Articular cartilage has poor repair potential and the tissue formed is mechanically incompetent. Mesenchymal stromal cells (MSCs) show chondrogenic properties and the ability to re-grow cartilage, however a viable human model for testing cartilage regeneration and repair is lacking. Here, we describe an ex vivo pre-clinical femoral head model for studying human cartilage repair using MSCs. Human femoral heads (FHs) were obtained following femoral neck fracture with ethical permission/patient consent and full-depth cartilage wells made using a 3mm biopsy punch. Pancreas-derived mesenchymal stromal cells (P-MSC) were prepared in culture media at ~5000 cells/20µl and added to each well and leakage prevented with fibrin sealant. After 24hrs, the sealant was removed and medium replaced with StemPro. TM. chondrogenesis differentiation medium. The FHs were incubated (37. o. C;5% CO. 2. ) for 3wks, followed by a further 3wks in standard medium with 10% human serum with regular medium changes throughout. Compared to wells with medium only, A-MSCs produced a thin film across the wells which was excised en-block, fixed with 4% paraformaldehyde and frozen for cryo-sectioning. The cell/tissue films varied in thickness ranging over 20-440µm (82±21µm; mean±SEM; N=3 FHs). The thickness of MSC films abutting the cartilage wells was variable but generally greater (15-1880µm) than across the wells, suggesting an attachment to native articular cartilage. Staining of the films using safranin O (for glycosaminoglycans; quantified using ImageJ) was variable (3±8%; mean±SEM; N=3) but in one experiment reached 20% of the adjacent cartilage. A preliminary assessment of the repair tissue gave an O'Driscoll score of 10/24 (24 is best). These preliminary results suggest the ex vivo femoral head model has promise for studying the capacity of MSCs to repair cartilage directly in human tissue, although optimising MSCs to produce hyaline-like tissue is essential. Supported by the CSO (TCS/17/32)

For the treatment of ununited fractures, we developed a system of delivering magnetic labelled mesenchymal stromal cells (MSCs) using an extracorporeal magnetic device. In this study, we transplanted ferucarbotran-labelled and luciferase-positive bone marrow-derived MSCs into a non-healing femoral fracture rat model in the presence of a magnetic field. The biological fate of the transplanted MSCs was observed using luciferase-based bioluminescence imaging and we found that the number of MSC derived photons increased from day one to day three and thereafter decreased over time. The magnetic cell delivery system induced the accumulation of photons at the fracture site, while also retaining higher photon intensity from day three to week four. Furthermore, radiological and histological findings suggested improved callus formation and endochondral ossification. We therefore believe that this delivery system may be a promising option for bone regeneration.

We used interconnected porous calcium hydroxyapatite ceramic to bridge a rabbit ulnar defect. Two weeks after inducing the defect we percutaneously injected rabbit bone marrow-derived mesenchymal stromal cells labelled with ferumoxide. The contribution of an external magnetic targeting system to attract these cells into the ceramic and their effect on subsequent bone formation were evaluated. This technique significantly facilitated the infiltration of ferumoxide-labelled cells into ceramic and significantly contributed to the enhancement of bone formation even in the chronic phase. As such, it is potentially of clinical use to treat fractures, bone defects, delayed union and nonunion

Objectives. Venous thromboembolism (VTE) is a major potential complication following orthopaedic surgery. Subcutaneously administered enoxaparin has been used as the benchmark to reduce the incidence of VTE. However, concerns have been raised regarding the long-term administration of enoxaparin and its possible negative effects on bone healing and bone density with an increase of the risk of osteoporotic fractures. New oral anticoagulants such as rivaroxaban have recently been introduced, however, there is a lack of information regarding how these drugs affect bone metabolism and post-operative bone healing. Methods. We measured the migration and proliferation capacity of mesenchymal stem cells (MSCs) under enoxaparin or rivaroxaban treatment for three consecutive weeks, and evaluated effects on MSC mRNA expression of markers for stress and osteogenic differentiation. Results. We demonstrate that enoxaparin, but not rivaroxaban, increases the migration potential of MSCs and increases their cell count in line with elevated mRNA expression of C-X-C chemokine receptor type 4 (CXCR4), tumor necrosis factor alpha (TNFα), and alpha-B-crystallin (CryaB). However, a decrease in early osteogenic markers (insulin-like growth factors 1 and 2 (IGF1, IGF2), bone morphogenetic protein2 (BMP2)) indicated inhibitory effects on MSC differentiation into osteoblasts caused by enoxaparin, but not by rivaroxaban. Conclusions. Our findings may explain the adverse effects of enoxaparin treatment on bone healing. Rivaroxaban has no significant impact on MSC metabolism or capacity for osteogenic differentiation in vitro. Cite this article: Dr H. Pilge. Enoxaparin and rivaroxaban have different effects on human mesenchymal stromal cells in the early stages of bone healing. Bone Joint Res 2016;5:95–100. DOI: 10.1302/2046-3758.53.2000595

Recent studies suggested that both the soluble protein of the mesenchymal stromal cell (MSC) secretome, as well as the secreted extracellular vesicles (EVs) promote bone regeneration. However, there is limited knowledge of the changes in MSC secretome vesicular fraction during aging. We therefore aimed to characterize the release profiles and cargo of EVs from MSCs of different chronological ages. Conditioned medium (CM) was collected from 13 bone marrow MSC strains (20-89 years) and from one MSC strain derived from human induced pluripotent stem cells (iPSCs). The EV-containing fraction was enriched with ultracentrifugation. The number of particles in the CM was evaluated by nanoparticle tracking analysis (NTA), and the number of EVs was evaluated by flow cytometry (FC) after staining with cell-mask-green and anti-CD81 antibody. EV cargo analysis was conducted using next-generation sequencing (NGS). Our data confirmed the release of EVs from all MSC strains used in the study. There were no correlations between the number of particles and the number of EVs released in the CM, and between the number of EVs released and the strain age. Nevertheless, some of the lowest concentrations of EVs were found in the CM of strains over 70 years of age, which exhibited a low/absent chondrogenic and osteogenic differentiation potential. In contrast, iPSC-MSCs, which exhibited a high growth and three-lineage differentiation potential, released a similar amount of EVs as the best performing bone marrow MSC strain. NGS analysis identified several microRNAs that were significantly enriched in EVs of young MSC strains exhibiting low senescence, and those that were enriched in EVs of strains exhibiting high differentiation potentials. Gender had no influence on microRNA profiles in EVs or releasing MSCs. Taken together, our data provides new insights into the properties of MSC vesicular secretome and its therapeutic potential during aging

Long bone fractures in patients with diabetes mellitus (DM) are slow to heal, often resulting in delayed reunion or non-union. It is reasonable to postulate that the underlying cause of these DM-associated complications is a reduced population of bone marrow progenitor cells and/or their dysfunction. With the hypothesis that the administration of healthy, allogeneic adult bone marrow-derived mesenchymal stromal cells (MSCs) can enhance DM fracture healing, the aim of this endeavour was to assess the efficacy of MSC administration to support fracture repair using two doses. Here 250,000 or 500,000 human bone marrow-derived MSCs were locally introduced to femoral fractures in diabetic mice, and the quality of de novo bone assessed 8 weeks later. Preliminary bone bridging was evident in all animals; however, a large circumferential reparative callus was consistently retained indicating non-union. Micro-CT analysis elucidated consistent callus dimensions, bone mineral density, bone volume/total volume in all groups, but an increase in bone surface area/bone volume in cell-treated fractures. Moreover, greater amounts of mature bone were identified in fractures treated with a low dose of MSCs. Four-point bending evaluation of the mechanical integrity of the repairing fracture indicated a statistically significant improvement in flexure strength and flexure modulus in DM fractures treated with 250,000 MSCs as compared to controls. An improvement in total energy required for failure was observed in both groups that received MSCs. Therefore, the administration of non-DM bone marrow-derived MSCs supported the development of more mature bone in the reparative callus, resulting in greater mechanical integrity

Early clinical studies investigating the effects of delivery of mesenchymal stromal cells (MSCs) to degenerated intervertebral discs have shown promising results, but with an incomplete understanding of the therapeutic mechanism(s) of action. To address this, we have developed a 3D co-culture system to unravel the biological interaction between MSCs and nucleus pulposus (NP) cells. Alginate constructs were created using a biphasic configuration consisting of a cylindrical shell with an encapsulated bead. Human NP cells were seeded in monolayer or encapsulated within alginate and cultured in hypoxia with variations of pH, osmolarity and growth factors (n = 6) to replicate healthy or degenerative conditions. Wells and gels were fixed and stained for ECM content, and retrieved cells and media were analysed for ECM and inflammatory factor expression. Encapsulated hNPCs showed no migration from either alginate structure and full bead separation was achieved over 14 days, reinforcing the construct as a separable 3D co-culture method. Addition of the degenerative growth factors TNFα and IL-1β as well as the adjustment of media pH to degenerative levels (pH 6.8) caused the hNPCs to decrease in size and proliferate significantly higher than control levels. TGF-β3 addition showed higher incidence of aggrecan deposition over addition of IL-1β. Addition of FGF2 altered cell morphology and ECM deposition including formation of pseudo lamellae, indicating a phenotype shift toward annulus fibrosis cells as shown in late-stage degenerative disc disease. The data from this study will be used in future MSC:NPC co-cultures to determine immunoregulatory interactions in a degenerative environment

Recapitulating tissue elasticity can direct mesenchymal stromal cell (MSC) differentiation; however, it is unclear how substrate elasticity affects MSC metabolism. It is hypothesized MSCs subjected to stiffnesses, atypical of standard tissue culture plastic, display altered metabolic phenotypes during differentiation. In this study, such alterations in MSC metabolic profiles, based on the fluorescence lifetime of NAD(P)H, a critical co-factor in energy production, were monitored using Fluorescence lifetime imaging microscopy (FLIM) as an evaluation tool. Polyacrylamide substrates with varying stiffnesses were fabricated to model the native elasticity of cartilage and bone. MSCs cultured on these substrates exhibited potent alterations in their metabolic status over a 14-day period that were detectable as early as day 3 using FLIM. Overall, soft substrates induced a more glycolytic response after 10 days of culture that persisted at day 14 (as measured by protein-bound NAD(P)H contributions to the lifetime decay). Similarly, by day 10; MSCs on intermediate-stiffness substrates favoured glycolysis. MSCs on stiffer substrates initially displayed a glycolytic phenotype followed by a transition to oxidative phosphorylation by day 10. Staining for mineralised nodules and glycosaminoglycans verified MSCs on stiffer substrates differentiating towards an osteogenic lineage, while MSCs on intermediate substrates showed similarities with differentiated chondrocytes. Overall, it can be concluded that matrix stiffness can induce metabolic perturbations in MSCs for up to 14 days. From this research, ideal culture conditions in which the metabolics of MSCs could be manipulated to promote maximum potency could potentially be defined in the future

Introduction. Current cell-based treatments and marrow stimulating techniques to repair articular cartilage defects are limited in restoring the tissue in its native composition. Despite progress in cartilage tissue engineering and chondrogenesis in vitro, the main limitation of this approach is the progression towards hypertrophy during prolonged culture in pellets or embedded in biomaterials. The objectives of this study were (A) to compare human bone marrow-derived mesenchymal stromal cells (hMSC) chondrogenesis and hypertrophy in pellet culture from single cells or cell spheroids and (B) to investigate the effect of tyramine-modified hyaluronic acid (THA) and collagen I (Col) content in composite hydrogels on the chondrogenesis and hypertrophy of encapsulated hMSC spheroids. Materials and Methods. Pellet cultures were prepared either from hMSC single cells (250’000 cells/pellet) or hMSC spheroids (282 cells/spheroid) at the same final cell concentration (250’000 cells/pellet = 887 spheroids/pellet). The effect of polymer concentration on encapsulated hMSC spheroids (887 spheroids/hydrogel) was investigated in THA-Col hydrogels (50μl) at the following concentrations (THA-Col mg/ml): Group (1) 12.5–2.5, (2) 16.7–1.7, (3) 12.5–1.7, (4) 16.7–2.5 mg/ml. All samples were cultured for 21 days in standard chondrogenic differentiation medium containing 10ng/ml TGF-β1. Chondrogenic differentiation and hypertrophy of both pellet cultures and hMSCs spheroids encapsulated in THA-Col were analysed using gene expression analysis (Aggrecan (ACAN), COL1A1, COL2A1, COL10A1), dimethylmethylene-Blue assay to quantify glycosaminoglycans (GAGs) retained in the samples and (immuno-) histological staining (Safranin-O, collagen II, aggrecan) on day 1 and day 21 (n=3 donors). Results. The culture of hMSCs in pellets based on single cells or spheroids resulted in an increase in chondrogenic-associated markers COL2A1 (2’900–3’400-fold) and ACAN (45–47-fold) compared to respective samples on day 1 in both groups. GAGs increased in spheroid pellets to 21.2±3.4 mg/ml and in single cell pellets to 20.8±6.6 mg/ml on day 21. Comparing the levels of hypertrophic markers, single cell pellets showed 7-fold and 20-fold higher expression of COL1A1 and COL10A1 than spheroid pellets on day 21. The encapsulation of hMSC spheroids in THA-Col resulted in an upregulation of chondrogenic-associated markers and GAG content in all hydrogels with differences in cell differentiation related to the Col and THA polymer ratio, while level of hypertrophy was comparable in all groups with values similar to the spheroid pellet group. Spheroids embedded in hydrogels with lower THA content (group 1 and 3) resulted in more pronounced chondrogenic phenotype marked by upregulation of COL2A1 (3’200–4’500-fold) and ACAN (152–179-fold) relative to the respective samples on day 1. Spheroids embedded in higher THA content hydrogels (group 2 and 4) showed less pronounced chondrogenesis marked by lower upregulation of COL2A1 (980–1800-fold) and ACAN (25–68-fold, relative to day 1 samples). This was confirmed by quantification of GAGs, increasing from 2.5±1.9 and 2.5±1.7 mg/ml (day 1) to 11.4±2.5 and 9.9±3.8 mg/ml on day 21 for groups 1 and 4, respectively. (Immuno-) histological stainings resulted in a more homogenous staining in lower THA content hydrogels compared to a more local matrix deposition in samples with higher THA content. Conclusion. The reduced level of hypertrophy in hMSC pellets prepared from cell spheroids compared to single cell pellets at same cell count might be related to the packing density of the cells with cells being more densely packed in single cell pellets compared to pellets from spheroids. Investigating the effect of polymer ratios on chondrogenesis, it seems that the THA content is the driving factor influencing hMSC chondrogenesis rather than Col content in THA-Col composites at comparable mechanical properties. This study highlights the feasibility to use hMSC spheroids as alternative approach to study in vitro chondrogenic differentiation and the suitability to investigate the effect of biomaterial composition on chondrogenesis and hMSC hypertrophy

Abstract

Bone marrow mesenchymal stromal cells were aspirated from immature male green fluorescent protein transgenic rats and cultured in a monolayer. Four weeks after the creation of the osteochondral defect, the rats were divided into three groups of 18: the control group, treated with an intra-articular injection of phosphate-buffered saline only; the drilling group, treated with an intra-articular injection of phosphate-buffered saline with a bone marrow-stimulating procedure; and the bone marrow mesenchymal stromal cells group, treated with an intra-articular injection of bone marrow mesenchymal stromal cells plus a bone marrow-stimulating procedure. The rats were then killed at 4, 8 and 12 weeks after treatment and examined. The histological scores were significantly better in the bone marrow mesenchymal stromal cells group than in the control and drilling groups at all time points (p < 0.05). The fluorescence of the green fluorescent protein-positive cells could be observed in specimens four weeks after treatment

Introduction. Bereft of their optimal tissue context, cells lose their phenotype, function and therapeutic potential during in vitro culture. Despite the fact that in vivo cells are exposed simultaneously to multiple signals, traditional ex vivo cultures are monofactorial. With these in mind, herein we assessed the combined effect of surface topography, substrate rigidity, collagen type I coating and macromolecular crowding in human tenocyte, skin fibroblast and bone marrow mesenchymal stromal cell cultures. Methods. Thermal imprinted was used to pattern (groove depth: 2,000 nm, groove width: 2,000 nm, line width: 2,000 nm) polydimethylsiloxane substrates of different rigidity (50 kPa, 130 kPa, 1,000 kPa). Grooved and planar substrates were subsequently coated with collagen type I and used to culture the aforementioned cell populations without and with macromolecular crowding (100 μg/ml carrageenan). After 3, 7 and 14 days in culture, cell morphology, viability, metabolic activity, proliferation, protein synthesis and deposition and gene expression analyses were conducted. Results. None of the variables assessed affected cell viability, metabolic activity and proliferation. Surface topography was found to be a potent regulator of cell morphology. Macromolecular crowding significantly increased extracellular matrix deposition, albeit in globular manner independently on whether grooved or planar substrates were used, possibly due to the low dimensionality of the grooves. Gene expression analysis made apparent that the 130 kPa and the 1,000 kPa grooved substrates under macromolecular crowding conditions maintained human tenocyte phenotype and directed human bone marrow mesenchymal stromal cells towards tendon-like lineage, respectively. None of the conditions assessed dramatically affected human skin fibroblast fate. Conclusions. Collectively, our data indicate that the physicochemical in vitro microenvironment modulators assessed herein are capable of maintaining human tenocyte phenotype and differentiating human bone marrow mesenchymal stromal cells towards tenogenic lineage, but not in trans-differentiating human skin fibroblasts

It is well known that environmental cues such as mechanical loading and/or cell culture medium composition affect tissue-engineered constructs resembling natural bone. These studies are mostly based on an initial setting of the influential parameter that will not be further changed throughout the study. Through the growth of the cells and the deposition of the extracellular matrix (ECM) the initial environmental conditions of the cells will change, and with that also the loads on the cells will change. This study investigates how changes of mechanical load or media composition during culture influences the differentiation and ECM production of mesenchymal stromal cells seeded on porous 3D silk fibroin scaffolds. ECM formation, ECM mineralization and cell differentiation in 3D tissue-engineered bone were analyzed using microscopic tools. Our results suggest that mechanical stimuli are necessary to differentiate human mesenchymal stromal cells of both bone marrow and adipose tissue origin into ECM producing osteoblasts which ultimately become ECM-embedded osteocytes. However, the influence of this stimulus seems to fade quickly after the onset of the culture. Constructs which were initially cultured under mechanical loading continued to deposit minerals at a similar growth rate once the mechanical stimulation was stopped. On the other hand, cell culture medium supplementation with FBS was identified as an extremely potent biochemical cue that influences the mechanosensitivity of the cells with regards to cell differentiation, ECM secretion and mineral deposition. Only through a thorough understanding on these influences over time will we be able to predictably control tissue development in vitro

The HIPGEN study funded under EU Horizon 2020 (Grant 7792939) has the aim to investigate the potential of the first regenerative cell therapy for the improvement of recovery after muscle injury in hip fracture patients. For this aim we intramuscularly injected placental derived mesenchymal stromal cells during hip fracture arthroplasty. Despite not having reached the primary endpoint, which was the Short Physical Performance Battery, we could observe an increase in abductor muscle strength and a faster return to balance looking at symmetry in insole measurements during follow up

Nanovesicle-based therapy is increasingly being pursued as a safe, cell-free strategy to combat various immunological, musculoskeletal and neurodegenerative diseases. Small secreted extracellular vesicles (sEVs) obtained from multipotent mesenchymal stromal cells (MSCs) are of particular interest for therapeutic use since they convey anti-inflammatory, anti-scarring and neuroprotective activities to the recipient cells. Cell-derived vesicles (CDVs) produced by a proprietary extrusion process are surrounded by a lipid bilayer membrane with correct membrane topology, display biological activities similar to MSC-derived EVs and may find specific application for organ-targeted drug delivery systems. Translation of nanovesicle-based therapeutics into clinical application requires quantitative and reproducible analysis of bioactivity and stability, and the potential for GMP-compliant manufacturing. Manufacturing and regulatory considerations as well as preclinical models to support clinical translation will be discussed

The biological understanding for the disease progression osteoarthritis (OA) has uncovered specific biomarkers from either synovial fluid, articular chondrocytes or synoviocytes that can be used to diagnose the disease. Examples of these biomarkers include interleukin-1β (IL-1β) or collagen II fragments (1, 2). In parallel, isolation of chondrocytes or bone marrow derived mesenchymal stromal cells (MSCs) has yielded cell-based strategies that have shown long- term beneficial effects in a specific cohort of patients, specifically in traumatic cartilage lesions (2). This latter finding shows that patient stratification of OA is an important tool to both match patients for a specific treatment and to develop novel therapies, especially disease modifying drugs. In order to create disease stage specific therapies, the use of next generation analysis tools such as RNAseq and metabolomics, has the potential to decipher specific cellular and molecular endotypes. Alongside greater understanding of the clinical phenotype (e.g. imaging, pain, co- morbidities), therapies can be designed to alleviate the symptoms of OA at specific points of the disease in patients. This talk will outline the current biological understanding of OA and discuss how patient stratification could assist in the design of innovative therapies for the disease. Acknowledgements: This presentation was supported by the COST action, CA21110 – Building an open European Network on Osteoarthritis Research (NetwOArk)

The use of mesenchymal stromal cells (MSCs) in regenerative medicine and tissue engineering is well established, given their properties of self-renewal and differentiation. However, several studies have shown that these properties diminish with age, and understanding the pathways involved are important to provide regenerative therapies in an ageing population. In this PRISMA systematic review, we investigated the effects of chronological donor ageing on the senescence of MSCs. We identified 3023 studies after searching four databases including PubMed, Web of Science, Cochrane, and Medline. Nine studies met the inclusion and exclusion criteria and were included in the final analyses. These studies showed an increase in the expression of p21, p53, p16, ROS, and NF- B with chronological age. This implies an activated DNA damage response (DDR), as well as increased levels of stress and inflammation in the MSCs of older donors. Additionally, highlighting the effects of an activated DDR in cells from older donors, a decrease in the expression of proliferative markers including Ki67, MAPK pathway elements, and Wnt/ -catenin pathway elements was observed. Furthermore, we found an increase in the levels of SA- -galactosidase, a specific marker of cellular senescence. Together, these findings support an association between chronological age and MSC senescence. The precise threshold for chronological age where the reported changes become significant is yet to be defined and should form the basis for further scientific investigations. The outcomes of this review should direct further investigations into reversing the biological effects of chronological age on the MSC senescence phenotype

Critical-sized bone defects remain challenging in the clinical setting. Autologous bone grafting remains preferred by clinicians. However, the use of autologous tissue is associated with donor-site morbidity and limited accessibility to the graft tissue. Advances in the development of synthetic bone substitutes focus on improving their osteoinductive properties. Whereas osteoinductivity has been demonstrated with ceramics, it is still a challenge in case of polymeric composites. One of the approaches to improve the regenerative properties of biomaterials, without changing their synthetic character, is the addition of inorganic ions with known osteogenic and angiogenic properties. We have previously reported that the use of a bioactive composite with high ceramic content composed of poly(ethyleneoxide terephthalate)/poly(butylene terephthalate) (1000PEOT70PBT30, PolyActive, PA) and 50% beta-tricalcium phosphate (β-TCP) with the addition of zinc in a form of a coating of the TCP particles can enhance the osteogenic differentiation of human mesenchymal stromal cells (hMSCs) (3). To further support the regenerative properties of these scaffolds, inorganic ions with known angiogenic properties, copper or cobalt, were added to the coating solution. β-TCP particles were immersed in a zinc and copper or zinc and cobalt solution with a concentration of 15 or 45 mM. 3D porous scaffolds composed of 1000PEOT70PBT30 and pure or coated β-TCP were additively manufactured by 3D fibre deposition. The osteogenic and angiogenic properties of the fabricated scaffolds were tested in vitro through culture with hMSCs and human umbilical vein endothelial cells, respectively. The materials were further evaluated through ectopic implantation in an in vivo mini-pig model. The early expression of relevant osteogenic gene markers (collagen-1, osteocalcin) of hMSCs was upregulated in the presence of lower concentration of inorganic ions. Further analysis will focus on the evaluation of ectopic bone formation and vascularisation of these scaffolds after implantation in a mini-pig ectopic intramuscular model

Osteochondral injuries are a recognised factor in the development of osteoarthritis (OA). Mesenchymal stromal cells (MSCs) represent a promising biological therapeutic option as an OA-modifying treatment, and they also secrete factors that may have an anti-catabolic effect and/or encourage endogenous repair. We aim to study the effects of (i) intra-articular injection of human bone-marrow-derived MSCs and (ii) their secretome on recovery in a murine knee osteochondral injury model. The MSC secretome was generated by stimulating human bone-marrow-derived MSCs with tumour necrosis factor alpha (TNFα). Mice (n=48) were injected with i) MSC secretome, ii) MSCs or iii) cell culture medium (control). Pain was assessed by activity monitoring, and cartilage repair, subchondral bone volume and synovial inflammation were evaluated using histology and microCT. Both MSC- and MSC-secretome-injected mice showed significant pain reduction at day 7 when compared to control mice, but only the MSC-injected mice maintained a significant improvement over the controls at day 28. Cartilage repair was significantly improved in MSC-injected mice. No significant effects were observed with regards to synovial inflammation or subchondral bone volume. The MSC secretome demonstrates regenerative effects but this does not appear to be as sustained as a MSC cell therapy. Further studies are required to investigate if this can be overcome using different dosing regiments for injection of the MSC secretome. As we further understand the regenerative properties of the MSC secretome, we may be able to enhance the clinical translatability of these therapies. Direct intra-articular injection of MSCs for the treatment of OA also appears promising as a potential future strategy for OA management. Acknowledgements: MS is supported by a grant from the Wellcome Trust (PhD Programme for Clinicians)

Mesenchymal stem cells (MSCs) have the potential to repair and regenerate damaged tissues in response to injury, such as fracture or other tissue injury. Bone marrow and adipose tissue are the major sources of MSCs. Previous studies suggested that the regenerative activity of stem cells can be enhanced by exposure to tissue microenvironments. The aim of our project was to investigate whether extracellular matrix (ECM) engineered from human induced pluripotent stem cells-derived mesenchymal-like progenitors (hiPSCs-MPs) can enhance the regenerative potential of human bone marrow mesenchymal stromal cells (hBMSCs). ECM was engineered from hiPSC-MPs. ECM structure and composition were characterized before and after decellularization using immunofluorescence and biochemical assays. hBMSCs were cultured on the engineered ECM, and differentiated into osteogenic, chondrogenic and adipogenic lineages. Growth and differentiation responses were compared to tissue culture plastic controls. Decellularization of ECM resulted in efficient cell elimination, as observed in our previous studies. Cultivation hBMSCs on the ECM in osteogenic medium significantly increased hBMSC growth, collagen deposition and alkaline phosphatase activity. Furthermore, expression of osteogenic genes and matrix mineralization were significantly higher compared to plastic controls. Chondrogenic micromass culture on the ECM significantly increased cell growth and expression of chondrogenic markers, including glycosaminoglycans and collagen type II. Adipogenic differentiation of hBMSCs on the ECM resulted in significantly increased hBMSC growth, but significantly reduced lipid vacuole deposition compared to plastic controls. Together, our studies suggest that BMSCs differentiation into osteogenic and chondrogenic lineages can be enhanced, whereas adipogenic activity is decreased by the culture on engineered ECM. Contribution of specific matrix components and underlying mechanisms need to be further elucidated. Our studies suggest that the three-lineage differentiation of aged BMSCs can be modulated by culture on hiPSC-engineered ECM. Further studies are aimed at scaling-up to three-dimensional ECM constructs for osteochondral tissue regeneration