The Nobel Prize in Physiology or Medicine for 2025 was awarded on October 6, 2025, by the Nobel Assembly at Karolinska Institutet to three scientists: Mary E. Brunkow (USA, Institute for Systems Biology), Fred Ramsdell (USA, Sonoma Biotherapeutics), and Shimon Sakaguchi (Japan, Osaka University). They shared the prize “for their discoveries concerning peripheral immune tolerance,” specifically for identifying the role of regulatory T cells (Tregs) in preventing the immune system from attacking the body’s own tissues. This work has profound implications for understanding and treating autoimmune diseases like type 1 diabetes, multiple sclerosis, and rheumatoid arthritis, as well as improving cancer immunotherapies and organ transplants.

Background: The Immune System’s Balancing Act

The human immune system is a double-edged sword—essential for fighting infections but potentially destructive if it turns against our own cells, leading to autoimmune disorders. Early research focused on “central immune tolerance,” where immature immune cells (like T cells) that recognize the body’s own proteins are eliminated in the thymus gland during development. However, this process isn’t foolproof; some self-reactive T cells escape into the bloodstream. The laureates’ breakthroughs revealed a second layer of protection: peripheral immune tolerance, enforced by specialized “security guards” called regulatory T cells. These cells actively suppress harmful immune responses in tissues throughout the body, maintaining balance without over-suppressing defenses against real threats like viruses or tumors.

Key Contributions of the Laureates

Their discoveries, spanning the 1980s to early 2000s, built on each other to define Tregs and the gene that controls them. Here’s a breakdown:

• Shimon Sakaguchi’s Pioneering Identification of Tregs (1980s–1990s):
  Working at Kyoto University, Sakaguchi observed that not all T cells promote inflammation; some seemed to dampen it. In a landmark 1995 experiment, he isolated a subset of T cells expressing both CD4 and CD25 markers (CD4+CD25+ T cells) from mice. When transferred to immune-deficient “nude” mice (which lack a functional thymus and suffer severe autoimmunity), these cells prevented fatal autoimmune attacks on skin, lungs, and other organs. This proved they were regulatory suppressors. Sakaguchi named them “regulatory T cells” (Tregs), challenging the prevailing view that all T cells were pro-inflammatory. His work laid the foundation, though initially met with skepticism from the scientific community.
• Mary E. Brunkow and Fred Ramsdell’s Genetic Breakthrough (1990s–2000s):
  At the University of Washington, Brunkow and Ramsdell studied a rare genetic disorder called IPEX (Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked) in humans, and its mouse equivalent “scurfy” syndrome. Affected males develop severe, multi-organ autoimmunity and die young due to uncontrolled T cell attacks. By 2001, they pinpointed the culprit: mutations in the FOXP3 gene on the X chromosome, which encodes a transcription factor essential for Treg development and function. In scurfy mice, FOXP3 is defective, leading to absent or dysfunctional Tregs and rampant autoimmunity. Their findings confirmed Tregs as a distinct lineage and linked FOXP3 as their “master regulator.” This genetic insight explained why IPEX is X-linked (affecting mostly boys) and opened doors to gene therapies.

Why This Matters: Impact on Medicine

• Autoimmune Diseases: Tregs fail in conditions like lupus or Crohn’s disease. Therapies boosting Tregs (e.g., low-dose IL-2 drugs) are in clinical trials, inspired by this work.
• Cancer and Transplants: Enhancing Tregs can prevent graft rejection or temper overzealous immune attacks on tumors in immunotherapy (e.g., CAR-T cells).
• Broader Legacy: Their research has influenced over 50,000 scientific papers, accelerating Treg-based treatments. Notably, Brunkow is the 14th woman to win this prize, highlighting progress in gender diversity.

The prize includes a medal, diploma, and about 11 million Swedish kronor (roughly $1 million USD), shared equally. For more details, see the official Nobel summary. 1 This year’s award underscores immunology’s shift toward precision regulation of immunity, potentially transforming lives for millions with immune disorders.