The study, published in Science Immunology under the title “Multiomic analysis of malignant ascites defines RORC-expressing dendritic cells at sites of metastasis,” was co-led by Steven Blum, MD, and jointly supervised by Gary Reynolds, MD, PhD, Samuel Klempner, MD, and Alexandra-Chloé Villani, PhD, at the Krantz Family Center for Cancer Research within the Mass General Brigham Cancer Institute and the Mass General Brigham Center for Immunology & Inflammatory Diseases. The work was driven by a stark clinical reality. Roughly one in ten patients with advanced solid tumors will develop malignant ascites, and the complication is especially common in gastric and gastroesophageal cancers, where patients typically survive only a few months once fluid begins to accumulate. Ascites is associated with poor responses to both chemotherapy and immunotherapy, yet the reasons for that treatment resistance have remained poorly understood.

The clinical burden of ascites extends well beyond its grim prognosis. Because the fluid presses on the stomach and intestines, patients often cannot eat comfortably, and the repeated paracentesis procedures used to drain it are physically exhausting and logistically demanding. For many patients, the researchers note, drainage is essentially the only available option, and the removed fluid is simply thrown away. That practice, the team realized, represented a missed opportunity of unusual magnitude. Unlike a tumor biopsy, which samples a single fixed piece of tissue, ascites is a living suspension of the entire metastatic ecosystem: cancer cells floating freely, immune cells drawn to the site of disease, stromal elements, and a rich soup of soluble signaling proteins, all collected non-invasively in the course of routine care.

To exploit that opportunity, the investigators collected paired ascites and blood specimens directly from patients with stomach and esophageal cancers and built an experimental pipeline designed to capture both the abundant and the vanishingly rare members of that cellular community. The approach combined single-cell RNA sequencing, which records which genes are active in each individual cell, with measurements of proteins on cell surfaces, sequencing of immune receptors, and profiling of the soluble proteins present in both blood and ascites fluid. The resulting primary dataset encompassed more than 500,000 cells, an unusually deep census of a metastatic environment, and the team went on to develop methods for physically isolating one of the rare populations they discovered so that it could be studied in primary tumors and lymph nodes as well.

The first conceptual conclusion of the study is deceptively simple but potentially far-reaching: malignant ascites is not merely plasma that has leaked out of the bloodstream carrying a few stray cancer cells. Instead, the analysis showed it to be a distinct and highly organized immune environment in its own right, one that actively shapes how the immune system responds to cancer. The immune cells found in ascites were not a passive reflection of what circulates in blood; they carried gene-expression programs, surface-protein signatures, and receptor repertoires consistent with a locally organized immunological niche. Some of the signaling pathways the team identified are already targetable with existing drugs, raising the possibility that such agents could be repositioned for ascites patients, potentially by delivering them directly into the abdominal cavity where the fluid collects.

The most striking discovery, however, was a rare population of dendritic cells expressing the gene RORC, which the authors termed RORC-expressing dendritic cells, or RORC DCs. Dendritic cells are the sentinels of the immune system, best known for gobbling up debris and presenting fragments of it to T cells, thereby launching immune attacks. The RORC DCs found in ascites represented only a tiny fraction of the immune cells present, yet they were remarkably consistent: the cells were detected in 22 of the 23 patients in the initial cohort. That near-universal presence, despite their scarcity, suggested they are not random contaminants but a reproducible feature of the metastatic peritoneal environment.

What makes the finding biologically provocative is what these cells resemble. RORC DCs bear similarities to a family of immune cells recently characterized in mice and humans that has been shown to help establish immune tolerance, the set of mechanisms that prevents the immune system from attacking harmless substances or the body’s own tissues. Those tolerance-associated cells are exceptionally rare in healthy human tissues and had never previously been connected to cancer. The authors’ experiments suggest that RORC DCs may represent an underappreciated cell type that promotes immune tolerance within tumors, essentially a cellular brake on anti-cancer immunity operating at the very sites where metastatic disease takes hold.

That hypothesis reframes a fundamental tension in immunology. Tolerance mechanisms are not pathological; they are essential, protecting everyone from autoimmunity and destructive inappropriate inflammation. But cancer, the researchers propose, may exploit the very same biology. In metastatic disease, a normally protective immune program could help create an environment in which T cells are less effective at recognizing and killing cancer cells, which would help explain why patients with ascites respond so poorly to immunotherapies designed to unleash T cells. If RORC DCs are indeed helping to impose that tolerant state, they offer an entirely new area of biology to investigate and, potentially, an entirely new class of therapeutic target.

The authors are careful to note that the findings will not change clinical care today. The work is an early step, and its immediate value lies in reimagining how patients with malignant ascites are studied and, eventually, treated. The team has identified pathways and candidate therapeutic targets that are now being evaluated in patient-derived laboratory models, with the hope that the most promising approaches can be brought back to patients through clinical trials. Key open questions include what RORC DCs are actually doing in the ascites environment, where they originate, and whether their activity can be safely modified. Answering those questions, the researchers argue, could uncover new ways to strengthen anti-cancer immunity in a patient population they describe as severely underserved.

Beyond the specific discovery, the study carries a broader methodological lesson for oncology. Ascites arises not only in gastric and gastroesophageal cancers but across many tumor types that metastasize to the abdominal cavity, and the researchers found that the immune populations they identified in stomach and esophageal cancer samples were also present in ascites from other cancers and at other sites of metastatic disease. That suggests the fluid could serve as a general-purpose sampling platform for metastatic immunology, one that requires no additional invasive procedure beyond the drainage patients already undergo. A substance long treated as waste, in other words, may prove to be one of the most detailed maps available of how cancer and the immune system negotiate their uneasy coexistence, and the rare cells drifting within it may hold clues to why the immune system so often stands down precisely when it is needed most.

Subject of Research: Immune cell analysis of malignant ascites in metastatic cancer

Article Title: New study turns attention to a common problem across cancers: malignant ascites

Article References: New study turns attention to a common problem across cancers: malignant ascites. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: malignant ascites, dendritic cells, RORC, immune tolerance, metastatic cancer, single-cell RNA sequencing, gastric cancer, gastroesophageal cancer, immunotherapy, tumor microenvironment, Science Immunology, Mass General Brigham