Stiff substrates enhance monocytic cell capture through E-selectin but not P-selectin

Let's Get Rolling: Precise Control of Microfluidic Assay Conditions to Recapitulate Selectin-Mediated Rolling Interactions of the Leukocyte Adhesion Cascade

The leukocyte adhesion cascade governs the recruitment of circulating immune cells from the vasculature to distal sites. The initial adhesive interactions between cell surface ligands displaying sialyl-LewisX (sLeX) and endothelial E- and P-selectins serve to slow the cells down enough to interact more closely with the surface, polarize, and exit into the tissues. Therefore, precise microfluidic assays are critical in modeling how well immune cells can interact and "roll" on selectins to slow down enough to complete further steps of the cascade. Here, we present a systematic protocol for selectin mediated rolling on recombinant surfaces and endothelial cell monolayers on polyacrylamide gels of varying stiffness. We also describe step-by-step the protocol for setting up and performing the experiment and how to analyze and present the data collected. This protocol serves to simplify and detail the procedure needed to investigate the initial selectin-mediated interactions of immune cells with the vasculature. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Preparing dishes for cell rolling experiments Basic Protocol 2: Fabrication of polyacrylamide gels for cell rolling experiments Alternate Protocol 1: Protein conjugation with N6 linker Alternate Protocol 2: HUVEC culturing for monolayers Basic Protocol 3: Conducting cell rolling experiments on polyacrylamide gels Basic Protocol 4: ImageJ analysis of cell rolling movies Basic Protocol 5: Quantification of Fc site density on a surface (e.g., for Fc chimeras).

Matrix stiffness, endothelial dysfunction and atherosclerosis

Atherosclerosis (AS) is the leading cause of the human cardiovascular diseases (CVDs). Endothelial dysfunction promotes the monocytes infiltration and inflammation that participate fundamentally in atherogenesis. Endothelial cells (EC) have been recognized as mechanosensitive cells and have different responses to distinct mechanical stimuli. Emerging evidence shows matrix stiffness-mediated EC dysfunction plays a vital role in vascular disease, but the underlying mechanisms are not yet completely understood. This article aims to summarize the effect of matrix stiffness on the pro-atherosclerotic characteristics of EC including morphology, rigidity, biological behavior and function as well as the related mechanical signal. The review also discusses and compares the contribution of matrix stiffness-mediated phagocytosis of macrophages and EC to AS progression. These advances in our understanding of the relationship between matrix stiffness and EC dysfunction open the avenues to improve the prevention and treatment of now-ubiquitous atherosclerotic diseases.

Receptor-Ligand Binding: Effect of Mechanical Factors

Gaining insight into the in situ receptor-ligand binding is pivotal for revealing the molecular mechanisms underlying the physiological and pathological processes and will contribute to drug discovery and biomedical application. An important issue involved is how the receptor-ligand binding responds to mechanical stimuli. This review aims to provide an overview of the current understanding of the effect of several representative mechanical factors, such as tension, shear stress, stretch, compression, and substrate stiffness on receptor-ligand binding, wherein the biomedical implications are focused. In addition, we highlight the importance of synergistic development of experimental and computational methods for fully understanding the in situ receptor-ligand binding, and further studies should focus on the coupling effects of these mechanical factors.

Microfluidic model of monocyte extravasation reveals the role of hemodynamics and subendothelial matrix mechanics in regulating endothelial integrity

Extravasation of circulating cells is an essential process that governs tissue inflammation and the body's response to pathogenic infection. To initiate anti-inflammatory and phagocytic functions within tissues, immune cells must cross the vascular endothelial barrier from the vessel lumen to the subluminal extracellular matrix. In this work, we present a microfluidic approach that enables the recreation of a three-dimensional, perfused endothelial vessel formed by human endothelial cells embedded within a collagen-rich matrix. Monocytes are introduced into the vessel perfusate, and we investigate the role of luminal flow and collagen concentration on extravasation. In vessels conditioned with the flow, increased monocyte adhesion to the vascular wall was observed, though fewer monocytes extravasated to the collagen hydrogel. Our results suggest that the lower rates of extravasation are due to the increased vessel integrity and reduced permeability of the endothelial monolayer. We further demonstrate that vascular permeability is a function of collagen hydrogel mass concentration, with increased collagen concentrations leading to elevated vascular permeability and increased extravasation. Collectively, our results demonstrate that extravasation of monocytes is highly regulated by the structural integrity of the endothelial monolayer. The microfluidic approach developed here allows for the dissection of the relative contributions of these cues to further understand the key governing processes that regulate circulating cell extravasation and inflammation.

The principles of directed cell migration

Cells have the ability to respond to various types of environmental cues, and in many cases these cues induce directed cell migration towards or away from these signals. How cells sense these cues and how they transmit that information to the cytoskeletal machinery governing cell translocation is one of the oldest and most challenging problems in biology. Chemotaxis, or migration towards diffusible chemical cues, has been studied for more than a century, but information is just now beginning to emerge about how cells respond to other cues, such as substrate-associated cues during haptotaxis (chemical cues on the surface), durotaxis (mechanical substrate compliance) and topotaxis (geometric features of substrate). Here we propose four common principles, or pillars, that underlie all forms of directed migration. First, a signal must be generated, a process that in physiological environments is much more nuanced than early studies suggested. Second, the signal must be sensed, sometimes by cell surface receptors, but also in ways that are not entirely clear, such as in the case of mechanical cues. Third, the signal has to be transmitted from the sensing modules to the machinery that executes the actual movement, a step that often requires amplification. Fourth, the signal has to be converted into the application of asymmetric force relative to the substrate, which involves mostly the cytoskeleton, but perhaps other players as well. Use of these four pillars has allowed us to compare some of the similarities between different types of directed migration, but also to highlight the remarkable diversity in the mechanisms that cells use to respond to different cues provided by their environment.

Integrin cross-talk modulates stiffness-independent motility of CD4+ T lymphocytes

To carry out their physiological responsibilities, CD4+ T lymphocytes interact with various tissues of different mechanical properties. Recent studies suggest that T cells migrate upstream on surfaces expressing intracellular adhesion molecule-1 (ICAM-1) through interaction with leukocyte function-associated antigen-1 (α_Lβ₂) (LFA-1) integrins. LFA-1 likely behaves as a mechanosensor, and thus we hypothesized that substrate mechanics might affect the ability of LFA-1 to support upstream migration of T cells under flow. Here we measured motility of CD4+ T lymphocytes on polyacrylamide gels with predetermined stiffnesses containing ICAM-1, vascular cell adhesion molecule-1 (VCAM-1), or a 1:1 mixture of VCAM-1/ICAM-1. Under static conditions, we found that CD4+ T cells exhibit an increase in motility on ICAM-1, but not on VCAM-1 or VCAM-1/ICAM-1 mixed, surfaces as a function of matrix stiffness. The mechanosensitivity of T-cell motility on ICAM-1 is overcome when VLA-4 (very late antigen-4 [α₄β₁]) is ligated with soluble VCAM-1. Last, we observed that CD4+ T cells migrate upstream under flow on ICAM-1-functionalized hydrogels, independent of substrate stiffness. In summary, we show that CD4+ T cells under no flow respond to matrix stiffness through LFA-1, and that the cross-talk of VLA-4 and LFA-1 can compensate for deformable substrates. Interestingly, CD4+ T lymphocytes migrated upstream on ICAM-1 regardless of the substrate stiffness, suggesting that flow can compensate for substrate stiffness.

miRNA-mediated macrophage behaviors responding to matrix stiffness and ox-LDL

Atherosclerosis is one of the leading causes of morbidity and mortality, mainly due to the immune response triggered by the recruitment of monocytes/macrophages in the artery wall. Accumulating evidence have shown that matrix stiffness and oxidized low-density lipoproteins (ox-LDL) play important roles in atherosclerosis through modulating cellular behaviors. However, whether there is a synergistic effect for ox-LDL and matrix stiffness on macrophages behavior has not been explored yet. In this study, we developed a model system to investigate the synergistic role of ox-LDL and matrix stiffness on macrophage behaviors, such as migration, inflammatory and apoptosis. We found that there was a matrix stiffness-dependent behavior of monocyte-derived macrophages stimulated with ox-LDL. What's more, macrophages were more sensitive to ox-LDL on the stiff matrices compared to cells cultured on the soft matrices. Through next-generation sequencing, we identified miRNAs in response to matrix stiffness and ox-LDL and predicted pathways that showed the capability of miRNAs in directing macrophages fates. Our study provides a novel understanding of the important synergistic role of ox-LDL and matrix stiffness in modulating macrophages behaviors, especially through miRNAs signaling pathways, which could be potential key regulators in atherosclerosis and immune-targeted therapies.

Stiffness Sensing by Cells

Physical stimuli are essential for the function of eukaryotic cells, and changes in physical signals are important elements in normal tissue development as well as in disease initiation and progression. The complexity of physical stimuli and the cellular signals they initiate are as complex as those triggered by chemical signals. One of the most important, and the focus of this review, is the effect of substrate mechanical properties on cell structure and function. The past decade has produced a nearly exponentially increasing number of mechanobiological studies to define how substrate stiffness alters cell biology using both purified systems and intact tissues. Here we attempt to identify common features of mechanosensing in different systems while also highlighting the numerous informative exceptions to what in early studies appeared to be simple rules by which cells respond to mechanical stresses.

Human Neutrophils Will Crawl Upstream on ICAM-1 If Mac-1 Is Blocked

The recruitment of neutrophils to sites of inflammatory insult is a hallmark of the innate immune response. Neutrophil recruitment is regulated by a multistep process that includes cell rolling, activation, adhesion, and transmigration through the endothelium commonly referred to as the leukocyte adhesion cascade. After selectin-mediated braking, neutrophils migrate along the activated vascular endothelium on which ligands, including intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), are expressed. Previous studies have shown that two cells that commonly home from blood vessel to tissue-T cells and hematopoietic stem and progenitor cells-use the integrin lymphocyte functional antigen-1 (LFA-1) to migrate against the direction of shear flow once adherent on ICAM-1 surfaces. Like T cells and hematopoietic stem and progenitor cells, neutrophils express LFA-1, but they also express macrophage-1 antigen (Mac-1), which binds to ICAM-1. Previous reports have shown that neutrophils will not migrate against the direction of flow on ICAM-1, but we hypothesized this was due to the influence of Mac-1. Here, we report that both the HL-60 neutrophil-like cell line and primary human neutrophils can migrate against the direction of fluid flow on ICAM-1 surfaces via LFA-1 if Mac-1 is blocked; otherwise, they migrate downstream. We demonstrate this both on ICAM-1 surfaces and on activated endothelium. In sum, both LFA-1 and Mac-1 binding ICAM-1 play a critical role in determining the direction of neutrophil migration along the endothelium, and their interaction may play an important role in controlling neutrophil trafficking during inflammation.

On Stability of Specific Adhesion of Particles to Membranes in Simple Shear Flow

Adhesion of carrier particles to the luminal surface of endothelium under hemodynamic flow conditions is critical for successful vascular drug delivery. Endothelial cells line the inner surface of blood vessels. The effect of mechanical behavior of this compliant surface on the adhesion of blood-borne particles is unknown. In this contribution, we use a phase-plane method, first developed by Hammer and Lauffenburger [Biophysical Journal, 52, 475 (1987)], to analyze the stability of specific adhesion of a spherical particle to a compliant interface layer. We construct a phase diagram that predicts the state of particle adhesion, subjected to an incident simple shear flow, in terms of interfacial elasticity, shear rate, binding affinity of cell adhesive molecules, and their surface density. The main conclusion is that the local deformation of the flexible interface inhibits the stable adhesion of the particle. In comparison with adhesion to a rigid substrate, a greater ligand density is required to establish a stable adhesion between a particle and a compliant interface. The results can be used for the rational design of particles in vascular drug delivery.

Motile Dendritic Cells Sense and Respond to Substrate Geometry

Dendritic cell (DC) migration is required for efficient presentation of antigen to T cells and the initiation of an adaptive immune response. In spite of its importance, many aspects of DC migration have not been characterized. DCs encounter a variety of environments with different stiffness and geometry, but the effect of these parameters on DC migration has not yet been determined. We addressed this question by comparing DC motility on standard migration surfaces (polydimethylsiloxane (PDMS)-coated coverslips) and micropost array detectors (mPADs). These two surfaces differ in both stiffness and geometry. We found that DC migration was affected by substrate type, with significant increases in speed and significant decreases in persistence time on mPADs made of PDMS as compared to spin-coated PDMS coverslips. To determine whether the geometry or compliance of the post arrays was responsible for these changes in DC migration, we quantified DC motility on mPADs of identical geometry but different stiffness. Migration was indistinguishable on these mPADs, suggesting that DCs are responsive to geometry of ligand presentation and not stiffness. Further, by micropatterning ligands on flat PDMS surfaces in similar geometries to the mPAD arrays, we determined that DCs respond to the geometry of printed ligand. Finally, we used a variety of small molecule inhibitors to identify pathways involved in geometry sensing. We saw a significant role for myosin contractility and α₅β₁ integrin engagement. We also noted significant reorganization of the actin cytoskeleton into dynamic actin rings when DCs were motile on posts. From these experiments, we conclude that DCs are insensitive to substrate compliance in the range tested but respond to changes in geometry via a mechanism that involves integrin function, myosin contractility, and remodeling of the actin cytoskeleton. As a possible explanation, we postulate a consistent role for filopodial extension and contraction as the driver of DC motility.

Migration against the direction of flow is LFA-1-dependent in human hematopoietic stem and progenitor cells

The recruitment of immune cells during inflammation is regulated by a multi-step cascade of cell rolling, activation, adhesion and transmigration through the endothelial barrier. Similarly, hematopoietic stem and progenitor cells (HSPCs) use this pathway to migrate and home to the bone marrow. After selectin-mediated braking, HSPCs migrate on adhesion ligands presented by the vascular endothelium including ICAM-1, VCAM-1 or MAdCAM-1. Here, we report that both the KG1a stem cell line and primary bone marrow CD34⁺ HSPCs can migrate against the direction of fluid flow on surfaces coated with cell adhesion molecules (CAMs), a behavior thus far only reported in T lymphocytes. We demonstrate that KG1a cells and primary HSPCs migrate upstream on surfaces presenting ICAM-1, downstream on surfaces presenting VCAM-1, and both upstream and downstream on surfaces presenting MAdCAM-1. In addition, we demonstrate that KG1a cells and HSPCs display upstream migration both on surfaces with multiple CAMs, as well as on human umbilical vein endothelial cell (HUVEC) monolayers. By blocking with monoclonal antibodies, we show that lymphocyte function-associated antigen-1 (LFA-1) is the key receptor responsible for upstream migration on the endothelium during the trafficking of HSPCs to the bone marrow.This article has an associated First Person interview with the first author of the paper.

Role for Mechanotransduction in Macrophage and Dendritic Cell Immunobiology

Tissue homeostasis is not only controlled by biochemical signals but also through mechanical forces that act on cells. Yet, while it has long been known that biochemical signals have profound effects on cell biology, the importance of mechanical forces has only been recognized much more recently. The types of mechanical stress that cells experience include stretch, compression, and shear stress, which are mainly induced by the extracellular matrix, cell-cell contacts, and fluid flow. Importantly, macroscale tissue deformation through stretch or compression also affects cellular function.Immune cells such as macrophages and dendritic cells are present in almost all peripheral tissues, and monocytes populate the vasculature throughout the body. These cells are unique in the sense that they are subject to a large variety of different mechanical environments, and it is therefore not surprising that key immune effector functions are altered by mechanical stimuli. In this chapter, we describe the different types of mechanical signals that cells encounter within the body and review the current knowledge on the role of mechanical signals in regulating macrophage, monocyte, and dendritic cell function.