Session Item

Turning immunologically cold tumors hot to sensitize them to immune checkpoint inhibitors is a current clinical challenge. Defects in DNA repair, especially in homologous recombination (HR), can trigger the intracellular immune response by accumulation of cytosolic DNA due to an increase in the number of DNA double strand breaks (DSBs). Thus, a combination of a HR-defect and radiation, chemotherapeutics or DNA damage response inhibitors might enhance the activation of the intracellular immune response, thereby maximizing its anti-tumor activity. This project therefore aims to identify a new combination of radio-chemotherapy in HR-impaired tumor cells to increase the intracellular immune signaling, potentially creating a new synthetic lethality effect.

Isogenic MCF7 and MDA-MB231 clones with different BRCA1 status were generated using CRISPR/Cas9. The HR-capacities were determined by plasmid-reconstruction assay and cell cycle distributions were analyzed by propidium iodide staining. For determination of the radio- and chemoresistance a colony formation assay was used. RPA and yH2AX foci formation was analyzed by immunostaining. To analyze DNA replication, the DNA fiber assay was conducted. Cytosolic DNA was measured using the PicoGreen Assay. PD-L1 expression on the cell surface was analyzed by multicolor flow cytometry.

All BRCA1 targeted clones showed a significant reduction in the HR-capacity (p ≤0.001), while no changes in the cell cycle were induced. Interestingly, the resistance to radiation (IR) and mitomycin C varied. The MCF7 9.2 and MDA-MB231 9.11 clones were resistant to both DNA damaging agents (D₃₇ = 3.8 Gy; IC₅₀ = 0.5 µg/mL), while the MCF7 14.3 and MDA-MB231 7.22 clones were sensitive (D₃₇ = 2 Gy; IC₅₀ = 0.25 µg/mL). As this could not be explained by different HR-capacities, we investigated whether there were variations in the DNA replication stress (RS) level. And indeed, the resistant MCF7 9.2 clone showed significantly lower level of RS (p = ≤0.01). Low level of RS might correlate with fewer DSBs, which could be confirmed by low formation of RPA and yH2AX foci. Strikingly, the resistant clones showed no increase in cytosolic DNA after IR, thus no activation of the intracellular immune signaling, while the sensitive clones did (p ≤0.001). It seems therefore likely that the resistance is associated with the impaired activation of the intracellular immune response. Additionally, we could observe a significantly increased PD-L1 surface expression post IR in the resistant clones (p = ≤0.01), further suppressing the anti-tumor effects of the immune system.

Our results indicate that radiochemoresistance in HR-impaired cell lines might be associated with a reduced activation of the intracellular immune signaling and active suppression of the anti-tumor effects. Increasing the RS level and combining IR with immune checkpoint inhibitors might offer a new therapeutic approach to treat tumors resistant to conventional radiochemotherapy.