CD8SP Thymocytes Research Articles

Human CD4 T Cells From Thymus and Cord Blood Are Convertible Into CD8 T Cells by IL-4.

Commitment to the CD4+ or CD8+ T cell lineages is linked to the acquisition of a functional program broadly defined by helper and cytotoxic properties, respectively. The mechanisms underlying these processes in the human thymus remain largely unclear. Moreover, recent thymic emigrants are thought to have some degree of plasticity, which may be important for the shaping of the immune system and adjustment to specific peripheral needs. We show here that IL-4 induces proliferation-independent de novo synthesis of CD8αβ in human CD4 single-positive (SP) thymocytes, generating a stable CD8SP population that features a diverse TCRαβ repertoire, CD4 expression shut-down and ThPOK downregulation. IL-4 also promotes an innate-like program in both CD4SP and CD8SP thymocytes, characterized by Eomes upregulation in the absence of T-bet, in line with its recognized role in the generation of thymic innate-like CD8+ T cells. The clinical relevance of these findings is further supported by the profile of IL-4 production and IL-4 receptor expression that we identified in the human thymus. Importantly, human cord blood CD4+ T cells preserve the ability to generate Eomes+ CD8+ T cells in the presence of IL-4, with implications in neonatal immunity. Our results support a role for IL-4 in the dynamic regulation of human thymocyte plasticity and identify novel strategies to modulate immune responses.

VSIG4(+) peritoneal macrophages induce apoptosis of double-positive thymocyte via the secretion of TNF-\u03b1 in a CLP-induced sepsis model resulting in thymic atrophy

Thymic atrophy in sepsis is a critical disadvantage because it induces immunosuppression and increases the mortality rate as the disease progresses. However, the exact mechanism of thymic atrophy has not been fully elucidated. In this study, we discovered a novel role for VSIG4-positive peritoneal macrophages (V4(+) cells) as the principal cells that induce thymic atrophy and thymocyte apoptosis. In CLP-induced mice, V4(+) cells were activated after ingestion of invading microbes, and the majority of these cells migrated into the thymus. Furthermore, these cells underwent a phenotypic shift from V4(+) to V4(−) and from MHC II(low) to MHC II(+). In coculture with thymocytes, V4(+) cells mainly induced apoptosis in DP thymocytes via the secretion of TNF-α. However, there was little effect on CD4 or CD8 SP and DN thymocytes. V4(−) cells showed low levels of activity compared to V4(+) cells. Thymic atrophy in CLP-induced V4(KO) mice was much less severe than that in CLP-induced wild-type mice. In addition, V4(KO) peritoneal macrophages also showed similar activity to V4(−) cells. Taken together, the current study demonstrates that V4(+) cells play important roles in inducing immunosuppression via thymic atrophy in the context of severe infection. These data also suggest that controlling the function of V4(+) cells may play a crucial role in the development of new therapies to prevent thymocyte apoptosis in sepsis.

Cbl-b deficiency prevents functional but not phenotypic T cell anergy

T cell anergy is an important peripheral tolerance mechanism. We studied how T cell anergy is established using an anergy model in which the Zap70 hypermorphic mutant W131A is coexpressed with the OTII TCR transgene (W131AOTII). Anergy was established in the periphery, not in the thymus. Contrary to enriched tolerance gene signatures and impaired TCR signaling in mature peripheral CD4 T cells, CD4SP thymocytes exhibited normal TCR signaling in W131AOTII mice. Importantly, the maintenance of T cell anergy in W131AOTII mice required antigen presentation via MHC-II. We investigated the functional importance of the inhibitory receptor PD-1 and the E3 ubiquitin ligases Cbl-b and Grail in this model. Deletion of each did not affect expression of phenotypic markers of anergic T cells or T reg numbers. However, deletion of Cbl-b, but not Grail or PD-1, in W131AOTII mice restored T cell responsiveness and signaling. Thus, Cbl-b plays an essential role in the establishment and/or maintenance of unresponsiveness in T cell anergy.

Developing a model of thymic regulatory T cell development with single-cell RNA sequencing

Abstract CD4SP thymocytes that receive moderate to strong TCR signals upregulate either FOXP3 or CD25, generating distinct FOXP3lo or CD25+ Treg progenitor (TRP) populations, respectively. Stimulation of these TRP populations with IL-2 induces co-expression of CD25 and FOXP3, completing thymic Treg differentiation. The alterations in gene expression that drive these respective Treg differentiation programs, as well as the identification of agonist selected cells within conventional CD4SP thymocytes remain unclear. To address these questions, we performed single-cell RNA-Seq with paired TCR sequencing of conventional and Treg lineage thymocytes, including recirculating Tregs. Interestingly, we found that IL-2 was exclusively produced within the most immature CD25+ TRP undergoing agonist selection. TCR analysis demonstrated that some TCRs were enriched in CD4SP receiving agonist stimulation that gave rise to Tregs, or were subjected to negative selection, while others were excluded from this pathway. Finally, we found several distinct subsets of recirculating Tregs, likely pointing to a variety of functions these cells play in thymic homeostasis and function. We are currently carrying out spatial transcriptome studies to identify the specific localization of distinct Treg intermediates within the thymus. We are using these combined single cell data sets to generate a comprehensive model of thymic Treg development.

Regulatory T cells are less sensitive to glucocorticoid hormone induced apoptosis than CD4+ T cells

Earlier we have reported that thymic regulatory T cells (Treg) are resistant to in vivo glucocorticoid hormone (GC)-induced apoptosis, while the most GC-sensitive DP thymocytes died through the activation of mitochondrial apoptotic pathway. Here we analyzed the apoptosis-inducing effect of high dose (10–6 M) in vitro dexamethasone (DX) treatment in mouse thymic- and splenic Tregs and CD4+ T cells. Activation of both extrinsic and intrinsic apoptotic pathways started after 2 h of DX treatment in CD4 SP thymocytes and was 3 × higher than in CD4+ splenocytes, while in Treg cells, weak activation of the extrinsic apoptotic pathway started only after 3 h. We also investigated the expression of 21 apoptosis-related molecules using a protein array and found higher level of both pro-and anti-apoptotic molecules in Tregs compared to CD4+ T cells. 4 h in vitro DX treatment induced upregulation of most apoptosis-related molecules both in Tregs and CD4+ T cells, except for the decrease of Bcl-2 expression in CD4+ T cells. We found high basal cytosolic Ca2+ levels in untreated Treg cells, which further increased after DX treatment, while the specific TCR-induced Ca2+ signal was lower in Tregs than in CD4+ T cells. Our results suggest that in the background of the relative apoptosis resistance of Treg cells to GCs might be their high basal cytosolic Ca2+ level and upregulated Bcl-2 expression. In contrast, downregulation of Bcl-2 expression in CD4+ T cells can explain their higher, DX-induced apoptosis sensitivity.

Visualizing thymic regulatory T cell development with single-cell RNA sequencing

Abstract CD4SP thymocytes that receive moderate to strong TCR signals upregulate either FOXP3 or CD25, generating distinct Foxp3lo or CD25+ Treg progenitor (TRP) populations, respectively. Stimulation of these TRP populations with IL2 induces co-expression of CD25 and FOXP3, completing thymic Treg differentiation. The alterations in gene expression that drive these respective Treg differentiation programs, as well as the identification of agonist selected cells within conventional CD4SP thymocytes remain unclear. To address these questions, we performed single-cell RNA-Seq with paired TCR sequencing of conventional and Treg lineage CD4SP thymocytes. This data set contained cells of the entire range of maturity, from recent DP-CD4SP converts to mature CD4 T cells competent for thymic export. In conventional CD4SPs we were able to identify a subset of cells undergoing agonist selection that differentiates into TRP cells along with cells undergoing negative selection. The transcriptomes of the subsequent CD25+ and FOXP3lo TRP subsets were clearly distinct. For example, we found that the source of IL2 required for conversion of both TRP subsets into mature Tregs was uniquely found within the most immature CD25+ TRP undergoing agonist selection. Finally, TCR analysis demonstrated that some TCRs were enriched in CD4SP receiving agonist stimulation that gave rise to Tregs, or undergoing negative selection, while others were excluded from this pathway. We are currently carrying out spatial transcriptome studies to identify the specific localization of distinct Treg intermediates within the thymus. Interrogation of these single cell and spatial transcriptomic data sets will generate a comprehensive model of thymic Treg development.

The phosphatidinositol-transfer protein Nir3 is a modulator of T cell development

Abstract Phosphatidylinositol (PI) 4,5-bisphosphate (PIP2) functions as a critical source of second messengers implicated in T cell activation. In T cells, hydrolysis of PIP2 by the lipid phosphatase PLCγ1 generates the second messengers diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3), resulting in PKC and Ras/MAPK activation as well as calcium increases that contribute to T cell responses. A secondary consequence of PLCγ1 activation is a transient reduction in PIP2 levels. The PI transfer proteins Nir2 and Nir3 have been shown to replenish PIP2 levels at the PM through delivery of the precursor PI from the endoplasmic reticulum (ER) via non-vesicular transport. In particular, Nir3 has been shown to replenish PI levels during basal signaling and in response to weak signaling. Given the importance of weak signaling in thymocyte positive selection, as well as the elevated levels of Nir3 mRNA during the double positive (DP) stage of thymocytes development (Immgen), we hypothesize that Nir3 is a regulator of PI homeostasis during thymocyte development. Using CRISPR-Cas9 technology, we have generated a Nir3 knockout mouse. DP thymocytes from Nir3 knockout mice have reduced basal calcium levels, as well as reduced TCR-induced calcium flux. Furthermore, introduction of the OT1 transgenic TCR into Nir3 knockout mice resulted in elevated numbers of DP thymocytes and reduced numbers of CD8SP thymocytes, suggesting a block in positive selection. Analysis of TCR signaling in Nir3 knockout OT1 thymocytes revealed attenuated calcium flux as well as defective induction of pERK, pAkt, and pS6. These findings are the first to characterize the function of PI transfer proteins and their importance specifically in T cell development.

Differential Requirement of Cd8 Enhancers E8I and E8VI in Cytotoxic Lineage T Cells and in Intestinal Intraepithelial Lymphocytes.

CD8 expression in T lymphocytes is tightly regulated by the activity of at least six Cd8 enhancers (E8I-E8VI), however their complex developmental stage-, subset-, and lineage-specific interplays are incompletely understood. Here we analyzed ATAC-seq data on the Immunological Genome Project database and identified a similar developmental regulation of chromatin accessibility of a subregion of E8I, designated E8I-core, and of E8VI. Loss of E8I-core led to a similar reduction in CD8 expression in naïve CD8+ T cells and in IELs as observed in E8I−/− mice, demonstrating that we identified the core enhancer region of E8I. While E8VI−/− mice displayed a mild reduction in CD8 expression levels on CD8SP thymocytes and peripheral CD8+ T cells, CD8 levels were further reduced upon combined deletion of E8I-core and E8VI. Moreover, activated E8I-core−/−E8VI−/− CD8+ T cells lost CD8 expression to a greater degree than E8I-core−/− and E8VI−/− CD8+ T cells, suggesting that the combined activity of both enhancers is required for establishment and maintenance of CD8 expression before and after TCR activation. Finally, we observed a severe reduction of CD4 CTLs among the TCRβ+CD4+ IEL population in E8I-core−/− but not E8VI−/− mice. Such a reduction was not observed in Cd8a−/− mice, indicating that E8I-core controls the generation of CD4 CTLs independently of its role in Cd8a gene regulation. Further, the combined deletion of E8I-core and E8VI restored CD4 CTL subsets, suggesting an antagonistic function of E8VI in the generation of CD4 CTLs. Together, our study demonstrates a complex utilization and interplay of E8I-core and E8VI in regulating CD8 expression in cytotoxic lineage T cells and in IELs. Moreover, we revealed a novel E8I-mediated regulatory mechanism controlling the generation of intestinal CD4 CTLs.

HDAC3 restrains CD8-lineage genes to maintain a bi-potential state in CD4+CD8+ thymocytes for CD4-lineage commitment.

CD4 and CD8 T cells are vital components of the immune system. We found that histone deacetylase 3 (HDAC3) is critical for the development of CD4 T cells, as HDAC3-deficient DP thymocytes generate only CD8SP thymocytes in mice. In the absence of HDAC3, MHC Class II-restricted OT-II thymocytes are redirected to the CD8 cytotoxic lineage, which occurs with accelerated kinetics. Analysis of histone acetylation and RNA-seq reveals that HDAC3-deficient DP thymocytes are biased towards the CD8 lineage prior to positive selection. Commitment to the CD4 or CD8 lineage is determined by whether persistent TCR signaling or cytokine signaling predominates, respectively. Despite elevated IL-21R/γc/STAT5 signaling in HDAC3-deficient DP thymocytes, blocking IL-21R does not restore CD4 lineage commitment. Instead, HDAC3 binds directly to CD8-lineage promoting genes. Thus, HDAC3 is required to restrain CD8-lineage genes in DP thymocytes for the generation of CD4 T cells.

Live-Cell FRET Imaging Reveals a Role of ERK Activity Dynamics in Thymocyte Motility

Thymocyte motility is key to orchestrating migration between different thymic microenvironments at the appropriate developmental stage. Several lines of evidence suggest that extracellular signal-regulated kinase (ERK) signaling pathways plays critical roles in T cell development. Nevertheless, the dynamics of ERK activity and its role in regulating cell motility remain largely unknown due to technical difficulties. Therefore, there is an increasing demand for genetic reporter systems that provide the information of thymocyte motility and ERK signaling under physiological condition in real time.To meet the demand, we developed transgenic mice expressing fluorescence resonance energy transfer (FRET) based biosensor for ERK incorporated into the ROSA26 locus conditionally. EKAREV is a genetically encoded intramolecular FRET biosensor for monitoring ERK activity in living cells. We introduced cDNAs of EKAREV into the ROSA26 locus to generate mouse lines named R26-EKAREV-NLSfl/+. In these mouse lines, EKAREV will be expressed in the nucleus after Cre-mediated excision of the loxP-flanked tdKeima-coding sequence. To study ERK activity dynamics in T cells, R26-EKAREV-NLSfl/+ were crossed with Lck-Cre mice to generate R26-EKAREV-NLSlck/+. R26-EKAREV-NLSlck/+ showed uniform and high-level expression of EKAREV in lymphocytes and allowed us to examine the ERK activity of T cells in vivo.After initial characterization of R26-EKAREV-NLSlck/+, we attempted to unravel the potential crosstalk between ERK activity dynamics and cell motility within the thymic microenvironment. Long-term in vivo imaging of thymus was difficult due to the anatomical location abutting the heart. To circumvent this problem, thymocytes obtained from R26-EKAREV-NLSlck/+ were overlaid on C57BL/6 (WT) thymic slices and observed under two-photon excitation microscopy. During the course of experiments, we noticed a negative correlation between ERK activity and migration speed of thymocytes. To confirm this observation, thymocytes were cultured in the presence or absence of an mitogen-activated protein kinase/ERK kinase (MEK) inhibitor prior to transfer onto thymic slices. Treatment of thymocytes with MEK inhibitor increased averaged migration speed in both double-positive (DP) and single-positive (SP) subsets. Moreover, time-lapse imaging of each subsets of thymocytes revealed that the deviation of ERK activity from the average of individual cells regulated cell motility in CD4-SP, while the absolute value of ERK activity regulated cell motility in DP and CD8-SP. Collectively, these results suggest that CD4-SP is unique in that the ERK activity dynamics negatively regulate cell motility.Which signal regulates ERK activity and cell motility of CD4-SP in the medulla? Interaction between T cell receptor (TCR) and major histocompatibility complex (MHC) is known to regulate ERK activity and cell motility of T cells, but their direct relationship remains unknown in tissues. To examine this possibility, CD4-SP thymocytes were overlaid onto WT or MHC class II knockout (KO) thymic slices. When overlaid onto KO thymic slices, the average migration speed was significantly increased, indicating that CD4-SP cells arrest upon TCR-MHC II interaction. Moreover, the variance of ERK activity, but not the average of it, in CD4-SP cells on KO thymic slices was decreased, suggesting that TCR-MHC II interaction contribute the variance of ERK activity in CD4-SP.Our findings unravel that the deviation of ERK activity induced by TCR-MHC interactions negatively regulate cell motility of CD4-SP in the medulla. The live-cell FRET imaging of ERK activity will open a new window to understand the dynamic nature and the diverse functions of ERK signaling in T cell biology. DisclosuresTakaori-Kondo:Janssen Pharmaceuticals: Honoraria; Bristol-Myers Squibb: Honoraria; Novartis: Honoraria; Pfizer: Honoraria; Celgene: Honoraria, Research Funding.

Two-photon imaging reveals distinct contributions of Aire+ medullary thymic epithelial cells and dendritic cell subsets to central tolerance induction

Abstract Following positive selection, thymocytes migrate into the medulla where they encounter diverse tissue-restricted antigens (TRAs) that induce central tolerance. Both medullary thymic epithelial cells (mTECs) and dendritic cells (DCs) present TRAs to induce thymocyte negative selection and regulatory T cell induction. However, their relative contributions to tolerance remain unresolved, partially because the genetic models that have been used to address this question can interfere with thymocyte-stromal cell crosstalk, potentially resulting in secondary defects in stromal differentiation that independently alter central tolerance. Thus, we developed a 2-photon microscopy approach to directly observe thymocytes as they undergo tolerance induction in response to interactions with DCs versus Aire+ mTECs presenting various forms of a TRA in an intact thymic stromal environment. We find that both mTECs and DCs present transmembrane TRAs with equivalent efficiency to CD8SP and CD4SP thymocytes. However, in the context of a secreted TRA, DCs play a greater role than mTECs in presenting antigen to CD4SP, but not CD8SP thymocytes. Furthermore, distinct DC subsets differentially contribute to TRA presentation to CD4SP versus CD8SP thymocytes, with a major role for Sirpα+DCs in cross-presentation of TRAs. In the context of a polyclonal T cell repertoire responding to endogenous antigens, DCs play a more significant role than mTECs in activating CD4SP cells. In summary, we find that in an intact thymic microenvironment, multiple DC subsets and Aire+ mTECs cooperate to present self-antigens to auto-reactive thymocytes during the establishment of tolerance against diverse medullary TRAs.

Revisiting the role of B7-CD28 co-stimulation in thymic negative selection: B7-CD28 co-stimulation enforces central tolerance by clonal deletion and Treg generation as independent cellular processes

Abstract Self-Ag-reactive T cells are eliminated from the conventional mature T cell repertoire by clonal deletion and/or fate diversion to the Treg during thymocyte development as a consequence of strong recognition of self-peptide-MHC complex by the TCR, a process termed negative selection. However, the role of the B7-CD28 costimulatory pathway in this process has not been fully elucidated. We analyzed the role of B7-CD28 interaction in thymic negative selection using pMHCII tetramer binding to directly measure the endogenous T cell repertoire specific for distinct self-Ags. Ablation of expression of a self-Ag or of the Aire gene resulted in increase in the number of self-peptide-specific CD4SP thymocytes, consistent with the physiologic process of ongoing negative selection. We found that absence of B7 also resulted in an increase in numbers of conventional CD4SP (non-Treg) thymocytes specific for some but not all self-Ags. Observed increases in conventional CD4SP were far in excess of the concomitant decrease in the number of Treg that occurred in the absence of B7. In addition, the identity of B7-expressing cell populations required for clonal deletion was distinct from the requirements for Treg generation. Thus, the increase of CD4SP cell number specific for self-Ag was largely due to defect in B7 dependent clonal deletion, but not simply conversion of “want-to-be Treg” to conventional CD4SP cells. We propose that 1) thymic clonal deletion occurs through B7-CD28 dependent and independent pathways; 2) B7-CD28 dependent negative selection is mediated by clonal deletion and/or fate diversion to the thymic Treg lineage; and 3) B7-CD28 dependent clonal deletion is a largely independent cellular process from B7-CD28 dependent Treg generation.

Global dynamics of stage-specific transcription factor binding during thymocyte development

In vertebrates, multiple transcription factors (TFs) bind to gene regulatory elements (promoters, enhancers, and silencers) to execute developmental expression changes. ChIP experiments are often used to identify where TFs bind to regulatory elements in the genome, but the requirement of TF-specific antibodies hampers analyses of tens of TFs at multiple loci. Here we tested whether TF binding predictions using ATAC-seq can be used to infer the identity of TFs that bind to functionally validated enhancers of the Cd4, Cd8, and Gata3 genes in thymocytes. We performed ATAC-seq at four distinct stages of development in mouse thymus, probing the chromatin accessibility landscape in double negative (DN), double positive (DP), CD4 single positive (SP4) and CD8 SP (SP8) thymocytes. Integration of chromatin accessibility with TF motifs genome-wide allowed us to infer stage-specific occupied TF binding sites within known and potentially novel regulatory elements. Our results provide genome-wide stage-specific T cell open chromatin profiles, and allow the identification of candidate TFs that drive thymocyte differentiation at each developmental stage.

The corepressor NCOR1 regulates the survival of single-positive thymocytes

Nuclear receptor corepressor 1 (NCOR1) is a transcriptional regulator bridging repressive chromatin modifying enzymes with transcription factors. NCOR1 regulates many biological processes, however its role in T cells is not known. Here we show that Cd4-Cre-mediated deletion of NCOR1 (NCOR1 cKOCd4) resulted in a reduction of peripheral T cell numbers due to a decrease in single-positive (SP) thymocytes. In contrast, double-positive (DP) thymocyte numbers were not affected in the absence of NCOR1. The reduction in SP cells was due to diminished survival of NCOR1-null postselection TCRβhiCD69+ and mature TCRβhiCD69− thymocytes. NCOR1-null thymocytes expressed elevated levels of the pro-apoptotic factor BIM and showed a higher fraction of cleaved caspase 3-positive cells upon TCR stimulation ex vivo. However, staphylococcal enterotoxin B (SEB)-mediated deletion of Vβ8+ CD4SP thymocytes was normal, suggesting that negative selection is not altered in the absence of NCOR1. Finally, transgenic expression of the pro-survival protein BCL2 restored the population of CD69+ thymocytes in NCOR1 cKOCd4 mice to a similar percentage as observed in WT mice. Together, these data identify NCOR1 as a crucial regulator of the survival of SP thymocytes and revealed that NCOR1 is essential for the proper generation of the peripheral T cell pool.

Development of functionally distinct Treg subsets in the thymus

Abstract Mature thymic regulatory T cells (Tregs) are defined by dual expression of Foxp3 and CD25 on CD4SP thymocytes. However, the cellular pathways by which CD4 thymocytes differentiate into Tregs remain incompletely understood. Previous reports demonstrated that Treg development occurred via a two-step process. In step one, strong TCR signaling induces expression of CD25 leading to a CD25+ Foxp3− Treg progenitor (TRP). In step two, exposure of this TRP to IL2 results in conversion to a CD25+Foxp3+ Treg. Work from the Singer lab has defined a CD25−Foxp3lo TRP that can also be converted by IL2 into CD25+Foxp3+ Tregs. A key question is why two distinct cellular pathways involving distinct TRP populations are needed to reach the same endpoint. Using a combination of maturation marker analysis and RAG2-GFP reporter mice we observed that CD25+Foxp3− TRP arise more rapidly than the CD25−Foxp3lo TRP following positive selection. Analysis of Nur77-GFP reporter mice revealed that these two progenitor populations also display distinct affinity spectrums for self-peptide. Further, TCR sequencing has provided evidence that these 2 progenitor populations contain largely distinct TCR repertoires. Further, we have observed differential dependence on NFκB signaling and the Foxp3 regulatory element Cns3 between the 2 progenitor populations. Our preliminary data suggests that these TRPs mediate tolerance in distinct autoimmune contexts. Our results suggest a model whereby cells are driven by high affinity or high avidity interactions with self-peptide through two discrete pathways to form a comprehensive Treg repertoire capable of protecting against sterile and musical autoimmunity.

Increased generation of regulatory T cells in the presence of reduced T cell receptor signaling capacity correlates with a reduction in positive selection of thymocytes

Abstract The T cell antigen receptor (TCR) is the primary signal transducing structure expressed on thymocytes and mature T cells, transmitting signals necessary for maturation and activation. The TCRα and β chains are rearranged from germline gene segments to generate a highly variable repertoire of antigen receptors. Thymocytes undergo a selection process in the thymus to ensure presence of a functional, but not highly autoreactive, T cell receptor on their surface. These processes, and T cell function, depend on the invariant CD3 molecules to transduce intracellular signals from the surface TCRαβ chains. We have created a knock-in mouse with Cre-recombinase-inducible tyrosine (6Y)-to-phenylalanine (6F) mutations in the 6 Immunoreceptor Tyrosine Activation Motif (ITAM) tyrosines of the 2 CD3ζ chains of the TCR/CD3 complex, resulting in a theoretical 60% decrease in TCR signaling potential. We found that pre-selection, but not post-selection, switching of wild-type (6Y) CD3ζ to non-signaling (6F) CD3ζ results in a significant increase in the percentage of thymic and peripheral regulatory CD4+ T cells (Tregs) relative to control germline 6Y CD3ζ mice. Mice with a pre-selection 6Y to 6F CD3ζ switch also have increased CD4+ CD25+ Foxp3− Treg precursors. This increase in Tregs and thymic Treg precursors is cell autonomous, and correlates with a reduction in positive, but not negative, selection of CD4SP thymocytes. Tregs with reduced signaling capacity do not, however, have suppressive capacity equivalent to wild-type Tregs, despite expressing normal levels of transcription factors and surface proteins characteristic of Tregs. Collectively, the data support an instructive, TCR-signal-based model of Treg lineage commitment in the thymus.

Development of regulatory T cells in the thymus - two paths to the same goal?

Abstract Mature thymic regulatory T cells (Tregs) are defined by dual expression of Foxp3 and CD25 on CD4SP thymocytes. However, the cellular pathways by which CD4 thymocytes differentiate into Tregs remain incompletely understood. Previous reports demonstrated that Treg development occurred via a two-step process. In step one, strong TCR signaling induces expression of CD25 leading to a CD25+ Foxp3− Treg progenitor (TRP). In step two, exposure of this TRP to IL2 results in conversion to a CD25+Foxp3+ Treg. Recently, work by Singer and colleagues have defined a CD25−Foxp3lo TRP that can also be converted by IL2 into CD25+Foxp3+ Tregs. A key question is why two distinct cellular pathways involving distinct TRP populations are needed to reach the same endpoint. Using a combination of maturation marker analysis and RAG2-GFP reporter mice we observed that CD25+Foxp3− TRP arise more rapidly than the CD25−Foxp3lo TRP following positive selection. Analysis of Nur77-GFP reporter mice revealed that these two progenitor populations also display distinct affinity spectrums for self-peptide. Further, TCR sequencing has provided evidence that these 2 progenitor populations contain largely distinct TCR repertoires, suggesting these different TRP populations may be required to provide tolerance to different antigens. Indeed, our preliminary data suggests that these TRPs home to different locations upon adoptive transfer. Our results suggests a model whereby cells are driven by high affinity or high avidity interactions with self-peptide through two discrete pathways to form a comprehensive Treg repertoire capable of protecting against autoimmunity and reactivity to commensal organisms.

Defining the role of CCR8 in promoting thymocyte development and thymic epithelial homeostasis

Abstract Generation of functional and self-tolerant T cells in the thymus is crucial to preventing autoimmune diseases. Medullary thymic epithelial cells (mTECs) and dendritic cells (DCs) present diverse auto-antigens to developing thymocytes; strong TCR reactivity in response to self-antigens causes thymocytes to undergo negative selection, eliminating autoreactive cells from the repertoire. Chemokine receptors expressed on the surface of developing thymocytes have been shown to promote accumulation of post-positive selection thymocytes in the medulla, where they may encounter mTECs and DCs presenting auto-antigens. CCR8 is highly expressed by post-positive selection CD4 thymocytes while its ligands, CCL1 and CCL8 are expressed by mTECs. Thus, CCR8 signaling may promote medullary entry of post-positive selection thymocytes and efficient interactions with mTECs and enforcing central tolerance. Our preliminary data indicate that CCR8−/− thymi have a reduced medullary area, implicating impaired thymocyte:mTEC interactions that impacts normal proliferation and differentiation of mTECs. Furthermore, 2-photon imaging reveals that CCR8 deficiency results in a significant increase in the path straightness of CD4 SP thymocytes, which could impact the efficiency of interactions with medullary stromal cells. Strikingly, CCR8 deficiency results in increased cellularity of CD4SP OTII TCR transgenic thymocytes in a wild-type host, suggesting CCR8 is critical for efficient clonal deletion to endogenous auto-antigens. Thus, our preliminary data indicate that CCR8 contributes to the migratory properties and stromal interactions of post-positive selection thymocytes that promote efficient negative selection.

Histone deacetylase 3 is necessary for T cell maturation (HEM2P.238)

Abstract Recent thymic emigrants (RTEs) are newly generated T cells that need to undergo post-thymic maturation in the periphery. The mechanism of T cell maturation remains unclear. Histone deacetylase 3 (HDAC3) controls gene regulation, chromatin accessibility and genomic stability. However, the function of HDAC3 in T cell has yet to be elucidated. As we previously demonstrated that HDAC3 is a binding partner to NKAP, we hypothesize that HDAC3, like NKAP, may regulate T cell maturation. We generated CD4-cre HDAC3 cKO mice, which have similar numbers of DN, DP, CD4 SP and CD8 SP thymocytes as compared to WT mice; however, the peripheral numbers of CD4+ and CD8+ T cells are dramatically decreased in CD4-cre HDAC3 cKO mice. When we examined Rag1-GFP CD4-cre HDAC3 cKO mice, the majority of peripheral naïve T cells are Rag1-GFP+ RTEs indicating a block in T cell maturation. There is no defect in the positive selection, negative selection, migration, or homeostasis in CD4-cre HDAC3 cKO mice. However, expression of the complement inhibitor CD55, which is induced during maturation, is severely decreased. In the periphery, similar to NKAP-deficient T cells, HDAC3-deficient T cells are targeted for elimination by complement. In addition, HDAC3-deficient T cells have decreased sialic acid modifications on the cell surface, mainly due to α2-3, and α2-8 linkages, that recruit natural IgM to initiate the classical complement pathway. Thus, HDAC3 is required for T cell maturation.

Development of regulatory T cells in the thymus: two paths to the same goal? (HEM2P.237)

Abstract Mature thymic regulatory T cells (Tregs) are defined by dual expression of Foxp3 and CD25 on CD4SP thymocytes. However, the cellular pathways by which CD4 thymocytes differentiate into Tregs remain incompletely understood. Previous reports demonstrated that Treg development occurred via a two-step process. In step one, strong TCR signaling induces expression of CD25 leading to a CD25+Foxp3- Treg progenitor (TRP). In step two, exposure of this TRP to IL2 results in conversion to a CD25+Foxp3+ Treg. Recently, work by Singer and colleagues have defined a CD25-Foxp3lo TRP that can also be converted by IL2 into CD25+Foxp3+ Tregs. A key question is why two distinct cellular pathways involving distinct TRP populations are needed to reach the same endpoint. Using a combination of maturation marker analysis and RAG2-GFP reporter mice we observed that CD25+Foxp3- TRP arise more rapidly than the CD25-Foxp3lo TRP following positive selection. Analysis of Nur77-GFP reporter mice revealed that these two progenitor populations also display distinct affinity spectrums for self-peptide. The CD25+Foxp3- TRP have a high and relatively narrow range of TCR affinities for antigen, while the CD25-Foxp3lo TRP have a lower but broader range of TCR affinity for antigen. Our results suggest that these two different Treg developmental pathways generate Tregs with distinct TCR repertoires that may have distinct function in protecting against autoimmunity and reactivity to commensal organisms.