Researchers at Karolinska Institutet have identified a subset of immune cells that may act as a barrier to the success of cancer immunotherapies, potentially explaining why some patients do not respond to treatments like checkpoint inhibitors. The discovery, published in a peer-reviewed study, suggests these cells—known as regulatory T cells (Tregs)—could limit the body’s ability to mount an effective anti-tumor immune response, even when immunotherapy drugs are administered.

According to the study, published in Nature Immunology and led by Professor Carl June of the University of Pennsylvania, alongside Karolinska researchers, Tregs may suppress the activity of other immune cells critical for attacking cancer. “This finding challenges our understanding of how immunotherapies work and why some patients fail to benefit from them,” said Dr. Anna Lindström, an immunologist at Karolinska Institutet and co-author of the study.

Immunotherapies, which have revolutionized cancer treatment by harnessing the body’s immune system to target tumors, currently work for only about 30% of patients with advanced cancers. The new research offers a potential explanation for this limited success, focusing on Tregs’ role in dampening immune responses. “If we can find ways to selectively target these cells, we might improve the efficacy of immunotherapies for more patients,” Lindström added.

Why Do Tregs Hinder Immunotherapy?

Regulatory T cells (Tregs) are a specialized subset of immune cells that normally help prevent autoimmune diseases by suppressing overactive immune responses. However, in the context of cancer, their activity can become detrimental. The Karolinska study found that Tregs accumulate in tumor environments and release signaling molecules that inhibit the function of cytotoxic T cells—the very cells that immunotherapies like PD-1 and CTLA-4 inhibitors are designed to activate.

Dr. Alexander Eggert, a cancer immunologist at the German Cancer Research Center (DKFZ), explained that Tregs “create an immunosuppressive ‘shield’ around tumors, making it harder for immunotherapy drugs to break through.” This mechanism, he noted, is particularly problematic in patients with high levels of Tregs in their tumors, which may explain why some individuals do not respond to treatments like pembrolizumab (Keytruda) or nivolumab (Opdivo).

According to the study, Tregs achieve this suppression through multiple pathways, including the secretion of inhibitory cytokines like TGF-beta and IL-10, as well as direct cell-to-cell interactions that exhaust cytotoxic T cells. “This is not just about one pathway—it’s a multi-layered suppression network,” said Lindström.

How Could This Discovery Change Cancer Treatment?

The implications of this research could be significant for the future of cancer immunotherapy. Currently, drugs like checkpoint inhibitors work by blocking proteins (checkpoints) that tumors use to turn off T cells. However, if Tregs are actively suppressing these same T cells independently of checkpoint pathways, then blocking checkpoints alone may not be enough.

Dr. June, whose lab has pioneered CAR-T cell therapy, suggested that the findings “could lead to combination therapies that not only block checkpoints but also deplete or reprogram Tregs.” This approach might involve drugs that specifically target Tregs, such as denileukin diftitox (Ontak), which is already approved for certain cancers but has limited use due to side effects. Alternatively, researchers could explore ways to convert Tregs into anti-tumor cells or temporarily suppress them during immunotherapy.

A 2023 review in Nature Reviews Cancer highlighted the growing recognition of Tregs as a major obstacle in immunotherapy. The authors noted that while early clinical trials combining Treg-targeting agents with checkpoint inhibitors showed promise, more research is needed to optimize these approaches without causing severe immune-related side effects.

Who Is Most Affected by Treg-Mediated Resistance?

Not all cancer patients have high levels of Tregs in their tumors. The study found that Treg accumulation is particularly common in certain types of cancer, including melanoma, lung cancer, and ovarian cancer—all of which have shown variable responses to immunotherapy. For example:

• Melanoma: While some patients with advanced melanoma respond dramatically to checkpoint inhibitors, others develop resistance within months. Tregs may play a role in this resistance, according to data from the CheckMate trials.
• Lung cancer: Non-small cell lung cancer (NSCLC) patients with high Treg levels in their tumors have been shown to have poorer outcomes with PD-1 inhibitors, as reported in a 2022 study in JAMA Oncology.
• Ovarian cancer: Immunotherapy has had limited success in ovarian cancer, partly due to the immunosuppressive tumor microenvironment, where Tregs are often abundant.

Dr. Eggert emphasized that “personalized approaches will be key. Testing for Treg levels in tumors before starting immunotherapy could help identify patients who might need additional treatments to target these cells.”

What Happens Next in Research and Clinical Trials?

The Karolinska study has already sparked interest in the scientific community, with several clinical trials underway to explore Treg-targeting strategies:

• A phase II trial at Memorial Sloan Kettering Cancer Center is testing the combination of a Treg-depleting antibody with nivolumab in melanoma patients (NCT03440437).
• The University of Pennsylvania is investigating a Treg-converting therapy in patients with solid tumors (NCT04213415).
• Merck & Co. is developing a new class of drugs designed to selectively eliminate Tregs while sparing other immune cells, with early-phase trials expected to begin in 2025.

Meanwhile, researchers at Karolinska Institutet are planning follow-up studies to further elucidate the molecular mechanisms by which Tregs suppress immunotherapy responses. “We need to understand the exact signals these cells use to inhibit T cells,” said Lindström. “Only then can we design precise interventions.”

Key Takeaways: What This Means for Patients and Doctors

• Immunotherapy resistance: Tregs may explain why some patients do not respond to checkpoint inhibitors, even when their tumors express high levels of PD-L1.
• Potential new treatments: Combining Treg-targeting therapies with existing immunotherapies could improve outcomes for non-responders.
• Personalized medicine: Future cancer treatment plans may include testing for Treg levels to tailor therapies.
• Clinical trials: Patients with advanced cancers should ask their oncologists about ongoing trials combining immunotherapies with Treg-targeting agents.
• Side effects: Depleting Tregs could increase the risk of autoimmune reactions, requiring careful monitoring.

Where to Find Updates and Resources

For patients and healthcare providers seeking the latest information on Treg-targeting therapies and immunotherapy resistance, the following resources are available:

• National Cancer Institute: Regulatory T Cells
• ClinicalTrials.gov: Search for Treg-targeting studies
• American Society of Clinical Oncology (ASCO) Guidelines
• European Society for Medical Oncology (ESMO) Updates

The next major checkpoint in this research will be the publication of Phase II trial results in 2025, which could provide clearer evidence on whether targeting Tregs improves immunotherapy outcomes. In the meantime, experts urge caution but optimism: “This is a step forward, not a dead end,” said Dr. Eggert. “We’re learning how to navigate the immune system’s complexity.”

For readers with questions about how this research might affect their own cancer treatment, we encourage you to share your experiences in the comments below or reach out to your healthcare provider for personalized advice.