Self-reactive human CD4 T cell clones form unusual immunological synapses

Effects of High Efficacy Multiple Sclerosis Disease Modifying Drugs on the Immune Synapse: A Systematic Review

BACKGROUND

Co-signaling and adhesion molecules are important elements for creating immune synapses between T lymphocytes and antigen-presenting cells; they positively or negatively regulate the interaction between a T cell receptor with its cognate antigen, presented by the major histocompatibility complex.

OBJECTIVES

We conducted a systematic review on the effects of High Efficacy Disease Modifying Drugs (HEDMDs) for Multiple Sclerosis (MS) on the co-signaling and adhesion molecules that form the immune synapse.

METHODS

We searched EMBASE, MEDLINE, and other sources to identify clinical or preclinical reports on the effects of HEDMDs on co-signaling and adhesion molecules that participate in the formation of immune synapses in patients with MS or other autoimmune disorders. We included reports on cladribine tablets, anti- CD20 monoclonal antibodies, S1P modulators, inhibitors of Bruton's Tyrosine Kinase, and natalizumab.

RESULTS

In 56 eligible reports among 7340 total publications, limited relevant evidence was uncovered. Not all co-signaling and adhesion molecules have been studied in relation to every HEDMD, with more data being available on the anti-CD20 monoclonal antibodies (that affect CD80, CD86, GITR and TIGIT), cladribine tablets (affecting CD28, CD40, ICAM-1, LFA-1) and the S1P modulators (affecting CD86, ICAM-1 and LFA-1) and less on Natalizumab (affecting CD80, CD86, CD40, LFA-1, VLA-4) and Alemtuzumab (affecting GITR and CTLA-4).

CONCLUSION

The puzzle of HEDMD effects on the immune synapse is far from complete. The available evidence suggests that distinguishing differences exist between drugs and are worth pursuing further.

Explainable machine learning for profiling the immunological synapse and functional characterization of therapeutic antibodies

Therapeutic antibodies are widely used to treat severe diseases. Most of them alter immune cells and act within the immunological synapse; an essential cell-to-cell interaction to direct the humoral immune response. Although many antibody designs are generated and evaluated, a high-throughput tool for systematic antibody characterization and prediction of function is lacking. Here, we introduce the first comprehensive open-source framework, scifAI (single-cell imaging flow cytometry AI), for preprocessing, feature engineering, and explainable, predictive machine learning on imaging flow cytometry (IFC) data. Additionally, we generate the largest publicly available IFC dataset of the human immunological synapse containing over 2.8 million images. Using scifAI, we analyze class frequency and morphological changes under different immune stimulation. T cell cytokine production across multiple donors and therapeutic antibodies is quantitatively predicted in vitro, linking morphological features with function and demonstrating the potential to significantly impact antibody design. scifAI is universally applicable to IFC data. Given its modular architecture, it is straightforward to incorporate into existing workflows and analysis pipelines, e.g., for rapid antibody screening and functional characterization.

INPP5E regulates CD3ζ enrichment at the immune synapse by phosphoinositide distribution control

The immune synapse, a highly organized structure formed at the interface between T lymphocytes and antigen-presenting cells (APCs), is essential for T cell activation and the adaptive immune response. It has been shown that this interface shares similarities with the primary cilium, a sensory organelle in eukaryotic cells, although the roles of ciliary proteins on the immune synapse remain elusive. Here, we find that inositol polyphosphate-5-phosphatase E (INPP5E), a cilium-enriched protein responsible for regulating phosphoinositide localization, is enriched at the immune synapse in Jurkat T-cells during superantigen-mediated conjugation or antibody-mediated crosslinking of TCR complexes, and forms a complex with CD3ζ, ZAP-70, and Lck. Silencing INPP5E in Jurkat T-cells impairs the polarized distribution of CD3ζ at the immune synapse and correlates with a failure of PI(4,5)P2 clearance at the center of the synapse. Moreover, INPP5E silencing decreases proximal TCR signaling, including phosphorylation of CD3ζ and ZAP-70, and ultimately attenuates IL-2 secretion. Our results suggest that INPP5E is a new player in phosphoinositide manipulation at the synapse, controlling the TCR signaling cascade.

Bispecific Antibody Format and the Organization of Immunological Synapses in T Cell-Redirecting Strategies for Cancer Immunotherapy

T cell-redirecting strategies have emerged as effective cancer immunotherapy approaches. Bispecific antibodies (bsAbs) are designed to specifically recruit T cells to the tumor microenvironment and induce the assembly of the immunological synapse (IS) between T cells and cancer cells or antigen-presenting cells. The way that the quality of the IS might predict the effectiveness of T cell-redirecting strategies, including those mediated by bsAbs or by chimeric antigen receptors (CAR)-T cells, is currently under discussion. Here we review the organization of the canonical IS assembled during natural antigenic stimulation through the T cell receptor (TCR) and to what extent different bsAbs induce T cell activation, canonical IS organization, and effector function. Then, we discuss how the biochemical parameters of different formats of bsAbs affect the effectivity of generating an antigen-induced canonical IS. Finally, the quality of the IS assembled by bsAbs and monoclonal antibodies or CAR-T cells are compared, and strategies to improve bsAb-mediated T cell-redirecting strategies are discussed.

Self-antigens, benign autoimmunity and type 1 diabetes: a beta-cell and T-cell perspective

PURPOSE OF REVIEW

Recent work using immunopeptidomics and deconvolution of the antigenic reactivity of islet-infiltrating CD8+ T cells has expanded our knowledge about the autoimmune target epitopes of type 1 diabetes. The stem-like properties of autoimmune CD8+ T cells have also been described. We here propose a possible link between these findings.

RECENT FINDINGS

Weak major histocompatibility complex (MHC)-binding epitopes list among the major targets of human islet-infiltrating CD8+ T cells, likely resulting in low peptide-MHC presentation that delivers weak T-cell receptor (TCR) signals, especially in the face of low-affinity autoimmune TCRs. These weak TCR signals may favor the maintenance of the partially differentiated stem-like phenotype recently described for islet-reactive CD8+ T cells in the blood and pancreatic lymph nodes. These weak TCR signals may also be physiological, reflecting the need for self-peptide-MHC contacts to maintain homeostatic T-cell survival and proliferation. These features may underlie the universal state of benign autoimmunity that we recently described, which is characterized by islet-reactive, naïve-like CD8+ T cells circulating in all individuals.

SUMMARY

These observations provide novel challenges and opportunities to develop circulating T-cell biomarkers for autoimmune staging. Therapeutic halting of islet autoimmunity may require targeting of stem-like T cells to blunt their self-regeneration.

T-cell trans-synaptic vesicles are distinct and carry greater effector content than constitutive extracellular vesicles

The immunological synapse is a molecular hub that facilitates the delivery of three activation signals, namely antigen, costimulation/corepression and cytokines, from antigen-presenting cells (APC) to T cells. T cells release a fourth class of signaling entities, trans-synaptic vesicles (tSV), to mediate bidirectional communication. Here we present bead-supported lipid bilayers (BSLB) as versatile synthetic APCs to capture, characterize and advance the understanding of tSV biogenesis. Specifically, the integration of juxtacrine signals, such as CD40 and antigen, results in the adaptive tailoring and release of tSV, which differ in size, yields and immune receptor cargo compared with steadily released extracellular vesicles (EVs). Focusing on CD40L+ tSV as model effectors, we show that PD-L1 trans-presentation together with TSG101, ADAM10 and CD81 are key in determining CD40L vesicular release. Lastly, we find greater RNA-binding protein and microRNA content in tSV compared with EVs, supporting the specialized role of tSV as intercellular messengers.

Beyond GWAS-Could Genetic Differentiation within the Allograft Rejection Pathway Shape Natural Immunity to COVID-19?

COVID-19 infections pose a serious global health concern so it is crucial to identify the biomarkers for the susceptibility to and resistance against this disease that could help in a rapid risk assessment and reliable decisions being made on patients' treatment and their potential hospitalisation. Several studies investigated the factors associated with severe COVID-19 outcomes that can be either environmental, population based, or genetic. It was demonstrated that the genetics of the host plays an important role in the various immune responses and, therefore, there are different clinical presentations of COVID-19 infection. In this study, we aimed to use variant descriptive statistics from GWAS (Genome-Wide Association Study) and variant genomic annotations to identify metabolic pathways that are associated with a severe COVID-19 infection as well as pathways related to resistance to COVID-19. For this purpose, we applied a custom-designed mixed linear model implemented into custom-written software. Our analysis of more than 12.5 million SNPs did not indicate any pathway that was significant for a severe COVID-19 infection. However, the Allograft rejection pathway (hsa05330) was significant (p = 0.01087) for resistance to the infection. The majority of the 27 SNP marking genes constituting the Allograft rejection pathway were located on chromosome 6 (19 SNPs) and the remainder were mapped to chromosomes 2, 3, 10, 12, 20, and X. This pathway comprises several immune system components crucial for the self versus non-self recognition, but also the components of antiviral immunity. Our study demonstrated that not only single variants are important for resistance to COVID-19, but also the cumulative impact of several SNPs within the same pathway matters.

The perception and response of T cells to a changing environment are based on the law of initial value

αβ T cells are critical components of the adaptive immune system and are capable of inducing sterilizing immunity after pathogen infection and eliminating transformed tumor cells. The development and function of T cells are controlled through the T cell antigen receptor, which recognizes peptides displayed on major histocompatibility complex (MHC) molecules. Here, we review how T cells generate the ability to recognize self-peptide-bound MHC molecules and use signals derived from these interactions to instruct cellular development, activation thresholds, and functional specialization in the steady state and during immune responses. We argue that the basic tenants of T cell development and function follow Weber-Fetcher's law of just noticeable differences and Wilder's law of initial value. Together, these laws argue that the ability of a system to respond and the quality of that response are scalable to the basal state of that system. Manifestation of these laws in T cells generates clone-specific activation thresholds that are based on perceivable differences between homeostasis and pathogen encounter (self versus nonself discrimination), as well as poised states for subsequent differentiation into specific effector cell lineages.

LFA-1 and kindlin-3 enable the collaborative transport of SLP-76 microclusters by myosin and dynein motors

Integrin engagement within the immune synapse enhances T cell activation, but our understanding of this process is incomplete. In response to T cell receptor (TCR) ligation, SLP-76 (LCP2), ADAP (FYB1) and SKAP55 (SKAP1) are recruited into microclusters and activate integrins via the effectors talin-1 and kindlin-3 (FERMT3). We postulated that integrins influence the centripetal transport and signaling of SLP-76 microclusters via these linkages. We show that contractile myosin filaments surround and are co-transported with SLP-76 microclusters, and that TCR ligand density governs the centripetal movement of both structures. Centripetal transport requires formin activity, actomyosin contraction, microtubule integrity and dynein motor function. Although immobilized VLA-4 (α4β1 integrin) and LFA-1 (αLβ2 integrin) ligands arrest the centripetal movement of SLP-76 microclusters and myosin filaments, VLA-4 acts distally, while LFA-1 acts in the lamellum. Integrin β2, kindlin-3 and zyxin are required for complete centripetal transport, while integrin β1 and talin-1 are not. CD69 upregulation is similarly dependent on integrin β2, kindlin-3 and zyxin, but not talin-1. These findings highlight the integration of cytoskeletal systems within the immune synapse and reveal extracellular ligand-independent roles for LFA-1 and kindlin-3. This article has an associated First Person interview with the first author of the paper.

Overexpression of the PTPN22 Autoimmune Risk Variant LYP-620W Fails to Restrain Human CD4+ T Cell Activation

A missense mutation (R620W) of protein tyrosine phosphatase nonreceptor type 22 (PTPN22), which encodes lymphoid-tyrosine phosphatase (LYP), confers genetic risk for multiple autoimmune diseases including type 1 diabetes. LYP has been putatively demonstrated to attenuate proximal T and BCR signaling. However, limited data exist regarding PTPN22 expression within primary T cell subsets and the impact of the type 1 diabetes risk variant on human T cell activity. In this study, we demonstrate endogenous PTPN22 is differentially expressed and dynamically controlled following activation. From control subjects homozygous for the nonrisk allele, we observed 2.1- (p < 0.05) and 3.6-fold (p < 0.001) more PTPN22 transcripts in resting CD4⁺ memory and regulatory T cells (Tregs), respectively, over naive CD4⁺ T cells, with expression peaking 24 h postactivation. When LYP was overexpressed in conventional CD4⁺ T cells, TCR signaling and activation were blunted by LYP-620R (p < 0.001) but only modestly affected by the LYP-620W risk variant versus mock-transfected control, with similar results observed in Tregs. LYP overexpression only impacted proliferation following activation by APCs but not anti-CD3- and anti-CD28-coated microbeads, suggesting LYP modulation of pathways other than TCR. Notably, proliferation was significantly lower with LYP-620R than with LYP-620W overexpression in conventional CD4⁺ T cells but was similar in Treg. These data indicate that the LYP-620W variant is hypomorphic in the context of human CD4⁺ T cell activation and may have important implications for therapies seeking to restore immunological tolerance in autoimmune disorders.

Nine residues in HLA-DQ molecules determine with susceptibility and resistance to type 1 diabetes among young children in Sweden

HLA-DQ molecules account over 50% genetic risk of type 1 diabetes (T1D), but little is known about associated residues. Through next generation targeted sequencing technology and deep learning of DQ residue sequences, the aim was to uncover critical residues and their motifs associated with T1D. Our analysis uncovered (αa1, α44, α157, α196) and (β9, β30, β57, β70, β135) on the HLA-DQ molecule. Their motifs captured all known susceptibility and resistant T1D associations. Three motifs, “DCAA-YSARD” (OR = 2.10, p = 1.96*10⁻²⁰), “DQAA-YYARD” (OR = 3.34, 2.69*10⁻⁷²) and “DQDA-YYARD” (OR = 3.71, 1.53*10⁻⁶) corresponding to DQ2.5 and DQ8.1 (the latter two motifs) associated with susceptibility. Ten motifs were significantly associated with resistance to T1D. Collectively, homozygous DQ risk motifs accounted for 43% of DQ-T1D risk, while homozygous DQ resistant motifs accounted for 25% protection to DQ-T1D risk. Of the identified nine residues five were within or near anchoring pockets of the antigenic peptide (α44, β9, β30, β57 and β70), one was the N-terminal of the alpha chain (αa1), one in the CD4-binding region (β135), one in the putative cognate TCR-induced αβ homodimerization process (α157), and one in the intra-membrane domain of the alpha chain (α196). Finding these critical residues should allow investigations of fundamental properties of host immunity that underlie tolerance to self and organ-specific autoimmunity.

Type 1 diabetes mellitus as a disease of the β-cell (do not blame the immune system?)

Type 1 diabetes mellitus is believed to result from destruction of the insulin-producing β-cells in pancreatic islets that is mediated by autoimmune mechanisms. The classic view is that autoreactive T cells mistakenly destroy healthy ('innocent') β-cells. We propose an alternative view in which the β-cell is the key contributor to the disease. By their nature and function, β-cells are prone to biosynthetic stress with limited measures for self-defence. β-Cell stress provokes an immune attack that has considerable negative effects on the source of a vital hormone. This view would explain why immunotherapy at best delays progression of type 1 diabetes mellitus and points to opportunities to use therapies that revitalize β-cells, in combination with immune intervention strategies, to reverse the disease. We present the case that dysfunction occurs in both the immune system and β-cells, which provokes further dysfunction, and present the evidence leading to the consensus that islet autoimmunity is an essential component in the pathogenesis of type 1 diabetes mellitus. Next, we build the case for the β-cell as the trigger of an autoimmune response, supported by analogies in cancer and antitumour immunity. Finally, we synthesize a model ('connecting the dots') in which both β-cell stress and islet autoimmunity can be harnessed as targets for intervention strategies.

A LAT-Based Signaling Complex in the Immunological Synapse as Determined with Live Cell Imaging Is Less Stable in T Cells with Regulatory Capability

Peripheral immune regulation is critical for the maintenance of self-tolerance. Here we have investigated signaling processes that distinguish T cells with regulatory capability from effector T cells. The murine Tg4 T cell receptor recognizes a peptide derived from the self-antigen myelin basic protein. T cells from Tg4 T cell receptor transgenic mice can be used to generate effector T cells and three types of T cells with regulatory capability, inducible regulatory T cells, T cells tolerized by repeated in vivo antigenic peptide exposure or T cells treated with the tolerogenic drug UCB9608 (a phosphatidylinositol 4 kinase IIIβ inhibitor). We comparatively studied signaling in all of these T cells by activating them with the same antigen presenting cells presenting the same myelin basic protein peptide. Supramolecular signaling structures, as efficiently detected by large-scale live cell imaging, are critical mediators of T cell activation. The formation of a supramolecular signaling complex anchored by the adaptor protein linker for activation of T cells (LAT) was consistently terminated more rapidly in Tg4 T cells with regulatory capability. Such termination could be partially reversed by blocking the inhibitory receptors CTLA-4 and PD-1. Our work suggests that attenuation of proximal signaling may favor regulatory over effector function in T cells.

The Role of T Cell Receptor Signaling in the Development of Type 1 Diabetes

T cell receptor (TCR) signaling influences multiple aspects of CD4⁺ and CD8⁺ T cell immunobiology including thymic development, peripheral homeostasis, effector subset differentiation/function, and memory formation. Additional T cell signaling cues triggered by co-stimulatory molecules and cytokines also affect TCR signaling duration, as well as accessory pathways that further shape a T cell response. Type 1 diabetes (T1D) is a T cell-driven autoimmune disease targeting the insulin producing β cells in the pancreas. Evidence indicates that dysregulated TCR signaling events in T1D impact the efficacy of central and peripheral tolerance-inducing mechanisms. In this review, we will discuss how the strength and nature of TCR signaling events influence the development of self-reactive T cells and drive the progression of T1D through effects on T cell gene expression, lineage commitment, and maintenance of pathogenic anti-self T cell effector function.

Next-Generation HLA Sequence Analysis Uncovers Seven HLA-DQ Amino Acid Residues and Six Motifs Resistant to Childhood Type 1 Diabetes

HLA-DQA1 and -DQB1 genes have significant and potentially causal associations with autoimmune type 1 diabetes (T1D). To follow up on the earlier analysis on high-risk HLA-DQ2.5 and DQ8.1, the current analysis uncovers seven residues (αa1, α157, α196, β9, β30, β57, and β70) that are resistant to T1D among subjects with DQ4-, 5-, 6-, and 7-resistant DQ haplotypes. These 7 residues form 13 common motifs: 6 motifs are significantly resistant, 6 motifs have modest or no associations (P values >0.05), and 1 motif has 7 copies observed among control subjects only. The motifs "DAAFYDG," "DAAYHDG," and "DAAYYDR" have significant resistance to T1D (odds ratios [ORs] 0.03, 0.25, and 0.18; P = 6.11 × 10^-24, 3.54 × 10^-15, and 1.03 × 10^-21, respectively). Remarkably, a change of a single residue from the motif "DAAYHDG" to "DAAYHSG" (D to S at β57) alters the resistance potential, from resistant motif (OR 0.15; P = 3.54 × 10^-15) to a neutral motif (P = 0.183), the change of which was significant (Fisher P value = 0.0065). The extended set of linked residues associated with T1D resistance and unique to each cluster of HLA-DQ haplotypes represents facets of all known features and functions of these molecules: antigenic peptide binding, peptide-MHC class II complex stability, β167-169 RGD loop, T-cell receptor binding, formation of homodimer of α-β heterodimers, and cholesterol binding in the cell membrane rafts. Identification of these residues is a novel understanding of resistant DQ associations with T1D. Our analyses endow potential molecular approaches to identify immunological mechanisms that control disease susceptibility or resistance to provide novel targets for immunotherapeutic strategies.

Relationship of 2D Affinity to T Cell Functional Outcomes

T cells are critical for a functioning adaptive immune response and a strong correlation exists between T cell responses and T cell receptor (TCR): peptide-loaded MHC (pMHC) binding. Studies that utilize pMHC tetramer, multimers, and assays of three-dimensional (3D) affinity have provided advancements in our understanding of T cell responses across different diseases. However, these technologies focus on higher affinity and avidity T cells while missing the lower affinity responders. Lower affinity TCRs in expanded polyclonal populations almost always constitute a significant proportion of the response with cells mediating different effector functions associated with variation in the proportion of high and low affinity T cells. Since lower affinity T cells expand and are functional, a fully inclusive view of T cell responses is required to accurately interpret the role of affinity for adaptive T cell immunity. For example, low affinity T cells are capable of inducing autoimmune disease and T cells with an intermediate affinity have been shown to exhibit an optimal anti-tumor response. Here, we focus on how affinity of the TCR may relate to T cell phenotype and provide examples where 2D affinity influences functional outcomes.

The contribution of thymic tolerance to central nervous system autoimmunity

Autoimmune diseases of the central nervous system (CNS) are associated with high levels of morbidity and economic cost. Research efforts have previously focused on the contribution of the peripheral adaptive and innate immune systems to CNS autoimmunity. However, a failure of thymic negative selection is a necessary step in CNS-reactive T cells escaping into the periphery. Even with defective thymic or peripheral tolerance, the development of CNS inflammation is rare. The reasons underlying this are currently poorly understood. In this review, we examine evidence implicating thymic selection in the pathogenesis of CNS autoimmunity. Animal models suggest that thymic negative selection is an important factor in determining susceptibility to and severity of CNS inflammation. There are indirect clinical data that suggest thymic function is also important in human CNS autoimmune diseases. Specifically, the association between thymoma and paraneoplastic encephalitis and changes in T cell receptor excision circles in multiple sclerosis implicate thymic tolerance in these diseases. We identify potential associations between CNS autoimmunity susceptibility factors and thymic tolerance. The therapeutic manipulation of thymopoiesis has the potential to open up new treatment modalities, but a better understanding of thymic tolerance in CNS autoimmunity is required before this can be realised.

Composition and structure of synaptic ectosomes exporting antigen receptor linked to functional CD40 ligand from helper T cells

Planar supported lipid bilayers (PSLB) presenting T cell receptor (TCR) ligands and ICAM-1 induce budding of extracellular microvesicles enriched in functional TCR, defined here as synaptic ectosomes (SE), from helper T cells. SE bind peptide-MHC directly exporting TCR into the synaptic cleft, but incorporation of other effectors is unknown. Here, we utilized bead supported lipid bilayers (BSLB) to capture SE from single immunological synapses (IS), determined SE composition by immunofluorescence flow cytometry and enriched SE for proteomic analysis by particle sorting. We demonstrate selective enrichment of CD40L and ICOS in SE in response to addition of CD40 and ICOSL, respectively, to SLB presenting TCR ligands and ICAM-1. SE are enriched in tetraspanins, BST-2, TCR signaling and ESCRT proteins. Super-resolution microscopy demonstrated that CD40L is present in microclusters within CD81 defined SE that are spatially segregated from TCR/ICOS/BST-2. CD40L+ SE retain the capacity to induce dendritic cell maturation and cytokine production.

The role of thymic tolerance in CNS autoimmune disease

The contributions of the peripheral adaptive and innate immune systems to CNS autoimmunity have been extensively studied. However, the role of thymic selection in these conditions is much less well understood. The thymus is the primary lymphoid organ for the generation of T cells; thymic mechanisms ensure that cells with an overt autoreactive specificity are eliminated before they emigrate to the periphery and control the generation of thymic regulatory T cells. Evidence from animal studies demonstrates that thymic T cell selection is important for establishing tolerance to autoantigens. However, there is a considerable knowledge gap regarding the role of thymic selection in autoimmune conditions of the human CNS. In this Review, we critically examine the current body of experimental evidence for the contribution of thymic tolerance to CNS autoimmune diseases. An understanding of why dysfunction of either thymic or peripheral tolerance mechanisms rarely leads to CNS inflammation is currently lacking. We examine the potential of de novo T cell formation and thymic selection as novel therapeutic avenues and highlight areas for future study that are likely to make these targets the focus of future treatments.

A temporal thymic selection switch and ligand binding kinetics constrain neonatal Foxp3+ Treg cell development

The neonatal thymus generates Foxp3⁺ regulatory T (tT_reg) cells that are critical in controlling immune homeostasis and preventing multiorgan autoimmunity. The role of antigen specificity on neonatal tT_reg cell selection is unresolved. Here we identify 17 self-peptides recognized by neonatal tT_reg cells, and reveal ligand specificity patterns that include self-antigens presented in an age- and inflammation-dependent manner. Fate-mapping studies of neonatal peptidyl arginine deiminase type IV (Padi4)-specific thymocytes reveal disparate fate choices. Neonatal thymocytes expressing T cell receptors that engage IA^b-Padi4 with moderate dwell times within a conventional docking orientation are exported as tT_reg cells. In contrast, Padi4-specific T cell receptors with short dwell times are expressed on CD4⁺ T cells, while long dwell times induce negative selection. Temporally, Padi4-specific thymocytes are subject to a developmental stage-specific change in negative selection, which precludes tT_reg cell development. Thus, a temporal switch in negative selection and ligand binding kinetics constrains the neonatal tT_reg selection window.

A Protocol to Study T-Cell Signaling in an Immune Synapse by Microscopy

The immune synapse is a complex cellular structure that enables cell-cell communications between immune cells, mainly at the interface of an effector T-cell and an antigen-presenting cell (APC) that expresses the appropriate peptide-MHC complexes. With progressive technological advances, there has been increasing interest in understanding molecular events that take place in motile T-lymphocytes at the immune synapse. Here, we provide an optimized method to induce the formation of an immune synapse between a T-cell and an APC in vitro. The experimental protocol described herein would be useful in characterizing the role of cell surface molecules and downstream signaling pathways in the context of cell-to-cell communications between T-cells and APCs.

Altered Binding of Tumor Antigenic Peptides to MHC Class I Affects CD8+ T Cell-Effector Responses

T-cell priming occurs when a naïve T cell recognizes cognate peptide-MHC complexes on an activated antigen-presenting cell. The circumstances of this initial priming have ramifications on the fate of the newly primed T cell. Newly primed CD8⁺ T cells can embark onto different trajectories, with some becoming short-lived effector cells and others adopting a tissue resident or memory cell fate. To determine whether T-cell priming influences the quality of the effector T-cell response to tumors, we used transnuclear CD8⁺ T cells that recognize the melanoma antigen TRP1 using TRP1^high or TRP1^low TCRs that differ in both affinity and fine specificity. From a series of altered peptide ligands, we identified a point mutation (K8) in a nonanchor residue that, when analyzed crystallographically and biophysically, destabilized the peptide interaction with the MHC binding groove. In vitro, the K8 peptide induced robust proliferation of both TRP1^high and TRP1^low CD8⁺ T cells but did not induce expression of PD-1. Cytokine production from K8-stimulated TRP1 cells was minimal, whereas cytotoxicity was increased. Upon transfer into B16 tumor-bearing mice, the reference peptide (TRP1-M9)- and K8-stimulated TRP1 cells were equally effective at controlling tumor growth but accomplished this through different mechanisms. TRP1-M9-stimulated cells produced more IFNγ, whereas K8-stimulated cells accumulated to higher numbers and were more cytotoxic. We, therefore, conclude that TCR recognition of weakly binding peptides during priming can skew the effector function of tumor-specific CD8⁺ T cells.

Dendritic cell-expressed common gamma-chain recruits IL-15 for trans-presentation at the murine immunological synapse

Background: Mutations of the common cytokine receptor gamma chain (γc) cause Severe Combined Immunodeficiency characterized by absent T and NK cell development. Although stem cell therapy restores these lineages, residual immune defects are observed that may result from selective persistence of γc-deficiency in myeloid lineages. However, little is known about the contribution of myeloid-expressed γc to protective immune responses.  Here we examine the importance of γc for myeloid dendritic cell (DC) function. Methods: We utilize a combination of in vitro DC/T-cell co-culture assays and a novel lipid bilayer system mimicking the T cell surface to delineate the role of DC-expressed γc during DC/T-cell interaction. Results: We observed that γc in DC was recruited to the contact interface following MHCII ligation, and promoted IL-15Rα colocalization with engaged MHCII. Unexpectedly, trans-presentation of IL-15 was required for optimal CD4+T cell activation by DC and depended on DC γc expression. Neither recruitment of IL-15Rα nor IL-15 trans-signaling at the DC immune synapse (IS), required γc signaling in DC, suggesting that γc facilitates IL-15 transpresentation through induced intermolecular cis associations or cytoskeletal reorganization following MHCII ligation. Conclusions: These findings show that DC-expressed γc is required for effective antigen-induced CD4+ T cell activation. We reveal a novel mechanism for recruitment of DC IL-15/IL-15Rα complexes to the IS, leading to CD4+ T cell costimulation through localized IL-15 transpresentation that is coordinated with antigen-recognition.

Heterogeneity of circulating CD8 T-cells specific to islet, neo-antigen and virus in patients with type 1 diabetes mellitus

Auto-reactive CD8 T-cells play an important role in the destruction of pancreatic β-cells resulting in type 1 diabetes (T1D). However, the phenotype of these auto-reactive cytolytic CD8 T-cells has not yet been extensively described. We used high-dimensional mass cytometry to phenotype autoantigen- (pre-proinsulin), neoantigen- (insulin-DRIP) and virus- (cytomegalovirus) reactive CD8 T-cells in peripheral blood mononuclear cells (PBMCs) of T1D patients. A panel of 33 monoclonal antibodies was designed to further characterise these cells at the single-cell level. HLA-A2 class I tetramers were used for the detection of antigen-specific CD8 T-cells. Using a novel Hierarchical Stochastic Neighbor Embedding (HSNE) tool (implemented in Cytosplore), we identified 42 clusters within the CD8 T-cell compartment of three T1D patients and revealed profound heterogeneity between individuals, as each patient displayed a distinct cluster distribution. Single-cell analysis of pre-proinsulin, insulin-DRIP and cytomegalovirus-specific CD8 T-cells showed that the detected specificities were heterogeneous between and within patients. These findings emphasize the challenge to define the obscure nature of auto-reactive CD8 T-cells.

Dendritic cell-expressed common gamma-chain recruits IL-15 for trans-presentation at the murine immunological synapse

Background: Mutations of the common cytokine receptor gamma chain (γc) cause Severe Combined Immunodeficiency characterized by absent T and NK cell development. Although stem cell therapy restores these lineages, residual immune defects are observed that may result from selective persistence of γc-deficiency in myeloid lineages. However, little is known about the contribution of myeloid-expressed γc to protective immune responses.  Here we examine the importance of γc for myeloid dendritic cell (DC) function. Methods: We utilize a combination of in vitro DC/T-cell co-culture assays and a novel lipid bilayer system mimicking the T cell surface to delineate the role of DC-expressed γc during DC/T-cell interaction. Results: We observed that γc in DC was recruited to the contact interface following MHCII ligation, and promoted IL-15Rα colocalization with engaged MHCII. Unexpectedly, trans-presentation of IL-15 was required for optimal CD4+T cell activation by DC and depended on DC γc expression. Neither recruitment of IL-15Rα nor IL-15 trans-signaling at the DC immune synapse (IS), required γc signaling in DC, suggesting that γc facilitates IL-15 transpresentation through induced intermolecular cis associations or cytoskeletal reorganization following MHCII ligation. Conclusions: These findings show that DC-expressed γc is required for effective antigen-induced CD4+ T cell activation. We reveal a novel mechanism for recruitment of DC IL-15/IL-15Rα complexes to the IS, leading to CD4+ T cell costimulation through localized IL-15 transpresentation that is coordinated with antigen-recognition.

The immunological synapse as a pharmacological target

The development of T cell mediated immunity relies on the assembly of a highly specialized interface between T cell and antigen presenting cell (APC), known as the immunological synapse (IS). IS assembly is triggered when the T cell receptor (TCR) binds to specific peptide antigen presented in association to the major histocompatibility complex (MHC) by the APC, and is followed by the spatiotemporal dynamic redistribution of TCR, integrins, co-stimulatory receptors and signaling molecules, allowing for the fine-tuning and integration of the signals that lead to T cell activation. The knowledge acquired to date about the mechanisms of IS assembly underscores this structure as a robust pharmacological target. The activity of molecules involved in IS assembly and function can be targeted by specific compounds to modulate the immune response in a number of disorders, including cancers and autoimmune diseases, or in transplanted patients. Here, we will review the state-of-the art of the current therapies which exploit the IS to modulate the immune response.

Quantitative T-cell repertoire analysis of peripheral blood mononuclear cells from lung cancer patients following long-term cancer peptide vaccination

Therapeutic cancer peptide vaccination is an immunotherapy designed to elicit cytotoxic T-lymphocyte (CTL) responses in patients. A number of therapeutic vaccination trials have been performed, nevertheless there are only a few reports that have analyzed the T-cell receptors (TCRs) expressed on tumor antigen-specific CTLs. Here, we use next-generation sequencing (NGS) to analyze TCRs of vaccine-induced CTL clones and the TCR repertoire of bulk T cells in peripheral blood mononuclear cells (PBMCs) from two lung cancer patients over the course of long-term vaccine therapy. In both patients, vaccination with two epitope peptides derived from cancer/testis antigens (upregulated lung cancer 10 (URLC10) and cell division associated 1 (CDCA1)) induced specific CTLs expressing various TCRs. All URLC10-specific CTL clones tested showed Ca2+ influx, IFN-γ production, and cytotoxicity when co-cultured with URLC10-pulsed tumor cells. Moreover, in CTL clones that were not stained with the URLC10/MHC-multimer, the CD3 ζ chain was not phosphorylated. NGS of the TCR repertoire of bulk PBMCs demonstrated that the frequency of vaccine peptide-specific CTL clones was near the minimum detectable threshold level. These results demonstrate that vaccination induces antigen-specific CTLs expressing various TCRs at different time points in cancer patients, and that some CTL clones are maintained in PBMCs during long-term treatment, including some with TCRs that do not bind peptide/MHC-multimer.

NFM Cross-Reactivity to MOG Does Not Expand a Critical Threshold Level of High-Affinity T Cells Necessary for Onset of Demyelinating Disease

Of interest to the etiology of demyelinating autoimmune disease is the potential to aberrantly activate CD4⁺ T cells due to cross-recognition of multiple self-epitopes such as has been suggested for myelin oligodendrocyte glycoprotein epitope 35-55 (MOG_35-55) and neurofilament medium protein epitope 15-35 (NFM_15-35). NFM_15-35 is immunogenic in C57BL/6 mice but fails to induce demyelinating disease by polyclonal T cells despite having the same TCR contact residues as MOG_35-55, a known encephalitogenic Ag. Despite reported cross-reactivity with MOG-specific T cells, the polyclonal response to NFM_15-35 did not expand threshold numbers of MOG_38-49 tetramer-positive T cells. Furthermore, NFM lacked functional synergy with MOG to promote experimental autoimmune encephalomyelitis because NFM-deficient synonymous with knockout mice developed an identical disease course to wild-type mice after challenge with MOG_35-55 Single-cell analysis of encephalitogenic T cells using the peptide:MHC monomer-based two-dimensional micropipette adhesion frequency assay confirmed that NFM was not a critical Ag driving demyelinating disease because NFM_18-30-specific T cells in the CNS were predominantly reactive to MOG_38-49 The absence of NFM contribution to disease allowed mapping of the amino acids required for encephalitogenicity and expansion of high-affinity, MOG-specific T cells that defined the polyclonal response. Alterations of N-terminal residues outside of the NFM_15-35 core nonamer promoted expansion of high-affinity, MOG_38-49 tetramer-positive T cells and promoted consistent experimental autoimmune encephalomyelitis induction, unlike mice challenged with NFM_15-35 Although NFM_15-35 is immunogenic and cross-reactive with MOG at the polyclonal level, it fails to expand a threshold level of encephalitogenic, high-affinity MOG-specific T cells.

Autoreactive T cells in type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease that causes severe loss of pancreatic β cells. Autoreactive T cells are key mediators of β cell destruction. Studies of organ donors with T1D that have examined T cells in pancreas, the diabetogenic insulitis lesion, and lymphoid tissues have revealed a broad repertoire of target antigens and T cell receptor (TCR) usage, with initial evidence of public TCR sequences that are shared by individuals with T1D. Neoepitopes derived from post-translational modifications of native antigens are emerging as novel targets that are more likely to evade self-tolerance. Further studies will determine whether T cell responses to neoepitopes are major disease drivers that could impact prediction, prevention, and therapy. This Review provides an overview of recent progress in our knowledge of autoreactive T cells that has emerged from experimental and clinical research as well as pathology investigations.

Analyzing Actin Dynamics at the Immunological Synapse

T cell signaling is inextricably linked to actin cytoskeletal dynamics at the immunological synapse (IS). This process can be imaged in living T cells expressing GFP actin or fluorescent F-actin binding proteins. Because of its planar nature, the IS provides a unique opportunity to image events as they happen, monitoring changes in actin retrograde flow in T cells interacting with different stimulatory surfaces or after pharmacological treatments. Here, we described the imaging methods and analytical procedures used to measure actin velocity across the IS in T cells spreading on planar stimulatory surfaces.

Understanding Autoimmune Diabetes through the Prism of the Tri-Molecular Complex

The strongest susceptibility allele for Type 1 Diabetes (T1D) is human leukocyte antigen (HLA), which supports a central role for T cells as the drivers of autoimmunity. However, the precise mechanisms that allow thymic escape and peripheral activation of beta cell antigen-specific T cells are still largely unknown. Studies performed with the non-obese diabetic (NOD) mouse have challenged several immunological dogmas, and have made the NOD mouse a key experimental system to study the steps of immunodysregulation that lead to autoimmune diabetes. The structural similarities between the NOD I-A^g7 and HLA-DQ8 have revealed the stability of the T cell receptor (TCR)/HLA/peptide tri-molecular complex as an important parameter in the development of autoimmune T cells, as well as afforded insights into the key antigens targeted in T1D. In this review, we will provide a summary of the current understanding with regard to autoimmune T cell development, the significance of the antigens targeted in T1D, and the relationship between TCR affinity and immune regulation.

The Immune Synapse: Past, Present, and Future

Immunological synapses are specialized cell-cell junctions characterized by (1) close apposition of the immune cell membrane with the membrane of another cell driven by adaptive or innate immune recognition, (2) adhesion, (3) stability, and (4) directed secretion. This phenomenon was first recognized in the 1970s and the early 1980s through electron microscopy of ex vivo functioning immune cells. Progressive advances in fluorescence microscopy and molecular immunology in the past 20 years have led to rapid progress on understanding the modes of cell-cell interaction and underlying molecular events. This volume contains a diverse range of protocols that can be applied to the study of the immunological synapses and related immune cell junctions both in vitro and in vivo; and in disease settings in animal models and humans. We have also included chapters on critical molecular tools such as protein expression and mRNA electroporation that underpin or expand imaging approaches, although they are not specific to the study of immune synapses. We hope that these chapters will be of use to people entering the field as well as seasoned practitioners looking to expand their repertoire of methods.

Studying Dynamic Plasma Membrane Binding of TCR-CD3 Chains During Immunological Synapse Formation Using Donor-Quenching FRET and FLIM-FRET

Over the last decade, advancements in the time and space resolution of microscopy technologies have enabled dissection of the molecular events involved in T cell Immunological Synapse (IS) formation. Using a combination of Förster Resonance Energy Transfer (FRET) and Fluorescence Lifetime Imagining Microscopy (FLIM), we have demonstrated dynamic plasma membrane binding by cytoplasmic domains of T cell receptor (TCR)-associated CD3 chains and other T cell transmembrane receptors. We have developed methods for imaging such membrane binding both at steady state and during receptor triggering at the IS. Plasma membrane binding by cytoplasmic domains may represent a novel mechanism for regulating the signaling function of important receptors in the immune system.

Low-affinity CD4+ T cells are major responders in the primary immune response

A robust primary immune response has been correlated with the precursor number of antigen-specific T cells, as identified using peptide MHCII tetramers. However, these tetramers identify only the highest-affinity T cells. Here we show the entire CD4+ T-cell repertoire, inclusive of low-affinity T cells missed by tetramers, using a T-cell receptor (TCR) signalling reporter and micropipette assay to quantify naive precursors and expanded populations. In vivo limiting dilution assays reveal hundreds more precursor T cells than previously thought, with higher-affinity tetramer-positive T cells, comprising only 5-30% of the total antigen-specific naive repertoire. Lower-affinity T cells maintain their predominance as the primary immune response progresses, with no enhancement of survival of T cells with high-affinity TCRs. These findings demonstrate that affinity for antigen does not control CD4+ T-cell entry into the primary immune response, as a diverse range in affinity is maintained from precursor through peak of T-cell expansion.

Regulatory and T Effector Cells Have Overlapping Low to High Ranges in TCR Affinities for Self during Demyelinating Disease

Having regulatory T cells (Tregs) with the same Ag specificity as the responding conventional T cells is thought to be important in maintaining peripheral tolerance. It has been demonstrated that during experimental autoimmune encephalomyelitis there are myelin oligodendrocyte glycoprotein (MOG)--specific Tregs that infiltrate into the CNS. However, the affinity of naturally occurring polyclonal Tregs for any self-antigen, let alone MOG, has not been analyzed in the periphery or at the site of autoimmune disease. Utilizing the highly sensitive micropipette adhesion frequency assay, which allows one to determine on a single-cell basis the affinity and frequency of polyclonal Ag-specific T cells directly ex vivo, we demonstrate that at peak disease MOG-specific Tregs were progressively enriched in the draining cervical lymph nodes and CNS as compared with spleen. These frequencies were greater than the frequencies measured by tetramer analysis, indicative of the large fraction of lower affinity T cells that comprise the MOG-specific conventional T cell (Tconv) and Treg response. Of interest, the self-reactive CD4(+) Tconvs and Tregs displayed overlapping affinities for MOG in the periphery, yet in the CNS, the site of neuroinflammation, Tconvs skew toward higher affinities. Most of the MOG-specific Tregs in the CNS possessed the methylation signature associated with thymic-derived Tregs. These findings indicate that thymic-derived Treg affinity range matches that of their Tconvs in the periphery and suggest a change in TCR affinity as a potential mechanism for autoimmune progression and escape from immune regulation.

Plasma membrane-associated superstructure: Have we overlooked a new type of organelle in eukaryotic cells?

A variety of intriguing plasma membrane-associated regions, including focal adhesions, adherens junctions, tight junctions, immunological synapses, neuromuscular junctions and the primary cilia, among many others, have been described in eukaryotic cells. Emphasizing their importance, alteration in their molecular structures induces or correlates with different pathologies. These regions display surface proteins connected to intracellular molecules, including cytoskeletal component, which maintain their cytoarchitecture, and signalling proteins, which regulate their organization and functions. Based on the molecular similarities and other common features observed, we suggest that, despite differences in external appearances, all these regions are just the same superstructure that appears in different locations and cells. We hypothesize that this superstructure represents an overlooked new type of organelle that we call plasma membrane-associated superstructure (PMAS). Therefore, we suggest that eukaryotic cells include classical organelles (e.g. mitochondria, Golgi and others) and also PMAS. We speculate that this new type of organelle might be an innovation associated to the emergence of eukaryotes. Finally we discuss the implications of the hypothesis proposed.

Antigen Presentation, Autoantigens, and Immune Regulation in Multiple Sclerosis and Other Autoimmune Diseases

Antigen presentation is in the center of the immune system, both in host defense against pathogens, but also when the system is unbalanced and autoimmune diseases like multiple sclerosis (MS) develop. It is not just by chance that a major histocompatibility complex gene is the major genetic susceptibility locus in MS; a feature that MS shares with other autoimmune diseases. The exact etiology of the disease, however, has not been fully understood yet. T cells are regarded as the major players in the disease, but most probably a complex interplay of altered central and peripheral tolerance mechanisms, T-cell and B-cell functions, characteristics of putative autoantigens, and a possible interference of environmental factors like microorganisms are at work. In this review, new data on all these different aspects of antigen presentation and their role in MS will be discussed, probable autoantigens will be summarized, and comparisons to other autoimmune diseases will be drawn.

Taming the TCR: antigen-specific immunotherapeutic agents for autoimmune diseases

Current treatments for autoimmune diseases are typically non-specific anti-inflammatory agents that affect not only the autoreactive cells but also the parts of the immune system that are required to maintain health. There is a need for the development of antigen-specific therapeutic agents that can effectively prevent the autoimmune attack while leaving the rest of the immune system functioning as normal. The simplest way to achieve this is using the autoantigen itself as a tolerizing agent; however, there is some risk involved with administering a potentially pathogenic antigen. In this review, we focus instead on the development and use of modified T cell receptor (TCR) ligands, in which the peptide ligand is modified to change the response by the T cell from a disease inducing to a protective response, and still retain the antigen-specificity necessary to target the autoreactive T cells. We review the use of modified TCR ligands as therapeutic agents in animal models of autoimmunity and in human autoimmune disease, and finally consider how they need to be improved in order to use them effectively in patients with autoimmune disease.

T/B-cell interactions are more transient in response to weak stimuli in SLE-prone mice

Changes in immune function during the course of systemic lupus erythematosus (SLE) are well characterized. Class-switched antinuclear antibodies are the hallmark of SLE, and T/B-cell interactions are thus critical. However, changes in immune function contributing to disease susceptibility are unknown. Here, we have analyzed primary T and B cells from a mouse model of SLE prior to the onset of disease. To allow cognate T-cell activation with low affinity, we have developed a lower potency peptide ligand for the OTII TCR. T- and B-cell couples formed less frequently and retained their polarity less efficiently preferentially in response to low-affinity stimulation in SLE-prone mice. This matched decreased recruitment of actin and Vav1 and an enhanced PKCΘ recruitment to the cellular interface in T cells. The induction of the GC B-cell marker GL7 was increased in T/B cell couples from SLE-prone mice when the T-cell numbers were limited. However, the overall gene expression changes were marginal. Taken together, the enhanced cell-couple transience may allow a more efficient sampling of a large number of T/B cell couples, preferentially in response to limiting stimuli, therefore enhancing the immune reactivity in the development of SLE.

The immunological synapse

The molecular interactions underlying regulation of the immune response take place in a nanoscale gap between T cells and antigen-presenting cells, termed the immunological synapse. If these interactions are regulated appropriately, the host is defended against a wide range of pathogens and deranged host cells. If these interactions are disregulated, the host is susceptible to pathogens or tumor escape at one extreme and autoimmunity at the other. Strategies targeting the synapse have helped to establish immunotherapy as a mainstream element in cancer treatment. This Masters' primer will cover the basics of the immunological synapse and some of the applications to tumor immunology.

Kupfer-type immunological synapses in vivo: Raison D'être of SMAC

T cells engage with antigen-presenting cells to form immunological synapses. These intimate contacts are characterized by the complex arrangement of molecules at the intercellular interface, which has been described as the supramolecular activation cluster (SMAC). However, due to T cells functioning without SMAC formation and the difficulties of studying these complex arrangements in vivo, its biological importance has been questioned. In light of recent data, we focus this review on the putative functionality of SMACs in T-cell synaptic contacts in vivo and emphasize the therapeutic potential of SMAC manipulation in immune-driven diseases.

Progression of relapsing-remitting demyelinating disease does not require increased TCR affinity or epitope spread

In this study, we investigate the basis of T cell recognition of myelin that governs the progression from acute symptoms into disease remission, relapse, and chronic progression in a secondary progressive model of demyelinating disease. Until now, the frequency and affinity of myelin-reactive CD4 T cells that elicit relapsing-remitting disease have not been quantified. The micropipette adhesion frequency assay was used to obtain a sensitive and physiologically relevant two-dimensional measurement of frequency and TCR affinity for myelin, as the inherent low affinity does not allow the use of specific peptide:MHC-II tetramers for this purpose. We found the highest affinity and frequency of polyclonal myelin oligodendrocyte glycoprotein-reactive cells infiltrate the CNS during acute disease, whereas affinities during remission, relapse, and chronic disease are not significantly different from each other. Frequency analysis revealed that the vast majority of CNS-infiltrating CD4 T cells are myelin oligodendrocyte glycoprotein reactive at all time points, demonstrating epitope spread is not a predominant factor for disease progression. Furthermore, time points at which mice were symptomatic were characterized by an infiltration of Th17 cells in the CNS, whereas symptom remission showed an enrichment of cells producing IFN-γ. Also, the ratio of regulatory T cells to Foxp3(-) CD4 T cells was significantly higher in the CNS at remission than during acute disease. The results of this study indicate that a high frequency of T cells specific for a single myelin Ag, rather than increased TCR affinity or epitope spread, governs the transition from acute symptoms through remission, relapse, and chronic disease states.

TCR affinity and tolerance mechanisms converge to shape T cell diabetogenic potential

Autoreactive T cells infiltrating the target organ can possess a broad TCR affinity range. However, the extent to which such biophysical parameters contribute to T cell pathogenic potential remains unclear. In this study, we selected eight InsB9-23-specific TCRs cloned from CD4(+) islet-infiltrating T cells that possessed a relatively broad range of TCR affinity to generate NOD TCR retrogenic mice. These TCRs exhibited a range of two-dimensional affinities (∼ 10(-4)-10(-3) μm(4)) that correlated with functional readouts and responsiveness to activation in vivo. Surprisingly, both higher and lower affinity TCRs could mediate potent insulitis and autoimmune diabetes, suggesting that TCR affinity does not exclusively dictate or correlate with diabetogenic potential. Both central and peripheral tolerance mechanisms selectively impinge on the diabetogenic potential of high-affinity TCRs, mitigating their pathogenicity. Thus, TCR affinity and multiple tolerance mechanisms converge to shape and broaden the diabetogenic T cell repertoire, potentially complicating efforts to induce broad, long-term tolerance.

The actin-driven spatiotemporal organization of T-cell signaling at the system scale

T cells are activated through interaction with antigen-presenting cells (APCs). During activation, receptors and signaling intermediates accumulate in diverse spatiotemporal distributions. These distributions control the probability of signaling interactions and thus govern information flow through the signaling system. Spatiotemporally resolved system-scale investigation of signaling can extract the regulatory information thus encoded, allowing unique insight into the control of T-cell function. Substantial technical challenges exist, and these are briefly discussed herein. While much of the work assessing T-cell spatiotemporal organization uses planar APC substitutes, we focus here on B-cell APCs with often stark differences. Spatiotemporal signaling distributions are driven by cell biologically distinct structures, a large protein assembly at the interface center, a large invagination, the actin-supported interface periphery as extended by smaller individual lamella, and a newly discovered whole-interface actin-driven lamellum. The more than 60 elements of T-cell activation studied to date are dynamically distributed between these structures, generating a complex organization of the signaling system. Signal initiation and core signaling prefer the interface center, while signal amplification is localized in the transient lamellum. Actin dynamics control signaling distributions through regulation of the underlying structures and drive a highly undulating T-cell/APC interface that imposes substantial constraints on T-cell organization. We suggest that the regulation of actin dynamics, by controlling signaling distributions and membrane topology, is an important rheostat of T-cell signaling.

Polarized release of T-cell-receptor-enriched microvesicles at the immunological synapse

The recognition events that mediate adaptive cellular immunity and regulate antibody responses depend on intercellular contacts between T cells and antigen-presenting cells (APCs). T-cell signalling is initiated at these contacts when surface-expressed T-cell receptors (TCRs) recognize peptide fragments (antigens) of pathogens bound to major histocompatibility complex molecules (pMHC) on APCs. This, along with engagement of adhesion receptors, leads to the formation of a specialized junction between T cells and APCs, known as the immunological synapse, which mediates efficient delivery of effector molecules and intercellular signals across the synaptic cleft. T-cell recognition of pMHC and the adhesion ligand intercellular adhesion molecule-1 (ICAM-1) on supported planar bilayers recapitulates the domain organization of the immunological synapse, which is characterized by central accumulation of TCRs, adjacent to a secretory domain, both surrounded by an adhesive ring. Although accumulation of TCRs at the immunological synapse centre correlates with T-cell function, this domain is itself largely devoid of TCR signalling activity, and is characterized by an unexplained immobilization of TCR-pMHC complexes relative to the highly dynamic immunological synapse periphery. Here we show that centrally accumulated TCRs are located on the surface of extracellular microvesicles that bud at the immunological synapse centre. Tumour susceptibility gene 101 (TSG101) sorts TCRs for inclusion in microvesicles, whereas vacuolar protein sorting 4 (VPS4) mediates scission of microvesicles from the T-cell plasma membrane. The human immunodeficiency virus polyprotein Gag co-opts this process for budding of virus-like particles. B cells bearing cognate pMHC receive TCRs from T cells and initiate intracellular signals in response to isolated synaptic microvesicles. We conclude that the immunological synapse orchestrates TCR sorting and release in extracellular microvesicles. These microvesicles deliver transcellular signals across antigen-dependent synapses by engaging cognate pMHC on APCs.

CD6 as a therapeutic target in autoimmune diseases: successes and challenges

The transmembrane surface glycoprotein CD6 was one of the first antigens identified on T lymphocytes. The recognition of its involvement in T-cell signaling processes heralds the potential of CD6 as a target for therapy in a number of pathologies associated with imbalances in T-cell function. Its tissue distribution, cellular expression, and overall molecular structure are well described, and the interaction with its physiological ligand CD166 has been determined to the amino-acid level. Nevertheless, the involvement of CD6 in signaling pathways remains poorly characterized and its biological function is controversial; still unresolved are whether CD6 is a co-stimulatory molecule in T-cell activation or, similar to the related CD5 antigen, a modulator of intracellular signaling. Here we revisit the earliest attempts of modulating immune function using CD6 monoclonal antibodies, and review the current thinking behind the recent developments in immunotherapy targeting CD6. Notwithstanding the promises and hopes brought by monoclonals already in clinical trials, the fact is that very little is known about the mechanism of action of these reagents, whether they enhance the physiological role of the receptor or whether they may induce a completely novel biochemical response that might, nevertheless, be beneficially used to treat human immune pathology.

CD8+ dendritic cell-mediated tolerance of autoreactive CD4+ T cells is deficient in NOD mice and can be corrected by blocking CD40L

DCs are important mediators of peripheral tolerance for the prevention of autoimmunity. Chimeric αDEC-205 antibodies with attached antigens allow in vivo antigen-specific stimulation of T cells by CD8(+) DCs, resulting in tolerance in nonautoimmune mice. However, it is not clear whether DC-mediated tolerance induction occurs in the context of ongoing autoimmunity. We assessed the role of CD8(+) DCs in stimulation of autoreactive CD4(+) T cells in the NOD mouse model of type 1 diabetes. Targeting of antigen to CD8(+) DCs via αDEC-205 led to proliferation and expansion of β-cell specific BDC2.5 T cells. These T cells also produced IL-2 and IFN-γ and did not up-regulate FoxP3, consistent with an activated rather than tolerant phenotype. Similarly, endogenous BDC peptide-reactive T cells, identified with I-A(g7) tetramers, did not become tolerant after antigen delivery via αDEC-205: no deletion or Treg induction was observed. We observed that CD8(+) DCs from NOD mice expressed higher surface levels of CD40 than CD8(+) DCs from C57BL/6 mice. Blockade of CD40-CD40L interactions reduced the number of BDC2.5 T cells remaining in mice, 10 days after antigen targeting to CD8 DCs, and blocked IFN-γ production by BDC2.5 T cells. These data indicate that the ability of autoreactive CD4(+) T cells to undergo tolerance mediated by CD8(+) DCs is defective in NOD mice and that blocking CD40-CD40L interactions can restore tolerance induction.

Analysis, Isolation, and Activation of Antigen-Specific CD4(+) and CD8(+) T Cells by Soluble MHC-Peptide Complexes

T cells constitute the core of adaptive cellular immunity and protect higher organisms against pathogen infections and cancer. Monitoring of disease progression as well as prophylactic or therapeutic vaccines and immunotherapies call for conclusive detection, analysis, and sorting of antigen-specific T cells. This is possible by means of soluble recombinant ligands for T cells, i.e., MHC class I-peptide (pMHC I) complexes for CD8(+) T cells and MHC class II-peptide (pMHC II) complexes for CD4(+) T cells and flow cytometry. Here we review major developments in the development of pMHC staining reagents and their diverse applications and discuss perspectives of their use for basic and clinical investigations.