Reentry of T cells to the adult thymus is restricted to activated T cells

A guide to thymic selection of T cells

The thymus is an evolutionarily conserved organ that supports the development of T cells. Not only does the thymic environment support the rearrangement and expression of diverse T cell receptors but also provides a unique niche for the selection of appropriate T cell clones. Thymic selection ensures that the repertoire of available T cells is both useful (being MHC-restricted) and safe (being self-tolerant). The unique antigen-presentation features of the thymus ensure that the display of self-antigens is optimal to induce tolerance to all types of self-tissue. MHC class-specific functions of CD4+ T helper cells, CD8+ killer T cells and CD4+ regulatory T cells are also established in the thymus. Finally, the thymus provides signals for the development of several minor T cell subsets that promote immune and tissue homeostasis. This Review provides an introductory-level overview of our current understanding of the sophisticated thymic selection mechanisms that ensure T cells are useful and safe.

Maternal acellular pertussis vaccination in mice impairs cellular immunity to Bordetella pertussis infection in offspring

Given the resurgence of pertussis, several countries have introduced maternal tetanus, diphtheria, and acellular pertussis (aP) vaccination during pregnancy to protect young infants against severe pertussis. Although protective against the disease, the effect of maternal aP vaccination on bacterial colonization of the offspring is unknown. Here, we used a mouse model to demonstrate that maternal aP immunization, either before or during pregnancy, protects pups from lung colonization by Bordetella pertussis. However, maternal aP vaccination resulted in significantly prolonged nasal carriage of B. pertussis by inhibiting the natural recruitment of IL-17-producing resident memory T cells and ensuing neutrophil influx in the nasal tissue, especially of those with proinflammatory and cytotoxic properties. Prolonged nasal carriage after aP vaccination is due to IL-4 signaling, as prolonged nasal carriage is abolished in IL-4Rα-/- mice. The effect of maternal aP vaccination can be transferred transplacentally to the offspring or via breastfeeding and is long-lasting, as it persists into adulthood. Maternal aP vaccination may, thus, augment the B. pertussis reservoir.

CD147 deficiency in T cells prevents thymic involution by inhibiting the EMT process in TECs in the presence of TGFβ

Thymic involution during aging is a major cause of decreased T-cell production and reduced immunity. Here, we show that the loss of CD147 on T cells prevents thymic senescence, resulting in slowed shrinkage of the thymus with age and increased production of naive T cells. This phenotype is the result of slowing of the epithelial–mesenchymal transition (EMT) process in thymic epithelial cells (TECs), which eventually leads to reduced adipocyte accumulation. In an in vitro coculture system, we found that TGFβ is an important factor in the EMT process in TECs and that it can reduce the expression of E-cadherin through p-Smad2/FoxC2 signaling. Moreover, CD147 on T cells can accelerate the decline in E-cadherin expression by interacting with Annexin A2 on TECs. In the presence of TGFβ, Annexin A2 and E-cadherin colocalize on TECs. However, CD147 on T cells competitively binds to Annexin A2 on TECs, leading to the isolation of E-cadherin. Then, the isolated E-cadherin is easily phosphorylated by phosphorylated Src kinase, the phosphorylation of which was induced by TGFβ, and finally, p-E-cadherin is degraded. Thus, in the thymus, the interaction between T cells and TECs contributes to thymic involution with age. In this study, we illuminate the mechanism underlying the triggering of the EMT process in TECs and show that inhibiting TGFβ and/or CD147 may serve as a strategy to hinder age-related thymic involution.

Prevalence of CD8/CD4 Ratio in the Fetal Thymic Parenchyme in Down’s Syndrome

Aim: The maturation of most T- lymphocyte precursors takes place within the meshwork of thymic epithelial cells. Different steps of this process can be defined by immunologic phenotyping. The prothymocytes are positive for the terminal deoxynucleotidyl transferase (TdT) and give rise to cortical thymocytes, which express CD1, CD2, CD3, CD5, and both CD4 and CD8. These CD4 and CD8 double-positive cortical thymocytes differentiate into two lineages: CD4+ or CD8+ lymphocytes of the thymic medulla, by the tenth week of gestation. Our study points towards the determination of the CD8 cytotoxic/suppressor capacity of the fetal thymus in Down’s syndrome. Experimental design: A quantitative comparison of T-lymphocytes (CD3, CD4, and CD8) in the thymic parenchyme in embryos after voluntary abortion during 2nd trimester of gestation and embryos with Down’s syndrome, respectively, was performed. Results: Our results showed: 1) A statistically significant depletion in the total number of T-cells (CD3 positive) in the cases of embryos with Down’s syndrome over those after voluntary abortion, during the second trimester of gestation (p<0.0001, t-test). 2) A significant difference in the CD8/CD4 ratio in the cases of embryos with Down’s syndrome, during the second trimester of gestation which was numerically stronger with the progress of fetal development (20th week: p<0.025; 24th week: p<0.01, chi-square). Conclusions: The occurrence of increased CD8/CD4 ratio in the cases with Down’s syndrome, in the second trimester of gestation, underlines the cytotoxic / suppressor property of the thymus in the affected fetuses.

ERK Signaling Controls Innate-like CD8+ T Cell Differentiation via the ELK4 (SAP-1) and ELK1 Transcription Factors

In mouse thymocyte development, signaling by the TCR through the ERK pathway is required for positive selection of conventional naive T cells. The Ets transcription factor ELK4 (SAP-1), an ERK-regulated cofactor of the SRF transcription factor, plays an important role in positive selection by activating immediate-early genes such as the Egr transcription factor family. The role of ELK4-SRF signaling in development of other T cell types dependent on ERK signaling has been unclear. In this article, we show that ELK4, and its close relative ELK1, act cell autonomously in the thymus to control the generation of innate-like αβ CD8+ T cells with memory-like characteristics. Mice lacking ELK4 and ELK1 develop increased numbers of innate-like αβ CD8+ T cells, which populate the periphery. These cells develop cell autonomously rather than through expansion of PLZF+ thymocytes and concomitantly increased IL-4 signaling. Their development is associated with reduced TCR-mediated activation of ELK4-SRF target genes and can be partially suppressed by overexpression of the ELK4-SRF target gene EGR2. Consistent with this, partial inhibition of ERK signaling in peripheral CD8+T cells promotes the generation of cells with innate-like characteristics. These data establish that low-level ERK signaling through ELK4 (and ELK1) promotes innate-like αβ CD8+ T cell differentiation, tuning conventional versus innate-like development.

Recirculating Th2 cells induce severe thymic dysfunction via IL-4/STAT6 signaling pathway

Thymic involution happened early in life, but a certain ratio of activated CD4⁺ T cells will persistently recirculate into the thymus from the periphery and it have been suggested to be able to inhibit the development of embryonic thymocytes. Our present study was aimed to elucidate the specific mechanism how activated CD4⁺ T cells could influence upon developing thymocytes by using fetal thymic organ culture (FTOC) and kidney capsule transplantation. Our results demonstrated that Th2 cells were found to play a fundamental role in the inhibition of embryonic thymocyte development since a very low concentration of Th2 cells could obviously reduce the total number of thymocytes. And this effect was not tenable in other Th cell type. Notably, IL-4, the major cytokine secreted by Th2 cells, was suggested the key factor playing the inhibition role. In addition to reduced cell population, the proportion of double positive (DP) T cells was also heavily decreased. Furthermore, we demonstrated that it was the downstream effector signal transducer and activator of transcription 6 (STAT6) of IL-4 partially manipulate this inhibition. Together, these findings reveal a novel influence of Th2 cells re-entering the thymus on thymic involution.

Depletion-Resistant CD4 T Cells Enhance Thymopoiesis During Lymphopenia

Lymphoablation is routinely used in transplantation, and its success is defined by the balance of pathogenic versus protective T cells within reconstituted repertoire. While homeostatic proliferation and thymopoiesis may both cause T cell recovery during lymphopenia, the relative contributions of these mechanisms remain unclear. The goal of this study was to investigate the role of the thymus during T cell reconstitution in adult allograft recipients subjected to lymphoablative induction therapy. Compared with euthymic mice, thymectomized heart allograft recipients demonstrated severely impaired CD4 and CD8 T cell recovery and prolonged heart allograft survival after lymphoablation with murine anti-thymocyte globulin (mATG). The injection with agonistic anti-CD40 mAb or thymus transplantation only partially restored T cell reconstitution in mATG-treated thymectomized mice. After mATG depletion, residual CD4 T cells migrated into the thymus and enhanced thymopoiesis. Conversely, depletion of CD4 T cells before lymphoablation inhibited thymopoiesis at the stage of CD4^- CD8^- CD44^hi CD25⁺ immature thymocytes. This is the first demonstration that the thymus and peripheral CD4 T cells cooperate to ensure optimal T cell reconstitution after lymphoablation. Targeting thymopoiesis through manipulating functions of depletion-resistant helper T cells may thus improve therapeutic benefits and minimize the risks of lymphoablation in clinical settings.

Thymic homing of activated CD4+ T cells induces degeneration of the thymic epithelium through excessive RANK signaling

Activated T cells have been shown to be able to recirculate into the thymus from the periphery. The present study was aimed to elucidate the functional consequences of thymic homing of activated T cells upon developing thymocytes and thymic epithelial cells (TEC). In the presence of activated T cells, especially CD4⁺ T cells, T cell development was found to be inhibited in thymic organ cultures with markedly reduced cellularity. Thymic transplantation demonstrated that the inhibitory effect was most likely due to a defective microenvironment. As the major component of the thymic stroma, the TEC compartment was severely disturbed after prolonged exposure to the activated T cells. In addition to reduced cell proliferation, TEC differentiation was heavily skewed to the mTEC lineage. Furthermore, we demonstrated that RANKL highly expressed by activated CD4⁺ T cells was primarily responsible for the detrimental effects. Presumably, excessive RANK signaling drove overproduction of mTECs and possibly exhaustion of epithelial progenitors, thereby facilitating the deterioration of the epithelial structures. These findings not only reveal a novel activity of activated T cells re-entering the thymus, but also provide a new perspective for understanding the mechanism underlying thymic involution.

Murine models of transplantation tolerance through mixed chimerism: advances and roadblocks

Organ transplantation is the treatment of choice for patients with end-stage organ failure, but chronic immunosuppression is taking its toll in terms of morbidity and poor efficacy in preventing late graft loss. Therefore, a drug-free state would be desirable where the recipient permanently accepts a donor organ while remaining otherwise fully immunologically competent. Mouse studies unveiled mixed chimerism as an effective approach to induce such donor-specific tolerance deliberately and laid the foundation for a series of clinical pilot trials. Nevertheless, its widespread clinical implementation is currently prevented by cytotoxic conditioning and limited efficacy. Therefore, the use of mouse studies remains an indispensable tool for the development of novel concepts with potential for translation and for the delineation of underlying tolerance mechanisms. Recent innovations developed in mice include the use of pro-apoptotic drugs or regulatory T cell (T_reg ) transfer for promoting bone marrow engraftment in the absence of myelosuppression and new insight gained in the role of innate immunity and the interplay between deletion and regulation in maintaining tolerance in chimeras. Here, we review these and other recent advances in murine studies inducing transplantation tolerance through mixed chimerism and discuss both the advances and roadblocks of this approach.

The thymus and rheumatology: should we care?

Purpose of review

The purpose of this review is to discuss the mechanisms of central and peripheral tolerance in relation to T-cell mediated autoimmunity in rheumatoid arthritis (RA).

Recent findings

The well established association between major histocompatibility complex class II and RA has led us to understand that T cells, and the adaptive immune response, are important in the pathogenesis of disease. In order for autoimmune disease to develop, there is a breach of tolerance to self antigen and the mechanisms of both central and peripheral tolerance aim to prevent this. Here, we review evidence from mouse models indicating that alterations in T-cell receptor signalling thresholds during thymic selection may be linked to the escape of T cells that mediate autoimmune arthritis. In addition, we summarize the role of dendritic cells and Foxp3+ regulatory T cells in both peripheral and thymic tolerance, and highlight their relevance to what we know about the aetiology of RA.

Summary

Mechanisms of central tolerance in the thymus and peripheral tolerance are in place to control autoreactive T cells and to prevent the development of autoimmune disease. We anticipate that a better understanding of these mechanisms will lead to the development of better, antigen-specific therapeutics to restore tolerance.

The kinases NDR1/2 act downstream of the Hippo homolog MST1 to mediate both egress of thymocytes from the thymus and lymphocyte motility

The serine and threonine kinase MST1 is the mammalian homolog of Hippo. MST1 is a critical mediator of the migration, adhesion, and survival of T cells; however, these functions of MST1 are independent of signaling by its typical effectors, the kinase LATS and the transcriptional coactivator YAP. The kinase NDR1, a member of the same family of kinases as LATS, functions as a tumor suppressor by preventing T cell lymphomagenesis, which suggests that it may play a role in T cell homeostasis. We generated and characterized mice with a T cell-specific double knockout of Ndr1 and Ndr2 (Ndr DKO). Compared with control mice, Ndr DKO mice exhibited a substantial reduction in the number of naïve T cells in their secondary lymphoid organs. Mature single-positive thymocytes accumulated in the thymus in Ndr DKO mice. We also found that NDRs acted downstream of MST1 to mediate the egress of mature thymocytes from the thymus, as well as the interstitial migration of naïve T cells within popliteal lymph nodes. Together, our findings indicate that the kinases NDR1 and NDR2 function as downstream effectors of MST1 to mediate thymocyte egress and T cell migration.

CMV reactivation drives posttransplant T-cell reconstitution and results in defects in the underlying TCRβ repertoire

Although cytomegalovirus (CMV) reactivation has long been implicated in posttransplant immune dysfunction, the molecular mechanisms that drive this phenomenon remain undetermined. To address this, we combined multiparameter flow cytometric analysis and T-cell subpopulation sorting with high-throughput sequencing of the T-cell repertoire, to produce a thorough evaluation of the impact of CMV reactivation on T-cell reconstitution after unrelated-donor hematopoietic stem cell transplant. We observed that CMV reactivation drove a >50-fold specific expansion of Granzyme B(high)/CD28(low)/CD57(high)/CD8(+) effector memory T cells (Tem) and resulted in a linked contraction of all naive T cells, including CD31(+)/CD4(+) putative thymic emigrants. T-cell receptor β (TCRβ) deep sequencing revealed a striking contraction of CD8(+) Tem diversity due to CMV-specific clonal expansions in reactivating patients. In addition to querying the topography of the expanding CMV-specific T-cell clones, deep sequencing allowed us, for the first time, to exhaustively evaluate the underlying TCR repertoire. Our results reveal new evidence for significant defects in the underlying CD8 Tem TCR repertoire in patients who reactivate CMV, providing the first molecular evidence that, in addition to driving expansion of virus-specific cells, CMV reactivation has a detrimental impact on the integrity and heterogeneity of the rest of the T-cell repertoire. This trial was registered at www.clinicaltrials.gov as #NCT01012492.

Concomitant analysis of Helios and Neuropilin-1 as a marker to detect thymic derived regulatory T cells in naïve mice

Regulatory T (Treg) cells are characterized by the expression of CD4, CD25 and the intracellular Foxp3. However, these markers do not indicate whether Treg cells are thymic derived Treg (tTreg) cells or peripherally induced Treg (pTreg) cells. Recently, Helios and Neuropilin-1 (Nrp1) has been reported as potential markers for tTreg cells. Herein, we used flow cytometry to examine the proportion of CD4(+)CD8(-)CD25(+) Treg cells expressing Helios, Nrp1 and Foxp3 in thymus, pancreatic draining lymph nodes (PDLNs) and spleen of CD-1 mice, and thymus of NOD and C57BL/6 mice. The frequency of Helios(+) cells was higher than that of Nrp1(+) cells in CD4(+)CD8(-)CD25(+) and CD4(+)CD8(-)CD25(+)Foxp3(+) Treg cells in thymus. Interestingly, the proportion of IL-10(+), Ebi3(+)and CTLA-4(+) cells was higher in Helios(+) than Nrp1(+) tTreg cells. The anti-apoptotic activity of Helios(+) tTreg cells was higher in thymus compared to Nrp1(+) tTreg cells. Nrp1 seems to be expressed at a later developmental stage compared to Helios and Foxp3. Furthermore, the expression of Nrp1 in CD4(+)CD25(+) T cells of younger mice did not increase after stimulating them in vitro with anti-CD3 and -CD28. Thus, under these conditions, Helios could be considered a more reliable marker for distinguishing tTreg cells from pTreg cells than Nrp1.

Stable interactions and sustained TCR signaling characterize thymocyte-thymocyte interactions that support negative selection

Negative selection is one of the primary mechanisms that render T cells tolerant to self. Thymic dendritic cells play an important role in negative selection, in line with their ability to induce migratory arrest and sustained TCR signals. Thymocytes themselves display self-peptide/MHC class I complexes, and although there is evidence that they can support clonal deletion, it is not clear whether they do so directly via stable cell-cell contacts and sustained TCR signals. In this study, we show that murine thymocytes can support surprisingly efficient negative selection of Ag-specific thymocytes. Furthermore, we observe that agonist-dependent thymocyte-thymocyte interactions occurred as stable, motile conjugates led by the peptide-presenting thymocyte and in which the trailing peptide-specific thymocyte exhibited persistent elevations in intracellular calcium concentration. These data confirm that self-Ag presentation by thymocytes is an additional mechanism to ensure T cell tolerance and further strengthen the correlation between stable cellular contacts, sustained TCR signals, and efficient negative selection.

Both retention and recirculation contribute to long-lived regulatory T-cell accumulation in the thymus

Natural Treg cells acquire their lineage-determining transcription factor Foxp3 during development in the thymus and are important in maintaining immunologic tolerance. Here, we analyzed the composition of the thymic Treg-cell pool using RAG2-GFP/FoxP3-RFP dual reporter mice and found that a population of long-lived GFP(-) Treg cells exists in the thymus. These long-lived Treg cells substantially increased with age, to a point where they represent >90% of the total thymic Treg-cell pool at 6 months of age. In contrast, long-lived conventional T cells remained at ∼ 15% of the total thymic pool at 6 months of age. Consistent with these studies, we noticed that host-derived Treg cells represented a large fraction (∼ 10%) of the total thymic Treg-cell pool in bone marrow chimeras, suggesting that long-lived Treg cells also reside in the thymus of these mice. The pool of long-lived Treg cells in the thymus was sustained by retention of Treg cells in the thymus and by recirculation of peripheral Treg cells back into the thymus. These long-lived thymic Treg cells suppressed T-cell proliferation to an equivalent extent to splenic Treg cells. Together, these data demonstrate that long-lived Treg cells accumulate in the thymus by both retention and recirculation.

B6.g7 mice reconstituted with BDC2·5 non-obese diabetic (BDC2·5NOD) stem cells do not develop autoimmune diabetes

In BDC2·5 non-obese diabetic (BDC2·5NOD) mice, a spontaneous model of type 1 diabetes, CD4(+) T cells express a transgene-encoded T cell receptor (TCR) with reactivity against a pancreatic antigen, chromogranin. This leads to massive infiltration and destruction of the pancreatic islets and subsequent diabetes. When we reconstituted lethally irradiated, lymphocyte-deficient B6.g7 (I-A(g7+)) Rag(-/-) mice with BDC2·5NOD haematopoietic stem and progenitor cells (HSPC; ckit(+)Lin(-)Sca-1(hi)), the recipients exhibited hyperglycaemia and succumbed to diabetes. Surprisingly, lymphocyte-sufficient B6.g7 mice reconstituted with BDC2·5NOD HSPCs were protected from diabetes. In this study, we investigated the factors responsible for attenuation of diabetes in the B6.g7 recipients. Analysis of chimerism in the B6.g7 recipients showed that, although B cells and myeloid cells were 98% donor-derived, the CD4(+) T cell compartment contained ∼50% host-derived cells. These host-derived CD4(+) T cells were enriched for conventional regulatory T cells (Tregs ) (CD25(+) forkhead box protein 3 (FoxP3)(+)] and also for host- derived CD4(+)CD25(-)FoxP3(-) T cells that express markers of suppressive function, CD73, FR4 and CD39. Although negative selection did not eliminate donor-derived CD4(+) T cells in the B6.g7 recipients, these cells were functionally suppressed. Thus, host-derived CD4(+) T cells that emerge in mice following myeloablation exhibit a regulatory phenoytpe and probably attenuate autoimmune diabetes. These cells may provide new therapeutic strategies to suppress autoimmunity.

Thymus-resident memory CD8+ T cells mediate local immunity

The thymus is a primary lymphoid organ responsible for production and selection of T cells. Nonetheless, mature T cells and in particular activated T cells can reenter the thymus. Here, we identified memory CD8(+) T cells specific for lymphocytic choriomeningitis virus or vaccinia virus in the thymus of mice long-time after the infection. CD8(+) T cells were mainly located in the thymic medulla, but also in the cortical areas. Interestingly, virus-specific memory CD8(+) T cells in the thymus expressed the cell surface markers CD69 and CD103 that are characteristic of tissue-resident memory T cells in a time-dependent manner. Kinetic analyses and selective depletion of peripheral CD8(+) T cells by antibodies further revealed that thymic virus-specific memory CD8(+) T cells did not belong to the circulating pool of lymphocytes. Finally, we demonstrate that these thymus-resident virus-specific memory CD8(+) T cells efficiently mounted a secondary proliferative response, exhibited immediate effector functions and were able to protect the thymus from lymphocytic choriomeningitis virus reinfection. In conclusion, the present study not only describes for the first time virus-specific memory CD8(+) T cells with characteristics of tissue-resident memory T (T(RM)) cells in a primary lymphoid organ but also extends our knowledge about local T-cell immunity in the thymus.

Analysis of the dynamics of infiltrating CD4(+) T cell subsets in the heart during experimental Trypanosoma cruzi infection

Chagas disease, caused by the protozoan parasite Trypanosoma cruzi, affects several million people in Latin America. Myocarditis, observed during both the acute and chronic phases of the disease, is characterized by an inflammatory mononuclear cell infiltrate that includes CD4⁺ T cells. It is known that Th1 cytokines help to control infection. The role that T_reg and Th17 cells may play in disease outcome, however, has not been completely elucidated. We performed a comparative study of the dynamics of CD4⁺ T cell subsets after infection with the T. cruzi Y strain during both the acute and chronic phases of the disease using susceptible BALB/c and non-susceptible C57BL/6 mice infected with high or low parasite inocula. During the acute phase, infected C57BL/6 mice showed high levels of CD4⁺ T cell infiltration and expression of Th1 cytokines in the heart associated with the presence of T_reg cells. In contrast, infected BALB/c mice had a high heart parasite burden, low heart CD4⁺ T cell infiltration and low levels of Th1 and inflammatory cytokines, but with an increased presence of Th17 cells. Moreover, an increase in the expression of IL-6 in susceptible mice was associated with lethality upon infection with a high parasite load. Chronically infected BALB/c mice continued to present higher parasite burdens than C57BL/6 mice and also higher levels of IFN-γ, TNF, IL-10 and TGF-β. Thus, the regulation of the Th1 response by T_reg cells in the acute phase may play a protective role in non-susceptible mice irrespective of parasite numbers. On the other hand, Th17 cells may protect susceptible mice at low levels of infection, but could, in association with IL-6, be pathogenic at high parasite loads.

Host-derived CD4+ T cells attenuate stem cell-mediated transfer of autoimmune arthritis in lethally irradiated C57BL/6.g7 mice

OBJECTIVE

In the K/BxN mouse model of inflammatory arthritis, T cells carrying a transgenic T cell receptor initiate disease by helping B cells to produce arthritogenic anti-glucose-6-phosphate isomerase (anti-GPI) autoantibodies. We found that lethally- irradiated lymphocyte-deficient C57BL/6 (B6).g7 (I-A(g7) +) recombinase-activating gene-deficient (Rag(-/-)) mice reconstituted with K/BxN hematopoietic stem and progenitor cells exhibit arthritis by week 4. In contrast, healthy B6.g7 recipients of K/BxN hematopoietic stem and progenitor cells show only mild arthritis, with limited extent and duration. The objective of this study was to investigate the factors responsible for the attenuation of arthritis in B6.g7 recipients.

METHODS

Antibody responses were measured by enzyme-linked immunosorbent assay. Fluorescence-activated cell sorting analyses were performed for testing chimerism, expression of markers of activation and suppression, tetramer binding, and intracellular cytokines in CD4+ T cells. Suppressive activity of CD4+ T cells was studied by adoptive transfer.

RESULTS

Titers of anti-GPI antibodies in reconstituted B6.g7 mice were ∼60-fold lower than in reconstituted B6.g7 Rag(-/-) mice. Examination of chimerism in the reconstituted B6.g7 mice showed that B cells and myeloid cells in these mice were donor derived, but CD4+ T cells were primarily host derived and enriched for cells expressing the conventional regulatory markers CD25 and FoxP3. Notably, CD4+CD25-FoxP3- T cells expressed markers of suppressive function (CD73 and folate receptor 4), and delayed disease after adoptive transfer. Activation of donor-derived CD4+ T cells was reduced, and thymic deletion of these cells appeared increased.

CONCLUSION

Despite myeloablation, host CD4+ T cells having a regulatory phenotype emerge in these mice and attenuate autoimmunity.

Modeling the intrathymic pathogenesis of myasthenia gravis

Myasthenia gravis is (MG) a prototypic autoimmune disease; the immune effector mechanisms and autoantigenic target have been delineated. However, the events that lead to the abrogation of self-tolerance to neuromuscular acetylcholine receptors (nAChRs) remain a mystery. The thymus gland has long been considered to hold the key to solving this mystery, although the nature of its involvement remains to be elucidated. The nAChR was one of the first self-proteins associated with a defined autoimmune disease that was found to be expressed on thymic stromal populations. The studies described herein represent our efforts to determine how this "promiscuous" autoantigen expression may be involved in the immunopathogenesis of MG. We review our work, characterizing the expression of the nAChR alpha subunit in the thymus, and advance a hypothesis and experimental model, which explore how intrathymic expression of this autoantigen may contribute to the immunopathogenesis of this autoimmune disease.

MCP-1/CCR2 interactions direct migration of peripheral B and T lymphocytes to the thymus during acute infectious/inflammatory processes

Mature lymphocyte immigration into the thymus has been documented in mouse, rat, and pig models, and highly increases when cells acquire an activated phenotype. Entrance of peripheral B and T cells into the thymus has been described in healthy and pathological situations. However, it has not been proposed that leukocyte recirculation to the thymus could be a common feature occurring during the early phase of a Th1 inflammatory/infectious process when a large number of peripheral cells acquire an activated phenotype and the cellularity of the thymus is seriously compromised. The data we present here demonstrate that in well-established Th1 models triggered by different types of immunogens, for example, LPS treatment (a bacterial product), Candida albicans infection (a fungus), and after Trypanosoma cruzi infection (a parasite), a large number of mature peripheral B and T cells enter the thymus. This effect is dependent on, but not exclusive of, the available space in the thymus. Our data also demonstrate that MCP-1/CCR2 (where MCP-1 is monocyte chemoattractant protein-1) interaction is responsible for the infiltration of peripheral cells to the thymus in these Th1-inflammatory/infectious situations. Finally, systemic expression of IL-12 and IL-18 produced during the inflammatory process is ultimately responsible for these migratory events.

The NF-κB1 transcription factor prevents the intrathymic development of CD8 T cells with memory properties

The role of specific members of the NF-κB family of transcription factors in CD8 T-cell selection and development is largely unknown. Here, we show that mice lacking NF-κB1 develop a unique population of conventional CD8 single-positive (SP) thymocytes with memory T cell-like properties that populate peripheral immune organs. Development of this memory-like population is not due to PLZF(+) thymocytes and instead coincides with changes in CD8 T-cell selection. These include a reduction in the efficiency of negative selection and a dependence on MHC class Ia or Ib expressed by haematopoietic cells. These findings indicate that NF-κB1 regulates multiple events in the thymus that collectively inhibit the excess development of CD8(+) thymocytes with memory cell characteristics.

The molecular signature underlying the thymic migration and maturation of TCRαβ+ CD4+ CD8 thymocytes

BACKGROUND

After positive selection, the newly generated single positive (SP) thymocytes migrate to the thymic medulla, where they undergo negative selection to eliminate autoreactive T cells and functional maturation to acquire immune competence and egress capability.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate the genetic program underlying this process, we analyzed changes in gene expression in four subsets of mouse TCRαβ(+)CD4(+)CD8(-) thymocytes (SP1 to SP4) representative of sequential stages in a previously defined differentiation program. A genetic signature of the migration of thymocytes was thus revealed. CCR7 and PlexinD1 are believed to be important for the medullary positioning of SP thymocytes. Intriguingly, their expression remains at low levels in the newly generated thymocytes, suggesting that the cortex-medulla migration may not occur until the SP2 stage. SP2 and SP3 cells gradually up-regulate transcripts involved in T cell functions and the Foxo1-KLF2-S1P(1) axis, but a number of immune function-associated genes are not highly expressed until cells reach the SP4 stage. Consistent with their critical role in thymic emigration, the expression of S1P(1) and CD62L are much enhanced in SP4 cells.

CONCLUSIONS

These results support at the molecular level that single positive thymocytes undergo a differentiation program and further demonstrate that SP4 is the stage at which thymocytes acquire the immunocompetence and the capability of emigration from the thymus.

Peripheral T cells re-enter the thymus and interfere with central tolerance induction

The thymus mainly contains developing thymocytes that undergo thymic selection. In addition, some mature activated peripheral T cells can re-enter the thymus. We demonstrated in this study that adoptively transferred syngeneic Ag-specific T cells can enter the thymus of lymphopenic mice, where they delete thymic dendritic cells and medullary thymic epithelial cells in an Ag-specific fashion, without altering general thymic functions. This induced sustained thymic release of autoreactive self-Ag-specific T cells suggested that adoptively transferred activated T cells can specifically alter the endogenous T cell repertoire by erasing negative selection of their own specificities. Especially in clinical settings in which adoptively transferred T cells cause graft-versus-host disease or graft-versus-leukemia, as well as in adoptive tumor therapies, these findings might be of importance, because the endogenous T cell repertoire might be skewed to contribute to both manifestations.

Modulation of immune cell function by α(1)-adrenergic receptor activation

The sympathetic nervous system regulates human immune system functions through epinephrine (Epi) and norepinephrine (NE) activation of adrenergic receptors (AR) expressed on immunocompetent cell populations. The anti-inflammatory effects that are most often attributed to increased sympathetic activity have been shown to occur through β₂- and α₂-AR stimulation. However, dichotomous AR effects on immune system function are becoming increasingly apparent. Reports of α₁-AR expression on immune cell populations have been conflicting due to a lack of specific antibodies or subtype-selective receptor ligands. This has made α₁-AR identification difficult and further characterization of α₁-AR subtype expression limited. Nevertheless, there is some evidence suggesting an induction of α₁-AR expression on immunocompetent cells under certain physiological conditions and disease states. Also, the function of α₁-AR activation to modulate immune responses is just beginning to emerge in the literature. Changes in the secretion of inflammatory mediators as well as increased cell migration and differentiation have been described following α₁-AR stimulation on immunocompetent cells. These observations demonstrate the significance of α₁-AR activity in immune cell biology and emphasize the importance for understanding α₁-AR effects on the immune system.

Cutting edge: spontaneous development of IL-17-producing gamma delta T cells in the thymus occurs via a TGF-beta 1-dependent mechanism

In naive animals, gammadelta T cells are innate sources of IL-17, a potent proinflammatory cytokine mediating bacterial clearance as well as autoimmunity. However, mechanisms underlying the generation of these cells in vivo remain unclear. In this study, we show that TGF-beta1 plays a key role in the generation of IL-17(+) gammadelta T cells and that it mainly occurs in the thymus particularly during the postnatal period. Interestingly, IL-17(+) gammadelta TCR(+) thymocytes were mainly CD44(high)CD25(low) cells, which seem to derive from double-negative 4 gammadelta TCR(+) cells that acquired CD44 and IL-17 expression. Our findings identify a novel developmental pathway during which IL-17-competent gammadelta T cells arise in the thymus by a TGF-beta1-dependent mechanism.

Thymic selection and lineage commitment of CD4(+)Foxp3(+) regulatory T lymphocytes

Regulatory T lymphocytes play a central role in the control of a variety of immune-responses. Their absence in humans and in experimental animal models leads to severe autoimmune and inflammatory disorders. Consistent with their major role in prevention of autoimmune pathology, their repertoire is enriched in autospecific cells. Probably the majority of regulatory T cells develop in the thymus. How T cell-precursors choose between the conventional versus regulatory T cell lineages remains an unanswered question. More is known about selection of regulatory T cell precursors. Positive selection of these cells is favored by high affinity interactions with MHC class II/peptide ligands expressed by thymic epithelial or dendritic cells. They are also known to be relatively resistant to negative selection. These two parameters allow for the generation of the autoreactive regulatory T cell repertoire, and clearly distinguish selection-criteria of conventional versus regulatory T cell-precursors. It will now be important to elucidate the molecular mechanisms involved in the intrathymic choice of the regulatory T cell-lineage.

T-cell seeding: neonatal transfer of anti-myelin basic protein T-cell lines renders Fischer rats susceptible later in life to the active induction of experimental autoimmune encephalitis

Fischer strain rats resist active induction of experimental autoimmune encephalomyelitis (EAE) following immunization with guinea-pig myelin basic protein (MBP) in complete Freund's adjuvant (CFA). Nevertheless, we now report that an encephalitogenic CD4(+) anti-MBP T-cell line could be developed from actively immunized Fischer rats. Adoptive transfer of the activated line mediated acute EAE in adult Fischer rats, but not in 1-day-old rats. Moreover, we found that both resting and activated anti-MBP T cells injected 1 day post-natally rendered these rats susceptible later in life to the active induction of EAE by immunization with MBP/CFA. The actively induced EAE manifested the accelerated onset of a secondary, memory-type response. Resting anti-MBP T cells injected even up to 2 weeks post-natally produced no clinical signs but seeded 50-100% of the recipients for an active encephalitogenic immune response to MBP. An earlier T-cell injection (1-2 days) produced a higher incidence and stronger response. The transferred resting T cells entered the neonatal spleen and thymus and proliferated there but did not change the total anti-MBP precursor number in adults. Splenocytes harvested from rats that were injected neonatally but not exposed to MBP in vivo proliferated strongly and produced significant amounts of interferon-gamma to MBP in vitro. Similar results were observed in rats injected with resting T-cell lines reactive to ovalbumin, suggesting that the neonatal injection of resting T cells specific for a self or for a foreign antigen can seed the immune system with the potential for an enhanced effector response to that antigen later in life.

PSGL-1 function in immunity and steady state homeostasis

The substantial importance of P-selectin glycoprotein ligand 1 (PSGL-1) in leukocyte trafficking has continued to emerge beyond its initial identification as a selectin ligand. PSGL-1 seemed to be a relatively simple molecule with an extracellular mucin domain extended as a flexible rod, teleologically consistent with its primary role in tethering leukocytes to endothelial selectins. The rolling interaction between leukocyte and endothelium mediated by this selectin-PSGL-1 interaction requires branched O-glycan extensions on specific PSGL-1 amino acid residues. In some cells, such as neutrophils, the glycosyltransferases involved in formation of the O-glycans are constitutively expressed, while in other cells, such as T cells, they are expressed only after appropriate activation. Thus, PSGL-1 supports leukocyte recruitment in both innate and adaptive arms of the immune response. A complex array of amino acids within the selectins engage multiple sugar residues of the branched O-glycans on PSGL-1 and provide the molecular interactions responsible for the velcro-like catch bonds that support leukocyte rolling. Such binding of PSGL-1 can also induce signaling events that influence cell phenotype and function. Scrutiny of PSGL-1 has revealed a better understanding of how it performs as a selectin ligand and yielded unexpected insights that extend its scope from supporting leukocyte rolling in inflammatory settings to homeostasis including stem cell homing to the thymus and mature T-cell homing to secondary lymphoid organs. PSGL-1 has been found to bind homeostatic chemokines CCL19 and CCL21 and to support the chemotactic response to these chemokines. Surprisingly, the O-glycan modifications of PSGL-1 that support rolling mediated by selectins in inflammatory conditions interfere with PSGL-1 binding to homeostatic chemokines and thereby limit responsiveness to the chemotactic cues used in steady state T-cell traffic. The multi-level influence of PSGL-1 on cell traffic in both inflammatory and steady state settings is therefore substantially determined by the orchestrated addition of O-glycans. However, central as specific O-glycosylation is to PSGL-1 function, in vivo regulation of PSGL-1 glycosylation in T cells remains poorly understood. It is our purpose herein to review what is known, and not known, of PSGL-1 glycosylation and to update understanding of PSGL-1 functional scope.

Role of ovarian hormones in age-associated thymic involution revisited

A commonly held view that ovarian hormones are causally involved in age-associated thymic involution has been recently challenged. In particular, their relevance in the progression of thymic involution has been disputed. To reassess this issue 10-month-old rats with well advanced thymic involutive changes were ovariectomized (Ovx), and after 1 month thymic cellularity, thymocyte development and levels of recent thymic emigrants (RTEs) were examined in peripheral blood and spleen. In addition, the distribution of major conventional and regulatory T-cell subsets was analyzed in the same peripheral lymphocyte compartments. Ovariectomy increased thymic weight and cellularity above the levels in both 10-month-old and age-matched controls indicating that ovarian hormone ablation not only prevented further progression of thymic involution, but also reversed it. The increased thymic cellularity was accompanied by altered thymocyte differentiation/maturation culminating in increased thymic output of naïve T cells as indicated by elevated levels of both CD4+ and CD8+ RTEs in peripheral blood and spleen. The changes in T-cell development produced: (i) a disproportional increase in cellularity across thymocyte subsets, so that relative proportions of cells at all maturational stages preceding the CD4+CD8+ T cell receptor (TCR)alphabeta(low) stage were reduced; the relative numbers of CD4+CD8+ TCRalphabeta(low) cells entering positive selection and their immediate CD4+CD8+ TCRalphabeta(high) descendents were increased, while those of the most mature CD4+CD8- and CD4-CD8+ TCRalphabeta(high) cells remained unaltered; (ii) enhanced cell proliferation across all thymocyte subsets and (iii) reduced apoptosis of cells within the CD4+CD8+ thymocyte subset. The augmented thymic output of naïve T cells in Ovx rats most likely reflected an early disinhibition of thymocyte development followed by increased positive/reduced negative selection, at least partly, due to raised thymocyte surface Thy-1 expression. The greater number of CD4+CD25+Foxp3+ cells in both thymus and peripheral blood suggested augmented thymic production of these cells. In addition, an increased CD4+/CD8+ cell ratio was found in the spleen of Ovx rats. Thus, ovarian hormone ablation led not only to increased diversity of the T-cell repertoire, but also to a new balance among distinct T-cell subsets in the periphery.

Enhanced selection of FoxP3+ T-regulatory cells protects CTLA-4-deficient mice from CNS autoimmune disease

It is generally acknowledged that cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4/CD152) plays a pivotal role in the regulation of T-cell activation and the establishment of self-tolerance in the periphery. CTLA-4-deficient (CTLA-4KO) mice develop a lymphoproliferative disorder and die within 4 weeks of birth, suggesting a role for CTLA-4 in T-cell homeostasis or the development and activity of T-regulatory (Treg) cells. To study the role of CTLA-4 in the control of experimental autoimmune encephalomyelitis (EAE), we have generated a CTLA-4KO mouse in which >90% of all CD4(+) T cells bear a Vbeta8.2 transgenic T-cell receptor that is specific for myelin basic protein peptide Ac1-9 (ASQKRPSQR). These mice do not develop spontaneous lymphoproliferative disease or EAE and are resistant to disease induction. This correlates with a higher frequency of functional FoxP3(+) Treg cells in the spleen and thymus of CTLA-4KO mice. The absence of CTLA-4-mediated suppression of CD28 signaling resulted in the early expression of FoxP3 on double-positive cells in the thymic cortex. We conclude that CTLA-4 is not essential for the peripheral function of FoxP3(+) Treg cells but plays a pivotal role in their thymic selection.

Self-antigen prevents CD8 T cell effector differentiation by CD134 and CD137 dual costimulation

We compared how CD4 vs CD8 cells attain the capacity to express the effector cytokine IFN-gamma under both immunogenic and tolerogenic conditions. Although the Ifng gene locus was epigenetically repressed in naive Ag-inexperienced CD4 cells, it had already undergone partial remodeling toward a transcriptionally competent configuration in naive CD8 cells. After TCR stimulation, CD8 cells fully remodeled the Ifng locus and gained the capacity to express high levels of IFN-gamma more rapidly than CD4 cells. Enforced dual costimulation through OX40 and 4-1BB redirected CD8 cells encountering soluble exogenous peptide to expand and differentiate into IFN-gamma and TNF-alpha double-producing effectors rather than becoming tolerant. Despite this and the stronger tendency of CD8 compared with CD4 cells to differentiate into IFN-gamma-expressing effectors, when parenchymal self-Ag was the source of tolerizing Ag, enforced dual costimulation selectively boosted expansion but did not push effector differentiation in CD8 cells while both expansion and effector differentiation were dramatically boosted in CD4 cells. Notably, enforced dual costimulation was able to push effector differentiation in CD8 cells encountering cognate parenchymal self-Ag when CD4 cells were simultaneously engaged. Thus, the ability of enforced OX40 plus 4-1BB dual costimulation to redirect CD8 cells to undergo effector differentiation was unexpectedly influenced by the source of tolerizing Ag and help was selectively required to facilitate CD8 cell effector differentiation when the tolerizing Ag derived from self.

Re-entry of mature T cells to the thymus: an epiphenomenon?

In certain situations mature post-thymic T cells are able to leave their residence in the secondary lymphoid tissues and re-enter the thymus. The physiological significance of this phenomenon is discussed.

T-lymphocyte interaction with stromal, bone and hematopoietic cells in the bone marrow

Mature T cells in the bone marrow (BM) are in constant exchange with the blood pool. Within the BM, T-cell recognition of antigen presented by dendritic cell (DC) can occur, nevertheless it is thought that BM T cells mostly receive non-antigenic signals by either stimulatory, for example, interleukin (IL)-7, IL-15, tumor necrosis factor family members, or inhibitory molecules, for example, transforming growth factor-beta. The net balance is in favor of T-cell proliferation. Indeed, the percentage of proliferating T cells is higher in the BM than in spleen and lymph nodes, both within CD4 and CD8 T cells. High numbers of memory T cells proliferate in the BM, as they preferentially home to the BM and have an increased turnover as compared with naive T cells. I propose here that the BM plays an essential role in maintaining normal peripheral T-lymphocyte numbers and antigen-specific memory for both CD4 and CD8 T cells. I also discuss BM T-cell contribution to the homeostasis of bone metabolism as well as of hematopoiesis. It emerges that BM T cells play unexpected roles in several diseases, for example AIDS and osteoporosis. A better knowledge on BM T cells has implications for currently used clinical interventions, for example, vaccination, BM transplantation, mesenchymal stem cell-based therapies.

MHC class II-dependent T-T interactions create a diverse, functional and immunoregulatory reaction circle

Unlike conventional T cells, innate-like T cells such as natural killer (NK) T cells are selected by homotypic T-cell interactions. Recently, a few reports have shown that T-T CD4(+) T cells can be generated in a similar manner to that for NKT cells. These two types of cells share common functional properties such as rapid response to antigenic encounters and the potential for a panoply of cytokine secretion. However, T-T CD4(+) T cells differ from NKT cells in that they are restricted by highly polymorphic major histocompatibility complex (MHC) II molecules and have a diverse T-cell receptor repertoire. Additional example of T-T interactions was recently reported in which peripheral T cells re-circulate to the thymus and participate in the thymocyte selection process. In this review, we dissect the cellular mechanisms underlying the production of T-T CD4(+) and NKT cells, with particular emphasis on the differences between these two T-cell prototypes. Finally, we propose that T-T CD4(+) T cells serve two major functions: one as an acute-phase reactant against viral infection and the other is the generation of anti-ergotypic CD4(+) T cells for regulatory purposes. All of these features make it possible to create a diverse set of functional cells through MHC class II-restricted T-T interactions.

Back to the thymus: peripheral T cells come home

The thymus has long been known as the generative organ for the T-cell arm of the immune system. To perform this role, the thymus was thought to require protection from antigenic and cellular insult from the 'outside world', with the notable exception of the continual influx of progenitor cells required to initiate the complicated process of T-cell differentiation. Overwhelming evidence that mature T cells can recirculate and persist in the thymus has required us to revamp this earlier view of the thymus as detached from outside influence. In this review, we consider the evidence for T-cell recirculation into the thymus, discuss the likely means and location of mature T-cell entry, and speculate on the potential consequences of such close apposition between differentiating thymocytes and mature recirculating lymphocytes.

The immunopathology of thymic GVHD

The clinical success of allogeneic hematopoietic stem cell transplantation (HSCT) depends on the appropriate reconstitution of the host's immune system. While recovery of T-cell immunity may occur in transplant recipients via both thymus-dependent and thymus-independent pathways, the regeneration of a population of phenotypically naive T cells with a broad receptor repertoire relies entirely on the de novo generation of T-cells in the thymus. Preclinical models and clinical studies of allogeneic HSCT have identified the thymus as a target of graft-versus-host disease (GVHD), thus limiting T-cell regeneration. The present review focuses on recent insight into how GVHD affects thymic structure and function and how this knowledge may aid in the design of new strategies to improve T-cell reconstitution following allogeneic HSCT.

Thymic emigration: when and how T cells leave home

The thymus supports the differentiation of multiple distinct T cell subsets that play unique roles in the immune system. CD4 and CD8 alpha/beta T cells, gamma/delta T cells, NKT cells, regulatory T cells, and intraepithelial lymphocytes all develop in the thymus and must leave it to provide their functions elsewhere in the body. This article will review recent research indicating differences in the time and migration patterns of T cell subsets found in the thymus. Additionally, we review current understanding of the molecules involved in thymocyte emigration, including the sphingolipid receptor S1P(1) and its regulation by the Krüppel-like transcription factor KLF2.

Peripheral T lymphocytes recirculating back into the thymus can mediate thymocyte positive selection

The thymus continuously produces T lymphocytes that contribute to the maintenance of the peripheral T cell pool. Since peripheral recirculating T cells represent a very minor population among total thymocytes in normal animals, the relationship between the thymus and secondary lymphoid organs is generally considered unidirectional. Recently, several reports have described the presence of recirculating T cells in the thymus, raising issues regarding their possible function. In this article, we show that the niche for recirculating T cells in the thymus, i.e., their absolute number, is the same in lymphopenic and normal mice. Using a novel combination of TCR-transgenic mice in which the ligand necessary for positive selection of host T cells is only expressed by transferred donor T cells, we show that mature T cells recirculating back to the thymus can mediate positive selection.

Human innate Mycobacterium tuberculosis-reactive alphabetaTCR+ thymocytes

The control of Mycobacterium tuberculosis (Mtb) infection is heavily dependent on the adaptive Th1 cellular immune response. Paradoxically, optimal priming of the Th1 response requires activation of priming dendritic cells with Th1 cytokine IFN-γ. At present, the innate cellular mechanisms required for the generation of an optimal Th1 T cell response remain poorly characterized. We hypothesized that innate Mtb-reactive T cells provide an early source of IFN-γ to fully activate Mtb-exposed dendritic cells. Here, we report the identification of a novel population of Mtb-reactive CD4⁻ αβTCR⁺ innate thymocytes. These cells are present at high frequencies, respond to Mtb-infected cells by producing IFN-γ directly ex vivo, and display characteristics of effector memory T cells. This novel innate population of Mtb-reactive T cells will drive further investigation into the role of these cells in the containment of Mtb following infectious exposure. Furthermore, this is the first demonstration of a human innate pathogen-specific αβTCR⁺ T cell and is likely to inspire further investigation into innate T cells recognizing other important human pathogens.

Mycobacterium tuberculosis (Mtb) infects about one-third of the world's population. Most people who are exposed remain healthy, but control of this intracellular bacterium requires a robust cellular immune response. Production of the pro-inflammatory cytokine IFN-γ from cells in the adaptive immune response is critically important in the immune control of Mtb. However, this cytokine is also essential in initiating an optimal adaptive immune response. We hypothesized that innate cells could provide an early source of IFN-γ to aid in generation of an optimal adaptive immune response. We looked for IFN-γ producing cells in human neonates that were unlikely to have been previously exposed to either Mtb or other environmental mycobacteria. Here, we report the identification of a novel T cell population from the thymus that produces IFN-γ in response to Mtb-infected cells. Mtb-reactive thymocytes are present at high frequencies, are present in nearly all newborns tested, and display characteristics of T cells normally associated with a memory response. This novel innate population of Mtb-reactive cells will drive further investigation into the role of these cells in the containment of Mtb following infectious exposure and is likely to inspire further investigation into innate T cells recognizing other important human pathogens.

TGF-beta signaling in thymic epithelial cells regulates thymic involution and postirradiation reconstitution

The thymus constitutes the primary lymphoid organ responsible for the generation of naive T cells. Its stromal compartment is largely composed of a scaffold of different subsets of epithelial cells that provide soluble and membrane-bound molecules essential for thymocyte maturation and selection. With senescence, a steady decline in the thymic output of T cells has been observed. Numeric and qualitative changes in the stromal compartment of the thymus resulting in reduced thymopoietic capacity have been suggested to account for this physiologic process. The precise cellular and molecular mechanisms underlying thymic senescence are, however, only incompletely understood. Here, we demonstrate that TGF-beta signaling in thymic epithelial cells exerts a direct influence on the cell's capacity to support thymopoiesis in the aged mouse as the physiologic process of thymic senescence is mitigated in mice deficient for the expression of TGF-beta RII on thymic epithelial cells. Moreover, TGF-beta signaling in these stromal cells transiently hinders the early phase of thymic reconstitution after myeloablative conditioning and hematopoietic stem cell transplantation. Hence, inhibition of TGF-beta signaling decelerates the process of age-related thymic involution and may hasten the reconstitution of regular thymopoiesis after hematopoietic stem cell transplantation.