BACKGROUND: Genetically modified dendritic cells (DCs) modulate the alloimmunity of T lymphocytes by regulating antigen presentation. METHODS: We generated mice with specific deletion of the X-box-binding protein 1 (XBP1) allele in bone marrow cells and cultured bone marrow-derived DCs (Xbp1-/- BMDCs) from these animals. We then tested the phenotype of Xbp1(-/-) BMDCs, evaluated their capability to activate allogeneic T cells and investigated their mechanistic actions. We developed a mouse model of allogeneic heart transplantation in which recipients received PBS, Xbp1(-/-) BMDCs, a suboptimal dose of cyclosporine A (CsA), or Xbp1(-/-) BMDCs combined with a suboptimal dose of CsA to evaluate the effects of Xbp1(-/-) BMDC transfusion on alloimmunity and on the survival of heart allografts. RESULTS: The deletion of XBP1 in BMDCs exploited the IRE1-dependent decay of TAPBP mRNA to reduce the expression of MHC-I on the cell surface, altered the capability of BMDCs to activate CD8+ T cells, and ultimately suppressed CD8+ T-cell-mediated allogeneic rejection. The adoptive transfer of Xbp1(-/-) BMDCs inhibited CD8+ T-cell-mediated rejection. In addition, XBP1-deficient BMDCs were weak stimulators of allogeneic CD4+ T cells despite expressing high levels of MHC-II and costimulatory molecules on their cell surface. Moreover, the adoptive transfer of Xbp1(-/-) BMDCs inhibited the production of circulating donor-specific IgG. The combination of Xbp1(-/-) BMDCs and CsA treatment significantly prolonged the survival of allografts compared to CsA alone. CONCLUSIONS: The deletion of XBP1 induces immunosuppressive BMDCs, and treatment with these immunosuppressive BMDCs prevents alloimmune rejection and improves the outcomes of heart transplantation. This finding provides a promising therapeutic target in combating transplant rejection and expands knowledge of inducing therapeutic DCs. J Heart Lung Transplant 2022;41:1660-1671 (c) 2022 International Society for Heart and Lung Transplantation. All rights reserved.