Donor NK cells improve survival and reduce GVHD severity after allogeneic BMT. (A) Donor natural killer (NK) cells prolong survival of mice receiving donor conventional T cells (Tcon + NK) compared with Tcon only (Tcon). Mice were sacrificed because of morbidity according to euthanasia criteria at the indicated time points. Mice receiving irradiation plus T cell–depleted bone marrow (TCD-BM) only and mice receiving irradiation only (XRT) are shown as controls. Tcon + NK animals had significantly prolonged survival compared with Tcon mice (P < .05). Data are from 5 mice per group, and are representative of 3 experiments. (B) Mice were scored for GVHD symptoms after transplantation. Tcon + NK mice had significantly lower average graft-versus-host disease (GVHD) scores than Tcon mice (*P < .05). Data are shown for day 20 after T-cell transplantation, and are representative of 2 experiments.