Concept of BM GVHD. BM appears to be a target for GVHD, with hematopoietic niches, especially those occupied by osteoblasts, being impaired soon after allo-HSCT. The damage is mediated by donor CD4⁺ T cells, resulting in BM suppression, including B-lymphopoiesis disruption. The concept of BM GVHD provides an explanation for the symptoms associated with BM dysfunction, such as infections after delayed immune reconstitution. Depletion of CD4⁺ T cells with anti-CD4 mAb could be a promising treatment to enhance BM recovery by minimizing GVHD while preserving the GVT effect. OB indicates osteoblast; HSC, hematopoietic stem cell; M, myeloid cells; E, erythroid cells; pT, T-cell precursor; pB, B-cell precursor; imB, immature B; and MSC, mesenchymal stem cell.