By Billy Candra 
In a significant advance for cancer research, scientists at the Sloan Kettering Institute (SKI) at Memorial Sloan Kettering Cancer Center (MSK) have unveiled a groundbreaking explanation for a long-standing paradox in colorectal cancer (CRC) immunology. While large numbers of regulatory T (Treg) cells, known for their immune-suppressing functions, are typically associated with worse outcomes in most solid tumors, their abundance in colorectal cancer has historically correlated with improved patient survival. This perplexing exception, which has puzzled researchers for years, has now been demystified by the discovery that not all Treg cells are created equal. The findings, published in the prestigious scientific journal Immunity, reveal two distinct subtypes of Treg cells within colorectal tumors, each playing an opposing role in disease progression. This discovery is poised to revolutionize immunotherapy strategies for the most common forms of colorectal cancer and potentially extend to other cancers arising in tissues constantly exposed to environmental stresses, such as the skin, stomach, mouth, and throat.
The Enigma of Colorectal Cancer and Treg Cells
For decades, the scientific community has grappled with the contradictory role of regulatory T cells in colorectal cancer. Treg cells are a critical component of the immune system, acting as "brakes" to prevent excessive immune responses and maintain immune tolerance—the ability of the immune system to distinguish between harmful pathogens and the body’s own healthy tissues or beneficial microbes. In the context of cancer, however, this immune-suppressing function often becomes detrimental, as Treg cells can hinder the immune system’s ability to recognize and destroy tumor cells. Their presence in high numbers in many solid tumors, including melanoma, lung, and renal cancers, is a well-established indicator of poor prognosis, as they effectively shield cancer cells from immune attack.
Yet, colorectal cancer stood as a perplexing outlier. Clinical observations consistently showed that patients with a higher infiltration of Treg cells in their colorectal tumors often experienced longer survival times. This counterintuitive correlation challenged prevailing immunological paradigms and presented a major hurdle for developing effective immunotherapies for CRC, particularly for the vast majority of patients whose tumors do not respond to existing checkpoint inhibitors. The lack of a clear understanding of this phenomenon underscored the urgent need for deeper investigation into the complex tumor microenvironment of colorectal cancer.
A Decades-Long Quest Culminates in Breakthrough
The seminal study, led by co-senior authors Alexander Rudensky, PhD, Chair of the Immunology Program at MSK and a Howard Hughes Medical Institute Investigator, and computational biologist Christina Leslie, PhD, represents the culmination of more than two decades of pioneering research by Dr. Rudensky’s lab. Dr. Rudensky is widely recognized as one of the world’s foremost experts on regulatory T cells, having significantly contributed to our fundamental understanding of their development, function, and role in immune tolerance and disease. His lab’s foundational work laid the groundwork for this breakthrough, gradually unraveling the intricate mechanisms by which Treg cells are generated and how they exert their influence on immune responses, including those directed against cancer.
The study’s 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 Dr. Leslie’s lab, meticulously investigated the cellular landscape of colorectal tumors. Their collaborative efforts, blending cutting-edge immunology with advanced computational biology, were instrumental in dissecting the heterogeneity within the Treg cell population.
Unveiling the Two Faces of Treg Cells: Protectors vs. Promoters
The key to unlocking the colorectal cancer paradox lay in the discovery that tumor-associated Treg cells are not a monolithic entity. Instead, they comprise two distinct subtypes with diametrically opposed effects on tumor growth. This critical differentiation hinges on the production of a specific signaling molecule, or cytokine, called interleukin-10 (IL-10).
Through a series of sophisticated experiments utilizing a meticulously developed mouse model that closely mimics human colorectal tumors, the research team identified:
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IL-10-positive Treg cells: These cells were found to restrain tumor growth. Their beneficial action stems from their ability to reduce the activity of Th17 cells, another immune cell type that produces interleukin-17 (IL-17). IL-17 is known to act as a growth signal for tumors, promoting their proliferation and survival. By suppressing Th17 cell activity and, consequently, IL-17 production, IL-10-positive Treg cells effectively slow down tumor progression. Importantly, these protective Treg cells were more commonly found in the healthy tissue surrounding the tumor, suggesting a role in maintaining local tissue homeostasis. When these IL-10-positive Treg cells were selectively removed in experimental models, tumors exhibited accelerated growth, underscoring their crucial beneficial role.
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IL-10-negative Treg cells: In stark contrast, these cells had a detrimental effect, actively fueling tumor growth. Their mechanism of action involves suppressing powerful immune defenders, particularly CD8+ T cells, which are renowned for their direct cancer-killing capabilities. By inhibiting these cytotoxic T lymphocytes, IL-10-negative Treg cells create an immunosuppressive environment that allows cancer cells to evade destruction. This harmful subtype was predominantly located within the tumor itself, indicating their direct involvement in shaping the tumor microenvironment. The selective elimination of IL-10-negative Treg cells resulted in significant tumor shrinkage, confirming their role as drivers of disease progression.
"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," explained Dr. Rudensky. "It’s these beneficial Treg cells that make the difference, and this underscores the need for selective approaches."
Validation Through Patient Data and Clinical Implications
To ensure the relevance of their findings to human disease, the research team rigorously validated their discoveries using tumor samples from actual colorectal cancer patients. This analysis confirmed the presence of the same two distinct populations of IL-10-positive and IL-10-negative Treg cells in human tumors.
Furthermore, a retrospective analysis of outcomes for over 100 colorectal cancer patients provided compelling clinical corroboration. Patients whose tumors contained higher levels of the beneficial IL-10-positive Treg cells exhibited significantly longer survival. Conversely, patients whose tumors harbored a greater abundance of the harmful IL-10-negative Treg cells experienced poorer outcomes. This strong correlation between Treg cell subtype distribution and patient prognosis definitively links the laboratory findings to real-world clinical implications.
"This research shows how important these positive cells are," stated Dr. Huang. "And it highlights the need to develop therapies that can selectively eliminate the harmful Tregs while preserving the helpful ones." This sentiment encapsulates the profound shift in therapeutic strategy that this study advocates for: moving away from broad immune suppression or activation towards highly targeted, precision immunology.
Colorectal Cancer: A Formidable Foe and the Immunotherapy Challenge
Colorectal cancer (CRC) remains a major global health concern. According to the American Cancer Society, it is the second leading cause of cancer-related death when men and women are counted together, accounting for approximately 53,000 deaths annually in the United States alone. Globally, there are over 1.9 million new cases each year.
The study specifically focused on the most common form of the disease, which constitutes about 80% to 85% of all colorectal cancers: microsatellite stable (MSS) tumors with proficient mismatch repair (MMRp). These tumors are characterized by relatively stable DNA, which unfortunately makes them largely unresponsive to current checkpoint inhibitor immunotherapies. Checkpoint inhibitors, such as pembrolizumab or nivolumab, work by blocking proteins that prevent T cells from attacking cancer, thereby "releasing the brakes" on the immune system. While highly effective for a subset of cancers, particularly those with high microsatellite instability (MSI-H) and mismatch repair deficiency (MMRd)—which account for only about 15-20% of CRC cases and can often be treated with immunotherapy alone, allowing many patients to avoid surgery, chemotherapy, and radiation—they have shown limited benefit for the majority of MSS-MMRp CRC patients. This disparity has left a significant unmet need for effective immunotherapeutic options for the vast majority of individuals diagnosed with CRC. The MSK study offers a promising new avenue to address this critical gap.
Targeting CCR8: A New Horizon for Precision Immunotherapy
The findings point towards a tangible and actionable path for improving treatment for the majority of colorectal cancer patients. The researchers made a crucial observation: the harmful IL-10-negative Treg cells express high levels of a specific protein on their surface called CCR8. These are precisely the cells that suppress the anti-tumor immune response and are predominantly found within the tumor microenvironment.
This discovery builds upon earlier groundbreaking work from Dr. Rudensky’s lab, led by breast cancer surgeon George Plitas, MD, which showed that CCR8 is also highly expressed on tumor-infiltrating Treg cells in breast cancer and numerous other human malignancies. That research had already suggested the potential for developing therapeutic antibodies to selectively deplete these harmful Treg cells. The premise is elegant: by specifically targeting CCR8-positive Treg cells, it might be possible to remove the immune-suppressing "bad" Tregs, thereby unleashing the body’s own immune system to effectively attack tumors, all while preserving the beneficial Treg cells that maintain immune tolerance in healthy tissues.
"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 emphasized. Indeed, the promise of CCR8 targeting has already translated into significant clinical development. Multiple clinical trials are currently underway at MSK and other leading institutions worldwide, evaluating this novel approach both as a monotherapy and in combination with existing immunotherapies. The new study on colorectal cancer further strengthens the scientific rationale and clinical urgency for pursuing this strategy, not only for CRC but potentially for a broader spectrum of cancers.
Broader Applicability: Similar Immune Patterns in Barrier Tissues
Beyond colorectal cancer, the researchers broadened their investigation to examine a large dataset of T cells from 16 different cancer types. This extensive analysis revealed that similar divisions between IL-10-positive and IL-10-negative Treg cells were present in several other cancers affecting barrier tissues, including the skin (e.g., squamous cell carcinoma), 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," noted Dr. Mitra, who led the data analysis. This insight suggests a conserved immunological mechanism across these tissues, implying that the dual role of Treg cells might be a general feature in cancers arising in organs that are continually challenged by external factors. Consequently, therapies designed to remove IL-10-negative Treg cells in colorectal cancer might also prove effective against these other cancers. This significantly expands the potential impact of the MSK team’s discovery, offering hope for a wider range of cancer patients.
Metastasis: A Different Battlefield Requiring Nuanced Strategies
The research also delved into the immune landscape of metastatic colorectal cancer, specifically examining tumors that had spread to the liver. Here, the team observed a distinct immune pattern that underscores the complexity of advanced disease. In these metastatic lesions, the harmful IL-10-negative Treg cells significantly outnumbered the helpful IL-10-positive cells. Crucially, unlike primary tumors, removing all Treg cells in this metastatic context actually caused the tumors to shrink.
This finding highlights a critical nuance: the immune microenvironment can vary significantly depending on the stage and location of the cancer. While selective targeting of IL-10-negative Tregs is paramount in primary tumors to preserve beneficial cells, a more comprehensive Treg depletion might be warranted in metastatic settings where the balance has shifted overwhelmingly towards immune suppression. This result strongly emphasizes the need for highly adaptive and personalized treatment strategies that account for both the specific tissue involved and the stage of disease progression.
The Future of Precision Immunotherapy
The groundbreaking work from MSK represents a pivotal moment in our understanding of tumor immunology and opens exciting new avenues for the development of more effective and precise cancer immunotherapies. By dissecting the heterogeneity of Treg cells and identifying specific markers like CCR8, researchers are moving closer to a future where cancer treatments can be tailored not just to the type of cancer, but to the specific immune landscape within each patient’s tumor.
The ongoing clinical trials testing CCR8-depleting antibodies are a direct testament to the translational power of this research. If successful, these therapies could offer a much-needed lifeline for the millions of colorectal cancer patients who currently have limited immunotherapy options, particularly those with MSS-MMRp tumors. Moreover, the potential applicability to other barrier tissue cancers underscores the broad impact of these fundamental immunological insights.
As science continues to unravel the intricate dance between cancer and the immune system, studies like this from Memorial Sloan Kettering Cancer Center pave the way for a new era of precision oncology, where the immune system’s power can be harnessed with unprecedented specificity to fight cancer more effectively than ever before.
Authors, Funding, and Disclosures
Additional authors contributing to this significant 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 played crucial roles in facilitating and supporting the research, providing essential technological and analytical capabilities.
This research was made possible through generous funding from several prestigious organizations, 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.
Dr. Rudensky serves on scientific advisory boards and holds equity in several biotechnology and pharmaceutical companies, including 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.
It is important to note that 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, highlighting the direct translational potential and intellectual property generated from their pioneering work.