Lymphoid Periphery Research Articles

Cutting Edge: Defective Aerobic Glycolysis Defines the Distinct Effector Function in Antigen-Activated CD8+ Recent Thymic Emigrants.

Recent thymic emigrants (RTEs) are the youngest peripheral T cells that have completed thymic selection and egress to the lymphoid periphery. RTEs are functionally distinct from their more mature but still naive T cell counterparts, because they exhibit dampened proliferation and reduced cytokine production upon activation. In this article, we show that, compared with more mature but still naive T cells, RTEs are impaired in their ability to perform aerobic glycolysis following activation. Impaired metabolism underlies the reduced IFN-γ production observed in activated RTEs. This failure to undergo Ag-induced aerobic glycolysis is caused by reduced mTORC1 activity and diminished Myc induction in RTEs. Critically, exogenous IL-2 restores Myc expression in RTEs, driving aerobic glycolysis and IFN-γ production to the level of mature T cells. These results reveal a previously unknown metabolic component to postthymic T cell maturation.

Recent thymic emigrants are tolerized in the absence of inflammation.

T cell development requires a period of postthymic maturation. Why this is the case has remained a mystery, particularly given the rigors of intrathymic developmental checkpoints, successfully traversed by only ∼5% of thymocytes. We now show that the first few weeks of T cell residence in the lymphoid periphery define a period of heightened susceptibility to tolerance induction to tissue-restricted antigens (TRAs), the outcome of which depends on the context in which recent thymic emigrants (RTEs) encounter antigen. After encounter with TRAs in the absence of inflammation, RTEs exhibited defects in proliferation, diminished cytokine production, elevated expression of anergy-associated genes, and diminished diabetogenicity. These properties were mirrored in vitro by enhanced RTE susceptibility to regulatory T cell-mediated suppression. In the presence of inflammation, RTEs and mature T cells were, in contrast, equally capable of inducing diabetes, proliferating, and producing cytokines. Thus, recirculating RTEs encounter TRAs during a transitional developmental stage that facilitates tolerance induction, but inflammation converts antigen-exposed, tolerance-prone RTEs into competent effector cells.

Impaired metabolic reprograming in antigen-activated recent thymic emigrants

Abstract Recent thymic emigrants (RTEs) are the youngest peripheral T cells that have completed thymic selection and egress to the lymphoid periphery. However, T cell maturation does not end in the thymus; rather, RTEs require 2 – 3 weeks in the lymphoid periphery to achieve functional maturity and enter the mature but naïve (MN) T cell pool. Following antigen encounter, RTEs are functionally distinct from MN T cells as they exhibit dampened proliferation, altered cytokine production, and increased expression of some anergy-associated genes. RTEs are a clinically important population as these cells contribute significantly to the T cell pool in neonates and adults recovering from HIV infection or chemotherapy. Here we show that, compared to MN T cells, RTEs are impaired in their ability to switch from oxidative phosphorylation to aerobic glycolysis following antigenic stimulation. This impaired metabolic reprograming contributes to the reduced proliferation and cytokine production observed in antigen-activated RTEs. This failure to undergo antigen-induced metabolic reprograming is associated with reduced expression of the transcription factor Myc. Critically, high levels of IL-2 are able to rescue the expression of Myc in RTEs resulting in proper metabolic reprograming that mimics their mature counterparts. These results suggest that an altered metabolic phenotype underlies the functional differences observed between RTEs and MN T cells and highlights a novel point for therapeutic intervention for neonates and individuals recovering from lymphoablation.

Commentary: Memory CD8(+) T cells colocalize with IL-7(+) stromal cells in bone marrow and rest in terms of proliferation and transcription.

Commentary: Memory CD8(+) T cells colocalize with IL-7(+) stromal cells in bone marrow and rest in terms of proliferation and transcription.

Tolerance induction in recent thymic emigrants (BA11P.121)

Abstract Recent thymic emigrants (RTEs) comprise the subset of peripheral T cells that have most recently undergone thymic maturation and egress to the lymphoid periphery. RTEs are an important lymphocyte population to study because they represent the majority of T cells in neonates and contribute to T cell diversity throughout life. Using RAG2p-GFP transgenic (Tg) mice to identify RTEs in the periphery, our lab has discovered that T cell development continues for several weeks after cells have left the thymus and entered the lymphoid periphery. We hypothesize that post-thymic maturation represents a period of heightened susceptibility to tolerance induction. We tested this notion using T cell receptor (TCR) Tg ovalbumin (OVA)-specific CD4 (OT-II) and CD8 (OT-I) RTE and MN T cells transferred into mice expressing a membrane-bound form of the OVA protein in the pancreatic islets and kidney proximal tubular cells (RIP-mOVA Tg). Following co-transfer of congenically marked OT-II and OT-I RTE and MN T cells to RIP-mOVA Tg mice, we found that, compared to mature T cells, both CD4 and CD8 RTEs proliferate, accumulate, and secrete less IL-2 and IFNg, but express elevated levels anergy-associated genes. Finally, both OT-II and OT-I RTEs were less diabetogenic than their mature T cell counterparts when transferred into RIP-mOVA adoptive hosts. These results further support our hypothesis that post-thymic maturation represents a period of heightened susceptibility to tolerance induction.

Recent Thymic Emigrants and Mature Naive T Cells Exhibit Differential DNA Methylation at Key Cytokine Loci

Recent thymic emigrants (RTEs) are the youngest T cells in the lymphoid periphery and exhibit phenotypic and functional characteristics distinct from those of their more mature counterparts in the naive peripheral T cell pool. We show in this study that the Il2 and Il4 promoter regions of naive CD4(+) RTEs are characterized by site-specific hypermethylation compared with those of both mature naive (MN) T cells and the thymocyte precursors of RTEs. Thus, RTEs do not merely occupy a midpoint between the thymus and the mature T cell pool, but represent a distinct transitional T cell population. Furthermore, RTEs and MN T cells exhibit distinct CpG DNA methylation patterns both before and after activation. Compared with MN T cells, RTEs express higher levels of several enzymes that modify DNA methylation, and inhibiting methylation during culture allows RTEs to reach MN T cell levels of cytokine production. Collectively, these data suggest that the functional differences that distinguish RTEs from MN T cells are influenced by epigenetic mechanisms and provide clues to a mechanistic basis for postthymic maturation.

IL-10 transcription is negatively regulated by BAF180, a component of the SWI/SNF chromatin remodeling enzyme

BackgroundSWI/SNF chromatin remodeling enzymes play a critical role in the development of T helper lymphocytes, including Th2 cells, and directly program chromatin structure at Th2 cytokine genes. Different versions of SWI/SNF complexes, including BAF and PBAF, have been described based on unique subunit composition. However, the relative role of BAF and PBAF in Th cell function and cytokine expression has not been reported.ResultsHere we examine the role of the PBAF SWI/SNF complex in Th cell development and gene expression using mice deficient for a PBAF-specific component, BAF180. We find that T cell development in the thymus and lymphoid periphery is largely normal when the BAF180 gene is deleted late in thymic development. However, BAF180-deficient Th2 cells express high levels of the immunoregulatory cytokine IL-10. BAF180 binds directly to regulatory elements in the Il-10 locus but is replaced by BAF250 BAF complexes in the absence of BAF180, resulting in increased histone acetylation and CBP recruitment to the IL-10 locus.ConclusionsThese results demonstrate that BAF180 is a repressor of IL-10 transcription in Th2 cells and suggest that the differential recruitment of different SWI/SNF subtypes can have direct consequences on chromatin structure and gene transcription.

Homeostatic signals do not drive post-thymic T cell maturation

Recent thymic emigrants, the youngest T cells in the lymphoid periphery, undergo a 3week-long period of functional and phenotypic maturation before being incorporated into the pool of mature, naïve T cells. Previous studies indicate that this maturation requires T cell exit from the thymus and access to secondary lymphoid organs, but is MHC-independent. We now show that post-thymic T cell maturation is independent of homeostatic and costimulatory pathways, requiring neither signals delivered by IL-7 nor CD80/86. Furthermore, while CCR7/CCL19,21-regulated homing of recent thymic emigrants to the T cell zones within the secondary lymphoid organs is not required for post-thymic T cell maturation, an intact dendritic cell compartment modulates this process. It is thus clear that, unlike T cell development and homeostasis, post-thymic maturation is focused not on interrogating the T cell receptor or the cell’s responsiveness to homeostatic or costimulatory signals, but on some as yet unrecognized property.

MHC Drives TCR Repertoire Shaping, but not Maturation, in Recent Thymic Emigrants

After developing in the thymus, recent thymic emigrants (RTEs) enter the lymphoid periphery and undergo a maturation process as they transition into the mature naive (MN) T cell compartment. This maturation presumably shapes RTEs into a pool of T cells best fit to function robustly in the periphery without causing autoimmunity; however, the mechanism and consequences of this maturation process remain unknown. Using a transgenic mouse system that specifically labels RTEs, we tested the influence of MHC molecules, key drivers of intrathymic T cell selection and naive peripheral T cell homeostasis, in shaping the RTE pool in the lymphoid periphery. We found that the TCRs expressed by RTEs are skewed to longer CDR3 regions compared with those of MN T cells, suggesting that MHC does streamline the TCR repertoire of T cells as they transition from the RTE to the MN T cell stage. This conclusion is borne out in studies in which the representation of individual TCRs was followed as a function of time since thymic egress. Surprisingly, we found that MHC is dispensable for the phenotypic and functional maturation of RTEs.

Post-thymic T cells undergo maturation in secondary lymphoid organs (82.11)

Abstract T cell development was traditionally thought to occur in its entirety in the thymus, giving rise to fully developed naïve T cells ready to patrol the lymphoid periphery. However, characterization of T cells that have recently left the thymus was hampered by their lack of a unique cell surface marker. With the aid of new murine model in which those recent thymic emigrants (RTEs) are specifically labeled, RTEs been shown to undergo further changes in the lymphoid periphery prior to gaining full immunocompetence. With this model, our lab has demonstrated that RTEs are functionally dampened and phenotypically immature relative to the rest of the mature naïve T cell pool. To aid in understanding the significance of this continued peripheral maturation, the mechanism driving this process is under investigation. We have found that exit from the thymus is required for full RTE maturation, and that this maturation process occurs progressively. We have also found that RTE maturation takes place within secondary lymphoid organs. In secondary lymphoid organs, IL-7 and MHC, homeostatic factors for naïve T cells, are dispensable for maturation, while dendritic cells do influence the maturation process. This work suggests that the process of RTE maturation is a dynamic process that is actively regulated by factors contained within the lymphoid periphery. This research was supported by grants from the Cancer Research Institute and the National Institutes of Health.

Uneven colonization of the lymphoid periphery by T cells that undergo early TCR{alpha} rearrangements.

A sparse population of thymocytes undergoes TCRalpha gene rearrangement early in development, before the double-positive stage. The potential of these cells to contribute to the peripheral T cell pool is unknown. To examine the peripheral T cell compartment expressing a repertoire biased to early TCR gene rearrangements, we developed a mouse model in which TCRalpha rearrangements are restricted to the double-negative stage of thymocyte development. These mice carry floxed RAG2 alleles and a Cre transgene driven by the CD4 promoter. As expected, conventional T cell development is compromised in such Cre(+) RAG2(fl/fl) mice, and the TCRalphabeta(+) T cells that develop are limited in their TCRalpha repertoire, preferentially using early rearranging Valpha genes. In the gut, the Thy-1(+)TCRalphabeta(+) intraepithelial lymphocyte (IEL) compartment is surprisingly intact, whereas the Thy-1(-)TCRalphabeta(+) subset is almost completely absent. Thus, T cells expressing a TCRalpha repertoire that is the product of early gene rearrangements can preferentially populate distinct IEL compartments. Despite this capacity, Cre(+) RAG2(fl/fl) T cell progenitors cannot compete with wild-type T cell progenitors in mixed bone marrow chimeras, suggesting that in normal mice, there is only a small contribution to the peripheral T cell pool by cells that have undergone early TCRalpha rearrangements. In the absence of wild-type competitors, aggressive homeostatic proliferation in the IEL compartment can promote a relatively normal Thy-1(+) TCRalphabeta(+) T cell pool from the limited population derived from Cre(+) RAG2(fl/fl) progenitors.

Cutting Edge: Contact with Secondary Lymphoid Organs Drives Postthymic T Cell Maturation

T cell development, originally thought to be completed in the thymus, has recently been shown to continue for several weeks in the lymphoid periphery. The forces that drive this peripheral maturation are unclear. The use of mice transgenic for GFP driven by the RAG2 promoter has enabled the ready identification and analysis of recent thymic emigrants. Here, we show that recent thymic emigrant maturation is a progressive process and is promoted by T cell exit from the thymus. Further, we show that this maturation occurs within secondary lymphoid organs and does not require extensive lymphocyte recirculation.

Thymic output in aged mice

Using GFP to mark recent thymic emigrants (RTEs) in mice carrying a GFP transgene driven by the recombination-activating gene 2 promoter, we demonstrate that RTEs are readily detectable even in 2-year-old mice, despite the fact that the proportion of the peripheral T cell pool comprised of RTEs declines with age. Although the number of RTEs decreases after reaching a peak at 6 weeks of age, thymic output as a function of thymic size is surprisingly age-independent. The CD4:CD8 ratio of RTEs declines with age, partly because of a striking decrease in steady-state proliferation of CD4+ RTEs in older mice. RTEs in aged mice undergo phenotypic maturation in the lymphoid periphery with delayed kinetics compared with young mice. RTEs from aged mice secrete less IL-2, proliferate less well, and achieve only weak expression of early-activation markers compared with more mature naïve peripheral T cells from the same mice. The proportion of GFP- cells in the CD4+ and CD8+ thymic compartments increases with age, partly as a result of leakiness in the aged thymus, allowing reentry of naïve peripheral T cells.

Continued maturation of thymic emigrants in the periphery.

Developing thymocytes are selected for recognition of molecules encoded by the major histocompatibility complex, purged of self-reactive cells and committed to either the CD4 or CD8 lineage. The 1% of thymocytes that complete these tasks emigrate and join the population of peripheral lymphocytes. Whether T cell maturation is complete at the time of thymic exit has been a subject of debate. Using mice transgenic for green fluorescent protein driven by the recombination activating gene 2 promoter to identify recent thymic emigrants, we now show that T cell differentiation continues post-thymically, with progressive maturation of both surface phenotype and immune function. In addition, the relative contribution of CD4 and CD8 recent thymic emigrants was modulated as they entered the peripheral T cell pool. Thus, T cell maturation and subset contribution are both finalized in the lymphoid periphery.

Memory T-cell competition for bone marrow seeding.

The presence in the bone marrow of memory CD8 T cells is well recognized. However, it is still largely unclear how T-cell migration from the lymphoid periphery to the bone marrow is regulated. In the present report, we show that antigen-specific CD4 T cells, as well as antigen-specific CD8 T cells, localize to the bone marrow of immunized mice, and are sustained there over long periods of time. To investigate the rules governing T-cell migration to the bone marrow, we generated chimeric mice in which the lymphoid periphery contained two genetically or phenotypically distinct groups of T cells, one of which was identical to the host. We then examined whether a distinct type of T cell had an advantage over the others in the colonization of bone marrow. Our results show that whereas ICAM1 and CD18 molecules are both involved in homing to lymph nodes, neither is crucial for T-cell bone marrow colonization. We also observed that memory-phenotype CD44high T cells, but not virgin-type CD44−/low T cells, preferentially home to the bone marrow upon adoptive transfer to normal young mice, but not to thymectomized old recipients where an existing memory T-cell pool precludes their free access. Thus, T-cell colonization of the bone marrow uses distinct molecules from those implicated in lymph node homing, and is regulated both by the properties of the T cell and by the competitive efficacy of other T cells inhabiting the same, saturable niche. This implies that the homing potential of an individual lymphocyte is not merely an intrinsic property of the cell, but rather a property of the lymphoid system taken as a whole.

Bone marrow CD8 T cells are in a different activation state than those in lymphoid periphery.

In addition to its well-established use for hematopoiesis reconstitution, bone marrow is considered with increasing interest as a possible source of mature cells for adoptive therapies, in particular for the immunotherapy of cancer. Nevertheless, the peculiarities of bone marrow T cells in comparison with those in lymphoid periphery are still largely unknown. In this report, we show for the first time that bone marrow CD8 T cells are in a different activation state than those in peripheral lymphoid organs. Firstly, we observed that mouse bone marrow contains a significantly higher percentage of blasts within the CD8 T cells than either spleen or lymph nodes, yet such enrichment is not due to recent antigenic stimulation. Secondly, when we challenged bone marrow CD8 T cells from immunized mice with their antigen in vitro, they displayed a faster response than those from the spleen. Thus, we suggest that the bone marrow could be a preferential source of CD8 T cells for adoptive therapies in those cases in which highly active effectors are required.

Receptor revision in peripheral T cells creates a diverse V beta repertoire.

In Vbeta5 transgenic mice, the age-dependent accumulation of Vbeta5(-)CD4(+) T cells expressing endogenous Vss elements represents an exception to the rule of strict allelic exclusion at the TCRbeta locus. The appearance of these cells is limited to the lymphoid periphery and is driven by a peripherally expressed tolerogen. Expression of the lymphoid-specific components of the recombinase machinery and the presence of recombination intermediates strongly suggest that TCR revision rescues tolerogen-reactive peripheral T cells from deletion. Here, we report that the appearance of Vbeta5(-)CD4(+) T cells is CD28-dependent. In addition, we find that the TCR repertoire of this unusual population of T cells in individual Vbeta5 transgenic mice is surprisingly diverse, both at the level of surface protein and at the nucleotide level within a given family of V(D)Jbeta rearrangements. This faithful recreation of the nontransgenic repertoire suggests that endogenous Vbeta-expressing populations do not arise from expansion of an initially rare subset. Furthermore, the undersized N regions in revised TCR genes distinguish these sequences from those generated in the adult thymus. The diversity of the revised TCRs, the minimal mouse-to-mouse variation in the expressed endogenous Vbeta repertoire, the atypical length of junctional sequences, and the CD28 dependence of the accumulation of Vbeta5(-)CD4(+) T cells all point to their extrathymic origin. Thus, tolerogen-driven receptor revision in peripheral T cells can expand the TCR repertoire extrathymically, thereby contributing to the flexibility of the immune repertoire.

Anergic CD8+ T Cells Can Persist and Function In Vivo

Using a mouse model system, we demonstrate that anergic CD8+ T cells can persist and retain some functional capabilities in vivo, even after the induction of tolerance. In TCR Vbeta5 transgenic mice, mature CD8+Vbeta5+ T cells transit through a CD8lowVbeta5low deletional intermediate during tolerance induction. CD8low cells are characterized by an activated phenotype, are functionally compromised in vitro, and are slated for deletion in vivo. We now demonstrate that CD8low cells derive from a proliferative compartment, but do not divide in vivo. CD8low cells persist in vivo with a t1/2 of 3-5 days, in contrast to their in vitro t1/2 of 0.5-1 day. During this unexpectedly long in vivo life span, CD8low cells are capable of producing IFN-gamma in vivo despite their inability to proliferate or to kill target cells in vitro. CD8low cells also accumulate at sites of inflammation, where they produce IFN-gamma. Therefore, rather than withdrawing from the pool of functional CD8+ T cells, anergic CD8low cells retain a potential regulatory role despite losing their capacity to proliferate. The ability of anergic cells to persist and function in vivo adds another level of complexity to the process of tolerance induction in the lymphoid periphery.

Chronic Modulation of the TCR Repertoire in the Lymphoid Periphery

Using TCR V beta 5 transgenic mice as a model system, we demonstrate that the induction of peripheral tolerance can mold the TCR repertoire throughout adult life. In these mice, three distinct populations of peripheral T cells are affected by chronic selective events in the lymphoid periphery. First, CD4+V beta 5+ T cells are deleted in the lymphoid periphery by superantigens encoded by mouse mammary tumor viruses-8 and -9 in an MHC class II-dependent manner. Second, mature CD8+V beta 5+ T cells transit through a CD8lowV beta 5low deletional intermediate during tolerance induction by a process that depends upon neither mouse mammary tumor virus-encoded superantigens nor MHC class II expression. Third, a population of CD4-CD8-V beta 5+ T cells arises in the lymphoid periphery in an age-dependent manner. We analyzed the TCR V alpha repertoire of each of these cellular compartments in both V beta 5 transgenic and nontransgenic C57BL/6 mice as a function of age. This analysis revealed age-related changes in the expression of V alpha families among different cellular compartments, highlighting the dynamic state of the peripheral immune repertoire. Our work indicates that the chronic processes maintaining peripheral T cell tolerance can dramatically shape the available TCR repertoire.

The induction of peripheral tolerance by the chronic activation and deletion of CD4+V beta 5+ cells.

In C57BL/6 mice transgenic for a rearranged gene encoding a V beta 5+ beta-chain of the TCR, transgene expression among CD4+ cells decreases with age, such that approximately 40% of CD4+ cells express an endogenous beta-chain gene in 8-mo-old mice. A similar deletion of V beta 5+ cells is observed among CD4+ cells from nontransgenic littermates. V beta 5+ T cells are deleted intrathymically in I-E+Mtv-9+ strains of mice, but this chronic deletion occurs in the lymphoid periphery, in the absence of I-E. We now demonstrate the increased expression of the activation markers CD44 and VLA-4 among CD4+V beta 5+ cells, in the absence of either an increase in size or IL-2 receptor expression. Functional as well as phenotypic differences distinguish CD4+ from CD8+ cells in older V beta 5+ transgenic mice. Relative to their CD8+ counterparts, CD4+V beta 5+ cells are hyporesponsive to plate-bound anti-V beta 5 Abs, and this anergy is partially reversible by the addition of exogenous IL-2. These data suggest the deletion of CD4+V beta 5+ cells is the result of a process that includes their activation, loss of function, and their eventual removal. To investigate the involvement of the principal V beta 5 superantigen Mtv-9 in this chronic deletion, we have derived several lines of V beta 5+I-E-Mtv-9- mice. Transgene expression also declines with age in CD4+ T cells in these mice, clearly demonstrating that the chronic deletion of CD4+V beta 5+ cells does not require Mtv-9. There is considerable variation in the kinetics and efficiency of CD4+V beta 5+ deletion between lines of Mtv-9- transgenic mice that is not from differences in the profiles of endogenous mammary tumor proviruses nor readily explained by environmental differences that influence proviral expression. These results suggest the existence of genetic factors other than mammary tumor proviruses that influence the deletion of CD4+V beta 5+ cells in the absence of I-E.