Interactions between NKT cells and Tregs are required for tolerance to combined bone marrow and organ transplants

We used a model of combined bone marrow and heart transplantation, in which tolerance and stable chimerism is induced after conditioning with fractionated irradiation of the lymphoid tissues and anti–T-cell antibodies. Graft acceptance and chimerism required host CD4+CD25+ Treg production of IL-10 that was in-turn enhanced by host invariant natural killer (NK) T-cell production of IL-4. Up-regulation of PD-1 on host Tregs, CD4+CD25− conventional T (Tcon) cells, and CD8+ T cells was also enhanced by NKT cell production of IL-4. Up-regulated PD-1 expression on Tregs was linked to IL-10 secretion, on CD8+ T cells was linked to Tim-3 expression, and on CD4+ Tcon cells was associated with reduced IFNγ secretion. Changes in the expression of PD-1 were induced by the conditioning regimen, and declined after bone marrow transplantation. In conclusion, NKT cells in this model promoted changes in expression of negative costimulatory receptors and anti-inflammatory cytokines by Tregs and other T-cell subsets in an IL-4–dependent manner that resulted in tolerance to the bone marrow and organ grafts.