Despite the importance of thymic stromal cells to T-cell development, relatively little is known about their biology. Here, we use single-cell analysis of stromal cells to analyze extensive changes in the number and composition of thymic stroma throughout life, revealing a surprisingly dynamic population. Phenotypic progression of thymic epithelial subsets was assessed at high resolution in young mice to provide a developmental framework. The cellular and molecular requirements of adult epithelium were studied, using various mutant mice to demonstrate new cross talk checkpoints dependent on RelB in the cortex and CD40 in the medulla. With the use of Ki67 and BrdU labeling, the turnover of thymic epithelium was found to be rapid, but then diminished on thymic involution. The various defects in stromal turnover and composition that accompanied involution were rapidly reversed following sex steroid ablation. Unexpectedly, mature cortical and medullary epithelium showed a potent capacity to stimulate naive T cells, comparable to that of thymic dendritic cells. Overall, these studies show that the thymic stroma is a surprisingly dynamic population and may have a more direct role in negative selection than previously thought. IntroductionT-cell development in the thymus is essential for the establishment and maintenance of the adaptive immune system. Thymic stromal cells mediate various phases of thymocyte development to produce mature T cells capable of responding to foreign antigen while remaining tolerant of self. It is well established that different subsets of stromal cells form microenvironments in the thymic subcapsule, cortex, and medulla to facilitate distinct thymocyte maturational steps. 1 The maintenance of thymic microenvironments requires reciprocal interactions between thymocytes and stromal cells, termed thymic cross talk. [2][3][4] Although much is known about the biology of thymocytes, our understanding of thymic stromal cells is lacking.The heterogeneity of murine thymic stromal cells has been demonstrated using monoclonal antibodies specific for various stromal determinants (eg, Godfrey et al 5 ). These have proven useful in defining the epithelium, endothelium, fibroblasts, dendritic cells (DCs), and macrophages within thymic microenvironments. The cortex is characterized by a meshwork of reticular, cortical thymic epithelial cells (cTECs) that can be identified by distinctive patterns of intracellular (eg, MTS-44, K8) and surface (eg, Ly51) antigen expression. [5][6][7] Early T-cell development involves the migration of immature double-negative (DN, CD4 Ϫ CD8 Ϫ ) thymocytes through the thymic cortex toward the subcapsule in close contact with cTECs. 8 At the subcapsule, lining fibroblasts facilitate DN thymocyte development through the provision of extracellular matrix components. 9 Maturing thymocytes then return through the cortex where cTECs mediate the process of positive selection, rescuing T-cell receptor positive (TCR ϩ ) double-positive (DP; CD4 ϩ CD8 ϩ ) thymocytes capable of interactin...

Lymph node stromal cells (LNSCs) can induce potent, antigen-specific T cell tolerance under steady-state conditions. Although expression of various peripheral tissue–restricted antigens (PTAs) and presentation to naive CD8+ T cells has been demonstrated, the stromal subsets responsible have not been identified. We report that fibroblastic reticular cells (FRCs), which reside in the T cell zone of the LN, ectopically express and directly present a model PTA to naive T cells, inducing their proliferation. However, we found that no single LNSC subset was responsible for PTA expression; rather, each subset had its own characteristic antigen display. Studies to date have concentrated on PTA presentation under steady-state conditions; however, because LNs are frequently inflammatory sites, we assessed whether inflammation altered stromal cell–T cell interactions. Strikingly, FRCs showed reduced stimulation of T cells after Toll-like receptor 3 ligation. We also characterize an LNSC subset expressing the highest levels of autoimmune regulator, which responds potently to bystander inflammation by up-regulating PTA expression. Collectively, these data show that diverse stromal cell types have evolved to constitutively express PTAs, and that exposure to viral products alters the interaction between T cells and LNSCs.

Upon TCR-mediated positive selection, developing thymocytes relocate within the thymus from the cortex to the medulla for further differentiation and selection. However, it is unknown how this cortex–medulla migration of thymocytes is controlled and how it controls T cell development. Here we show that in mice deficient for CCR7 or its ligands mature single-positive thymocytes are arrested in the cortex and do not accumulate in the medulla. These mutant mice are defective in forming the medullary region of the thymus. Thymic export of T cells in these mice is compromised during the neonatal period but not in adulthood. Thymocytes in these mice show no defects in maturation, survival, and negative selection to ubiquitous antigens. TCR engagement of immature cortical thymocytes elevates the cell surface expression of CCR7. These results indicate that CCR7 signals are essential for the migration of positively selected thymocytes from the cortex to the medulla. CCR7-dependent cortex–medulla migration of thymocytes plays a crucial role in medulla formation and neonatal T cell export but is not essential for maturation, survival, negative selection, and adult export of thymocytes.

The thymus undergoes age-related atrophy, coincident with increased circulating sex steroids from puberty. The impact of thymic atrophy is most profound in clinical conditions that cause a severe loss in peripheral T cells with the ability to regenerate adequate numbers of naive CD4+ T cells indirectly correlating with patient age. The present study demonstrates that androgen ablation results in the complete regeneration of the aged male mouse thymus, restoration of peripheral T cell phenotype and function and enhanced thymus regeneration following bone marrow transplantation. Importantly, this technique is also applicable to humans, with analysis of elderly males undergoing sex steroid ablation therapy for prostatic carcinoma, demonstrating an increase in circulating T cell numbers, particularly naive (TREC+) T cells. Collectively these studies represent a fundamentally new approach to treating immunodeficiency states in humans.

Endogenous thymic regeneration is a crucial function that allows for renewal of immune competence after stress, infection or immunodepletion. The mechanisms governing this regeneration, however, remain poorly understood. Here we detail a framework of thymic regeneration centred on IL-22 and triggered by depletion of CD4+CD8+ double positive (DP) thymocytes. Intrathymic levels of IL-22 were increased following thymic insult, and thymic recovery was impaired in IL-22-deficient mice. IL-22, which signalled through thymic epithelial cells (TECs) and promoted their proliferation and survival, was upregulated by radio-resistant RORγ(t)+CCR6+NKp46− lymphoid tissue-inducer cells (LTi) after thymic injury in an IL-23 dependent manner. Importantly, administration of IL-22 enhanced thymic recovery following total body irradiation (TBI). These studies reveal mechanisms of endogenous thymic repair and offer innovative regenerative strategies for improving immune competence.

Chronic thymus involution associated with aging results in less efficient T-cell development and decreased emigration of naïve T cells to the periphery. Thymic decline in the aged is linked to increased morbidity and mortality in a wide range of clinical settings. Negative consequences of these effects on global health make it of paramount importance to understand the mechanisms driving thymic involution and homeostatic processes across the lifespan. There is growing evidence that thymus tissue is plastic and that the involution process might be therapeutically halted or reversed. We present here progress on the exploitation of thymosuppressive and thymostimulatory pathways using factors such as keratinocyte growth factor, interleukin 7 or sex steroid ablation for therapeutic thymus restoration and peripheral immune reconstitution in adults.

To evaluate the efficacy of autologous adipose-derived mesenchymal stem cell (ADMSC) therapy on pain, function and disease modification in knee osteoarthritis. Methods: 30 participants with symptomatic knee osteoarthritis were randomized into three groups. Two treatment groups received intra-articular ADMSC therapy consisting of either a single injection (100 × 10 6 ADMSCs) or two injections (100 × 10 6 ADMSCs at baseline and 6 months). The third group served as control and continued conservative management. Results: No serious adverse events were observed. Both treatment groups receiving ADMSCs showed clinically significant pain and functional improvement at completion of follow-up at 12 months. Radiological analysis using the Magnetic Resonance Imaging Osteoarthritis Knee Score indicated modification of disease progression. Conclusion: Autologous ADMSC therapy appears to be a safe and effective therapy for knee osteoarthritis and may have the potential to prevent disease progression.Trial registration number: ACTRN12614000814673

Acute respiratory distress syndrome is characterized by loss of lung tissue as a result of inflammation and fibrosis. Augmenting tissue repair by the use of mesenchymal stem cells may be an important advance in treating this condition. We evaluated the role of term human umbilical cord cells derived from Wharton's jelly with a phenotype consistent with mesenchymal stem cells (uMSCs) in the treatment of a bleomycininduced mouse model of lung injury. uMSCs were administered systemically, and lungs were harvested at 7, 14, and 28 days post-bleomycin. Injected uMSCs were located in the lung 2 weeks later only in areas of inflammation and fibrosis but not in healthy lung tissue. The administration of uMSCs reduced inflammation and inhibited the expression of transforming growth factor-␤, interferon-␥, and the proinflammatory cytokines macrophage migratory inhibitory factor and tumor necrosis factor-␣. Collagen concentration in the lung was significantly reduced by uMSC treatment, which may have been a consequence of the simultaneous reduction in Smad2 phosphorylation (transforming growth factor-␤ activity). uMSCs also increased matrix metalloproteinase-2 levels and reduced their endogenous inhibitors, tissue inhibitors of matrix metalloproteinases, favoring a pro-degradative milieu following collagen deposition. Notably, injected human lung fibroblasts did not influence either collagen or matrix metalloproteinase levels in the lung. The results of this study suggest that uMSCs have antifibrotic properties and may augment lung repair if used to treat acute respiratory distress syndrome. (Am J