The interplay between biochemical mediators and mechanotransduction in chondrocytes: Unravelling the differential responses in primary knee osteoarthritis.

In primary or idiopathic osteoarthritis (OA), it is unclear which factors trigger the shift of articular chondrocyte activity from pro-anabolic to pro-catabolic. In fact, there is a controversy about the aetiology of primary OA, either mechanical or inflammatory. Chondrocytes are mechanosensitive cells, that integrate mechanical stimuli into cellular responses in a process known as mechanotransduction. Mechanotransduction occurs thanks to the activation of mechanosensors, a set of specialized proteins that convert physical cues into intracellular signalling cascades. Moderate levels of mechanical loads maintain normal tissue function and have anti-inflammatory effects. In contrast, mechanical over- or under-loading might lead to cartilage destruction and increased expression of pro-inflammatory cytokines. Simultaneously, mechanotransduction processes can regulate and be regulated by pro- and anti-inflammatory soluble mediators, both local (cells of the same joint, i.e., the chondrocytes themselves, infiltrating macrophages, fibroblasts or osteoclasts) and systemic (from other tissues, e.g., adipokines). Thus, the complex process of mechanotransduction might be altered in OA, so that cartilage-preserving chondrocytes adopt a different sensitivity to mechanical signals, and mechanic stimuli positively transduced in the healthy cartilage may become deleterious under OA conditions. This review aims to provide an overview of how the biochemical exposome of chondrocytes can alter important mechanotransduction processes in these cells. Four principal mechanosensors, i.e., integrins, Ca2+ channels, primary cilium and Wnt signalling (canonical and non-canonical) were targeted. For each of these mechanosensors, a brief summary of the response to mechanical loads under healthy or OA conditions is followed by a concise overview of published works that focus on the further regulation of the mechanotransduction pathways by biochemical factors. In conclusion, this paper discusses and explores how biological mediators influence the differential behaviour of chondrocytes under mechanical loads in healthy and primary OA

Investigation of the combined effects of a catabolic microenvironment and complex mechanical loading on intervertebral disc degeneration

INTRODUCTION Intervertebral disc (IVD) degeneration is the cause of around half of all low back pain cases in young adults, however the initiating and risk factors are poorly understood, limiting development of personalized therapies 1,2 . The balance between anabolic and catabolic processes is essential for healthy turnover of IVD extracellular matrix (ECM), and disruption of this process through elevated catabolic activity leads to disease progression 2 . Although the e!ects of pro-inflammatory cytokines and mechanical loading has been investigated within the IVD 3 , it is unknown how IVD response to complex mechanical loading is a!ected by the presence of cytokines. Thus, we aimed to investigate the combined e!ects of dynamic compression and torsion with catabolic cytokine interleukin 1 beta (IL-1β) and inhibitory cytokine interleukin 1 receptor antagonist (IL-1Ra), on bovine IVDs using ex vivo culture, magnetic resonance imaging (MRI), and finite element (FE) modeling. METHODS Whole bovine IVDs obtained within 3-4 hours port-mortem from a local abattoir were isolated and the IVDs cultured in a customized two degrees-of-freedom bioreactor applying diurnal dynamic compression (0.1-0.5 MPa) and torsion (6 degrees) under normal (HG DMEM), catabolic (10 ng/ml IL-1β) and inhibitory (10 ng/ml IL-1Ra) media conditions for one week. Static compression (0.1 MPa) under the same media conditions were used as a control. Before and after culture, the IVDs were imaged using 3T MRI, which was used to create subject-specific FE models. Downstream analyses included height measurement, qPCR, glycosaminoglycan (GAG) quantification, and cell metabolic activity. For statistical analysis (when n>3), nonparametric distribution was assumed and a Kruskal–Wallis test then Dunn’s multiple comparisons test were performed, and a P < 0.05 considered statistically significant. RESULTS Following one week of culture, IVD height decreased in all conditions (Figure 1a & b), however, this disc height decreases was less pronounced within IVDs stimulated with IL-1Ra. Cellular metabolic activity in the nucleus pulposus (NP) decreased in all conditions, but the di!erence was only significant in IVDs stimulated with IL-1β and complex dynamic loading (Figure 1c). Similarly, the GAG content decreased in the NP of all conditions, but was not significant. Gene expression of anabolic genes, i.e. collagen type II (COL II) decreased (Figure 1e), while expression of catabolic genes, i.e. matrix metalloproteinase 3 (MMP3) increased in the NP tissue for all conditions except in the control static IVDs and in the IL-1Ra stimulated IVDs, respectively (Figure 1d). DISCUSSION We hypothesized that catabolic cytokines, i.e. IL-1β, in the microenvironment of the IVD are su#cient to negatively alter the cellular response to complex loading, leading to further downstream degeneration. However, markers of degeneration were found in all conditions, which could indicate that loading was supraphysiological and catabolic alone. This will be investigated further using subject-specific FE models developed from the MRI images. Additionally, short half-lives of cytokines could have prevented them from di!using through the IVD e!ectively, thus limiting the cellular response. This will be further investigated using immunohistochemistry and ELISA. Nevertheless, the results convey that static or complex dynamic loading is su#cient to induce catabolism with or without cytokine stimulation

Complex mechanical loading and inflammation in intervertebral disc degeneration

Introduction Intervertebral disc (IVD) degeneration is the cause of around half of all low back pain cases in young adults, however the initiating and risk factors are poorly understood, limiting development of personalized therapies. [1] Although the effects of pro-inflammatory cytokines and mechanical loading has been investigated within the IVD, [2] it is unknown how IVD response to complex mechanical loading is affected by the presence of cytokines. Thus, we aimed to investigate the combined effects of dynamic compression and torsion with catabolic cytokine interleukin 1 beta (IL-1β) and inhibitory cytokine interleukin 1 receptor antagonist (IL-1Ra), on bovine IVDs using ex vivo culture, magnetic resonance imaging (MRI), and finite element (FE) modeling. Methods Whole bovine IVDs obtained within 3-4 hours portmortem from a local abattoir were isolated and the IVDs cultured in a customized two degrees-of-freedom bioreactor applying diurnal dynamic compression (0.1- 0.5 MPa) and torsion (6°) under normal (HG DMEM), catabolic (10 ng/ml IL-1β) and inhibitory (10 ng/ml IL1Ra) media conditions for one week. Static compression (0.1 MPa) under the same media conditions were used as a control. Before and after culture, the IVDs were imaged using 3T MRI, which was used to create subjectspecific FE models. Downstream analyses included height measurement, qPCR, glycosaminoglycan (GAG) quantification, and cell metabolic activity. For statistical analysis (when n>3), nonparametric distribution was assumed and a Kruskal–Wallis test then Dunn’s multiple comparisons test were performed, and a P < 0.05 considered statistically significant. Results Following one week of culture, IVD height decreased in all conditions (Figure 1a & b), however, this disc height decreases was less pronounced within IVDs stimulated with IL-1Ra. Cellular metabolic activity in the nucleus pulposus (NP) decreased in all conditions, but the difference was only significant in IVDs stimulated with IL-1β and complex dynamic loading (Figure 1c). Similarly, the GAG content decreased in the NP of all conditions, but was not significant. Gene expression of anabolic genes, i.e. collagen type II (COL II) decreased (Figure 1e), while expression of catabolic genes, i.e. matrix metalloproteinase 3 (MMP3) increased in the NP tissue for all conditions except in control static IVDs and in IL-1Ra stimulated IVDs, respectively (Figure 1d). Introduction Intervertebral disc (IVD) degeneration is the cause of around half of all low back pain cases in young adults, however the initiating and risk factors are poorly understood, limiting development of personalized therapies. [1] Although the effects of pro-inflammatory cytokines and mechanical loading has been investigated within the IVD, [2] it is unknown how IVD response to complex mechanical loading is affected by the presence of cytokines. Thus, we aimed to investigate the combined effects of dynamic compression and torsion with catabolic cytokine interleukin 1 beta (IL-1β) and inhibitory cytokine interleukin 1 receptor antagonist (IL-1Ra), on bovine IVDs using ex vivo culture, magnetic resonance imaging (MRI), and finite element (FE) modeling. Methods Whole bovine IVDs obtained within 3-4 hours portmortem from a local abattoir were isolated and the IVDs cultured in a customized two degrees-of-freedom bioreactor applying diurnal dynamic compression (0.1- 0.5 MPa) and torsion (6°) under normal (HG DMEM), catabolic (10 ng/ml IL-1β) and inhibitory (10 ng/ml IL1Ra) media conditions for one week. Static compression (0.1 MPa) under the same media conditions were used as a control. Before and after culture, the IVDs were imaged using 3T MRI, which was used to create subjectspecific FE models. Downstream analyses included height measurement, qPCR, glycosaminoglycan (GAG) quantification, and cell metabolic activity. For statistical analysis (when n>3), nonparametric distribution was assumed and a Kruskal–Wallis test then Dunn’s multiple comparisons test were performed, and a P < 0.05 considered statistically significant. Results Following one week of culture, IVD height decreased in all conditions (Figure 1a & b), however, this disc height decreases was less pronounced within IVDs stimulated with IL-1Ra. Cellular metabolic activity in the nucleus pulposus (NP) decreased in all conditions, but the difference was only significant in IVDs stimulated with IL-1β and complex dynamic loading (Figure 1c). Similarly, the GAG content decreased in the NP of all conditions, but was not significant. Gene expression of anabolic genes, i.e. collagen type II (COL II) decreased (Figure 1e), while expression of catabolic genes, i.e. matrix metalloproteinase 3 (MMP3) increased in the NP tissue for all conditions except in control static IVDs and in IL-1Ra stimulated IVDs, respectively (Figure 1d). Introduction Intervertebral disc (IVD) degeneration is the cause of around half of all low back pain cases in young adults, however the initiating and risk factors are poorly understood, limiting development of personalized therapies. [1] Although the effects of pro-inflammatory cytokines and mechanical loading has been investigated within the IVD, [2] it is unknown how IVD response to complex mechanical loading is affected by the presence of cytokines. Thus, we aimed to investigate the combined effects of dynamic compression and torsion with catabolic cytokine interleukin 1 beta (IL-1β) and inhibitory cytokine interleukin 1 receptor antagonist (IL-1Ra), on bovine IVDs using ex vivo culture, magnetic resonance imaging (MRI), and finite element (FE) modeling. Methods Whole bovine IVDs obtained within 3-4 hours portmortem from a local abattoir were isolated and the IVDs cultured in a customized two degrees-of-freedom bioreactor applying diurnal dynamic compression (0.1- 0.5 MPa) and torsion (6°) under normal (HG DMEM), catabolic (10 ng/ml IL-1β) and inhibitory (10 ng/ml IL1Ra) media conditions for one week. Static compression (0.1 MPa) under the same media conditions were used as a control. Before and after culture, the IVDs were imaged using 3T MRI, which was used to create subjectspecific FE models. Downstream analyses included height measurement, qPCR, glycosaminoglycan (GAG) quantification, and cell metabolic activity. For statistical analysis (when n>3), nonparametric distribution was assumed and a Kruskal–Wallis test then Dunn’s multiple comparisons test were performed, and a P <0.05 considered statistically significant. Results Following one week of culture, IVD height decreased in all conditions (Figure 1a & b), however, this disc height decreases was less pronounced within IVDs stimulated with IL-1Ra. Cellular metabolic activity in the nucleus pulposus (NP) decreased in all conditions, but the difference was only significant in IVDs stimulated with IL-1β and complex dynamic loading (Figure 1c). Similarly, the GAG content decreased in the NP of all conditions, but was not significant. Gene expression of anabolic genes, i.e. collagen type II (COL II) decreased (Figure 1e), while expression of catabolic genes, i.e. matrix metalloproteinase 3 (MMP3) increased in the NP tissue for all conditions except in control static IVDs and in IL-1Ra stimulated IVDs, respectively (Figure 1d).Discussion We hypothesized that catabolic cytokines, i.e. IL-1β, in the microenvironment of the IVD are sufficient to negatively alter the cellular response to complex loading, leading to further downstream degeneration. However, markers of degeneration were found in all conditions, which could indicate that loading was supraphysiological and catabolic alone. This will be investigated further using subject-specific FE models developed from the MRI images. Additionally, short half-lives of cytokines could have prevented them from diffusing through the IVD effectively, thus limiting the cellular response. This will be further investigated using immunohistochemistry and ELISA. Nevertheless, the results convey that static or complex dynamic loading is sufficient to induce catabolism with or without cytokine stimulation. References 1. Baumgartner L et al., J. Int J Mol Sci. 2021; 22(2):703. 2. Walter BA et al., PLOS ONE. 2015; 10(3): e0118358.5

Conditioned medium of intervertebral disc cells inhibits osteogenesis on autologous bone-marrow-derived mesenchymal stromal cells and osteoblasts

INTRODUCTION: Low back pain (LBP) is a significant global burden and is associated with the degeneration of the spine and human intervertebral discs (hIVD). Current surgical treatment for hIVD degeneration is the removal of the affected tissue with a cage to promote spinal fusion and relieve discomfort. Despite progress in the treatment of LBP, however, in ~30% of all cases this procedure ends in non-fusion and painful pseudo-athrosis after operation. Previous research from our laboratory showed morphogenetic protein (BMP) antagonists secreted by the intervertebral disc have been potentially associated with inhibiting the process of osteogenesis. However, the co-cultured cells did not come from the same donor. In this study, we investigated the hypothesis that IVD cells secrete BMP inhibitors that inhibit osteogenesis in autologous osteoblast (hOB) and bone marrow mesenchymal stem cell (hMSC). METHODS: Conditioned Medium (CM) collected from primary hIVD cells in 3D alginate culture was co-cultured with seven donor-matched hOB and hMSC in 2D culture. Osteogenesis after 10 days was then quantified at the transcript level using qPCR to measure the expression of bone makers and BMP antagonists, and at the protein level by alkaline phosphatase (ALP) activity. Additionally, they were evaluated histologically by alizarin red (ALZR) staining on Day 21. The relationship between ALP activity, osteogenesis and Noggin expression in hOB or hMSC or hIVD was investigated to uncover the potential causes. RESULTS: ALP activity significantly decreased and the formation of calcium deposits in alizarin red staining was inhibited after culture with CM derived from hIVD. Interestingly, less changes of bone makers and BMP inhibitors’ expression was found in hOB or hMSC on Day 10. Noggin was relatively higher expressed (Average fold change: AF, 6.9; CEP, 10.0; NP, 6.3; relative to autologous hOB. AF, 2.3; CEP, 3.4; NP, 3.2; relative to autologous hMSC.) in hIVD compared to hOB or hMSC. DISCUSSION: The up-regulation of Noggin mRNA (and possibly other BMP inhibitors) in residual hIVD tissue after spinal fusion surgery is potentially a potent reason for prevention of successful osteogenesis. Similar results were found previously with allogenic co-cultures. However, in previous study there were merely trends of inhibition. Here we show a significant decrease with autologous donor-matched samples

In-Vitro and In-Silico Investigation of Dynamic Compression on Cartilage Endplate Cells in Agarose

INTRODUCTION: The cartilage endplate (CEP) covers the top and bottom of the intervertebral disc (IVD) and acts to transmit compressive loads and transport water, nutrients, and waste in and out of the disc. Early cartilage endplate (CEP) degeneration is likely to play a key role in IVD degeneration, but little is known about CEP mechanobiology and its changes in degeneration. Investigating these changes is essential to elucidate how the CEP contributes to IVD pathology. METHODS: In-vitro: Bovine-tail CEP cells were expanded until passage three. Afterwards, a 1:1 mixture of CEP cells and agarose was pipetted into silicon molds to create 2% agarose and 1x107 cells/ml carriers, 6 mm diameter and 3 mm thickness, and cultured two days for phenotype recovery. Cell-agarose carriers were placed in custom-made chambers, stimulated with 10 ng/ml TGF-β1 throughout the entirety of the experiment and dynamically compressed up to 7% strain for one hour at 1.5 Hz every day for up to 14 days. Those not dynamically loaded experienced the constant weight of the chamber lid exerting ~5.1 Pa per carrier. Carriers were collected on Days 0, 7, and 14 for downstream analysis of cell viability, gene expression, and glycosaminoglycan (GAG) content. In-silico: A 2D axisymmetric porohyperelastic, compressible, Neo-Hookean finite element model (FEM) of a cell-agarose carrier was developed in Abaqus using literature-derived material properties and loaded with dynamic compression as in the in-vitro experiment. A previously developed mechanotransduction network model was used to predict protein activation levels by initial mechanoreceptor perturbations standing for dynamic compression (α5β1, αvβ3), physioosmotic pressure (TRPV4), tensile strain (αvβ5), plus chondrogenic media (TGF-β). Predicted protein activation was normalized by baseline conditions. RESULTS: After seven and 14 days of culture, cell-agarose carriers in all conditions demonstrated significantly increased expression of anabolic genes aggrecan (ACAN) 2-200x, collagen II (COL II) 31000x, and GAG/DNA content 2.5-5x, alongside decreased expression of catabolic gene matrix metalloproteinase 3 (MMP3). Reaction forces from FEM (0.07N) matched force data collected during loading (0.06N). The FEM showed that hydrostatic pressure varies from center to edge of carrier. General trends of increased/decreased gene expression and protein activation matched between experimental and network model results. DISCUSSION & CONCLUSIONS: A novel framework coupling 3D cell culture with in-silico methods is presented, in which the FEM provides details about loads experienced at each point within the carrier, while the network model uses these mechanical cues and environmental perturbations to predict protein expression and identify key proteins for future analysis. Keywords: Intervertebral disc / spine and their disorders, Biomechanics / biophysical stimuli and mechanotransductio

Direction Dependent Mass Transport Through the Cartilage Endplate

INTRODUCTION: Intervertebral Disc (IVD) Degeneration can result from chemical changes in the Cartilage Endplate (CEP) and it might suggest that pain is related to CEP weaknesses and imperfections. The CEP has a crucial role in keeping the IVD healthy by acting as the main gateway of nutrients and waste in and out of that avascular region. Yet, among the spinal tissues, CEPs receive the least amount of attention in scientific literature. The purpose of this study is to follow a combined in vitro and imaging-driven in silico approach to obtain an improved understanding of the CEP functionality regarding mass transport. METHODS: In vitro: 6 CEPs were harvested from a fresh bovine tail. Eight mm diameter biopsy samples were prepared and enabled to free swell prior to testing in Dulbecco's Modified Eagle Medium (DMEM). Each sample was fitted tightly into a silicone tube with constant inlet and outlet pressures. DMEM was passed for 10min twice in the forward and reverse direction corresponding to the flow directions into and out of the IVD in vivo respectively. The amount of passed fluid was collected to determine the flow rate. In silico: The samples were incubated in 40% Hexabrix, a contrast agent, for 48 hours and then imaged by a GE Nanotom® M nanoCT device. From the reconstructed nanoCT images, 1 mm diameter diameter subsamples were selected to create 3D models of the pore structure at different locations in the CEPs. To date, one model was meshed and imported into OpenFOAM® to perform Computational Fluid Dynamics (CFD) Simulations. The other models are still under development. RESULTS: The in vitro experiment showed that the average flow rate through that CEP samples was 6.86 mm3/sec and 4.84 mm3/sec in the forward and reverse directions respectively meaning that the reverse flow was 70.55% of the forward flow. The CFD analysis on one subsample showed that the flow rates were 15.1 mm3/sec and 10.7 mm3/sec meaning that the reverse flow was 70.86% of the forward flow. DISCUSSION & CONCLUSIONS: The results from both the in vitro experiment and the in silico simulation showed that the CEP has a tendency to resist flow differently according the direction of the flow where flow into the IVD was higher than that out of the IVD. This combination of imagebased in silico modelling with in vitro experiments is a first step towards a better quantification of the mass transport across the CEP in and out of IVDs

Sulfated hydrogels as primary intervertebral disc cell culture systems

INTRODUCTION: Intervertebral disc (IVD) degeneration is a key contributor for low back pain, a leading cause of disability worldwide1. During degeneration, IVD aging is accelerated, leading to progressive structural changes, including blood vessel and nerve ingrowth that promote discogenic pain2. In vitro studies require novel biomaterials that mimic the IVD extracellular matrix (ECM) to provide mechanical support and a reservoir of cytokines and growth factors. As proteoglycans with their attached sulfated glycosaminoglycans (GAGs) are one of the major components of the ECM, the ECM’s sulfation state could be a key factor for IVD cell-fate3. Thus, we aim to explore human NP cell fate using a novel sulfated alginate model with varying degrees of sulfation (DS). METHODS: Primary human NP cells were expanded, mixed with solutions of i) 2.5% of standard alginate, ii) 0.1 DS, and iii) 0.2 DS alginate (4 x 106 cells/ml) and casted in 27 l cylindrical-shaped carriers (4 mm diameter, 2 mm height). Carriers were cultured for two weeks for phenotype recovery and were collected with the culture media on day 0, 7 and 14. RESULTS: A significant decrease of cell density (p<0.05) was observed in 0.2 DS alginate after 7 and 14 days of culture. Similarly, cell viability was significantly reduced (p<0.05) in 0.2 DS alginate after 7 days of culture (N=4). In addition, cell metabolic activity tended to be decreased in 0.2 DS alginate compared to standard alginate after 14 days of culture. Surprisingly, ECM remodeling factors such as MMP2 and TIMP1 were slightly upregulated in the 0.1 DS group (N=1), whereas catabolic cytokines were downregulated in the 0.1% DS group. DISCUSSION & CONCLUSIONS: We demonstrate significant cellular differences between 0.2 DS alginate vs standard alginate and 0.1 DS alginate. Particularly, a significant decrease in cell density, metabolic activity and viability were observed in the 0.2 DS alginate after 7 days of culture. According to the secretome, the sulfated alginate group seems to possess increased catabolic ECM remodeling with lower secretion of catabolic factors, suggesting less responsive NP cells to ECM structural changes. Overall, standard alginate seems to be the best option for NP cell 3D culture models. ACKNOWLEDGEMENTS: This project was supported by the Marie Skłodowska Curie International Training Network “disc4all” under the grant agreement #955735. REFERENCES: 1FY. Wang et al (2020) JOR Spine 5:1186. 2P. Bermudez-Lekerika et al (2022) Front Cell Dev Biol 29(10):924692. 3E. Lazarus et al (2021) Cells 10(12):3568. Keywords: Hydrogels and injectable systems, In vitro microenvironment

Cartilaginous endplates: A comprehensive review on a neglected structure in intervertebral disc research

The cartilaginous endplates (CEP) are key components of the intervertebral disc (IVD) necessary for sustaining the nutrition of the disc while distributing mechanical loads and preventing the disc from bulging into the adjacent vertebral body. The size, shape, and composition of the CEP are essential in maintaining its function, and degeneration of the CEP is considered a contributor to early IVD degeneration. In addition, the CEP is implicated in Modic changes, which are often associated with low back pain. This review aims to tackle the current knowledge of the CEP regarding its structure, composition, permeability, and mechanical role in a healthy disc, how they change with degeneration, and how they connect to IVD degeneration and low back pain. Additionally, the authors suggest a standardized naming convention regarding the CEP and bony endplate and suggest avoiding the term vertebral endplate. Currently, there is limited data on the CEP itself as reported data is often a combination of CEP and bony endplate, or the CEP is considered as articular cartilage. However, it is clear the CEP is a unique tissue type that differs from articular cartilage, bony endplate, and other IVD tissues. Thus, future research should investigate the CEP separately to fully understand its role in healthy and degenerated IVDs. Further, most IVD regeneration therapies in development failed to address, or even considered the CEP, despite its key role in nutrition and mechanical stability within the IVD. Thus, the CEP should be considered and potentially targeted for future sustainable treatments

pUL21 is a viral phosphatase adaptor that promotes herpes simplex virus replication and spread.

The herpes simplex virus (HSV)-1 protein pUL21 is essential for efficient virus replication and dissemination. While pUL21 has been shown to promote multiple steps of virus assembly and spread, the molecular basis of its function remained unclear. Here we identify that pUL21 is a virus-encoded adaptor of protein phosphatase 1 (PP1). pUL21 directs the dephosphorylation of cellular and virus proteins, including components of the viral nuclear egress complex, and we define a conserved non-canonical linear motif in pUL21 that is essential for PP1 recruitment. In vitro evolution experiments reveal that pUL21 antagonises the activity of the virus-encoded kinase pUS3, with growth and spread of pUL21 PP1-binding mutant viruses being restored in adapted strains where pUS3 activity is disrupted. This study shows that virus-directed phosphatase activity is essential for efficient herpesvirus assembly and spread, highlighting the fine balance between kinase and phosphatase activity required for optimal virus replication.Wellcome Trust Senior Research Fellowship (219447/Z/19/Z), Wellcome Trust Senior Research Fellowship (106207/Z/14/Z), Biotechnology and Biological Sciences Research Council Research Grant (BB/M021424/1), Sir Henry Dale Fellowship, jointly funded by the Wellcome Trust and the Royal Society (098406/Z/12/B)

Immuno-modulatory effects of intervertebral disc cells

Low back pain is a highly prevalent, chronic, and costly medical condition predominantly triggered by intervertebral disc degeneration (IDD). IDD is often caused by structural and biochemical changes in intervertebral discs (IVD) that prompt a pathologic shift from an anabolic to catabolic state, affecting extracellular matrix (ECM) production, enzyme generation, cytokine and chemokine production, neurotrophic and angiogenic factor production. The IVD is an immune-privileged organ. However, during degeneration immune cells and inflammatory factors can infiltrate through defects in the cartilage endplate and annulus fibrosus fissures, further accelerating the catabolic environment. Remarkably, though, catabolic ECM disruption also occurs in the absence of immune cell infiltration, largely due to native disc cell production of catabolic enzymes and cytokines. An unbalanced metabolism could be induced by many different factors, including a harsh microenvironment, biomechanical cues, genetics, and infection. The complex, multifactorial nature of IDD brings the challenge of identifying key factors which initiate the degenerative cascade, eventually leading to back pain. These factors are often investigated through methods including animal models, 3D cell culture, bioreactors, and computational models. However, the crosstalk between the IVD, immune system, and shifted metabolism is frequently misconstrued, often with the assumption that the presence of cytokines and chemokines is synonymous to inflammation or an immune response, which is not true for the intact disc. Therefore, this review will tackle immunomodulatory and IVD cell roles in IDD, clarifying the differences between cellular involvements and implications for therapeutic development and assessing models used to explore inflammatory or catabolic IVD environments