FSTL3 is a biomarker of poor prognosis and associated with immunotherapy resistance in ovarian cancer.

High-grade serous ovarian carcinoma (HGSOC) is associated with high mortality rates due to late-stage diagnosis and limited treatment options. We investigated the role of FSTL3 in ovarian cancer progression both as a prognostic biomarker and as a potential therapeutic target.We measured levels of follistatin (FST) and follistatin-like 3 (FSTL3) in 96 ovarian cancer patient ascites samples and found that FSTL3 overexpression was more predominant than FST and associated with poorer survival outcomes. Mice implanted with an HGSOC syngeneic cell line bearing common alterations in ovarian cancer (KRAS^{G12 V}, P53^R172H, CCNE1^oe, AKT2^oe) had increasing levels of FST and FSTL3 in serum during tumor growth. Further alteration of this model to generate a knockout of FST (KPCA.FSTKO) and an overexpression of human FSTL3 (KPCA.FSTKO_hFSTL3) revealed that FSTL3 expression was associated with a more fibrotic tumor microenvironment, correlating with an increased abundance of cancer-associated myofibroblasts (myCAFs), and cancer cells with a more mesenchymal phenotype. Tumors overexpressing FSTL3 also had significantly less immunocyte infiltration, reduced intratumoral T-cell abundance, and increased CD8+ T cell exhaustion. FSTL3 overexpression completely abrogated tumor response to PPC treatment (Prexasertib combined with PD-1 and CTLA-4 blockade) compared to controls, suggesting that FSTL3 may be involved in immunotherapy resistance. In conclusion, this study suggests a role for FSTL3 as a prognostic marker and as therapeutic target in HGSOC, where it may play a role in promoting a mesenchymal tumor phenotype, maintaining an immunosuppressive tumor microenvironment, and driving immunotherapy resistance.