Programmed cell death of NK cells in response to tumor cell challenge. RAG-1(-/-)C57BL6/j mice were injected with 10⁶/mouse of tumor cells intravenously and with 2 daily doses of BrdU intraperitoneally. Mice were killed at 48 hours after tumor cell challenge for analysis. (A) BrdU incorporation and DNA contents of NK cells. Spleen cells were stained with NK1.1 mAb and after fixation and permeabilization, stained with anti-BrdU mAb and 7-AAD for DNA content. Data shown were NK1.1⁺ cells with scatters of viable cells and are representative of 4 mice in each group. Note that all of the cells with subdiploid DNA contents were BrdU⁺. (B,C) Analysis of apoptosis by annexin V-staining of NK cells. Spleen cells were stained with mAbs specific for NK1.1 and Ly49D, or with annexin V. Data shown in panel B are contour graphs of gated NK1.1⁺ cells with scatters of viable lymphocytes. The percentages of annexin V⁺ cells among total NK population as well as Ly49D⁺ and Ly49D⁺ NK cells were presented in panel C. Only the mice that received MHC⁺B7⁺ tumor cells showed a significant increase in annexin V⁺ cells when compared with those mice that received PBS. No other comparison showed a statistically significant difference. Data shown are means ± SEM.