Expression of the CD8αα homodimer, CD57, and perforin on CD8^dull subsets. (Left panels) Shown are the percentages of cells in either the CD8^dullCD4⁺, CD8^dullCD4⁻, or the CD8^bright subsets that express CD8αα homodimers or CD57. Subjects had been treated for HD 3 to 25 years before analysis. Four had elevated levels (<19/μL) of the CD8^dullCD4⁺ cells; three had elevated levels (<61/μL) of the CD8^dullCD4⁻cells; two of these subjects had elevated levels of both subsets. The gray box indicates the median and interquartile range for the expression of CD57 on CD8^bright cells in a group of 19 healthy controls. The gray bars below and above this box are the 10th and 90th percentiles of the distribution. (Right panels). T cells from two healthy individuals who have enough CD4⁺CD8^dull (top plots) or CD4⁻CD8^dull (bottom) were stained for intracellular perforin in addition to the immunophenotyping stains. The CD8^dull populations exhibit a high degree of perforin, including the CD57⁻ cells within the CD4⁻CD8^dull population. These data suggest that the CD8^dull cells may have cytotoxic function in the body. The perforin expressing (CD57⁺) CD4⁺ T cells in the CD8⁻ fraction were entirely negative for CD8, ie, they are not contaminating cells from the CD8^dull fraction and represent yet another phenotypically distinct subset of cells.