FIGURE 1.

Human NK cell proliferation was dependent on homotypic 2B4/CD48 interactions among NK cells. A, human NK cells were isolated from PBMCs and their surface expression of 2B4, CD48, CD2, and CD58 was assessed by flow cytometry. B-D, isolated NK cells were cultured with IL-2 (300 units/ml) for 10 days in the presence of anti-2B4, anti-CD48, anti-CD2, or anti-CD58 mAb (5 μg/ml). Mouse IgG1 (mIgG1) or IgM (not shown) were used as matching isotype controls. The effect of mIgG1 or mIgM on cell proliferation was comparable with that of the untreated. B, live cell numbers were counted by trypan blue exclusion. Relative cell numbers compared with the isotype control are presented as mean (%) ± S.D. n = 9. ***, p < 0.001 (one sample t test). C, cells were incubated with 1 μCi of [³H]thymidine for the last 18 h in culture with IL-2 and incorporated [³H]thymidine into the DNA was measured. n = 4. **, p < 0.01 and ***, p < 0.001 determined by ANOVA combined with a Tukey post hoc test. D, cells were stained with FITC-conjugated Annexin V and propidium iodide and analyzed by flow cytometry. The values represent the percentage of cells in each quadrant. The results are representatives of 5 independent experiments. The mean ± S.D. from these 5 experiments are summarized in the table.