Mystery Solved: Scientists Discover Why Colorectal Cancer Defies Immune System Rules

Colorectal cancer breaks the usual immune rules, with certain regulatory T cells linked to improved survival.

In many solid tumors, having a large number of regulatory T (Treg) cells is linked to worse outcomes. These cells can weaken the immune system’s ability to recognize and attack cancer.

Colorectal cancer is an unusual exception. In this disease, tumors packed with Treg cells are actually tied to better survival, even though researchers have not fully understood the reason.

A new study from the Sloan Kettering Institute at Memorial Sloan Kettering Cancer Center (MSK) sheds light on this contradiction. The findings suggest a path toward improving immunotherapy for most people with colorectal cancer, and they may also apply to cancers that develop in tissues such as the skin and the lining of the stomach, mouth, and throat.

According to results published December 15 in Immunity, a leading immunology journal, the key factor is not simply how many Treg cells are present, but which kinds of Treg cells are in the tumor.

“Instead of the regulatory T cells promoting tumor growth, as they do in most cancers, in colorectal cancer we discovered there are actually two distinct subtypes of Treg cells that play opposing roles — one restrains tumor growth, while the other fuels it,” says Alexander Rudensky, PhD, co-senior author of the study and chair of the Immunology Program at MSK. “It’s these beneficial Treg cells that make the difference, and this underscores the need for selective approaches.”

The study was co-led by first authors Xiao Huang, PhD, a postdoctoral researcher in the Rudensky Lab; Dan Feng, MD, PhD, a former MSK Medical Oncology fellow currently at the Icahn School of Medicine at Mount Sinai; and Sneha Mitra, PhD, a postdoctoral researcher in the lab of computational biologist Christina Leslie, PhD, the paper’s other senior author.

The work builds on two decades of research by Dr. Rudensky, one of the world’s leading Treg researchers. His work helped establish that regulatory T cells enforce “immune tolerance” — they help the immune system tell friend from foe, preventing overreactions to our own cells, beneficial microbes, and harmless foods. And further research by his lab has illuminated the mechanisms of Treg cell generation and function, as well as their role in cancer.

Investigating the Most Common Type of Colorectal Cancer

Colorectal cancer is the second leading cause of cancer death when numbers for men and women are combined, according to the American Cancer Society.

For this study, the researchers focused on a type of colorectal cancer that accounts for 80% to 85% of all colorectal cancers — microsatellite stable (MSS) with proficient mismatch repair (MMRp), meaning the tumors’ DNA is relatively stable. These cancers are largely resistant to checkpoint inhibitor immunotherapies.

Previous groundbreaking research at MSK found checkpoint inhibitors alone could successfully treat rectal cancer and several other cancers with the opposite tumor type — those with high microsatellite instability (MSI-H) and mismatch repair deficiency (MMRd). This allows doctors to spare many patients from surgery, chemotherapy, and radiation.

Two Distinct Subpopulations of Tregs: One Helpful, One Harmful

Here the team employed an MSK-developed mouse model that accurately recreates the common mutations, behaviors, and immune cell composition of human colorectal cancer. They found that the regulatory T cells associated with the cancer are split between two types: Cells that make a signaling molecule (cytokine) called interleukin-10 (IL-10) and cells that don’t.

Through a series of sophisticated experiments that selectively eliminated each type of cell, the researchers discovered:

• IL-10-positive Tregs help hold tumor growth in check. They work by dampening the activity of a different type of T cell, called Th17 cells — these produce interleukin 17 (IL-17), which acts as a growth factor for the tumor. They’re more abundant in healthy tissue adjacent to a tumor.
  • When IL-10-positive cells were removed, tumor growth accelerated.
• IL-10-negative Tregs, on the other hand, suppress immune defenders — especially CD8+ T cells with strong anti-cancer capabilities. This subtype of Tregs is largely found within the tumor itself.
  • When IL-10-negative cells were removed, the tumors shrank.

The researchers validated their laboratory findings using samples from people with colorectal tumors. Here, too, they found two distinct populations of IL-10-positive and IL-10-negative cells. And, in an analysis of outcomes in more than 100 colorectal cancer patients, those with more of the “good” IL-10-positive Tregs lived longer, while those with more “bad” IL-10-negative cells fared worse.

“This research shows how important these positive cells are,” Dr. Huang says. “And it highlights the need to develop therapies that can selectively eliminate the harmful Tregs while preserving the helpful ones.”

Findings Point Toward New, Selective Treatment by Targeting CCR8

The research does point to a potential opportunity to improve outcomes for the majority of colorectal cancer patients, says Dr. Rudensky, who is also a Howard Hughes Medical Institute Investigator.

The IL-10-negative cells — the immunosuppressive ones primarily found in tumors — express high levels of a protein called CCR8, the team found.

Previous research from Dr. Rudensky’s lab — led by breast cancer surgeon George Plitas, MD — found high levels of CCR8 displayed by tumor Treg cells in breast cancer and many other types of human cancer. Those findings suggested that harmful Treg cells might be selectively targeted with antibodies — depleting them and opening the tumor up to attack by other immune cells, while sparing helpful Treg cells.

“This idea of using CCR8-depleting antibodies, which was pioneered at MSK, is the main target of global efforts to bring regulatory T cell–based immunotherapy to the clinic,” Dr. Rudensky says.

Numerous clinical trials are underway at MSK and elsewhere to test the approach as a standalone treatment and in combination with other immunotherapies.

The new study adds evidence of the strategy’s potential against colorectal cancer and perhaps other cancers as well.

Other Cancers Share Similar Features

Looking beyond colorectal cancer, the researchers searched for similar divisions between IL-10-positive and IL-10-negative cells in a large dataset of T cells spanning 16 different cancer types — and found them in several other cancers that affect the skin and the lining of the mouth, throat, and stomach.

“What these tissues have in common is that immune cells play a critical role in constantly defending and repairing them as they’re exposed to microbes and environmental stresses,” says Dr. Mitra, who led the complex data analysis. Dr. Mitra is co-mentored by Dr. Leslie and Dr. Rudensky.

Approaches that selectively target IL-10-negative cells in colorectal cancer might also be effective against these other barrier-tissue cancers, the researchers say.

A Different Pattern in Metastasis

Interestingly, a different pattern emerged when the team looked at colorectal cancer that had spread to the liver.

Here, IL-10-negative cells far outnumbered their positive, helpful counterparts. As a result, in contrast to primary tumors, removing all Treg cells led to shrinkage of metastasized tumors. This finding points to a need for tissue- and context-specific therapeutic strategies in colorectal cancer, the researchers say.

Reference: “Opposing functions of distinct regulatory T cell subsets in colorectal cancer” by Xiao Huang, Dan Feng, Sneha Mitra, Emma S. Andretta, Nima B. Hooshdaran, Aazam P. Ghelani, Eric Y. Wang, Joe N. Frost, Victoria R. Lawless, Aparna Vancheswaran, Qingwen Jiang, Cheryl Mai, Karuna Ganesh, Christina S. Leslie and Alexander Y. Rudensky, 15 December 2025, Immunity.
DOI: 10.1016/j.immuni.2025.11.014

The research was supported by the National Cancer Institute (P30 CA008748, U54 CA274492, T32 CA009512), the National Institute of Allergy and Infectious Diseases (AI034206), the Ludwig Center for Cancer Immunotherapy at MSK, the Howard Hughes Medical Institute, the Cancer Research Institute, and a Marie-Joseé Kravis Fellowship in Quantitative Biology.

Dr. Rudensky is a scientific advisory board member of and holds equity in Sonoma Biotherapeutics, RAPT Therapeutics, Coherus Oncology, Santa Ana Bio, Odyssey Therapeutics, and Nilo Therapeutics; he is also a scientific advisory board member of Amgen, BioInvent, and Vedanta Biosciences. He has served as a consultant for AbbVie and is an editor of the Journal of Experimental Medicine and an editorial advisor to Immunity.

Dr. Rudensky and Dr. Plitas are inventors on patents and patent applications held by MSK related to CCR8-based therapeutic depletion of tumoral Treg cells and novel antibodies against CCR8.

Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.