Adoptively transferred MDSCs derived from TLI/ATS/CTX-conditioned recipients prevent MHC-mismatched lethal GVHD. (A) Protocol for MDSC adoptive transfer studies: (1) CD45.1 congenic B6 mice received TLI/ATS/CTX followed by (2) WT BALB/c BMT. At 7 days following BMT, H-2K^d-negB220^negCD11b⁺Gr-1^highCD11c^neg MDSCs were sorted from spleens of recipients in (2) and infused IV (3) into adoptive WT (CD45.2) B6 recipients of 1000 cGy TBI (4), followed by (5) WT BALB/c BMT. Adoptive hosts were either euthanized at day 7 for FACS analysis and histopathologic assessment of key GVHD target organs, or monitored for 100 days for survival and signs of GVHD. BMT at day −7 = 50 × 10⁶ bone marrow cells + 60 × 10⁶ spleen cells from BALB/c donors; BMT at day 0 = 10 × 10⁶ bone marrow cells + 10 × 10⁶ spleen cells from BALB/c donors. (B) Mean ± SEM absolute number (log 10) H-2K^d+TCRαβ⁺CD8⁺ donor GVHD effector cells in colons of adoptive hosts receiving BM alone (negative control), BMT + PBS vehicle (positive control), or BMT + 1 × 10⁶ or 2 × 10⁶ sorted recipient MDSCs. Data are cumulative (n = 3 experiments). (C) Mean ± SEM absolute number (log 10) H-2K^d+TCRαβ⁺CD8⁺ donor GVHD effector cells in spleens (left), MLN (middle), and colon (right) at day 7 post-BMT in adoptive hosts administered BMT + PBS vehicle, BMT + 2 × 10⁶ sorted MDSCs, or BM alone. Data represent n = 3 experiments. (D) Kaplan-Meier cumulative survival (%) of adoptive hosts shown in panel A. Data are from n = 4 representative experiments. (E) Mean ± SEM body mass (g) of mice in panel D. Data are from n = 4 representative experiments. Plus sign indicates time point at which 3 or fewer survivors remain in the specified treatment group. (F) Mean ± SEM cumulative GVHD scores (left) and colon GVHD scores (right) of experimental groups shown in panel D. Data are cumulative (n = 8 experiments).