A landmark discovery at Memorial Sloan Kettering Cancer Center has solved a 30-year mystery in cancer immunology: why immune cells that typically protect tumors actually help patients with colorectal cancer survive longer. Scientists identified two distinct subtypes of regulatory T cells (Tregs) with opposite functions, opening the door to more precise immunotherapies for the second leading cause of cancer-related death worldwide. Why Has Colorectal Cancer Defied the Immune Rules? For decades, cancer researchers observed a baffling contradiction. In most solid tumors like melanoma and lung cancer, high concentrations of regulatory T cells signal poor outcomes because these cells suppress the immune system's ability to attack cancer. Yet in colorectal cancer, the opposite was true: patients with more Tregs often lived longer. The reason this matters is that approximately 80 to 85 percent of colorectal cancers are classified as microsatellite stable (MSS) with proficient mismatch repair (MMRp). These tumors are considered "cold," meaning they have fewer mutations and are largely ignored by the immune system. Checkpoint inhibitors, drugs that release the immune system's "brakes," have historically failed to produce significant results in the vast majority of these patients. What Are the Two Types of Tregs, and How Do They Work Differently? The research team, led by co-senior authors Alexander Rudensky, PhD, and Christina Leslie, PhD, used sophisticated mouse models and high-resolution single-cell analysis to identify two primary subpopulations of Tregs within colorectal tumors. Each subtype plays a dramatically different role in cancer progression. - IL-10-Positive Tregs (Beneficial): These cells produce interleukin-10 (IL-10), a signaling molecule with anti-inflammatory properties. They suppress Th17 cells, which produce interleukin-17 (IL-17). In the colon, IL-17 acts as a potent growth factor for cancer cells, so by keeping Th17 cells in check, these beneficial Tregs indirectly slow tumor progression. - IL-10-Negative Tregs (Harmful): These cells function like Tregs in other cancers, suppressing CD8+ T cells, which are the immune system's primary cancer killers. Located deep within the tumor mass, these harmful Tregs are the main obstacle to an effective anti-tumor immune response. - The CCR8 Protein Distinction: The harmful IL-10-negative Tregs express high levels of a protein called CCR8, while beneficial IL-10-positive Tregs do not express this protein to the same degree. This distinction is critical for developing targeted treatments. Dr. Xiao Huang, a postdoctoral researcher and co-first author of the study, explained the significance of this finding: "It's not just about the quantity of the immune cells. It's about the specific functional state of those cells within the unique environment of the organ". "It's not just about the quantity of the immune cells. It's about the specific functional state of those cells within the unique environment of the organ," noted Dr. Xiao Huang, postdoctoral researcher at Memorial Sloan Kettering Cancer Center. Dr. Xiao Huang, Postdoctoral Researcher at Memorial Sloan Kettering Cancer Center How Did Researchers Validate These Findings? The breakthrough emerged through a multi-stage experimental process that moved from mouse models to human tissue validation. When researchers removed all Treg cells from mouse tumors, the cancers actually grew faster, confirming that some Tregs were indeed protective. When they selectively removed only the IL-10-producing Tregs, tumor growth accelerated due to unchecked IL-17. Conversely, when they targeted the IL-10-negative Tregs, tumors shrank as CD8+ T cells finally attacked the malignancy. The team then analyzed tumor samples and clinical data from over 100 colorectal cancer patients. The human data mirrored the mouse results: patients with a higher ratio of IL-10-positive beneficial Tregs had significantly longer survival, while those with a preponderance of IL-10-negative harmful Tregs faced poorer outcomes. What Does This Mean for Future Cancer Treatment? The identification of CCR8 as a marker on harmful Tregs provides a clear target for the next generation of cancer treatments. Current immunotherapies often act broadly, affecting large portions of the immune system and frequently causing unwanted autoimmune side effects. By using antibodies that specifically target CCR8, clinicians may be able to selectively deplete the harmful Tregs within the tumor while leaving the protective Tregs intact. This strategy was pioneered in earlier research at Memorial Sloan Kettering led by Dr. George Plitas and Dr. Rudensky, which found high CCR8 expression in breast cancer Tregs. The current study confirms that this target is equally relevant, if not more so, for colorectal cancer. Several clinical trials are currently underway globally, including at Memorial Sloan Kettering, to test CCR8-depleting antibodies both as standalone therapy and in combination with existing checkpoint inhibitors. The research team extended their analysis to a massive dataset of T cells across 16 different cancer types and discovered that the division between IL-10-positive and IL-10-negative Tregs is not unique to the colon. Similar patterns were observed in cancers arising from "barrier tissues," areas of the body constantly exposed to the outside environment, such as the skin, stomach lining, and tissues of the mouth and throat. In these tissues, the immune system has evolved to be highly regulated to prevent chronic inflammation from exposure to microbes and environmental toxins. This discovery transforms how researchers think about immunotherapy resistance in colorectal cancer. Rather than viewing all regulatory T cells as obstacles to overcome, scientists can now distinguish between cells that help and cells that harm, paving the way for more precise treatments that work with the body's natural immune mechanisms rather than against them.
