Mechanical regulation of lymphocyte activation and function

Abstract

Mechanosensing, or how cells sense and respond to the physical environment, is crucial for many aspects of biological function, ranging from cell movement during development to cancer metastasis, the immune response and gene expression driving cell fate determination. Relevant physical stimuli include the stiffness of the extracellular matrix, contractile forces, shear flows in blood vessels, complex topography of the cellular microenvironment and membrane protein mobility. Although mechanosensing has been more widely studied in non-immune cells, it has become increasingly clear that physical cues profoundly affect the signaling function of cells of the immune system. In this Review, we summarize recent studies on mechanical regulation of immune cells, specifically lymphocytes, and explore how the force-generating cytoskeletal machinery might mediate mechanosensing. We discuss general principles governing mechanical regulation of lymphocyte function, spanning from the molecular scale of receptor activation to cellular responses to mechanical stimuli.

Keywords: B lymphocytes; Cytoskeletal forces; Mechanoimmunology; Mechanosensing; Signaling; T lymphocytes.

Fig. 1.

Mechanical regulation of immune receptor activation. (A) Kinetic proofreading model of T cell receptor activation. Left, cartoon depicting the TCR–pMHC bond at the IS. Right, if the TCR binds to a specific peptide, the longer bond lifetime leads to phosphorylation of ITAMs by Lck, triggering T cell activation. Binding to a non-specific peptide does not allow for ITAM phosphorylation, aborting T cell activation. (B) At the T cell–APC contact site, actin and myosin exert force on the TCR and the surrounding microvillus. The applied force alters the conformation of the TCR, enabling recruitment of signaling molecules. (C) Bond lifetime (typically measured in seconds) versus applied force for a catch bond (blue) and a slip bond (red). (D) Schematic of the BCR. (E) Graphical representation of force dependence of B cell activation. IgM BCRs (red) show low levels of activation at low force. Activation increases as IgM BCRs experience higher forces and then plateau. IgG or IgE BCRs show little or no force-dependent activation. Figure created with BioRender.com

Fig. 2.

Force generation in T and B cells. (A) Actin and microtubule cytoskeletal networks at the lymphocyte IS. The expansion depicts distinct cytoskeletal arrangements in T and B cells. (B,C) Spatio-temporal force generation in T and B cells. Left, heatmaps representing spatial force patterning. Warm and cool colors depict higher and lower forces, respectively. T and B cells show coordinated centripetal force patterning with the highest forces exerted towards the synapse periphery. Dotted red circles indicate actin-dependent 3D protrusive forces observed in T cells. B cells uniquely show a pool of central non-coordinated forces likely originating from protrusive actin patches. Right, force versus time graphs for different lymphocyte types [Jurkat CD4⁺ T cells plated on activating 1.2 kPa hydrogels, based on data from Hui et al. (2015), and primary human CD4⁺ T cells on micropillar arrays (spring constant: 1.4 nN/μm), based on data from Bashour et al. (2014) (B), and DT40 B cells interacting with antigen-coated 1 kPa hydrogels, based on data from Wang et al. (2018) (C)]. Figure created with BioRender.com.

Fig. 3.

Experimental approaches to assess mechanical regulation of immune function. Left, the effect of APC stiffness has been traditionally studied using planar polyacrylamide hydrogels and elastomeric surfaces. Newer methodologies include 2D and 3D biomimetic alginate-RGD (Arg-Gly-Asp peptide) hydrogel scaffolds and antigen-coated lipid droplets with tunable surface tension. Right, surfaces, often PDMS-based, with topographical features such as nanoridges and grids, as well as micropillars, are used to examine the effect of APC topography on lymphocyte function. Alternative approaches include antigen-functionalized ZnO nanowires with various lengths and porous anodic aluminum oxide (AAO) membranes. APC geometry has been mimicked using spherical and ellipsoidal poly(lactide-glycolide) microparticles. Bottom, studies on ligand mobility frequently employ supported lipid bilayers, which allow lateral mobility of receptor ligands. Ligand distribution has been investigated with 2D patterning approaches such as microcontact printing and block copolymer micellar nanolithography. Figure created with BioRender.com.