Requirement for interactions of natural killer T cells and myeloid-derived suppressor cells for transplantation tolerance.

The goal of the study was to elucidate the cellular and molecular mechanisms by which a clinically applicable immune tolerance regimen of combined bone marrow and heart transplants in mice results in mixed chimerism and graft acceptance. The conditioning regimen of lymphoid irradiation and anti-T cell antibodies changed the balance of cells in the lymphoid tissues to create a tolerogenic microenvironment favoring the increase of natural killer T (NKT) cells, CD4+ CD25+ regulatory T cells and Gr-1+ CD11b+ myeloid-derived suppressor cells (MDSCs), over conventional T cells (Tcons). The depletion of MDSCs abrogated chimerism and tolerance, and add back of these purified cells was restorative. The conditioning regimen activated the MDSCs as judged by the increased expression of arginase-1, IL-4Rα and programmed death ligand 1, and the activated cells gained the capacity to suppress the proliferation of Tcons to alloantigens in the mixed leukocyte reaction. MDSC activation was dependent on the presence of host invariant NKT cells. The conditioning regimen polarized the host invariant NKT cells toward IL-4 secretion, and MDSC activation was dependent on IL-4. In conclusion, there was a requirement for MDSCs for chimerism and tolerance, and their suppressive function was dependent on their interactions with NKT cells and IL-4.

The changed balance of regulatory and naive T cells promotes tolerance after TLI and anti-T-cell antibody conditioning.

The goal of the study was to determine how the changed balance of host naïve and regulatory T cells observed after conditioning with total lymphoid irradiation (TLI) and antithymocyte serum (ATS) promotes tolerance to combined organ and bone marrow transplants. Although previous studies showed that tolerance was dependent on host natural killer T (NKT) cells, this study shows that there is an additional dependence on host CD4(+)CD25(+) Treg cells. Depletion of the latter cells before conditioning resulted in rapid rejection of bone marrow and organ allografts. The balance of T-cell subsets changed after TLI and ATS with TLI favoring mainly NKT cells and ATS favoring mainly Treg cells. Combined modalities reduced the conventional naïve CD4(+) T cells 2800-fold. The host type Treg cells that persisted in the stable chimeras had the capacity to suppress alloreactivity to both donor and third party cells in the mixed leukocyte reaction. In conclusion, tolerance induction after conditioning in this model depends upon the ability of naturally occurring regulatory NKT and Treg cells to suppress the residual alloreactive T cells that are capable of rejecting grafts.

IL-10 receptor dysfunction in macrophages during chronic inflammation.

The immunosuppressive activity of interleukin-10 (IL-10) makes this cytokine a potentially important clinical tool to reduce inflammatory responses in various diseases. Its efficacy as a therapeutic modality is dependent on the responsiveness of immune cells. We report that macrophages from mice chronically infected with the LP-BM5 retrovirus had a reduced capacity to respond to IL-10 in vitro. The ability of IL-10 to inhibit lipopolysaccharide-induced production of tumor necrosis factor (TNF) alpha and IL-6 was significantly reduced in both alveolar and peritoneal macrophages from infected versus uninfected mice. IL-10 hyporesponsiveness was not related to direct infection by the retrovirus, because bone marrow-derived macrophages infected in vitro with LP-BM5 were as responsive to IL-10 as were uninfected bone marrow-derived macrophages. TNF-alpha appeared to contribute to development of IL-10 hyporesponsiveness, because exposure of normal macrophages to TNF-alpha but not interferon-gamma reduced macrophage responsiveness to IL-10. Reverse transcriptase-PCR and flow cytometry demonstrated normal expression of the alpha and beta chains of the IL-10 receptor in macrophages from infected mice, suggesting that IL-10 hyporesponsiveness is not related to a change in receptor expression. The potential role of reduced IL-10 responsiveness in the chronicity of inflammation in this and other diseases is discussed.