto density plots (black shading) of total CD8⁺ T cells. Percentages represent the fraction of overlaid cells that fell within each quadrant. (C) The memory distributions of T-bet^neg (blue), T-bet^dull (yellow), and T-bet^bright (red) populations, as determined by CD27, CD45RO, and CD57, were established for HIV-negative subjects (n = 8). Bars represent the means and error bars indicate SDs. (D-E) Representative flow cytometric plots showing T-bet against both perforin and Grz B among total CD8⁺ T cells from an HIV-negative (D) and an HIV-positive (E) subject. Percentages represent the fraction of perforin or Grz B that was negative, dull, or bright for T-bet expression.