The T cell receptor (TCR) interacts with peptide-major histocompatibility complexes (pMHC) to discriminate pathogens from self-antigens and trigger adaptive immune responses. Direct physical contact is required between the T cell and the antigen-presenting cell (APC) for cross-junctional binding where the TCR and pMHC are anchored on two-dimensional (2D) membranes of the apposing cells1. Despite their 2D nature, TCR-pMHC binding kinetics have only been analyzed three-dimensionally (3D) with a varying degree of correlation with the T cell responsiveness2-4. Here we use two mechanical assays5,6 to show high 2D affinities between a TCR and its antigenic pMHCs driven by rapid on-rates. Compared to their 3D counterparts, 2D affinities and on-rates of the TCR for a panel of pMHC ligands possess far broader dynamic ranges that match that of their corresponding T cell responses. The best 3D predictor of response is the off-rate, with agonist pMHC dissociating the slowest2-4. In contrast, 2D off-rates are up to 8,300-fold faster, with the agonist pMHC dissociating the fastest. Our 2D data suggest rapid antigen sampling by T cells and serial engagement of a few agonist pMHCs by TCRs in a large self pMHC background. Thus, the cellular environment amplifies the TCR-pMHC binding to generate broad affinities and rapid kinetics that determine T-cell responsiveness.

Summary TCR–pMHC interactions initiate adaptive immune responses, but the mechanism of how such interactions under force induce T-cell signaling is unclear. We show that force prolongs lifetimes of single TCR–pMHC bonds for agonists (catch bonds) but shortens those for antagonists (slip bonds). Both magnitude and duration of force are important as the highest Ca2+ responses were induced by 10 pN via both pMHC catch bonds whose lifetime peaks at this force and anti-TCR slip bonds whose maximum lifetime occurs at 0 pN. High Ca2+ levels require early and rapid accumulation of bond lifetimes whereas short-lived bonds that slow early accumulation of lifetimes correspond to low Ca2+ responses. Our data support a model where force on the TCR induces signaling events depending on its magnitude, duration, frequency, and timing, such that agonists form catch bonds that trigger the T cell digitally, whereas antagonists form slip bonds that fail to activate.

T cell receptor (TCR) engagement of peptide-major histocompatibility complex (MHC) is essential to adaptive immunity, but it is unknown if TCR signaling responses are influenced by the binding topology of the TCR-peptide-MHC complex. We developed yeast-displayed peptide-MHC libraries that enabled us to identify new peptide sequences reactive with a single TCR. Structural analysis showed that four peptides bound to the TCR with distinct 3-dimensional (3D) and 2D affinities, using entirely different binding chemistries. Three of the peptides that shared a common docking mode, where key TCR-MHC germline interactions are preserved, induced TCR signaling. The fourth peptide failed to induce signaling, and was recognized in a substantially different TCR-MHC binding mode that apparently exceeded geometric tolerances compatible with signaling. We suggest that the ‘stereotypical’ TCR-MHC docking paradigm evolved from productive signaling geometries, and that TCR signaling can be modulated by peptides that are recognized in alternative TCR-pMHC binding orientations.

SUMMARY The T cell receptor (TCR) and CD8 bind peptide-major histocompatibility complex (pMHC) glycoproteins to initiate adaptive immune responses, yet the trimolecular binding kinetics at the T cell membrane is unknown. Using a micropipette adhesion frequency assay, we show that this kinetic has two stages. The first consists of TCR-dominant binding to agonist pMHC. This triggers a second stage consisting of a step increase in adhesion following a one second delay. The second-stage binding requires Src family kinase activity to initiate CD8 binding to the same pMHC engaged by the TCR. This induced-trimeric-cooperative interaction enhances adhesion synergistically to favor potent ligands, which further amplifies discrimination. Our data reveal a TCR-CD8 positive feedback loop involved in initial signaling steps that is sensitive to a single pMHC, is rapid, reversible, synergistic, and peptide-discriminative.

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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 coreceptor 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 to 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.

Summary Two-dimensional (2D) kinetic analysis directly measures molecular interactions at cell-cell junctions, thereby incorporating inherent cellular effects. By comparison, three-dimensional (3D) analysis probes the intrinsic physical chemistry of interacting molecules isolated from the cell. To understand how T-cell tumor reactivity relates to 2D and 3D binding parameters and to directly compare them, we performed kinetic analyses of a panel of human T-cell receptors (TCR) interacting with a melanoma self-antigen peptide (gp100209–217) bound to major histocompatibility complex (pMHC) in the absence and presence of coreceptor CD8. We found that while 3D parameters are inadequate to predict T-cell function, 2D parameters (which do not correlate with their 3D counterparts) show a far broader dynamic range and significantly improved correlation with T-cell function. Thus, our data support the general notion that 2D parameters of TCR–pMHC–CD8 interactions determine T-cell responsiveness and suggest a potential 2D-based strategy to screen TCRs for tumor immunotherapy.

The cycloaddition reaction of CO2 with various epoxides to generate cyclic carbonates is one of the most promising and efficient approaches for CO2 fixation. Typical imidazolium‐based ionic liquids possessing electrophilic cations and nucleophilic halogen anions have been identified as excellent and environmentally friendly candidates for synergistically activating epoxides to convert CO2. Therefore, the feasible construction of a series of imidazolium‐functionalized organic cationic polymers can bridge the gap between homogeneous and heterogeneous catalysis, thereby obtaining highly selective CO2 adsorption and simultaneous conversion ability. This Review describes the recent advancements made with regard to the design and synthesis of this type of polymeric networks having imidazolium functionality. They are considered as an outstanding heterogeneous catalyst for the cycloaddition of CO2 to epoxides. Based on the perspective from the design of building blocks to the synthesis of cationic polymers, the focus mainly lies on how to introduce imidazole units into the material backbone via a covalent linking approach and how to incorporate other active sites capable of activating CO2 and/or epoxides into such polymeric materials.