By Lina carolina 
In the complex landscape of cancer immunology, a perplexing anomaly has long challenged researchers: while regulatory T cells (Tregs) are typically associated with poorer prognoses across most solid tumors by suppressing anti-cancer immune responses, they exhibit a counterintuitive role in colorectal cancer. Tumors with a higher presence of these immune "brakes" in colorectal cancer are often linked to improved patient survival, a stark deviation from established understanding. Now, a groundbreaking study from the Sloan Kettering Institute at Memorial Sloan Kettering Cancer Center (MSK) has illuminated the underlying mechanism behind this phenomenon, identifying two distinct subtypes of Tregs with opposing functions, a discovery that promises to reshape immunotherapy strategies for colorectal cancer and potentially other malignancies.
For decades, the prevailing dogma in cancer immunology dictated that Tregs, crucial for maintaining immune tolerance and preventing autoimmune reactions, inadvertently aided tumor progression by dampening the body’s natural defense mechanisms. Their abundance within tumor microenvironments was a consistent marker of aggressive disease and reduced patient survival. However, the consistent observation of the opposite trend in colorectal cancer—more Tregs correlating with better outcomes—presented a persistent enigma. This discrepancy fueled years of intensive research, seeking to reconcile this outlier behavior with broader immunological principles.
The breakthrough, published in the prestigious scientific journal Immunity, offers a comprehensive explanation: not all Tregs are created equal. The MSK study meticulously demonstrates that the number of Tregs is less critical than their specific functional phenotype. This nuanced understanding has profound implications for the development of more targeted and effective immunotherapies, particularly for the majority of colorectal cancer patients whose tumors do not respond well to current checkpoint inhibitor treatments. The findings also suggest a potential applicability to cancers originating in tissues like the skin, stomach, mouth, and throat, which share certain immunological characteristics with colorectal cancer.
"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 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." Dr. Rudensky, a globally recognized authority on regulatory T cells, has dedicated over two decades to unraveling their intricate functions. His foundational work established the concept of immune tolerance mediated by Tregs, explaining how they differentiate between harmful pathogens and the body’s own tissues, commensal microbes, and dietary antigens, thereby preventing autoimmune disease. His lab’s subsequent research has progressively elucidated the development, function, and influence of Tregs in cancer.
A Decades-Long Quest Culminates in a Paradigm Shift
The ambitious study was spearheaded by a dedicated team of researchers, with 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, serving as co-first authors. Dr. Leslie was the study’s other senior author.
The research builds upon a rich history of investigation into the immune system’s role in cancer. The identification of regulatory T cells and their broad immunosuppressive functions was a significant milestone in immunology, occurring in the late 1990s and early 2000s. Early studies in the 2000s began to link Treg infiltration with poor prognosis in various cancers, a trend that became a cornerstone of cancer immunology research. However, the consistent observation of an inverse correlation in colorectal cancer began to emerge in studies from the mid-2000s onward, prompting questions about the specificity of Treg function in different tumor types.
Understanding the Common Colorectal Cancer Profile
Colorectal cancer remains a significant global health challenge, ranking as the second leading cause of cancer-related mortality for both men and women when combined, according to the American Cancer Society. In 2023, it is estimated that over 150,000 new cases will be diagnosed in the United States alone, with a substantial proportion resulting in death.
This particular study focused on the most prevalent form of colorectal cancer, known as microsatellite stable (MSS) with proficient mismatch repair (MMRp). This subtype, accounting for approximately 80% to 85% of all colorectal cancers, is characterized by a relatively stable genome. Crucially, these MSS/MMRp tumors typically exhibit limited responsiveness to current mainstay immunotherapies, specifically checkpoint inhibitors. These inhibitors, such as PD-1 and CTLA-4 blockers, work by releasing the brakes on other immune cells, allowing them to attack cancer. Their efficacy is generally higher in the less common microsatellite instable (MSI-H) and mismatch repair deficient (MMRd) colorectal cancers, where the immune system is already more primed to recognize tumor cells. The stark difference in treatment response between these subtypes has been a key driver for seeking alternative or complementary therapeutic strategies for MSS/MMRp colorectal cancer.
Unveiling the Dual Nature of Regulatory T Cells
To dissect the unique immunological environment of MSS/MMRp colorectal cancer, the MSK team employed a sophisticated mouse model meticulously engineered to replicate the genetic alterations, behavioral patterns, and immune milieu of human colorectal tumors. This model provided a controlled platform for detailed experimental manipulation.
Their investigation revealed a critical distinction within the tumor-infiltrating Tregs: they could be categorized into two primary groups based on their production of a key signaling molecule, or cytokine, known as interleukin-10 (IL-10). One subset of Tregs produced IL-10, while the other did not.
Through a series of precise experiments involving the selective elimination of each Treg subtype, the researchers elucidated their opposing impacts on tumor progression.
The Protective Role of IL-10-Producing Tregs
The study identified IL-10-producing Tregs as beneficial players. These cells exert their tumor-restraining effect by modulating the activity of another immune cell type, Th17 cells. Th17 cells are known to produce interleukin-17 (IL-17), a cytokine that acts as a potent growth stimulant for tumors. By dampening Th17 cell activity, IL-10-positive Tregs effectively slow tumor proliferation. These protective Tregs were predominantly found in the healthy tissue surrounding the tumor, acting as an external guardian. When these IL-10-positive Tregs were experimentally depleted, the researchers observed a significant acceleration in tumor growth, underscoring their crucial role in tumor suppression.
The Detrimental Impact of IL-10-Negative Tregs
Conversely, IL-10-negative Tregs demonstrated a pro-tumorigenic function. These cells actively suppress potent immune effector cells, most notably CD8+ T cells, which are the primary warriors of the anti-cancer immune response. By inhibiting these cancer-fighting lymphocytes, the IL-10-negative Tregs create an immunosuppressive shield within the tumor microenvironment, allowing cancer cells to evade immune detection and destruction. This harmful subtype was primarily localized within the tumor itself, directly within the hostile territory. The elimination of IL-10-negative Tregs led to a marked reduction in tumor size, providing compelling evidence of their pro-tumorigenic activity.
Human Data Validates In Vivo Findings
To confirm the relevance of these findings to human patients, the research team analyzed tumor samples from individuals diagnosed with colorectal cancer. Their analysis corroborated the existence of these two distinct Treg populations, IL-10-positive and IL-10-negative, within human tumors.
Furthermore, the study retrospectively analyzed the clinical outcomes of over 100 colorectal cancer patients. The results were striking: patients with higher levels of the beneficial IL-10-positive Tregs exhibited significantly longer survival rates. Conversely, those whose tumors harbored a greater abundance of the detrimental IL-10-negative Tregs experienced poorer prognoses.
"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 reflects a critical shift in therapeutic strategy, moving from broad immunosuppression to a highly targeted approach.
Targeting CCR8: A Promising Avenue for Novel Therapies
The insights gleaned from this research point towards a promising new therapeutic strategy for enhancing treatment outcomes for the vast majority of colorectal cancer patients, according to Dr. Rudensky, who also holds a prestigious position as a Howard Hughes Medical Institute Investigator.
A key discovery in this study was the high expression of a protein known as CCR8 on the surface of the detrimental IL-10-negative Treg cells. These are the Tregs that actively suppress the immune response and are predominantly found within the tumor mass. This finding aligns with earlier pioneering work from Dr. Rudensky’s lab, led by breast cancer surgeon George Plitas, MD, which demonstrated that CCR8 is also highly expressed on tumor-associated Tregs in breast cancer and a wide spectrum of other human cancers. That earlier research had posited that antibodies could be developed to specifically target and eliminate these harmful Tregs, thereby unleashing the patient’s own immune system to mount a more effective attack against the cancer, while crucially sparing the beneficial Tregs.
"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. This innovative approach aims to selectively remove the immunosuppressive Tregs that shield the tumor, thereby restoring anti-tumor immunity.
Broader Implications and Future Directions
The findings from the MSK study are not confined to colorectal cancer. The researchers extended their investigation by analyzing a comprehensive dataset of T cells from 16 different cancer types. This comparative analysis revealed similar patterns of IL-10-positive and IL-10-negative Treg populations in several cancers affecting barrier tissues, including those of the skin, 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," explained Dr. Mitra, who was instrumental in leading the data analysis for the study and is co-mentored by Dr. Leslie and Dr. Rudensky. This shared immunological characteristic suggests that therapies designed to eliminate IL-10-negative Tregs in colorectal cancer could potentially be effective against these other barrier tissue-associated cancers as well, opening up a broad new front in cancer immunotherapy.
However, the study also revealed a critical distinction in the immune landscape of metastatic colorectal cancer. When examining colorectal cancer that had spread to the liver, the researchers observed a different immune balance. In these advanced metastatic tumors, IL-10-negative Tregs significantly outnumbered their beneficial counterparts. In this context, the experimental removal of all Tregs led to a reduction in metastatic tumor size, a stark contrast to the findings in primary tumors. This observation underscores the critical need for treatment strategies that are tailored to the specific tissue of origin and the stage of the disease, acknowledging that the tumor microenvironment can evolve significantly as cancer progresses.
The groundbreaking research has been supported by substantial funding from prestigious institutions, including the National Cancer Institute, the National Institute of Allergy and Infectious Diseases, the Ludwig Center for Cancer Immunotherapy at MSK, the Howard Hughes Medical Institute, the Cancer Research Institute, and a Marie-José Kravis Fellowship in Quantitative Biology.
The research team acknowledges the significant contributions of additional authors, including 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 also played pivotal roles in facilitating this complex study.
Dr. Rudensky’s extensive contributions to the field are further recognized by his roles on scientific advisory boards and his equity in several pioneering biotechnology companies, including Sonoma Biotherapeutics, RAPT Therapeutics, Coherus Oncology, Santa Ana Bio, Odyssey Therapeutics, and Nilo Therapeutics. He also serves on the scientific advisory board for Amgen, BioInvent, and Vedanta Biosciences, has consulted for AbbVie, and holds editorial positions with leading scientific journals, including the Journal of Experimental Medicine and Immunity. Furthermore, Dr. Rudensky and Dr. Plitas are listed as inventors on patents and patent applications held by MSK related to CCR8-based therapeutic depletion of tumoral Tregs and novel antibodies targeting CCR8, highlighting the direct translational potential of their discoveries.
In conclusion, this seminal study from MSK has not only resolved a long-standing paradox in colorectal cancer immunology but has also paved the way for a new era of precision immunotherapy. By dissecting the functional dichotomy of regulatory T cells, researchers are now equipped with a deeper understanding to develop therapies that can selectively disarm the harmful elements of the immune system while preserving its vital protective components, offering renewed hope for patients battling this formidable disease and potentially many others. The ongoing clinical trials evaluating CCR8-targeting antibodies represent a critical step in translating these profound scientific insights into tangible clinical benefits.
