IL-15 drives the generation of neonatal CD56⁺ T cells from CD56^– populations. (A) CD56-depleted populations were cultured with IL-15, harvested on days 4, 7, 14, 21, and 28, and labeled with CD3 and CD56. Results show the generation of CD56⁺ T cells as a percentage of total lymphocytes. (B) CD56-depleted cells were cultured with IL-15 for 14 days and labeled with CD3, CD56, and 1 of 17 other antibodies. By gating dotplots of CD3 versus CD56, 3 distinct subpopulations could be analyzed: CD56⁺CD3⁺ T cells, CD56⁺CD3^– NK cells, and CD56^–CD3⁺ T cells. The cell surface and intracellular cytokine phenotype of generated CD56⁺ T cells was compared with that of IL-15–expanded CD56^– T cells and circulating T cells (CD56⁺ and CD56^–). Data, shown for 9 antibodies, are expressed as percent positivity within each cell population. (C) CD56-depleted cells were labeled with CFSE prior to culture with IL-15 for 7 days. CFSE-labeled cells were also cultured with low levels of IL-4 (dotted line) in order to maintain a viable but nondividing control population. Cells were stained for CD3 and CD56, and cell division within the 3 cell populations was determined by flow cytometry as in panel B. Bars indicate the percentage of cells that have undergone at least one division. Results are means (± SEMs) of 5 to 9 samples (A-B) or representative of 6 samples (C).