Results

Neuroblastoma-bearing animals develop splenomegaly, indicating an active role of the peripheral immune system during MYCN-driven neuroblastoma development. Flow-cytometric analysis of spleens showed accumulation of CD11b⁺Ly6c⁺Ly6g⁺ myeloid-derived suppressor cells (MDSC) in tumor-bearing mice compared to controls. The myeloid cell populations found in tumor-bearing animals expressed high levels of PD1-binding ligand, PD-L1, suggesting a mechanism of T-cell inhibition and a possible target for immunotherapy. In spleens, total T cell numbers were reduced, while the CD4/CD8 ratio and expression of activation markers remained unchanged. RegulatoryT cells were present in tumor tissues, but their frequency was not enhanced in spleens of cancer-bearing animals, indicating that MDSCs play a major role in peripheral immune suppression and tumor progression. Sorted from spleens of tumor-bearing mice, these MDSCs were capable of suppressing T cell proliferation in vitro through the production of indolamine-2,3-dioxygenase (IDO) and inducible nictric oxide synthase (iNOS). To dissect the mechanisms of MDSC-induction in neuroblastoma, naïve bone marrow cells were cultured with neuroblastoma tumor cell line supernatants. This led to the development of suppressive cells with an "MDSC-like" phenotype. This system will be further exploited to define key factors enhancing the accumulation of MDSCs in this malignancy.