The Body's Peacemakers

Introduction

The immune system is a remarkably complex and sophisticated defence network that protects the body against a constant barrage of pathogens, such as bacteria, viruses, and fungi. A key feature of this system is its ability to distinguish between 'self'—the body's own cells and tissues—and 'non-self'—foreign invaders. This capacity for self-recognition is crucial for maintaining health. When this delicate balance is disrupted, and the immune system mistakenly attacks the body's own components, it can lead to a host of debilitating and often chronic autoimmune diseases. For decades, immunologists have sought to understand the mechanisms that prevent such self-destructive immune responses. The 2025 Nobel Prize in Physiology or Medicine honours three scientists whose pioneering work has provided a profound insight into this fundamental question. Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi have been recognised for their discoveries concerning peripheral immune tolerance, a critical process that ensures the immune system maintains peace within the body. Their research has not only unravelled a key mystery of immunology but has also opened up exciting new possibilities for treating a wide array of human diseases.

The Historical Quest for Immunological Tolerance

The concept of immunological tolerance, the state of unresponsiveness of the immune system to substances or tissues that have the capacity to elicit an immune response, has a rich history. The intellectual seeds of this idea were sown long before the molecular and cellular mechanisms were understood. In the early 20th century, the prevailing view, encapsulated by Paul Ehrlich's term "horror autotoxicus," was that the body had an intrinsic inability to produce antibodies against its own tissues ^{25, 29}. This concept, while not entirely accurate in its mechanistic underpinnings, highlighted the fundamental question of how the immune system avoids self-destruction.

A pivotal moment in the history of immunology came in 1945 with the work of Ray D. Owen ^{3, 5, 11}. He observed that dizygotic (non-identical) twin cattle that had shared a placental circulation in utero were chimaeras, meaning they possessed a mixture of each other's red blood cells throughout their lives without any signs of immune rejection ¹¹. This finding suggested that exposure to foreign antigens during early development could lead to a state of specific tolerance.

This natural experiment of nature was followed by the groundbreaking experimental work of Sir Frank Macfarlane Burnet and Frank Fenner, who in 1949 proposed the concept of immunological tolerance as a learned process ¹¹. They hypothesised that the immune system learns to distinguish self from non-self during embryonic development. This theoretical framework was experimentally validated in 1953 by Rupert Billingham, Leslie Brent, and Sir Peter Medawar ^{3, 22}. They demonstrated that injecting cells from one strain of mouse into a neonatal mouse of another strain would induce a state of tolerance, allowing the recipient mouse to later accept skin grafts from the donor strain without rejection ^{3, 11}. This work, for which Burnet and Medawar were awarded the Nobel Prize in Physiology or Medicine in 1960, firmly established the concept of acquired immunological tolerance and laid the foundation for future research in this field.

These early discoveries led to the development of the clonal selection theory by Burnet, which posited that lymphocytes with receptors specific for self-antigens are eliminated or 'deleted' during their development ⁵. This process, now known as central tolerance, was thought to be the primary mechanism by which the immune system prevents autoimmunity.

Central Tolerance: The Thymic School of Immunity

The thymus, a small organ located behind the breastbone, plays a central role in the maturation of T cells, a type of white blood cell that is a key player in the adaptive immune response. It is within the thymus that T cells undergo a rigorous 'education' process to ensure they can effectively recognise and respond to foreign invaders while remaining tolerant to the body's own tissues ^{9, 12, 14, 18}. This process of central tolerance is a critical first line of defence against autoimmunity.

T cell progenitors, known as thymocytes, are generated in the bone marrow and migrate to the thymus to mature ¹². During their development, they undergo a process of genetic recombination that generates a vast and diverse repertoire of T cell receptors (TCRs), each capable of recognising a specific antigen. This random process, however, inevitably produces some T cells with TCRs that can bind to self-antigens.

The thymus employs a two-step selection process to weed out these potentially harmful self-reactive T cells. The first step is positive selection, which ensures that only T cells with TCRs that can recognise the body's own major histocompatibility complex (MHC) molecules are allowed to survive. MHC molecules are responsible for presenting antigens to T cells. T cells that cannot recognise MHC molecules are useless and are eliminated through a process of programmed cell death called apoptosis.

The second and more critical step for preventing autoimmunity is negative selection ^{9, 12, 14}. During this process, thymocytes that bind too strongly to self-antigens presented by MHC molecules in the thymus are also eliminated through apoptosis. This clonal deletion of self-reactive T cells is a crucial mechanism for establishing central tolerance.

A key player in this process is the autoimmune regulator (Aire) gene, which is expressed in the thymus and promotes the expression of a wide range of tissue-specific antigens that are normally found only in peripheral organs ²⁷. This allows the developing T cells to be 'tested' against a broad array of self-antigens, ensuring that T cells reactive to these antigens are deleted before they can enter the circulation. Mutations in the Aire gene lead to a rare but severe autoimmune disease called autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED), which is characterised by the presence of autoantibodies and lymphocytic infiltration in multiple organs.

While central tolerance is a highly effective mechanism, it is not foolproof. Some self-reactive T cells can escape thymic deletion and enter the peripheral circulation ¹³. This is partly because not all self-antigens are expressed in the thymus, even with the help of Aire. Therefore, there must be additional mechanisms in the periphery to control these potentially dangerous cells and prevent them from causing autoimmune disease. The discovery of these peripheral tolerance mechanisms is the monumental achievement for which Brunkow, Ramsdell, and Sakaguchi have been awarded the Nobel Prize.

Shimon Sakaguchi and the Discovery of Regulatory T Cells

In the late 1980s and early 1990s, the field of immunology was largely focused on the mechanisms of T cell activation and the role of different T cell subsets in orchestrating immune responses. The prevailing view was that immune tolerance was primarily established in the thymus through the deletion of self-reactive T cells. However, some researchers began to question whether this was the whole story.

Shimon Sakaguchi, born in 1951 in Japan, was one of these inquisitive minds ^{8, 10, 19, 23}. After obtaining his M.D. and Ph.D. from Kyoto University, he pursued postdoctoral studies at Johns Hopkins University and Stanford University in the United States before returning to Japan ^{8, 10, 15, 19}. His early work focused on understanding the mechanisms of autoimmunity.

Sakaguchi was intrigued by a long-standing observation in immunology: the removal of the thymus from mice shortly after birth (neonatal thymectomy) led to the development of severe autoimmune diseases affecting multiple organs ². This suggested that the thymus was not only a site for T cell maturation but also for the generation of a cell population that actively suppresses autoimmune responses.

In a series of elegant experiments in the mid-1990s, Sakaguchi set out to identify this elusive cell population. He hypothesised that if such a suppressive cell population existed, then transferring T cells from a healthy adult mouse to a neonatally thymectomised mouse should prevent the development of autoimmune disease. His experiments confirmed this hypothesis.

He then went a step further to characterise these suppressive T cells. He noticed that a small subset of CD4+ T cells, a type of helper T cell, expressed high levels of a molecule called CD25, which is a component of the receptor for interleukin-2 (IL-2), a key cytokine for T cell proliferation ⁴⁸. At the time, CD25 was primarily considered a marker of activated T cells. However, Sakaguchi found that this CD4+CD25+ T cell population was present in healthy mice and had potent suppressive activity.

In a landmark paper published in 1995, Sakaguchi demonstrated that the removal of this CD4+CD25+ T cell subset from a population of T cells before transferring them into a mouse lacking a thymus resulted in the development of autoimmune disease ⁴⁸. Conversely, the co-transfer of the CD4+CD25+ T cells with the remaining T cells prevented autoimmunity. This was the first definitive evidence for the existence of a specialised population of T cells with a dedicated suppressive function. Sakaguchi named these cells "regulatory T cells" or Tregs ^{1, 19}.

This discovery was met with some scepticism from the scientific community, as the concept of "suppressor T cells" had fallen out of favour in the 1980s due to a lack of well-defined markers and reproducible experimental evidence. However, Sakaguchi's meticulous work and the clear identification of the CD4+CD25+ phenotype provided a solid foundation for the re-emergence of this crucial concept in immunology.

Mary E. Brunkow, Fred Ramsdell, and the Scurfy Mouse

While Sakaguchi was unravelling the existence of regulatory T cells in Japan, a parallel line of investigation was being pursued in the United States by Mary E. Brunkow and Fred Ramsdell. Their research, which focused on a rare and fatal genetic disorder in mice, would provide the crucial missing piece of the puzzle.

Mary E. Brunkow, born in 1961, received her PhD in molecular biology from Princeton University ¹⁹. Fred Ramsdell, born in 1960, earned his PhD in immunology from the University of California, Los Angeles ^{6, 34}. In the late 1990s, they were both working at the biotechnology company Celltech Chiroscience in Bothell, Washington ³¹.

Their research centred on a mutant mouse strain known as "scurfy" ⁴¹. These mice suffer from a severe and rapidly fatal autoimmune disease characterised by an overactive immune system, enlarged lymph nodes and spleen, and inflammatory infiltrates in various organs, particularly the skin. The scurfy phenotype was known to be caused by a single gene mutation on the X chromosome, but the identity of this gene remained a mystery.

Brunkow and Ramsdell reasoned that identifying the gene responsible for the scurfy phenotype could provide valuable insights into the mechanisms that control immune responses and prevent autoimmunity. Using the tools of molecular genetics, they embarked on a painstaking effort to clone the scurfy gene.

In 2001, their hard work paid off. They successfully identified the gene and named its protein product "scurfin" ¹⁷. They found that the scurfy mice had a mutation in this gene that rendered the protein non-functional. This discovery was a major breakthrough, as it provided a direct genetic link to a severe autoimmune disease.

Crucially, Brunkow and Ramsdell also found that the human equivalent of the scurfy gene was mutated in patients with a rare and devastating autoimmune disease called IPEX syndrome (Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked) ^{6, 17, 31}. This finding underscored the importance of this gene in controlling immune responses in both mice and humans.

The gene they identified was later found to be a member of the forkhead box family of transcription factors and was renamed Foxp3 ^{4, 17, 41, 43, 44, 47}. Transcription factors are proteins that bind to DNA and control the expression of other genes. The discovery of Foxp3 as the causative gene for the scurfy and IPEX syndromes was a pivotal moment in immunology, but its connection to Sakaguchi's regulatory T cells was yet to be made.

The Synthesis: Foxp3 as the Master Regulator of Tregs

The discoveries of Sakaguchi and the team of Brunkow and Ramsdell were like two pieces of a complex puzzle that were waiting to be joined. The final, crucial connection was made by Sakaguchi himself, in a brilliant synthesis of the two lines of research.

Following the identification of Foxp3 as the gene mutated in scurfy mice, Sakaguchi and his team investigated whether this gene was expressed in the regulatory T cells he had discovered. In a landmark study published in 2003, they demonstrated that Foxp3 is specifically and highly expressed in CD4+CD25+ regulatory T cells ²⁷. They also showed that the forced expression of Foxp3 in conventional T cells could convert them into regulatory T cells with suppressive activity ²⁹.

This was the "eureka" moment that tied everything together. It became clear that Foxp3 was the master regulator of regulatory T cell development and function ^{1, 13, 16, 17, 41, 43, 44, 47}. The severe autoimmune disease seen in scurfy mice and IPEX patients was a direct consequence of the absence of functional regulatory T cells due to mutations in the Foxp3 gene.

This synthesis provided a unifying framework for understanding peripheral immune tolerance. It was now clear that the immune system has a dedicated lineage of professional suppressor cells, the regulatory T cells, and that their development and function are critically dependent on the transcription factor Foxp3. This discovery not only validated Sakaguchi's earlier work but also provided a molecular handle to study and manipulate these crucial cells.

The Mechanisms of Regulatory T Cell Suppression

The discovery of regulatory T cells and their master regulator, Foxp3, opened up a new and exciting field of research aimed at understanding how these cells exert their suppressive effects. It is now known that Tregs employ a variety of mechanisms to control immune responses and maintain tolerance ^{1, 7, 16, 21, 25, 26}. These mechanisms can be broadly divided into those that are dependent on cell-to-cell contact and those that involve the secretion of inhibitory molecules.

One of the key contact-dependent mechanisms involves the molecule CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), which is highly expressed on the surface of Tregs. CTLA-4 can bind to the B7 molecules (CD80 and CD86) on antigen-presenting cells (APCs), such as dendritic cells, and prevent them from activating other T cells. This effectively puts the brakes on the immune response at its very initiation.

Tregs can also suppress immune responses by producing inhibitory cytokines, such as interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β) ^{16, 21}. IL-10 is a potent anti-inflammatory cytokine that can inhibit the production of pro-inflammatory cytokines by other immune cells. TGF-β has a wide range of effects on the immune system, including inhibiting the proliferation and activation of T cells and promoting the differentiation of other regulatory T cell subsets.

Another important mechanism of Treg suppression is the consumption of interleukin-2 (IL-2) ²⁹. Tregs express high levels of the high-affinity IL-2 receptor (CD25), which allows them to effectively compete with other T cells for this essential growth factor. By soaking up the available IL-2, Tregs can starve other T cells and prevent their proliferation and expansion.

Tregs can also induce the death of other immune cells through the production of granzymes and perforin, molecules that are also used by cytotoxic T cells to kill infected cells ¹⁶. This mechanism allows Tregs to directly eliminate overactive or self-reactive T cells.

Furthermore, Tregs can modulate the function of antigen-presenting cells, rendering them less effective at activating other T cells ²¹. They can also promote the generation of other types of regulatory cells, thereby amplifying their suppressive effects in a process known as infectious tolerance ⁴⁶.

The multiplicity of these suppressive mechanisms highlights the central role of Tregs in maintaining immune homeostasis and preventing autoimmunity. The specific mechanisms used by Tregs may vary depending on the context, such as the type of immune response and the tissue environment.

The Laureates: A Closer Look at the Pioneers of Peripheral Tolerance

The 2025 Nobel Prize in Physiology or Medicine is a testament to the dedication, perseverance, and intellectual brilliance of three remarkable scientists.

Shimon Sakaguchi, currently a Distinguished Professor at the Immunology Frontier Research Center at Osaka University, Japan, is widely regarded as the father of the field of regulatory T cells ^{8, 10, 19, 23, 34}. His pioneering work in the 1990s laid the foundation for our current understanding of peripheral immune tolerance. His numerous accolades, including the William B. Coley Award, the Gairdner Foundation International Award, and the Crafoord Prize, reflect the profound impact of his research on the field of immunology ^{8, 23}.

Mary E. Brunkow is currently a senior programme manager at the Institute for Systems Biology in Seattle, Washington ¹⁹. Her expertise in molecular biology was instrumental in the successful cloning of the Foxp3 gene. Her work has not only contributed to our understanding of immune regulation but has also had a significant impact on the field of human genetics.

Fred Ramsdell is currently a scientific advisor at Sonoma Biotherapeutics in San Francisco and the Parker Institute for Cancer Immunotherapy ^{6, 31, 32, 34}. With a long and distinguished career in both academia and the biotechnology industry, he has been at the forefront of translational immunology research. His work on Foxp3 and regulatory T cells has been instrumental in the development of new therapeutic strategies for autoimmune diseases and cancer. He is also a recipient of the Crafoord Prize, which he shared with Sakaguchi and Alexander Rudensky in 2017 ^{6, 31, 32}.

The collaborative and complementary nature of the laureates' work is a shining example of how scientific progress is often achieved through the convergence of different lines of inquiry. Sakaguchi's functional studies on regulatory T cells, combined with Brunkow and Ramsdell's genetic and molecular discoveries, provided a complete and compelling picture of a fundamental mechanism of immune regulation.

Therapeutic Implications: Harnessing the Power of Regulatory T Cells

The discovery of regulatory T cells and the Foxp3 gene has not only revolutionised our understanding of immunology but has also opened up a new frontier in the treatment of a wide range of human diseases. The ability to manipulate the number and function of Tregs holds immense therapeutic potential for conditions characterised by either excessive or insufficient immune responses.

In the context of autoimmune diseases, where the immune system mistakenly attacks the body's own tissues, the goal is to boost the number or function of Tregs to restore immune tolerance. Several therapeutic strategies are currently being explored to achieve this. One approach is the adoptive transfer of Tregs, which involves isolating a patient's own Tregs, expanding them in the laboratory, and then re-infusing them back into the patient ^{2, 4, 24, 36, 38}. This approach has shown promise in early clinical trials for a number of autoimmune diseases, including type 1 diabetes, lupus, and multiple sclerosis ^{24, 28, 39}.

Another strategy is to use drugs that can selectively promote the expansion or function of Tregs in the body. Low-dose interleukin-2 (IL-2) therapy is one such approach ^{29, 33}. As Tregs express high levels of the IL-2 receptor, they are more sensitive to low doses of IL-2 than other T cells. This allows for the selective expansion of Tregs without activating other potentially harmful immune cells.

In the field of organ transplantation, the induction of tolerance to the transplanted organ is a major goal to avoid the need for lifelong immunosuppressive drugs, which can have serious side effects ³⁶. Treg-based therapies are being investigated as a way to achieve this. The infusion of Tregs, either from the donor or the recipient, has shown promise in preclinical models and is now being tested in clinical trials for kidney and liver transplantation ^{2, 33, 39}.

Conversely, in the context of cancer, the goal is to reduce the number or function of Tregs to unleash the immune system's ability to attack and destroy tumour cells ^{1, 4}. Tumours often exploit Tregs to create an immunosuppressive microenvironment that protects them from immune surveillance. Therefore, therapies that can deplete or inhibit Tregs are being developed as a new form of cancer immunotherapy. Several clinical trials are currently underway to evaluate the efficacy of these approaches, often in combination with other cancer treatments such as checkpoint inhibitors.

Challenges and Future Directions

Despite the enormous therapeutic potential of regulatory T cells, there are still a number of challenges that need to be overcome before these therapies can become widely available.

One of the main challenges in adoptive Treg therapy is the difficulty in isolating and expanding a pure and stable population of Tregs ^{20, 26, 30, 37}. There is a risk that the expanded Tregs could lose their suppressive function or even convert into pro-inflammatory cells after being infused back into the patient. Researchers are working on developing better methods for Treg isolation and expansion, as well as strategies to ensure their stability and function in vivo.

Another challenge is to develop ways to target Tregs to specific tissues or organs where they are needed. This would help to maximise their therapeutic effects while minimising potential side effects. One promising approach is to genetically engineer Tregs to express receptors that can recognise specific antigens in the target tissue.

In the context of cancer therapy, the challenge is to selectively deplete or inhibit Tregs in the tumour microenvironment without affecting their beneficial role in preventing autoimmunity in other parts of the body. Researchers are exploring various strategies to achieve this, such as developing antibodies that can specifically target Tregs in the tumour or using drugs that can block the function of Tregs in a localised manner.

The future of research in peripheral immune tolerance is bright. Scientists are continuing to unravel the complex molecular and cellular mechanisms that govern the function of regulatory T cells. This knowledge will be crucial for developing more effective and safer Treg-based therapies. The ongoing clinical trials will provide valuable information on the efficacy and safety of these therapies in different disease settings.

The work of Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi has fundamentally changed our view of the immune system. They have shown that the immune system is not just a collection of killer cells that are programmed to destroy foreign invaders, but also a highly regulated network of cells that work together to maintain a delicate balance between immunity and tolerance. Their discoveries have not only solved a long-standing mystery in immunology but have also provided us with new tools to combat some of the most challenging diseases of our time. The legacy of their work will continue to inspire and guide researchers for many years to come, as we strive to harness the power of the body's own peacemakers to promote health and well-being.

Conclusion

The 2025 Nobel Prize in Physiology or Medicine rightfully celebrates a trio of scientists whose discoveries have profoundly reshaped our understanding of the immune system. The elucidation of the role of regulatory T cells and the master transcription factor Foxp3 in peripheral immune tolerance by Mary E. Brunkow, Fred Ramsdell, and Shimon Sakaguchi represents a paradigm shift in immunology. Their work has moved the field beyond a primary focus on the destructive capacity of the immune system to a more nuanced appreciation of the intricate mechanisms that actively maintain self-tolerance and prevent autoimmunity.

The journey from the early observations of immunological tolerance in cattle twins to the molecular dissection of regulatory T cell function is a testament to the power of scientific inquiry. The laureates' discoveries have not only provided a satisfying explanation for a fundamental biological question but have also opened up a wealth of new opportunities for therapeutic intervention. The ongoing development of Treg-based therapies for autoimmune diseases, organ transplantation, and cancer holds the promise of a new era of precision medicine, where the immune system can be fine-tuned to restore health and combat disease.

The work of Brunkow, Ramsdell, and Sakaguchi is a powerful reminder that some of the most important discoveries in science are those that reveal the elegant and often surprising ways in which nature maintains balance and order. Their legacy will undoubtedly be measured not only by the depth of our new understanding of the immune system but also by the number of lives that will be improved and saved by the therapies that are born from their groundbreaking research.