Pregnancy-Associated Plasma Protein-A (PAPP-A) in Ewing Sarcoma: Role in Tumor Growth and Immune Evasion

Background: Ewing sarcoma (EWS) manifests one of the lowest somatic mutation rates of any cancer, leading to a scarcity of druggable mutations and neoantigens. Immunotherapeutics targeting differentially expressed cell surface antigens could provide therapeutic benefit for such tumors. Pregnancy-associated plasma protein A (PAPP-A) is a cell membrane-associated proteinase produced by the placenta that promotes fetal growth by inducing insulinlike growth factor (IGF) signaling. Methods: By comparing RNA expression of cell surface proteins in EWS (n ¼ 120) versus normal tissues (n ¼ 42), we comprehensively characterized the surfaceome of EWS to identify highly differentially expressed molecules. Using CRISPR/ Cas-9 and anti-PAPP-A antibodies, we investigated biological roles for PAPP-A in EWS in vitro and in vivo in NSG xenograft models and performed RNA-sequencing on PAPPA knockout clones (n ¼ 5) and controls (n ¼ 3). All statistical tests were two-sided. Results: EWS surfaceome analysis identified 11 highly differentially overexpressed genes, with PAPPA ranking second in differential expression. In EWS cell lines, genetic knockout of PAPPA and treatment with anti-PAPP-A antibodies revealed an essential survival role by regulating local IGF-1 bioavailability. MAb-mediated PAPPA inhibition diminished EWS growth in orthotopic xenografts (leg area mm² at day 49 IgG2a control (CTRL) [n ¼ 14], mean ¼ 397.0, SD ¼ 86.1 vs anti-PAPP-A [n ¼ 14], mean ¼ 311.7, SD ¼ 155.0; P ¼ .03; median OS anti-PAPP-A ¼ 52.5 days, 95% CI ¼ 46.0 to 63.0 days vs IgG2a ¼ 45.0 days, 95% CI ¼ 42.0 to 52.0 days; P ¼ .02) and improved the efficacy of anti-IGF-1R treatment (leg area mm² at day 49 anti-PAPP-A þ anti-IGF-1R [n ¼ 15], mean ¼ 217.9, SD ¼ 148.5 vs IgG2a-CTRL; P < .001; median OS anti-PAPP-A þ anti-IGF1R ¼ 63.0 days, 95% CI ¼ 52.0 to 67.0 days vs IgG2a-CTRL; P < .001). Unexpectedly, PAPPA knockout in EWS cell lines induced interferon (IFN)-response genes, including proteins associated with antigen processing/presentation. Consistently, gene expression profiles in PAPPA-low EWS tumors were enriched for immune response pathways. Conclusion: This work provides a comprehensive characterization of the surfaceome of EWS, credentials PAPP-A as a highly differentially expressed therapeutic target, and discovers a novel link between IGF-1 signaling and immune evasion in cancer, thus implicating shared mechanisms of immune evasion between EWS and the placenta.