STING agonism overcomes STAT3-mediated immunosuppression and adaptive resistance to PARP inhibition in ovarian cancer

Background: PARP inhibition (PARPi) has demonstrated potent therapeutic efficacy in patients with BRCA-mutant ovarian cancer. However, acquired resistance to PARPi remains a major challenge in the clinic. Methods: PARPi-resistant ovarian cancer mouse models were generated by long-term treatment of olaparib in syngeneic Brca1-deficient ovarian tumors. STAT3-mediated immunosuppression was investigated in vitro by co-culture experiments and in vivo by analysis of immune cells in the TME of human and mouse PARPi-resistant tumors. Whole genome transcriptome analysis was performed to assess the anti-tumor immunomodulatory effect of STING (stimulator of interferon genes) agonists on myeloid cells in the TME of PARPi-resistant ovarian tumors. A STING agonist was used to overcome STAT3-mediated immunosuppression and acquired PARPi resistance in syngeneic and PDX models of ovarian cancer. Results: In this study, we uncover an adaptive resistance mechanism to PARP inhibition mediated by tumor associated macrophages (TAMs) in the tumor microenvironment (TME). Markedly increased populations of pro-tumor macrophages are found in BRCA-deficient ovarian tumors that rendered resistance to PARPi in both murine models and patients. Mechanistically, PARP inhibition elevates the STAT3 signaling pathway in tumor cells, which in turn promotes pro-tumor polarization of TAMs. STAT3 ablation in tumor cells mitigates polarization of pro-tumor macrophages and increases tumor-infiltrating T-cells upon PARP inhibition. These findings are corroborated in patient-derived, PARPi-resistant BRCA1-mutant ovarian tumors. Importantly, STING agonists reshape the immunosuppressive TME by reprogramming myeloid cells and overcome the TME-dependent adaptive resistance to PARPi in ovarian cancer. This effect is further enhanced by addition of PD-1 blockade. Conclusions: We elucidate an adaptive immunosuppression mechanism rendering resistance to PARPi in BRCA1-mutant ovarian tumors. This is mediated by enrichment of pro-tumor TAMs propelled by PARPi-induced STAT3 activation in tumor cells. We also provide a new strategy to reshape the immunosuppressive TME with STING agonist and overcome PARPi resistance in ovarian cancer. Overall design: PBM and PBM-R tumors: transcriptome analysis were performed on bulk tumors from PBM and PBM-R tumor-bearing mice. PBM tumors were generated by orthotopic injection of PBM tumor cells into FVB/NJ mice. PBM-R are refactory tumors generated in PBM tumor-bearing mice after long-term treatment with olaparib (50 mg/kg/day). Myeloid cells: about 1 x10^6 PBM-R tumor cells were intraperitoneally injected to FVB/NJ mice. About 2-3 weeks after injection, mice were grouped and treated with control, olaparib, MSA-2 and MSA-2 in combination with MSA-2 for 24 hours. After treatment, myeloid cells (CD45+CD11b+) were isolated from the ascites of each mouse (n=3 per group) by EasySep Mouse CD11b positive Selection Kit II (STEMCELL Technology, #18970) and subjected for transcriptome analysis.