Reciprocal crosstalk between Th17 and mesothelial cells promotes metastasis‐associated adhesion of ovarian cancer cells

Abstract Background IL‐17A and TNF synergistically promote inflammation and tumorigenesis. Their interplay and impact on ovarian carcinoma (OC) progression are, however, poorly understood. We addressed this question focusing on mesothelial cells, whose interaction with tumor cells is known to play a pivotal role in transcoelomic metastasis formation. Methods Flow‐cytometry and immunohistochemistry experiments were employed to identify cellular sources of IL‐17A and TNF. Changes in transcriptomes and secretomes were determined by bulk and single cell RNA sequencing as well as affinity proteomics. Functional consequences were investigated by microscopic analyses and tumor cell adhesion assays. Potential clinical implications were assessed by immunohistochemistry and survival analyses. Results We identified Th17 cells as the main population of IL‐17A‐ and TNF producers in ascites and detected their accumulation in early omental metastases. Both IL‐17A and its receptor subunit IL‐17RC were associated with short survival of OC patients, pointing to a role in clinical progression. IL‐17A and TNF synergistically induced the reprogramming of mesothelial cells towards a pro‐inflammatory mesenchymal phenotype, concomitantly with a loss of tight junctions and an impairment of mesothelial monolayer integrity, thereby promoting cancer cell adhesion. IL‐17A and TNF synergistically induced the Th17‐promoting cytokines IL‐6 and IL‐1β as well as the Th17‐attracting chemokine CCL20 in mesothelial cells, indicating a reciprocal crosstalk that potentiates the tumor‐promoting role of Th17 cells in OC. Conclusions Our findings reveal a novel function for Th17 cells in the OC microenvironment, which entails the IL‐17A/TNF‐mediated induction of mesothelial‐mesenchymal transition, disruption of mesothelial layer integrity and consequently promotion of OC cell adhesion. These effects are potentiated by a positive feedback loop between mesothelial and Th17 cells. Together with the observed clinical associations and accumulation of Th17 cells in omental micrometastases, our observations point to a potential role in early metastases formation and thus to new therapeutic options.