MHC class II deficiency induces chronic GVHD with associated reduced Treg number. (A) Clinical scores of lethally irradiated B6D2F1 recipients transplanted with 5 × 10⁶ TCD BM cells from B6.H2-Ab1^−/− or WT donor mice (n = 14-15, 3 experiments). (B) Representative images of H&E staining of skin taken at day 70 after BMT and GVHD histopathology scores quantified in the skin (n = 4-5, 1 experiment). (C) The frequency and number of CD3⁺CD4⁺FoxP3⁺ cells were analyzed in the spleen of the recipients at day 70 after BMT (n = 8-10, 2 experiments).