NK cells mediate the elimination of donor cells after anti-CD40L mAb treatment. C57BL/6 mice were treated with anti-CD40L mAb in combination with 10⁶ BALB/c BMCs at day 0. At day 23 and day 27 the mice received an injection with PBS (n = 2), anti-NK1.1 mAb (n = 4), or anti-CD8 mAb (n = 4). At day 28 an in vivo cytotoxicity assay was performed and peripheral-blood samples were collected 2 days later for FACS analysis. The PBS-treated C57BL/6 mice eliminated 94.2% ± 0.2% of the CFSE-labeled BALB/c splenocytes within 2 days, whereas the NK1.1-depleted C57BL/6 mice eliminated 32.1% ± 10.9% of the CFSE-labeled BALB/c splenocytes within 2 days. The C57BL/6 mice depleted for CD8⁺ cells eliminated 89.6% ± 1.8% of the CFSE-labeled BALB/c splenocytes within 2 days. The displayed histograms are representative of each group.