The CD4 transmembrane GGXXG and juxtamembrane (C/F)CV+C motifs mediate pMHCII-specific signaling independently of CD4-LCK interactions.

CD4+ T cell activation is driven by five-module receptor complexes. The T cell receptor (TCR) is the receptor module that binds composite surfaces of peptide antigens embedded within MHCII molecules (pMHCII). It associates with three signaling modules (CD3γε, CD3δε, and CD3ζζ) to form TCR-CD3 complexes. CD4 is the coreceptor module. It reciprocally associates with TCR-CD3-pMHCII assemblies on the outside of a CD4+ T cells and with the Src kinase, LCK, on the inside. Previously, we reported that the CD4 transmembrane GGXXG and cytoplasmic juxtamembrane (C/F)CV+C motifs found in eutherian (placental mammal) CD4 have constituent residues that evolved under purifying selection (Lee et al., 2022). Expressing mutants of these motifs together in T cell hybridomas increased CD4-LCK association but reduced CD3ζ, ZAP70, and PLCγ1 phosphorylation levels, as well as IL-2 production, in response to agonist pMHCII. Because these mutants preferentially localized CD4-LCK pairs to non-raft membrane fractions, one explanation for our results was that they impaired proximal signaling by sequestering LCK away from TCR-CD3. An alternative hypothesis is that the mutations directly impacted signaling because the motifs normally play an LCK-independent role in signaling. The goal of this study was to discriminate between these possibilities. Using T cell hybridomas, our results indicate that: intracellular CD4-LCK interactions are not necessary for pMHCII-specific signal initiation; the GGXXG and (C/F)CV+C motifs are key determinants of CD4-mediated pMHCII-specific signal amplification; the GGXXG and (C/F)CV+C motifs exert their functions independently of direct CD4-LCK association. These data provide a mechanistic explanation for why residues within these motifs are under purifying selection in jawed vertebrates. The results are also important to consider for biomimetic engineering of synthetic receptors.

CAR and TCR form individual signaling synapses and do not cross-activate, however, can co-operate in T cell activation.

In engineered T cells the CAR is co-expressed along with the physiological TCR/CD3 complex, both utilizing the same downstream signaling machinery for T cell activation. It is unresolved whether CAR-mediated T cell activation depends on the presence of the TCR and whether CAR and TCR mutually cross-activate upon engaging their respective antigen. Here we demonstrate that the CD3ζ CAR level was independent of the TCR associated CD3ζ and could not replace CD3ζ to rescue the TCR complex in CD3ζ KO T cells. Upon activation, the CAR did not induce phosphorylation of TCR associated CD3ζ and, vice versa, TCR activation did not induce CAR CD3ζ phosphorylation. Consequently, CAR and TCR did not cross-signal to trigger T cell effector functions. On the membrane level, TCR and CAR formed separate synapses upon antigen engagement as revealed by total internal reflection fluorescence (TIRF) and fast AiryScan microscopy. Upon engaging their respective antigen, however, CAR and TCR could co-operate in triggering effector functions through combinatorial signaling allowing logic "AND" gating in target recognition. Data also imply that tonic TCR signaling can support CAR-mediated T cell activation emphasizing the potential relevance of the endogenous TCR for maintaining T cell capacities in the long-term.

Negative Regulation of Zap70 by Lck Forms the Mechanistic Basis of Differential Expression in CD4 and CD8 T Cells.

Lck and Zap70, two non-receptor tyrosine kinases, play a crucial role in the regulation of membrane proximal TCR signaling critical for thymic selection, CD4/CD8 lineage choice and mature T cell function. Signal initiation upon TCR/CD3 and peptide/MHC interaction induces Lck-mediated phosphorylation of CD3 ITAMs. This is necessary for Zap70 recruitment and its phosphorylation by Lck leading to full Zap70 activation. In its native state Zap70 maintains a closed conformation creating an auto-inhibitory loop, which is relieved by Lck-mediated phosphorylation of Y315/Y319. Zap70 is differentially expressed in thymic subsets and mature T cells with CD8 T cells expressing the highest amount compared to CD4 T cells. However, the mechanistic basis of differential Zap70 expression in thymic subsets and mature T cells is not well understood. Here, we show that Zap70 is degraded relatively faster in DP and mature CD4 T cells compared to CD8 T cells, and inversely correlated with relative level of activated Zap70. Importantly, we found that Zap70 expression is negatively regulated by Lck activity: augmented Lck activity resulting in severe diminution in total Zap70. Moreover, Lck-mediated phosphorylation of Y315/Y319 was essential for Zap70 degradation. Together, these data shed light on the underlying mechanism of Lck-mediated differential modulation of Zap70 expression in thymic subsets and mature T cells.

Enhancing and inhibitory motifs regulate CD4 activity.

CD4+ T cells use T cell receptor (TCR)-CD3 complexes, and CD4, to respond to peptide antigens within MHCII molecules (pMHCII). We report here that, through ~435 million years of evolution in jawed vertebrates, purifying selection has shaped motifs in the extracellular, transmembrane, and intracellular domains of eutherian CD4 that enhance pMHCII responses, and covary with residues in an intracellular motif that inhibits responses. Importantly, while CD4 interactions with the Src kinase, Lck, are viewed as key to pMHCII responses, our data indicate that CD4-Lck interactions derive their importance from the counterbalancing activity of the inhibitory motif, as well as motifs that direct CD4-Lck pairs to specific membrane compartments. These results have implications for the evolution and function of complex transmembrane receptors and for biomimetic engineering.

Cholesterol inhibits TCR signaling by directly restricting TCR-CD3 core tunnel motility.

Cholesterol molecules specifically bind to the resting αßTCR to inhibit cytoplasmic CD3ζ ITAM phosphorylation through sequestering the TCR-CD3 complex in an inactive conformation. The mechanisms of cholesterol-mediated inhibition of TCR-CD3 and its activation remain unclear. Here, we present cryoelectron microscopy structures of cholesterol- and cholesterol sulfate (CS)-inhibited TCR-CD3 complexes and an auto-active TCR-CD3 variant. The structures reveal that cholesterol molecules act like a latch to lock CD3ζ into an inactive conformation in the membrane. Mutations impairing binding of cholesterol molecules to the tunnel result in the movement of the proximal C terminus of the CD3ζ transmembrane helix, thereby activating the TCR-CD3 complex in human cells. Together, our data reveal the structural basis of TCR inhibition by cholesterol, illustrate how the cholesterol-binding tunnel is allosterically coupled to TCR triggering, and lay a foundation for the development of immunotherapies through directly targeting the TCR-CD3 complex.

The T cell receptor displays lateral signal propagation involving non-engaged receptors.

T cells are highly sensitive to low levels of antigen, but how this sensitivity is achieved is currently unknown. Here, we imaged proximal TCR-CD3 signal propagation with single molecule localization microscopy (SMLM) in T cells activated with nanoscale clusters of TCR stimuli. We observed the formation of large TCR-CD3 clusters that exceeded the area of the ligand clusters, and required multivalent interactions facilitated by TCR-CD3 phosphorylation for assembly. Within these clustered TCR-CD3 domains, TCR-CD3 signaling spread laterally for ∼500 nm, far beyond the activating site, via non-engaged receptors. Local receptor density determined the functional cooperativity between engaged and non-engaged receptors, but lateral signal propagation was not influenced by the genetic deletion of ZAP70. Taken together, our data demonstrates that clustered ligands induced the clustering of non-ligated TCR-CD3 into domains that cooperatively facilitate lateral signal propagation.

The Full Model of the pMHC-TCR-CD3 Complex: A Structural and Dynamical Characterization of Bound and Unbound States.

The machinery involved in cytotoxic T-cell activation requires three main characters: the major histocompatibility complex class I (MHC I) bound to the peptide (p), the T-cell receptor (TCR), and the CD3 complex, a multidimer interfaced with the intracellular side. The pMHC:TCR interaction has been largely studied by means of both experimental and computational models, giving a contribution in understanding the complexity of the TCR triggering. Nevertheless, a detailed study of the structural and dynamical characterization of the full complex (pMHC:TCR:CD3 complex) is still missing due to a lack of structural information of the CD3-chains arrangement around the TCR. Very recently, the determination of the TCR:CD3 complex structure by means of Cryo-EM technique has given a chance to build the entire system essential in the activation of T-cells, a fundamental mechanism in the adaptive immune response. Here, we present the first complete model of the pMHC interacting with the TCR:CD3 complex, built in a lipid environment. To describe the conformational behavior associated with the unbound and the bound states, all-atom Molecular Dynamics simulations were performed for the TCR:CD3 complex and for two pMHC:TCR:CD3 complex systems, bound to two different peptides. Our data point out that a conformational change affecting the TCR Constant ß (Cß) region occurs after the binding to the pMHC, revealing a key role of this region in the propagation of the signal. Moreover, we found that TCR reduces the flexibility of the MHC I binding groove, confirming our previous results.

Structural variability and concerted motions of the T cell receptor - CD3 complex.

We investigate the structural and orientational variability of the membrane-embedded T cell receptor (TCR) - CD3 complex in extensive atomistic molecular dynamics simulations based on the recent cryo-EM structure determined by Dong et al., 2019. We find that the TCR extracellular (EC) domain is highly variable in its orientation by attaining tilt angles relative to the membrane normal that range from 15° to 55°. The tilt angle of the TCR EC domain is both coupled to a rotation of the domain and to characteristic changes throughout the TCR - CD3 complex, in particular in the EC interactions of the Cß FG loop of the TCR, as well as in the orientation of transmembrane helices. The concerted motions of the membrane-embedded TCR - CD3 complex revealed in our simulations provide atomistic insights on conformational changes of the complex in response to tilt-inducing forces on antigen-bound TCRs.

Multi-scale simulations of the T cell receptor reveal its lipid interactions, dynamics and the arrangement of its cytoplasmic region.

The T cell receptor (TCR-CD3) initiates T cell activation by binding to peptides of Major Histocompatibility Complexes (pMHC). The TCR-CD3 topology is well understood but the arrangement and dynamics of its cytoplasmic tails remains unknown, limiting our grasp of the signalling mechanism. Here, we use molecular dynamics simulations and modelling to investigate the entire TCR-CD3 embedded in a model membrane. Our study demonstrates conformational changes in the extracellular and transmembrane domains, and the arrangement of the TCR-CD3 cytoplasmic tails. The cytoplasmic tails formed highly interlaced structures while some tyrosines within the immunoreceptor tyrosine-based activation motifs (ITAMs) penetrated the hydrophobic core of the membrane. Interactions between the cytoplasmic tails and phosphatidylinositol phosphate lipids in the inner membrane leaflet led to the formation of a distinct anionic lipid fingerprint around the TCR-CD3. These results increase our understanding of the TCR-CD3 dynamics and the importance of membrane lipids in regulating T cell activation.

Axicabtagene ciloleucel and brexucabtagene autoleucel in relapsed and refractory diffuse large B-cell and mantle cell lymphomas.

Axicabtagene ciloleucel and brexucabtagene autoleucel are anti-CD19 T-cell therapies that utilize the same second-generation chimeric antigen receptor with a CD28 costimulatory subunit. They have demonstrated high rates of response in high-risk patients with relapsed and refractory B-cell malignancies in multicenter clinical trials, including diffuse large B-cell and mantle cell lymphomas. The high clinical activity has led to the US FDA approval of axicabtagene ciloleucel for diffuse large B-cell lymphoma, and brexucabtagene autoleucel for mantle cell lymphoma. While they are highly effective, they have significant toxicities, including cytokine release syndrome and neurologic toxicities, which can be severe and require specialized management. This review will discuss the development, efficacy and safety of axicabtagene ciloleucel and brexucabtagene autoleucel in B-cell lymphomas.

Predicting antigen specificity of single T cells based on TCR CDR3 regions.

It has recently become possible to simultaneously assay T-cell specificity with respect to large sets of antigens and the T-cell receptor sequence in high-throughput single-cell experiments. Leveraging this new type of data, we propose and benchmark a collection of deep learning architectures to model T-cell specificity in single cells. In agreement with previous results, we found that models that treat antigens as categorical outcome variables outperform those that model the TCR and antigen sequence jointly. Moreover, we show that variability in single-cell immune repertoire screens can be mitigated by modeling cell-specific covariates. Lastly, we demonstrate that the number of bound pMHC complexes can be predicted in a continuous fashion providing a gateway to disentangle cell-to-dextramer binding strength and receptor-to-pMHC affinity. We provide these models in the Python package TcellMatch to allow imputation of antigen specificities in single-cell RNA-seq studies on T cells without the need for MHC staining.

Nef-Mediated CD3-TCR Downmodulation Dampens Acute Inflammation and Promotes SIV Immune Evasion.

The inability of Nef to downmodulate the CD3-T cell receptor (TCR) complex distinguishes HIV-1 from other primate lentiviruses and may contribute to its high virulence. However, the role of this Nef function in virus-mediated immune activation and pathogenicity remains speculative. Here, we selectively disrupted this Nef activity in SIVmac239 and analyzed the consequences for the virological, immunological, and clinical outcome of infection in rhesus macaques. The inability to downmodulate CD3-TCR does not impair viral replication during acute infection but is associated with increased immune activation and antiviral gene expression. Subsequent early reversion in three of six animals suggests strong selective pressure for this Nef function and is associated with high viral loads and progression to simian AIDS. In the absence of reversions, however, viral replication and the clinical course of infection are attenuated. Thus, Nef-mediated downmodulation of CD3 dampens the inflammatory response to simian immunodeficiency virus (SIV) infection and seems critical for efficient viral immune evasion.

Actin polymerization regulates recruitment of Nck to CD3ε upon T-cell receptor triggering.

Following T-cell antigen receptor (TCR) engagement, rearrangement of the actin cytoskeleton supports intracellular signal transduction and T-cell activation. The non-catalytic region of the tyrosine kinase (Nck) molecule is an adapter protein implicated in TCR-induced actin polymerization. Further, Nck is recruited to the CD3ε subunit of the TCR upon TCR triggering. Here we examine the role of actin polymerization in the recruitment of Nck to the TCR. To this end, Nck binding to CD3ε was quantified in Jurkat cells using the proximity ligation assay. We show that inhibition of actin polymerization using cytochalasin D delayed the recruitment of Nck1 to the TCR upon TCR triggering. Interestingly, CD3ε phosphorylation was also delayed. These findings suggest that actin polymerization promotes the recruitment of Nck to the TCR, enhancing downstream signaling, such as phosphorylation of CD3ε.

Ultrasmall silica nanoparticles directly ligate the T cell receptor complex.

The impact of ultrasmall nanoparticles (<10-nm diameter) on the immune system is poorly understood. Recently, ultrasmall silica nanoparticles (USSN), which have gained increasing attention for therapeutic applications, were shown to stimulate T lymphocytes directly and at relatively low-exposure doses. Delineating underlying mechanisms and associated cell signaling will hasten therapeutic translation and is reported herein. Using competitive binding assays and molecular modeling, we established that the T cell receptor (TCR):CD3 complex is required for USSN-induced T cell activation, and that direct receptor complex-particle interactions are permitted both sterically and electrostatically. Activation is not limited to αß TCR-bearing T cells since those with γδ TCR showed similar responses, implying that USSN mediate their effect by binding to extracellular domains of the flanking CD3 regions of the TCR complex. We confirmed that USSN initiated the signaling pathway immediately downstream of the TCR with rapid phosphorylation of both ζ-chain-associated protein 70 and linker for activation of T cells protein. However, T cell proliferation or IL-2 secretion were only triggered by USSN when costimulatory anti-CD28 or phorbate esters were present, demonstrating that the specific impact of USSN is in initiation of the primary, nuclear factor of activated T cells-pathway signaling from the TCR complex. Hence, we have established that USSN are partial agonists for the TCR complex because of induction of the primary T cell activation signal. Their ability to bind the TCR complex rapidly, and then to dissolve into benign orthosilicic acid, makes them an appealing option for therapies targeted at transient TCR:CD3 receptor binding.

Structural basis of assembly of the human T cell receptor-CD3 complex.

The αß T cell receptor (TCR), in association with the CD3γε-CD3δε-CD3ζζ signalling hexamer, is the primary determinant of T cell development and activation, and of immune responses to foreign antigens. The mechanism of assembly of the TCR-CD3 complex remains unknown. Here we report a cryo-electron microscopy structure of human TCRαß in complex with the CD3 hexamer at 3.7 Å resolution. The structure contains the complete extracellular domains and all the transmembrane helices of TCR-CD3. The octameric TCR-CD3 complex is assembled with 1:1:1:1 stoichiometry of TCRαß:CD3γε:CD3δε:CD3ζζ. Assembly of the extracellular domains of TCR-CD3 is mediated by the constant domains and connecting peptides of TCRαß that pack against CD3γε-CD3δε, forming a trimer-like structure proximal to the plasma membrane. The transmembrane segment of the CD3 complex adopts a barrel-like structure formed by interaction of the two transmembrane helices of CD3ζζ with those of CD3γε and CD3δε. Insertion of the transmembrane helices of TCRαß into the barrel-like structure via both hydrophobic and ionic interactions results in transmembrane assembly of the TCR-CD3 complex. Together, our data reveal the structural basis for TCR-CD3 complex assembly, providing clues to TCR triggering and a foundation for rational design of immunotherapies that target the complex.

Identification of drug-specific public TCR driving severe cutaneous adverse reactions.

Drug hypersensitivity such as severe cutaneous adverse reactions (SCAR), including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), could be life-threatening. Here, we enroll SCAR patients to investigate the T cell receptor (TCR) repertoire by next-generation sequencing. A public αßTCR is identified from the cytotoxic T lymphocytes of patients with carbamazepine-SJS/TEN, with its expression showing drug/phenotype-specificity and an bias for HLA-B*15:02. This public αßTCR has binding affinity for carbamazepine and its structural analogs, thereby mediating the immune response. Adoptive transfer of T cell expressing this public αßTCR to HLA-B*15:02 transgenic mice receiving oral administration of carbamazepine induces multi-organ injuries and symptoms mimicking SCAR, including hair loss, erythema, increase of inflammatory lymphocytes in the skin and blood, and liver and kidney dysfunction. Our results not only demonstrate an essential role of TCR in the immune synapse mediating SCAR, but also implicate potential clinical applications and development of therapeutics.

Macrophage migration inhibitory factor regulates TLR4 expression and modulates TCR/CD3-mediated activation in CD4+ T lymphocytes.

Toll-like receptor 4 (TLR4) is involved in CD4+ T lymphocyte-mediated pathologies. Here, we demonstrate that CD4+ T lymphocytes express functional TLR4 that contributes to their activation, proliferation and cytokine secretion. In addition, we demonstrate that TLR4-induced responses are mediated by macrophage migration inhibitory factor (MIF), a pro-inflammatory cytokine. We also demonstrate that MIF regulates suboptimal TCR/CD3-mediated activation of T lymphocytes. On one hand, MIF prevents excessive TCR/CD3-mediated activation of CD4+ T lymphocytes under suboptimal stimulation conditions and, on the other hand, MIF enables activated CD4+ T lymphocytes to sense their microenvironment and adapt their effector response through TLR4. Therefore, MIF appears to be a major regulator of the activation of CD4+ T lymphocytes and the intensity of their effector response. TLR4-mediated activation is thus an important process for T cell-mediated immunity.

"Waltz" of Cell Membrane-Coated Nanoparticles on Lipid Bilayers: Tracking Single Particle Rotation in Ligand-Receptor Binding.

Understanding the binding of nanoparticles to receptors on biomembranes is critical to the development and screening of therapeutic materials. A prevailing understanding is that multivalent ligand-receptor binding leads to slower and confined translational motion of nanoparticles. In contrast, we report in this study distinct types of rotational dynamics of nanoparticles during their seemingly similar translational confinements in ligand-receptor binding. Our nanoparticles are fluorescently anisotropic and camouflaged with T cell membranes. As they bind to ligands on planar lipid bilayers, the particles transition from back-and-forth rocking motion to circling and eventually confined circling motion, while "hopping" between translational confinements. Both rotational and translational motions of the nanoparticles become more confined at higher ligand density. The time-dependent changes in particle rotation reveal different stages in the progression of multivalent binding between the cell-membrane coated nanoparticles and their ligands. Our work also demonstrates the promise of using combined rotational and translational single particle tracking to resolve biological interactions that could be "hidden" in translational measurements alone.

Biophysical Characterization of CD6-TCR/CD3 Interplay in T Cells.

Activation of the T cell receptor (TCR) on the T cell through ligation with antigen-MHC complex of an antigen-presenting cell (APC) is an essential process in the activation of T cells and induction of the subsequent adaptive immune response. Upon activation, the TCR, together with its associated co-receptor CD3 complex, assembles in signaling microclusters that are transported to the center of the organizational structure at the T cell-APC interface termed the immunological synapse (IS). During IS formation, local cell surface receptors and associated intracellular molecules are reorganized, ultimately creating the typical bull's eye-shaped pattern of the IS. CD6 is a surface glycoprotein receptor, which has been previously shown to associate with CD3 and co-localize to the center of the IS in static conditions or stable T cell-APC contacts. In this study, we report the use of different experimental set-ups analyzed with microscopy techniques to study the dynamics and stability of CD6-TCR/CD3 interaction dynamics and stability during IS formation in more detail. We exploited antibody spots, created with microcontact printing, and antibody-coated beads, and could demonstrate that CD6 and the TCR/CD3 complex co-localize and are recruited into a stimulatory cluster on the cell surface of T cells. Furthermore, we demonstrate, for the first time, that CD6 forms microclusters co-localizing with TCR/CD3 microclusters during IS formation on supported lipid bilayers. These co-localizing CD6 and TCR/CD3 microclusters are both radially transported toward the center of the IS formed in T cells, in an actin polymerization-dependent manner. Overall, our findings further substantiate the role of CD6 during IS formation and provide novel insight into the dynamic properties of this CD6-TCR/CD3 complex interplay. From a methodological point of view, the biophysical approaches used to characterize these receptors are complementary and amenable for investigation of the dynamic interactions of other membrane receptors.

Peptide-MHC (pMHC) binding to a human antiviral T cell receptor induces long-range allosteric communication between pMHC- and CD3-binding sites.

T cells generate adaptive immune responses mediated by the T cell receptor (TCR)-CD3 complex comprising an αß TCR heterodimer noncovalently associated with three CD3 dimers. In early T cell activation, αß TCR engagement by peptide-major histocompatibility complex (pMHC) is first communicated to the CD3 signaling apparatus of the TCR-CD3 complex, but the underlying mechanism is incompletely understood. It is possible that pMHC binding induces allosteric changes in TCR conformation or dynamics that are then relayed to CD3. Here, we carried out NMR analysis and molecular dynamics (MD) simulations of both the α and ß chains of a human antiviral TCR (A6) that recognizes the Tax antigen from human T cell lymphotropic virus-1 bound to the MHC class I molecule HLA-A2. We observed pMHC-induced NMR signal perturbations in the TCR variable (V) domains that propagated to three distinct sites in the constant (C) domains: 1) the Cß FG loop projecting from the Vß/Cß interface; 2) a cluster of Cß residues near the Cß αA helix, a region involved in interactions with CD3; and 3) the Cα AB loop at the membrane-proximal base of the TCR. A biological role for each of these allosteric sites is supported by previous mutational and functional studies of TCR signaling. Moreover, the pattern of long-range, ligand-induced changes in TCR A6 revealed by NMR was broadly similar to that predicted by the MD simulations. We propose that the unique structure of the TCR ß chain enables allosteric communication between the TCR-binding sites for pMHC and CD3.