Impaired allogeneic T-cell priming is due to ADCC of IVIg-DCs by NK cells. (A) Matured CTRL-DCs, IVIg-DCs, or HSA-DCs were cultured for 18 hours with allogeneic NK cells (ratio, 1:6) with (black bars) or without (white bars) 10 μg/mL CD16-blocking antibody (5D2). DC death was determined by 7-AAD uptake. The increased death of IVIg-DCs was abrogated by blocking the FcγRIII on NK cells (N = 5; Wilcoxon test for paired data, *P < .05; NS indicates not significant). (B) Matured CTRL-DCs (□) and IVIg-DCs (■) were cultured for 18 hours with allogeneic NK cells (ratio, 1:6) with or without 10 μg/mL 5D2 antibody. Absolute numbers of viable DCs were calculated by determining the ratio of 7-AAD⁻ DCs to detected beads and then multiplying this ratio by the number of beads in the tube. Data are depicted as mean with SE (N = 7; Wilcoxon test for paired data, *P < .05; NS indicates not significant [P = .14]). (C) Matured IVIg-DCs (–) or CTRL-DCs (—) were cultured for 5 days with allogeneic T cells and allogeneic NK cells from the same donor in the absence (left graph) or presence (right graph) of CD16-blocking antibody (5D2) (10 μg/mL). Addition of 5D2 restored the capacity of IVIg-DCs to stimulate allogeneic T cells (N = 6; Wilcoxon test for paired data, **P < .01, *P < .05). (D) Matured IVIg-DCs (–) or CTRL-DCs (—) were cultured for 5 days with allogeneic T cells alone (left graph) or with allogeneic T cells and autologous NK cells (right graph). In the latter case, DCs and NK cells are from the same donor. In addition, DCs were cultured with autologous NK cells and allogeneic T cells, and CD16-blocking antibody (5D2) (10 μg/mL) was added to the culture (right below graft). Addition of 5D2 restored the capacity of IVIg-DCs to stimulate allogeneic T cells (N = 3; **P < .01, *P < .05). Error bars in panels A, C, and D represent SD.