Mechanosensing in macrophages and dendritic cells in steady-state and disease

Abstract

Macrophages and dendritic cells are myeloid cells that play critical roles in immune responses. Macrophages help to maintain homeostasis through tissue regeneration and the clearance of dead cells, but also mediate inflammatory processes against invading pathogens. As the most potent antigen-presenting cells, dendritic cells are important in connecting innate to adaptive immune responses via activation of T cells, and inducing tolerance under physiological conditions. While it is known that macrophages and dendritic cells respond to biochemical cues in the microenvironment, the role of extracellular mechanical stimuli is becoming increasingly apparent. Immune cell mechanotransduction is an emerging field, where accumulating evidence suggests a role for extracellular physical cues coming from tissue stiffness in promoting immune cell recruitment, activation, metabolism and inflammatory function. Additionally, many diseases such as pulmonary fibrosis, cardiovascular disease, cancer, and cirrhosis are associated with changes to the tissue biophysical environment. This review will discuss current knowledge about the effects of biophysical cues including matrix stiffness, topography, and mechanical forces on macrophage and dendritic cell behavior under steady-state and pathophysiological conditions. In addition, we will also provide insight on molecular mediators and signaling pathways important in macrophage and dendritic cell mechanotransduction.

Keywords: Hippo signalling; Piezo1; TRPV4; dendritic cells; integrins; macrophages; mechanotransduction; substrate stiffness.

FIGURE 1

Proposed mechanotransduction pathways in macrophages and dendritic cells. Multiple mechanisms are implicated to translate mechanical cues into cellular responses in macrophages and dendritic cells (DCs) (Martino et al., 2018). Integrins can sense extracellular matrix stiffness changes, which can translate into activation of small GTPases and subsequent modification of the subcellular organization and structure of F-actin filaments. This is linked to activation of the Hippo signaling pathway by increasing YAP/TAZ activation, leading to the upregulation of genes involved in glycolysis, amino acid metabolism, cellular proliferation and cell survival. Mechanosensitive ion channels are expressed in macrophages and DCs, including Piezo1 (Shaheen et al., 2017) in both, and TRPV4 (Oakes et al., 2009) in macrophages. Physical forces change the tension in the plasma membrane of cells, causing the ion channels to open. Piezo1 and TRPV4 opening allows the entry of extracellular Ca²⁺ ions into the cells. The influx of Ca²⁺ ions can activate the protein kinase C (PKC)/mitogen-activated protein kinase (MAPK) pathway or calcineurin/NF-κb pathway, leading to the upregulation of transcriptional programs that can increase cellular proliferation, differentiation, and inflammatory responses.

FIGURE 2

Impact of substrate stiffness on macrophages and dendritic cells. Softer substrates increase the roundness of macrophages. There is an increase PRM-like phenotype with an increase in IL-10 production and arginase-1 activity following IL-4 stimulation. Migration speed of macrophages grown on more pliant substrates is decreased and podosome-dependent. Macrophages cultured on stiffer substrates have a stretched morphology, with increased pro-inflammatory phenotype and secretion of pro-inflammatory cytokines TNF-α and IL-6. Phagocytosis capability has been suggested to increase, but still remains unclear. They acquire a fast, podosome-independent mode of migration after growth on stiffer surfaces. Dendritic cells (DCs) grown on softer substrates appear to have enhanced migratory capacity with increased expression of chemokine receptor 7 (CCR7). There is a decreased ability to produce pro-inflammatory cytokines TNF-α, IL-1β, and IL-6, and decrease in activation state. Metabolically, there is a decrease in glycolytic capacity. DCs cultured on stiffer substrates have greater ability to prime CD4⁺ and CD8⁺ T cells. They have increased capability to produce TNF-α, IL-1β, and IL-6 and have an enhanced activation state with increased expression of CD80/86 and CD40. Metabolically, they have increased glyocolytic capacity and express more intermediates of glycolysis.

FIGURE 3

Methods for studying effects of mechanical stimuli on macrophages and dendritic cells. (1) Hydrogels used to study the effect of substrate stiffness of various compliance on immune cell development, differentiation and function. Common hydrogels include poly-dimethyl-siloxane hydrogels (as described in (Lee et al., 2022)), and polyacrylamide hydrogels (Shaheen et al., 2017). Lucite chamber used to apply pressure to cells. Pressure can be controlled using an airtight Lucite box with an inlet valve for gas application, and an outlet valve connected to a manometer. Boxes are prewarmed to 37°C to prevent internal temperature and pressure fluctuations (as described in (Craig et al., 2008)) (Oakes et al., 2009). Flexcell system to apply strain to cells. Cyclic strain can be applied by deformation of the Bioflex well plate through regulated air vacuum supplied to the bottom of the plate, causing the membrane to stretch (Adlerz et al., 2016). Microfluidics channel to apply shear stress on cells. Cells are cultured within the lower main channel, while medium is injected into the upper channel. Cells get exposed to shear stress generated through the bridge channel (as described in (Kang et al., 2021)), and mechanical forces experienced by cells get calculated using computational simulation.