Carly M. Van Wagoner

Despite the clear efficacy of immune checkpoint blockade, a subset of melanoma patients receives no clinical benefit from treatment. Boiling histotripsy (BH), a mechanical form of focused ultrasound, allows for non‐invasive and non‐ionizing tissue destruction. As mechanically fractionating tumors with BH may potentiate T cell‐mediated anti‐tumor responses, we addressed gaps in knowledge regarding the impact BH has on antigen‐presenting cell (APC) presence in the tumor microenvironment (TME), antigen acquisition, and conventional dendritic cell (cDC) activation states in settings where BH does or does not induce a primary tumor growth response. When BH treatment immediately inhibited melanoma outgrowth, APC presence in the TME was significantly reduced, tumor antigen acquisition amongst APCs remained elevated when compared to controls, and intratumoral immune activation was observed distinctly in the cDC1 subset. Cavitation scoring based on real‐time B‐mode imaging signatures characteristic of BH‐induced tissue destruction demonstrated that BH‐mediated destruction strongly correlates with tumor antigen presence in the draining lymph nodes 24 h post‐ablation. Collectively, these findings reveal that immediate BH‐driven tumor growth control in a melanoma model stratifies distinct intratumoral immune states; positioning early growth response as a biologically informative prognostic indicator of BH‐induced intratumoral immune activation.

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer (BrCa), owing to its lack of targetable receptors and resistance to chemical and molecularly targeted therapeutic approaches. While chemotherapy and surgical resection remain the standard of care, these interventions have significant side effects and varying patient outcomes. Thermally ablative focused ultrasound (T-FUS)—a non-invasive and non-ionizing therapy that utilizes targeted acoustic energy to debulk tumors—has displayed immunomodulatory effects in BrCa. However, T-FUS as a monotherapy has had limited clinical efficacy in TNBC due to the presence of anti-inflammatory immunosuppressive myeloid cells (isMCs). We hypothesized that the elimination of isMCs or initiating tumoricidal activity from them would lead to augmented activity of T-FUS. Thus, we interrogated the ability of myeloablative chemotherapies and antibodies; myeloid recruiting chemokine receptor blockade; and TLR agonists to remodel the tumor myeloid populations. Consistent with our previous studies, we found that while myeloablative chemotherapies decreased circulating isMCs, they had little impact on intratumoral isMCs. In contrast, antibodies targeting Ly6C and Ly6G ablated intratumoral isMCs and systemic isMCs, yet their effect was transient and was accompanied by a surprising depletion of T cells. While targeting CCR2, the dominant chemokine receptor for intratumoral isMC diminished a large subset of immunosuppressive cells within the TME; it also depleted T cells and dendritic cells. Contrary to previous studies, TLR stimulation failed to repolarize myeloid cells into a pro-inflammatory, tumoricidal phenotype but did lead to their depletion from the tumor microenvironment (TME) and mobilization of conventional dendritic cells to the draining lymph nodes. We therefore hypothesized that combining isMC depletion and TLR-driven immune activation would enhance FUS efficacy; however, this combinatorial regimen did not enhance overall survival or control tumor volume after T-FUS treatment. Thus, the BrCa TME is highly resistant to approaches intended to remodel the myeloid cell component which fail to synergize with T-FUS-mediated tumor ablation.