IMMU-52. TUMOR TREATING FIELDS (TTFIELDS) INDUCE IMMUNOGENIC CELL DEATH RESULTING IN ENHANCED ANTITUMOR EFFICACY WHEN COMBINED WITH ANTI-PD-1 THERAPY

Abstract

Tumor Treating Fields (TTFields) are an effective anti-neoplastic treatment modality delivered via noninvasive application of low intensity (1-3 V/cm), intermediate frequency (100-300 kHz), alternating electric fields. TTFields are employed as a regional treatment modality using insulated transducer arrays applied to the skin, with the intent to kill tumor cells and reduce local recurrence. This therapy is approved for the treatment of patients with glioblastoma. Previous investigations have shown that TTFields disrupt microtubules and septin filaments, both of which govern key processes in mitosis. The outcomes of mitosis under TTFields application include abnormal chromosome segregation, which trigger different forms of cell death. In this study we evaluated whether TTFields-induced cell death can be immunogenic. We demonstrate that cancer cells that die during TTFields application exhibit endoplasmic reticulum (ER) stress leading to calreticulin translocation to the cell surface and release of damageassociated molecular patterns including the chromatin-binding protein HMGB1 and adenosine triphosphate. Further, we show that TTFieldstreated cells promote phagocytosis by dendritic cells (DCs) and maturation of DCs under co-culture conditions. In vivo, the combined treatment of lung tumor-bearing mice with TTFields in combination with the immune checkpoint inhibitor anti-PD-1, significantly improved therapeutic efficacy compared to the control group or TTFields and anti-PD-1 alone. Significant increase in the number of tumor infiltrating immune cells was observed in the TTFields plus anti-PD-1 group. These infiltrating cells, specifically macrophages and DCs, demonstrated upregulation of surface PD-L1 expression. Correspondingly, cytotoxic T-cells isolated from these tumors have shown higher levels of IFN-gamma production relative to untreated mice. Collectively, our results suggest that TTFields application induces both ER stress and autophagy, resulting in immunogenic cell death. Combining TTFields with anti-PD-1 may therefore achieve tumor control by further enhancing antitumor immunity.