Affinity of thymic self-peptides for the TCR determines the selection of CD8(+) T lymphocytes in the thymus

Rapid cloning of antigen-specific T-cell receptors by leveraging the cis activation of T cells

It is commonly understood that T cells are activated via trans interactions between antigen-specific T-cell receptors (TCRs) and antigenic peptides presented on major histocompatibility complex (MHC) molecules on antigen-presenting cells. By analysing a large number of T cells at the single-cell level on a microwell array, we show that T-cell activation can occur via cis interactions (where TCRs on the T cell interact with the antigenic peptides presented on MHC class-I molecules on the same cell), and that such cis activation can be used to detect antigen-specific T cells and clone their TCR within 4 d. We used the detection-and-cloning system to clone a tumour-antigen-specific TCR from peripheral blood mononuclear cells of healthy donors. TCR cloning by leveraging the cis activation of T cells may facilitate the development of TCR-engineered T cells for cancer therapy.

The early proximal αβ TCR signalosome specifies thymic selection outcome through a quantitative protein interaction network

During αβ T cell development, T cell antigen receptor (TCR) engagement transduces biochemical signals through a protein-protein interaction (PPI) network that dictates dichotomous cell fate decisions. It remains unclear how signal specificity is communicated, instructing either positive selection to advance cell differentiation or death by negative selection. Early signal discrimination might occur by PPI signatures differing qualitatively (customized, unique PPI combinations for each signal), quantitatively (graded amounts of a single PPI series), or kinetically (speed of PPI pathway progression). Using a novel PPI network analysis, we found that early TCR-proximal signals distinguishing positive from negative selection appeared to be primarily quantitative in nature. Furthermore, the signal intensity of this PPI network was used to find an antigen dose that caused a classic negative selection ligand to induce positive selection of conventional αβ T cells, suggesting that the quantity of TCR triggering was sufficient to program selection outcome. Because previous work had suggested that positive selection might involve a qualitatively unique signal through CD3δ, we reexamined the block in positive selection observed in CD3δ⁰ mice. We found that CD3δ⁰ thymocytes were inhibited but capable of signaling positive selection, generating low numbers of MHC-dependent αβ T cells that expressed diverse TCR repertoires and participated in immune responses against infection. We conclude that the major role for CD3δ in positive selection is to quantitatively boost the signal for maximal generation of αβ T cells. Together, these data indicate that a quantitative network signaling mechanism through the early proximal TCR signalosome determines thymic selection outcome.

Targeted suppression of autoreactive CD8+ T-cell activation using blocking anti-CD8 antibodies

CD8⁺ T-cells play a role in the pathogenesis of autoimmune diseases such as multiple sclerosis and type 1 diabetes. However, drugs that target the entire CD8⁺ T-cell population are not desirable because the associated lack of specificity can lead to unwanted consequences, most notably an enhanced susceptibility to infection. Here, we show that autoreactive CD8⁺ T-cells are highly dependent on CD8 for ligand-induced activation via the T-cell receptor (TCR). In contrast, pathogen-specific CD8⁺ T-cells are relatively CD8-independent. These generic differences relate to an intrinsic dichotomy that segregates self-derived and exogenous antigen-specific TCRs according to the monomeric interaction affinity with cognate peptide-major histocompatibility complex class I (pMHCI). As a consequence, “blocking” anti-CD8 antibodies can suppress autoreactive CD8⁺ T-cell activation in a relatively selective manner. These findings provide a rational basis for the development and in vivo assessment of novel therapeutic strategies that preferentially target disease-relevant autoimmune responses within the CD8⁺ T-cell compartment.

Th1/Th17 cytokine profiles in patients with reactive arthritis/undifferentiated spondyloarthropathy

OBJECTIVE

Data on synovial fluid (SF) cytokine concentrations in patients with reactive arthritis (ReA) or undifferentiated spondyloarthropathy (uSpA) are limited and contradictory. We measured levels of several proinflammatory and immunoregulatory cytokines in SF and sera from patients with ReA/uSpA.

METHODS

Interleukin 17 (IL-17), IL-6, interferon-g (IFN-g), and IL-12p40, and immunoregulatory cytokines IL-10 and transforming growth factor-beta (TGF-beta) were assayed using ELISA in SF specimens from 51 patients with ReA/uSpA (ReA 21, uSpA 30), 40 patients with rheumatoid arthritis (RA), and 11 patients with osteoarthritis (OA). IL-17, IL-6, IFN-g, and IL-10 levels were also measured in paired sera samples from patients with ReA/uSpA.

RESULTS

SF concentrations of IL-17, IL-6, TGF-beta, and IFN-g were significantly higher in patients with ReA/uSpA as compared to RA patients (for IL-17 median 46 pg/ml, range < 7.8-220 vs median < 7.8 pg/ml, range < 7.8-136, p < 0.05; for TGF-beta median 4.2 ng/ml, range 1.32-12 vs median 3.01 ng/ml, range 0.6-9.6, p < 0.01; for IL-6 median 58 ng/ml, range 2-540 vs median 34.5 ng/ml, range < 0.009-220, p < 0.05; for IFN-g median 290 pg/ml, range < 9.4-1600 vs median 100 pg/ml, range < 9.4-490, p < 0.05). SF levels of IL-10 were comparable but the ratio of IFN-g/IL-10 was significantly higher in ReA/uSpA patients than RA patients (median 3.18, range 0.06-200 for ReA/uSpA vs median 1.0, range 0.03-26.9 for RA; p < 0.05). IL-17, IL-6, IL-10, and IFN-g SF levels were significantly higher than paired serum levels in ReA/uSpA patients (p < 0.01 for IL-17, p < 0.0001 for IL-6, p < 0.0001 for IL-10, and p < 0.001 for IFN-g).

CONCLUSION

Increased IL-17, IL-6, TGF-beta, and IFN-g concentrations in ReA/uSpA than in RA suggest that Th1 and Th17 cells could be the major agents in inflammation in ReA/uSpA.

TCR-alpha CDR3 loop audition regulates positive selection

How positive selection molds the T cell repertoire has been difficult to examine. In this study, we use TCR-beta-transgenic mice in which MHC shapes TCR-alpha use. Differential AV segment use is directly related to the constraints placed on the composition of the CDR3 loops. Where these constraints are low, efficient selection of alphabeta pairs follows. This mode of selection preferentially uses favored AV-AJ rearrangements and promotes diversity. Increased constraint on the alpha CDR3 loops leads to inefficient selection associated with uncommon recombination events and limited diversity. Further, the two modes of selection favor alternate sets of AJ segments. We discuss the relevance of these findings to the imprint of self-MHC restriction and peripheral T cell activation.

Ephrin-B1 Is Critical in T-cell Development

Eph kinases are the largest family of receptor tyrosine kinases, and their ligands, ephrins (EFNs), are also cell surface molecules. In this study, we investigated the role of EFNB1 and the Ephs it interacts with (collectively called EFNB1 receptors) in mouse T-cell development. In the thymus, CD8 single positive (SP) and CD4CD8 double positive (DP) cells expressed high levels of EFNB1 and EFNB1 receptors, whereas CD4 SP cells had moderate expression of both. Soluble EFNB1-Fc in fetal thymus organ culture caused significant subpopulation ratio skew, with increased CD4 SP and CD8 SP and decreased DP percentage, while the cellularity of the thymus remained constant. Moreover, in EFNB1-treated fetal thymus organ culture, CD117(+), CD25(+), DP, CD4 SP, and CD8 SP cells all had significantly enhanced proliferation history, according to bromodeoxyuridine uptake. In vitro culture of isolated thymocytes revealed that EFNB1-Fc on solid-phase protected thymocytes from anti-CD3-induced apoptosis, with concomitant augmentation of several antiapoptotic factors, particularly in CD4 SP and CD8 SP cells; on the other hand, soluble EFNB1-Fc promoted anti-CD3-induced apoptosis, as was the case in vivo. This study reveals that EFNB1 and EFNB1 receptors are critical in thymocyte development.

Tetramer analysis of human autoreactive CD4-positive T cells

Self-reactivity is an intrinsic property of the human immune system. Autoreactive T cells derive directly from the developmental requirement for TCR engagement by self-antigens during lymphocyte maturation. The fundamental questions implicating these autoreactive cells in human autoimmunity then, are not "Where do they come from?", but rather "Why do they persist?", "How do they become activated?", and "How are they regulated or deleted?". New technologies, in which peptide-MHC (pMHC) ligands used for T-cell recognition are utilized as soluble fluorescent multimers, now permit the direct visualization of antigen-specific autoreactive T-lymphocytes. By using multimer technology to study self-reactive cells present in autoimmune patients and control individuals, a very broad range of autoreactive potential has been identified.

A role for the granzyme B inhibitor serine protease inhibitor 6 in CD8+ memory cell homeostasis

Generation and maintenance of protective immunological memory is the goal of vaccination programs. It has recently become clear that CD8+ memory T cells are derived directly from CTLs. The mechanisms underlying this transformation and the subsequent survival of memory cells are not completely understood. However, some effector molecules required by CTLs to eliminate infected cells have also been shown to control the number of Ag-specific cells. We report that memory cells express high levels of serine protease inhibitor (Spi) 6, an inhibitor of the effector molecule granzyme B, and that Spi6 can protect T cells from granzyme B-mediated apoptosis. In mouse models, both elevated expression of Spi6 and the complete absence of granzyme B in CD8+ T cells led to an increase in memory cells after infection with lymphocytic choriomeningitis virus. This was not the result of increased levels of antilymphocytic choriomeningitis virus CD8+ T cells during the expansion or contraction phases, but rather transgenic Spi6 directly influenced the survival of CD8+ memory T cells. We propose that expression of protective molecules, like Spi6, serves to shield metabolically active CD8+ memory T cells from their own effector molecules.

Distinct footprints of TCR engagement with highly homologous ligands

T cell receptor engagement promotes proliferation, differentiation, survival, or death of T lymphocytes. The affinity/avidity of the TCR ligand and the maturational stage of the T cell are thought to be principal determinants of the outcome of TCR engagement. We demonstrate in this study that the same mouse TCR preferentially uses distinct residues of homologous peptides presented by the MHC molecules to promote specific cellular responses. The preference for distinct TCR contacts depends on neither the affinity/avidity of TCR engagement (except in the most extreme ranges), nor the maturity of engaged T cells. Thus, different portions of the TCR ligand appear capable of biasing T cells toward specific biological responses. These findings explain differences in functional versatility of TCR ligands, as well as anomalies in the relationship between affinity/avidity of the TCR for the peptide/MHC and cellular responses of T cells.

Serine protease inhibitor 2A is a protective factor for memory T cell development

An essential event in the development of memory CD8(+) T lymphocytes is the escape of progenitors from programmed cell death, but how this is mediated is unclear. Here we report that the gene encoding serine protease inhibitor 2A (Spi2A), an inhibitor of lysosomal executioner proteases dependent on transcription factor NF-kappaB, is upregulated in memory cell precursors. Spi2A upregulation protected lymphocytic choriomeningitis virus-specific memory progenitors from programmed cell death. Thus, Spi2A promotes the survival of cytotoxic T lymphocytes, allowing them to differentiate into memory CD8 T cells. These findings suggest a model in which commitment to the memory lineage is facilitated by the upregulation of protective genes.

Duration of Alloantigen Presentation and Avidity of T Cell Antigen Recognition Correlate with Immunodominance of CTL Response to Minor Histocompatibility Antigens

CD8 T lymphocytes (CTL) responsive to immunodominant minor histocompatibility (minor H) Ags are thought to play a disproportionate role in allograft rejection in MHC-identical solid and bone marrow transplant settings. Although many studies have addressed the mechanisms underlying immunodominance in models of infectious diseases, cancer immunotherapy, and allograft immunity, key issues regarding the molecular basis of immunodominance remain poorly understood. In this study, we exploit the minor H Ag system to understand the relationship of the various biochemical parameters of Ag presentation and recognition to immunodominance. We show that the duration of individual minor H Ag presentation and the avidity of T cell Ag recognition influence the magnitude and, hence, the immunodominance of the CTL response to minor H Ags. These properties of CTL Ag presentation and recognition that contribute to immunodominance have implications not only for tissue transplantation, but also for autoimmunity and tumor vaccine design.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

Rare, structurally homologous self-peptides promote thymocyte positive selection

Although it is clear that positive selection of T cells involves recognition of specific self-peptide/MHC complexes, the nature of these self-ligands and their relationship to the cognate antigen are controversial. Here we used two complementary strategies to identify naturally occurring self-peptides able to induce positive selection of T cells bearing a specific T cell receptor, OT-I. Both the bioassay- and bioinformatics-based strategies identified the same self-peptides, derived from F-actin capping protein and beta-catenin. These peptides displayed charge conservation at two key TCR contact residues. The biological activity of 43 other self-peptides and of complex peptide libraries directly correlated to the extent of conservation at TCR contact residues. These results demonstrate that selecting self-peptides are rare and can be identified by homology-based search strategies.

A naturally processed mitochondrial self-peptide in complex with thymic MHC molecules functions as a selecting ligand for a viral-specific T cell receptor

Peptide fragments of self-proteins bound to major histocompatibility complex molecules within the thymus are important for positively selecting T cell receptor (TCR)-bearing CD4(+)CD8(+) double positive (DP) thymocytes for further maturation. The relationship between naturally processed thymic self-peptides and TCR-specific cognate peptides is unknown. Here we employ HPLC purification of peptides released from H-2K(b) molecules of the C57BL/6 thymus in conjunction with mass spectrometry (MS) and functional profiling to identify a naturally processed K(b)-bound peptide positively selecting the N15 TCR specific for the vesicular stomatitis virus octapeptide (VSV8) bound to K(b). The selecting peptide was identified in 1 of 80 HPLC fractions and shown by tandem MS (MS/MS) sequencing to correspond to residues 68-75 of the MLRQ subunit of the widely expressed mitochondrial NADH ubiquinone oxidoreductase (NUbO(68-75)). Of note, the peptide differs at six of its eight residues from the cognate peptide VSV8 and functions as a weak agonist for mature CD8 single positive (SP) N15 T cells, with activity 10,000-fold less than VSV8. In N15 transgenic (tg) recombinase activating gene 2(-/)- transporter associated with antigen processing 1(-/)- fetal thymic organ culture, NUbO(68-75) induces phenotypic and functional differentiation of N15 TCR bearing CD8 SP thymocytes. Failure of NUbO(68-75) to support differentiation of a second K(b)-restricted TCR indicates that its inductive effects are not general.