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

CD8 T cells contain a distinct subset of CD8(+)CD28(-) cells. These cells a re not present at birth and their frequency increases with age. They freque ntly contain expanded clones using various TCR alpha beta receptors and the se 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 perfor in by three-color flow cytometry, in particular when this subset is expande d. Together with their known ability to exert potent re-directed cytotoxici ty, this indicates that CD8(+)CD28(-) T cells comprise cytotoxic effector c ells. 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. Moreov er, ex vivo CD8(+)CD28(+) cells contain the majority of CD8 blasts, support ing 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 vi tro. 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 be come perforin(+) effector cells.