Regulatory T cells and Beyond: Shimon Sakaguchi’s 2025 Nobel Prize in Physiology or Medicine

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The 2025 Nobel Prize in Physiology or Medicine, awarded to Shimon Sakaguchi, Mary Brunkow, and Fred Ramsdell, marks a defining milestone in modern immunology and the culmination of decades of inquiry into the mechanisms of immune tolerance. Sakaguchi has led a distinguished career in immunology, and his unwavering dedication to investigating the long-debated hypothesis of “suppressor T cells” culminated in the discovery of regulatory T (Treg) cells, which are specialised lymphocytes that maintain peripheral immune tolerance. Building on the early framework of central tolerance, his landmark 1995 study identified the CD4+CD25+ Treg cell subset as critical in preventing autoimmune pathology. Complementary work by Brunkow and Ramsdell subsequently identified the FOXP3 gene, which Sakaguchi later demonstrated encodes the master transcription factor defining Treg lineage specification, thereby revealing the genetic architecture of immune restraint. The delineation of Treg biology has transformed both fundamental immunology and the therapeutic landscape, informing emerging strategies in autoimmunity, organ transplantation, and oncology. Ongoing clinical trials, including those exploring low-dose interleukin-2 (IL-2LD) therapy and chimeric antigen receptor T regulatory (CAR-Treg) cell engineering, highlight the promise of harnessing immune restraint while underscoring the complexity of safely translating these discoveries into clinical benefit. Persistent challenges remain in ensuring Treg cell lineage stability, optimising therapeutic windows, and elucidating the intricate molecular pathways governing suppression. Sakaguchi’s determination to pursue an idea once dismissed as artefactual exemplifies scientific rigour and intellectual courage. His discoveries have redefined the conceptual and therapeutic landscape of immunology, established Treg cells as a cornerstone of immune homeostasis, and opened new avenues for precision immunotherapy across a spectrum of diseases.