Cell-to-cell contact is required for the activation of NK cells by γδ T lymphocytes. Trans-well experiments were performed by culturing purified NK cells in lower wells coated with hIgG1 (2.5 μg/mL). Expanded γδ T lymphocytes were added either to the lower wells (cell-to-cell contact) or to the upper wells (soluble factors). The ratio of NK to γδ T cells was 4:1. After 48 hours of culture, the expression of CD69 and CD54 was analyzed by flow cytometry. The bar diagrams depict the percentage of CD69 and CD54 expressing cells in gated NK populations. Representative data from 1 of 3 independent experiments is shown.