Parsing digital or analogue TCR performance through piconewton forces

αβ T-cell receptors (TCRs) recognize aberrant peptides bound to major histocompatibility complex molecules (pMHCs) on unhealthy cells, amplifying specificity and sensitivity through physical load placed on the TCR-pMHC bond during immunosurveillance. To understand this mechanobiology, TCRs stimulated by abundantly and sparsely arrayed epitopes (NP 366-374 /D b and PA 224-233 /D b , respectively) following in vivo influenza A virus infection were studied with optical tweezers. While certain NP repertoire CD8 T lymphocytes require many ligands for activation, others are digital, needing just few. Conversely, all PA TCRs perform digitally, exhibiting pronounced bond lifetime increases through sustained, energizing volleys of structural transitioning. Optimal digital performance is superior in vivo, correlating with ERK phosphorylation, CD3 loss, and activation marker upregulation in vitro . Given neoantigen array paucity, digital TCRs are likely critical for immunotherapies.

One Sentence Summary

Quality of ligand recognition in a T-cell repertoire is revealed through application of physical load on clonal T-cell receptor (TCR)-pMHC bonds.

Lon degrades stable substrates slowly but with enhanced processivity, redefining the attributes of a successful AAA+ protease

Lon is a widely distributed AAA+ (ATPases associated with diverse cellular activities) protease known for degrading poorly folded and damaged proteins and is often classified as a weak protein unfoldase. Here, using a Lon-degron pair from Mesoplasma florum (MfLon and MfssrA, respectively), we perform ensemble and single-molecule experiments to elucidate the molecular mechanisms underpinning MfLon function. Notably, we find that MfLon unfolds and degrades stably folded substrates and that translocation of these unfolded polypeptides occurs with a ∼6-amino-acid step size. Moreover, the time required to hydrolyze one ATP corresponds to the dwell time between steps, indicating that one step occurs per ATP-hydrolysis-fueled "power stroke." Comparison of MfLon to related AAA+ enzymes now provides strong evidence that HCLR-clade enzymes function using a shared power-stroke mechanism and, surprisingly, that MfLon is more processive than ClpXP and ClpAP. We propose that ample unfoldase strength and substantial processivity are features that contribute to the Lon family's evolutionary success.

Characterizing Biophysical Parameters of Single TCR-pMHC Interactions Using Optical Tweezers

αβ T cells are mechanosensors that leverage bioforces during immune surveillance for highly sensitive and specific antigen discrimination. Single-molecule studies are used to profile the initial TCRαβ-pMHC binding event, and various biophysical parameters can be identified. Isolating purified TCRαβ and pMHC molecules on a coverslip allows for direct measurements of the kinetics and conformational changes in the system and removes cellular components along the load pathway that may interfere with or mask subtle changes. Optical tweezers provide high resolution position and force information that map the bonding profile, including catch bond, and the ability to measure distinct conformational changes driven by forces. The present method describes the single-molecule optical tweezers assay setup, considerations, and execution. This model can be used for various TCR-pMHC pairs or expanded to measure a wide variety of receptor-ligand interactions operative in multiple biological systems.

Measuring αβ T-Cell Receptor-Mediated Mechanosensing Using Optical Tweezers Combined with Fluorescence Imaging

T-cell antigen receptors (TCRs) are mechanosensors, which initiate a signaling cascade upon ligand recognition resulting in T-cell differentiation, homeostasis, effector and regulatory functions. An optical trap combined with fluorescence permits direct monitoring of T-cell triggering in response to force application at various concentrations of peptide-bound major histocompatibility complex molecules (pMHC). The technique mimics physiological shear forces applied as cells crawl across antigen-presenting surfaces during immune surveillance. True single molecule studies performed on single cells profile force-bond lifetime, typically seen as a catch bond, and conformational change at the TCR-pMHC bond on the surface of the cell upon force loading. Together, activation and single molecule single cell studies provide chemical and physical triggering thresholds as well as insight into catch bond formation and quaternary structural changes of single TCRs. The present methods detail assay design, preparation, and execution, as well as data analysis. These methods may be applied to a wide range of pMHC-TCR interactions and have potential for adaptation to other receptor-ligand systems.

Single Molecule Force Spectroscopy Reveals Distinctions in Key Biophysical Parameters of αβ T-Cell Receptors Compared with Chimeric Antigen Receptors Directed at the Same Ligand

Chimeric antigen receptor (CAR) T-cell therapies exploit facile antibody-mediated targeting to elicit useful immune responses in patients. This work directly compares binding profiles of CAR and αβ T-cell receptors (TCR) with single cell and single molecule optical trap measurements against a shared ligand. DNA-tethered measurements of peptide-major histocompatibility complex (pMHC) ligand interaction in both CAR and TCR exhibit catch bonds with specific peptide agonist peaking at 25 and 14 pN, respectively. While a conformational transition is regularly seen in TCR-pMHC systems, that of CAR-pMHC systems is dissimilar, being infrequent, of lower magnitude, and irreversible. Slip bonds are observed with CD19-specific CAR T-cells and with a monoclonal antibody mapping to the MHC α2 helix but indifferent to the bound peptide. Collectively, these findings suggest that the CAR-pMHC interface underpins the CAR catch bond response to pMHC ligands in contradistinction to slip bonds for CARs targeting canonical ligands.

Screw loosening in an in vitro mid fibular osteotomy model under dynamic loading conditions

The purpose of this study was to evaluate the effect of cyclic loading on screw fixation in an experimental bone plating model. The test specimens consist of plated porcine fibulae subjected to cyclic compressive, bending and torsional loading. Breakaway torque measurements of orthopedic screws are found to be significantly less than the screws tightening torque. The breakaway torque for a given screw and tapped bone hole is found to be consistent after repeated tightening, and is proposed as a viable approach to quantify bone screw loosening. After cyclic loading at moderate levels no screw loosening was identified, but instead an apparent paradoxical tightening, as observed in breakaway torque measurements of the screws. Cyclic loading of a plated fibular fracture was not found to cause screw loosening unless accompanied by gross failure as found under excessive load levels.

Subtalar repositional arthrodesis for adult acquired flatfoot

Arthrodesis of the subtalar joint is well recognized treatment option for moderate or severe flatfoot associated with adult acquired flatfoot secondary to posterior tibial tendon dysfunction. The success of the subtalar arthrodesis is dependent on restoration of normal bony relationships in the hindfoot and midfoot. For this reason, a distinction is made between a repositional arthrodesis and the traditional in situ type of arthrodesis. An in vitro study of the adult acquired flatfoot identifies an anteroposterior subluxation of the subtalar articulation that can be corrected durably and reliably through a repositional talocalcaneal arthrodesis. Intraoperative reduction techniques are useful in obtaining reduction of the peritalar subluxation. There are certain clinical features that help identify patients with flatfoot deformities who are good candidates for subtalar fusion. As the pathoanatomy of the flatfoot deformity is better elucidated, treatment techniques are modified to better address the key elements of the deformity. A repositional subtalar arthrodesis was shown to produce excellent correction in a moderate to severe in vitro flatfoot example in the authors' clinical series.

Calcaneal malunions: results of a prognostic computed tomography classification system

Following nonoperative treatment of calcaneal fractures, some patients may develop a disabling malunion with associated posttraumatic arthritis of the subtalar joint, impingement of the peroneal tendons, and hindfoot mal-alignment. We present a computed tomography classification system for calcaneal malunions which guides treatment and is of prognostic significance. A prospective study was performed using this classification system on a series of 26 malunions treated over a 45-month period. Three distinct types of malunions were identified: type I, lateral wall exostosis without subtalar arthrosis; type II, lateral wall exostosis with subtalar arthrosis; and type III, lateral wall exostosis, subtalar arthrosis, and a varus malunion. The surgical treatment was determined by a protocol based on the specific type of malunion encountered. Results were evaluated using the Maryland Foot Score. There were 18 excellent, 5 good, and 3 fair results. Although outcomes deteriorated as malunion complexity increased, significant clinical improvement as a result of reconstructive surgery was noted in even the worst types of malunion. This algorithm is consistent, prognostic, and useful for the orthopaedic surgeon presented with a symptomatic calcaneal malunion of one of these types.