Tumor Treating Fields (TTFields) Delivery to Macrophages Promotes a Pro-Inflammatory Phenotype

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<h3>Purpose/Objective(s)</h3> Tumor Treating Fields (TTFields) are electric fields that disrupt cancer cell division. In addition to an established anti-mitotic mechanism of action, cancer cell death following application of TTFields has previously been shown to stimulate anti-tumor immunity and promote dendritic cell maturation. In the current research, the effect of TTFields on the phenotypic regulation of M1 and M2 macrophages was investigated. <h3>Materials/Methods</h3> Bone marrow–derived macrophages (BMDMs) were generated from the femurs and tibias of 5–8-week-old Balb\C mice. TTFields were applied for 24 h at a frequency of 150 kHz to BMDMs stimulated with LPS+IFN-γ (M1 polarization) or IL-4 (M2 polarization). Surface expression of the macrophage biomarker F4/80 and activation markers CD80, major histocompatibility complex class II (MHC II), inducible nitric oxide synthase (iNOS), CD206, and ARG-1 were examined by flow cytometry. To analyze the heterogeneity of the stimulated macrophages, multiplexed secretion assays were employed measuring levels of CXCL1 (KC), IL-18, IL-23, IL-12p70, IL6, TNF-α, IL-12p40, TGF-β1, CCL22 (MDC), IL-10, IL-6, G-CSF, CCL17 (TARC), and IL-1β. <h3>Results</h3> A significant increase in the percentage of cells expressing the pro-inflammatory M1 markers CD80⁺ and MHC II^high was seen following TTFields application to the polarized BMDMs, while expression of the M2 markers CD206 and ARG-1 was significantly decreased. Cell supernatants of M1 and M2 BMDMs treated with TTFields all showed a pro-inflammatory secretion pattern, with increased levels of CXCL1, IL-18, IL-23, IL-12p70, TNF-α, IL-12p40, CCL22, G-CSF, CCL17 and IL-1β. Overall, this indicates that TTFields induced phenotype skewing of M2 polarized BMDMs to the M1 phenotype. <h3>Conclusion</h3> TTFields treatment displays a novel immunoregulatory role and promotes a pro-inflammatory phenotype in vitro. Future investigations will focus on defining the underlying mechanism of this phenotypic skewing.