The absence of Siglec-G expression on host APCs enhances GVHD. To make BM chimeras, WT-B6 Ly5.1, WT-B6 Ly5.2, and Siglec-G^−/− animals were lethally irradiated with 11 Gy and infused with 5 × 10⁶ BM cells and 5 × 10⁶ splenocytes from syngeneic WT-B6 Ly5.2, WT-B6 Ly5.1, or Siglec-G^−/− donors. Four months later, [B6 Ly5.2→B6 Ly5.1], [B6 Ly5.1→B6Ly5.2], [Siglec-G^−/−→B6Ly5.2], or [B6 Ly5.2→Siglec-G^−/−] animals were irradiated with 9 Gy and received 2.5 × 10⁶ CD90⁺ T cells along with 5 × 10⁶ TCD-BM cells from either syngeneic B6 or allogeneic MHC-mismatched BALB/c donors, or received 1.5 × 10⁶ CD90⁺ T cells along with 5 × 10⁶ TCD-BM cells from either syngeneic B6 or allogeneic MHC-matched, multiple MiHA-mismatched, C3H.sw donors. (A) Survival (left) and clinical GVHD score (right) (n = 6 to 18 per group, pooled from 2 experiments) in MHC-mismatched BALB/c→B6 model. (B) Survival (left) and clinical GVHD score (right) (n = 8 to 12 per group, pooled from 2 experiments) in MHC-matched, multiple MiHA-mismatched C3H.sw→B6 model. (C) Donor T-cell (H-2k^d+CD4⁺CD8⁺) expansion in the spleen on day 7 after allo-HCT (n = 3 to 6 per group, pooled from 2 experiments) in MHC-mismatched BALB/c→B6 model. (D) Donor T-cell (CD229.1⁺CD4⁺CD8⁺) expansion in the spleen on day 14 after allo-HCT (n = 3 to 6 per group, pooled from 2 experiments). (E) Donor T-cell (H-2d⁺CD3⁺) expansion in the spleen and MLNs on day 1 after allo-HCT (n = 3 per group, pooled from 2 experiments). (F) Serum TNF-α levels on day 1 after allo-HCT (n = 3 to 6 per group, pooled from 2 experiments). (G) Donor T-cell (H-2d⁺CD3⁺) expansion in the spleen and intraepitherial lymphocytes on day 3 after allo-HCT (n = 3 per group, pooled from 2 experiments). (H) Donor IFN-γ⁺CD3⁺ T-cell expansion in the spleen on day 3 after allo-HCT (n = 3 per group, pooled from 2 experiments). (I) Serum IFN-γ levels on day 3 after allo-HCT (n = 3 to 6 per group, pooled from 2 experiments). (D-I) The bars show the mean ± SEM.