125I seed brachytherapy synergized with PD-1 inhibitor imparts potent antitumor immune responses in hepatocellular carcinoma.

Background and purpose: Hepatocellular carcinoma (HCC) exhibits a complex immunosuppressive network, leading to resistance to immune checkpoint inhibitors (ICIs). To counter this, we introduced a novel method, brachytherapy of 125I seed implantation prior to PD-1 inhibitor therapy, aiming to reprogram these inhibitory lymphocyte subsets, which synergized with PD-1 inhibitor to generate robust antitumor immune responses in HCC. Methods and materials: Mouse orthotopic and subcutaneous HCC models were constructed with H22 cells. 125I seeds were implanted into the tumors for irradiation, subsequently. Apoptosis of tumor cells and tissues were measured by flow cytometry (FC) and TdT-mediated dUTP nick end labeling (TUNEL). Immunogenic cell death (ICD) molecules and immune checkpoints were detected with flow cytometer (FCM), confocal fluorescence microscopy, ELISA, chemiluminescence assay and immunofluorescence (IF). Changes of lymphocyte subsets were tested by FC and polychromatic immunofluorescence (PIF). Cytokine release was detected by enzyme-linked immunosorbent assay (ELISA). Results: 125I seed irradiation reversed myeloid cell aggregation, preventing their differentiation into suppressive dendritic cells (DCs), tumor-associated macrophages (TAMs), and myeloid-derived suppressor cells (MDSCs). This reprogramming led to the activation of a T cell-mediated immune response and enhanced susceptibility to anti-PD-1 therapy. The combination of 125I seed brachytherapy with anti-PD-1 therapy elicited a robust anti-tumor immune response, stimulated an abscopal effect suppressing unirradiated distant tumors, and generated memory immunity that resisted tumor rechallenge. Conclusions: Our brachy-primed immunotherapy approach offers a promising and effective treatment strategy, which may improve the immunotherapeutic efficacy in HCC.