Hemophagocytosis and lack of perforin detection and NK-cell cytotoxicity in case reports A and B. (A) Bone marrow evaluation of triplet A (case report A; i-iii) and triplet B (case report B; iv-vi), showing hemophagocytosis of bone marrow elements (eg, red blood cells and neutrophils) by histiocytes. All pictures are from May-Grünwald/Giemsa–stained slides and are shown at × 400 magnification. (B) Perforin and granzymes A and B antibody reactivity in CD56⁺CD8^– NK cells from control (top row), triplet A (middle row), and triplet B (bottom row). Percentage of positive cells is shown in each quadrant. GzmA indicates granzyme A; GzmB, granzyme B. (C) Representative flow-based cytotoxicity results, demonstrating NK-cell–induced apoptosis (FSC^lo/7-AAD^+hi) of major histocompatibility complex (MHC) class I–negative target cells (K562) in control, triplet A, and triplet B. Percentage of apoptotic target cells is shown in the R3 gate of each panel. FSC indicates forward scatter; 7-AAD, 7-aminoactinomycin D. (D) Summary of NK-cell killing, demonstrating the average percentage of apoptotic target cells detected in the R3 gate for each subject. Effector-target (E / T) ratio was 50:1 for all samples. Values shown are the mean ± SD. **P < .01, NS = not significant.