Soft Polydimethylsiloxane-Supported Lipid Bilayers for Studying T Cell Interactions

Target cell tension regulates macrophage trogocytosis

Macrophages are known to engulf small membrane fragments, or trogocytose, target cells and pathogens, rather than fully phagocytose them. However, little is known about what causes macrophages to choose trogocytosis versus phagocytosis. Here, we report that cortical tension of target cells is a key regulator of macrophage trogocytosis. At low tension, macrophages will preferentially trogocytose antibody-opsonized cells, while at high tension they tend towards phagocytosis. Using model vesicles, we demonstrate that macrophages will rapidly switch from trogocytosis to phagocytosis when membrane tension is increased. Stiffening the cortex of target cells also biases macrophages to phagocytose them, a trend that can be countered by increasing antibody surface density and is captured in a mechanical model of trogocytosis. This work suggests that a distinct molecular pathway for trogocytosis is not required to explain differences in trogocytosis among target cell types and points to a mechanism for target cells to modulate trogocytosis.

Lipid membranes supported by polydimethylsiloxane substrates with designed geometry

The membrane curvature of cells and intracellular compartments continuously adapts to enable cells to perform vital functions, from cell division to signal trafficking. Understanding how membrane geometry affects these processes in vivo is challenging because of the biochemical and geometrical complexity as well as the short time and small length scales involved in cellular processes. By contrast, in vitro model membranes with engineered curvature would provide a versatile platform for this investigation and applications to biosensing and biocomputing. Here, we present a strategy that allows fabrication of lipid membranes with designed shape by combining 3D micro-printing and replica-molding lithography with polydimethylsiloxane to create curved micrometer-sized scaffolds with virtually any geometry. The resulting supported lipid membranes are homogeneous and fluid. We demonstrate the versatility of the system by fabricating structures of interesting combinations of mean and Gaussian curvature. We study the lateral phase separation and how local curvature influences the effective diffusion coefficient. Overall, we offer a bio-compatible platform for understanding curvature-dependent cellular processes and developing programmable bio-interfaces for living cells and nanostructures.

Functional Integration of Synthetic Cells into 3D Microfluidic Devices for Artificial Organ-On-Chip Technologies

Microfluidics plays a pivotal role in organ-on-chip technologies and in the study of synthetic cells, especially in the development and analysis of artificial cell models. However, approaches that use synthetic cells as integral functional components for microfluidic systems to shape the microenvironment of natural living cells cultured on-chip are not explored. Here, colloidosome-based synthetic cells are integrated into 3D microfluidic devices, pioneering the concept of synthetic cell-based microenvironments for organs-on-chip. Methods are devised to create dense and stable networks of silica colloidosomes, enveloped by supported lipid bilayers, within microfluidic channels. These networks promote receptor-ligand interactions with on-chip cultured cells. Furthermore, a technique is introduced for the controlled release of growth factors from the synthetic cells into the channels, using a calcium alginate-based hydrogel formation within the colloidosomes. To demonstrate the potential of the technology, a modular plug-and-play lymph-node-on-a-chip prototype that guides the expansion of primary human T cells by stimulating receptor ligands on the T cells and modulating their cytokine environment is presented. This integration of synthetic cells into microfluidic systems offers a new direction for organ-on-chip technologies and suggests further avenues for exploration in potential therapeutic applications.

Soft Synthetic Cells with Mobile Membrane Ligands for Ex Vivo Expansion of Therapy-Relevant T Cell Phenotypes

The expansion of T cells ex vivo is crucial for effective immunotherapy but currently limited by a lack of expansion approaches that closely mimic in vivo T cell activation. Taking inspiration from bottom-up synthetic biology, a new synthetic cell technology is introduced based on dispersed liquid-liquid phase-separated droplet-supported lipid bilayers (dsLBs) with tunable biochemical and biophysical characteristics, as artificial antigen presenting cells (aAPCs) for ex vivo T cell expansion. These findings obtained with the dsLB technology reveal three key insights: first, introducing laterally mobile stimulatory ligands on soft aAPCs promotes expansion of IL-4/IL-10 secreting regulatory CD8+ T cells, with a PD-1 negative phenotype, less prone to immune suppression. Second, it is demonstrated that lateral ligand mobility can mask differential T cell activation observed on substrates of varying stiffness. Third, dsLBs are applied to reveal a mechanosensitive component in bispecific Her2/CD3 T cell engager-mediated T cell activation. Based on these three insights, lateral ligand mobility, alongside receptor- and mechanosignaling, is proposed to be considered as a third crucial dimension for the design of ex vivo T cell expansion technologies.

Quantifying biomolecular organisation in membranes with brightness-transit statistics

Cells crucially rely on the interactions of biomolecules at their plasma membrane to maintain homeostasis. Yet, a methodology to systematically quantify biomolecular organisation, measuring diffusion dynamics and oligomerisation, represents an unmet need. Here, we introduce the brightness-transit statistics (BTS) method based on fluorescence fluctuation spectroscopy and combine information from brightness and transit times to elucidate biomolecular diffusion and oligomerisation in both cell-free in vitro and in vitro systems incorporating living cells. We validate our approach in silico with computer simulations and experimentally using oligomerisation of EGFP tethered to supported lipid bilayers. We apply our pipeline to study the oligomerisation of CD40 ectodomain in vitro and endogenous CD40 on primary B cells. While we find a potential for CD40 to oligomerize in a concentration or ligand depended manner, we do not observe mobile oligomers on B cells. The BTS method combines sensitive analysis, quantification, and intuitive visualisation of dynamic biomolecular organisation.

Mechanical regulation of lymphocyte activation and function

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.

Nuclear curvature determines Yes-associated protein localization and differentiation of mesenchymal stem cells

Controlling mesenchymal stem cell (MSC) differentiation remains a critical challenge in MSCs' therapeutic application. Numerous biophysical and mechanical stimuli influence stem cell fate; however, their relative efficacy and specificity in mechanically directed differentiation remain unclear. Yes-associated protein (YAP) is one key mechanosensitive protein that controls MSC differentiation. Previous studies have related nuclear mechanics with YAP activity, but we still lack an understanding of what nuclear deformation specifically regulates YAP and its relationship with mechanical stimuli. Here, we report that maximum nuclear curvature is the most precise biophysical determinant for YAP mechanotransduction-mediated MSC differentiation and is a relevant parameter for stem cell-based therapies. We employed traction force microscopy and confocal microscopy to characterize the causal relationships between contractility and nuclear deformation in regulating YAP activity in MSCs. We observed that an increase in contractility compresses nuclei anisotropically, whereby the degree of asymmetric compression increased the bending curvature of the nuclear membrane. We then examined membrane curvature and tension using thin micropatterned adhesive substrate lines and an FRET-based tension sensor, revealing the direct role of curvature in YAP activity driven by both active and passive nuclear import. Finally, we employed micropatterned lines to control nuclear curvature and precisely direct MSC differentiation. This work illustrates that nuclear curvature subsumes other biophysical aspects to control YAP-mediated differentiation in MSCs and may provide a deterministic solution to some of the challenges in mesenchymal stem cell therapies.

Unprecedented Approach of Fabrication and Analysis of a Bioactive PDMS/Hydroxyapatite/Graphene Nanocomposite Scaffold with a Vascular Channel to Combat Carcinogenesis

In the present investigation, natural bone-derived hydroxyapatite (HA, 2 wt %) and/or exfoliated graphene (Gr, 0.1 wt %)-embedded polydimethylsiloxane (PDMS) elastomeric films were prepared using a vascular method. The morphology, mechanical properties, crystallinity, and chemical structure of the composite films were evaluated. The in vitro biodegradation kinetics of the films indicates their adequate physiological stability. Most of the results favored PDMS/HA/Gr as a best composite scaffold having more than 703% elongation. A simulation study of the microfluidic vascular channel of the PDMS/HA/Gr scaffold suggests that the pressure drop at the outlet became greater (from 1.19 to 0.067 Pa) unlike velocity output (from 0.071 to 0.089 m/s), suggesting a turbulence-free laminar flow. Our bioactive scaffold material, PDMS/HA/Gr, showed highest cytotoxicity toward the lung cancer and breast cancer cells through Runx3 protein-mediated cytotoxic T lymphocyte (CTL) generation. Our data and predicted mechanism also suggested that the PDMS/HA/Gr-supported peripheral blood mononuclear cells (PBMCs) not only increased the generation of CTL but also upregulated the expression of RUNX3. Since the PDMS/HA/Gr scaffold-supported Runx3 induced CTL generation caused maximum cell cytotoxicity of breast cancer (MCF-7) and lung cancer (A549) cells, PDMS/HA/Gr can be treated as an excellent potential candidate for CTL-mediated cancer therapy.

PDMS as a Substrate for Lipid Bilayers

PDMS (polydimethylsiloxane) is a cheap, optically clear polymer that is elastic and can be easily and quickly fabricated into a wide array of microscale and nanoscale architectures, making it a versatile substrate for biophysical experiments on cell membranes. It is easy to imagine many new experiments will be devised that require a bilayer to be placed upon a substrate that is flexible or easily cast into a desired geometry, such as in lab-on-a-chip, organ-on-chip, and microfluidic applications, or for building accurate membrane models that replicate the surface structure and elasticity of the cytoskeleton. However, PDMS has its limitations, and the extent to which the behavior of membranes is affected on PDMS has not been fully explored. We use AFM and fluorescence optical microscopy to investigate the use of PDMS as a substrate for the formation and study of supported lipid bilayers (SLBs). Lipid bilayers form on plasma-treated PDMS and show free diffusion and normal phase transitions, confirming its suitability as a model bilayer substrate. However, lipid-phase separation on PDMS is severely restricted due to the pinning of domains to surface roughness, resulting in the cessation of lateral hydrodynamic flow. We show the high-resolution porous structure of PDMS and the extreme smoothing effect of oxygen plasma treatment used to hydrophilize the surface, but this is not flat enough to allow domain formation. We also observe bilayer degradation over hour timescales, which correlates with the known hydrophobic recovery of PDMS, and establish a critical water contact angle of 30°, above which bilayers degrade or not form at all. Care must be taken as incomplete surface oxidation and hydrophobic recovery result in optically invisible membrane disruption, which will also be transparent to fluorescence microscopy and lipid diffusion measurements in the early stages.

Antigen discrimination by T cells relies on size-constrained microvillar contact

T cells use finger-like protrusions called 'microvilli' to interrogate their targets, but why they do so is unknown. To form contacts, T cells must overcome the highly charged, barrier-like layer of large molecules forming a target cell's glycocalyx. Here, T cells are observed to use microvilli to breach a model glycocalyx barrier, forming numerous small (<0.5 μm diameter) contacts each of which is stabilized by the small adhesive protein CD2 expressed by the T cell, and excludes large proteins including CD45, allowing sensitive, antigen dependent TCR signaling. In the absence of the glycocalyx or when microvillar contact-size is increased by enhancing CD2 expression, strong signaling occurs that is no longer antigen dependent. Our observations suggest that, modulated by the opposing effects of the target cell glycocalyx and small adhesive proteins, the use of microvilli equips T cells with the ability to effect discriminatory receptor signaling.

The magnitude of LFA-1/ICAM-1 forces fine-tune TCR-triggered T cell activation

T cells defend against cancer and viral infections by rapidly scanning the surface of target cells seeking specific peptide antigens. This key process in adaptive immunity is sparked upon T cell receptor (TCR) binding of antigens within cell-cell junctions stabilized by integrin (LFA-1)/intercellular adhesion molecule-1 (ICAM-1) complexes. A long-standing question in this area is whether the forces transmitted through the LFA-1/ICAM-1 complex tune T cell signaling. Here, we use spectrally encoded DNA tension probes to reveal the first maps of LFA-1 and TCR forces generated by the T cell cytoskeleton upon antigen recognition. DNA probes that control the magnitude of LFA-1 force show that F>12 pN potentiates antigen-dependent T cell activation by enhancing T cell-substrate engagement. LFA-1/ICAM-1 mechanical events with F>12 pN also enhance the discriminatory power of the TCR when presented with near cognate antigens. Overall, our results show that T cells integrate multiple channels of mechanical information through different ligand-receptor pairs to tune function.

The interplay between membrane topology and mechanical forces in regulating T cell receptor activity

T cells are critically important for host defense against infections. T cell activation is specific because signal initiation requires T cell receptor (TCR) recognition of foreign antigen peptides presented by major histocompatibility complexes (pMHC) on antigen presenting cells (APCs). Recent advances reveal that the TCR acts as a mechanoreceptor, but it remains unclear how pMHC/TCR engagement generates mechanical forces that are converted to intracellular signals. Here we propose a TCR Bending Mechanosignal (TBM) model, in which local bending of the T cell membrane on the nanometer scale allows sustained contact of relatively small pMHC/TCR complexes interspersed among large surface receptors and adhesion molecules on the opposing surfaces of T cells and APCs. Localized T cell membrane bending is suggested to increase accessibility of TCR signaling domains to phosphorylation, facilitate selective recognition of agonists that form catch bonds, and reduce noise signals associated with slip bonds.

A Study of the Effects of Plasma Surface Treatment on Lipid Bilayers Self-Spreading on a Polydimethylsiloxane Substrate under Different Treatment Times

Plasma-treated poly(dimethylsiloxane) (PDMS)-supported lipid bilayers are used as functional tools for studying cell membrane properties and as platforms for biotechnology applications. Self-spreading is a versatile method for forming lipid bilayers. However, few studies have focused on the effect of plasma treatment on self-spreading lipid bilayer formation. In this paper, we performed lipid bilayer self-spreading on a PDMS surface with different treatment times. Surface characterization of PDMS treated with different treatment times is evaluated by AFM and SEM, and the effects of plasma treatment of the PDMS surface on lipid bilayer self-spreading behavior is investigated by confocal microscopy. The front-edge velocity of lipid bilayers increases with the plasma treatment time. By theoretical analyses with the extended-DLVO modeling, we find that the most likely cause of the velocity change is the hydration repulsion energy between the PDMS surface and lipid bilayers. Moreover, the growth behavior of membrane lobes on the underlying self-spreading lipid bilayer was affected by topography changes in the PDMS surface resulting from plasma treatment. Our findings suggest that the growth of self-spreading lipid bilayers can be controlled by changing the plasma treatment time.