Mechanosensing drives acuity of αβ T-cell recognition

Abstract

T lymphocytes use surface αβ T-cell receptors (TCRs) to recognize peptides bound to MHC molecules (pMHCs) on antigen-presenting cells (APCs). How the exquisite specificity of high-avidity T cells is achieved is unknown but essential, given the paucity of foreign pMHC ligands relative to the ubiquitous self-pMHC array on an APC. Using optical traps, we determine physicochemical triggering thresholds based on load and force direction. Strikingly, chemical thresholds in the absence of external load require orders of magnitude higher pMHC numbers than observed physiologically. In contrast, force applied in the shear direction (∼10 pN per TCR molecule) triggers T-cell Ca 2+ flux with as few as two pMHC molecules at the interacting surface interface with rapid positional relaxation associated with similarly directed motor-dependent transport via ∼8-nm steps, behaviors inconsistent with serial engagement during initial TCR triggering. These synergistic directional forces generated during cell motility are essential for adaptive T-cell immunity against infectious pathogens and cancers.mechanosensor | T-cell receptor | T-cell activation | optical tweezers | cellular force relaxation T he T-cell receptor (TCR) expressed on T lymphocytes of the adaptive immune system is a stout and squat (12-nm wide × 8-nm tall) multisubunit surface complex with a ligand binding moiety that is an αβ disulfide-linked heterodimer buttressed by the associated invariant CD3 subunits (1-3). The αβ chains are each encoded by V and J gene segments and in the case of the β, a D segment as well (4). The clone-specific TCR unique to each T lymphocyte endows mammals with the capacity to detect perturbations in host cellular function resulting from myriad infectious pathogens, physical damage (thermal, irradiation, etc.), or premalignant or malignant cellular transformations while averting strong self-reactivities that could induce autoimmunity (5).Signaling is initiated through ligation of the clonotype by its cognate antigenic peptide bound to an MHC molecule (pMHC) and displayed on the surface of antigen-presenting cells (APCs) (6, 7). Ligation impacts disposition and function of the associated CD3 dimers (CD3 γ, CD3 δ, and CD3ζζ) as well as the transmembrane domains of the TCR heterodimer and CD3 subunits that interdigitate in the membrane to signal into the cytoplasm (8, 9). A cascade of intracellular events involving phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) on the CD3 cytoplasmic domains with subsequent ZAP70 activation and downstream signaling follows (10, 11). Membrane-associated CD8 and CD4 coreceptors, marking cytotoxic T lymphocytes (CTLs) and helper T cells, respectively, function to bring the membrane-anchored Src family tyrosine kinase Lck to the TCR-pMHC complex for the initiation of ITAM phosphorylation. Signaling, in turn, leads to a transient rise of cytosolic Ca 2+ and other biochemical events resulting in transcriptional activation, ultimately resulting in developmental decisions or effector functi...

Molecular mechanisms of T cell sensitivity to antigen

“…In summary, although forces are likely to be generated at the contact interface, mechanisms may be operating to reduce the tension on the TCR/pMHC interaction. Although beyond the scope of this review, it has been suggested that applied forces on the TCR may be required to induce TCR signaling, but there is also evidence that TCR signaling can take place without pMHC ligands and hence an applied force . The role of force in mechanically transducing signals is beyond the scope of this work and we refer the reader elsewhere…”

ERM-Dependent Assembly of T-Cell Receptor Signaling and Co-stimulatory Molecules on Microvilli Prior to Activation

“…The connection of TCR components to the actin filaments inside microvilli also allows T cells to exert significant forces on these components during the first few seconds following encounter of the APC, and these forces may stabilize the TCR-MHC-peptide bonds, as suggested by recent biophysical measurements (77)(78)(79)(80). As the T cell interaction with the APC persists, the phosphorylated ERM linkers become inactivated by dephosphorylation (63) and release the connection of the cortical actin cytoskeleton and the plasma membrane in the vicinity of the TCR machineries.…”

Lymphocyte mechanotransduction: The regulatory role of cytoskeletal dynamics in signaling cascades and effector functions

“…Observing mechanotransduction in the immune system is more complex than in adherent cells; however, recent technological advances have enabled scientists to directly measure forces exerted by leukocytes during an immune repsonse. 165,166 It is becoming evident that external forces are sensed by lymphocytes during activation and inhibition at the IS, and these are converted into biochemical signals that impact the immune response. Often, it is the cytoskeleton that mediates between external physical stimuli and signaling pathways.…”