Abstract 6134: Immunomodulatory <i>PIGR</i> <i>+</i> <i>RORC</i> <i>+</i> dendritic cells (PRDCs) are enriched in malignant ascites

Abstract Background: Peritoneal carcinomatosis and malignant ascites are frequently caused by solid tumors, and the presence of ascites is associated with a poor prognosis and resistance to immune checkpoint inhibitors. While some immunosuppressive features of ascites have been described, the role of dendritic cell (DC) subsets in this microenvironment is unknown. Recently, a subset of DCs that express RORC have been found in normal mouse and human tissues, and they have been linked to tolerance of gut antigens. However, this DC population has not previously been associated with cancer or well characterized in humans. Methods: We performed single-cell RNA sequencing (scRNA-seq) with paired surface proteomics (CITE-seq), and secreted proteomics (n = 109 analytes) on ascites (n = 23) and matched peripheral blood (n = 15) from patients with gastric and gastroesophageal adenocarcinoma (GEA; n = 523,322 total cells). Allogeneic mixed lymphocyte reactions (MLRs) were performed using DCs from GEA and pancreas cancer patients with ascites. In vitro differentiation experiments leveraged cDCs from healthy donors and cultured them in media supplemented with ascites supernatant or paired plasma from GEA patients. Results: We identified a DC subset expressing PIGR and RORC (PRDCs) that were transcriptionally distinct from canonical DC subsets and similar to tolerogenic cell subsets. Analysis of &amp;gt;1x107 cells from public datasets suggest that PRDCs are found in other human tissues but enriched in malignant ascites from both GEA and ovarian cancers. Sorted PRDCs stimulated T-cell proliferation in MLRs. However, scRNAseq of T cells stimulated by PRDCs showed that those T cells were less effectively polarized due to reduced expression of effector cytokine genes (e.g., IFNG, IL5, IL13) compared to T cells stimulated by cDC2s. Spatial transcriptomics confirmed the presence of PRDC-like cells in human lymphoid tissues, and neighboring, proliferating CD4 T cells similarly showed reduced polarization signatures. Features of the PRDC transcriptional program could be induced by culturing blood-derived cDC1s from healthy donors in supernatant from patients with malignant ascites, suggesting that PRDCs are induced in the ascites microenvironment. Conclusions: PRDCs represent a conserved, immunomodulatory DC subset enriched in malignant ascites. PRDCs are capable of modulating T-cell responses and may represent a tolerogenic state induced by the ascites microenvironment. Targeting PRDCs or their differentiation pathways may offer new strategies to enhance anti-tumor immunity in patients with peritoneal metastases. Citation Format: Steven M. Blum, Thomas L. Chan, Neal P. Smith, Courtney Ambrose, Katherine H. Xu, Alice Tirard, Nandini Samanta, Sidney Martin, Roya Best, Elizabeth Tuttle, Christopher T. Stueber, Vaishnavi R. Yalala, Haley Barnes, Sean T. Bannon, Yuhui Song, Benjamin Y. Arnold, Mushriq Al-Jazrawe, Joseph J. Zhao, Kamil Slowikowski, Jessica Tantivit, Kasidet Manakongtreecheep, Lukas M. Altenburger, Simon Knott, Matthew R. Strickland, Michael G. Drage, Linda T. Nieman, Jesse S. Boehm, Raghav Sundar, Gary Reynolds, Samuel J. Klempner, Alexandra-Chloé Villani. Immunomodulatory PIGR + RORC + dendritic cells (PRDCs) are enriched in malignant ascites [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6134.