Abstract
Focused ultrasound (FUS) is a clinically relevant and attractive therapeutic, due to its non-invasive and non-ionizing nature. By employing FUS, solid malignancies can be targeted and destroyed via mechanical or thermal based approaches. Significant progress in the field has been made in the design of therapeutic transducers and imaging systems which allow for precision-based targeting, however, the immunological effects induced by FUS remain unclear. While FUS treatment can promote the physical debulking of a tumor, the cell death induced through ultrasonic insult needs to be better understood. Here, we demonstrate that boiling histotripsy (BH), a mechanically ablative form of FUS, enhances melanoma-specific T-cell activation and induces T-cell expansion in the tumor draining lymph nodes (TDLNs) independent of conventional dendritic cell (cDC) migration – the canonical form of tumor antigen transport required to initiate a T-cell response in the Sham, untreated state. This suggests that tumor antigen captured by LN-resident cDCs is capable of eliciting tumor-specific T-cell activation in response to a bolus of antigen disseminated to the TDLNs by BH. Importantly, we did observe variability in the strength of T-cell activation. As we know cDCs post BH are not reaching the same heightened activation state as cDCs that have been exposed to toll-like receptor (TLR) agonists, we have identified an area of the Cancer-Immunity Cycle that would benefit from an adjuvant therapy. Further studies are required to determine which antigen presenting cell (APC) subset is required for BH-mediated T-cell activation. In addition, we found that the level of tumor debulking and constrain of primary tumor growth resulting from BH treatment, promoted substantially different immune effects in the tumor microenvironment – particularly in the skewing of cDC1 and cDC2 activation. These results both define mechanisms of BH-induced anti-tumor immunity and inform the design of improved combinatorial therapies for melanoma. The notion that the amount of destruction elicited by FUS varies as properties of the sound waves, or ablative regimens are altered was also observed when we compared two regimens of thermally ablative FUS (TFUS) – one that induced ~20-30% tumor cell death and another that resulted in ~5% tumor cell destruction. We found that triple negative breast cancer tumors treated with the more ablative TFUS regimen had a larger amount of differentially expressed genes and pathways which sustained in the week following treatment. This was not observed with the less ablative strategy. The tune-abliity of FUS, and how exactly the sound waves are eliciting divergent immune responses remains to be assessed. In all, our studies provide clarity on the strengths and weakness of BH and TFUS as anti-tumorigenic immune stimulates and highlights areas of immune activation that could be further enhanced through dual immune-based therapies, such as TLR agonism and immune checkpoint blockade, to improve their efficacy and better leverage these FUS modalities as cancer therapeutics.
