A model for the maintenance of MC following allo-HSCT. (A) In an immunocompetent host, donor cells are processed by host DC and presented to host T cells, which then eliminate residual donor cells. This scenario can occur following entry of fetal cells into the maternal circulation during pregnancy or during rejection of solid organ grafts. (B) In the setting of profound host immune suppression following allo-HSCT, all host T cells are eliminated by conditioning so that a host alloreactive response to donor cells is abrogated. Instead, donor T cells recognize host peptide/major histocompatibility complex complexes on DC; this alloreactive response leads to both GVHD and the graft-versus-leukemia effect. (C) MC may be established in the setting of moderate host immune suppression following allo-HSCT. Here, residual host Treg cells may limit the activation of DCs, which then fail to induce a cytotoxic allogeneic immune response by donor T cells. Propagation of the allogeneic immune response is inhibited further by the presence of donor Treg and donor DCs that are rendered tolerogenic.