By Billy Candra 
In a significant paradigm shift for cancer immunotherapy, researchers at the Sloan Kettering Institute (SKI) at Memorial Sloan Kettering Cancer Center (MSK) have unveiled a groundbreaking discovery regarding regulatory T (Treg) cells in colorectal cancer (CRC). This new understanding challenges decades of conventional wisdom and offers a clear explanation for a long-standing paradox in cancer biology, potentially paving the way for more effective and targeted treatments, particularly for the most common forms of CRC that currently resist existing immunotherapies. The study, published in the prestigious scientific journal Immunity, reveals that not all Treg cells are created equal; instead, distinct subtypes play opposing roles, either restraining or fueling tumor growth, a finding that could redefine therapeutic strategies for a range of cancers.
The Treg Cell Paradox: A Long-Standing Enigma
For many years, the scientific community has understood regulatory T (Treg) cells as potent suppressors of the immune system. These specialized immune cells, critical for maintaining "immune tolerance," act as natural brakes, preventing the body’s defenses from attacking its own healthy tissues, beneficial microbes, and harmless environmental elements. In the context of most solid tumors, an abundance of Treg cells is generally associated with poorer patient outcomes. Their presence allows cancers to evade immune surveillance, weakening the body’s natural ability to mount an effective anti-tumor response. This understanding has led to therapeutic strategies aimed at depleting or inhibiting these immune suppressors to unleash the full power of the immune system against cancer.
However, colorectal cancer has always stood out as a perplexing exception to this rule. In a puzzling departure from other malignancies, numerous studies have shown that a higher density of Treg cells within colorectal tumors often correlates with improved patient survival. This counterintuitive observation has baffled researchers for years, hindering the development of Treg-targeted therapies for CRC and prompting intense investigation into the unique immunological landscape of this disease. Colorectal cancer, which is the second leading cause of cancer-related death when men and women are considered together, according to the American Cancer Society, represents a significant global health challenge, underscoring the urgency of understanding its distinct biological mechanisms.
A Decades-Long Quest Culminates in Breakthrough
The new study from MSK offers the definitive answer to this enigma, revealing a critical nuance in Treg cell biology. The key, as it turns out, is not merely the quantity of Treg cells, but their inherent heterogeneity and specific functions within the tumor microenvironment. "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," explains Alexander Rudensky, PhD, co-senior author of the study and chair of the Immunology Program at MSK, and a Howard Hughes Medical Institute Investigator. "It’s these beneficial Treg cells that make the difference, and this underscores the need for selective approaches."
This seminal work is the culmination of more than two decades of pioneering research by Dr. Rudensky, one of the world’s foremost authorities on regulatory T cells. His foundational contributions helped establish the critical role of Treg cells in maintaining immune tolerance, distinguishing self from non-self, and preventing autoimmune responses. Over the years, his laboratory has systematically elucidated the mechanisms governing Treg cell development, function, and their complex interplay with cancer progression. The current study builds directly upon this extensive body of knowledge, providing unprecedented detail into their context-dependent roles.
The research was a collaborative effort, led by first authors Xiao Huang, PhD, a postdoctoral researcher in the Rudensky Lab; Dan Feng, MD, PhD, a former MSK Medical Oncology fellow now 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, who served as the study’s other senior author. Their combined expertise in immunology, oncology, and computational biology proved instrumental in unraveling the intricate cellular dynamics within colorectal tumors.
Unmasking the Dual Nature: Two Treg Subtypes with Divergent Effects
To dissect the complexities of Treg cells in colorectal cancer, the research team utilized sophisticated mouse models developed at MSK that closely recapitulate the genetic alterations, pathological behavior, and immune microenvironment observed in human colorectal tumors. This allowed for precise manipulation and observation of immune cell populations in vivo.
Their meticulous investigations revealed that tumor-associated Treg cells could be broadly categorized into two main groups based on their cytokine production profiles: those that produce the signaling molecule interleukin-10 (IL-10) and those that do not. Through a series of carefully designed experiments involving the selective removal of each group, the researchers uncovered profound and opposing effects on tumor growth.
The IL-10-positive Treg cells emerged as the "beneficial" subtype, acting to slow tumor progression. They achieve this primarily by dampening the activity of another immune cell population, Th17 cells, which produce interleukin-17 (IL-17). IL-17 is known to act as a potent growth signal for tumors, promoting their proliferation and survival. By reducing Th17 cell activity and, consequently, IL-17 production, IL-10-positive Treg cells effectively restrain tumor growth. Importantly, these protective Treg cells were found to be more prevalent in the healthy tissue surrounding the tumor rather than deep within the malignant mass itself. When these IL-10-positive Treg cells were experimentally depleted, tumors in the mouse models exhibited significantly accelerated growth, underscoring their protective role.
Conversely, the IL-10-negative Treg cells were identified as the "harmful" subtype. These cells exert their detrimental effects by suppressing powerful anti-cancer immune defenders, most notably CD8+ T cells, which are renowned for their ability to directly recognize and kill cancer cells. By inhibiting CD8+ T cell activity, IL-10-negative Treg cells create an immunosuppressive environment that allows tumors to thrive unchecked. This harmful subtype was predominantly located within the tumor microenvironment, where its suppressive activities could have the most direct impact on cancer progression. The elimination of IL-10-negative Treg cells in experimental models resulted in a notable reduction in tumor size, confirming their role in fueling tumor growth.
Translational Validation: Human Data Corroborates Findings
To ensure the relevance of their preclinical findings, the MSK team rigorously validated their observations using tumor samples obtained from human colorectal cancer patients. This crucial translational step confirmed the presence of the two distinct populations of IL-10-positive and IL-10-negative Treg cells within human tumors, mirroring the patterns observed in the mouse models.
Further strengthening their conclusions, the researchers analyzed clinical outcomes for more than 100 colorectal cancer patients. The results were striking: patients whose tumors contained higher levels of the beneficial IL-10-positive Treg cells consistently demonstrated longer survival. Conversely, patients whose tumors were characterized by a greater abundance of the harmful IL-10-negative Treg cells experienced poorer clinical outcomes. "This research shows how important these positive cells are," Dr. Huang notes. "And it highlights the need to develop therapies that can selectively eliminate the harmful Tregs while preserving the helpful ones." This dual impact of Treg subsets provides a compelling explanation for the long-standing paradox of Treg cell correlation with improved survival in CRC, demonstrating that the beneficial effects of one subtype outweigh the detrimental effects of the other in primary tumors.
A New Therapeutic Frontier: Targeting CCR8 for Selective Treg Depletion
The identification of these functionally distinct Treg subtypes carries profound implications for the development of next-generation immunotherapies, particularly for microsatellite stable (MSS) colorectal cancer. MSS CRC, which accounts for approximately 80% to 85% of all colorectal cancers and exhibits proficient mismatch repair (MMRp), is notoriously resistant to current checkpoint inhibitor immunotherapies. This contrasts sharply with microsatellite unstable (MSI-H) colorectal cancers, which often respond exceptionally well to existing immunotherapies, allowing many patients to avoid aggressive surgery, chemotherapy, and radiation. The MSK study offers a promising avenue to extend the benefits of immunotherapy to the vast majority of CRC patients who currently have limited options.
A key aspect of this therapeutic strategy revolves around a specific protein marker. The researchers discovered that the harmful IL-10-negative Treg cells, which are primarily located within tumors and actively suppress the immune response, express high levels of a protein called CCR8. This finding is particularly significant because earlier work from Dr. Rudensky’s lab, spearheaded by breast cancer surgeon George Plitas, MD, had already identified CCR8 as highly expressed on tumor Treg cells in breast cancer and numerous other human cancers. That previous research had suggested that antibodies targeting CCR8 could be utilized to selectively deplete these harmful Treg cells without affecting the beneficial ones.
"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 states. By precisely eliminating the CCR8-positive, IL-10-negative Treg cells, the immune system could be unleashed to attack tumors more effectively, while preserving the protective IL-10-positive Treg cells that contribute to better outcomes. This selective approach represents a significant advancement over indiscriminate Treg depletion strategies, which could inadvertently remove beneficial cells and lead to adverse effects.
Encouragingly, multiple clinical trials are currently underway at MSK and other leading institutions worldwide, testing CCR8-targeting antibodies both as monotherapy and in combination with existing immunotherapies. The compelling evidence from this new study provides strong validation for pursuing this strategy specifically in colorectal cancer, and potentially across a broader spectrum of malignancies.
Broader Implications: Immune Patterns in Barrier Tissues
The MSK team further expanded their investigation beyond colorectal cancer, examining a vast dataset of T cells derived from 16 different cancer types. Their goal was to ascertain whether similar patterns of IL-10-positive and IL-10-negative Treg cell division existed elsewhere in the body. Remarkably, they identified similar divisions in several cancers affecting "barrier tissues," including the skin and the delicate linings 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," explains Dr. Mitra, who led the extensive data analysis and is co-mentored by Dr. Leslie and Dr. Rudensky. This common immunological imperative suggests that the dual role of Treg cells might be a conserved mechanism in tissues constantly interacting with the external environment. This finding significantly broadens the potential applicability of therapies designed to selectively remove IL-10-negative Treg cells, suggesting they could be effective not only in colorectal cancer but also in these other barrier tissue cancers where similar immune balances are at play.
A Critical Distinction in Metastatic Disease
While the study revealed a clear benefit in preserving IL-10-positive Treg cells in primary colorectal tumors, the researchers made another critical observation when examining metastatic colorectal cancer that had spread to the liver. In these advanced, secondary tumors, the immune landscape was distinctly different. The harmful IL-10-negative Treg cells significantly outnumbered the helpful IL-10-positive cells. In this metastatic context, unlike in primary tumors, the complete removal of all Treg cells resulted in the shrinking of metastatic tumors.
This finding highlights the crucial importance of considering both the tissue context and the stage of disease when devising therapeutic strategies. What is beneficial in a primary tumor may not be in a metastatic setting, where the balance of Treg subtypes is dramatically shifted. This underscores the need for highly nuanced and adaptable treatment approaches that account for the dynamic and heterogeneous nature of cancer progression.
Future Directions and Lasting Impact
The comprehensive findings from the Sloan Kettering Institute represent a pivotal moment in cancer immunology. By resolving the long-standing Treg cell paradox in colorectal cancer, this research not only offers a clear mechanistic explanation but also provides a tangible therapeutic target. The ability to selectively modulate Treg cell subsets—depleting the harmful ones while preserving the beneficial—opens a new chapter in precision immunotherapy. This approach holds immense promise for improving outcomes for the majority of colorectal cancer patients who currently lack effective immunotherapeutic options, and potentially for patients with other barrier tissue cancers.
The ongoing clinical trials evaluating CCR8-targeting antibodies are a direct testament to the translational power of this discovery, poised to bring this advanced understanding from the laboratory bench to the patient’s bedside. As research continues to refine our understanding of the tumor microenvironment and immune cell heterogeneity, selective immunomodulation, guided by discoveries like this, will undoubtedly become a cornerstone of future cancer treatment strategies, moving towards more personalized and effective interventions.
Additional authors contributing to this impactful study include Emma Andretta, Nima Hooshdaran, Aazam Ghelani, Eric Wang, Joe Frost, Victoria Lawless, Aparna Vancheswaran, Qingwen Jiang, Cheryl Mai, and Karuna Ganesh. The Integrated Genomics Operation and the Single Cell Research Initiative at MSK were instrumental in providing key technological support for the research.
This research received generous funding from multiple sources, including 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-Josée Kravis Fellowship in Quantitative Biology.
Disclosures: Dr. Rudensky serves on scientific advisory boards 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, has consulted for AbbVie, and serves as 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.
