2429 - Mechanistic Study of PARP Inhibitor Olaparib Combined with Low-Dose Radiotherapy and PD-1 Blockade in Reversing Tumor Immune Desertification and Therapeutic Resistance in Ovarian Cancer

Purpose/Objective(s): This study aimed to compare the effects of olaparib combined with PD-1 blockade versus olaparib combined with LDRT and PD-1 blockade in reversing immune desertification and overcoming therapeutic resistance in ovarian cancer, and to systematically elucidate the underlying molecular mechanisms.

Materials/Methods: In vivo and in vitro experiments were conducted using the ID8 ovarian cancer mouse model and ovarian cancer cell lines. Animals were assigned to three groups: a normal control group, an olaparib plus PD-1 blockade group, and an olaparib plus LDRT and PD-1 blockade group. Therapeutic responses were evaluated by monitoring tumor growth, while immune cell infiltration patterns and functional states within the tumor microenvironment were analyzed using single-cell RNA sequencing and multiparameter flow cytometry. At the cellular level, the effects of olaparib and LDRT on DNA damage accumulation, modes of cell death, and tumor cell immunogenicity were systematically assessed. Transcriptomic profiling was further performed to characterize global remodeling of the tumor immune microenvironment.

Results: The results demonstrated that olaparib combined with PD-1 blockade moderately suppressed tumor growth; however, the overall immune activation was limited, and the tumor microenvironment remained predominantly immunosuppressive. In contrast, the addition of LDRT resulted in a markedly enhanced antitumor effect, with significant inhibition of tumor growth. Flow cytometric analysis revealed that olaparib combined with LDRT and PD-1 blockade significantly increased intratumoral infiltration of CD8? T cells and NK cells and enhanced the expression of cytotoxic effector molecules. Concurrently, the proportions of immunosuppressive myeloid cells and M2-like tumor-associated macrophages were significantly reduced, indicating a transition of the tumor microenvironment from an immune-desert phenotype to an immune-inflamed state.Single-cell RNA sequencing further revealed that the triple combination therapy induced reprogramming of tumor-associated macrophages from an immunosuppressive phenotype toward a pro-inflammatory and antigen-presenting phenotype. Antigen processing and presentation pathways, as well as type I interferon signaling, were markedly activated in dendritic cells.

Conclusion: The combination of the PARP inhibitor olaparib with low-dose radiotherapy and PD-1 blockade markedly remodels the ovarian cancer tumor immune microenvironment through multifaceted immunoregulatory mechanisms, effectively reversing immune desertification and overcoming therapeutic resistance. LDRT serves as a critical immune reprogramming component in this combinatorial strategy, providing important experimental evidence for the integrated application of radiotherapy, targeted therapy, and immunotherapy.