Objective : Despite its revolutionary impact, the oncological benefits of checkpoint inhibition immunotherapy have been limited to selected patients with immunogenic cancers. Furthermore, the ability of tumor-directed therapies like radiation and thermal ablation to produce potentially therapeutic immune responses against cancer antigens has proven to be modest. We hypothesized that histotripsy, a novel non-invasive, non-thermal ablative modality that induces cell membrane disruption through ultrasound-generated acoustic cavitation, could stimulate adaptive immune responses to tumor neoantigens.
Methods : Immunocompentent mice with poorly immunogenic syngeneic melanoma or hepatocellular carcinoma flank tumors were treated with sham therapy, radiation, thermal ablation, or histotripsy ablation with or without concurrent anti-CTLA-4 checkpoint inhibition immunotherapy. Parallel experiments employed a two-tumor model consisting of bilateral flank melanoma tumors or solitary flank tumors plus pulmonary metastases established via tail vein injection. Treated and distant untreated (contralateral flank or pulmonary) tumors, lymph nodes, and spleens were analyzed by flow cytometry and by immunohistochemistry to quantify regional and systemic tumor-directed immune responses.
Results : Histotripsy ablation stimulated potent local and systemic tumor-specific CD8+ T cell responses. The magnitude of this immunostimulation was far greater than that seen with sham therapy, radiation or thermal ablation, and comparable to that seen with checkpoint inhibition immunotherapy. In the two-tumor model, only histotripsy ablation of unilateral flank tumors stimulated significant intratumoral CD8+ T cell infiltration and growth inhibition of distant, untreated flank and pulmonary tumors, indicating the unique induction of an abscopal immune response. This immunostimulatory effect was associated with local and systemic release of the inflammatory damage-associated molecular pattern high mobility group box 1 (HMGB1). Furthermore, the addition of histotripsy ablation markedly potentiated the therapeutic efficacy of anti-CTLA-4 checkpoint inhibition immunotherapy against both melanoma and hepatocellular carcinoma (Figure).
Conclusions : These preclinical observations demonstrate that non-invasive histotripsy ablation stimulates potent local, regional, and systemic tumor-specific immune responses. When combined with checkpoint inhibition, histotripsy-mediated immunostimulation may be capable of extending the clinical promise of immunotherapy to patients with otherwise immunoresistant cancers.