Study of Signal Transduction Pathways by Phospho-Protein Evaluation.

Flow cytometric evaluation of phosphorylation status of signal transduction molecules is a useful method to study T-cell signaling pathways. As mutations occurring in TCR complex molecules, common gamma chain family's cytokines, their receptors or molecules involved in these pathways can lead to severe immune system defects, the study of T-cell signal transduction can be applied to both basic and clinical/translational research areas. In the present chapter, we show two different protocols for the study of T- cell response to an antigen-like stimulus and to IL-2.

Stimulation strength controls the rate of initiation but not the molecular organisation of TCR-induced signalling.

Millions of naïve T cells with different TCRs may interact with a peptide-MHC ligand, but very few will activate. Remarkably, this fine control is orchestrated using a limited set of intracellular machinery. It remains unclear whether changes in stimulation strength alter the programme of signalling events leading to T cell activation. Using mass cytometry to simultaneously measure multiple signalling pathways during activation of murine CD8+ T cells, we found a programme of distal signalling events that is shared, regardless of the strength of TCR stimulation. Moreover, the relationship between transcription of early response genes Nr4a1 and Irf8 and activation of the ribosomal protein S6 is also conserved across stimuli. Instead, we found that stimulation strength dictates the rate with which cells initiate signalling through this network. These data suggest that TCR-induced signalling results in a coordinated activation program, modulated in rate but not organization by stimulation strength.

Amongst the different types of cells the body uses to protect itself, killer T cells have an unique role: they can detect and neutralize cells that have been become dangerous for the organism ­ for example, cells which are cancerous or hijacked by viruses. In a healthy organism, T cells circulate through the body in an inactivated state. When a disease emerges, receptors at the surface of the cells can detect elements coming from harmful agents; this stimulation then triggers a molecular cascade inside the T cell that leads to activation. This system is relatively simple, pairing a finite number of receptors with a limited set of internal components. At the same time, the activity of T cells is finely regulated, and their activation tightly controlled: they must kill enough cells to stop the illness without causing excess damage. How this is accomplished is still unclear. A T cell can recognize harmful agents that bind its receptors with differing strengths, but how this variability in stimulation strength affects the signaling processes within the cell is still poorly understood. To investigate this question, Ma et al. used an approach called mass cytometry and analyzed the internal processes of mouse killer T cells receiving different strengths of stimulation. This investigation revealed little change in the patterns of signaling in response to signals of different strength. Instead, what differed was the proportion of T cells that became activated, and how fast this process took place: stronger stimulations led to a larger population of killer T cells being activated more rapidly. Overall, this work sheds light on how killer T cells fine-tune their response to illness using only a simple system to control their activation.

MicroRNA-155 Expression Is Enhanced by T-cell Receptor Stimulation Strength and Correlates with Improved Tumor Control in Melanoma.

microRNAs are short noncoding RNAs that regulate protein expression posttranscriptionally. We previously showed that miR-155 promotes effector CD8+ T-cell responses. However, little is known about the regulation of miR-155 expression. Here, we report that antigen affinity and dose determine miR-155 expression in CD8+ T cells. In B16 tumors expressing a low-affinity antigen ligand, tumor-specific infiltrating CD8+ T cells showed variable miR-155 expression, whereby high miR-155 expression was associated with more cytokine-producing cells and tumor control. Moreover, anti-PD-1 treatment led to both increased miR-155 expression and tumor control by specific CD8+ T cells. In addition, miR-155 overexpression enhanced exhausted CD8+ T-cell persistence in the LCMV cl13 chronic viral infection model. In agreement with these observations in mouse models, miR-155 expression in human effector memory CD8+ T cells positively correlated with their frequencies in tumor-infiltrated lymph nodes of melanoma patients. Low miR-155 target gene signature in tumors was associated with prolonged overall survival in melanoma patients. Altogether, these results raise the possibility that high miR-155 expression in CD8+ tumor-infiltrating T cells may be a surrogate marker of the relative potency of in situ antigen-specific CD8+ T-cell responses.

Isolation and enrichment of mouse insulin-specific CD4+ T regulatory cells.

Polyclonal T regulatory cells (Treg - CD4+CD25+CD127lowFoxp3+) are used in several protocols for the treatment of type 1 diabetes (T1D), multiple sclerosis and graft-versus host disease in clinical trials. However, general opinion is that autoantigen-specific Treg could be more efficient in autoimmunity suppression due to their direct effect on pathogenic autoantigen-specific effector T cells. This study describes isolation and expansion of insulin-specific Treg in vitro. Insulin-specific Treg are uniformly distributed in lymphoid tissues however their number is extremely low. To enrich the proportion of insulin-specific Treg, pure CD4+ cells were co-cultured with insulin B chain peptide-loaded dendritic cells, isolated from mice that develop T1D spontaneously - NOD mice. Insulin-specific CD4+ cell expansion peaked after 48 h of incubation and was in favour of Treg. These cells were then sorted using insulin peptide-loaded MHC class II tetramers and cultured in vitro for 48 h in the presence of TCR stimulators, TGF-ß and IL-2. The proportion of gained insulin-specific cells with T regulatory phenotype (CD4+CD25highCD127lowGITR+FoxP3+) was in average between 93% and 97%. These cells have shown potent in vitro suppressive effect on T effector cells, produced IL-10 and TGF-ß and expressed PD-1 and CD39. Further proliferation of these insulin-specific Treg required the presence of dendritic cells, anti-CD3 antibody and IL-2. This study provides new, reproducible experimental design for the enrichment and expansion of insulin-specific Treg that can be used for the cell-based therapy of autoimmunity.

A power law function describes the time- and dose-dependency of Vγ9Vδ2 T cell activation by phosphoantigens.

Phosphoantigens stimulate Vγ9Vδ2 T cells after binding to BTN3A1 in target cells and cell-cell contact. We evaluated phosphoantigens including diphosphates, bisphosphonates, and prodrugs for ability to induce leukemia cells to stimulate Vγ9Vδ2 T cell interferon-γ secretion. Most compounds displayed time-dependent activity at exposure times between 15 and 240 min. Potency (EC50 values) ranged between 8.4 nM and >100 µM. The diphosphate C-HMBPP displayed a shallow dose-response slope (Hill slope = 0.71), while the bisphosphonate slopes were steep (Hill slopes > 2), and the prodrugs intermediate. The bis-acyloxyalkyl POM2-C-HMBP showed low nanomolar potency even at an exposure time of 1 min. Mixed aryl-POM prodrugs also retained excellent potency at 15 min, while aryl-amidates were time dependent below 240 min. The sum of the dose and time logarithms is often constant, while a power law function fits most compounds. Collectively, these findings illustrate the exquisite activity of prodrugs relative to diphosphates and bisphosphonates.

Arginase-1 is neither constitutively expressed in nor required for myeloid-derived suppressor cell-mediated inhibition of T-cell proliferation.

Although previous reports suggest that tumor-induced myeloid-derived suppressor cells (MDSC) inhibit T cells by L-arginine depletion through arginase-1 activity, we herein show that arginase-1 is neither inherently expressed in MDSC nor required for MDSC-mediated inhibition. Employing Percoll density gradients, large expansions of MDSC in the bone marrow of tumor-bearing mice were isolated and demonstrated potent inhibition in T-cell proliferation activated by TCR-ligation, Concanavalin A, PMA plus ionomycin, or IL-2. Despite demonstrating characteristic immunosuppressive capacity, these MDSC exhibit no arginase-1 expression and/or exert their inhibitory effects independent of arginase-1 activity. However, arginase-1 expression in MDSC can be induced by exposure to TCR-activated T cells or their culture medium, but not T cells activated by other means or growing tumor cells. Further investigation reveals multiple cytokines secreted by TCR-activated T cells as orchestrating two signaling-relay axes, IL-6-to-IL-4 and GM-CSF/IL-4-to-IL-10, leading to arginase-1 expression in MDSC. Specifically, IL-6 signaling increases IL-4R, enabling IL-4 to induce arginase-1 expression; similarly, GM-CSF in concert with IL-4 induces IL-10R, allowing IL-10-mediated induction. Surprisingly, our study indicates that induction of arginase-1 expression is not conducive to the critical MDSC-mediated inhibition toward T cells, which is rather dependent on direct cell contacts undiminished by PD-L1 blockade or SIRPα deficiency.

Catch Bonds at T Cell Interfaces: Impact of Surface Reorganization and Membrane Fluctuations.

Catch bonds are characterized by average lifetimes that initially increase with increasing tensile force. Recently, they have been implicated in T cell activation, where small numbers of antigenic receptor-ligand bonds at a cell-cell interface can stimulate a T cell. Here, we use computational methods to investigate small numbers of bonds at the interface between two membranes. We characterize the time-dependent forces on the bonds in response to changes in the membrane shape and the organization of other surface molecules. We then determine the distributions of bond lifetimes using recent force-dependent lifetime data for T cell receptors bound to various ligands. Strong agonists, which exhibit catch bond behavior, are markedly more likely to remain intact than an antagonist whose average lifetime decreases with increasing force. Thermal fluctuations of the membrane shape enhance the decay of the average force on a bond, but also lead to fluctuations of the force. These fluctuations promote bond rupture, but the effect is buffered by catch bonds. When more than one bond is present, the bonds experience reduced average forces that depend on their relative positions, leading to changes in bond lifetimes. Our results highlight the importance of force-dependent binding kinetics when bonds experience time-dependent and fluctuating forces, as well as potential consequences of collective bond behavior relevant to T cell activation.

Predominant contribution of DGKζ over DGKα in the control of PKC/PDK-1-regulated functions in T cells.

Diacylglycerol kinase (DGK)-mediated consumption of the diacylglycerol (DAG) generated in response to antigen recognition is an important mechanism to limit T-cell function. Targeting DGK activity presents new opportunities for therapeutic manipulation of the immune response, but assessment of individual DGK functions is complex. T cells express two DGK isoforms, DGKα and DGKζ, and there are no isoform-specific inhibitors. Here we used short interfering RNA-mediated gene silencing in human T cells and DGKα- and DGKζ-deficient mice to define DGK isoform-specific regulation of key signaling pathways during T-cell activation. Our results identify DGKζ as the predominant brake on basal/tonic conditions as well as on downstream T-cell receptor/co-stimulatory signals. DGKζ silencing triggers basal RasGTP activation and facilitates enhanced membrane stability of protein kinase C alpha as well as increased activity of AGC kinases. Downstream of T-cell receptor/co-stimulation, DGKζ silencing results in enhanced and maintained recruitment of PKC theta to the membrane, as well as phosphoinositide-dependent protein kinase-1 activation and scaffolding functions. Our studies identify a previously unrecognized DGKζ contribution as a negative regulator of the crosstalk between phospholipase C-gamma- and phosphoinositide 3-kinase-regulated pathways. This DGKζ input helps to explain previous observations in DGK-deficient mice and suggests that the development of isoform-specific DGK inhibitors is of great interest for the manipulation of distinct aspects of T-cell responses.

T helper 1 immunity requires complement-driven NLRP3 inflammasome activity in CD4⁺ T cells.

The NLRP3 inflammasome controls interleukin-1ß maturation in antigen-presenting cells, but a direct role for NLRP3 in human adaptive immune cells has not been described. We found that the NLRP3 inflammasome assembles in human CD4(+) T cells and initiates caspase-1-dependent interleukin-1ß secretion, thereby promoting interferon-γ production and T helper 1 (T(H)1) differentiation in an autocrine fashion. NLRP3 assembly requires intracellular C5 activation and stimulation of C5a receptor 1 (C5aR1), which is negatively regulated by surface-expressed C5aR2. Aberrant NLRP3 activity in T cells affects inflammatory responses in human autoinflammatory disease and in mouse models of inflammation and infection. Our results demonstrate that NLRP3 inflammasome activity is not confined to "innate immune cells" but is an integral component of normal adaptive T(H)1 responses.

Phase separation of signaling molecules promotes T cell receptor signal transduction.

Activation of various cell surface receptors triggers the reorganization of downstream signaling molecules into micrometer- or submicrometer-sized clusters. However, the functional consequences of such clustering have been unclear. We biochemically reconstituted a 12-component signaling pathway on model membranes, beginning with T cell receptor (TCR) activation and ending with actin assembly. When TCR phosphorylation was triggered, downstream signaling proteins spontaneously separated into liquid-like clusters that promoted signaling outputs both in vitro and in human Jurkat T cells. Reconstituted clusters were enriched in kinases but excluded phosphatases and enhanced actin filament assembly by recruiting and organizing actin regulators. These results demonstrate that protein phase separation can create a distinct physical and biochemical compartment that facilitates signaling.

The quest for spatio-temporal control of CAR T cells.

Chimeric antigen receptors (CARs) are synthetic receptors capable of directing potent antigen-specific anti-tumor T cell responses. A recent report by Wu et al. extends a series of strategies aiming to curb excessive T cell activity, utilizing in this instance a chemical dimerizer to aggregate antigen-binding, T cell-activating and costimulatory domains.

Remote control of therapeutic T cells through a small molecule-gated chimeric receptor.

There is growing interest in using engineered cells as therapeutic agents. For example, synthetic chimeric antigen receptors (CARs) can redirect T cells to recognize and eliminate tumor cells expressing specific antigens. Despite promising clinical results, these engineered T cells can exhibit excessive activity that is difficult to control and can cause severe toxicity. We designed "ON-switch" CARs that enable small-molecule control over T cell therapeutic functions while still retaining antigen specificity. In these split receptors, antigen-binding and intracellular signaling components assemble only in the presence of a heterodimerizing small molecule. This titratable pharmacologic regulation could allow physicians to precisely control the timing, location, and dosage of T cell activity, thereby mitigating toxicity. This work illustrates the potential of combining cellular engineering with orthogonal chemical tools to yield safer therapeutic cells that tightly integrate cell-autonomous recognition and user control.

Thymic regulatory T cell niche size is dictated by limiting IL-2 from antigen-bearing dendritic cells and feedback competition.

The thymic production of regulatory T cells (Treg cells) requires interleukin 2 (IL-2) and agonist T cell antigen receptor (TCR) ligands and is controlled by competition for a limited developmental niche, but the thymic sources of IL-2 and the factors that limit access to the niche are poorly understood. Here we found that IL-2 produced by antigen-bearing dendritic cells (DCs) had a key role in Treg cell development and that existing Treg cells limited new development of Treg cells by competing for IL-2. Our data suggest that antigen-presenting cells (APCs) that can provide both IL-2 and a TCR ligand constitute the thymic niche and that competition by existing Treg cells for a limited supply of IL-2 provides negative feedback for new production of Treg cells.

The role of T cell receptor signaling thresholds in guiding T cell fate decisions.

Canonical T cell receptor signal transduction has been extensively studied and dissected in cell lines and primary lymphocytes. However, a static depiction of this signaling cascade fails to capture the complex and dynamic process by which individual T cells discriminate TCR:peptide-MHC affinity, then integrate signals over time to drive discrete cellular behaviors such as thymic selection, proliferation, and cytokine production. Recent technological advances have made it possible to study complex lymphocyte behavior on a single cell level and are revealing how T cells interpret information about affinity and abundance of antigen in order to make life-and-death cell fate decisions individually and collectively.

MHC class I recognition by monocyte-/macrophage-specific receptors.

The most important transplantation antigens in the discrimination between "self" and "nonself" are encoded by genes in the major histocompatibility complex (MHC) locus (H-2 in mice). It has been assumed that T lymphocytes are the effector cells for allograft rejection, as athymic nude rodents fail to reject allografts. In 1988, we i.p. transplanted Meth A (H-2D(d)K(d)) tumor cells into C57BL/6 (H-2D(b)K(b)) mice and found macrophages to be cytotoxic against the allografts. In 1996, several groups using CD4 or CD8 knockout mice reported that non-T cells were the effector cells for the rejection of skin or organ allografts. In 1998, we ascertained that macrophages were the effector cells of skin allograft rejection. Recently, we isolated cDNA clones encoding monocyte/macrophage MHC receptors (MMRs) for H-2D(d) and H-2K(d); established H-2D(d)- and/or H-2K(d)-transgenic mice and lymphoma cells; and found, using MMR-deficient mice, that MMR and T-cell receptor were essential for the rejection of transgenic skin and lymphoma, respectively.

Accumulation of dynamic catch bonds between TCR and agonist peptide-MHC triggers T cell signaling.

TCR-pMHC interactions initiate adaptive immune responses, but the mechanism of how such interactions under force induce T cell signaling is unclear. We show that force prolongs lifetimes of single TCR-pMHC bonds for agonists (catch bonds) but shortens those for antagonists (slip bonds). Both magnitude and duration of force are important, as the highest Ca(2+) responses were induced by 10 pN via both pMHC catch bonds whose lifetime peaks at this force and anti-TCR slip bonds whose maximum lifetime occurs at 0 pN. High Ca(2+) levels require early and rapid accumulation of bond lifetimes, whereas short-lived bonds that slow early accumulation of lifetimes correspond to low Ca(2+) responses. Our data support a model in which force on the TCR induces signaling events depending on its magnitude, duration, frequency, and timing, such that agonists form catch bonds that trigger the T cell digitally, whereas antagonists form slip bonds that fail to activate.

Costimulation via the tumor-necrosis factor receptor superfamily couples TCR signal strength to the thymic differentiation of regulatory T cells.

Regulatory T cells (Treg cells) express members of the tumor-necrosis factor (TNF) receptor superfamily (TNFRSF), but the role of those receptors in the thymic development of Treg cells is undefined. We found here that Treg cell progenitors had high expression of the TNFRSF members GITR, OX40 and TNFR2. Expression of those receptors correlated directly with the signal strength of the T cell antigen receptor (TCR) and required the coreceptor CD28 and the kinase TAK1. The neutralization of ligands that are members of the TNF superfamily (TNFSF) diminished the development of Treg cells. Conversely, TNFRSF agonists enhanced the differentiation of Treg cell progenitors by augmenting responsiveness of the interleukin 2 receptor (IL-2R) and transcription factor STAT5. Costimulation with the ligand of GITR elicited dose-dependent enrichment for cells of lower TCR affinity in the Treg cell repertoire. In vivo, combined inhibition of GITR, OX40 and TNFR2 abrogated the development of Treg cells. Thus, expression of members of the TNFRSF on Treg cell progenitors translated strong TCR signals into molecular parameters that specifically promoted the development of Treg cells and shaped the Treg cell repertoire.