Cotransplantation With Myeloid-Derived Suppressor Cells Protects Cell Transplants: A Crucial Role of Inducible Nitric Oxide Synthase

Background Islet transplantation is an alternative to pancreas transplantation to cure type 1 diabetes, but both require chronic immunosuppression, which is often accompanied by deleterious side effects. The purpose of this study was to explore prolongation of islet allograft survival by cotransplantation with myeloid-derived suppressor cells (MDSCs) without requirement of immunosuppression and determine the role of inducible nitric oxide synthase (iNOS) produced by MDSCs in immune regulation. Methods Bone marrow cells were isolated from wild-type (WT) or iNOS(-/-) mice and cultured in the presence of granulocyte-macrophage colony-stimulating factor and hepatic stellate cells (HSCs), resulting in the generation of MDSCs. WT or iNOS(-/-) MDSCs were cotransplanted with islet allografts under the renal capsule of diabetic recipient mice. Results Addition of HSCs into DC culture promoted generation of MDSCs (instead of DCs). MDSCs had elevated expression of iNOS upon exposure to IFN- and inhibited T-cell responses in an MLR culture. Cotransplantation with WT MDSCs markedly prolonged survival of islet allografts, which was associated with reduced infiltration of CD8(+) T cells resulting from inhibited proliferative response. These effects were significantly attenuated when MDSCs were deficient in iNOS. Furthermore, iNOS(-/-) MDSCs largely lost their ability to protect islet allografts. Conclusions Cotransplantation with HSC-induced MDSCs significantly extends islet allograft survival through iNOS-mediated T-cell inhibition. The results demonstrate the potential use of in vitro generated MDSCs as a novel adjunctive immunotherapy for islet transplantation.