Most human thymic and peripheral-blood CD4+CD25+ regulatory T cells express 2 T-cell receptors

scRNA+TCR-seq reveals the pivotal role of dual receptor T lymphocytes in the pathogenesis of Kawasaki disease and during IVIG treatment

Introduction

Kawasaki disease (KD), a common cause of acquired heart disease in children in developed countries, is primarily treated with intravenous immunoglobulin (IVIG), but some children demonstrate IVIG resistance with increased coronary artery injury risk. T cells have been demonstrated to be involved in the pathogenesis of KD and its treatment with IVIG. However, the role and mechanism of dual TCR T lymphocytes in the occurrence of KD and IVIG therapy remain unclear.

Methods

This study, based on scRNA-seq combined with TCR-seq technology, clustered the peripheral blood mononuclear cells of 3 healthy controls and 6 KD patients before and after IVIG treatment. Comparative analysis was conducted to investigate the differences in the proportion of single/dual receptor T cells, the characteristics of CDR3 repertoires, cell types, and the expression of transcription factors among the three groups. The study aimed to explore the correlation between dual TCR T cells and KD as well as IVIG treatment.

Results

In our experimental results, we observed the presence of dual TCR T cells in all three groups. However, compared to the healthy control group and the IVIG-treated group, the KD patients before IVIG treatment exhibited a lower proportion of dual TCR T cells, with variability between samples, ranging from 4% to 15%. Notably, after IVIG treatment, the proportion of dual TCR T cells significantly increased, stabilizing above 12%, and these T cells also exhibited clonal expansion and a preference for V gene usage. In addition we found differences in dual TCR T cell subsets among the three groups, for example, IVIG treatment increases the proportion of dual TCR Treg cells, but it still remains below that of healthy control groups, significantly higher proportions of both dual TCR CD8 central and effector memory T cells in IVIG-treated KD patients, and differences in the expression of transcription factors between single and dual TCR T cells. These results suggest dual TCR T cells correlate with KD and IVIG treatment.

Conclusion

Dual TCR T lymphocytes, especially dual TCR CD8 T cells and Treg cells, play crucial roles in the pathogenesis of KD and during IVIG treatment, providing strong support for further elucidating KD pathogenesis and optimizing treatment strategies.

scRNA + TCR-seq revealed the dual TCR pTh17 and Treg T cells involvement in autoimmune response in ankylosing spondylitis

OBJECTIVE

Th17 and Treg play important roles in AS, but their single and dual TCR pairing types, ratios, and CDR3 characteristics remain unknown.

METHODS

Single-cell RNA + TCR-seq results from six AS patients were used to cluster T-cell subpopulations and analyze the single and dual TCR T cell ratio, diversity/clonality/overlap of CDR3, and expression of transcription factors.

RESULTS

1. AS patients have about 10% of dual TCR T cells, and SFMC have decreased diversity CDR3 libraries and significant clonal proliferation compared to PBMC. 2. Dual TCR ratio: memory T > naive T; pTh 17 > Th17; Treg /Th17/Th1/EM significantly higher than naive CD4 + T/CM, Pathogenic Th17 cells contain clonally proliferating single TCR and dual TCR cells. 3. The expression of single TCR and dual TCR transcription factors of each T cell subpopulation was basically the same, but there was differential expression of characteristic transcription factors, e.g. Foxp3, CTLA4, STAT5B, IL10RB, LAG3 in dual TCR Treg was higher than that of single TCR Treg; TNFSF10/12, TNFRSF4/14, CCL5, KLRB1 in dual TCR pTh17 were significantly higher than those in single TCR pTh17. 4. Between naive CD4 + T, pTh17, Th1 and Treg, there are partially identity identical tcr paired cells.

CONCLUSIONS

The high proportion of dual TCR T cells such as pTh17 and Treg in AS and the high expression of some transcription factors suggested a close association with self-response in AS; The overlap of CDR3 between Th1, Th17,pTh17, and Treg in AS suggested that the subpopulations may be differentiated from each other to regulate the inflammatory homeostasis and progression.

scRNA+TCR+BCR-seq revealed the proportions and gene expression patterns of dual receptor T and B lymphocytes in NPC and NLH

While the relationship between single receptor lymphocytes and cancer has been deeply researched, the origin and biological roles of dual receptor lymphocytes in tumor microenvironment (TME) remain largely unknown. And since nasopharyngeal carcinoma (NPC) is a type of cancer closely associated with immune infiltration, studying the TME of NPC holds particular significance. Utilizing single-cell RNA sequencing combined with T cell receptor (TCR) and B cell receptor (BCR) sequencing (scRNA + TCR + BCR-seq), we analyzed data from 7 patients with NPC and 3 patients with nasopharyngeal lymphatic hyperplasia (NLH). In our research, it was firstly found that the presence of dual receptor lymphocytes in both the TME of NPC and the inflammatory environment of NLH. We also confirmed their clonal expansion, suggesting their potential involvement in the immune response. Subsequently, we further discovered the lineage and the pairing characteristics. It was found that the dual receptor lymphocytes in NPC and NLH mainly originate from memory cells, and the predominant pairing type for dual TCR was β+α1+α2 and dual BCR was heavy+κ+λ. By further analyzing their gene expression, we compared the function of dual receptor cells with single receptor cells in the context of both NPC and NLH. This groundbreaking research has enhanced our comprehension of the features of dual-receptor cells and has contributed to a better understanding of the TME in NPC. By comparing with NLH, it illuminates part of the alterations in the process of malignant transformation in NPC. These findings present the potential to acquire improved diagnostic markers and treatment modalities.

Bayesian Inference of Allelic Inclusion Rates in the Human T Cell Receptor Repertoire

A small population of αβ T cells is characterized by the expression of more than one unique T cell receptor (TCR); this outcome is the result of "allelic inclusion," that is, inclusion of both α- or β-chain alleles during V(D)J recombination. Limitations in single-cell sequencing technology, however, have precluded comprehensive enumeration of these dual receptor T cells. Here, we develop and experimentally validate a fully Bayesian inference model capable of reliably estimating the true rate of α and β TCR allelic inclusion across two different emulsion-barcoding single-cell sequencing platforms. We provide a database composed of over 51,000 previously unpublished allelic inclusion TCR sequence sets drawn from eight healthy individuals and show that allelic inclusion contributes a distinct and functionally important set of sequences to the human TCR repertoire. This database and a Python implementation of our statistical inference model are freely available at our Github repository (https://github.com/JasonACarter/Allelic_inclusion).

Alemtuzumab in Multiple Sclerosis: Short- and Long-Term Effects of Immunodepletion on the Peripheral Treg Compartment

Treatment with alemtuzumab is followed by an early increase in Treg frequencies. Whether naïve and memory subsets are differentially affected and how depletion influences dysfunctional MS-Treg is unclear. In this study, we analyzed the effect of alemtuzumab on regulatory T-cells (Treg) in patients with multiple sclerosis (MS). For this purpose 182 blood samples from 25 MS patients were taken shortly before treatment and serially for up to 24 months after two alemtuzumab cycles. We studied Treg by flow cytometry (quantitation, phenotypical characterization), real-time polymerase chain reaction (T-cell receptor (TCR) excision circles [TREC] content), CDR3-spectratyping (clonal distribution), and proliferation assays (suppressive function). CD52-mediated cytolysis of Treg and conventional T-cells was determined by a complement-dependent cytolysis assay. Our studies revealed that 1 week post-alemtuzumab, Treg were depicted at constant frequencies among CD4⁺ T-cells. In contrast, Treg frequencies were massively increased at month 1. Post-depletional Treg exhibited a CD45RO⁺ memory phenotype, a skewed TCR repertoire, and contained minimum TREC numbers. Naïve Treg, thymic markers, and TCR-variability commenced to rise after 6 months but did not attain baseline levels. In vitro, Treg exhibited higher susceptibility to lysis than Tcon. Treg suppressive function constantly increased within 1 year when co-cultured with syngeneic T-cells, but remained stable against allogeneic T-cells from normal donors. Our findings suggest that (1) Treg are not spared from alemtuzumab-mediated depletion and thymopoiesis does not considerably contribute to long-term recovery, (2) either homeostatic proliferation and/or conversion from residual Tcon contributes to Treg expansion during the early post-treatment phase (3) the enhanced inhibitory effect of Treg following alemtuzumab is due to altered composition and reactivity of post-depletional Tcon.

Foxp3+ Regulatory and Conventional CD4+ T Cells Display Similarly High Frequencies of Alloantigen-Reactive Cells

Foxp3⁺ regulatory T cells (Tregs) play a major role in acquired immune tolerance to allogenic transplants. Their suppressive activity is thought to require T cell receptor (TCR)-driven antigen recognition; little, however, is known about the fraction of Tregs able to recognize alloantigens within this T cell subset primarily educated against self-antigens. Performing transfer experiments of Tregs or conventional T cells (Tconv) into both lymphoreplete and lymphopenic mice, we observed a similarly high proportion of cells signaling through their TCR and proliferating in allogenic hosts. Furthermore, using an in vivo proliferation assay with limited T cell numbers infused into lymphopenic mice, we found that the overall frequency of alloreactive Tregs was similar if not higher to that of alloreactive Tconv. Overall our study highlights a noticeably high level of alloreactive Foxp3⁺ regulatory T cells accounting for their predominant role in transplantation tolerance.

Humanized Mice Reveal New Insights Into the Thymic Selection of Human Autoreactive CD8+ T Cells

Thymic selection constitutes the first checkpoint in T-cell development to purge autoreactive T cells. Most of our understanding of this process comes from animal models because of the challenges of studying thymopoiesis and how T cell receptor (TCR) specificity impacts thymocyte phenotype in humans. We developed a humanized mouse model involving the introduction of autoreactive TCRs and cognate autoantigens that enables the analysis of selection of human T cells in human thymic tissue in vivo. Here, we describe the thymic development of MART1-specific autoreactive CD8⁺ T cells that normally escape deletion and how their phenotype and survival are affected by introduction of the missing epitope in the hematopoietic lineage. Expression of the epitope in a fraction of hematopoietic cells, including all major types of antigen-presenting cells (APCs), led to profound yet incomplete deletion of these T cells. Upregulation of PD-1 upon antigen encounter occurred through the different stages of thymocyte development. PD-1 and CCR7 expression were mutually exclusive in both transgenic and non-transgenic thymocytes, challenging the view that CCR7 is necessary for negative selection in humans. In the presence of antigen, MART1-reactive T cells down-regulated TCR, CD3, CD8, and CD4 in the thymus and periphery. Moreover, expression of secondary TCRs influences MHC class I-restricted T cells to develop as CD4⁺, particularly regulatory T cells. This new model constitutes a valuable tool to better understand the development of autoreactive T cells identified in different human autoimmune diseases and the role of different APC subsets in their selection.

Dual TCR T Cells: Identity Crisis or Multitaskers?

Dual TCR T cells are a common and natural product of TCR gene rearrangement and thymocyte development. As much as one third of the T cell population may have the capability to express two different TCR specificities on the cell surface. This discovery provoked a reconsideration of the classic model of thymic selection. Many potential roles for dual TCR T cells have since been hypothesized, including posing an autoimmune hazard, dominating alloreactive T cell responses, inducing allergy, and expanding the TCR repertoire to improve protective immunity. Yet, since the initial wave of publications following the discovery of dual TCR T cells, research in the area has slowed. In this study, we aim to provide a brief but comprehensive history of dual TCR T cell research, re-evaluate past observations in the context of current knowledge of the immune system, and identify key issues for future study.

Cutting Edge: Dual TCRα Expression Poses an Autoimmune Hazard by Limiting Regulatory T Cell Generation

Despite accounting for 10-30% of the T cell population in mice and humans, the role of dual TCR-expressing T cells in immunity remains poorly understood. It has been hypothesized that dual TCR T cells pose an autoimmune hazard by allowing self-reactive TCRs to escape thymic selection. We revisited this hypothesis using the NOD murine model of type 1 diabetes. We bred NOD mice hemizygous at both TCRα and β (TCRα^+/- β^+/-) loci, rendering them incapable of producing dual TCR T cells. We found that the lack of dual TCRα expression skewed the insulin-specific thymocyte population toward greater regulatory T (T_reg) cell commitment, resulting in a more tolerogenic T_reg to conventional T cell ratio and protection from diabetes. These data support a novel hypothesis by which dual TCR expression can promote autoimmunity by limiting agonist selection of self-reactive thymocytes into the T_reg cell lineage.

The highly alloreactive nature of dual TCR T cells

PURPOSE OF REVIEW

T cells can mediate allograft rejection and graft-versus-host disease (GVHD), but are necessary for tolerance and protective immunity. Identifying T-cell populations differentially responsible for these effects has been a goal in transplant research. This review describes investigation of a small subset of T cells naturally predisposed toward alloreactivity, cells expressing two T-cell receptors (TCRs).

RECENT FINDINGS

Rare peripheral T cells express two αβTCRs. Their impact on T-cell development and function has been uncertain. Recent work demonstrates an important role for these cells in mouse models and human hematopoietic stem cell transplant patients with acute GVHD. Dual receptor T cells are preferentially activated and expanded in vitro and in vivo by allogeneic stimulation. Genetic elimination of dual TCR expression results in loss of approximately half of the alloreactive repertoire and impedes the earliest steps of GVHD.

SUMMARY

Identification of dual TCR T cells as predisposed to alloreactivity provides an opportunity to examine responses limiting transplantation. Continued investigation will reveal significant fundamental features of T-cell alloreactivity and important information about the earliest events determining allograft rejection and self-tolerance.

Retention of duplicated ITAM-containing transmembrane signaling subunits in the tetraploid amphibian species Xenopus laevis

The ITAM-bearing transmembrane signaling subunits (TSS) are indispensable components of activating leukocyte receptor complexes. The TSS-encoding genes map to paralogous chromosomal regions, which are thought to arise from ancient genome tetraploidization(s). To assess a possible role of tetraploidization in the TSS evolution, we studied TSS and other functionally linked genes in the amphibian species Xenopus laevis whose genome was duplicated about 40 MYR ago. We found that X. laevis has retained a duplicated set of sixteen TSS genes, all except one being transcribed. Furthermore, duplicated TCRα loci and genes encoding TSS-coupling protein kinases have also been retained. No clear evidence for functional divergence of the TSS paralogs was obtained from gene expression and sequence analyses. We suggest that the main factor of maintenance of duplicated TSS genes in X. laevis was a protein dosage effect and that this effect might have facilitated the TSS set expansion in early vertebrates.

Regulatory T-Cell Development in the Human Thymus

The thymus generates a lineage-committed subset of regulatory T-cells (Tregs), best identified by the expression of the transcription factor FOXP3. The development of thymus-derived Tregs is known to require high-avidity interaction with MHC-self peptides leading to the generation of self-reactive Tregs fundamental for the maintenance of self-tolerance. Notwithstanding their crucial role in the control of immune responses, human thymic Treg differentiation remains poorly understood. In this mini-review, we will focus on the developmental stages at which Treg lineage commitment occurs, and their spatial localization in the human thymus, reviewing the molecular requirements, including T-cell receptor and cytokine signaling, as well as the cellular interactions involved. An overview of the impact of described thymic defects on the Treg compartment will be provided, illustrating the importance of these in vivo models to investigate human Treg development.

Fingolimod does not impair T-cell release from the thymus and beneficially affects Treg function in patients with multiple sclerosis

Background:

In multiple sclerosis (MS), disturbed T-cell homeostasis affects both conventional CD4+ T cells (Tcon) and regulatory T cells (Treg). Functionally, this is linked to a loss of Treg-suppressive properties. Concerns exist as to whether fingolimod might further aggravate Treg dysfunction by inhibiting thymic egress and, thus, promoting premature immunosenescence.

Objective:

The objective of this paper is to investigate whether fingolimod, by sequestration of developing cells in the thymus, might deteriorate numeric and/or functional disequilibrium of T-cell subtypes.

Methods:

We assessed numbers and phenotypes of blood Tcon and Treg in 74 MS patients treated with fingolimod and in 37 healthy donors. Treg and Tcon were also analyzed for immunoreactivity, suppressive function, sphingosine-1-phosphate-triggered (S1P) trafficking, and S1P-receptor expression. This was complemented by assessing surrogate markers of thymic T-cell development, including frequencies of cells expressing T-cell receptors (TCR) of dual specificity, and TCR diversity in Treg.

Results:

Fingolimod did not negatively affect naive T-cell phenotypes or markers of thymic T-cell development. By reducing CCR7-expressing Tcon, fingolimod increased relative proportions of Treg. As a result of this shift, fewer proliferative CCR7− Tcon became enriched and Treg-dysfunction was indirectly reversed.

Conclusion:

These observations argue against harmful interference of fingolimod with thymic T-cell output that, particularly in pediatric MS, might possibly counteract its beneficial effects.

In vivo analysis of helper T cell responses in patients with autoimmune polyendocrinopathy - candidiasis - ectodermal dystrophy provides evidence in support of an IL-22 defect

Autoimmune polyendocrinopathy - candidiasis - ectodermal dystrophy (APECED) is caused by mutations in the Autoimmune regulator (AIRE) gene and is associated with neutralizing anti-cytokine autoantibodies. We have used an in vivo challenge model to analyze antigen-specific CD4(+) T cell responses. Bacille Calmette-Guérin (BCG)-vaccinated patients and controls were injected tuberculin intradermally, skin blisters were induced by suction on the indurations and on unexposed skin, and the infiltrating cells harvested. The patients had a quantitatively normal CD4(+) T cell response and no significant abnormalities in the expression of T helper type (Th) 1- or Th2-related genes. The expression of interleukin (IL)-22, in contrast, was lower in the patients. Two patients, both with a pre-existing ocular keratopathy, experienced a relapse of keratoconjunctivitis, suggesting a possible immunological basis for this APECED component. Our in vivo data are compatible with a selective IL-22 defect in the activated CD4(+) T cells of APECED patients, affecting also unexposed skin in steady-state conditions.

The ability to rearrange dual TCRs enhances positive selection, leading to increased Allo- and Autoreactive T cell repertoires

Thymic selection is designed to ensure TCR reactivity to foreign Ags presented by self-MHC while minimizing reactivity to self-Ags. We hypothesized that the repertoire of T cells with unwanted specificities such as alloreactivity or autoreactivity are a consequence of simultaneous rearrangement of both TCRα loci. We hypothesized that this process helps maximize production of thymocytes capable of successfully completing thymic selection, but results in secondary TCRs that escape stringent selection. In T cells expressing two TCRs, one TCR can mediate positive selection and mask secondary TCR from negative selection. Examination of mice heterozygous for TRAC (TCRα(+/-)), capable of only one functional TCRα rearrangement, demonstrated a defect in generating mature T cells attributable to decreased positive selection. Elimination of secondary TCRs did not broadly alter the peripheral T cell compartment, though deep sequencing of TCRα repertoires of dual TCR T cells and TCRα(+/-) T cells demonstrated unique TCRs in the presence of secondary rearrangements. The functional impact of secondary TCRs on the naive peripheral repertoire was evidenced by reduced frequencies of T cells responding to autoantigen and alloantigen peptide-MHC tetramers in TCRα(+/-) mice. T cell populations with secondary TCRs had significantly increased ability to respond to altered peptide ligands related to their allogeneic ligand as compared with TCRα(+/-) cells, suggesting increased breadth in peptide recognition may be a mechanism for their reactivity. Our results imply that the role of secondary TCRs in forming the T cell repertoire is perhaps more significant than what has been assumed.

Dual receptor T cells mediate pathologic alloreactivity in patients with acute graft-versus-host disease

Acute graft-versus-host disease (aGVHD) results from a robust response of donor T cells transferred during hematopoietic stem cell transplantation (HSCT) to allogeneic peptide-major histocompatibility complex antigens. Previous investigations have not identified T cell subsets that selectively mediate either protective immunity or pathogenic alloreactivity. We demonstrate that the small subset of peripheral T cells that naturally express two T cell receptors (TCRs) on the cell surface contributes disproportionately to aGVHD in patients after allogeneic HSCT. Dual TCR T cells from patients with aGVHD demonstrate an activated phenotype and produce pathogenic cytokines ex vivo. Dual receptor clones from a patient with symptomatic aGVHD responded specifically to mismatched recipient human leukocyte antigens (HLAs), demonstrating pathologic alloreactivity. Human dual TCR T cells are strongly activated and expanded by allogeneic stimulation in vitro, and disproportionately contribute to the repertoire of T cells recognizing both major (HLA) and minor histocompatibility antigens, providing a mechanism for their observed activity in vivo in patients with aGVHD. These results identify dual TCR T cells as a target for focused analysis of a T cell subset mediating GVHD and as a potential prognostic indicator.

Distribution and clonality of the vα and vβ T-cell receptor repertoire of regulatory T cells in leukemia patients with and without graft versus host disease

Graft versus host disease (GVHD) is the main complication following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Recent data indicated that regulatory T (Treg) cells might relate to GVHD, and such functions might be mediated by certain T-cell receptor (TCR) subfamily of Treg cells. Thus, we analyzed the distribution and clonality of the TCR Vα and Vβ repertoire of Treg cells from leukemia patients with and without GVHD after allo-HSCT. Numerous TCR Vα subfamilies, including Vα1, Vα9, Vα13, Vα16-19, and Vα24-29, were absent in Treg cells after allo-HSCT. The usage numbers for the TCR Vα and Vβ subfamilies in Treg cells from patients without GVHD appeared more widely. The expression frequencies of Vα10 or Vα20 between both groups were significantly different. Moreover, the expression frequency of TCR Vβ2 subfamily in patients without GVHD was significantly higher than that in patients with GVHD. Oligoclonally expanded TCR Vα and Vβ Treg cells were identified in a few samples in both groups. Restricted utilization of the Vα and Vβ subfamilies and the absence of some important TCR rearrangements in Treg cells may be related to GVHD due to a lower regulating function of Treg subfamilies.

Characteristics of TCR/CD3 complex CD3{varepsilon} chains of regulatory CD4+ T (Treg) lymphocytes: role in Treg differentiation in vitro and impact on Treg in vivo

Tregs are anergic CD4(+)CD25(+)Foxp3(+) T lymphocytes exerting active suppression to control immune and autoimmune responses. However, the factors in TCR recognition underlying Treg differentiation are unclear. Based on our previous data, we hypothesized that Treg TCR/CD3 antigen receptor complexes might differ from those of CD4(+)CD25(-) Tconv. Expression levels of TCR/CD3, CD3ε,ζ chains, or other molecules involved in antigen signaling and the characteristics of CD3ε chains were analyzed in thymus or spleen Treg cells from normal mice. Tregs had quantitative and qualitatively distinct TCR/CD3 complexes and CD3ε chains. They expressed significantly lower levels of the TCR/CD3 antigen receptor, CD3ε chains, TCR-ζ chain, or the CD4 coreceptor than Tconv. Levels of kinases, adaptor molecules involved in TCR signaling, and early downstream activation pathways were also lower in Tregs than in Tconv. Furthermore, TCR/CD3 complexes in Tregs were enriched in CD3ε chains conserving their N-terminal, negatively charged amino acid residues; this trait is linked to a higher activation threshold. Transfection of mutant CD3ε chains lacking these residues inhibited the differentiation of mature CD4(+)Foxp3(-) T lymphocytes into CD4(+)Foxp3(+) Tregs, and differences in CD3ε chain recognition by antibodies could be used to enrich for Tregs in vivo. Our results show quantitative and qualitative differences in the TCR/CD3 complex, supporting the hyporesponsive phenotype of Tregs concerning TCR/CD3 signals. These differences might reconcile avidity and flexible threshold models of Treg differentiation and be used to implement therapeutic approaches involving Treg manipulation.

CTLA-4 controls the thymic development of both conventional and regulatory T cells through modulation of the TCR repertoire

Cytotoxic T lymphocyte-associated antigen-4 (CTLA-4; CD152) is of pivotal importance for self-tolerance, with deficiency or unfavorable polymorphisms leading to autoimmune disease. Tolerance to self-antigens is achieved through thymic deletion of highly autoreactive conventional T (Tconv) cells and generation of FoxP3(+) regulatory T (Treg) cells. The main costimulatory molecule, CD28, augments the negative selection of Tconv cells and promotes the generation of FoxP3(+) Treg cells. The role of its antagonistic homolog CTLA-4, however, remains a topic of debate. To address this topic, we investigated the thymic development of T cells in the presence and absence of CTLA-4 in a T-cell receptor (TCR) transgenic mouse model specific for the myelin basic protein peptide Ac1-9. We reveal that CTLA-4 is expressed in the corticomedullary region of the thymus. Its absence alters the response of CD4(+)CD8(-) thymocytes to self-antigen recognition, which affects the quantity of the Treg cells generated and broadens the repertoire of peripheral Tconv cells. T-cell repertoire alteration after deletion of CTLA-4 results from changes in TCR Vα and Jα segment selection as well as CDR3α composition in Tconv and Treg cells. CTLA-4, therefore, regulates the early development of self-reactive T cells in the thymus and plays a key role in central tolerance.

The ambiguity in immunology

In the present article, we discuss the various ambiguous aspects of the immune system that render this complex biological network so highly flexible and able to defend the host from different external invaders. This ambiguity stems mainly from the property of the immune system to be both protective and harmful. Immunity cannot be fully protective without producing a certain degree of damage (immunopathology) to the host. The balance between protection and tissue damage is, therefore, critical for the establishment of immune homeostasis and protection. In this review, we will consider as ambiguous, various immunological tactics including: (a) the opposing functions driving immune responses, immune-regulation, and contra-regulation, as well as (b) the phenomenon of chronic immune activation as a result of a continuous cross-presentation of apoptotic T cells by dendritic cells. All these plans participate principally to maintain a state of chronic low-level inflammation during persisting infections, and ultimately to favor the species survival.

Dual TCR expression biases lung inflammation in DO11.10 transgenic mice and promotes neutrophilia via microbiota-induced Th17 differentiation

A commonly used mouse model of asthma is based on i.p. sensitization to OVA together with aluminum hydroxide (alum). In wild-type BALB/c mice, subsequent aerosol challenge using this protein generates an eosinophilic inflammation associated with Th2 cytokine expression. By constrast, in DO11.10 mice, which are transgenic for an OVA-specific TCR, the same treatment fails to induce eosinophilia, but instead promotes lung neutrophilia. In this study, we show that this neutrophilic infiltration results from increased IL-17A and IL-17F production, whereas the eosinophilic response could be restored upon blockade of IFN-γ, independently of the Th17 response. In addition, we identified a CD4(+) cell population specifically present in DO11.10 mice that mediates the same inflammatory response upon transfer into RAG2(-/-) mice. This population contained a significant proportion of cells expressing an additional endogenous TCR α-chain and was not present in RAG2(-/-) DO11.10 mice, suggesting dual antigenic specificities. This particular cell population expressed markers of memory cells, secreted high levels of IL-17A, and other cytokines after short-term restimulation in vitro, and triggered a neutrophilic response in vivo upon OVA aerosol challenge. The relative numbers of these dual TCR lymphocytes increased with the age of the animals, and IL-17 production was abolished if mice were treated with large-spectrum antibiotics, suggesting that their differentiation depends on foreign Ags provided by gut microflora. Taken together, our data indicate that dual TCR expression biases the OVA-specific response in DO11.10 mice by inhibiting eosinophilic responses via IFN-γ and promoting a neutrophilic inflammation via microbiota-induced Th17 differentiation.

The interleukin-7 receptor α chain contributes to altered homeostasis of regulatory T cells in multiple sclerosis

Treg homeostasis is disturbed in multiple sclerosis (MS). Frequencies of recent thymic emigrant (RTE)-Treg are reduced and the disparity between RTE-Treg and long-lived memory Treg coincides with the MS-associated Treg defect, as shown previously. Recent studies demonstrate that IL-7 and thymic stromal lymphopoietin (TSLP) are critical for Treg maturation. Therefore, altered signaling through their receptors (IL-7R, TSLP receptor (TSLPR)), sharing the IL-7Rα-chain (IL-7Rα), might contribute to impaired Treg development. Using blood samples from 56 patients with MS and 33 healthy controls, we assessed IL-7Rα-expression on conventional T cells; frequencies, phenotypes and suppressive activities of Treg, plasma levels of IL-7 and soluble IL-7Rα; and screened for MS-associated IL-7RA gene polymorphism rs6897932. Moreover, we determined Treg expressing two different TCR Vα-chains designating thymus-originated cells. As TSLP/TSLPR signaling in thymic myeloid dendritic cells (MDCs) promotes Treg differentiation, we measured TSLPR expression on peripheral MDCs to indirectly test whether altered TSLPR expression might add to compromised Treg neogenesis. We found reduced IL-7Rα expression on conventional T cells and upregulated IL-7 plasma levels together with reduction of RTE-Treg frequencies and Treg function in MS, without clear genetic influence. Decreased IL-7Rα expression in MS correlated with declined dual-receptor-Treg and reduced MDC TSLPR expression, indicating contracted thymic Treg output. We suggest that altered IL-7R/TSLPR signaling contributes to impaired Treg neogenesis in MS, which is compensated by expanded memory-Treg and finally results in dysfunctional Treg.

T cells expressing two different T cell receptors form a heterogeneous population containing autoreactive clones

During T cell development both alleles of the T cell receptor (TCR) alpha locus are rearranged. As a result, a sizeable proportion of T cells can express two distinct TCRs, but the functional significance of this phenomenon remains controversial. Studies on transgenic mice with two TCRs have focused on the risk of immunopathology that such cells may pose, while some have suggested that most dual-specific T cells are nonfunctional or even protective. We tracked the fate and TCR repertoire of single- and dual-specific T cells within a normal polyclonal population undergoing lymphopenia-induced proliferation, a setting which has been shown to cause immunopathology and autoimmunity. After the expansion the repertoire of dual-specific T cells had become highly biased, with both prominent clonal expansions and the complete disappearance of other clones. Our results suggest that the normal repertoire of dual-specific T cells contains both nonfunctional cells and a small, 5% fraction of clones which display a much higher than average affinity to antigens normally tolerated as harmless. This heterogeneity may also help in reconciling some of the earlier, conflicting results.

GARP: a key receptor controlling FOXP3 in human regulatory T cells

Recent evidence suggests that regulatory pathways might control sustained high levels of FOXP3 in regulatory CD4(+)CD25(hi) T (T(reg)) cells. Based on transcriptional profiling of ex vivo activated T(reg) and helper CD4(+)CD25(-) T (T(h)) cells we have identified GARP (glycoprotein-A repetitions predominant), LGALS3 (lectin, galactoside-binding, soluble, 3) and LGMN (legumain) as novel genes implicated in human T(reg) cell function, which are induced upon T-cell receptor stimulation. Retroviral overexpression of GARP in antigen-specific T(h) cells leads to an efficient and stable re-programming of an effector T cell towards a regulatory T cell, which involves up-regulation of FOXP3, LGALS3, LGMN and other T(reg)-associated markers. In contrast, overexpression of LGALS3 and LGMN enhance FOXP3 and GARP expression, but only partially induced a regulatory phenotype. Lentiviral down-regulation of GARP in T(reg) cells significantly impaired the suppressor function and was associated with down-regulation of FOXP3. Moreover, down-regulation of FOXP3 resulted in similar phenotypic changes and down-regulation of GARP. This provides compelling evidence for a GARP-FOXP3 positive feedback loop and provides a rational molecular basis for the known difference between natural and transforming growth factor-beta induced T(reg) cells as we show here that the latter do not up-regulate GARP. In summary, we have identified GARP as a key receptor controlling FOXP3 in T(reg) cells following T-cell activation in a positive feedback loop assisted by LGALS3 and LGMN, which represents a promising new system for the therapeutic manipulation of T cells in human disease.

Analysis of indirect pathway CD4+ T cells in a patient with metastable tolerance to a kidney allograft: possible relevance to superior graft survival of HLA class II closely matched renal allografts

MHC class I mismatched, but class II matched kidney transplants are tolerogenic in large animal models. CD4(+) T regulatory cells specific for HLA-B1501-derived peptide p37-MA (DSDAASPRMAPRAPWIEQ) developed in a long term (>12 years) tolerant patient who received an HLA-B1501 mismatched, HLA class II closely matched renal allograft. We hypothesized that class II matching favored T regulatory cell development by allowing allopeptide presentation on either recipient (DR4/DQ7(+)) or donor (DR4/DQ8(+)) antigen presenting cells (APC). Indirect pathway CD4(+) T cell clones were generated from recipient PBMC by sorting antigen-stimulated, proliferating cells. Most clones responded to p37-MA-pulsed B1501(-) autologous, but not to DQ8(+)B1501(+) donor B-lymphoblastoid cell lines (B-LCL). However, some clones responded to both; in fact, one responded even more strongly to donor B-LCL than to p37-MA-pulsed autologous B-LCL. P37-MA contained a DQ8-binding motif and induced strong TH1 responses from DQ8 but not DQ6 transgenic mice. Microchimerism was found to be enriched in the dendritic cells (DC) cultured from adherent PBMC. This indicates donor APC could possibly present p37-MA peptide directly to p37-MA specific T cells. These data support the concept that, when the donor is MHC class II closely matched, a "hybrid" form of allorecognition (direct/indirect) occurs. This may favor the generation of a beneficial form of indirect pathway alloreactivity, i.e. allopeptide-specific CD4(+) T regulatory cells, in the context of long term DC microchimerism.

Phosphoantigen-activated V gamma 2V delta 2 T cells antagonize IL-2-induced CD4+CD25+Foxp3+ T regulatory cells in mycobacterial infection

Although Foxp3(+) T regulatory cells (Tregs) are well documented for their ability to suppress various immune cells, T-cell subsets capable of counteracting Tregs have not been demonstrated. Here, we assessed phosphoantigen-activated Vgamma2Vdelta2 T cells for the ability to interplay with Tregs in the context of mycobacterial infection. A short-term IL-2 treatment regimen induced marked expansion of CD4(+)CD25(+)Foxp3(+) T cells and subsequent suppression of mycobacterium-driven increases in numbers of Vgamma2Vdelta2 T cells. Surprisingly, activation of Vgamma2Vdelta2 T cells by adding phosphoantigen Picostim to the IL-2 treatment regimen down-regulated IL-2-induced expansion of CD4(+)CD25(+)Foxp3(+) T cells. Consistently, in vitro activation of Vgamma2Vdelta2 T cells by phosphoantigen plus IL-2 down-regulated IL-2-induced expansion of CD4(+)CD25(+)Foxp3(+) T cells. Interestingly, anti-IFN-gamma-neutralizing antibody, not anti-TGF-beta or anti-IL-4, reduced the ability of activated Vgamma2Vdelta2 T cells to down-regulate Tregs, suggesting that autocrine IFN-gamma and its network contributed to Vgamma2Vdelta2 T cells' antagonizing effects. Furthermore, activation of Vgamma2Vdelta2 T cells by Picostim plus IL-2 treatment appeared to reverse Treg-driven suppression of immune responses of phosphoantigen-specific IFNgamma(+) or perforin(+) Vgamma2Vdelta2 T cells and PPD-specific IFNgamma(+)alphabeta T cells. Thus, phos-phoantigen activation of Vgamma2Vdelta2 T cells antagonizes IL-2-induced expansion of Tregs and subsequent suppression of Ag-specific antimicrobial T-cell responses in mycobacterial infection.

Cutting edge: human CD4-CD8- thymocytes express FOXP3 in the absence of a TCR

The best candidate for regulatory T (Treg) cell lineage-determining factor is currently the Forkhead box transcription factor FOXP3. FOXP3 up-regulation has been linked to TCR-mediated signals, and in mice the abrogation of TCR expression or signals also prevents FoxP3 expression. In contrast, the TCR dependence of human FOXP3 is assumed but not established. In this study we show on a single cell level that 1.4% (range 0.1-3.8%) of CD4(-)CD8(-) thymocytes in healthy humans express FOXP3, two thirds of them without any detectable alphabeta TCR. These TCR(-)FOXP3(+) cells were mostly CD25(-) and did not express gammadelta TCR or B cell, NK cell, or monocyte-associated markers. Like mature Treg cells, they were mostly CD2(+)CD127(low) and expressed cytoplasmic CTLA-4. Our results suggest that in immature human thymocytes the expression of FOXP3 precedes surface TCR, in which case TCR-mediated signals cannot be responsible for the thymic up-regulation of FOXP3.

T cells with regulatory activity in hepatitis C virus infection: what we know and what we don't

The mechanism behind the apparent lack of effective antiviral immune response in patients with chronic hepatitis C virus (HCV) infection is poorly understood. Although multiple levels of abnormalities have been identified in innate and adaptive immunity, it remains unclear if any of the subpopulations of T cells with regulatory capacity (Tregs) contribute to the induction and maintenance of HCV persistence. In this review, we summarize the current knowledge about Tregs as they relate to HCV infection.

CD4+CD25+ regulatory T cell repertoire formation shaped by differential presentation of peptides from a self-antigen

We have used TCR transgenic mice directed to different MHC class II-restricted determinants from the influenza virus hemagglutinin (HA) to analyze how specificity for self-peptides can shape CD4+CD25+ regulatory T (Treg) cell formation. We show that substantial increases in the number of CD4+CD25+ Treg cells can occur when an autoreactive TCR directed to a major I-E(d)-restricted determinant from HA develops in mice expressing HA as a self-Ag, and that the efficiency of this process is largely unaffected by the ability to coexpress additional TCR alpha-chains. This increased formation of CD4+CD25+ Treg cells in the presence of the self-peptide argues against models that postulate selective survival rather than induced formation as mechanisms of CD4+CD25+ Treg cell formation. In contrast, T cells bearing a TCR directed to a major I-A(d)-restricted determinant from HA underwent little or no selection to become CD4+CD25+ Treg cells in mice expressing HA as a self-Ag, correlating with inefficient processing and presentation of the peptide from the neo-self-HA polypeptide. These findings show that interactions with a self-peptide can induce thymocytes to differentiate along a pathway to become CD4+CD25+ Treg cells, and that peptide editing by DM molecules may help bias the CD4+CD25+ Treg cell repertoire away from self-peptides that associate weakly with MHC class II molecules.

Detailed analyses of antibodies recognizing mitochondrial antigens suggest similar or identical mechanism for production of natural antibodies and natural autoantibodies

Because of their endosymbiotic evolutionary origin, proteins compartmentalized into mitochondria represent an interesting transition from prokaryotic foreign to essential self molecules. We investigated the presence of naturally occurring antibodies (nAbs) recognizing mitochondrial inner membrane enzymes. Epitope mapping analysis of a mitochondrial inner membrane enzyme, citrate synthase (CS) by synthetic overlapping peptides and phage display libraries using sera from healthy individuals and from patients having systemic autoimmune disease revealed CS recognizing nAbs with IgM isotype. We analyzed cross-reactive epitopes on human CS, bacterial CS, and various standard autoantigens. We have found that the fine epitope pattern on CS is different under physiological and pathological conditions. Moreover sera affinity purified on CS cross reacts with nucleosome antigen, which cross-reactivity could be mapped to a short epitope on human CS. These data indicate that in theory, nAbs "specific" for a given self antigen could fulfill the function of participating in innate defense mechanisms and at the same time recognize a target antigen in a systemic autoimmune disease. Thus, at the level of recognized epitopes there is a possible link between the innate like part and the adaptive-autoimmune arm of the humoral immune system.

Circulating naïve and CD4+CD25high regulatory T cells in patients with autoimmune pancreatitis

OBJECTIVE

Autoimmune pancreatitis (AIP) is a new clinical entity of pancreatic disorder. There are several immunologic and histological abnormalities specific for the disease, including increased levels of serum IgG4 and infiltration of lymphocytes and IgG4-positive plasmacytes. The role of IgG4 is unclear. Recently, regulatory T cells (Tregs) have been reported to be involved in the development of various autoimmune diseases as well as B cell shifting to IgG4-producing plasmacytes. To clarify the role of Tregs in the pathophysiology of AIP, we analyzed circulating Tregs in AIP.

METHODS

We recruited 27 patients with AIP for this study. For comparison, we also recruited 23 patients with other pancreatic disease and 32 healthy subjects as controls. We analyzed Tregs as CD4+CD25high and CD4+CD25+CD45RA+ (naïve) from peripheral blood by flow cytometry.

RESULTS

In peripheral blood, CD4+CD25high Tregs were significantly increased in AIP patients (3.01% T 1.77%) compared with alcoholic chronic pancreatitis (CP) (1.65% T 0.58%), idiopathic CP (1.53% T0.56%), and healthy control (1.72% T 0.81%, P G 0.05). Naïve Tregs significantly decreased in AIP (0.32% T 0.22%) compared with healthy control (0.83% T 0.65%) and CP group (alcoholic and idiopathic CP; 0.52% T 0.40%, P G 0.05). In untreated AIP patients,the number of CD4+CD25high Tregs and IgG4 are correlated (R =0.53, P G 0.05).

CONCLUSIONS

Increased numbers of CD4+CD25high Tregs may influence IgG4 production in AIP, whereas decreased numbers of naïve Tregs may be involved in the pathogenesis of AIP.

Cutting edge: size and diversity of CD4+CD25high Foxp3+ regulatory T cell repertoire in humans: evidence for similarities and partial overlapping with CD4+CD25- T cells

Both differentiation and function of CD4+CD25(high) naturally arising regulatory T cells (Treg), which play a key role in the control of autoimmunity, are thought to depend on TCR specificity. In the present study, we comparatively measured the alphabetaTCR repertoire sizes of human peripheral blood Treg and CD4+CD25- T cells by using a methodology based on PCR amplification and sequencing analysis. We show that Treg use a large unrestricted alphabeta TCR repertoire, the size and diversity of which are closely similar to those of CD4+CD25- T cells, with a mean estimated size of 3.5 x 10(6) distinct alphabeta TCR vs 4.7 x 10(6) distinct alphabetaTCR for CD4+CD25- T cells. In addition, a 24% overlap between the repertoires of these two CD4+ subsets in the periphery is found. These data emphasize the intersection between naturally occurring Treg and effector T cell peripheral repertoires and provide new insights into the ontogeny of Treg in humans.

Nonself-antigens are the cognate specificities of Foxp3+ regulatory T cells

The majority of regulatory Foxp3+CD4+ T cells naturally arises in the thymus. It has been proposed that T cell receptors (TCRs) on these cells recognize self-MHC class II-peptide complexes with high or higher affinity and that their specificities mirror specificities of autoreactive T cells. Here, we analyzed hundreds of TCRs derived from regulatory or nonregulatory T cells and found little evidence that the former population preferably recognizes self-antigens as agonists. Instead, these cells recognized foreign MHC-peptide complexes as often as nonregulatory T cells. Our results show that high-affinity, autoreactive TCRs are rare on all CD4+ T cells and suggest that selecting self-peptide is different from the peptide that activates the same regulatory T cells in the periphery.

The regulation of Foxp3 expression in regulatory CD4(+)CD25(+)T cells: multiple pathways on the road

Regulatory T cells (Treg cells) have been well documented to have a crucial physiological role in preventing the development of autoimmune diseases and keeping self-tolerance. Foxp3, a recently identified member of the forkhead transcription factors, serves as a master regulator for the development and function of CD4(+)CD25(+)Treg cells. Though it is well defined that Foxp3 expression is sufficient to program CD4(+)CD25(+)Treg cell development, the physiological factors initiating intracellular Foxp3 expression remain poorly understood so far. In the present manuscript, we try to summarize the recent advances regarding the regulatory roles of T-cell receptor (TCR), co-stimulatory molecules, interleukin-2 (IL-2), transforming growth factor-beta (TGF-beta) and beyond pathways on Foxp3 expression.