Reframing Thymic Biology: From AIRE to Mimetics, Thymflammation, Regeneration, and Beyond

The thymus represents a unique biological factory, crafting T cells that are both self-tolerant and capable of defending against pathogens. In this issue of Immunological Reviews, 11 leading experts provide an up-to-date synthesis of the molecular and cellular networks that orchestrate T cell development and selection, induce central tolerance, sustain thymic homeostasis and enable regeneration across the lifespan. For the past two decades, our understanding of central tolerance has been largely shaped by studies focused heavily on Aire+ medullary thymic epithelial cells (mTECs). These specialized stromal cells express nearly 90% of the coding genome, allowing presentation of an exceptionally broad repertoire of tissue-specific antigens (TSAs) [1-4]. This paradigm established promiscuous gene expression (PGE) as the dominant mechanism by which self-reactive thymocytes are either purged by clonal deletion [4] or diverted into the Foxp3+ T regulatory (Treg) lineage in the thymus [5] (Figure 1a). Yet, several recent advances have decisively moved the field beyond an Aire-centric framework of thymic tolerance. Specifically, the recent identification of thymic mimetic cells has revealed an additional (and largely Aire-independent) layer of self-representation within the thymus [6-8]. Notably, many of these mimetic populations arise developmentally from Aire+ mTECs that, under the control of tissue-specifying transcription factors, undergo programmed differentiation into specialized epithelial identities. These subsets further expand the self-antigen landscape beyond classical PGE by emulating large and complex tissue identities. Most prominently, they resemble cell types that form mucosal barriers (e.g., tuft-like, microfold-like, enterocyte-like, ionocyte-like, and cilia-like TECs), as well as cells of the skin (corneocyte-like TECs), muscle (myocyte-like TECs), and (neuro)endocrine tissues (endocrine-like TECs) (Figure 1a). Functionally, TEC mimetics are not only critical for induction of self-tolerance to these tissues, but were also shown to execute distinct functional programs that shape thymic homeostasis and cellular dynamics [6, 9, 10]. In parallel, accumulating evidence has uncovered a striking and unexpected feature of thymic physiology—the presence of constitutively active inflammatory circuits, most prominently tonic type-I interferon signaling [11-13]. This phenomenon, now conceptualized as “thymflammation” challenges the traditional view of the thymus as an immunologically quiescent organ [14]. Although its full physiological significance is still unfolding, it is increasingly clear that this intrinsic inflammatory tone is instrumental for establishing tolerance to inflammation-associated self-antigens (ISAs) (Figure 1b) and for coordinating intercellular crosstalk within the thymic niche. Finally, recent studies have firmly established that central tolerance is not the exclusive domain of mTECs. Instead, diverse stromal and hematopoietic populations, including dendritic cells and other specialized antigen-presenting subsets, cooperate to diversify antigen presentation, refine selection thresholds, and integrate contextual signals (Figure 1c). The volume opens with a highly engaging perspective by Gray and colleagues [15], who provide a clear and comprehensive narrative of the conceptual and technological milestones that have shaped modern thymus biology. They revisit the central role of thymic epithelial cells in guiding hematopoietic progenitors through lineage commitment and enforcing stringent tolerance checkpoints, emphasizing how these functions rely on profound TEC specialization and the establishment of distinct thymic microenvironments. Importantly, the authors chart how these microenvironments are dynamically remodeled across the lifespan, from development, through injury-induced regeneration, to age-associated involution, highlighting the recent discovery of a distinct population of age-associated TECs (aaTECs) that may fundamentally reshape thymic architecture and function in later life [16]. Furthermore, by emphasizing how advances such as multiplexed imaging and single-cell technologies have uncovered an unexpectedly diverse cellular landscape, this perspective offers a comprehensive and highly accessible overview that provides an essential conceptual framework for the entire issue. The issue then continues with a perspective by Alves and colleagues [17], who shift the focus to cortical thymic epithelial cells (cTECs) as the principal orchestrators of the earliest steps in thymocyte development and selection. Central to this process is the specialized antigen-processing machinery of cTECs, including the thymoproteasome and the generation of a unique self-peptidome, which defines the molecular “code” of positive selection and shapes the breadth and diversity of the TCR repertoire. This review provides the most recent insights into how cortical microenvironments imprint long-term functional properties on peripheral CD4+ and CD8+ T cells. The volume then continues with a review by Cowan and colleagues, who provide a clear and insightful account of the fates of positively selected thymocytes as they migrate from the cortex to the medulla. They detail how the medulla shapes the development of both conventional and non-conventional T-cell lineages, including regulatory T (Treg), invariant natural killer T (iNKT) cells, and others. The review also draws attention to the molecular and functional distinctions between newly generated thymic Tregs and mature Tregs that recirculate back to the thymus, including differences in the chemokine cues that govern their recruitment to the medulla. Finally, the authors discuss how the properties of the medulla evolve across the lifespan. This focus on the medulla as a dynamic instructional niche also provides a natural bridge to the next review by Peterson [18], who revisits the medulla as the central site of tolerance induction. Specifically, Peterson explores central tolerance through the lens of promiscuous gene expression and AIRE biology, integrating advances in our understanding of AIRE's transcriptional regulation, chromatin interactions, and nuclear organization with emerging evidence that it also orchestrates interferon-dependent programs within the thymus. The presence of neutralizing type I interferon autoantibodies in patients with APECED provides compelling evidence that tolerance to these cytokines is clinically consequential. Peterson further proposes that tonic type I interferon signaling is a conserved feature of the mammalian thymus, suggesting that inflammatory signals are an integral component of normal thymic physiology. In this framework, AIRE-driven interferon programs appear to influence not only thymocyte selection but also thymic homeostasis and thymocyte development. The theme of tonic interferon signaling sets the stage for two complementary reviews by Hogquist and Voboril, and by Kreslavsky. Hogquist and Voboril further dissect the role of type I interferons within the thymus, positioning them both as self-antigens and as active architects of the selection niche [19]. Specifically, through engagement of their receptors, type I interferons influence the differentiation and maturation of multiple antigen-presenting cells in the thymus, including mTECs, B cells, and macrophages. By shaping antigen presentation and the repertoire of self-peptide–MHC complexes available for selection, interferons become integral components of the molecular logic that sculpts the TCR repertoire. Kreslavsky further expands this framework beyond interferons, advancing the concept of “thymflammation” as a constitutive and highly organized feature of thymic biology [20]. Strikingly, even under homeostatic conditions, the thymus engages a broad spectrum of inflammatory modules typically associated with distinct classes of pathogens, such as type I/III interferons that are normally produced in the periphery in response to viral infection; type-2 cytokines characteristic of anti-parasitic immunity and/or tissue damage; and type-3 cytokines that are linked to antifungal defense (Figure 1b). Furthermore, the thymus is also characterized by active B cell-centered humoral circuits commonly mobilized during bacterial infection. The presence of such inflammatory circuitry in an organ largely shielded from pathogens presents a conceptual paradox. Kreslavsky proposes that it evolved to tolerize the immune system to host molecules that are barely detectable at baseline but surge (by order of magnitude) following diverse inflammatory triggers such as infection or tissue damage. Their constitutive thymic expression may therefore be critical to restrain cytokine-directed autoimmunity, as illustrated by the emergence of anti-interferon autoantibodies in AIRE-deficient patients or following severe viral infections such as COVID-19 [21]. By the same logic, the development of anti-IL-17 and anti-IL-22 autoantibodies in AIRE deficiency may reflect a dual insult: impaired central tolerance to these AIRE-dependent cytokines within the thymus, coupled with their marked peripheral overproduction during chronic Candida infection, which together precipitate and amplify the breach in tolerance to these inflammation-associated self-antigens. The theme of antigen-presenting cell specialization, introduced in the context of interferon-driven niche conditioning by Hogquist and Voboril, is further developed in a comprehensive review by Ehrlich and colleagues, who synthesize recent insights into the composition and specialization of these dendritic cell subsets, revealing a complex network of cDC1s, cDC2s, and plasmacytoid DCs that cooperate with epithelial cells to enforce tolerance [22]. The authors describe how cDC1s excel at cross-presenting mTEC-derived antigens (Figure 1c), while cDC2s import peripheral and blood-borne self-antigens, thereby extending the thymic antigenic repertoire beyond the epithelial transcriptome. They also highlight the molecular pathways that regulate dendritic cell activation within the thymus, including CD40–CD40L signaling and chemokine interactions such as XCL1–XCR1. Through these mechanisms, thymic DCs integrate positional cues and cellular cross-talk to ensure efficient antigen transfer and presentation. The resulting interplay between mTECs and DCs forms an antigen-presenting circuit that collectively achieves the near-complete representation of self, required for effective central tolerance. The issue then moves to one of the most striking recent advances in the field—the remarkable diversification of the mTEC lineage. In a fascinating perspective, Diane Mathis and colleagues illustrate this diversity through an in-depth exploration of thymic mimetics (Figure 1a), synthesizing evidence that these cells are not rare curiosities but rather a conserved and integral component of thymic architecture across vertebrate species [23]. While many mimetic subtypes are evolutionarily conserved, their frequency, degree of specialization, and repertoire composition vary, suggesting that species-specific immunological pressures have shaped their diversification. Furthermore, this perspective also highlights the potential clinical relevance of thymic mimetic cells for human health. For instance, it raises the possibility that alterations in specific mimetic subsets (e.g., CHRNA1-expressing neuroendocrine TECs) may contribute to diseases such as myasthenia gravis. Moreover, the remarkable plasticity of mTECs (exemplified by the diverse mimetic programs they adopt) positions them as intriguing candidates for regenerative medicine and cellular reprogramming. For example, mimetic programs resembling cochlear hair cells raise the possibility that TEC-derived cells could one day be exploited to restore hearing in individuals suffering from hair cell loss, a condition for which natural regeneration is currently absent. In addition, their role in shaping central tolerance positions them as potentially important players in combined organ–thymus transplantation and xenotransplantation strategies aimed at establishing durable donor-specific immune tolerance. Extending this regenerative perspective, Paola Bonfanti and her colleagues reframe thymus biology through the lens of stem cell and developmental principles, positioning the organ as a compelling model for epithelial regeneration [24]. Revisiting classical thymic reconstitution assays and the search for clonogenic progenitors, she examines the evidence for bona fide TEC stem cells and highlights the paradox of an involuting postnatal thymus that nonetheless retains cells capable of expansion and functional reconstitution. Their review dissects how epithelial potency shifts from embryonic development to postnatal homeostasis and regeneration post-injury, and considers the contributions of mesenchymal niches and the extracellular milieu to organ rebuilding. Together, these insights underscore the enormous translational potential of TEC stem cells for ex vivo thymus reconstruction, immune reconstitution, and next-generation immunotherapies. The issue then naturally transitions to the broader question of how the thymus repairs itself after injury. Dudakov and van den Brink provide an exhaustive and mechanistic synthesis of the cellular and molecular pathways that govern endogenous thymic regeneration [25]. They emphasize the paradoxical nature of the organ: exquisitely sensitive to insults such as stress, infection, and particularly ionizing radiation, yet endowed with a remarkable capacity for repair, albeit one that is often insufficient after profound damage leading to prolonged T cell lymphopenia. The review dissects distinct regenerative axes centered on defined stromal and immune circuits, including IL-22 produced by ILC-3 cells, BMP4 derived from endothelial cells, and type-2 cytokines supplied by TECs, fibroblasts, ILC-2, eosinophils, and regulatory T cells. Strikingly, many of these pathways converge on mechanisms that sense and interpret cell death, positioning damage detection as a unifying trigger of repair. By delineating both the promise and the intrinsic limitations of endogenous thymic regeneration, Dudakov and van den Brink outline what may ultimately be required to translate these pathways into effective thymus-boosting therapies. The appreciation of thymic complexity extends beyond epithelial and hematopoietic compartments to include non-epithelial stromal populations that provide critical architectural and signaling support. In their integrative review, Nitta and colleagues bring long-overdue attention to thymic fibroblasts, historically overlooked yet remarkably versatile cells that actively shape thymic function [26]. Distinct capsular and medullary fibroblast subsets produce chemokines, extracellular matrix components, and growth factors that guide thymocyte migration, support TEC maintenance, and contribute to thymic regeneration after injury. Beyond fibroblasts, the authors also provide insightful perspectives on broader mechanisms governing thymic regeneration and tolerance induction. Particularly timely is their discussion of the transcription factor Fezf2. Initially proposed to drive promiscuous gene expression in a manner analogous (though more modest) to AIRE [27], this view has been challenged previously [28] and accumulating evidence now points to a fundamentally different role. Rather than broadly promoting promiscuous gene expression, Fezf2 appears to regulate mTEC lineage diversification, controlling the differentiation of mimetic epithelial subsets, most notably thymic tuft-like cells [29]. This revised view refines our understanding of Fezf2 function, positioning it as a key regulator of mimetic epithelial differentiation. Collectively, the contributions in this volume integrate the latest molecular and cellular insights into thymus biology and redefine the organ as a highly dynamic, multicellular ecosystem. The thymus diversifies self-representation not only to tissue-specific antigens through AIRE-driven promiscuous gene expression and the acquisition of mimetic identities, but also to inflammation-associated self-antigens through thymflammation. At the same time, it is endowed with remarkable stem and progenitor potential, fueling its cellular diversity and regenerative capacity while opening new therapeutic avenues in regenerative medicine. Moving beyond an AIRE-centered view of central tolerance, these advances position the thymus at the intersection of immunology, developmental biology, and translational medicine. As emerging technologies continue to resolve its cellular heterogeneity and functional plasticity, thymus biology enters a new era, one in which understanding and harnessing this complexity may reshape strategies for treating autoimmunity, immune deficiency, transplantation, and immune reconstitution. The author declares no conflicts of interest. Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.