Objectives: Glioblastoma (GBM) is the most common and deadliest malignant brain cancer in adults. Tumor Treating Fields (TTFields) was approved in combination with adjuvant temozolomide chemotherapy for newly diagnosed GBM patients. The addition of TTFields resulted in a significant improvement in overall survival. TTFields are low-intensity alternating electric fields that are thought to disturb mitotic macromolecules' assembly. In many patients, a transient stage of increased peritumoral edema is often observed early during TTFields treatment followed subsequently by objective radiographic responses, suggesting that a major component of therapeutic efficacy by TTFields may be an immune mediated process. We hypothesize that TTFields activate the immune system by triggering pyroptosis and type I Interferon (IFN) response.
Methods: A panel of GBM cell lines were treated with TTFields at the clinically approved frequency of 200 kHz using an in vitro TTFields system. Cells were analyzed for the production of micronuclei and activation of both pyroptosis and STING pathways using immunostaining, quantitative PCR, ELISA and cytometry. Pre-treated mouse GBM cells were injected into B6 mouse brain, IVIS and immunophenotyping were performed post implantation.
Results: TTFields resulted in a significantly higher rate of micronuclei structures released into the cytoplasm, which were co-localized with two upstream dsDNA sensors AIM2 and cGAS. TTFields-activated micronuclei-dsDNA sensor complexes led to i) induction of pyroptotic cell death, as measured by LDH release assay, and through AIM2-recruited caspase1 activation and cleavage of pyroptosis-specific Gasdermin D; and ii) activation of STING pathway leading to the release of type I IFNs and pro-inflammatory cytokines downstream of the NFκB pathway. In a co-culture experiment of bone marrow cells with cells/supernatants obtained from GBM cells treated with TTFields, GBM cells depleted of AIM2 and STING failed to induce bone marrow cells. In mouse model, double knocking down of STING/AIM2 eliminated the tumor suppression effects caused by TTFields. TTFields pretreated wild type cells successfully elevated dendritic cell level in mouse cervical lymph nodes which can be reversed by double knocking down.
Conclusions: These results provide compelling evidence that TTFields activates the innate immune system in GBM cells, and a strong rationale for combining TTFields with immune checkpoint inhibitors to create a therapeutic synergy.
Citation Format: Dongjiang Chen, Nagheme Thomas, Jie Ren, Son Le, Mathew Sebastian, Changwang Deng, Dan Jin, David Tran. Co-activation of STING pathway and immunogenic cell death by tumor treating fields produces effective antitumor immunity in glioblastoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5524.