MDSC-mediated GVHD suppression is independent of MHC class I expression and does not interfere with the GVT effect. (A-C) Lethally irradiated B6.bm1 recipient mice were reconstituted with TCD-BM from B6 mice without or with B6-derived SCs and conjected with 1 × 10⁷ B6-derived WT or MHC Cl I^−/− (Cl I^−/−) MDSCs. Survival (A) and GVHD-scores (B) were determined and surviving animals/total animals treated are indicated in the survival curve in brackets. (A) TCD-BM + SC vs TCD-BM + SC + 1 × 10⁷ WT MDSC, *P ≤ .05; TCD-BM + SC vs TCD-BM + SC + 1 × 10⁷ Cl I^−/− MDSC, *P ≤ .05. (B) Error bars indicate ± SEM. (C) Paraffin sections of ileum and colon (left), liver (middle), and skin (right) of 8 to 10 animals/treatment group were analyzed for histologic signs of GVHD on the day mice were euthanized due to their moribund state or at the end of the experiment, *P ≤ .05, n.s., not significant. (D-F) Lethally irradiated B6.bm1 mice were transplanted with B6-derived TCD-BM in the presence or absence of B6-derived SCs and 1 × 10⁷ B6-derived MDSCs. CD8⁺ JM6-thymoma cells (H-2K^bm1) were conjected. Mice were analyzed for survival and surviving animals/total animals treated are indicated in brackets. TCD-BM + SC vs TCD-BM + SC + 1 × 10⁷ MDSC; ***P ≤ .001 (D). (D) Spleen and liver weights were determined either the day mice were euthanized due to their moribund state or at the end of the experiment; ***P ≤ .001 (E). (F) Presence of tumor cells in BM, liver, and spleen was determined by the expression of CD8. CD4 and CD8 expression was compared with nontransplanted B6.bm1 mice (untreated). FACS analysis is shown for 1 representative mouse out of at least 5 mice analyzed at the end of the experiment or the day mice were euthanized due to their moribund state.