Differentiation of human CD8 T cells:: implications for in vivo persistence of CD8+CD28- cytotoxic effector clones

CD8 T cells contain a distinct subset of CD8(+)CD28(-) cells. These cells are not present at birth and their frequency increases with age. They frequently contain expanded clones using various TCR alpha beta receptors and these clones can represent >50% of all CD8 cells, specially in old subjects or patients with chronic viral infections such as HIV-1. Herein, it is shown that a large fraction of CD8(+)CD28(-) cells expresses intracellular perforin by three-color flow cytometry, in particular when this subset is expanded. Together with their known ability to exert potent re-directed cytotoxicity, this indicates that CD8(+)CD28(-) T cells comprise cytotoxic effector cells. With BrdU labeling, we show that CD8(+)CD28(-) cells derive from CD8(+)CD28(+) precursors in vitro. In addition, sorted CD8(+)CD28(+) cells gave rise to a population of CD8(+)CD28(-) cells after allo-stimulation. Moreover, ex vivo CD8(+)CD28(+) cells contain the majority of CD8 blasts, supporting the notion that they contain the proliferative precursors of CD8(+)CD28(-) cells. CD95 (Fas) expression was lower in CD8(+)CD28(-) cells, and this subset was less prone to spontaneous apoptosis in ex vivo samples and more resistant to activation-induced cell death induced by a superantigen in vitro. Thus, the persistence of expanded clones in vivo in the CD8(+)CD28(-) subset may be explained by antigen-driven differentiation from CD8(+)CD28(+) memory precursors, with relative resistance to apoptosis as the clones become perforin(+) effector cells.