The 2025 Nobel Prize in Physiology or Medicine has been awarded to Mary E. Brunkow, Fred Ramsdell and Shimon Sakaguchi “for their discoveries concerning peripheral immune tolerance”. Their work identified regulatory T cells, the cells that keep the immune system from attacking the body’s own healthy tissue, and the gene that controls them. The Nobel Assembly at Karolinska Institutet announced the prize today.
What happened
The three laureates did not work as a single team, and their discoveries arrived in steps over about eight years. According to Karolinska Institutet, the story runs like this.
- In 1995, Shimon Sakaguchi identified a previously unknown class of immune cells: regulatory T cells.
- In 2001, Mary Brunkow and Fred Ramsdell found that a mutation in a gene called Foxp3 explained why a particular strain of mice was vulnerable to autoimmune disease. They also linked the same gene to a human disease, IPEX.
- In 2003, Sakaguchi connected the two lines of work, showing that Foxp3 governs the development of the regulatory T cells he had described eight years earlier.
Today Brunkow is at the Institute for Systems Biology in Seattle, Ramsdell is at Sonoma Biotherapeutics in San Francisco, and Sakaguchi is at Osaka University.
How it works
Your immune system has a hard job. It has to find and destroy invaders it has never met before, while leaving alone the cells that make up your own body. When that balance fails and the immune system turns on healthy tissue, the result is autoimmune disease.
It is easy to picture immunity only as attack: cells that recognise a threat and destroy it. The discoveries honoured today are about the brakes. Regulatory T cells act a bit like security guards inside a building full of armed staff. The staff are there to deal with intruders, but now and then one of them misidentifies a colleague as a threat. The guards step in and calm things down before real damage is done. The Nobel committee’s phrase “peripheral immune tolerance” describes that restraint, which keeps working throughout the body.
Foxp3 is the gene that makes these guards what they are. The mouse strain in Brunkow and Ramsdell’s study carried a faulty version, and the animals developed autoimmune problems. Finding the same gene behind a human disease showed that this was not a quirk of laboratory mice. Sakaguchi’s 2003 result then closed the loop: Foxp3 is what directs the development of regulatory T cells, so a broken Foxp3 means the guards are missing or defective.
By the numbers
| Item | Figure | Source |
|---|---|---|
| Laureates | 3 (Brunkow, Ramsdell, Sakaguchi) | Karolinska Institutet, 6 Oct 2025 |
| Discovery of regulatory T cells | 1995, Sakaguchi | Karolinska Institutet, 6 Oct 2025 |
| Foxp3 mutation linked to autoimmunity in mice and to IPEX | 2001, Brunkow and Ramsdell | Karolinska Institutet, 6 Oct 2025 |
| Foxp3 shown to govern regulatory T cell development | 2003, Sakaguchi | Karolinska Institutet, 6 Oct 2025 |
| Countries of the laureates’ institutions | 2 (United States and Japan) | Karolinska Institutet, 6 Oct 2025 |
Why it matters
What I like about this prize is that it rewards an idea that sounds almost obvious once you hear it: a defence system needs its own internal checks. Yet it took careful work across two continents to prove those checks exist, find the cells responsible and pin down the gene behind them.
The practical side is where the story is still being written. Karolinska Institutet says the discoveries have spurred treatments for cancer and autoimmune diseases that are now in clinical trials, and that they may improve outcomes in transplantation. The logic is easy to follow. In autoimmune disease you would want more of this restraint. In cancer, my reading is that you might sometimes want less of it, so the attackers can do their job. In transplants, calming the immune response could help the body accept a new organ.
I want to be precise here, because medical news gets inflated fast. These therapies are in clinical trials. That means they are still being tested, and the prize announcement does not say any of them is approved or able to cure a disease. The Nobel recognises the basic science, and that science is solid. Whether it becomes routine treatment is a question the trials will answer.
What comes next
Nobel week continues with the other science prizes. For medicine, though, the schedule that matters most is the one set by the clinical trials Karolinska Institutet mentions, where treatments built on regulatory T cells and Foxp3 are being tested in cancer and autoimmune disease.
Sources
- Karolinska Institutet news, “The 2025 Nobel Prize in Physiology or Medicine to Mary E. Brunkow, Fred Ramsdell and Shimon Sakaguchi”, 6 October 2025, https://news.ki.se/the-2025-nobel-prize-in-physiology-or-medicine-to-mary-e-brunkow-fred-ramsdell-and-shimon-sakaguchi
- The Nobel Assembly at Karolinska Institutet via nobelprize.org, press release on the 2025 Nobel Prize in Physiology or Medicine, 6 October 2025, https://www.nobelprize.org/prizes/medicine/2025/press-release/
- The Nobel Assembly at Karolinska Institutet, press release PDF, 6 October 2025, https://www.nobelprize.org/uploads/2025/10/press-medicineprize2025.pdf