Activated unlicensed U-NK cells promote donor BMC engraftment after allogeneic BMT. B6 mice were treated with mAb against licensed L-Ly49C/I⁺ NK cells (5E6) and/or unlicensed U-Ly49G2⁺ NK cells (4D11) with or without NK cell stimulation (IL2 or Poly I:C) 2 days before allogeneic BMT of 3 × 10⁶ B10.D2 donor BMCs. Anti-NK1.1 (PK136) was used as positive control of total engraftment. (A-B) The hematopoietic progenitor content of spleens (Total CFU-c/spleen) of IL2- (A) or Poly I:C-treated (B) host NK cells was assessed 7 days post-allogeneic BMT. Lethally irradiated NSG mice were IV injected with 3 × 10⁶ BALB/c (allogeneic: alloBMCs) or B6 (syngeneic: synBMCs) BMCs plus 1 × 10⁶ of in vitro activated licensed (CD45⁺CD3^–CD122⁺Ly49G2^–C/I⁺) or unlicensed (CD45⁺CD3^–CD122⁺Ly49G2⁺Ly49C/I^–) sorted NK cells. (C) CFU-c/spleen is shown 7 days post-BMT of NSG mice. (D) Total number of engrafted donor BMCs (H2d⁺ alloBMCs or H2b⁺ synBMCs) in NSG mice measured by flow cytometry is shown for the spleen. Activated sorted licensed or unlicensed B6 NK cells were also cultured in vitro with allogeneic (B10.D2) or syngeneic (B6) BMCs at a 1:1 ratio for 24 hours and then CFU-c was assessed in the absence of rGM-CSF. (E) CFU-c/BM for in vitro assay is shown. Data are representative of at least 2 experiments with 3 mice per group (A-D) or by triplicate (E) (mean ± SEM). One-way analysis of variance was used to assess significance (*P < .05, **P < .01, ***P < .001; n.d., not detected, n.s., not significant).