NK cells mediate the early elimination of major mismatched hematopoietic cells. (A) Data from an in vivo cytotoxicity assay performed in untreated C57BL/6 mice (n = 5) showed that 35.1% ± 11.5% of the CFSE-labeled BALB/c splenocytes were eliminated within 3 hours, 88.8% ± 3.5% within 1 day, 97.2% ± 1.0% within 2 days, and 98.7% ± 0.5% within 3 days after infusion. Data from an in vivo cytotoxicity assay performed in untreated T and NKT-cell-deficient C57BL/6 nu/nu mice (n = 5) showed that 44.6% ± 6.1% of the CFSE-labeled BALB/c nu/nu splenocytes were eliminated within 3 hours, 93.4% ± 2.4% within 1 day, and 99.0% ± 0.7% within 2 days after infusion. (B) Normal C57BL/6 mice and T and NKT-cell-deficient C57BL/6 nu/nu mice were treated with PBS or anti-NK1.1 mAb at day -5 and -1. At day 0, a mixture of differentially CFSE-labeled donor and syngeneic splenocytes was injected; 1 or 2 days later, FACS analysis on peripheral-blood samples was performed. The PBS-treated C57BL/6 mice (n = 5) eliminated 88.5% ± 8.2% of the CFSE-labeled BALB/c splenocytes within 2 days, whereas the NK-cell-depleted C57BL/6 mice (n = 5) eliminated 46.0% ± 2.6% of the CFSE-labeled BALB/c splenocytes within 2 days. The PBS-treated C57BL/6 nu/nu mice (n = 5) eliminated 79.8% ± 9.9% of the CFSE-labeled BALB/c splenocytes within 1 day, whereas the NK-cell-depleted C57BL/6 nu/nu mice eliminated 25.8% ± 8.2% of the CFSE-labeled BALB/c splenocytes within 2 days. The displayed histograms are representative of each group.