Activation of Dendritic Cells in a Sterile, Non-Invasive Manner via Alternating Electric Fields

Dendritic cells (DC) play critical roles in the immune response against tumors, prompting numerous attempts to utilize them in clinical studies. Tumor treating fields (TTFields), are alternating electric fields approved for treatment of glioblastoma (200 kHz) and pleural mesothelioma (150kHz). TTFields are known for their cytotoxic effects on proliferating cells, yet their impact on immune cells remains underexplored. To date, all reported immune-modulating effects of TTFields have been attributed to their induction of immunogenic signals in the treated cancer cells, e.g. by activation of STING or AIM2, or the induction of immunogenic cell death (ICD).

We tested the effects of TTFields on human DC. We found that some (not all) human cancer cell lines treated with TTFields activate human monocyte-derived DC (MoDC) and type-2 conventional DC (cDC2) upon DC-tumor co-culture outside TTFields (termed `indirect` DC activation). Unexpectedly, we found that in addition to the `indirect` activation, TTFields also exert a potent `direct` activation effect on all natural DC (nDC) subtypes. TTFields treatment of cDC1, cDC2, and plasmacytoid DC (pDC) using 150kHz, but not 200kHz, elicited DC activation comparable to that of LPS+R848 - a potent combination of TLR agonists. This `direct` activation effect was quantitatively stronger than the `indirect` activation effect by TTFields-treated cancer cells.

Using a novel miniaturized mixed lymphocyte reaction (mini-MLR) method we demonstrated that TTFields-treated nDC skewed naïve T helper cells towards a Th1-polarization, similarly to adjuvants used in antiviral vaccines or in cancer immunotherapy. We compared RNA-seq data from pre- and post-treatment tumor samples of GBM patients treated either with TTFields+chemoradiation or chemoradiation only (control). We found that the two pathways most strongly induced in TTFields vs. control, were the type-I and the gamma interferon pathways, both induced by some immune adjuvants. Strong correlations between activated-DCs and T helpers, or activated-DCs and effector-memory CTLs were exclusively found in the TTFields-treated group.

Our findings reveal a novel tumor-independent mechanism of action for TTFields, demonstrating that TTFields 150kHz act as a sterile, non-invasive in-situ adjuvant. Combining DCs with TTFields may transform TTFields from a locoregional therapy into a systemic one, with major clinical implications on TTFields-combined immunotherapeutic approaches.