gms | German Medical Science

Introduction: The treatment of glioblastoma multiforme (GBM) is a major challenge in neuro-oncology. Despite aggressive therapeutic strategies including surgery and radio-chemotherapy, median survival remains 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 GBM progression. Cell-bound NKG2D-ligands such as MHC class I related molecule A and B, 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 (sNKG2DLs), which then bind to NKG2Dr, tumor cells inhibit the killing potential of the effector cells. High levels of sNKG2DL are associated with a poor prognosis and tumor progression. Numerous studies documented the importance of the NKG2D-system in 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 (87.9% males) and 20 healthy controls (HC, 75% males) 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 sNKG2DL in HC 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. Furthermore, NKG2DL-expression on GBM-primary cells was studied via flow cytometry and westernblotting.

Results: GBM patients show reduced numbers of leukocytes and cytotoxic effector cells, but higher numbers of the immunosuppressive cell subsets in peripheral blood. 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. In line with this data, GBM primary cells release sNKG2DLs into the culture medium.

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 mechanisms regulating the NKG2D-system in GBM patients will be beneficial for the development of new therapeutic strategies targeting.

Note: Equally shared authorship for Guranda Chitadze and Charlotte Flueh.