Macrophages (Mφ) are immune cells that play a crucial role in both innate and adaptive immunity as they are involved in a wide range of physiological and pathological processes. Depending on the microenvironment and signals present, Mφ can polarize into either M1 or M2 phenotypes, with M1 macrophages exhibiting pro-inflammatory and cytotoxic effects, while M2 macrophages having immunosuppressive and tissue repair properties. Macrophages have been shown to play key roles in the development and progression or inhibition of various diseases, including cancer. For example, macrophages can stimulate tumor progression by promoting immunosuppression, angiogenesis, invasion, and metastasis. This work aimed to investigate the effect of extracellular vesicles (EVs)-derived from polarized macrophages on an osteosarcoma cell line. Monocytes were extracted from buffy coats and cultured in RPMI medium with platelet lysate or M-CSF. After 6 days of seeding, Mφ were differentiated into M1 and M2 with INF-γ/LPS and IL-4/IL-13, respectively. The medium with M1 or M2 derived EVs was collected and EVs were isolated by differential centrifugation and size exclusion chromatography and its morphology and size were characterized with SEM and NTA, respectively. The presence of typical EVs markers (CD9, CD63) was assessed by Western Blot. Finally, EVs from M1 or M2-polarized Mφ were added onto osteosarcoma cell cultures and their effect on cell viability and cell cycle, proliferation, and gene expression was assessed. The EVs showed the typical shape, size and surface markers of EVs. Overall, we observed that osteosarcoma cells responded differentially to EVs isolated from the M1 and M2-polarized Mφ. In summary, the use of Mφ-derived EVs for the treatment of osteosarcoma and other cancers deserves further study as it could benefit from interesting traits of EVs such as low immunogenicity, nontoxicity, and ability to pass through tissue barriers. Acknowledgements: Carlos III Health Institute and the European Social Fund for contract CP21/00136 and project PI22/01686

In relation to regenerative therapies in osteoarthritis and cartilage repair, mesenchymal stromal cells (MSCs) have immunomodulatory functions and influence macrophage behaviour. Macrophages exist as a spectrum of pro-(M1) and anti-(M2) inflammatory phenotypic subsets. In the context of cartilage repair, we investigated MSC-macrophage crosstalk, including specifically the priming of cartilage cells by macrophages to achieve a regenerative rather than fibrotic outcome. Human monocytes were isolated from blood cones and differentiated towards M1 and M2 macrophages. Monocytes (Mo), M1 and M2 macrophages were cultured directly and indirectly (trans-well system) with human bone marrow derived MSCs. MSCs were added during M1 polarisation and separately to already induced M1 cells. Outcomes (M1/M2 markers and ligands/receptors) were evaluated using RT-qPCR and flow cytometry. Influence on chondrogenesis was assessed by applying M1 and M2 macrophage conditioned media (CM) sequentially to cartilage derived cells (recapitulating an acute injury environment). RT-qPCR was used to evaluate chondrogenic/fibrogenic gene transcription. The ratio of M2 markers (CD206 or CD163) to M1 markers (CD38) increased when MSCs were added to Mo/M1 macrophages, regardless of culture system used (direct or indirect). Pro-inflammatory markers (including TNFβ) decreased. CXCR2 expression by both M1 macrophages and MSCs decreased when MSCs were added to differentiated M1 macrophages in transwell. When adding initially M1 CM (for 12 hours) followed by M2 CM (for 12 hours) sequentially to chondrocytes, there was a significant increase of Aggrecan and Collagen type 2 gene expression and decrease in fibroblastic cell surface markers (PDPN/CD90). Mo/M1 macrophages cultured with MSCs, directly or indirectly, are shifted towards a more M2 phenotype. Indirect culture suggests this effect can occur via soluble signaling mediators. Sequential exposure of M1CM followed by M2CM to chondrocytes resulted in increased chondrogenic and reduced fibrotic gene expression, suggesting that an acute pro-inflammatory stimulus may prime chondrocytes before repair

Abstract. Objectives. In relation to regenerative therapies in osteoarthritis and cartilage repair, mesenchymal stromal cells (MSCs) have immunomodulatory functions and influence macrophage behaviour. Macrophages exist as a spectrum of pro-(M1) and anti-(M2) inflammatory phenotypic subsets. In the context of cartilage repair, we investigated MSC-macrophage crosstalk, including specifically the priming of cartilage cells by macrophages to achieve a regenerative rather than fibrotic outcome. Methods. Human monocytes were isolated from blood cones and differentiated towards M1 and M2 macrophages. Monocytes (Mo), M1 and M2 macrophages were cultured directly and indirectly (trans-well system) with human bone marrow derived MSCs. MSCs were added during M1 polarisation and separately to already induced M1 cells. Outcomes (M1/M2 markers and ligands/receptors) were evaluated using RT-qPCR and flow cytometry. Influence on chondrogenesis was assessed by applying M1 and M2 macrophage conditioned media (CM) sequentially to cartilage derived cells (recapitulating an acute injury environment). RT-qPCR was used to evaluate chondrogenic/fibrogenic gene transcription. Results. The ratio of M2 markers (CD206 or CD163) to M1 markers (CD38) increased when MSCs were added to Mo/M1 macrophages, regardless of culture system used (direct or indirect). Pro-inflammatory markers (including TNFa) decreased. CXCR2 expression by both M1 macrophages and MSCs decreased when MSCs were added to differentiated M1 macrophages in transwell. When adding initially M1 CM (for 12 hours) followed by M2 CM (for 12 hours) sequentially to chondrocytes, there was a significant increase of Aggrecan and Collagen type 2 gene expression and decrease in fibroblastic cell surface markers (PDPN/CD90). Conclusions. Mo/M1 macrophages cultured with MSCs, directly or indirectly, are shifted towards a more M2 phenotype. Indirect culture suggests this effect can occur via soluble signaling mediators. Sequential exposure of M1CM followed by M2CM to chondrocytes resulted in increased chondrogenic and reduced fibrotic gene expression, suggesting that an acute pro-inflammatory stimulus may prime chondrocytes before repair. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

We investigated in vitro a mechanism by which particulate debris may induce bone resorption and cause implant loosening. We first studied two standard particles: latex, which is considered to be inert, and zymosan, which is inflammatory. Macrophages that phagocytosed either particle became activated, and stimulated 15 times as much bone resorption as did control macrophages. For activation to occur, 100 times more latex than zymosan had to be phagocytosed. We also found that bone cement and polyethylene particles activated macrophages in a similar manner, and that the necessary amounts of these were intermediate between those of latex and zymosan. None of the particles were toxic. It was concluded that implant loosening may result from bone resorption stimulated by mediators released by macrophages that have phagocytosed particles of bone cement or polyethylene

Metal particles and ions are liberated from the articular interface of metal-metal (MM) total hip arthroplasties. To better understand their cellular effect, we analyzed the internalization of these metal particles and ions by macrophages in vitro. Macrophages were exposed to metal particles isolated from MM prostheses cycled in a hip simulator and to metal ions. Cells were processed for transmission electron microscopy analysis. Results reveal the internalization of metal particles and Cr. 3+. ions in specifically localized cytoplasmic areas. This study is the first to reveal that metal particles of clinically relevant size and Cr. 3+. ions are internalized by an apparently active process. In order to minimize articular interface wear, metal-metal (MM) hip implants have been considered as an alternative to conventional metal-polyethylene bearings. While the local histological effects of the metallic particles and ions appear to be similar to that seen with metal-polyethylene hip replacements (i.e., a foreign-body macrophage response), little is known about the cellular effects of these metal particles and ions. The purpose of this study was to better understand the cellular effect of metal particles and ions, we analyzed their internalization by macrophages in vitro. J774 mouse macrophages were exposed to metal particles isolated from serum of MM prostheses cycled in a hip simulator and to Cr. 3+. (CrCl. 3. ) and Co. 2+. (CoCl. 2. ) ions. Cells were then processed for transmission electron microscopy analysis. Micrographs revealed the internalization of metal particles and Cr. 3+. ions in specifically localized cytoplasmic areas, suggesting that they are phagocytosed via an active pathway. Energy disperse X-ray analysis spectra of macrophages incubated with Cr. 3+. revealed a chromium phosphate composition. The same structure and composition were also observed when Cr. 3+. ions were incubated in culture medium without cells, suggesting that they were formed outside the cells. Co. 2+. ions did not form visibly agglomerated structures. This study is the first to reveal that metal particles of clinically relevant size are internalized by an apparently active process and that Cr. 3+. ions can be internalized by macrophages after binding to phosphorus or phosphoproteins. Kinetic studies are now necessary to better understand the mechanism of phagocytosis and the ultimate outcome of these particles and ions in macrophages

Purpose: Extruded tissue specimens excised during surgery on human intervertebral disc hernia and chondrocytes established and cultured from the excised tissue were observed via electron microscopy. Macrophages confirmed by CD68 immunostaining were added to the chondrocyte culture, and observed via electron microscopy. To observe, using an electron microscope, disc hernia degeneration at the cellular level as expressed in extruded tissue from a human intervertebral disc and cultured chondrocytes, and to investigate the mechanism of spontaneous regression and the effects of macrophages. Materials and Methods: Tissues excised during surgery were fixed in various fixatives for electron microscopy and immune electron microscopy to avoid divided and treated with collagenase, and chondrocytes were then isolated and cultured. Human heparinized peripheral blood was separated using Ficoll and cultured. After culture, macrophages were collected and confirmed by CD68 immunostaining. These macrophages were added to the chondrocyte culture and observed under an electron microscope. Results: Chondrocytes in the hernia is extruded region markedly differed from cultured chondrocytes. The tissue extruded from the intervertebral disc showed obvious degeneration such as changes in osmotic pressure. Macrophages were observed as the mechanism of spontaneous regression. Discussion: In a previous study, we used ELISA to measure MMP-3 levels and TIMP-1 levels in both mRNA and serum in patients, and found a correlation between the two. In this study, we observed the pathological state of a disc hernia at the cellular level. When chondrocytes from the same tissue were cultured under conditions similar to those in the intervertebral disc, the extruded tissue showed a clear difference. It was considered that membrane osmotic pressure affects intervertebral disc hernia in humans and that protein transmission occurs in endoplasmic reticulum. It was also considered that spontaneous regression is due to the infiltration of macrophages

Recent researches indicate that both M1 and M2 macrophages play vital roles in tissue repair and foreign body reaction processes. In this study, we investigated the dynamics of M1 macrophages in the induced membrane using a mouse femur critical-sized bone defect model.

The Masquelet method (M) and control (C) groups were established using C57BL/6J male mice (n=24). A 3mm-bone defect was created in the right femoral diaphysis followed by a Kirschner wire fixation, and a cement spacer was inserted into the defect in group M. In group C, the bone defect was left uninserted. Tissues around the defect were harvested at 1, 2, 4, and 6 weeks after surgery (n=3 in each group at each time point). Following Hematoxylin and eosin (HE) staining, immunohistochemical staining (IHC) was used to evaluate the CD68 expression as a marker of M1 macrophage. Iron staining was performed additionally to distinguish them from hemosiderin-phagocytosed macrophages.

In group M, HE staining revealed a hematoma-like structure, and CD68-positive cells were observed between the spacer and fibroblast layer at 1 week. The number of CD68-positive cells decreased at 2 weeks, while they were observed around the new bone at 4 and 6 weeks. In group C, fibroblast infiltration and fewer CD68-positive cells were observed in the bone defect without hematoma-like structure until 2 weeks, and no CD68-positive cells were observed at 4 and 6 weeks. Iron staining showed hemosiderin deposition in the surrounding area of the new bone in both groups at 4 and 6 weeks. The location of hemosiderin deposition was different from that of macrophage aggregation.

This study suggests that M1 macrophage aggregation is involved in the formation of induced membranes and osteogenesis and may be facilitated by the presence of spacers.

Purpose: The osteogenic effects of BMPs on mesenchymal stem cells (MSCs) are less profound in human as compared to rodent. The mechanism for this phenomenon is unclear. This study evaluated the effects of macrophages on proliferation and BMP-2 induced osteogenic differentiation of human MSCs. Method: MSCs were isolated from human bone marrow. Human monocytes THP-1 (human acute monocytic leukemia cell line) were induced into macrophages by phorbol myristate acetate. The conditioned media (CM) from monocytes and macrophages were collected separately. After treated with CM from monocytes or macrophages for 5 and 7 days, the proliferation rate of human MSCs was determined by WST-8 assay. A group without CM served as control. Pretreated human MSCs were then induced towards osteogenic differentiation by osteoinductive medium supplemented with 0.1ug/ml BMP-2. Expression levels of osteogenic markers were determined by real-time quantitative PCR. Alkaline phosphatase (ALP) activity and mineral deposition were assessed by p-NPP colorimetric kinetic assay and calcium assay, respectively. Results: The number of MSCs was significantly decreased in the group with macrophage CM at both 5 and 7 days (both p< 0.001) as compared with control group, but not in the group with monocytes CM. Expression levels of ALP and bone sialoprotein 2 in the macrophage CM group were significantly lower than those in the control group (p=0.003 and p< 0.001, respectively). ALP activity was also significantly lower in the group with macrophage CM than control group (p< 0.001). Although the expression levels of osteocalcin and RUNX2 as well as calcium deposition in the macrophage CM group were reduced, they did not reach statistical significance. Conclusion: Macrophages suppressed the proliferation of MSCs and inhibited BMP-2 induced osteogenic differentiation of human MSCs. In addition to known BMP antagonists, macrophages might be another important factor in suppressing the osteogenic effect of BMP-2 on human MSCs

Phagocytosis of wear particles by perimplant macrophages results in cytokine release and osteoclast activation and osteolysis. Some investigators have proposed that this response may be mediated by adherent endotoxin. The aim of this study was to determine the role of endotoxin in modulating pro-inflammatory cytokine mRNA expression of macrophages when stimulated with titanium particles using relative quantitative real-time polymerase chain reaction (rqRT-PCR). Human peripheral blood mononuclear cells were isolated from healthy subjects and plated in chamber slides. Three types of titanium particles were prepared; commercially pure titanium particles (cpTi), endotoxin stripped particles and endotoxin stripped particles with endotoxin (LPS) added back. Endotoxin levels of 450, 0 and 140 Eu/ml respectively were confirmed by high sensitivity Limulus Amebocyte Lysate assay. Macrophages were stimulated with particle concentrations of 0, 8.3, 83 and 830 particles per cell at time points 0 and 3 hours. LPS (200ng/ml) was used as a positive control. rqRT-PCR was performed using standard techniques. Stimulation of human macrophages with cpTi demonstrated a significant dose dependent increase in TNFα, IL-1A, IL-1B and, IL-6. (Kruskal-Wallis p=0.01, p=0.017, p=0.001 and p=0.013 respectively). IL-18 mRNA levels were not increased (P> 0.05). The expression of mRNA following stimulation with the highest dose of titanium particles was similar to that following LPS stimulation. Endotoxin-free cpTi particles did not elicit any increase in mRNA expression above base line levels (P > 0.05, all cytokines). This lack of response was rescued in endotoxin-stripped particles with LPS added back. Particle dose dependent increases in cytokine mRNA levels were observed for TNFα, IL-1A, IL-1B and, IL-6 mRNA but not IL-18 (p=0.01, p=0.01, p=0.01, p=0.05 and p=0.> 0.05 respectively). Our results show that adherent endotoxin plays a role in modulating particle induced pro-inflammatory cytokine mRNA expression in-vitro. Further study is required in evaluating the role of adherent endotoxin in vivo

Aim

Staphylococcus aureus (SA) can cause various infections and is associated with high morbidity and mortality rates of up to 40%. Antibiotic treatment often fails to eradicate SA infections even if the causative strain has been tested susceptible in vitro. The mechanisms leading to this persistence is still largely unknown. In our work, we to reveal SA interactions with host cells that allow SA to persist at the site of infection.

Introduction and Objective

Mesenchymal stem cells (MSC) are attractive candidates for bone regeneration approaches. Benefits of MSC therapy are mainly attributed to paracrine effects via soluble factors, exerting both immunoregulatory and regenerative actions. Encapsulation of MSC in hydrogels prepared with extracellular matrix (ECM) proteins has been proposed as a strategy to enhance their survival and potentiate their function after implantation. Functional activity of MSC can be regulated by the physical and mechanical properties of their microenvironment. In this work, we investigated whether matrix stiffness can modulate the crosstalk between MSC encapsulated in collagen hydrogels with macrophages and osteoblasts.

Silver nanoparticles (AgNPs) possess anti-inflammatory activities and have been widely deployed for promoting tissue repair. Here we explored the efficacy of AgNPs on functional recovery after spinal cord injury (SCI). Our data indicated that, in a SCI rat model, local AgNPs delivery could significantly recover locomotor function and exert neuroprotection through reducing of pro-inflammatory M1 survival. Furthermore, in comparison with Raw 264.7-derived M0 and M2, a higher level of AgNPs uptake and more pronounced cytotoxicity were detected in M1. RNA-seq analysis revealed the apoptotic genes in M1 were upregulated by AgNPs, whereas in M0 and M2, pro-apoptotic genes were downregulated and PI3k-Akt pathway signaling pathway was upregulated. Moreover, AgNPs treatment preferentially reduced cell viability of human monocyte-derived M1 comparing to M2, supporting its effect on M1 in human. Overall, our findings reveal AgNPs could suppress M1 activity and imply its therapeutic potential in promoting post-SCI motor recovery.

Introduction

Metal alloys have been commonly used for surgical applications due to their suitable mechanical characteristics and relatively good biocompatibility. However, direct cellular corrosion of orthopaedic implants remains a controversial topic and is still not fully understood. This study aims to examine a possible aspect of this corrosion mechanism by determining if macrophages can attach and directly affect the surfaces of 316L stainless steel, Ti6Al4V, and CoCrMo by releasing components of the alloy oxide layer.

Objectives

Up to 10% of fractures result in undesirable outcomes, for which female sex is a risk factor. Cellular sex differences have been implicated in these different healing processes. Better understanding of the mechanisms underlying bone healing and sex differences in this process is key to improved clinical outcomes. This study utilized a macrophage–mesenchymal stem cell (MSC) coculture system to determine: 1) the precise timing of proinflammatory (M1) to anti-inflammatory (M2) macrophage transition for optimal bone formation; and 2) how such immunomodulation was affected by male versus female cocultures.

Synovitis has been shown to play a role in pathophysiology of OA promoting cartilage destruction and pain. Synovium is mainly composed of synovial fibroblast (SF) and macrophage (SM) that guide synovial inflammation. Adipose stromal cells (ASC) promising candidate for cell therapy in OA are able to counteract inflammation. Two different subsets of macrophages have been described showing a pro-inflammatory (M1) and an anti-inflammatory (M2) phenotype. Macrophage markers: CD68, CD80 (M1-like) and CD206 (M2-like) were evaluated in osteoarthritic synovial tissue. GMP-clinical grade ASC were isolated from subcutaneous adipose tissue and M1-macrophages were differentiated from CD14+ obtained from peripheral blood of healthy donors. ASC were co-cultured in direct and indirect contact with activated (GM-CSF+IFNγ)-M1 macrophages for 48h. At the end of this co-culture we analyzed IL1β, TNFα, IL6, MIP1α/CCL3, S100A8, S100A9, IL10, CD163 and CD206 by qRT-PCR or immunoassay. PGE2 blocking experiments were performed. In moderate grade OA synovium we found similar percentages of CD80 and CD206. M1-activated macrophage factors IL1β, TNFα, IL6, MIP1α/CCL3, S100A8 and S100A9 were down-modulated both co-culture conditions. Moreover, ASC induced the typical M2 macrophage markers IL10, CD163 and CD206. Blocking experiments showed that TNFα, IL6, IL10, CD163 and CD206 were significantly modulated by PGE2. We confirmed the involvement of PGE2/COX2 also in CD14+ OA synovial macrophages. In conclusion we demonstrated that ASC are responsible for the switching of activated-M1-like to a M2-like anti-inflammatory phenotype, mainly through PGE2. This suggested a specific role of ASC as important determinants in therapeutic dampening of synovial inflammation in OA.

Joint injuries often result in inflammation and cartilage defects. When inflamed, the synovium secretes factors that prevent successful cartilage repair by inhibiting chondrogenic differentiation of progenitor cells. In particular the pro-inflammatory macrophages in the synovium are indicated to contribute to this anti-chondrogenic effect. Thus, we aimed to counteract the anti-chondrogenic effect of inflamed synovium by modulating synovial inflammation and its macrophages. Synovium tissue obtained from osteoarthritic patients undergoing a total knee replacement was cut into explants and cultured for 72 hours +/− 1 µM of the anti-inflammatory drug triamcinolone acetonide (TAA) (Sigma Aldrich). TAA significantly decreased gene expression of TNFA, IL1β and IL6, and increased expression of CCL18, IL1RA in synovial explants (all with p < 0.001). On the other hand, TAA significantly decreased the percentages of pro-inflammatory CD14+/CD80+ and CD14+/CD86+ macrophages in the synovium (both p < 0.001) as assessed by flow cytometry analyses. The percentages of anti-inflammatory CD14+/CD163+ macrophages, is significantly increased (p < 0.001) in TAA treated synovium. Conditioned medium (CM) from synovium explants downregulated the gene expression of cartilage matrix components collagen type-2 and aggrecan expression in chondrogenic MSCs. This chondrogenesis inhibiting effect was reduced by treating synovium with TAA during the production of the CM. Our findings indicate that reducing synovial inflammation might improve the joint environment for better cartilage repair, possibly by modulation of macrophage phenotypes.

Introduction Approximately one quarter of spinal cord injury patients will develop post-traumatic syringomyelia. This condition can produce devastating neurological deficits, and treatment is often not successful. The pathogenesis is unknown, however it is likely that initial cyst formation plays an important role in subsequent syrinx development. An up-regulated inflammatory process observed following contusive and transective spinal cord injury has been proposed as a contributory factor in secondary spinal cord damage. Specifically, a depletion or suppression of macrophages following injury is shown to preserve neurons and myelinated axons. This study examines the role of inflammation following excitotoxic spinal cord injury, a potent precursor to syrinx formation.

Methods Twenty-four male Sprague-Dawley rats were divided into six groups. Twenty rats received four 0.5 μL injections of 24 mg/ mL quisqualic acid and 1% Evans blue from the rostral C8 to the caudal T1 level. Ten microlitres of 250 mg/ mL kaolin were then injected into the subarachnoid space. Animals were sacrificed at 1, 5, 10, 20 or 50 days following the injections. There were four normal control animals. Spinal cord tissue was frozen and sectioned, and cytoplasmic antigen ED1 was detected immunohistochemically with anti-ED1 antibody. This antibody is specific to phagocytic macrophages and reactive microglia. The area and density of ED1 was semi-quantitated.

Results Few ED1 positive cells were observed in normal controls in the subarachnoid space (SAS) and cord vessels. Day 1 animals displayed ED1 positive cells within 50% of the subarachnoid space. ED1 positive cells within cord vessels were also slightly increased (10%). Day 5 animals showed strong staining through 50% of grey matter, predominantly on the side of injury. This was also observed in cord above and below the level of Quisqualic Acid injection. Arachnoiditis was observed by day 10 combined with strong staining through grey and white matter. ED1 positive staining area was again increased by day 20, comprising 70% grey matter on the injured and non-injured sides of the cord, which was limited to the level Quisqualic Acid injection. By day 50 moderate staining was observed in the SAS and white matter.

Discussion Cytoplasmic antigen ED1 cells were observed, reaching a peak at 20 days following excito-toxic spinal cord injury. Phagocytic macrophage proliferation might play a role in secondary spinal cord damage and initial cyst formation. The role of macrophages and the release of their inflammatory cytokines, reactive nitrogen and oxygen intermediates require further examination.

Introduction

Wear debris and metal ions originating from metal on metal hip replacements have been widely shown to recruit and activate macrophages. These cells secrete chemokines and pro-inflammatory cytokines that lead to an adverse local tissue reaction (ALTR), frequently requiring early revision. The mechanism for this response is still poorly understood. It is well documented that cobalt gives rise to apoptosis, necrosis and reactive oxygen species generation. Additionally, cobalt stimulates T cell migration, although the effect on macrophage motility remains unknown. This study tests the hypothesis that cobalt ions and nanoparticles affect macrophage migration stimulating an ALTR.

Despite the satisfactory short-term implant survivor-ship, there is an increasing concern that the metal-on-metal hip resurfacing arthroplasty (MoMHRA) release large amount of very small wear particles and metal ions. The periprosthetic soft-tissue masses such as pseudotumours are being increasingly reported. These were found be locally destructive, requiring revision surgery in most patients. It has been suggested that either an immune reaction or cytotoxic effect of chromium(Cr) or cobalt(Co) may play a role in its aetiology. However, the effect of the phagocytosis of implant-associated metal nanoparticles on macrophages has not been elucidated. The aim of this study was to investigate the in vitro viability and proliferative response of murine macrophages to clinically relevant metal nanoparticles and ions.

Materials and Methods: The RAW 264.7 murine macrophage cell line was cultured in MEM at a seeding density of 10E5 cells/cm2. Culture was set up in the presence of either:(1) negative control: medium alone;(2)Cobalt sulphate heptahyrate and chromium chloride hexahydrate (Sigma) at concentrations of 1uM, 10uM, 100uM;(3)Metal nanoparticles sized 30–35nm (American Elements) of cobalt, chromium and titanium at concentrations from 10E7 to 10E14 particles/ml.

At the end of day 1 and 4, two methods were used to quantify cell proliferation and viability. The AlamarBlue assay(Invitrogen) incorporates a fluorimetric growth indicator and the fluorescence signal correlates with metabolic activity of the cells. LIVE/DEAD stain kit(Molecular Probes) contains two fluorescent dyes to stain living cells green and dead cells red. The viability was calculated by the number of live cells divided by total cell numbers. Inter-group comparisons were performed using one-way ANOVA with Tukey post hoc test. Differences at p< 0.05 were considered to be significant.

Results: Compared with control, Alamar blue assay showed inhibition of cell proliferation in all three metal particles (p< 0.05). The Live/Dead staining showed Co nanoparticles were cytotoxic to most of cells Day 1 and Day 4 at 10E11/mL. At 10E13/mL, the Cr group showed cytotoxicity at day 4 (p< 0.05). There was no difference between Ti and control group. The Co2+ and Cr3+ ions led to inhibition to cell proliferation. At 10uM concentration, Co2+ caused a dramatic decrease in cell number. Live/Dead staining showed that Co2+ were toxic to cells (p< 0.05). Cr3+ group showed cytotoxicity at Day 4 (p< 0.05).

Discussion: This study demonstrates that Co and Cr nanoparticles and ions have dose-dependent proliferation and cytotoxic effects on the macrophages in vitro. The cytotoxicity occurred at the high concentration range that is found in the hip aspirates of MoMHRA patients with pseudotumours. This suggests the formation of pseudotumour may be the local sequelae of cytotoxicity due to increased production of metal wear nanoparticles.

Metal and their alloys have been widely used as implantable materials and prostheses in orthopaedic surgery. However, concerns exist as the metal nanoparticles released from wear of the prostheses cause clinical complications and in some cases result in catastrophic host tissue responses. The mechanism of nanotoxicity and cellular responses to wear metal nanoparticles are largely unknown. The aim of this study was to characterise macrophage phagocytosed cobalt/chromium metal nanoparticles both in vitro and in vivo, and investigate the consequent cytotoxicity. Two types of macrophage cell lines, murine RAW246.7 and human THP-1s were used for in vitro study, and tissues retrieved from pseudotumour patients caused by metal-on-metal hip resurfacing (MoMHR) were used for ex vivo observation. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) in combination with backscatter, energy-disperse X-ray spectrometer (EDS), focused ion beam (FIB) were employed to characterise phagocytosed metal nanoparticles. Alamar blue assay, cell viability assays in addition to confocal microscopy in combination with imaging analysis were employed to study the cytotoxiticy in vitro. The results showed that macrophages phagocytosed cobalt and chromium nanoparticles in vitro and the phagocytosed metal particles were confirmed by backscatter SEM+EDS and FIB+EDS. these particles were toxic to macrophages at a dose dependent manner. The analysis of retrieved tissue from revision of MoMHR showed that cobalt/chromium metal nanoparticles were observed exclusively in living macrophages and fragments of dead macrophages, but they were not seen within either live or dead fibroblasts. Dead fibroblasts were associated with dead and disintegrated macrophages and were not directly in contact with metal particles; chromium but not cobalt was the predominant component remaining in tissue. We conclude that as an important type of innate immune cells and phagocytes, macrophages play a key role in metal nanoparticles related cytotoxicity. Metal nanoparticles are taken up mainly by macrophages. They corrode in an acidic environment of the phagosomes. Cobalt that is more soluble than chromium may release inside macrophages to cause death of individual nanoparticle-overloaded macrophages. It is then released into the local environment and results in death of fibroblasts and is subsequently leached from the tissue.