Chemotherapy Induces an IL1β-dependent Neutrophil Recruitment that Promotes Chemoresistance in Metastatic Ovarian Cancer

High-grade serous carcinoma (HGSC) of the ovary acquires chemoresistance through diverse cancer cell-intrinsic and-extrinsic mechanisms, culminating in treatment-refractory intraperitoneal metastasis. How chemotherapy-induced remodeling of the tumor microenvironment modulates drug sensitivity remains unclear. In this study, we demonstrate that chemotherapy induced IL1β-dependent neutrophil accumulation in tumors, driving chemoresistance in HGSC. Using patient samples, bulk transcriptomic profiling before and after chemotherapy revealed post-treatment upregulation of IL1B , and single-cell RNA sequencing identified myeloid cells as its principal source. In a chemoresistant murine metastatic ovarian cancer model, chemotherapy increased neutrophils and neutrophil extracellular traps (NETs) in omentum tumors; these increases were abrogated in IL1β-deficient mice, with expansion of activated CD8+ T cells and tumor control. Neutrophil depletion in wild-type mice recapitulated the chemosensitive phenotype of IL1β-deficient mice. In vitro , IL1β did not alter cancer cell-intrinsic chemosensitivity, whereas NETs reduced the chemosensitivity of cancer cells. Additionally, the dominant IL1β receptor (IL1R1) was predominantly expressed in tumor-associated fibroblasts in humans and mice. Consistently, IL1R1-deficient mice exhibited chemosensitivity with decreased neutrophil accumulation and increased IFNγ⁺TNF⁺CD8⁺ T cells. We also found that chemotherapy upregulated CXCL2 in patients and that ablating IL1β-IL1R1 axis decreased CXCL2 expression in tumor-associated fibroblasts in mice. Finally, residual human HGSC tumor after chemotherapy showed increased neutrophils and a trend toward more NETs. Collectively, these findings illuminate a paradoxical, cancer cell-extrinsic mechanism in HGSC whereby chemotherapy itself amplifies chemoresistance and suggest that targeting chemotherapy-induced inflammation may help overcome treatment resistance.