Distinct influences of peptide-MHC quality and quantity on in vivo T-cell responses

ABCD of IA: A multi-scale agent-based model of T cell activation in inflammatory arthritis

Biomaterial-based agents have been demonstrated to regulate the function of immune cells in models of autoimmunity. However, the complexity of the kinetics of immune cell activation can present a challenge in optimizing the dose and frequency of administration. Here, we report a model of autoreactive T cell activation, which are key drivers in autoimmune inflammatory joint disease. The model is termed a multi-scale Agent-Based, Cell-Driven model of Inflammatory Arthritis (ABCD of IA). Using kinetic rate equations and statistical theory, ABCD of IA simulated the activation and presentation of autoantigens by dendritic cells, interactions with cognate T cells and subsequent T cell proliferation in the lymph node and IA-affected joints. The results, validated with in vivo data from the T cell driven SKG mouse model, showed that T cell proliferation strongly correlated with the T cell receptor (TCR) affinity distribution (TCR-ad), with a clear transition state from homeostasis to an inflammatory state. T cell proliferation was strongly dependent on the amount of antigen in antigenic stimulus event (ASE) at low concentrations. On the other hand, inflammation driven by Th17-inducing cytokine mediated T cell phenotype commitment was influenced by the initial level of Th17-inducing cytokines independent of the amount of arthritogenic antigen. The introduction of inhibitory artificial antigen presenting cells (iaAPCs), which locally suppress T cell activation, reduced T cell proliferation in a dose-dependent manner. The findings in this work set up a framework based on theory and modeling to simulate personalized therapeutic strategies in IA.

Hit-and-run vaccine system that overcomes limited neoantigen epitopes for efficient broad antitumor response

Neoantigen cancer vaccines have been envisioned as one of the most promising means for cancer therapies. However, identifying neoantigens for tumor types with low tumor mutation burdens continues to limit the effectiveness of neoantigen vaccines. Herein, we proposed a "hit-and-run" vaccine strategy which primes T cells to attack tumor cells decorated with exogenous "neo-antigens". This vaccine strategy utilizes a peptide nanovaccine to elicit antigen-specific T cell responses after tumor-specific decoration with a nanocarrier containing the same peptide antigens. We demonstrated that a poly(2-oxazoline)s (POx) conjugated with OVA257-264 peptide through a matrix metalloprotease 2 (MMP-2) sensitive linker could efficiently and selectively decorate tumor cells with OVA peptides in vivo. Then, a POx-based nanovaccine containing OVA257-264 peptides to elicit OVA-specific T cell responses was designed. In combination with this hit-and-run vaccine system, an effective vaccine therapy was demonstrated across tumor types even without OVA antigen expression. This approach provides a promising and uniform vaccine strategy against tumors with a low tumor mutation burden.

T-cell virtuosity in ''knowing thyself"

Major Histocompatibility Complex (MHC) I and II and the αβ T-cell antigen receptor (TCRαβ) govern fundamental traits of adaptive immunity. They form a membrane-borne ligand-receptor system weighing host proteome integrity to detect contamination by nonself proteins. MHC-I and -II exhibit the "MHC-fold", which is able to bind a large assortment of short peptides as proxies for self and nonself proteins. The ensuing varying surfaces are mandatory ligands for Ig-like TCRαβ highly mutable binding sites. Conserved molecular signatures guide TCRαβ ligand binding sites to focus on the MHC-fold (MHC-restriction) while leaving many opportunities for its most hypervariable determinants to contact the peptide. This riveting molecular strategy affords many options for binding energy compatible with specific recognition and signalling aimed to eradicated microbial pathogens and cancer cells. While the molecular foundations of αβ T-cell adaptive immunity are largely understood, uncertainty persists on how peptide-MHC binding induces the TCRαβ signals that instruct cell-fate decisions. Solving this mystery is another milestone for understanding αβ T-cells' self/nonself discrimination. Recent developments revealing the innermost links between TCRαβ structural dynamics and signalling modality should help dissipate this long-sought-after enigma.

Modulating Antigen Availability in Lymphoid Organs to Shape the Humoral Immune Response to Vaccines

Primary immune responses following vaccination are initiated in draining lymph nodes, where naive T and B cells encounter Ag and undergo coordinated steps of activation. For humoral immunity, the amount of Ag present over time, its localization to follicles and follicular dendritic cells, and the Ag's structural state all play important roles in determining the subsequent immune response. Recent studies have shown that multiple elements of vaccine design can impact Ag availability in lymphoid tissues, including the choice of adjuvant, physical form of the immunogen, and dosing kinetics. These vaccine design elements affect the transport of Ag to lymph nodes, Ag's localization in the tissue, the duration of Ag availability, and the structural integrity of the Ag. In this review, we discuss these findings and their implications for engineering more effective vaccines, particularly for difficult to neutralize pathogens.

Antigen perception in T cells by long-term Erk and NFAT signaling dynamics

Immune system threat detection hinges on T cells' ability to perceive varying peptide-major histocompatibility complex (pMHC) antigens. As the Erk and NFAT pathways link T cell receptor engagement to gene regulation, their signaling dynamics may convey information about pMHC inputs. To test this idea, we developed a dual reporter mouse strain and a quantitative imaging assay that, together, enable simultaneous monitoring of Erk and NFAT dynamics in live T cells over day-long timescales as they respond to varying pMHC inputs. Both pathways initially activate uniformly across various pMHC inputs but diverge only over longer (9+ h) timescales, enabling independent encoding of pMHC affinity and dose. These late signaling dynamics are decoded via multiple temporal and combinatorial mechanisms to generate pMHC-specific transcriptional responses. Our findings underscore the importance of long timescale signaling dynamics in antigen perception and establish a framework for understanding T cell responses under diverse contexts.

Development of protein-polymer conjugate nanoparticles for modulation of dendritic cell phenotype and antigen-specific CD4 T cell responses

Polymeric nanoparticles (NPs) comprised of poly(lactic-co-glycolic acid) (PLGA) have found success in modulating antigen (Ag)-specific T cell responses for the treatment multiple immunological diseases. Common methods by which Ags are associated with NPs are through encapsulation and surface conjugation; however, these methods suffer from several limitations, including uncontrolled Ag loading, burst release, and potential immune recognition. To overcome these limitations and study the relationship between NP design parameters and modulation of innate and Ag-specific adaptive immune cell responses, we developed ovalbumin (OVA) protein-PLGA bioconjugate NPs (acNP-OVA). OVA was first modified by conjugation with multiple PLGA polymers to synthesize OVA-PLGA conjugates, followed by precise combination with unmodified PLGA to form acNP-OVA with well-defined Ag loadings, reduced burst release, and reduced antibody recognition. Expression of MHC II, CD80, and CD86 on bone marrow-derived dendritic cells (BMDCs) increased as a function of acNP-OVA Ag loading. NanoString studies using BMDCs showed that PLGA NPs generally induced anti-inflammatory gene expression profiles independent of the Ag delivery method, where S100a9, Sell, and Ppbp were most significantly reduced. Co-culture studies using acNP-OVA-treated BMDCs and OT-II CD4+ T cells revealed that Ag-specific T cell activation, expansion, and differentiation were dependent on Ag loading and formulation parameters. CD25 expression was induced using acNP-OVA with the lowest Ag loading; however, the induction of robust CD4+ T cell proliferative and cytokine responses required acNP-OVA formulations with higher Ag loading, which was supported using a regulatory T cell (Treg) induction assay. The distinct differences in Ag loading required to achieve various T cell responses supported the concept of an Ag loading threshold for Ag-specific immunotherapy. We anticipate this work will help guide NP designs and aid in the future development of NP-based immunotherapies for Ag-specific immunomodulation.

What's the Catch? The Significance of Catch Bonds in T Cell Activation

One of the main goals in T cell biology has been to investigate how TCR recognition of peptide:MHC (pMHC) determines T cell phenotype and fate. Ag recognition is required to facilitate survival, expansion, and effector function of T cells. Historically, TCR affinity for pMHC has been used as a predictor for T cell fate and responsiveness, but there have now been several examples of nonfunctional high-affinity clones and low-affinity highly functional clones. Recently, more attention has been paid to the TCR being a mechanoreceptor where the key biophysical determinant is TCR bond lifetime under force. As outlined in this review, the fundamental parameters between the TCR and pMHC that control Ag recognition and T cell triggering are affinity, bond lifetime, and the amount of force at which the peak lifetime occurs.

Antigen perception in T cells by long-term Erk and NFAT signaling dynamics

Immune system threat detection hinges on T cells' ability to perceive varying peptide major-histocompatibility complex (pMHC) antigens. As the Erk and NFAT pathways link T cell receptor engagement to gene regulation, their signaling dynamics may convey information about pMHC inputs. To test this idea, we developed a dual reporter mouse strain and a quantitative imaging assay that, together, enable simultaneous monitoring of Erk and NFAT dynamics in live T cells over day-long timescales as they respond to varying pMHC inputs. Both pathways initially activate uniformly across various pMHC inputs, but diverge only over longer (9+ hrs) timescales, enabling independent encoding of pMHC affinity and dose. These late signaling dynamics are decoded via multiple temporal and combinatorial mechanisms to generate pMHC-specific transcriptional responses. Our findings underscore the importance of long timescale signaling dynamics in antigen perception, and establish a framework for understanding T cell responses under diverse contexts.

SIGNIFICANCE STATEMENT

To counter diverse pathogens, T cells mount distinct responses to varying peptide-major histocompatibility complex ligands (pMHCs). They perceive the affinity of pMHCs for the T cell receptor (TCR), which reflects its foreignness, as well as pMHC abundance. By tracking signaling responses in single living cells to different pMHCs, we find that T cells can independently perceive pMHC affinity vs dose, and encode this information through the dynamics of Erk and NFAT signaling pathways downstream of the TCR. These dynamics are jointly decoded by gene regulatory mechanisms to produce pMHC-specific activation responses. Our work reveals how T cells can elicit tailored functional responses to diverse threats and how dysregulation of these responses may lead to immune pathologies.

T cell receptor signaling in the differentiation and plasticity of CD4+ T cells

CD4⁺ T cells are critical components of the adaptive immune system. The T cell receptor (TCR) and co-receptor signaling cascades shape the phenotype and functions of CD4⁺ T cells. TCR signaling plays a crucial role in T cell development, antigen recognition, activation, and differentiation upon recognition of foreign- or auto-antigens. In specific autoimmune conditions, altered TCR repertoire is reported and can predispose autoimmunity with organ-specific inflammation and tissue damage. TCR signaling modulates various signaling cascades and regulates epigenetic and transcriptional regulation during homeostasis and disease conditions. Understanding the mechanism by which coreceptors and cytokine signals control the magnitude of TCR signal amplification will aid in developing therapeutic strategies to treat inflammation and autoimmune diseases. This review focuses on the role of the TCR signaling cascade and its components in the activation, differentiation, and plasticity of various CD4⁺ T cell subsets.

Emerging Roles of T Helper Cells in Non-Infectious Neuroinflammation: Savior or Sinner

CD4⁺ T cells, also known as T helper (Th) cells, contribute to the adaptive immunity both in the periphery and in the central nervous system (CNS). At least seven subsets of Th cells along with their signature cytokines have been identified nowadays. Neuroinflammation denotes the brain’s immune response to inflammatory conditions. In recent years, various CNS disorders have been related to the dysregulation of adaptive immunity, especially the process concerning Th cells and their cytokines. However, as the functions of Th cells are being discovered, it’s also found that their roles in different neuroinflammatory conditions, or even the participation of a specific Th subset in one CNS disorder may differ, and sometimes contrast. Based on those recent and contradictory evidence, the conflicting roles of Th cells in multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, epilepsy, traumatic brain injury as well as some typical mental disorders will be reviewed herein. Research progress, limitations and novel approaches concerning different neuroinflammatory conditions will also be mentioned and compared.

A DNA vaccine targeting VEE virus delivered by needle-free jet-injection protects macaques against aerosol challenge

We have previously shown that DNA vaccines expressing codon optimized alphavirus envelope glycoprotein genes protect both mice and nonhuman primates from viral challenge when delivered by particle-mediated epidermal delivery (PMED) or intramuscular (IM) electroporation (EP). Another technology with fewer logistical drawbacks is disposable syringe jet injection (DSJI) devices developed by PharmaJet, Inc. These needle-free jet injection systems are spring-powered and capable of delivering vaccines either IM or into the dermis (ID). Here, we evaluated the immunogenicity of our Venezuelan equine encephalitis virus (VEEV) DNA vaccine delivered by either the IM- or ID-DSJI devices in nonhuman primates. The protective efficacy was assessed following aerosol challenge. We found that a prime and single boost by either the IM or ID route resulted in humoral and cellular immune responses that provided significant protection against disease and viremia. Although the ID route utilized one-fifth the DNA dose used in the IM route of vaccination, and the measured humoral and cellular immune responses trended lower, the level of protection was high and performed as well as the IM route for several clinical endpoints.

The interplay between membrane topology and mechanical forces in regulating T cell receptor activity

T cells are critically important for host defense against infections. T cell activation is specific because signal initiation requires T cell receptor (TCR) recognition of foreign antigen peptides presented by major histocompatibility complexes (pMHC) on antigen presenting cells (APCs). Recent advances reveal that the TCR acts as a mechanoreceptor, but it remains unclear how pMHC/TCR engagement generates mechanical forces that are converted to intracellular signals. Here we propose a TCR Bending Mechanosignal (TBM) model, in which local bending of the T cell membrane on the nanometer scale allows sustained contact of relatively small pMHC/TCR complexes interspersed among large surface receptors and adhesion molecules on the opposing surfaces of T cells and APCs. Localized T cell membrane bending is suggested to increase accessibility of TCR signaling domains to phosphorylation, facilitate selective recognition of agonists that form catch bonds, and reduce noise signals associated with slip bonds.

Pre-existing chromatin accessibility and gene expression differences among naive CD4+ T cells influence effector potential

CD4+ T cells have a remarkable potential to differentiate into diverse effector lineages following activation. Here, we probe the heterogeneity present among naive CD4+ T cells before encountering their cognate antigen to ask whether their effector potential is modulated by pre-existing transcriptional and chromatin landscape differences. Single-cell RNA sequencing shows that key drivers of variability are genes involved in T cell receptor (TCR) signaling. Using CD5 expression as a readout of the strength of tonic TCR interactions with self-peptide MHC, and sorting on the ends of this self-reactivity spectrum, we find that pre-existing transcriptional differences among naive CD4+ T cells impact follicular helper T (TFH) cell versus non-TFH effector lineage choice. Moreover, our data implicate TCR signal strength during thymic development in establishing differences in naive CD4+ T cell chromatin landscapes that ultimately shape their effector potential.

Mechanobiology of T Cell Activation: To Catch a Bond

T cell activation is a critical event in the adaptive immune response, indispensable for cell-mediated and humoral immunity as well as for immune regulation. Recent years have witnessed an emerging trend emphasizing the essential role that physical force and mechanical properties play at the T cell interface. In this review, we integrate current knowledge of T cell antigen recognition and the different models of T cell activation from the perspective of mechanobiology, focusing on the interaction between the T cell receptor (TCR) and the peptide-major histocompatibility complex (pMHC) antigen. We address the shortcomings of TCR affinity alone in explaining T cell functional outcomes and the rising status of force-regulated TCR bond lifetimes, most notably the TCR catch bond. Ultimately, T cell activation and the ensuing physiological responses result from mechanical interaction between TCRs and the pMHC. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Strength and Numbers: The Role of Affinity and Avidity in the 'Quality' of T Cell Tolerance

The ability of T cells to identify foreign antigens and mount an efficient immune response while limiting activation upon recognition of self and self-associated peptides is critical. Multiple tolerance mechanisms work in concert to prevent the generation and activation of self-reactive T cells. T cell tolerance is tightly regulated, as defects in these processes can lead to devastating disease; a wide variety of autoimmune diseases and, more recently, adverse immune-related events associated with checkpoint blockade immunotherapy have been linked to a breakdown in T cell tolerance. The quantity and quality of antigen receptor signaling depend on a variety of parameters that include T cell receptor affinity and avidity for peptide. Autoreactive T cell fate choices (e.g., deletion, anergy, regulatory T cell development) are highly dependent on the strength of T cell receptor interactions with self-peptide. However, less is known about how differences in the strength of T cell receptor signaling during differentiation influences the 'function' and persistence of anergic and regulatory T cell populations. Here, we review the literature on this subject and discuss the clinical implications of how T cell receptor signal strength influences the 'quality' of anergic and regulatory T cell populations.

Opposing effects of T cell receptor signal strength on CD4 T cells responding to acute versus chronic viral infection

A hallmark of adaptive immunity is CD4 T cells’ ability to differentiate into specialized effectors. A long-standing question is whether T cell receptor (TCR) signal strength can dominantly instruct the development of Th1 and T follicular helper (Tfh) cells across distinct infectious contexts. We characterized the differentiation of murine CD4 TCR transgenic T cells responding to altered peptide ligand lymphocytic choriomeningitis viruses (LCMV) derived from acute and chronic parental strains. We found that TCR signal strength exerts opposite and hierarchical effects on the balance of Th1 and Tfh cells responding to acute versus persistent infection. TCR signal strength correlates positively with Th1 generation during acute but negatively during chronic infection. Weakly activated T cells express lower levels of markers associated with chronic T cell stimulation and may resist functional inactivation. We anticipate that the panel of recombinant viruses described herein will be valuable for investigating a wide range of CD4 T cell responses.

TCR Recognition of Peptide-MHC-I: Rule Makers and Breakers

T cells are a critical part of the adaptive immune system that are able to distinguish between healthy and unhealthy cells. Upon recognition of protein fragments (peptides), activated T cells will contribute to the immune response and help clear infection. The major histocompatibility complex (MHC) molecules, or human leukocyte antigens (HLA) in humans, bind these peptides to present them to T cells that recognise them with their surface T cell receptors (TCR). This recognition event is the first step that leads to T cell activation, and in turn can dictate disease outcomes. The visualisation of TCR interaction with pMHC using structural biology has been crucial in understanding this key event, unravelling the parameters that drive this interaction and their impact on the immune response. The last five years has been the most productive within the field, wherein half of current unique TCR-pMHC-I structures to date were determined within this time. Here, we review the new insights learned from these recent TCR-pMHC-I structures and their impact on T cell activation.

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.

In silico Design of Phl p 6 Variants With Altered Fold-Stability Significantly Impacts Antigen Processing, Immunogenicity and Immune Polarization

Introduction: Understanding, which factors determine the immunogenicity and immune polarizing properties of proteins, is an important prerequisite for designing better vaccines and immunotherapeutics. While extrinsic immune modulatory factors such as pathogen associated molecular patterns are well-understood, far less is known about the contribution of protein inherent features. Protein fold-stability represents such an intrinsic feature contributing to immunogenicity and immune polarization by influencing the amount of peptide-MHC II complexes (pMHCII). Here, we investigated how modulation of the fold-stability of the grass pollen allergen Phl p 6 affects its ability to stimulate immune responses and T cell polarization. Methods: MAESTRO software was used for in silico prediction of stabilizing or destabilizing point mutations. Mutated proteins were expressed in E. coli, and their thermal stability and resistance to endolysosomal proteases was determined. Resulting peptides were analyzed by mass spectrometry. The structure of the most stable mutant protein was assessed by X-ray crystallography. We evaluated the capacity of the mutants to stimulate T cell proliferation in vitro, as well as antibody responses and T cell polarization in vivo in an adjuvant-free BALB/c mouse model. Results: In comparison to wild-type protein, stabilized or destabilized mutants displayed changes in thermal stability ranging from -5 to +14°. While highly stabilized mutants were degraded very slowly, destabilization led to faster proteolytic processing in vitro. This was confirmed in BMDCs, which processed and presented the immunodominant epitope from a destabilized mutant more efficiently compared to a highly stable mutant. In vivo, stabilization resulted in a shift in immune polarization from TH2 to TH1/TH17 as indicated by higher levels of IgG2a and increased secretion of TNF-α, IFN-γ, IL-17, and IL-21. Conclusion: MAESTRO software was very efficient in detecting single point mutations that increase or reduce fold-stability. Thermal stability correlated well with the speed of proteolytic degradation and presentation of peptides on the surface of dendritic cells in vitro. This change in processing kinetics significantly influenced the polarization of T cell responses in vivo. Modulating the fold-stability of proteins thus has the potential to optimize and polarize immune responses, which opens the door to more efficient design of molecular vaccines.

Regulation of T Helper Cell Fate by TCR Signal Strength

T cells are critical in orchestrating protective immune responses to cancer and an array of pathogens. The interaction between a peptide MHC (pMHC) complex on antigen presenting cells (APCs) and T cell receptors (TCRs) on T cells initiates T cell activation, division, and clonal expansion in secondary lymphoid organs. T cells must also integrate multiple T cell-intrinsic and extrinsic signals to acquire the effector functions essential for the defense against invading microbes. In the case of T helper cell differentiation, while innate cytokines have been demonstrated to shape effector CD4⁺ T lymphocyte function, the contribution of TCR signaling strength to T helper cell differentiation is less understood. In this review, we summarize the signaling cascades regulated by the strength of TCR stimulation. Various mechanisms in which TCR signal strength controls T helper cell expansion and differentiation are also discussed.

Key Features Relevant to Select Antigens and TCR From the MHC-Mismatched Repertoire to Treat Cancer

Adoptive transfer of T cells transgenic for tumor-reactive T-cell receptors (TCR) is an attractive immunotherapeutic approach. However, clinical translation is so far limited due to challenges in the identification of suitable target antigens as well as TCRs that are concurrent safe and efficient. Definition of key characteristics relevant for effective and specific tumor rejection is essential to improve current TCR-based adoptive T-cell immunotherapies. We here characterized in-depth two TCRs derived from the human leukocyte antigen (HLA)-mismatched allogeneic repertoire targeting two different myeloperoxidase (MPO)-derived peptides presented by the same HLA-restriction element side by side comprising state of the art biochemical and cellular in vitro, in vivo, and in silico experiments. In vitro experiments reveal comparable functional avidities, off-rates, and cytotoxic activities for both TCRs. However, we observed differences especially with respect to cytokine secretion and cross-reactivity as well as in vivo activity. Biochemical and in silico analyses demonstrate different binding qualities of MPO-peptides to the HLA-complex determining TCR qualities. We conclude from our biochemical and in silico analyses of peptide-HLA-binding that rigid and high-affinity binding of peptides is one of the most important factors for isolation of TCRs with high specificity and tumor rejection capacity from the MHC-mismatched repertoire. Based on our results, we developed a workflow for selection of such TCRs with high potency and safety profile suitable for clinical translation.

Isolation of a Structural Mechanism for Uncoupling T Cell Receptor Signaling from Peptide-MHC Binding

TCR-signaling strength generally correlates with peptide-MHC binding affinity; however, exceptions exist. We find high-affinity, yet non-stimulatory, interactions occur with high frequency in the human T cell repertoire. Here, we studied human TCRs that are refractory to activation by pMHC ligands despite robust binding. Analysis of 3D affinity, 2D dwell time, and crystal structures of stimulatory versus non-stimulatory TCR-pMHC interactions failed to account for their different signaling outcomes. Using yeast pMHC display, we identified peptide agonists of a formerly non-responsive TCR. Single-molecule force measurements demonstrated the emergence of catch bonds in the activating TCR-pMHC interactions, correlating with exclusion of CD45 from the TCR-APC contact site. Molecular dynamics simulations of TCR-pMHC disengagement distinguished agonist from non-agonist ligands based on the acquisition of catch bonds within the TCR-pMHC interface. The isolation of catch bonds as a parameter mediating the coupling of TCR binding and signaling has important implications for TCR and antigen engineering for immunotherapy.

DNA methylation as a mediator of HLA-DRB1*15:01 and a protective variant in multiple sclerosis

The human leukocyte antigen (HLA) haplotype DRB1*15:01 is the major risk factor for multiple sclerosis (MS). Here, we find that DRB1*15:01 is hypomethylated and predominantly expressed in monocytes among carriers of DRB1*15:01. A differentially methylated region (DMR) encompassing HLA-DRB1 exon 2 is particularly affected and displays methylation-sensitive regulatory properties in vitro. Causal inference and Mendelian randomization provide evidence that HLA variants mediate risk for MS via changes in the HLA-DRB1 DMR that modify HLA-DRB1 expression. Meta-analysis of 14,259 cases and 171,347 controls confirms that these variants confer risk from DRB1*15:01 and also identifies a protective variant (rs9267649, p < 3.32 × 10^-8, odds ratio = 0.86) after conditioning for all MS-associated variants in the region. rs9267649 is associated with increased DNA methylation at the HLA-DRB1 DMR and reduced expression of HLA-DRB1, suggesting a modulation of the DRB1*15:01 effect. Our integrative approach provides insights into the molecular mechanisms of MS susceptibility and suggests putative therapeutic strategies targeting a methylation-mediated regulation of the major risk gene.

Differential T-cell receptor signals for T helper cell programming

Upon encounter with their cognate antigen, naive CD4 T cells become activated and are induced to differentiate into several possible T helper (Th) cell subsets. This differentiation depends on a number of factors including antigen-presenting cells, cytokines and co-stimulatory molecules. The strength of the T-cell receptor (TCR) signal, related to the affinity of TCR for antigen and antigen dose, has emerged as a dominant factor in determining Th cell fate. Recent studies have revealed that TCR signals of high or low strength do not simply induce quantitatively different signals in the T cells, but rather qualitatively distinct pathways can be induced based on TCR signal strength. This review examines the recent literature in this area and highlights important new developments in our understanding of Th cell differentiation and TCR signal strength.

CD4 T Cell Affinity Diversity Is Equally Maintained during Acute and Chronic Infection

TCR affinity for peptide MHC dictates the functional efficiency of T cells and their propensity to differentiate into effectors and form memory. However, in the context of chronic infections, it is unclear what the overall profile of TCR affinity for Ag is and if it differs from acute infections. Using the comprehensive affinity analysis provided by the two-dimensional micropipette adhesion frequency assay and the common indirect affinity evaluation methods of MHC class II tetramer and functional avidity, we tracked IA^b GP_61-80-specific cells in the mouse model of acute (Armstrong) and chronic (clone 13) lymphocytic choriomeningitis virus infection. In each response, we show CD4 T cell population affinity peaks at the effector phase and declines with memory. Of interest, the range and average relative two-dimensional affinity was equivalent between acute and chronic infection, indicating chronic Ag exposure did not skew TCR affinity. In contrast, functional and tetramer avidity measurements revealed divergent results and lacked a consistent correlation with TCR affinity. Our findings highlight that the immune system maintains a diverse range in TCR affinity even under the pressures of chronic Ag stimulation.

Quantification of HLA-DM-Dependent Major Histocompatibility Complex of Class II Immunopeptidomes by the Peptide Landscape Antigenic Epitope Alignment Utility

The major histocompatibility complex of class II (MHCII) immunopeptidome represents the repertoire of antigenic peptides with the potential to activate CD4⁺ T cells. An understanding of how the relative abundance of specific antigenic epitopes affects the outcome of T cell responses is an important aspect of adaptive immunity and offers a venue to more rationally tailor T cell activation in the context of disease. Recent advances in mass spectrometric instrumentation, computational power, labeling strategies, and software analysis have enabled an increasing number of stratified studies on HLA ligandomes, in the context of both basic and translational research. A key challenge in the case of MHCII immunopeptidomes, often determined for different samples at distinct conditions, is to derive quantitative information on consensus epitopes from antigenic peptides of variable lengths. Here, we present the design and benchmarking of a new algorithm [peptide landscape antigenic epitope alignment utility (PLAtEAU)] allowing the identification and label-free quantification (LFQ) of shared consensus epitopes arising from series of nested peptides. The algorithm simplifies the complexity of the dataset while allowing the identification of nested peptides within relatively short segments of protein sequences. Moreover, we apply this algorithm to the comparison of the ligandomes of cell lines with two different expression levels of the peptide-exchange catalyst HLA-DM. Direct comparison of LFQ intensities determined at the peptide level is inconclusive, as most of the peptides are not significantly enriched due to poor sampling. Applying the PLAtEAU algorithm for grouping of the peptides into consensus epitopes shows that more than half of the total number of epitopes is preferentially and significantly enriched for each condition. This simplification and deconvolution of the complex and ambiguous peptide-level dataset highlights the value of the PLAtEAU algorithm in facilitating robust and accessible quantitative analysis of immunopeptidomes across cellular contexts. In silico analysis of the peptides enriched for each HLA-DM expression conditions suggests a higher affinity of the pool of peptides isolated from the high DM expression samples. Interestingly, our analysis reveals that while for certain autoimmune-relevant epitopes their presentation increases upon DM expression others are clearly edited out from the peptidome.

Functional differences in airway dendritic cells determine susceptibility to IgE-sensitization

Respiratory IgE-sensitization to innocuous antigens increases the risk for developing diseases such as allergic asthma. Dendritic cells (DC) residing in the airways orchestrate the immune response following antigen exposure and their ability to sample and present antigens to naïve T cells in airway draining lymph nodes contributes to allergen-specific IgE-sensitization. In order to characterize inhaled antigen capture and presentation by DC subtypes in vivo, we used an adjuvant-free respiratory sensitization model using two genetically distinct rat strains, one of which is naturally resistant and the other naturally susceptible to allergic sensitization. Upon multiple exposures to ovalbumin (OVA), the susceptible strain developed OVA-specific IgE and airway inflammation, whereas the resistant strain did not. Using fluorescently tagged OVA and flow cytometry, we demonstrated significant differences in antigen uptake efficiency and presentation associated with either IgE-sensitization or resistance to allergen exposures in respective strains. We further identified CD4⁺ conventional DC (cDC) as the subset involved in airway antigen sampling in both strains, however, CD4⁺ cDC in the susceptible strain were less efficient in OVA sampling and displayed increased MHC-II expression compared with the resistant strain. This was associated with generation of an exaggerated Th2 response and a deficiency of airway regulatory T cells in the susceptible strain. These data suggest that subsets of cDC are able to induce either sensitization or resistance to inhaled antigens as determined by genetic background, which may provide an underlying basis for genetically determined susceptibility to respiratory allergic sensitization and IgE production in susceptible individuals.

Fine-Tuning of Optimal TCR Signaling in Tumor-Redirected CD8 T Cells by Distinct TCR Affinity-Mediated Mechanisms

Redirecting CD8 T cell immunity with self/tumor-specific affinity-matured T cell receptors (TCRs) is a promising approach for clinical adoptive T cell therapy, with the aim to improve treatment efficacy. Despite numerous functional-based studies, little is known about the characteristics of TCR signaling (i.e., intensity, duration, and amplification) and the regulatory mechanisms underlying optimal therapeutic T cell responses. Using a panel of human SUP-T1 and primary CD8 T cells engineered with incremental affinity TCRs against the cancer-testis antigen NY-ESO-1, we found that upon activation, T cells with optimal-affinity TCRs generated intense and sustained proximal (CD3ζ, LCK) signals associated with distal (ERK1/2) amplification-gain and increased function. In contrast, in T cells with very high affinity TCRs, signal initiation was rapid and strong yet only transient, resulting in poor MAPK activation and low proliferation potential even at high antigen stimulation dose. Under resting conditions, the levels of surface TCR/CD3ε, CD8β, and CD28 expression and of CD3ζ phosphorylation were significantly reduced in those hyporesponsive cells, suggesting the presence of TCR affinity-related activation thresholds. We also show that SHP phosphatases were involved along the TCR affinity gradient, but displayed spatially distinct regulatory roles. While PTPN6/SHP-1 phosphatase activity controlled TCR signaling initiation and subsequent amplification by counteracting CD3ζ and ERK1/2 phosphorylation, PTPN11/SHP-2 augmented MAPK activation without affecting proximal TCR signaling. Together, our findings indicate that optimal TCR signaling can be finely tuned by TCR affinity-dependent SHP-1 and SHP-2 activity, and this may readily be determined at the TCR/CD3 complex level. We propose that these TCR affinity-associated regulations represent potential protective mechanisms preventing high affinity TCR-mediated autoimmune diseases.

The avidity of cross-reactive virus-specific T cells for their viral and allogeneic epitopes is variable and depends on epitope expression

Virus-specific T cells can recognize allogeneic HLA (allo-HLA) through cross-reactivity of their T-cell receptor (TCR). In a transplantation setting, such allo-HLA cross-reactivity may contribute to harmful immune responses towards the allograft, provided that the cross-reactive T cells get sufficiently activated upon recognition of the allo-HLA. An important determinant of T-cell activation is TCR avidity, which to date, has remained largely unexplored for allo-HLA-cross-reactive virus-specific T cells. For this purpose, cold target inhibition assays were performed using allo-HLA-cross-reactive virus-specific memory CD8⁺ T-cell clones as responders, and syngeneic cells loaded with viral peptide and allogeneic cells as hot (radioactively-labeled) and cold (non-radioactively-labeled) targets. CD8 dependency of the T-cell responses was assessed using interferon γ (IFNγ) enzyme-linked immunosorbent assay (ELISA) in the presence and absence of CD8-blocking antibodies. At high viral-peptide loading concentrations, T-cell clones consistently demonstrated lower avidity for allogeneic versus viral epitopes, but at suboptimal concentrations the opposite was observed. In line, anti-viral reactivity was CD8 independent at high, but not at suboptimal viral-peptide-loading concentrations. The avidity of allo-HLA-cross-reactive virus-specific memory CD8⁺ T cells is therefore highly dependent on epitope expression, and as a consequence, can be both higher and lower for allogeneic versus viral targets under different (patho)physiological conditions.

The IRF4 Gene Regulatory Module Functions as a Read-Write Integrator to Dynamically Coordinate T Helper Cell Fate

Transcriptional regulation during CD4⁺ T cell fate decisions enables their differentiation into distinct states, guiding immune responses toward antibody production via Tfh cells or inflammation by Teff cells. Tfh-Teff cell fate commitment is regulated by mutual antagonism between the transcription factors Bcl6 and Blimp-1. Here we examined how T cell receptor (TCR) signals establish and arbitrate Bcl6-Blimp-1 counter-antagonism. We found that the TCR-signal-induced transcription factor Irf4 is essential for the differentiation of Bcl6-expressing Tfh and Blimp-1-expressing Teff cells. Increased TCR signaling raised Irf4 amounts and promoted Teff cell fates at the expense of Tfh ones. Importantly, orthogonal induction of Irf4 expression redirected Tfh cell fate trajectories toward those of Teff. Mechanistically, we linked greater Irf4 abundance with its recruitment toward low-affinity binding sites within Teff cell cis-regulatory elements, including those of Prdm1. We propose that the Irf4 locus functions as the "reader" of TCR signal strength, and in turn, concentration-dependent activity of Irf4 "writes" T helper fate choice.

Rapid G0/1 transition and cell cycle progression in CD8+ T cells compared to CD4+ T cells following in vitro stimulation

T-cell population consists of two major subsets, CD4⁺ T cells and CD8⁺ T cells, which can be distinguished by the expression of CD4 or CD8 molecules, respectively. Although they play quite different roles in the immune system, many of their basic cellular processes such as proliferation following stimulation are presumably common. In this study, we have carefully analyzed time-course of G0/1 transition as well as cell cycle progression in the two subsets of quiescent T-cell population following in vitro growth stimulation. We found that CD8⁺ T cells promote G0/1 transition more rapidly and drive their cell cycle progression faster compared to CD4⁺ T cells. In addition, expression of CD25 and effects of its blockade revealed that IL-2 is implicated in the rapid progression, but not the earlier G0/1 transition, of CD8⁺ T cells.

Active immunotherapy for TNF-mediated inflammation using self-assembled peptide nanofibers

Active immunotherapies raising antibody responses against autologous targets are receiving increasing interest as alternatives to the administration of manufactured antibodies. The challenge in such an approach is generating protective and adjustable levels of therapeutic antibodies while at the same time avoiding strong T cell responses that could lead to autoimmune reactions. Here we demonstrate the design of an active immunotherapy against TNF-mediated inflammation using short synthetic peptides that assemble into supramolecular peptide nanofibers. Immunization with these materials, without additional adjuvants, was able to break B cell tolerance and raise protective antibody responses against autologous TNF in mice. The strength of the anti-TNF antibody response could be tuned by adjusting the epitope content in the nanofibers, and the T-cell response was focused on exogenous and non-autoreactive T-cell epitopes. Immunization with unadjuvanted peptide nanofibers was therapeutic in a lethal model of acute inflammation induced by intraperitoneally delivered lipopolysaccharide, whereas formulations adjuvanted with CpG showed comparatively poorer protection that correlated with a more Th1-polarized response. Additionally, immunization with peptide nanofibers did not diminish the ability of mice to clear infections of Listeria monocytogenes. Collectively this work suggests that synthetic self-assembled peptides can be attractive platforms for active immunotherapies against autologous targets.

Reduced TCR-dependent activation through citrullination of a T-cell epitope enhances Th17 development by disruption of the STAT3/5 balance

Citrullination is a post‐translational modification of arginine that commonly occurs in inflammatory tissues. Because T‐cell receptor (TCR) signal quantity and quality can regulate T‐cell differentiation, citrullination within a T‐cell epitope has potential implications for T‐cell effector function. Here, we investigated how citrullination of an immunedominant T‐cell epitope affected Th17 development. Murine naïve CD4⁺ T cells with a transgenic TCR recognising p89‐103 of the G1 domain of aggrecan (agg) were co‐cultured with syngeneic bone marrow‐derived dendritic cells (BMDC) presenting the native or citrullinated peptides. In the presence of pro‐Th17 cytokines, the peptide citrullinated on residue 93 (R93Cit) significantly enhanced Th17 development whilst impairing the Th2 response, compared to the native peptide. T cells responding to R93Cit produced less IL‐2, expressed lower levels of the IL‐2 receptor subunit CD25, and showed reduced STAT5 phosphorylation, whilst STAT3 activation was unaltered. IL‐2 blockade in native p89‐103‐primed T cells enhanced the phosphorylated STAT3/STAT5 ratio, and concomitantly enhanced Th17 development. Our data illustrate how a post‐translational modification of a TCR contact point may promote Th17 development by altering the balance between STAT5 and STAT3 activation in responding T cells, and provide new insight into how protein citrullination may influence effector Th‐cell development in inflammatory disorders.

The era of bioengineering: how will this affect the next generation of cancer immunotherapy?

Background

Immunotherapy consists of activating the patient’s immune system to fight cancer and has the great potential of preventing future relapses thanks to immunological memory. A great variety of strategies have emerged to harness the immune system against tumors, from the administration of immunomodulatory agents that activate immune cells, to therapeutic vaccines or infusion of previously activated cancer-specific T cells. However, despite great recent progress many difficulties still remain, which prevent the widespread use of immunotherapy. Some of these limitations include: systemic toxicity, weak immune cellular responses or persistence over time and most ultimately costly and time-consuming procedures.

Main body

Synthetic and natural biomaterials hold great potential to address these hurdles providing biocompatible systems capable of targeted local delivery, co-delivery, and controlled and/or sustained release. In this review we discuss some of the bioengineered solutions and approaches developed so far and how biomaterials can be further implemented to help and shape the future of cancer immunotherapy.

Conclusion

The bioengineering strategies here presented constitute a powerful toolkit to develop safe and successful novel cancer immunotherapies.

Different TCR-induced T lymphocyte responses are potentiated by stiffness with variable sensitivity

T cells are mechanosensitive but the effect of stiffness on their functions is still debated. We characterize herein how human primary CD4⁺ T cell functions are affected by stiffness within the physiological Young’s modulus range of 0.5 kPa to 100 kPa. Stiffness modulates T lymphocyte migration and morphological changes induced by TCR/CD3 triggering. Stiffness also increases TCR-induced immune system, metabolism and cell-cycle-related genes. Yet, upon TCR/CD3 stimulation, while cytokine production increases within a wide range of stiffness, from hundreds of Pa to hundreds of kPa, T cell metabolic properties and cell cycle progression are only increased by the highest stiffness tested (100 kPa). Finally, mechanical properties of adherent antigen-presenting cells modulate cytokine production by T cells. Together, these results reveal that T cells discriminate between the wide range of stiffness values found in the body and adapt their responses accordingly.

DOI:http://dx.doi.org/10.7554/eLife.23190.001

Our immune system contains many cells that play various roles in defending the body against infection, cancer and other threats. For example, T cells constantly patrol the body ready to detect and respond to dangers. They do so by gathering cues from their surroundings, which can be specific chemical signals or physical properties such as the stiffness of tissues. Once the T cells are active they respond in several different ways including releasing hormones and dividing to produce more T cells.

Tissue stiffness varies considerably between different organs. Furthermore, disease can lead to changes in tissue stiffness. For example, tissues become more rigid when they are inflamed. The stiffness and other physical properties of the surfaces that T cells interact with affect how the cells respond when they detect a threat, but few details are known about exactly how these cues tune T cell responses. Saitakis et al. studied how human T cells respond to artificial surfaces of varying stiffness that mimic the range found in the body.

The experiments show that T cells that interact with stiff surfaces become more active than T cells that interact with softer surfaces. However, some responses are more sensitive to the stiffness of the surface than others. For example, the ability of the T cells to release hormones was affected by the whole range of stiffnesses tested in the experiments, whereas only very stiff surfaces stimulated the T cells to divide.

These findings show that T cells can detect the stiffness of surfaces in the body and use this to adapt how they respond to threats. Future challenges will be to find out how T cells sense the physical properties of their surroundings and investigate whether cell and tissue stiffness affects immune responses in the body. This will help us to understand how T cells fight infections and other threats, and could be used to develop new ways of boosting these cells to fight cancer and other diseases.

DOI:http://dx.doi.org/10.7554/eLife.23190.002

Influence of protein fold stability on immunogenicity and its implications for vaccine design

INTRODUCTION

In modern vaccinology and immunotherapy, recombinant proteins more and more replace whole organisms to induce protective or curative immune responses. Structural stability of proteins is of crucial importance for efficient presentation of antigenic peptides on MHC, which plays a decisive role for triggering strong immune reactions. Areas covered: In this review, we discuss structural stability as a key factor for modulating the potency of recombinant vaccines and its importance for antigen proteolysis, presentation, and stimulation of B and T cells. Moreover, the impact of fold stability on downstream events determining the differentiation of T cells into effector cells is reviewed. We summarize studies investigating the impact of protein fold stability on the outcome of the immune response and provide an overview on computational methods to estimate the effects of point mutations on protein stability. Expert commentary: Based on this information, the rational design of up-to-date vaccines is discussed. A model for predicting immunogenicity of proteins based on their conformational stability at different pH values is proposed.

Adequate immune response ensured by binary IL-2 and graded CD25 expression in a murine transfer model

The IL-2/IL-2Ralpha (CD25) axis is of central importance for the interplay of effector and regulatory T cells. Nevertheless, the question how different antigen loads are translated into appropriate IL-2 production to ensure adequate responses against pathogens remains largely unexplored. Here we find that at single cell level, IL-2 is binary (digital) and CD25 is graded expressed whereas at population level both parameters show graded expression correlating with the antigen amount. Combining in vivo data with a mathematical model we demonstrate that only this binary IL-2 expression ensures a wide linear antigen response range for Teff and Treg cells under real spatiotemporal conditions. Furthermore, at low antigen concentrations binary IL-2 expression safeguards by its spatial distribution selective STAT5 activation only of closely adjacent Treg cells regardless of their antigen specificity. These data show that the mode of IL-2 secretion is critical to tailor the adaptive immune response to the antigen amount.

DOI:http://dx.doi.org/10.7554/eLife.20616.001

T-cell stimuli independently sum to regulate an inherited clonal division fate

In the presence of antigen and costimulation, T cells undergo a characteristic response of expansion, cessation and contraction. Previous studies have revealed that population-level reproducibility is a consequence of multiple clones exhibiting considerable disparity in burst size, highlighting the requirement for single-cell information in understanding T-cell fate regulation. Here we show that individual T-cell clones resulting from controlled stimulation in vitro are strongly lineage imprinted with highly correlated expansion fates. Progeny from clonal families cease dividing in the same or adjacent generations, with inter-clonal variation producing burst-size diversity. The effects of costimulatory signals on individual clones sum together with stochastic independence; therefore, the net effect across multiple clones produces consistent, but heterogeneous population responses. These data demonstrate that substantial clonal heterogeneity arises through differences in experience of clonal progenitors, either through stochastic antigen interaction or by differences in initial receptor sensitivities.

Why do populations of highly similar T cells have heterogeneous division destinies in response to antigenic stimulus? Here the authors develop a multiplex-dye assay and a mathematical framework to test clonal heterogeneity and show distinction in division destiny is a result of inter-clonal variability as lineage imprinting ensures clones share similar proliferation fates.

TCR/pMHC Interaction: Phenotypic Model for an Unsolved Enigma

TCR-pMHC interaction is the keystone of the adaptive immune response. This process exhibits an impressive capacity of speed, sensitivity, and discrimination that allows detecting foreign pMHCs at very low concentration among much more abundant self-pMHC ligands. However, and despite over three decades of intensive research, the mechanisms by which this remarkable discrimination and sensitivity is attained remain controversial. In kinetic proofreading mechanisms (KPR), an increase of specificity occurs by reducing the sensitivity. To overcome this difficulty, more elaborate models including feedback processes or induced rebinding have been incorporated into the KPR scheme. Here a new approach based on the assumption that the proofreading chain behaves differently for foreign- and self-pMHC complexes has been integrated into a phenotypic model in which the complexes responsible for T cell activation stabilize (for foreign peptides) or weaken (for foreign peptides), resulting in a dramatic increase in sensitivity and specificity. Stabilization and destabilization of complexes may be caused by conformational changes, rebinding, or any other process leading to variations in the dissociation rate constants of the complexes transmitting the activation. The numerical solution and the analytical expression for the steady-state response as a function of k_off(i) (i = 0, 1, …, N, where C₀, C₁, …, C_N are the complexes in the proofreading chain) are provided. The activation chain speeds up, and larger increases in sensitivity and discrimination are obtained if the rate of activation along the proofreading chain increases for foreign pMHCs and decreases for self-ligands. Experimental implications and comparison with current models are discussed.

Type 1 diabetes vaccine candidates promote human Foxp3(+)Treg induction in humanized mice

Immune tolerance is executed partly by Foxp3⁺regulatory T (Treg) cells, which suppress autoreactive T cells. In autoimmune type 1 diabetes (T1D) impaired tolerance promotes destruction of insulin-producing β-cells. The development of autoantigen-specific vaccination strategies for Foxp3⁺Treg-induction and prevention of islet autoimmunity in patients is still in its infancy. Here, using human haematopoietic stem cell-engrafted NSG-HLA-DQ8 transgenic mice, we provide direct evidence for human autoantigen-specific Foxp3⁺Treg-induction in vivo. We identify HLA-DQ8-restricted insulin-specific CD4⁺T cells and demonstrate efficient human insulin-specific Foxp3⁺Treg-induction upon subimmunogenic vaccination with strong agonistic insulin mimetopes in vivo. Induced human Tregs are stable, show increased expression of Treg signature genes such as Foxp3, CTLA4, IL-2Rα and TIGIT and can efficiently suppress effector T cells. Such Foxp3⁺Treg-induction does not trigger any effector T cells. These T1D vaccine candidates could therefore represent an expedient improvement in the challenge to induce human Foxp3⁺Tregs and to develop novel precision medicines for prevention of islet autoimmunity in children at risk of T1D.

Type 1 diabetes is associated with the loss of self-tolerance to the insulin-producing β-cells in the pancreas. Here the authors show that vaccination with insulin mimetopes can induce human insulin-specific regulatory T cells to mediate tolerance in a humanized mouse model.

Low-Affinity Memory CD8+ T Cells Mediate Robust Heterologous Immunity

Heterologous immunity is recognized as a significant barrier to transplant tolerance. Whereas it has been established that pathogen-elicited memory T cells can have high or low affinity for cross-reactive allogeneic peptide-MHC, the role of TCR affinity during heterologous immunity has not been explored. We established a model with which to investigate the impact of TCR-priming affinity on memory T cell populations following a graft rechallenge. In contrast to high-affinity priming, low-affinity priming elicited fully differentiated memory T cells with a CD45RB(hi) status. High CD45RB status enabled robust secondary responses in vivo, as demonstrated by faster graft rejection kinetics and greater proliferative responses. CD45RB blockade prolonged graft survival in low affinity-primed mice, but not in high affinity-primed mice. Mechanistically, low affinity-primed memory CD8(+) T cells produced more IL-2 and significantly upregulated IL-2Rα expression during rechallenge. We found that CD45RB(hi) status was also a stable marker of priming affinity within polyclonal CD8(+) T cell populations. Following high-affinity rechallenge, low affinity-primed CD45RB(hi) cells became CD45RB(lo), demonstrating that CD45RB status acts as an affinity-based differentiation switch on CD8(+) T cells. Thus, these data establish a novel mechanism by which CD45 isoforms tune low affinity-primed memory CD8(+) T cells to become potent secondary effectors following heterologous rechallenge. These findings have direct implications for allogeneic heterologous immunity by demonstrating that despite a lower precursor frequency, low-affinity priming is sufficient to generate memory cells that mediate potent secondary responses against a cross-reactive graft challenge.

TCR Signal Strength Alters T-DC Activation and Interaction Times and Directs the Outcome of Differentiation

The ability of CD4+ T cells to differentiate into effector subsets underpins their ability to shape the immune response and mediate host protection. During T cell receptor-induced activation of CD4+ T cells, both the quality and quantity of specific activatory peptide/MHC ligands have been shown to control the polarization of naive CD4+ T cells in addition to co-stimulatory and cytokine-based signals. Recently, advances in two--photon microscopy and tetramer-based cell tracking methods have allowed investigators to greatly extend the study of the role of TCR signaling in effector differentiation under in vivo conditions. In this review, we consider data from recent in vivo studies analyzing the role of TCR signal strength in controlling the outcome of CD4+ T cell differentiation and discuss the role of TCR in controlling the critical nature of CD4+ T cell interactions with dendritic cells during activation. We further propose a model whereby TCR signal strength controls the temporal aspects of T-DC interactions and the implications for this in mediating the downstream signaling events, which influence the transcriptional and epigenetic regulation of effector differentiation.

Tuning Sensitivity of CAR to EGFR Density Limits Recognition of Normal Tissue While Maintaining Potent Antitumor Activity

Many tumors overexpress tumor-associated antigens relative to normal tissue, such as EGFR. This limits targeting by human T cells modified to express chimeric antigen receptors (CAR) due to potential for deleterious recognition of normal cells. We sought to generate CAR(+) T cells capable of distinguishing malignant from normal cells based on the disparate density of EGFR expression by generating two CARs from monoclonal antibodies that differ in affinity. T cells with low-affinity nimotuzumab-CAR selectively targeted cells overexpressing EGFR, but exhibited diminished effector function as the density of EGFR decreased. In contrast, the activation of T cells bearing high-affinity cetuximab-CAR was not affected by the density of EGFR. In summary, we describe the generation of CARs able to tune T-cell activity to the level of EGFR expression in which a CAR with reduced affinity enabled T cells to distinguish malignant from nonmalignant cells.

Digital signaling decouples activation probability and population heterogeneity

Digital signaling enhances robustness of cellular decisions in noisy environments, but it is unclear how digital systems transmit temporal information about a stimulus. To understand how temporal input information is encoded and decoded by the NF-κB system, we studied transcription factor dynamics and gene regulation under dose- and duration-modulated inflammatory inputs. Mathematical modeling predicted and microfluidic single-cell experiments confirmed that integral of the stimulus (or area, concentration × duration) controls the fraction of cells that activate NF-κB in the population. However, stimulus temporal profile determined NF-κB dynamics, cell-to-cell variability, and gene expression phenotype. A sustained, weak stimulation lead to heterogeneous activation and delayed timing that is transmitted to gene expression. In contrast, a transient, strong stimulus with the same area caused rapid and uniform dynamics. These results show that digital NF-κB signaling enables multidimensional control of cellular phenotype via input profile, allowing parallel and independent control of single-cell activation probability and population heterogeneity.

Regulatory T Cells in Autoimmune Diabetes: Mechanisms of Action and Translational Potential

Since the discovery of specialized T cells with regulatory function, harnessing the power of these cells to ameliorate autoimmunity has been a major goal. Here we collate the evidence that regulatory T cells (Treg) can inhibit Type 1 diabetes in animal models and humans. We discuss the anatomical sites and molecular mechanisms of Treg suppressive function in the Type 1 diabetes setting, citing evidence that Treg can function in both the pancreatic lymph nodes and within the pancreatic lesion. Involvement of the CTLA-4 pathway, as well as TGF-β and IL-2 deprivation will be considered. Finally, we summarize current efforts to manipulate Treg therapeutically in individuals with Type 1 diabetes. The translation of this research area from bench to bedside is still in its infancy, but the remarkable therapeutic potential of successfully manipulating Treg populations is clear to see.

Linking form to function: Biophysical aspects of artificial antigen presenting cell design

Artificial antigen presenting cells (aAPCs) are engineered platforms for T cell activation and expansion, synthesized by coupling T cell activating proteins to the surface of cell lines or biocompatible particles. They can serve both as model systems to study the basic aspects of T cell signaling and translationally as novel approaches for either active or adoptive immunotherapy. Historically, these reductionist systems have not been designed to mimic the temporally and spatially complex interactions observed during endogenous T cell-APC contact, which include receptor organization at both micro- and nanoscales and dynamic changes in cell and membrane morphologies. Here, we review how particle size and shape, as well as heterogenous distribution of T cell activating proteins on the particle surface, are critical aspects of aAPC design. In doing so, we demonstrate how insights derived from endogenous T cell activation can be applied to optimize aAPC, and in turn how aAPC platforms can be used to better understand endogenous T cell stimulation. This article is part of a Special Issue entitled: Nanoscale membrane organisation and signalling.

Salmonella as a model for non-cognate Th1 cell stimulation

Salmonella has been a model pathogen for examining CD4 T cell activation and effector functions for many years due to the strength of the Th1 cell response observed during Salmonella infections, the relative ease of use of Salmonella, the availability of Salmonella-specific T cell reagents, and the well-characterized nature of the model system, the pathogen, and the immune response elicited. Herein, we discuss the use of Salmonella as a model pathogen to explore the complex interaction of T cells with their inflammatory environment. In particular, we address the issue of bystander activation of naïve T cells and non-cognate stimulation of activated and memory T cells. Further, we compare and contrast our current knowledge of these non-cognate responses in CD8 versus CD4 T cells. Finally, we make a case for Salmonella as a particularly appropriate model pathogen in the study of non-cognate CD4 T cell responses based on the strength of the Th1 response during infection, the requirement for CD4 T cells in bacterial clearance, and the well-characterized inflammatory response to conserved molecular patterns induced by Salmonella infection.