Induction of Cytotoxic CD8+CD56+T Cells from Human Thymocytes by Interleukin-15

CD8+ CD56+ cells isolated from human peripheral blood lymphocytes have been shown recently to represent a population of cytotoxic active T cells. However, it is not known if these cells are intrathymically or extrathymically developed or how these cells are influenced by growth factors. In the present study, we investigated the effects of interleukin-2 (IL-2) and IL-15 on human thymocytes with respect to development of CD8+ CD56+ T cells. Freshly isolated thymocytes contain few CD8+ CD56+ cells, but the number of these cells increases significantly when thymocytes are grown in the presences of IL-15 or IL-2. However, IL-15 induced a significantly higher fraction of CD8+ CD56+ cells compared with IL-2. Thus, although IL-2 and IL-15 are known to have a number of redundant functions, we here demonstrate that IL-15 is superior to IL-2 in inducing CD8+ CD56+ T cells from cultures of thymocytes. The majority of the IL-15-grown CD8+ CD56+ cells were CD45R0+, representing a memory phenotype, and showed high expression of the IL-15R-complex and high numbers of CD69+ cells. Moreover, cytotoxic activity was confined to this cell population.