Ipilimumab triggers NK cells to kill melanoma cells in a chimeric murine xenograft model. NOD/SCID mice were s.c. injected with melanoma MECO cells (2×106) after incubation with Ipilimumab (IPI-MECO xenograft) or Rituximab (RIT-MECO xenograft) with human allogeneic NK cells (IPI-MECO/NK and RIT-MECO/NK xenografts). Human NK cells were ex vivo isolated from the peripheral blood of 3 different healthy donors and mixed at 1:1 NK:MECO ratio. Controls consisting of MECO cells alone or MECO cells mixed with NK cells (MECO and MECO/NK xenografts, respectively) were also injected. Tumor growth was monitored twice a week and plotted as tumor volumes (mean ± S.E.M.) (panels A, B and C). Data are representative of six independent xenografts with NK cells from the 3 donors (mean ± S.E.M.) from one of the two experiments performed.