Ifnlr1-signaling in recipient NK cells is responsible for the protection from GVHD mediated by hematopoietic cells. (A-B) Donor BALB/c^Luc T-cell expansion in WT and Ifnlr1^–/– recipients determined by bioluminescence 7 days post-BMT. Representative images (A) and quantification (n = 9 per group, combined from 3 experiments) (B). (C) Quantification of donor T cells in spleen at day 4 post-BMT (n = 8 per group, combined from 2 experiments). (D) Quantification of donor T cells in colon at day 4 post-BMT (n = 6, combined from 2 experiments). (E) Proportion of T cells at day 4 post-BMT that are donor vs host (n = 8, combined from 2 experiments). (F-G) Functional capacity of splenic DCs from naive B6 WT or Ifnlr1^–/– mice to stimulate BALB/c. CD4⁺ (F) or CD8⁺ (G) T cells in a mixed lymphocyte reaction (n = 3; data are from 1 of 2 representative experiments). BALB/c recipients were transplanted with WT or Ifnlr1^–/– BM + T cells at day 0 and B6.Tea^Luc T cells (reactive to BALB/c I-E^d derived TEa peptide expressed in donor B6 I-A^b) were transferred at day 12 to assess donor-derived APC function in the GI tract, as determined by bioluminescence of antigen-specific TEa T cells. (H-I) Representative bioluminescence (H) and quantification (n = 10, combined from 2 experiments) (I). (J) Proportions of donor T cells producing IFN-γ in spleen at day 4 post-BMT (n = 8, combined from 2 experiments). (K) Dividing capacity of splenic BALB/c T cells transplanted into WT or Ifnlr1^–/– recipients calculated at day 4 by carboxyfluorescein diacetate succinimidyl ester dilution (n = 14, combined from 3 experiments). (L) Proportion of AnnexinV⁺7AAD^– apoptotic donor T cells at day 4 as in panel K) (n = 7, combined from 2 experiments). (M) Number of NKp46⁺ cells in naive recipient mice (n = 8). (N) B6 WT or Ifnlr1^–/– recipients received αNK1.1 or IgG isotype and were transplanted with BALB/c BM + T cells. IFN-γ was determined in sera at day 4 post-BMT (n = 9 per group, combined from 2 experiments). B6 CD45.2⁺ WT or Ifnlr1^–/– recipients received αNK1.1 or IgG Isotype and were transplanted with CD45.1⁺ allogeneic BALB/c BM and CD45.1⁺CD45.2⁺ syngeneic B6 BM. (O) Representative fluorescence-activated cell sorting plots from NK-depleted recipients showing syngeneic vs allogeneic cells in spleen 48 hours post-BMT. (P) Index of cytotoxicity as described in "Methods" (n = 8, combined from 2 experiments). (Q) Index of cytotoxicity in spleen of WT and Ifnlr1^–/– recipient mice in addition to NKp46^Cre.Ifnlr1^fl.fl-negative and -positive recipients (n = 12, combined from 2 experiments). (R) Number of NKp46⁺ NK cells in NKp46^Cre.Ifnlr1^fl.fl-negative and -positive mice 48 hours after 1000 cGy irradiation. Data are presented as mean ± SEM. P values were calculated by using the 2-tailed Mann-Whitney t test. *P < .05, **P < .01, ***P < .001, ****P < .0001.