Mechanobiological Principles Influence the Immune Response in Regeneration: Implications for Bone Healing

Abstract

A misdirected or imbalanced local immune composition is often one of the reasons for unsuccessful regeneration resulting in scarring or fibrosis. Successful healing requires a balanced initiation and a timely down-regulation of the inflammation for the re-establishment of a biologically and mechanically homeostasis. While biomaterial-based approaches to control local immune responses are emerging as potential new treatment options, the extent to which biophysical material properties themselves play a role in modulating a local immune niche response has so far been considered only occasionally. The communication loop between extracellular matrix, non-hematopoietic cells, and immune cells seems to be specifically sensitive to mechanical cues and appears to play a role in the initiation and promotion of a local inflammatory setting. In this review, we focus on the crosstalk between ECM and its mechanical triggers and how they impact immune cells and non-hematopoietic cells and their crosstalk during tissue regeneration. We realized that especially mechanosensitive receptors such as TRPV4 and PIEZO1 and the mechanosensitive transcription factor YAP/TAZ are essential to regeneration in various organ settings. This indicates novel opportunities for therapeutic approaches to improve tissue regeneration, based on the immune-mechanical principles found in bone but also lung, heart, and skin.

Keywords: PIEZO1; TRPV4; YAP/TAZ; immune-mechanics; inflammation; mechano-transduction; mechanobiology; regeneration.

Figure 1

The mechanical niche defined by the extracellular matrix influences the crosstalk between non-hematopoietic cells and immune cells.

Figure 2

Forces acting on lung tissue changes the extracellular matrix (blue panel), or are perceived by non-hematopoietic (light orange panels) and immune cells (pink panel), thereby setting up fibrosis and pro-inflammatory responses. (A) Forces that are caused for example by mechanical ventilation act on lung tissue. This may lead to the cellular responses shown in (B–F). (B) Biglycan binds to TLR4 receptor on macrophages, thereby leading to the secretion of TNF-α and MIP-2. The latter acts as a chemoattractant for neutrophils. (C) Stretching of fibroblasts increases the amount of hyaluronan synthase 3 (HAS3), which produces low molecular weight hyaluronic acid (LMW-HA). Macrophages then bind LMW-HA via TLR4 and react with the secretion of IL-8 and IFN-β, both of which are associated with pro-inflammatory functions. (D) Cyclic stretch of alveolar epithelial type II cells (AEIIs) leads to nuclear localization of YAP/TAZ, and subsequently increases the proliferation rate. (E) Increased stiffness of the extracellular matrix (ECM) leads to nuclear localization of YAP in fibroblasts, which subsequently show enhanced production of ECM components. This might increase the stiffness of the ECM again and start a vicious cycle, that may end in fibrosis. (F) Calcium influx in macrophages via TRPV4 and crosstalk with LPS mediated signaling leads to increased secretion of pro-inflammatory cytokines such as TNF-α, ROS, and IL-6. The YAP symbol in the illustrations is representative for YAP and TAZ. CXCL, C-X-C motif ligand; IL, interleukin; IFN, interferon; LPS, lipopolysaccharides; MIP-2, macrophage inflammatory protein-2; TAZ, WW domain-containing transcription regulator 1; TNF-α, tumor necrosis factor alpha; TRPV4, transient receptor potential vanilloid 4; YAP, yes-associated protein 1.

Figure 3

Forces acting on heart tissue changes the extracellular matrix (blue panel), and are perceived by non-hematopoietic (light orange) and immune cells (pink panel), thereby leading to regenerative or tissue destructive processes. (A) Forces that are caused for example by transverse aortic constriction may lead to the cellular responses shown in (B–E). (B) The extracellular component agrin increases the stability of the ECM-integrin interaction, resulting in enhanced nuclear localization of YAP/TAZ in cardiomyocytes. Nuclear YAP/TAZ localization may then increase the proliferation rate of cardiomyocytes. (C) YAP/TAZ knock out in the epicardium leads to lower levels of INF-γ and a decrease of regulatory T cell (Treg) numbers. (D) Transverse aortic constriction leads to higher forces perceived by fibroblasts. Together with higher TGF-β levels, this decreases the MMP8 levels and the MMP8 cleavage product proline-glycin-prolin (PGP). Subsequently, the reduced PGP levels downregulate the recruitment and activation of monocytes and macrophages. (E) High shear stress during aortic valve stenosis leads to Ca²⁺ influx in monocytes, upon which they react with the secretion of pro-inflammatory mediators, such as IFN-β1, IL-1β, IL-1ra, IL-6, and IL-12. The YAP symbol in the illustrations is representative for YAP and TAZ. IL, interleukin; IFN, interferon; MMP8, Matrix metallopeptidases 8; TAZ, WW domain-containing transcription regulator 1; TGF-β, transforming growth factor β; YAP, yes-associated protein 1.

Figure 4

Forces acting on skin tissue during wound healing influences fibrotic, pro-inflammatory and anti-inflammatory processes by changing the extracellular matrix properties (blue panels). (A) Forces acting on a skin wound may lead to the cellular responses shown in (B and C), (B) Fibulin-5 increases the stability of the ECM-integrin connection and the formation of focal adhesions. This might promote nuclear YAP/TAZ localization and together with the additional input of TRPV4, this leads to the secretion of monocyte chemoattractant protein (MCP)-1. Monocytes will migrate along the MCP-1 gradient and secrete TGF-β, which acts back onto the fibroblast. If unchecked, this starts a vicious cycle leading to myofibroblast differentiation and fibrosis. (C) Left side: Stretched fibronectin conformation can function as a Damage Associated Molecular Pattern (DAMP) by biding to TLR4 on fibroblasts and lead to the secretion of the pro-inflammatory cytokine IL-8. Right side: Conversely, the stretched fibronectin conformation shows a higher binding affinity of IL-7. Higher levels of IL-7 can increase the stability and numbers of regulatory T cells (Tregs), which is often associated with an anti-inflammatory response. The YAP symbol in the illustrations is representative for YAP and TAZ. IL, interleukin; TAZ, WW domain-containing transcription regulator 1; TGF- β, transforming growth factor β; TLR4, toll-like receptor 4; TRPV4, transient receptor potential vanilloid 4; YAP, yes-associated protein 1.

Figure 5

Common immune-mechanical principles found in the lung, skin and heart could potentially impact the fracture healing cascade via ECM modulation (blue panels), non-hematopoietic cells (light orange panel) and immune cells (pink panel). (A) Forces acting on and within the fracture gap might lead to the cellular processes described in (B–E). (B) Fibronectin can be found in the growth plate and during new bone formation. Fibronectin could have similar roles in the bone as it was found for lung tissue, where stretched fibronectin conformation promoted a pro-inflammatory reaction by acting as a DAMP and increased the release of IL-8. Conversely, a stretched fibronectin conformation might also locally increase IL-7 levels, which leads to a higher regulatory T cell stability and numbers. (C) Agrin has been shown to promote heart regeneration and could also promote bone regeneration via osteochondral healing since Agrin mediated signaling leads to increased chondrogenic differentiation of mesenchymal stem cells. (D) The compression of osteoblasts has already been found to increase pro-inflammatory mediators such as IL-6, IL-8, IL-17, and Receptor Activator of NF-κB Ligand (RANKL). (E) Similar to the findings in the lung, Ca²⁺ influx via PIEZO1 in monocytes could lead to the secretion of CXCL2 in bone and attract polymorphonuclear leukocytes (left site). In macrophages, the calcium-mediated signaling via TRPV4 has been found in lung tissue to lead to reactive oxygen species (ROS) secretion. A comparable mechanism could potentially also lead to the ROS levels found in the fracture hematoma (right site). CXCL, C-X-C motif ligand; DAMP, damage-associated molecular pattern; IL, interleukin; Treg, regulatory T cell; TLR4, toll-like receptor 4; TRPV4, transient receptor potential vanilloid 4.

Figure 6

Mechanically or ECM triggered crosstalk between non-hematopoietic cells—immune cells—ECM regulates cell behavior. Immune cells can respond either directly or indirectly to mechanical forces. When immune cells sense the mechanical niche directly (1), they can transmit this information by the cytokine secretion, which either acts in an autocrine manner or on non-hematopoietic cells (2). This connects immune cells within a wound environment not only with each other, but also to non-hematopoietic cells such as smooth muscle cells, fibroblasts, epithelial cells and endothelial cells. Non-hematopoietic cells might then start to secrete ECM-components and change the ECM-composition (3) and subsequently its biophysical properties (4). This could close a feedback loop, and immune cells sense the altered ECM-composition/biophysical cues. Depending on the secreted cytokines of the immune cells, this feedback could be the first step into a vicious cycle or alternatively the first step toward a successful restoration of homeostasis. In the indirect case, tissue-resident cells embedded in the ECM first perceive the mechanical stimuli (5), translate them into a cellular response and might then transmit the stimuli to nearby immune cells by secreting cytokines and/or chemokines (6).