Abstract
INTRODUCTION: The treatment of glioblastoma multiforme (GBM) is a major challenge of neuro-oncology. Despite aggressive therapeutic strategies including surgery and radio-chemotherapy, median survival remains extremely low. This points to the urgent need for alternative treatment strategies such as immunotherapy. Immunological escape mechanisms, involving the Natural Killer Group 2, member D (NKG2D) receptor-ligand system, play a major role in tumor progression. Cell-bound NKG2D-ligands such as MHC class I related molecule A and B (MICA and MICB), and the UL-16 binding protein family (ULBP1-6) are recognized by the NKG2D-receptor (NKG2Dr) and trigger cytotoxic effector functions in NK-cells and T-cell subsets. By releasing soluble NKG2DL (sNKG2DL), which then bind to NKG2Dr, tumor cells inhibit the killing potential of the effector cells. Numerous studies documented the importance of NKG2D-system in vitro GBM-model systems. Here, we aimed to analyze NKG2D-system in GBM-patients ex vivo. MATERIALS AND METHODS: Until now, 37 GBM patients and 19 healthy controls (HCs) have been included in the study. A total of 24 GBM patients were on medication with dexamethasone prior to surgery. We analyzed serum levels of sNKG2DLs in HCs and GBM patients before and 3 months after surgery and radio-chemotherapy via Luminex-based multiplex assay. Absolute cell numbers of distinct immune cell subsets and the expression of NKG2Dr were analyzed by flow cytometry. NKG2DL-expression on GBM primary cell cultures was studied via flow cytometry and western-blotting. RESULTS: Our data show that in comparison to healthy controls GBM patients show reduced numbers of leukocytes and cytotoxic effector cells in peripheral blood, but elevated numbers of immunosuppressive cell subsets. Whereas NKG2Dr is not detectable in primary cell cultures, the tumor cells moderately express NKG2DL and release sNKG2DL into the culture medium. Serum levels of sNKG2DL are differentially modulated in GBM patients when comparing pre- to 3 months postop. This effect is independent of medication with dexamethasone. CONCLUSION: GBM patients seem to have impaired cytotoxic immune response compared to HCs. As NKG2DL are released by the tumor cells, this effect might also be generated via sNKG2DL. Nevertheless, serum levels of NKG2DL do not decline after treatment in most patients. A better understanding of the mechanisms regulating the NKG2D-system in GBM patients will be crucial for the development of new therapeutic strategies targeting the NKG2D system.