Recipient Myeloid-Derived Immunomodulatory Cells Induce PD-1 Ligand–Dependent Donor CD4+Foxp3+ Regulatory T Cell Proliferation and Donor–Recipient Immune Tolerance after Murine Nonmyeloablative Bone Marrow Transplantation

Abstract

We showed previously that nonmyeloablative total lymphoid irradiation/rabbit anti-thymocyte serum (TLI/ATS) conditioning facilitates potent donor–recipient immune tolerance following bone marrow transplantation (BMT) across MHC barriers via recipient invariant NKT (iNKT) cell-derived IL-4–dependent expansion of donor Foxp3+ naturally occurring regulatory T cells (nTregs). In this study, we report a more specific mechanism. Wild-type (WT) BALB/c (H-2d) hosts were administered TLI/ATS and BMT from WT or STAT6−/− C57BL/6 (H-2b) donors. Following STAT6−/− BMT, donor nTregs demonstrated no loss of proliferation in vivo, indicating that an IL-4–responsive population in the recipient, rather than the donor, drives donor nTreg proliferation. In graft-versus-host disease (GVHD) target organs, three recipient CD11b+ cell subsets (Gr-1highCD11c−, Gr-1intCD11c−, and Gr-1lowCD11c+) were enriched early after TLI/ATS + BMT versus total body irradiation/ATS + BMT. Gr-1lowCD11c+ cells induced potent H-2Kb+CD4+Foxp3+ nTreg proliferation in vitro in 72-h MLRs. Gr-1lowCD11c+ cells were reduced significantly in STAT6−/− and iNKT cell–deficient Jα18−/− BALB/c recipients after TLI/ATS + BMT. Depletion of CD11b+ cells resulted in severe acute GVHD, and adoptive transfer of WT Gr-1lowCD11c+ cells to Jα18−/− BALB/c recipients of TLI/ATS + BMT restored day-6 donor Foxp3+ nTreg proliferation and protection from CD8 effector T cell–mediated GVHD. Blockade of programmed death ligand 1 and 2, but not CD40, TGF-β signaling, arginase 1, or iNOS, inhibited nTreg proliferation in cocultures of recipient-derived Gr-1lowCD11c+ cells with donor nTregs. Through iNKT-dependent Th2 polarization, myeloid-derived immunomodulatory dendritic cells are expanded after nonmyeloablative TLI/ATS conditioning and allogeneic BMT, induce PD-1 ligand–dependent donor nTreg proliferation, and maintain potent graft-versus-host immune tolerance.