NKG2D-dependent anti-tumor effects of chemotherapy and radiotherapy against glioblastoma

Purpose: NKG2D is a potent activating immune cell receptor and glioma cells express the cognate ligands (NKG2DL). These ligands are inducible by cellular stress and temozolomide (TMZ) or irradiation (IR), the standard treatment of glioblastoma, could affect their expression. However, a role of NKG2DL for the efficacy of TMZ and IR has never been addressed. Experimental Design:We assessed the effect of TMZ and IR on NKG2DL in vitro and in vivo in a variety of murine and human glioblastoma models including glioma-initiating cells and a cohort of paired glioblastoma samples from patients before and after therapy. Functional effects were studied with immune cell assays. The relevance of the NKG2D system for the efficacy of TMZ and IR was assessed in vivo in syngeneic orthotopic glioblastoma models with blocking antibodies and NKG2D knockout mice. Results:TMZ or IR induced NKG2DL in vitro and in vivo in all glioblastoma models and glioblastoma patient samples had increased levels of NKG2DL after therapy with TMZ and IR. This enhanced the immunogenicity of glioma cells in a NGK2D-dependent manner, was independent from cytotoxic or growth inhibitory effects, attenuated by O6-methylguanine-DNA-methyltransferase (MGMT) and required the DNA damage response. The survival benefit afforded by TMZ or IR relied on an intact NKG2D system and was decreased upon inhibition of the NKG2D pathway. Conclusions: The immune system may influence the activity of convential cancer treatments with particular importance of the NKG2D pathway in glioblastoma. Our data provide a rationale to combine NKG2D-based immunotherapies with TMZ and IR.

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