CD70 reverse signaling in human NK cells. (A-B) CD70-negative- CD27-positive- Oci-Ly1 or primary B-ALL cells (patient 1, supplemental Table 1) were cocultured with FACS-sorted CD27-negative NK cells from healthy donors at a ratio of 5:1, in the presence of blocking anti-CD70 antibody or IgG. After 48 hours, IFN-γ and CD107α expression was analyzed. (A) Frequency of IFN-γ⁺ CD107α⁺ NK cells. (B) Frequency of Annexin-V⁺ NK cells. (C-J) Human B-ALL cells (5 × 10⁶; patient 2; supplemental Table 1) were injected IV into NSG mice and analyzed for engraftment in blood at the time points indicated. B-ALL–bearing mice with 5% to 10% engraftment were injected IV with FACS-sorted human NK cells (5 × 10⁶) and treated with anti-CD70 or IgG for 48 hours. (D) Human CD56⁺ CD3⁻ NK cell numbers in spleens of B-ALL–bearing mice 2 days after NK cell transfer. (E-I) Restimulation of CD19-MACS–depleted splenocytes with PMA/ionomycin. (E-F) Frequencies of IFN-γ⁺ (E) and IFN-γ⁺ CD107α⁺ (F) human NK cells. (G) Δ MFI, [MFI-stimulated – MFI-unstimulated], of IFN-γ in NK cells. (H) Frequency of CD107α⁺ NK cells. (I) Frequency of Annexin-V⁺ NK cells. (J) MFI of CD70 on NK cells. Data are shown as mean ± SEM of triplicates per group in panels A-B and 3 animals per group in panels C-J representative of 3 independent experiments. Statistics: Student t test (A-B,D-J). hNK, human NK; PB, peripheral blood.