Analysis of Myeloid Derived Suppressor Cells and the Role of Inducible Nitric Oxide Synthase in Renal Transplantation Across MHC Mismatched Mice

Despite recent advances in immunology and post-operative care, transplant recipients continue to face major complications. This is due to immune-mediated destruction of graft tissue and drug toxicity caused by chronic immunosuppression. A solution to both of these issues is the induction of long-term tolerance to donor antigens. Recently, renal allograft tolerance has been achieved between HLA-disparate individuals in the clinic without the need for immunosuppression using a mixed chimerism protocol. However, the mechanisms and cellular components involved in the initiation of regulatory and deletional tolerance in this model remains to be understood. The aim of this study is to identify suppressive mechanisms that myeloid-derived suppressor cells (MDSCs) utilize to induce regulatory tolerance. MDSCs were enriched from the bone marrow using a negative selection protocol, and cultured with stimulating cytokines. Flow cytometry was then used to assess the expression of surface and intracellular molecules. Moreover, DBA donor kidneys were transplanted into B6 recipients and were euthanized after 1, 3, and 6 weeks. Nitrosylation was assessed by immunohistochemistry (IHC) using anti-nitrotyrosine and anti-iNOS unconjugated antibodies. CD11b+ cells were isolated from 6 week kidney allografts, and analyzed with IHC and flow cytometry. Cell culture showed that when MDSCs are stimulated they upregulate PD-L1, podoplanin, and F4/80. Interestingly, Ly6C expression is dramatically reduced. Additionally, when these cells are cultured with naïve T cells they are able to promote the induction of T regulatory cells (Tregs). Moreover, accepted renal allografts are distinct from rejected allografts with respect to nitrosylation. The presence of iNOS+ cells in the graft shows that nitric oxide (NO) production is occurring within the kidney microenvironment. CD11b+ cells within the Treg-rich organized lymphoid structures (TOLS) of the graft are one source of NO production. Our findings provide insight into the phenotype and potential role of these tissue effector cells in solid organ transplantation