Mechanosensing through immunoreceptors

Abstract

The immune response is orchestrated by a variety of immune cells. The function of each cell is determined by the collective signals from various immunoreceptors, whose expression and activity depend on the developmental stages of the cell and its environmental context. Recent studies have highlighted the presence of mechanical force on several immunoreceptor-ligand pairs and the important role of force in regulating their interaction and function. In this Perspective, we use the T cell antigen receptor as an example with which to review the current understanding of the mechanosensing properties of immunoreceptors. We discuss the types of forces that immunoreceptors may encounter and the effects of force on ligand bonding, conformational change and the triggering of immunoreceptors, as well as the effects of force on the downstream signal transduction, cell-fate decisions and effector function of immune cells.

Fig. 1 |. Immune cells are subjected to external forces from, and apply endogenous forces to, their surroundings, which can be measured by biophysical techniques.

a, A lymphocyte rolling on the endothelium experiences forces. Blood flow applies a tangential force F_s to the cell center and a rotational torque T_s, which are supported by the tether force F_t at the rear with an inclined angle. The tangential component F_tx balances F_s. The normal component F_ty generates a torque F_ty × r × sinθ to balance T_s and the torque of the tangential force F_tx × r × cosθ. F_ty also induces a reaction F_c from the endothelium. b,c, During migration (b) and formation of the immunological synapse (c), a lymphocyte can apply endogenous forces, generated by actomyosin contraction and actin retrograde flow, on the ECM (b) or the APC (c). d, The TCR and CD8, localized at the tip of the microvillus of a T cell interacting with an APC, bind pMHC in the center, while LFA-1 binds to ICAM-1 at the periphery. CD45 is uniformly distributed. e–g, TFM (e), mPADs (f) and molecular tension probes (g) measure the endogenous force generated by cells and applied on receptors. Force is measured from the displacements of beads embedded in an elastic substrate to which the cell adheres (TFM), the deflections of elastic pillars to which the cell attaches (mPAD) and the unquenched fluorescence (molecular tension probe). Cellular-level forces on many receptors are measured by TFM and mPAD, which reflect only the tangential force components, because only lateral bead displacements and pillar deflections can be visualized by the microscope’s top view. By comparison, molecular tension probes measure tension along the long axis of the receptors. h, Tension gauge tethers fail when force exceeds a designed threshold, which thus limits the level of tension that the cell can apply on its receptors.

Fig. 2 |. Single-molecule force techniques for applying forces to immuneoreceptors on immune cells.

a–d, Atomic-force microscopy (a), biomembrane force probes (b), optical tweezers (c) and magnetic tweezers (d) apply force to single receptors on cells and analyze the effects on receptor–ligand interactions, protein conformational changes and cellular functions. Atomic-force microscopy (a), biomembrane force probes (b) and magnetic tweezers (d) tend to apply force normal to the bead–cell (or substrate) interface by pulling them apart. Optical tweezers (c) apply a force F_s by moving the cell normally or tangentially relative to the bead–cell interface, pulling the receptor along the normal direction or an inclined direction with a force F_t. F_t is equal to F_s in the normal case but is larger than F_s in the tangential case. The mechanical analysis in the tangential case is similar to that in Fig. 1a. The inclined angle, determined by the relative dimensions of the microvillus length l and the bead radius r by the equation F_t = F_s/sin{cos−¹[1/(1 + l/r)]}, is needed to generate both the normal force component F_ty and tangential force component F_tx to balance the forces (F_c and F_s) and torques (F_ty × r × sinθ and – F_tx × r × cosθ) on the bead. e, Normal motion of the T cell away from the substrate stretches the microvillus and pulls the TCR upward with a cell force F_c against the bond force F_t. f, Tangential motion of the T cell parallel to the substrate surface bends the microvillus and the TCR, assuming the root of the microvillus and the membrane anchor of the TCR can support bending moments (i.e., behave as a built-in end). g, If the microvillus root or the TCR membrane anchor, or both, behave(s) as a hinge incapable of supporting bending moment, the microvillus and/or the TCR would tilt to adjust its (their) orientation(s) to bear tension along its (their) long axis (axes).

Fig. 3 |. Force-regulated pMHC conformations determine TCR–MHC dynamic bonds.

a, When a TCR forms a catch bond with an agonistic pMHC (top), increasing force prolongs the bond lifetime at low forces, but the bond changes to a slip bond at high forces. When a TCR forms a slip bond with antagonistic pMHC (bottom), increasing force monotonically shortens the bond lifetime. b, A TCR catch bond with agonistic pMHC is initiated by force-enhanced engagement between the TCR’s CDR3 loops and the peptide’s ‘hotspot’ residues, followed by a force-induced betterment of the complementarity of the MHC–TCR interface (top). Force also induces separation of β₂m from the MHC heavy chain, which leads to MHC extension and rotation of the α₁α₂ domains toward the TCR; this further strengthens the TCR–MHC engagement allosterically. For a weak ligand, force is unable to induce the conformational changes noted above in the pMHC, which leads to slip bonds (bottom). c,d, Cancer-associated somatic mutations (c) or genotype variations (d) encoding mutant HLA molecules may also affect the susceptibility of MHC to conformational changes under force. Examples of the former are HLA-A2 F8V and A236, which form more hydrogen bonds between β₂m and the MHC heavy chains (c). As an example of the latter, HLA-B15 has more hydrogen bonds between β₂m and the MHC heavy chain than does HLA-B44 (d). Diagrams in Fig. 3c,d are modified from ref. .

Fig. 4 |. TCR mechanotransduction via dynamic catch.

a, Lifetime-versus-force curves of various bonds. Dynamic catch is a catch bond that results from the binding of CD8 to a negative-selection pMHC (–p) pre-engaged by a TCR ($t_{\text{TCR}+\text{CD}8}^{-\text{p}}$; purple). This is an emergent property from cooperative binding rather than a property intrinsic to the two arms of the trimeric complex, because both TCR–pMHC bimolecular interactions ($t_{\text{TCR}}^{-\text{p}}$; cyan) and pMHC–CD8 bimolecular interactions (t_CD8; red) behave as slip bonds. Interaction of the TCR with a positive-selection pHMC (+p) also forms a slip bond ($t_{\text{TCR}}^{+\text{p}}$; green) with a lifetime slightly shorter than that of the bond with a negative-selection pMHC. However, it does not induce dynamic catch formation. b, Up to three extracellular interactions participate in the binding of a thymocyte to pMHC, which involve the TCR (left), CD8 (middle), or both (right). c, Minimal model for dynamic catch formation. For elucidation of the inner workings of the dynamic catch, intracellular interactions are shown here, which include the binding of Lck to TCR–CD3 pre-bound by pMHC and the phosphorylation of TCR–CD3 by Lck (Step 1; left), the recruitment of CD8 to phosphorylated TCR–CD3 by Lck (Step 2; middle), and the binding of CD8 to pMHC cooperatively to stabilize the trimeric complex (Step 3; right). During the recognition of antigen by the TCR and signal initiation, mechanical forces may apply to both extracellular interactions and intracellular interactions, which may elicit catch bonds or slip bonds, induce protein conformational change and regulate enzymatic activity.