Cbl-b mitigates the responsiveness of naive CD8+ T cells that experience extensive tonic T cell receptor signaling

Naive T cells experience tonic T cell receptor (TCR) signaling in response to self-antigens presented by major histocompatibility complex (MHC) in secondary lymphoid organs. We investigated how relatively weak or strong tonic TCR signals influence naive CD8+ T cell responses to stimulation with foreign antigens. The heterogeneous expression of Nur77-GFP, a transgenic reporter of tonic TCR signaling, in naive CD8+ T cells suggests variable intensities or durations of tonic TCR signaling. Although the expression of genes associated with acutely stimulated T cells was increased in Nur77-GFPHI cells, these cells were hyporesponsive to agonist TCR stimulation compared with Nur77-GFPLO cells. This hyporesponsiveness manifested as diminished activation marker expression and decreased secretion of IFN-γ and IL-2. The protein abundance of the ubiquitin ligase Cbl-b, a negative regulator of TCR signaling, was greater in Nur77-GFPHI cells than in Nur77-GFPLO cells, and Cbl-b deficiency partially restored the responsiveness of Nur77-GFPHI cells. Our data suggest that the cumulative effects of previously experienced tonic TCR signaling recalibrate naive CD8+ T cell responsiveness. These changes include gene expression changes and negative regulation partially dependent on Cbl-b. This cell-intrinsic negative feedback loop may enable the immune system to restrain naive CD8+ T cells with higher self-reactivity.

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.

Force in Immunology: There Is Often a Catch

This issue of The Journal of Immunology features a collection of three Brief Reviews and a nomenclature topic piece on “Force in Immunology: There Is Often a Catch” (1–4).

A single-amino acid substitution in the adaptor LAT accelerates TCR proofreading kinetics and alters T-cell selection, maintenance and function

Mature T cells must discriminate between brief interactions with self-peptides and prolonged binding to agonists. The kinetic proofreading model posits that certain T-cell antigen receptor signaling nodes serve as molecular timers to facilitate such discrimination. However, the physiological significance of this regulatory mechanism and the pathological consequences of disrupting it are unknown. Here we report that accelerating the normally slow phosphorylation of the linker for activation of T cells (LAT) residue Y136 by introducing an adjacent Gly135Asp alteration (LATG135D) disrupts ligand discrimination in vivo. The enhanced self-reactivity of LATG135D T cells triggers excessive thymic negative selection and promotes T-cell anergy. During Listeria infection, LATG135D T cells expand more than wild-type counterparts in response to very weak stimuli but display an imbalance between effector and memory responses. Moreover, despite their enhanced engagement of central and peripheral tolerance mechanisms, mice bearing LATG135D show features associated with autoimmunity and immunopathology. Our data reveal the importance of kinetic proofreading in balancing tolerance and immunity.

Immunogen-Specific Strengths and Limitations of the Activation-Induced Marker Assay for Assessing Murine Antigen-Specific CD4+ T Cell Responses

The activation-induced marker (AIM) assay is a cytokine-independent technique to identify Ag-specific T cells based on the upregulated expression of activation markers after Ag restimulation. The method offers an alternative to intracellular cytokine staining in immunological studies, in which limited cytokine production makes the cell subsets of interest difficult to detect. Studies of lymphocytes in human and nonhuman primates have used the AIM assay to detect Ag-specific CD4+ and CD8+ T cells. However, there is a lack of validation of the strengths and limitations of the assay in murine (Mus musculus) models of infection and vaccination. In this study, we analyzed immune responses of TCR-transgenic CD4+ T cells, including lymphocytic choriomeningitis virus-specific SMARTA, OVA-specific OT-II, and diabetogenic BDC2.5-transgenic T cells, and measured the ability of the AIM assay to effectively identify these cells to upregulate AIM markers OX40 and CD25 following culture with cognate Ag. Our findings indicate that the AIM assay is effective for identifying the relative frequency of protein immunization-induced effector and memory CD4+ T cells, whereas the AIM assay had reduced ability to identify specific cells induced by viral infection, particularly during chronic lymphocytic choriomeningitis virus infection. Evaluation of polyclonal CD4+ T cell responses to acute viral infection demonstrated that the AIM assay can detect a proportion of both high- and low-affinity cells. Together, our findings indicate that the AIM assay can be an effective tool for relative quantification of murine Ag-specific CD4+ T cells to protein vaccination, while demonstrating its limitations during conditions of acute and chronic infection.

Memory in repetitive protein–protein interaction series

Interactions between proteins coordinate biological processes in an organism and may impact its responses to changing environments and diseases through feedback systems. Feedback systems function by using changes in the past to influence behaviors in the future, which we refer to here as memory. Here, we summarized several observations made, ideas conceptualized, and mathematical models developed for quantitatively analyzing memory effects in repetitive protein–protein interactions (PPIs). Specifically, we consider how proteins on the cell or in isolation retain information about prior interactions to impact current interactions. The micropipette, biomembrane force probe, and atomic force microscopic techniques were used to repeatedly assay PPIs. The resulting time series were analyzed by a previous and two new models to extract three memory indices of short (seconds), intermediate (minutes), and long (hours) timescales. We found that interactions of cell membrane, but not soluble, T cell receptor (TCR) with peptide-major histocompatibility complex (pMHC) exhibits short-term memory that impacts on-rate, but not off-rate of the binding kinetics. Peptide dissociation from MHC resulted in intermediate- and long-term memories in TCR–pMHC interactions. However, we observed no changes in kinetic parameters by repetitive measurements on living cells over intermediate timescales using stable pMHCs. Parameters quantifying memory effects in PPIs could provide additional information regarding biological mechanisms. The methods developed herein also provide tools for future research.

Strong Basal/Tonic TCR Signals Are Associated with Negative Regulation of Naive CD4+ T Cells

T cells experience varying intensities of tonic or basal TCR signaling in response to self-peptides presented by MHC (self-pMHC) in vivo. We analyzed four subpopulations of mouse naive CD4⁺ cells that express different levels of Nur77-GFP and Ly6C, surrogate markers that positively and inversely correlate with the strength of tonic TCR signaling, respectively. Adoptive transfer studies suggest that relatively weak or strong Nur77-GFP intensity in thymocytes tends to be maintained in mature T cells. Two-dimensional affinity measurements were lowest for Nur77-GFP^loLy6C⁺ cells and highest for Nur77-GFP^hiLy6C^- cells, highlighting a positive correlation between apparent TCR affinity and tonic TCR signal strength. Despite experiencing the strongest tonic TCR signaling, Nur77-GFP^hiLy6C^- cells were least responsive to multiple concentrations of a cognate or suboptimal pMHC. Gene expression analyses suggest that Nur77-GFP^hiLy6C^- cells induce a gene expression program that has similarities with that of acutely stimulated T cells. However, strong tonic TCR signaling also correlates with increased expression of genes with inhibitory functions, including coinhibitory receptors. Similarly, assay for transposase-accessible chromatin with sequencing analyses suggested that increased tonic TCR signal strength correlated with increased chromatin accessibility associated with genes that have positive and inhibitory roles in T cell activation. Strikingly, Nur77-GFP^hiLy6C^- cells exhibited differential accessibility within regions of Cd200r1 and Tox that were similar in location to differentially accessible regions previously identified in exhausted CD8⁺ T cells. We propose that constitutive strong tonic TCR signaling triggers adaptations detectable at both the transcriptional and epigenetic levels, ultimately contributing to the tuning of T cell responsiveness.

The influence of antigen recognition on Treg suppressive function

Tregs play an essential role in preventing autoimmunity by maintaining effector T cell balance and self-tolerance. Antigen stimulation through T cell receptor (TCR) makes Treg suppression more efficacious and as such is generally viewed as a strong predictor of Treg function. In demyelinating disease Tregs with the most effective suppressive capacity are self-antigen specific, however, it is unclear how the parameters of self-antigen stimulation (affinity, force, and bond lifetime) modulate Treg suppressive potency. Here we utilize the well-described myelin oligodendrocyte glycoprotein (MOG) 35–55 peptide driven experimental autoimmune encephalomyelitis (EAE) to address how Treg self-antigen interactions under chronic inflammation influence suppressive potency. Recently Tregs have been shown to perform trogocytosis to remove MHC molecules from the surface of antigen presenting cells in an antigen specific manner. We hypothesize that trogocytosis correlates with optimal Treg antigen specific suppression in demyelinating disease. To test suppressive capacity of self-antigen specific Tregs we utilized a MOG knockout mouse model of EAE. In this system we show that MOG knockout Tregs are less potent suppressors when challenged with EAE despite identical affinity for MOG35–55 as wild type Tregs. In addition, we show that MOG knockout Tregs perform trogocytosis less readily than wild type Tregs demonstrating that the presence of self-antigen MOG35–55 is necessary for development of optimal Treg generation. An understanding of how Tregs respond to antigen may allow the design of therapeutic strategies to better harness Treg power in combatting autoimmune conditions.

Supported by grants from NIH (5R01NS071518-09)

MOG variant 45D does not properly activate high-affinity, MOG reactive effector T cells but allows for high-affinity FoxP3+ Treg function

Although it’s well appreciated that autoreactive T cells can drive autoimmune disease, how T cells precisely distinguish and respond to epitopes is unclear. For example, T cells can proliferate in response to multiple epitopes of myelin oligodendrocyte glycoprotein (MOG), but not all epitopes are encephalitogenic. Here, we investigate why T cells specific for MOG 35–55 are encephalitogenic. We compare the wildtype MOG epitope (wtMOG) to a single amino acid substitute, 45D, which was engineered to expresses the same T cell receptor (TCR) contact residues as wtMOG but is mutated such that its affinity for MHC II is decreased. We employed the micropipette adhesion frequency assay to demonstrate that a monoclonal, MOG-specific, high-affinity TCR binds to 45D with the same affinity as wtMOG. Despite conserved TCR contact residues, the MOG 45D variant is not encephalitogenic and fails to prime high-affinity tetramer-positive TCR clones. We also extended our polyclonal analysis to FoxP3+ regulatory T cells (Tregs) and found that, compared to wtMOG, 45D priming increases the frequency of MOG-specific Tregs. We further show that these Tregs are functional and suppress the proliferation of MOG-specific T cells. Our work begins to dissect the differential T cell receptor kinetic properties that allow for a response, or lack thereof, in Tregs and Teffs to myelin peptides.

Supported by NIH R01 NS071518-09

Plasmodium infection elevates risk of severe secondary bacterial disease by altering the immunological landscape of the lung

Lower respiratory infections cause an estimated two million sepsis deaths yearly, with >50% attributed to the opportunistic pathogen Streptococcus pneumoniae. Sub-Saharan Africa, the site of most severe Streptococcus burden, is also ravaged by the malaria-causing Plasmodium parasite family, which afflicted 241 million people and killed 627,000 in 2020. Compared to healthy children, those with asymptomatic malaria have an elevated risk to severe bacterial infections, but the underlying mechanism(s) remain elusive. To fill this knowledge gap, we infected C57BL/6J mice with P. berghei NK65-NY either 7, 14, or 21 days prior to inoculation with S. pneumoniae 6304. Coinfected mice exhibited a 40–60% fatality rate, with death beginning 1–4 days post-bacterial infection. In contrast, singly infected groups survived the duration of the experiment. Coinfected mice exhibited equivalent parasitemia but significantly higher bacterial burden in the lung compared to mono-infections. Neutrophil recruitment and ROS production was altered in coinfected mice, which we predict is due to the formerly described effects of the Plasmodium metabolic product hemozoin. As such, we hypothesize that neutrophil control of S. pneumoniae is impaired. Since malaria has been shown to increase blood brain barrier permeability, we used Evans blue dye to test for a similar effect at the alveolar-capillary interface. Lung weight significantly increased as early as 6 days post-infection, while permeability trended towards a slight increase, peaking at day 9. Thus, we hypothesize that Plasmodium infection increases pathogenicity of Streptococcus infection due to a combination of impaired bacterial control and increased vascular leak in the lungs.

This work was supported by the NIH (ROl AI123425-01A1).

Breakdown of the blood brain barrier in Experimental Cerebral Malaria is caused by CD8+ T cells with low affinity TCRs

Malaria is a worldwide health concern killing over 400,000 people each year, many of them children under 5 years old. One severe outcome of Plasmodium infection is cerebral malaria caused by a dysregulation of the immune response in the central nervous system. Using an established mouse model of cerebral malaria called experimental cerebral malaria(ECM) using Plasmodium berghei ANKA (PbA) infections of C57BL/6J mice have shown that breakdown of the blood brain barrier is mediated by IFN-g producing CD8 T cells. Here, using novel techniques to determine the affinity of the MHC:peptide:TCR interaction we have discovered that although there are high affinity cells in the population trafficking to the CNS, surprisingly a large majority of these PbA-reactive pathogenic CD8 T cells have low affinity TCRs, and a large percentage of them cannot be detected by tetramer. Importantly we show that these low affinity cells are also producing IFN-g suggesting they are pathogenic. Through adoptive transfer experiments we show that high and low affinity T cells remain stable in their affinity profile. Furthermore we show here that low affinity cells are needed for breakdown of the blood brain barrier – CD8 T cells bearing high affinity TCRs for the GAP50 peptide of Plasmodium have an impaired ability to cause ECM compared to low affinity CD8 T cells. This work demonstrates that low affinity cells are the major component of the immunopathology causing ECM when infected with PbA and contrasts with current dogma which suggests that high affinity cells are the main cells responding in these types if infections.

Supported by grants from NIH (R01 AI167422)

Tuning T cell receptor sensitivity through catch bond engineering

Adoptive cell therapy using engineered T cell receptors (TCRs) is a promising approach for targeting cancer antigens, but tumor-reactive TCRs are often weakly responsive to their target ligands, peptide-major histocompatibility complexes (pMHCs). Affinity-matured TCRs can enhance the efficacy of TCR-T cell therapy but can also cross-react with off-target antigens, resulting in organ immunopathology. We developed an alternative strategy to isolate TCR mutants that exhibited high activation signals coupled with low-affinity pMHC binding through the acquisition of catch bonds. Engineered analogs of a tumor antigen MAGE-A3-specific TCR maintained physiological affinities while exhibiting enhanced target killing potency and undetectable cross-reactivity, compared with a high-affinity clinically tested TCR that exhibited lethal cross-reactivity with a cardiac antigen. Catch bond engineering is a biophysically based strategy to tune high-sensitivity TCRs for T cell therapy with reduced potential for adverse cross-reactivity.

The magnitude of LFA-1/ICAM-1 forces fine-tune TCR-triggered T cell activation

T cells defend against cancer and viral infections by rapidly scanning the surface of target cells seeking specific peptide antigens. This key process in adaptive immunity is sparked upon T cell receptor (TCR) binding of antigens within cell-cell junctions stabilized by integrin (LFA-1)/intercellular adhesion molecule-1 (ICAM-1) complexes. A long-standing question in this area is whether the forces transmitted through the LFA-1/ICAM-1 complex tune T cell signaling. Here, we use spectrally encoded DNA tension probes to reveal the first maps of LFA-1 and TCR forces generated by the T cell cytoskeleton upon antigen recognition. DNA probes that control the magnitude of LFA-1 force show that F>12 pN potentiates antigen-dependent T cell activation by enhancing T cell-substrate engagement. LFA-1/ICAM-1 mechanical events with F>12 pN also enhance the discriminatory power of the TCR when presented with near cognate antigens. Overall, our results show that T cells integrate multiple channels of mechanical information through different ligand-receptor pairs to tune function.

MHC class II tetramers engineered for enhanced binding to CD4 improve detection of antigen-specific T cells

The ability to identify T cells that recognize specific peptide antigens bound to major histocompatibility complex (MHC) molecules has enabled enumeration and molecular characterization of the lymphocytes responsible for cell-mediated immunity. Fluorophore-labeled peptide:MHC class I (p:MHCI) tetramers are well-established reagents for identifying antigen-specific CD8+ T cells by flow cytometry, but efforts to extend the approach to CD4+ T cells have been less successful, perhaps owing to lower binding strength between CD4 and MHC class II (MHCII) molecules. Here we show that p:MHCII tetramers engineered by directed evolution for enhanced CD4 binding outperform conventional tetramers for the detection of cognate T cells. Using the engineered tetramers, we identified about twice as many antigen-specific CD4+ T cells in mice immunized against multiple peptides than when using traditional tetramers. CD4 affinity-enhanced p:MHCII tetramers, therefore, allow direct sampling of antigen-specific CD4+ T cells that cannot be accessed with conventional p:MHCII tetramer technology. These new reagents could provide a deeper understanding of the T cell repertoire.

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.

Canonical T cell receptor docking on peptide-MHC is essential for T cell signaling

T cell receptor (TCR) recognition of peptide-major histocompatibility complexes (pMHCs) is characterized by a highly conserved docking polarity. Whether this polarity is driven by recognition or signaling constraints remains unclear. Using "reversed-docking" TCRβ-variable (TRBV) 17⁺ TCRs from the naïve mouse CD8⁺ T cell repertoire that recognizes the H-2D^b-NP₃₆₆ epitope, we demonstrate that their inability to support T cell activation and in vivo recruitment is a direct consequence of reversed docking polarity and not TCR-pMHCI binding or clustering characteristics. Canonical TCR-pMHCI docking optimally localizes CD8/Lck to the CD3 complex, which is prevented by reversed TCR-pMHCI polarity. The requirement for canonical docking was circumvented by dissociating Lck from CD8. Thus, the consensus TCR-pMHC docking topology is mandated by T cell signaling constraints.

Dietary fish oil directly impact viral immunity by modifying CD8 T cell membrane flexibility and bond lifetime under force

Fish oil supplements enriched in omega-3s are increasingly consumed for their health benefits and exhibit immunosuppressive effects on both innate and adaptive immunity. While omega-3s have been shown to reduced inflammatory metabolites, we focused on the direct effects of a high omega-3 diet on T cell activation during viral infection. Using lymphocytic choriomeningitis (LCMV) infection we analyzed CD8 T cells response to antigen. The omega-3 based diet manipulated structural lipid content in plasma membranes leading to stiffer T cells which subsequently resulted in reduced frequency and activation of the prevalence immunodominant epitopes. We further found that a high omega-3 diet decreased 2D affinity and number of tetramer positive polyclonal as well as CD8 T cell receptor transgenic cells at peak anti-viral immunity. In addition to affinity, the bond lifetime under force of antigen recognition was also decreased. This resulted in decreased numbers of SLEC and effector cytokines without impeding viral clearance. Our work indicates that a major effect of dietary omega-3 fatty acids on T cells is the modification of the cell membrane to dampen the initial recognition of antigen leading to decreased but effective immunity.

Re-thinking the Plasmodium mouse model: using wild derived mice to interrogate asymptomatic malaria

Malaria is a major world health issue, with over 200 million clinical cases per year that lead to over 200,000 deaths. Though there have been advances in treatment and control of malaria in symptomatic patients, nearly 60–75% of Plasmodium infections are asymptomatic (no overt symptoms), leading to a large proportion of ignored infections that have a large influence on transmission. However, the term asymptomatic is misleading, as many of these individuals display evidence of mild anemia and vascular activation, leading to a higher risk for co-morbidities such as infection with typhoidal Salmonella. Unfortunately, the immunological mechanisms governing the asymptomatic phenotype are poorly understood, partially due to a lack of reliable immunologically intact asymptomatic animal models. Here, we utilize a unique genetically diverse wild-derived mouse model to address the immunological underpinnings of asymptomatic malaria. This model allows us to disentangle the relative roles of the genetics and environment in the generation of immune responses to Plasmodium infections. Results show the wild-derived mice have varying levels of anemia associated with Plasmodium yoelii XNL infection, with some mice exhibiting an asymptomatic phenotype with mild anemia independent of parasitemia. Anemia levels positively correlate with the production of TNF-a and IL-10, which is consistent with patterns seen in children in Cameroon. Furthermore, the CD4 T cells producing IL-10 and IFN-g were also correlated with the severity of anemia, also a pattern seen in human malaria. These results demonstrate that the wild-derived mice can be used to model asymptomatic malaria and allow us to interrogate the immune mechanisms leading to anemia.

Induction with MHC variant peptide MOG 45D generates MOG-reactive T cells but no clinical disease

Demyelinating disease can be driven by CD4+ T cells specific for myelin oligodendrocyte glycoprotein (MOG). We have previously decreased the affinity of MOG for Class II MHC by mutating the P6 anchor residue from an S to a D amino acid, creating the 45D MHC variant peptide. 45D is incapable of inducing experimental autoimmune encephalomyelitis (EAE) despite its CD4 epitope bearing the same T cell receptor contact residues as the wildtype MOG epitope (wtMOG). Although the wtMOG and 45D peptides generate CD4 T cells specific for MOG in the periphery and CNS, there is an overall decrease in the number of high-affinity CD4 T cells in response to 45D priming. Analysis of the responding T cell populations to 45D also found increases in MOG-reactive Tregs and decreases in MOG-specific CD8 T cells. Thus, the variant peptide 45D alters the MOG-specific T cell response in favor of peripheral tolerance mechanisms that ultimately result in the absence of clinical disease.

CD8αα expression increases during demyelinating disease progression

Experimental autoimmune encephalomyelitis (EAE) is a well-described mouse model for multiple sclerosis (MS), a demyelinating autoimmune disorder of the central nervous system (CNS). While the role of CD4 T cells is well established, other immune cell types, particularly CD8 T cells, are key contributors to disease pathogenesis. Our study aims to categorize the functional contributions of CD8 T cell subsets to EAE kinetics and disease outcomes. The use of myelin oligodendrocyte glycoprotein (MOG) 35–55 peptide alongside an adjuvant to inoculate mice, provides a reproducible disease model. Demyelinating disease severity was analyzed at defined timepoints including, onset (d10–14), peak (d17–21), early chronic (d28–35) and late chronic (d80–120) stages. The total number of CD3+ T cells in the CNS remained consistent during EAE. However, it goes from a CD4 T cell dominated response at peak EAE to an equalized 1:1 ratio of CD4 and CD8 T cells at late chronic stage. Furthermore, differentiation of CD8 T cell subsets based on CD8α and CD8β protein expression revealed that CD8αα T cells numbers increase to comprise ~40% of the CD8+ population at late chronic EAE. In addition, CD8βLO cells become a prominent subset during late chronic stage, with a concomitant decrease in CD8αβHI T cells. Moreover, CD8αα T cells are activated and exhibit a central memory phenotype along with downregulation of exhaustion markers by FACS analysis, and yet have significantly reduced cytokine expression. These results are consistent with single cell RNA sequencing data obtained at the peak and late chronic time points. Thus, CD8αα T cells constitute a phenotypically distinct class of CD8 T cell subsets, with a blunted effector role in demyelinating disease outcome.

Targeting transcriptional coregulator OCA-B/Pou2af1 blocks activated autoreactive T cells in the pancreas and type 1 diabetes

The transcriptional coregulator OCA-B promotes expression of T cell target genes in cases of repeated antigen exposure, a necessary feature of autoimmunity. We hypothesized that T cell-specific OCA-B deletion and pharmacologic OCA-B inhibition would protect mice from autoimmune diabetes. We developed an Ocab conditional allele and backcrossed it onto a diabetes-prone NOD/ShiLtJ strain background. T cell-specific OCA-B loss protected mice from spontaneous disease. Protection was associated with large reductions in islet CD8+ T cell receptor specificities associated with diabetes pathogenesis. CD4+ clones associated with diabetes were present but associated with anergic phenotypes. The protective effect of OCA-B loss was recapitulated using autoantigen-specific NY8.3 mice but diminished in monoclonal models specific to artificial or neoantigens. Rationally designed membrane-penetrating OCA-B peptide inhibitors normalized glucose levels and reduced T cell infiltration and proinflammatory cytokine expression in newly diabetic NOD mice. Together, the results indicate that OCA-B is a potent autoimmune regulator and a promising target for pharmacologic inhibition.

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.

An Engineered T Cell Receptor Variant Realizes the Limits of Functional Binding Modes

T cell receptors (TCRs) orchestrate cellular immunity by recognizing peptides presented by a range of major histocompatibility complex (MHC) proteins. Naturally occurring TCRs bind the composite peptide/MHC surface, recognizing peptides that are structurally and chemically compatible with the TCR binding site. Here we describe a molecularly evolved TCR variant that binds the human class I MHC protein HLA-A2 independent of the bound peptide, achieved by a drastic perturbation of the TCR binding geometry that places the molecule far from the peptide binding groove. This unique geometry is unsupportive of normal T cell signaling. A substantial divergence between affinity measurements in solution and in two dimensions between proximal cell membranes leads us to attribute the lack of signaling to steric hindrance that limits binding in the confines of a cell-cell interface. Our results provide an example of how receptor binding geometry can impact T cell function and provide further support for the view that germline-encoded residues in TCR binding loops evolved to drive productive TCR recognition and signaling.

Localized hydrogel delivery of dendritic cells for attenuation of multiple sclerosis in a murine model

In multiple sclerosis (MS), abnormally activated immune cells responsive to myelin proteins result in widespread damage throughout the central nervous system (CNS) and ultimately irreversible disability. Immunomodulation by delivering dendritic cells (DCs) utilizes a potent and rapid MS disease progression driver therapeutically. Here, we investigated delivering DCs for disease severity attenuation using an experimental autoimmune encephalomyelitis preclinical MS model. DCs treated with interleukin-10 (IL-10) (DC10s) were transplanted using in situ gelling poly(ethylene glycol)-based hydrogel for target site localization. DC delivery increased hydrogel longevity and altered the injection site recruited, endogenous immune cell profile within 2 days postinjection. Furthermore, hydrogel-mediated DC transplantation efficacy depended on the injection-site. DCs delivered to the neck local to MS-associated CNS-draining cervical lymph nodes attenuated paralysis, compared to untreated controls, while delivery to the flank did not alter paralysis severity. This study demonstrates that local delivery of DC10s modulates immune cell recruitment and attenuates disease progression in a preclinical model of MS.

IL-21 from high-affinity CD4 T cells drives differentiation of brain-resident CD8 T cells during persistent viral infection

Development of tissue-resident memory (T_RM) CD8 T cells depends on CD4 T cells. In polyomavirus central nervous system infection, brain CXCR5^hi PD-1^hi CD4 T cells produce interleukin-21 (IL-21), and CD8 T cells lacking IL-21 receptors (IL21R^-/-) fail to become bT_RM IL-21⁺ CD4 T cells exhibit elevated T cell receptor (TCR) affinity and higher TCR density. IL21R^-/- brain CD8 T cells do not express CD103, depend on vascular CD8 T cells for maintenance, are antigen recall defective, and lack T_RM core signature genes. CD4 T cell-deficient and IL21R^-/- brain CD8 T cells show similar deficiencies in expression of genes for oxidative metabolism, and intrathecal delivery of IL-21 to CD4 T cell-depleted mice restores expression of electron transport genes in CD8 T cells to wild-type levels. Thus, high-affinity CXCR5^hi PD-1^hi CD4 T cells in the brain produce IL-21, which drives CD8 bT_RM differentiation in response to a persistent viral infection.

Insights from an in vitro derived T cell receptor

Heterodimeric alpha-beta T cell receptors (TCRs) play a key role in cell-mediated immune responses of adaptive immunity. T cells continuously probe peptides presented by major histocompatibility complex (MHC) found on the cell surface. T cell activation initiates through the binding of antigenic peptides on MHCs. Yeast display technology has proven an effective way to gain insight into the underpinnings of peptide-MHC specificity enabling the switching of TCR affinity and specificity to unrelated antigens through directed evolution. Interestingly, in vitro directed evolution also generates mutants that lose peptide specificity. Here, we provide further detail of one of these cross-reactive TCRs, S3–4, which binds tightly but with a highly unusual geometry. Despite binding with an affinity that is characteristic of a strong agonist, S3–4’s odd binding geometry does not support T cell signaling. Further investigation with functional experiments and 2D kinetics shows that lack of signaling by S3–4 is ultimately attributable to the TCR’s inability to reach ligand. Although S3–4 is an example from a constrained system generated outside the bounds of usual immune function, our results show how unusual binding can influence T cell function and further demonstrate how divergences between 3D and 2D binding parameters can emerge.

A Critical Insulin TCR Contact Residue Selects High-Affinity and Pathogenic Insulin-Specific T Cells

Type 1 diabetes is an autoimmune-mediated disease that culminates in the targeted destruction of insulin-producing β-cells. CD4 responses in NOD mice are dominated by insulin epitope B:9-23 (InsB_9-23) specificity, and mutation of the key T-cell receptor (TCR) contact residue within the epitope prevents diabetes development. However, it is not clear how insulin self-antigen controls the selection of autoimmune and regulatory T cells (Tregs). Here we demonstrate that mutation of insulin epitope results in escape of highly pathogenic T cells. We observe an increase in antigen reactivity, clonality, and pathogenicity of insulin-specific T cells that develop in the absence of cognate antigen. Using a single TCR system, we demonstrate that Treg development is greatly diminished in mice with the Y16A mutant epitope. Collectively, these results suggest that the tyrosine residue at position 16 is necessary to constrain TCR reactivity for InsB_9-23 by both limiting the development of pathogenic T cells and supporting the selection of Tregs.

A Hybrid Insulin Epitope Maintains High 2D Affinity for Diabetogenic T Cells in the Periphery

β-Cell antigen recognition by autoreactive T cells is essential in type 1 diabetes (T1D) pathogenesis. Recently, insulin hybrid peptides (HIPs) were identified as strong agonists for CD4 diabetogenic T cells. Here, using BDC2.5 transgenic and NOD mice, we investigated T-cell recognition of the HIP2.5 epitope, which is a fusion of insulin C-peptide and chromogranin A (ChgA) fragments, and compared it with the WE14 and ChgA_{29 -42} epitopes. We measured in situ two-dimensional affinity on individual live T cells from thymus, spleen, pancreatic lymph nodes, and islets before and after diabetes. Although preselection BDC2.5 thymocytes possess higher affinity than splenic BDC2.5 T cells for all three epitopes, peripheral splenic T cells maintained high affinity only to the HIP2.5 epitope. In polyclonal NOD mice, a high frequency (∼40%) of HIP2.5-specific islet T cells were identified at both prediabetic and diabetic stages comprising two distinct high- and low-affinity populations that differed in affinity by 100-fold. This high frequency of high- and low-affinity HIP2.5 T cells in the islets potentially represents a major risk factor in diabetes pathogenesis.

Understanding TCR affinity, antigen specificity, and cross-reactivity to improve TCR gene-modified T cells for cancer immunotherapy

Adoptive cell transfer (ACT) using T cell receptor (TCR) gene-modified T cells is an exciting and rapidly evolving field. Numerous preclinical and clinical studies have demonstrated various levels of feasibility, safety, and efficacy using TCR-engineered T cells to treat cancer and viral infections. Although evidence suggests their use can be effective, to what extent and how to improve these therapeutics are still matters of investigation. As TCR affinity has been generally accepted as the central role in defining T cell specificity and sensitivity, selection for and generation of high affinity TCRs has remained a fundamental approach to design more potent T cells. However, traditional methods for affinity-enhancement by random mutagenesis can induce undesirable cross-reactivity causing on- and off-target adverse events, generate exhausted effectors by overstimulation, and ignore other kinetic and cellular parameters that have been shown to impact antigen specificity. In this Focussed Research Review, we comment on the preclinical and clinical potential of TCR gene-modified T cells, summarize our contributions challenging the role TCR affinity plays in antigen recognition, and explore how structure-guided design can be used to manipulate antigen specificity and TCR cross-reactivity to improve the safety and efficacy of TCR gene-modified T cells used in ACT.

Discriminative T cell recognition of cross-reactive islet-antigens is associated with HLA-DQ8 transdimer-mediated autoimmune diabetes

Human leukocyte antigen (HLA)-DQ8 transdimer (HLA-DQA1*0501/DQB1*0302) confers exceptionally high risk in autoimmune diabetes. However, little is known about HLA-DQ8 transdimer-restricted CD4 T cell recognition, an event crucial for triggering HLA-DQ8 transdimer-specific anti-islet immunity. Here, we report a high degree of epitope overlap and T cell promiscuity between susceptible HLA-DQ8 and HLA-DQ8 transdimer. Despite preservation of putative residues for T cell receptor (TCR) contact, stronger disease-associated responses to cross-reactive, immunodominant islet epitopes are elicited by HLA-DQ8 transdimer. Mutagenesis at the α chain of HLA-DQ8 transdimer in complex with the disease-relevant GAD65_250-266 peptide and in silico analysis reveal the DQ α52 residue located within the N-terminal edge of the peptide-binding cleft for the enhanced T cell reactivity, altering avidity and biophysical affinity between TCR and HLA-peptide complexes. Accordingly, a structurally promiscuous but nondegenerate TCR-HLA-peptide interface is pivotal for HLA-DQ8 transdimer-mediated autoimmune diabetes.

Distinct waves of CD8 T cell affinity during demyelinating disease progression

The animal model of Multiple Sclerosis, Experimental Autoimmune Encephalomyelitis (EAE), requires CD4 T cells to induce demyelinating disease. However, CD8 T cells are ever present during EAE and have been hypothesized to have both effector and suppressor functions. To understand autoimmune disease progression, we analyzed the affinity and genetic profile of the CD4 and CD8 T cell response over time. Affinity analysis of CD8 T cells during the autoimmune disease course revealed the greatest affinity for myelin at peak disease (d21-23), whereas affinities were significantly lower at onset (d14) and chronic (d36-38) timepoints. Conversely, affinity analysis of CD4 T cells revealed no significant changes at the same time points. Additionally, CD8 T cells displayed a smaller, 10-fold range in affinity, suggestive of a restricted TCR clonal response. CD4 affinities covered a much wider 10,000-fold range, consisting of low and high affinity clones. To further understand the dynamics of the T cell population during disease, we performed single cell RNA-sequencing. Heterogenous populations of CD4 and CD8 T cells were observed at peak EAE disease, including a genetically distinct population of FoxP3+, CD25+ CD8 T cells which accounted for 3.33% the total CD8 population. These genomic data paired with the known associations between affinity and T cell fate suggest phenotypically and functionally distinct waves of CD8 T cells with limited clonality infiltrate the CNS during EAE progression.

Diets rich in omega-3s decrease CD8 T cell 2-dimensional affinity and short lived effector cell development and function during viral infection

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), which are key components of fish oil, are increasingly consumed for their health benefits and have been shown to dramatically influence chronic disease. Although omega-3s can lead to the production of less inflammatory metabolites, we focused on the direct effects of a high omega-3 diet on T cell activation during viral infection. Specifically, we analyzed T cell recognition of antigen and the resulting functional responses during lymphocytic choriomeningitis (LCMV) infection. The omega-3 based diet manipulated structural lipid content in plasma membranes leading to reduced T cell frequency and changes in the prevalence of immunodominant epitopes. We further found that a high omega-3 diet decreased 2D affinity and number of tetramer positive polyclonal as well as CD8 T cell receptor transgenic cells at peak anti-viral immunity. In addition to affinity, the bond lifetime of antigen recognition was also decreased. This resulted in decreased numbers of SLEC and effector cytokines without impeding viral clearance. Our work indicate that a major effect of dietary omega-3 fatty acids on T cells is the modification of the cell membrane to dampen the initial recognition of antigen leading to decreased but effective immunity.

Antigen dose affects disease kinetics in a murine model of demyelinating disease

Multiple sclerosis is an autoimmune disorder caused by immune cell damage to the myelin sheath lining nerve cells, leading to neuronal damage and dysfunction. The murine model of the disease - experimental autoimmune encephalomyelitis (EAE) - is induced by inoculation with MOG (myelin oligodendrocyte glycoprotein) peptide and an adjuvant, leading to recapitulation of disease that is mediated primarily by CD4 T cells. In this study, we look at the effect of MOG dosage on demyelinating disease kinetics in WT and MOG-deficient animals, as MOG-deficient mice have different numbers of MOG specific CD4 Teff and Tregs. The T cells from MOG deficient animals have a higher affinity for MOG, expand faster, and cause exacerbated disease in an adoptive transfer model compared to CD4 T cells from wildtype mice. Our results indicate that the dose of MOG peptide used to induce disease can also modulate disease course. Using tenfold lower doses changes T cell and disease kinetics resulting in disease that resolves compared to controls. Taken together, we can use varying doses of MOG as a tool to probe CD8 and CD4 T cell expansion and monitor migration of immune cells to the CNS. Further understanding of the cellular tides during disease course enables us to gain insight on how cell ratios in different sites contribute to disease pathogenesis. Using this information, we are developing mathematical modeling tools that could be used to analyze and predict disease progression.

Incorporation of genetic diversity provides protection from cerebral malaria in Mus musculus

Since their creation over a century ago inbred mouse strains have been the workhorse of immunology and due to their reduced biological variability have led to countless biological breakthroughs. However, this reduced variability by its very nature is not necessarily representative of a genetically diverse (outbred) population. Here we try to understand how increased genetic diversity in the Mus musculus system may lead to more protective immunity, through a heightened but not over reactive response. Breeding together wild derived mice, that have been maintained for genetic diversity, and C57Bl/6 mice we are able to look at how the incorporation of diversity into the host genome can alter known C57Bl/6 immune function. The F1 mice from the cross are infected with the well characterized pathogen system Plasmodium bergehi ANKA, which induces cerebral malaria and death of C57Bl/6 animals in ~6 days post infection. In contrast, we show a large reduction of virulence in the F1 mice (<5% morbidity) that corresponds with differential immune response allowing for suppression of disease. Furthermore, F1 mice appear to produce a more controlled reaction that promotes the differentiation of more memory-like cells and fewer effector-like cells. These data suggest that the incorporation of genetic diversity into the C57Bl/6 inbred animal model system drastically alters Plasmodium infection outcome.

PD-1 Dynamically Regulates Inflammation and Development of Brain-Resident Memory CD8 T Cells During Persistent Viral Encephalitis

Programmed cell death-1 (PD-1) receptor signaling dampens the functionality of T cells faced with repetitive antigenic stimulation from chronic infections or tumors. Using intracerebral (i.c.) inoculation with mouse polyomavirus (MuPyV), we have shown that CD8 T cells establish a PD-1^hi, tissue-resident memory population in the brains (bT_RM) of mice with a low-level persistent infection. In MuPyV encephalitis, PD-L1 was expressed on infiltrating myeloid cells, microglia and astrocytes, but not on oligodendrocytes. Engagement of PD-1 on anti-MuPyV CD8 T cells limited their effector activity. NanoString gene expression analysis showed that neuroinflammation was higher in PD-L1^-/- than wild type mice at day 8 post-infection, the peak of the MuPyV-specific CD8 response. During the persistent phase of infection, however, the absence of PD-1 signaling was found to be associated with a lower inflammatory response than in wild type mice. Genetic disruption and intracerebroventricular blockade of PD-1 signaling resulted in an increase in number of MuPyV-specific CD8 bT_RM and the fraction of these cells expressing CD103, the αE integrin commonly used to define tissue-resident T cells. However, PD-L1^-/- mice persistently infected with MuPyV showed impaired virus control upon i.c. re-infection with MuPyV. Collectively, these data reveal a temporal duality in PD-1-mediated regulation of MuPyV-associated neuroinflammation. PD-1 signaling limited the severity of neuroinflammation during acute infection but sustained a level of inflammation during persistent infection for maintaining control of virus re-infection.

Mechano-regulation of Peptide-MHC Class I Conformations Determines TCR Antigen Recognition

TCRs recognize cognate pMHCs to initiate T cell signaling and adaptive immunity. Mechanical force strengthens TCR-pMHC interactions to elicit agonist-specific catch bonds to trigger TCR signaling, but the underlying dynamic structural mechanism is unclear. We combined steered molecular dynamics (SMD) simulation, single-molecule biophysical approaches, and functional assays to collectively demonstrate that mechanical force induces conformational changes in pMHCs to enhance pre-existing contacts and activates new interactions at the TCR-pMHC binding interface to resist bond dissociation under force, resulting in TCR-pMHC catch bonds and T cell activation. Intriguingly, cancer-associated somatic mutations in HLA-A2 that may restrict these conformational changes suppressed TCR-pMHC catch bonds. Structural analysis also indicated that HLA polymorphism might alter the equilibrium of these conformational changes. Our findings not only reveal critical roles of force-induced conformational changes in pMHCs for activating TCR-pMHC catch bonds but also have implications for T cell-based immunotherapy.

A TCR mechanotransduction signaling loop induces negative selection in the thymus

The T cell antigen receptor (TCR) expressed on thymocytes interacts with self-peptide major histocompatibility complex (pMHC) ligands to signal apoptosis or survival. Here, we found that negative-selection ligands induced thymocytes to exert forces on the TCR and the co-receptor CD8 and formed cooperative TCR-pMHC-CD8 trimolecular 'catch bonds', whereas positive-selection ligands induced less sustained thymocyte forces on TCR and CD8 and formed shorter-lived, independent TCR-pMHC and pMHC-CD8 bimolecular 'slip bonds'. Catch bonds were not intrinsic to either the TCR-pMHC or the pMHC-CD8 arm of the trans (cross-junctional) heterodimer but resulted from coupling of the extracellular pMHC-CD8 interaction to the intracellular interaction of CD8 with TCR-CD3 via associated kinases to form a cis (lateral) heterodimer capable of inside-out signaling. We suggest that the coupled trans-cis heterodimeric interactions form a mechanotransduction loop that reinforces negative-selection signaling that is distinct from positive-selection signaling in the thymus.

Differential IL-2 expression defines developmental fates of follicular versus nonfollicular helper T cells

In response to infection, naïve CD4⁺ T cells differentiate into two subpopulations: T follicular helper (T_FH) cells, which support B cell antibody production, and non-T_FH cells, which enhance innate immune cell functions. Interleukin-2 (IL-2), the major cytokine produced by naïve T cells, plays an important role in the developmental divergence of these populations. However, the relationship between IL-2 production and fate determination remains unclear. Using reporter mice, we found that differential production of IL-2 by naïve CD4⁺ T cells defined precursors fated for different immune functions. IL-2 producers, which were fated to become T_FH cells, delivered IL-2 to nonproducers destined to become non-T_FH cells. Because IL-2 production was limited to cells receiving the strongest T cell receptor (TCR) signals, a direct link between TCR-signal strength, IL-2 production, and T cell fate determination has been established.

2D Kinetic Analysis of TCR and CD8 Coreceptor for LCMV GP33 Epitopes

The LCMV GP33 CD8 epitope has long been one of the most widely used antigens in viral immunology. Of note, almost all of the in vitro analyses of CD8 T cell responses to this epitope make use of an altered peptide ligand (APL) in which the cysteine from the original 9-mer peptide (KAVYNFATC) is substituted by a methionine at position 41 (KAVYNFATM). In addition, it is possible that the antigen processed during natural LCMV infection is an 11-mer peptide (KAVYNFATCGI) rather than the widely used 9-mer. Although previous affinity measurements using purified proteins for these antigen variants revealed minimal differences, we applied highly sensitive two dimensional (2D) biophysical based techniques to further dissect TCR interaction with these closely related GP33 variants. The kinetic analyses of affinity provided by the 2D micropipette adhesion frequency assay (2D-MP) and bond lifetime under force analyzed using a biomembrane force probe (BFP) revealed significant differences between 41M, 41C and the 11-mer 41CGI antigen. We found a hierarchy in 2D affinity as 41M peptide displayed augmented TCR 2D affinity compared to 41C and 41CGI. These differences were also maintained in the presence of CD8 coreceptor and when analysis of total TCR:pMHC and CD8:pMHC bonds were considered. Moreover, the three ligands displayed dramatic differences in the bond lifetimes generated under force, in particular the 41CGI variant with the lowest 2D affinity demonstrated a 15-fold synergistic contribution of the CD8 coreceptor to overall bond lifetime. Our analyses emphasize the sensitivity of single cell and single bond 2D kinetic measurements in distinguishing between related agonist peptides.

T Cell Receptor-Major Histocompatibility Complex Interaction Strength Defines Trafficking and CD103+ Memory Status of CD8 T Cells in the Brain

T cell receptor–major histocompatibility complex (TCR–MHC) affinities span a wide range in a polyclonal T cell response, yet it is undefined how affinity shapes long-term properties of CD8 T cells during chronic infection with persistent antigen. Here, we investigate how the affinity of the TCR–MHC interaction shapes the phenotype of memory CD8 T cells in the chronically Toxoplasma gondii-infected brain. We employed CD8 T cells from three lines of transnuclear (TN) mice that harbor in their endogenous loci different T cell receptors specific for the same Toxoplasma antigenic epitope ROP7. The three TN CD8 T cell clones span a wide range of affinities to MHCI–ROP7. These three CD8 T cell clones have a distinct and fixed hierarchy in terms of effector function in response to the antigen measured as proliferation capacity, trafficking, T cell maintenance, and memory formation. In particular, the T cell clone of lowest affinity does not home to the brain. The two higher affinity T cell clones show differences in establishing resident-like memory populations (CD103⁺) in the brain with the higher affinity clone persisting longer in the host during chronic infection. Transcriptional profiling of naïve and activated ROP7-specific CD8 T cells revealed that Klf2 encoding a transcription factor that is known to be a negative marker for T cell trafficking is upregulated in the activated lowest affinity ROP7 clone. Our data thus suggest that TCR–MHC affinity dictates memory CD8 T cell fate at the site of infection.

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.

Anti-inflammatory effects of PUFA’s on T cells reduce TCR affinity for antigen

Polyunsaturated fatty acids (PUFAs) have been shown to modulate inflammation yet little is known on their specific effects on recognition of antigen by T cells despite. Since the T cell receptor (TCR) and its peptide-MHC (pMHC) antigenic ligand are embedded in the membrane, incorporation of omega-3 fatty acids into lipid membranes could likely change the 2D kinetics of TCR:pMHC engagement and the membrane environment. We used the 2D micropipette adhesion frequency assay to define the proximal kinetics of antigen recognition by T cells on a single cell basis. We found that dietary fish oil modifies the structural lipid content in plasma membranes as measured by mass spectrometry, resulting in reduced TCR affinity for antigen at the clonal and polyclonal level. The reduced affinity led to deceased frequency and effector functions from both CD4 and CD8 T cells at the peak immune response to viral (LCMV) infection. Thus, our work provides mechanistic insight for the anti-inflammatory effects of PUFAs by altering plasma membrane composition which leads to decreased 2D kinetics of antigen recognition in T cells and dampened response.

CD45RB Status Defines TCR Priming Affinity and CD8+ T Cell Memory Persistence

T cell receptor (TCR) affinity plays a critical role in shaping T cell differentiation, but few studies have investigated the role of TCR affinity in the context of the endogenous CD8+ T cell memory pool. While naïve CD8+ T cells express primarily large CD45 isoforms (denoted as CD45RBhi), CD8+ T cell memory cells are traditionally described as expressing small isoforms (CD45RBlo). Interestingly, although CD8+ T cell memory to acute infection with LCMV included CD45RBlo cells, we found that over half of the CD8+ T cell memory pool remained CD45RBhi. The 2D micropipette affinity assay revealed that CD45RBhi memory cells had a 29-fold lower affinity for antigen relative to CD45RBlo cells. CD45RBloand CD45RBhi memory cells expressed similar levels of granzyme B and IFN-γ, but RNASeq analysis revealed that these populations differentially expressed over 150 genes. Phenotypically, the CD45RBhi population contained a higher frequency of the CD62LhiCD27hiCD127hiKLRG1lo persistent memory phenotype compared to CD45RBlo cells. We found that from early to late memory time-points, portions of each of the CD45RBhi and CD45RBlo populations underwent conversion to CD62Lhi, demonstrating that CD45RB status is distinct from the TCM phenotype. Adoptive transfer of purified CD45RBhi and CD45RBlo populations into naïve mice revealed that CD45RBlo memory cells underwent significant cell loss relative to CD45RBhi memory cells. Over time, the majority of CD45RBlo cells homeostatically converted to the CD45RBhipersistent memory phenotype. Together, these data demonstrate a novel role for CD45RB expression as a marker of TCR priming affinity, and describe a novel way in which CD45RB status can functionally categorize the CD8+ T cell memory pool.

Low affinity CD8+ T cells are a critical component in the development of experimental cerebral malaria

Malaria is responsible for at least 500 million deaths annually. Of those who become infected with malaria, around 1% develop cerebral malaria, which is fatal in 15% – 20% of patients. Therefore, understanding the immune mechanisms contributing to the lethality of cerebral malaria are critical for developing appropriate therapies. Infection with Plasmodium berghei ANKA in C57BL/6 mice induces experimental cerebral malaria (ECM), which requires activated CD8 T cells. In these experiments, we used the highly sensitive two-dimensional (2D) micropipette adhesion frequency assay to define the affinity and frequency of the immunodominant glideosome associated protein 50 (GAP50)40–48 epitope. We found both high and low affinity GAP50 specific CD8 T cells are found in the spleen (d5) and brain (d6) of ANKA infected mice. However, the 2D micropipette assay was more sensitive than tetramer staining with respective measurements of 20% vs. 1.2% of CD8 T cells in the spleen and 30% vs. 13.2% of CD8 T cells in the brain of ANKA infected mice measured as specific for GAP50. Furthermore, 67% of the antigen specific CD8 T cells in the spleen and 50% in the brain had low affinity for GAP50. These data reveal that tetramer vastly underestimates the number of antigen specific cells responding during ANKA infection. Infection with the NK65 strain of P. bergei results in similar levels of parasitemia and frequency of CD8 T cells in the brain, but ECM does not develop. Unlike ANKA infection, micropipette analysis of CD8 T cells from the brains of NK65 revealed only high affinity CD8 T cells for GAP50. These data indicate that under appreciated low affinity CD8 T cells are a vital component of the pathogenic and lethal response observed in experimental cerebral malaria.

Activation induced decrease and recovery of TCR 2D affinity and bond lifetime

Shortly after encountering antigen, T cells are considered transiently unresponsive or refractory to subsequent simulation. To determine whether antigen stimulation also changes the cell surface biophysical interaction between TCR and pMHC, we assessed the 2D TCR affinity and bond lifetime under force between this receptor ligand pair. Virus (LCMV) specific SMARTA transgenic CD4+ T cells were stimulated with peptide in vitro and 2D TCR affinity and bond lifetime determined at various time points after stimulation using the 2D micropipette adhesion frequency assay and the biomembrane force probe (BFP) respectively. We found activated T cells downregulated 2D affinity at twelve and twenty-four hours after seeing antigen with affinity recovering to naïve levels by forty-eight hours. Unlike pMHC tetramer staining which was dependent on TCR expression levels, the decrease in affinity was independent of antigen dose and degree of TCR downregulation. TCR:pMHC bond lifetime under force was also reduced early after antigen encounter with a steady recovery observed with time. Our data demonstrate a decrease in bond lifetime and relative 2D affinity of TCR and pMHC result from early T cell activation events followed by a recovery phase with both changes occurring independent of activation induced modulation of TCR expression. Thus, TCR affinity for pMHC is dynamic in the 2D T cell membrane context with cellular events allowing the T cell to fine-tune the ability of TCR to interact with antigen.

Dual Molecular Mechanisms Govern Escape at Immunodominant HLA A2-Restricted HIV Epitope

Serial accumulation of mutations to fixation in the SLYNTVATL (SL9) immunodominant, HIV p17 Gag-derived, HLA A2-restricted cytotoxic T lymphocyte epitope produce the SLFNTIAVL triple mutant “ultimate” escape variant. These mutations in solvent-exposed residues are believed to interfere with TCR recognition, although confirmation has awaited structural verification. Here, we solved a TCR co-complex structure with SL9 and the triple escape mutant to determine the mechanism of immune escape in this eminent system. We show that, in contrast to prevailing hypotheses, the main TCR contact residue is 4N and the dominant mechanism of escape is not via lack of TCR engagement. Instead, mutation of solvent-exposed residues in the peptide destabilise the peptide–HLA and reduce peptide density at the cell surface. These results highlight the extraordinary lengths that HIV employs to evade detection by high-affinity TCRs with a broad peptide-binding footprint and necessitate re-evaluation of this exemplar model of HIV TCR escape.

Clinical and immunologic evaluation of three metastatic melanoma patients treated with autologous melanoma-reactive TCR-transduced T cells

Malignant melanoma incidence has been increasing for over 30 years, and despite promising new therapies, metastatic disease remains difficult to treat. We describe preliminary results from a Phase I clinical trial (NCT01586403) of adoptive cell therapy in which three patients received autologous CD4⁺ and CD8⁺ T cells transduced with a lentivirus carrying a tyrosinase-specific TCR and a marker protein, truncated CD34 (CD34t). This unusual MHC Class I-restricted TCR produces functional responses in both CD4⁺ and CD8⁺ T cells. Parameters monitored on transduced T cells included activation (CD25, CD69), inhibitory (PD-1, TIM-3, CTLA-4), costimulatory (OX40), and memory (CCR7) markers. For the clinical trial, T cells were activated, transduced, selected for CD34t⁺ cells, then re-activated, and expanded in IL-2 and IL-15. After lymphodepleting chemotherapy, patients were given transduced T cells and IL-2, and were followed for clinical and biological responses. Transduced T cells were detected in the circulation of three treated patients for the duration of observation (42, 523, and 255 days). Patient 1 tolerated the infusion well but died from progressive disease after 6 weeks. Patient 2 had a partial response by RECIST criteria then progressed. After progressing, Patient 2 was given high-dose IL-2 and subsequently achieved complete remission, coinciding with the development of vitiligo. Patient 3 had a mixed response that did not meet RECIST criteria for a clinical response and developed vitiligo. In two of these three patients, adoptive transfer of tyrosinase-reactive TCR-transduced T cells into metastatic melanoma patients had clinical and/or biological activity without serious adverse events.

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.

Targeted loss of SHP1 in murine thymocytes dampens TCR signaling late in selection

SHP1 is a tyrosine phosphatase critical to proximal regulation of TCR signaling. Here, analysis of CD4-Cre SHP1(fl/fl) conditional knockout thymocytes using CD53, TCRβ, CD69, CD4, and CD8α expression demonstrates the importance of SHP1 in the survival of post selection (CD53(+) ), single-positive thymocytes. Using Ca(2+) flux to assess the intensity of TCR signaling demonstrated that SHP1 dampens the signal strength of these same mature, postselection thymocytes. Consistent with its dampening effect, TCR signal strength was also probed functionally using peptides that can mediate selection of the OT-I TCR, to reveal increased negative selection mediated by lower-affinity ligand in the absence of SHP1. Our data show that SHP1 is required for the survival of mature thymocytes and the generation of the functional T-cell repertoire, as its absence leads to a reduction in the numbers of CD4(+) and CD8(+) naïve T cells in the peripheral lymphoid compartments.

MOG35-55, not NFM15-35, is the critical autoantigen for inducing demyelinating autoimmune disease

Here we examine functional significance of CD4+ T cell cross-recognition between MOG35-55 (myelin oligodendrocyte glycoprotein) and NFM15-35 (neurofilament medium polypeptide) in demyelinating autoimmune disease. These epitopes are interesting for synergist potential in promoting autoimmunity due to identical amino acids at proposed TCR contacts while displaying variances at MHC. Surprisingly, NFM15-35 peptide is not encephalitogenic in polyclonal models and we proposed to understand why by studying how NFM15-35 effects the known encephalitogenic MOG CD4+ T cell population using peptide:MHC (pMHC) technologies. We found a NFM15-35 challenge was less potent than MOG35-55 in expanding tetramer positive, MOG38-49:I-Ab T cells. Diminished tetramer staining indicated the expanded polyclonal T cells from NFM challenge were of low affinity for MOG38-49, because tetramer enriches for T cells with high affinity for pMHC. We also found that NFM18-30 specific cells in the CNS of mice with MOG35-55 induced EAE predominantly cross-recognized MOG38-49:I-Ab using the 2-dimensional micropipette adhesion frequency assay. These data suggested MOG35-55, not NFM15-35, was the critical encephalitogenic antigen and we confirmed this by showing that demyelinating disease in NFM deficient mice exhibited identical disease course to wild type mice. Interestingly, alterations of NFM15-35 N-terminal amino acids expanded T cells that cross-recognized MOG38-49 tetramer, a phenotype concomitant with restoring encephalitogenic potential in altered NFM15-35 peptides. Overall, we support MOG35-55 as the critical autoantigen for induced demyelinating disease with the contribution of NFM15-35 being dictated by cross-recognition of MOG.