Toll-like receptors-2 and 4 are overexpressed in an experimental model of particle-induced osteolysis

Friend or foe? Inflammation and the foreign body response to orthopedic biomaterials

The use of biomaterials and implants for joint replacement, fracture fixation, spinal stabilization and other orthopedic indications has revolutionized patient care by reliably decreasing pain and improving function. These surgical procedures always invoke an acute inflammatory reaction initially, that in most cases, readily subsides. Occasionally, chronic inflammation around the implant develops and persists; this results in unremitting pain and compromises function. The etiology of chronic inflammation may be specific, such as with infection, or be unknown. The histological hallmarks of chronic inflammation include activated macrophages, fibroblasts, T cell subsets, and other cells of the innate immune system. The presence of cells of the adaptive immune system usually indicates allergic reactions to metallic haptens. A foreign body reaction is composed of activated macrophages, giant cells, fibroblasts, and other cells often distributed in a characteristic histological arrangement; this reaction is usually due to particulate debris and other byproducts from the biomaterials used in the implant. Both chronic inflammation and the foreign body response have adverse biological effects on the integration of the implant with the surrounding tissues. Strategies to mitigate chronic inflammation and the foreign body response will enhance the initial incorporation and longevity of the implant, and thereby, improve long-term pain relief and overall function for the patient. The seminal research performed in the laboratory of Dr. James Anderson and co-workers has provided an inspirational and driving force for our laboratory's work on the interactions and crosstalk among cells of the mesenchymal, immune, and vascular lineages, and orthopedic biomaterials. Dr. Anderson's delineation of the fundamental biologic processes and mechanisms underlying acute and chronic inflammation, the foreign body response, resolution, and eventual functional integration of implants in different organ systems has provided researchers with a strategic approach to the use of biomaterials to improve health in numerous clinical scenarios.

Macrophage Polarization and the Osteoimmunology of Periprosthetic Osteolysis

PURPOSE OF REVIEW

Joint replacement has revolutionized the treatment of end-stage arthritis. We highlight the key role of macrophages in the innate immune system in helping to ensure that the prosthesis-host interface remains biologically robust.

RECENT FINDINGS

Osteoimmunology is of great interest to researchers investigating the fundamental biological and material aspects of joint replacement. Constant communication between cells of the monocyte/macrophage/osteoclast lineage and the mesenchymal stem cell-osteoblast lineage determines whether a durable prosthesis-implant interface is obtained, or whether implant loosening occurs. Tissue and circulating monocytes/macrophages provide local surveillance of stimuli such as the presence of byproducts of wear and can quickly polarize to pro- and anti-inflammatory phenotypes to re-establish tissue homeostasis. When these mechanisms fail, periprosthetic osteolysis results in progressive bone loss and painful failure of mechanical fixation. Immune modulation of the periprosthetic microenvironment is a potential intervention to facilitate long-term durability of prosthetic interfaces.

The Expression Levels of Toll-like Receptors after Metallic Particle and Ion Exposition in the Synovium of a Murine Model

To date, the exact role of specific Toll-like receptors (TLRs) in regulating immune reactivity to metallic byproducts of orthopedic implants has not been fully clarified. In light of the situation, our objective in this investigation was to assess the expression levels of surface TLRs after metallic particle and ion exposure in an established animal model. Ten female BALB/c mice in each group received intra-articular injections of phosphate buffer (PBS) (control), metallic particles (MP), and metallic ions (MI), respectively. Seven days later, immunohistochemical staining was undertaken in the synovial layer of the murine knee joints using anti-TLR 1, 2, 4, 5, and 6 polyclonal antibodies. In addition to increased cellular infiltrates and a hyperplastic synovial membrane, the MP group showed significantly elevated TLR expression compared to the control group and had higher TLR 1-, 4-, and 6-positive cells than the MI group (p < 0.0167). TLR 4- and TLR 6-positive cells were significantly augmented for the MI group compared to the control group (p < 0.0167). Additionally, greenish corrosion particles found in the necrotic tissue suggested that metallic particles might release a certain level of locally toxic metallic ions in vivo.

The use of bone turnover markers for monitoring the treatment of osteoporosis in postmenopausal females undergoing total knee arthroplasty: a prospective randomized study

BACKGROUND

Osteoporosis (OP) and osteoarthritis (OA) commonly coexist in postmenopausal females. The decrease in bone density and increase in bone resorption in postmenopausal females with OP may consequently affect the surgical outcome of total knee arthroplasty (TKA). However, clinicians often ignore monitoring the treatment of OP in the perioperative management of TKA. Bone turnover marker (BTM) can timely and accurately reflect bone metabolism to monitor the treatment of OP. The purpose of this study was to investigate the effect of BTM monitoring to guide the treatment of OP in postmenopausal females undergoing TKA.

METHODS

Postmenopausal females with OP who underwent primary unilateral TKA were randomly divided into two groups (monitoring group and control group), given oral medication (alendronate, calcitriol, and calcium), and followed for 1 year. In the monitoring group, serum BTMs (C-telopeptide of type I collagen (CTX-I), N-terminal propeptide of type I procollagen (PINP), and 25(OH)D) were assessed preoperatively and repeated postoperatively; alendronate was withdrawn when CTX-I and PINP reached the reference interval; and calcitriol and calcium were withdrawn when 25(OH)D reached the reference interval. In the control group, oral medication was implemented for a uniform duration of 3 months. During the 1-year follow-up, the mean maximum total point motion (MTPM) of the tibial component, bone mineral density (BMD), visual analog scale (VAS) score, range of motion, and Oxford Knee Score (OKS) score were obtained.

RESULTS

In the monitoring group, BTM monitoring prolonged the medication duration, but did not cause more adverse reactions than in the control group. The mean MTPM values at 6 m and 12 m in the monitoring group were lower than those in the control group, and the BMD at 12 m in the monitoring group was significantly higher than that in the control group. Patients in the monitoring group had lower VAS scores at 6 m and higher OKS scores at 6 m and 12 m than those in the control group.

CONCLUSION

In postmenopausal females with osteoporosis undergoing primary TKA, the application of BTM monitoring to guide the treatment of osteoporosis can enhance bone density, maintain prosthesis stability, and improve surgical outcome.

TRIAL REGISTRATION

ChiCTR ChiCTR-INR-17010495 . Registered on 22 January 2017.

circRNA expression pattern and ceRNA network in the pathogenesis of aseptic loosening after total hip arthroplasty

Increasing evidence has demonstrated that circular RNA (circRNA) exerts important function in the pathogenesis of some diseases. While, the contributions of circRNAs to aseptic loosening after total hip arthroplasty (THA) remain largely unknown. Our research is to explore the differentially expressed circRNAs (DEcircRNAs) and elucidate complex regulated mechanism of circRNAs in aseptic loosening. The DEcircRNAs were identified by RNA sequencing (RNA-seq) analysis. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was adopted to corroborate these DEcircRNAs. The potential function of circRNAs in aseptic loosening tissue was identified by competing endogenous RNA (ceRNA) analysis. Enrichment analysis was performed for target mRNAs and host genes of the DEcircRNAs by Gene Oncology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). 257 DEcircRNAs were obtained from RNA-seq results. Following the RT-qPCR corroboration, 6 circRNAs (hsa_circ_0007482, hsa_circ_0005232, hsa_circ_0000994, hsa_circ_0000690, hsa_circ_0058092 and hsa_circ_0004496) were selected for further analysis. By circRNA-miRNA and miRNA-mRNA prediction, 6 circRNAs, 138 miRNAs and 1667 mRNAs were identified. Then, circRNA-miRNA-mRNA network was established. The result of GO and KEGG enrichment analysis suggested that the circRNAs were related with some biological functions and pathways of aseptic loosening. A novel pathogenesis and treatment strategy about aseptic loosening after THA was revealed from our study of circRNA-miRNA-mRNA network.

Exosomal lncRNAs NONMMUT000375.2 and NONMMUT071578.2 derived from titanium particle treated RAW264.7 cells regulate osteogenic differentiation of MC3T3-E1 cells

Periprosthetic osteolysis and the subsequent aseptic loosening can lead to the failure of joint replacement. Wear particles are well known to be the initiative cause inducing osteolysis through enhancing osteoclast-mediated bone resorption and reducing osteogenic differentiation. The purpose of this study was to investigate the effects of osteoclast-secreted exosomal long noncoding RNAs (lncRNAs) on osteogenesis in the process of particle-induced osteolysis. RAW264.7 cells were treated by titanium particles (TI). The inflammatory cytokines were increased, and expression of Receptor Activator of Nuclear Factor-κB and Nuclear factor of activated T cells c1 were also increased, indicating osteoclast differentiation occurred. The purified exosomes from RAW264.7 cells induced with TI inhibited osteogenic differentiation of MC3T3-E1 cells. RNA sequencing generated lncRNAs expression profiles (458 up-regulated and 1641 down-regulated) of the exosomes derived from RAW264.7 cells treated with TI. Based on the results of gene ontology/Kyoto Encyclopedia of Genes and Genomes analysis and quantitative real time polymerase chain reaction validation, we confirmed two candidate lncRNAs, NONMMUT000375.2 and NONMMUT071578.2. The regulation network presented that some vital genes involved in osteoclast differentiation, such as Bcl2, Wnt11, TGF-β, and Pdk1, were under the regulation of NONMMUT000375.2 and NONMMUT071578.2. Taken together, exosomes derived from TI treated RAW264.7 cells inhibit the osteogenic activity of MC3T3-E1 cells. Exosomal lncRNAs, NONMMUT000375.2 and NONMMUT071578.2 may potentially play their roles in promoting osteoclast differentiation and suppressing osteogenesis, which aggravates the osteoclastogenesis/osteogenesis imbalance.

Comparative Analysis of NF-κB in the MyD88-Mediated Pathway After Implantation of Titanium Alloy and Stainless Steel and the Role of Regulatory T Cells

OBJECTIVE

Development of immunologically smart implants, integrated to biological systems, is a key aim to minimize the inflammatory response of the host to biomaterial implants.

METHODS

The aim of this study is to investigate the influence of titanium alloy and stainless steel implants on immunological responses in rats by comparative analysis of nuclear factor kappa B (NF-κB) profiles in the activation of inflammatory signaling pathways and the role of CD4+CD25+Foxp3+.

RESULTS

Both Ti alloy and stainless steel alloy group implantation affect Toll-like receptors-4 pathways and CD4+CD25+ regulatory T cells in different ways.

CONCLUSIONS

Results show that NF-κB/p65 and NF-κB1/p50 possess potential as a therapeutic target in the prevention of adverse reactions to metal, especially for controlling inflammation after the implantation.

GEO-PGS composite shows synergistic and complementary effect on Escherichia coli and improvement of intestinal dysfunction

Palygorskite (PGS) is a kind of clay minerals with the property of absorbent capacity, and ginger essential oil (GEO) is a kind of natural antibacterial substances. In the present study PGS was used as carrier of GEO, and thus, a kind of new anti-bacterial composite GEO-PGS has been obtained. Characterization, inhibitory effect of GEO-PGS on Escherichia coli (E. coli) and its function of improvement of intestinal health would be investigated. Results showed that characterization analysis of GEO-PGS (FTIR, TG-DSC, BET, Zeta potential, specific surface area, total pore volume and size, TEM observation) demonstrated combination of GEO and PGS, and GEO was absorbed on the surface of PGS, partially filled the micropores of PGS. GEO-PGS had obvious inhibitory effect on E.coli, in combination of the antibacterial activity of GEO and bacteria-absorbed capability of PGS. GEO-PGS also had ameliorating effect on enteritis and intestinal dysfunction in vivo, which might be related to the inhibition of gene expression of inflammatory cytokines (TLR2, IL-6, TNFα, and IL-8). In conclusion, the novel composite GEO-PGS has the potential usage as functional component having effect of improving intestinal health.

Diagnosis and management of implant debris-associated inflammation

Introduction: Total joint replacement is one of the most common, safe, and efficacious operations in all of surgery. However, one major long-standing and unresolved issue is the adverse biological reaction to byproducts of wear from the bearing surfaces and modular articulations. These inflammatory reactions are mediated by the innate and adaptive immune systems.Areas covered: We review the etiology and pathophysiology of implant debris-associated inflammation, the clinical presentation and detailed work-up of these cases, and the principles and outcomes of non-operative and operative management. Furthermore, we suggest future strategies for prevention and novel treatments of implant-related adverse biological reactions.Expert opinion: The generation of byproducts from joint replacements is inevitable, due to repetitive loading of the implants. A clear understanding of the relevant biological principles, clinical presentations, investigative measures and treatments for implant-associated inflammatory reactions and periprosthetic osteolysis will help identify and treat patients with this issue earlier and more effectively. Although progressive implant-associated osteolysis is currently a condition that is treated surgically, with further research, it is hoped that non-operative biological interventions could prolong the lifetime of joint replacements that are otherwise functional and still salvageable.

Periprosthetic Osteolysis: Mechanisms, Prevention and Treatment

Clinical studies, as well as in vitro and in vivo experiments have demonstrated that byproducts from joint replacements induce an inflammatory reaction that can result in periprosthetic osteolysis (PPOL) and aseptic loosening (AL). Particle-stimulated macrophages and other cells release cytokines, chemokines, and other pro-inflammatory substances that perpetuate chronic inflammation, induce osteoclastic bone resorption and suppress bone formation. Differentiation, maturation, activation, and survival of osteoclasts at the bone-implant interface are under the control of the receptor activator of nuclear factor kappa-Β ligand (RANKL)-dependent pathways, and the transcription factors like nuclear factor κB (NF-κB) and activator protein-1 (AP-1). Mechanical factors such as prosthetic micromotion and oscillations in fluid pressures also contribute to PPOL. The treatment for progressive PPOL is only surgical. In order to mitigate ongoing loss of host bone, a number of non-operative approaches have been proposed. However, except for the use of bisphosphonates in selected cases, none are evidence based. To date, the most successful and effective approach to preventing PPOL is usage of wear-resistant bearing couples in combination with advanced implant designs, reducing the load of metallic and polymer particles. These innovations have significantly decreased the revision rate due to AL and PPOL in the last decade.

Cytokine Profile in Patients with Aseptic Loosening of Total Hip Replacements and Its Relation to Metal Release and Metal Allergy

Metal release from total hip replacements (THRs) is associated with aseptic loosening (AL). It has been proposed that the underlying immunological response is caused by a delayed type IV hypersensitivity-like reaction to metals, i.e., metal allergy. The purpose of this study was to investigate the immunological response in patients with AL in relation to metal release and the prevalence of metal allergy. THR patients undergoing revision surgery due to AL or mechanical implant failures were included in the study along with a control group consisting of primary THR patients. Comprehensive cytokine analyses were performed on serum and periimplant tissue samples along with metal analysis using inductive coupled plasma mass spectrometry (ICP-MS). Patient patch testing was done with a series of metals related to orthopedic implant. A distinct cytokine profile was found in the periimplant tissue of patients with AL. Significantly increased levels of the proinflammatory cytokines IL-1β, IL-2, IL-8, IFN-γ and TNF-α, but also the anti-inflammatory IL-10 were detected. A general increase of metal concentrations in the periimplant tissue was observed in both revision groups, while Cr was significantly increased in patient serum with AL. No difference in the prevalence of metal sensitivity was established by patch testing. Increased levels of IL-1β, IL-8, and TNF-α point to an innate immune response. However, the presence of IL-2 and IFN-γ indicates additional involvement of T cell-mediated response in patients with AL, although this could not be detected by patch testing.

LncRNA TSIX promotes osteoblast apoptosis in particle-induced osteolysis by down-regulating miR-30a-5p

OBJECTIVE

This study aims to investigate the role of TSIX/miR-30a-5p axis in particle-induced osteolysis (PIO).

METHOD

PIO mouse model was established by the implantation of Co-Cr-Mo metal particles (CoPs). MC3T3-E1 cells received CoPs stimulation. Bone mineral density (BMD) in the skull was detected to evaluate PIO development. The expression of TSIX and miR-30a-5p was detected by using qRT-PCR. Osteoblast apoptosis was measured using flow cytometry. RNA pull-down was used to verify the regulatory relationship between TSIX and miR-30a-5p.

RESULT

The results showed that BMD of the skull in PIO mice was significantly decreased compared with control mice, which indicated that the PIO model was established successfully. Moreover, CoPs could up-regulate TSIX level, down-regulate miR-30a-5p expression, and promote osteoblast apoptosis in vivo and in vitro. The results also found that TSIX negatively regulated miR-30a-5p expression, and knockdown of TSIX inhibited Runx2 expression. As expected, miR-30a-5p inhibitor could reverse the inhibition of si-TSIX on osteoblast apoptosis.

CONCLUSION

TSIX played a pivotal role in PIO development by negatively regulating miR-30a-5p.

Inhibitory effect of Zanthoxylum bungeanum essential oil (ZBEO) on Escherichia coli and intestinal dysfunction

The inhibitory effects of Zanthoxylum bungeanum essential oil (ZBEO) on Escherichia coli (E. coli) in vitro and in vivo were investigated, as well as its function of improvement of intestinal health. The results of in vitro studies, such as minimal inhibitory concentration (MIC) analysis, agar disc diffusion test and growth curve analysis of E. coli, showed that ZBEO had an excellent inhibitory effect on the growth of E. coli, which may be related to the loss of the normal shape of the cell membranes and the leakage of intracellular constituents, on the basis of SEM observation and cell constituents' release assay. ZBEO also had an inhibitory effect on enteritis and intestinal dysfunction induced by infection of E. coli in vivo, and histopathological observation indicated that ZBEO could markedly ameliorate the structural destruction of intestinal tissues, which might be related to its inhibitory effect on the gene expression of inflammatory cytokines (TLR2, TLR4, TNF_α and IL-8). In conclusion, ZBEO showed an excellent inhibitory effect on E. coli both in vitro and in vivo, suggesting the potential application of ZBEO as a kind of functional component having the effects of improving intestinal function and health.

Danger of frustrated sensors: Role of Toll-like receptors and NOD-like receptors in aseptic and septic inflammations around total hip replacements

The innate immune sensors, Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), can recognize not only exogenous pathogen-associated molecular patterns (PAMPs), but also endogenous molecules created upon tissue injury, sterile inflammation, and degeneration. Endogenous ligands are called damage-associated molecular patterns (DAMPs), and include endogenous molecules released from activated and necrotic cells as well as damaged extracellular matrix. TLRs and NLRs can interact with various ligands derived from PAMPs and DAMPs, leading to activation and/or modulation of intracellular signalling pathways. Intensive research on the innate immune sensors, TLRs and NLRs, has brought new insights into the pathogenesis of not only various infectious and rheumatic diseases, but also aseptic foreign body granuloma and septic inflammation of failed total hip replacements (THRs). In this review, recent knowledge is summarized on the innate immune system, including TLRs and NLRs and their danger signals, with special reference to their possible role in the adverse local host response to THRs.

Translational potential of this article: A clear understanding of the roles of Toll-like receptors and NOD-like receptors in aseptic and septic loosening of joint replacements will facilitate potential strategies to mitigate these events, thereby extending the longevity of implants in humans.

TLR4 (not TLR2) dominate cognate TLR activity associated with CoCrMo implant particles

Innate immune reactions to orthopedic implant debris are the primary cause of total joint replacement (TJR) failure over the long term (15-20 years). The role of pathogen associated pattern recognition receptors (i.e., TLRs) in regulating immune reactivity to metal implant particles remains controversial. Do different TLRs (i.e., TLR2 vs. TLR4) activated by their respective ligands in concert with metal implant debris elicit equivalent innate immune responses? In this investigation, our in vitro and in vivo data indicate that Gram-negative PAMPs are more pro-inflammatory than Gram-positive PAMPs. In vitro results indicated TLR4 activation in concert with CoCrMo orthopedic implant debris (CoCrMo/LPS+) challenged primary macrophages resulted in significantly greater inflammatory responses than CoCrMo/PAM3CSK+ (TLR2). Similarly, in vivo results indicated CoCrMo/LPS+ TLR4 challenge induced a twofold increase in inflammation-induced bone resorption (osteolysis) than CoCrMo/PAM3CSK+ (p < 0.01) or CoCrMo (p < 0.03) alone in an established murine calvaria model. This points to a more potent TLR4-based effect of CoCrMo/LPS+ on innate immune responses, that is, IL-1ß, TNF-α, and resulting osteolysis. Differential CoCrMo/LPS+ induced osteolysis compared to CoCrMo/PAM3CSK+, reveals inherent differences in TLR4 versus TLR2 activation which are relevant to (i) how different types of implant debris elicit differential reactivity, (ii) how TLR2 Gram-positive bacteria benefits from less immune activation possibly due to the down-regulation of TLR2 surface expression, that subsequently impacts Gram-positive infections in TJRs, and (iii) how using TLR4 LPS (a Gram-negative agonist) may not accurately model Gram-positive bacteria responses, alone and/or with specific types of implant particles, particularly CoCrMo alloy. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1007-1017, 2017.

The Biologic Response to Bearing Materials

Total joint arthroplasty (TJA) is a common and highly successful orthopaedic procedure for which surgeons can use different bearing materials. The materials used for TJA must be both biocompatible to minimize adverse local tissue reactions and robust enough to support weight bearing during common daily activities. Modern bearing materials for TJA are made from metals and their alloys, polymers, and ceramics. The orthopaedic surgeon should be knowledgeable about the biologic response to the different bearing materials used for TJA, as well as the wear by-products generated.

Aging Affects Bone Marrow Macrophage Polarization: Relevance to Bone Healing

Macrophages are an important component of the inflammatory cascade by initiating and modulating the processes leading to tissue regeneration and bone healing. Depending on the local environment, macrophages can be polarized into M1 (pro-inflammatory) or M2 (anti-inflammatory) phenotypes. In order to assess the effects of aging on macrophage function, bone marrow macrophage polarization using primary bone marrow macrophages (BMMs) from young (8 weeks old) and aged (72 weeks old) wild-type male C57BL/6J mice was analyzed. Fluorescence-activated cell sorting (FACS) analysis (CD11b, iNOS, CD206), qRT-PCR (iNOS, TNF-α, CD206, Arginase 1), and ELISA (TNF-α, IL-1ra) were performed to compare the M1 and M2 phenotypic markers in young and aged mouse macrophages. Once M1 and M2 macrophage phenotypes were confirmed, the results showed that TNF-α mRNA was significantly upregulated in aged M1s after interferon gamma (INF-γ) exposure. Arginase 1 and CD206 mRNA expression were still upregulated with IL4 stimulation in aged macrophages, but to a lesser extend than those from younger animals. TNF-α secretion was also significantly increased in aged M1s compared to young M1s, following lipopolysaccharide (LPS) exposure. However, the IL-1ra secretion did not increase accordingly in aged mice. The results demonstrate that, compared to younger animals, aging of bone marrow derived macrophages increases the resting levels of oxidative stress, and the ratios of pro- to anti-inflammatory markers. These age-related changes in macrophage polarization may explain in part the attenuated response to adverse stimuli and delay in processes such as fracture healing seen in the elderly.

LAY SUMMARY

Bone healing is a complex process that involves both biological and mechanical factors. Macrophages are key cells that regulate the events involved in bone healing, especially the initial inflammatory phase. In this biological cascade of events, macrophages present as different functional phenotypes including uncommitted (M0), pro-inflammatory (M1), and anti-inflammatory (M2), a process called macrophage polarization. A clear understanding of the effects of aging on macrophage polarization is critical to modulating adverse events such as fractures, atraumatic bone loss, and tissue regeneration in an aging population.

The biological response to orthopedic implants for joint replacement. II: Polyethylene, ceramics, PMMA, and the foreign body reaction

Novel evidence-based prosthetic designs and biomaterials facilitate the performance of highly successful joint replacement (JR) procedures. To achieve this goal, constructs must be durable, biomechanically sound, and avoid adverse local tissue reactions. Different biomaterials such as metals and their alloys, polymers, ceramics, and composites are currently used for JR implants. This review focuses on (1) the biological response to the different biomaterials used for TJR and (2) the chronic inflammatory and foreign-body response induced by byproducts of these biomaterials. A homeostatic state of bone and surrounding soft tissue with current biomaterials for JR can be achieved with mechanically stable, infection free and intact (as opposed to the release of particulate or ionic byproducts) implants. Adverse local tissue reactions (an acute/chronic inflammatory reaction, periprosthetic osteolysis, loosening and subsequent mechanical failure) may evolve when the latter conditions are not met. This article (Part 2 of 2) summarizes the biological response to the non-metallic materials commonly used for joint replacement including polyethylene, ceramics, and polymethylmethacrylate (PMMA), as well as the foreign body reaction to byproducts of these materials. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1685-1691, 2017.

Aging, inflammation, stem cells, and bone healing

Complex interactions among cells of the monocyte-macrophage-osteoclast lineage and the mesenchymal stem cell-osteoblast lineage play a major role in the pathophysiology of bone healing. Whereas the former lineage directs inflammatory events and bone resorption, the latter represents a source of cells for bone regeneration and immune modulation. Both of these lineages are affected by increasing age, which is associated with higher baseline levels of inflammatory mediators, and a significant reduction in osteogenic capabilities. Given the above, fracture healing, osteoporosis, and other related events in the elderly present numerous challenges, which potentially could be aided by new therapeutic approaches to modulate both inflammation and bone regeneration.

Evidence that osteocyte perilacunar remodelling contributes to polyethylene wear particle induced osteolysis

Periprosthetic osteolysis (PO) leading to aseptic loosening, is the most common cause of failure of total hip replacement (THR) in the mid- to long-term. Polyethylene (PE) particulates from the wear of prosthesis liners are bioactive and are implicated in the initiation and or progression of osteolysis. Evidence exists that cells of the osteoblast/osteocyte lineage are affected by PE particles and contribute to the catabolic response by promoting osteoclastic bone resorption. In this study, we hypothesised that osteocytes contribute directly to PO by removing bone from their perilacunar matrix. Osteocyte responses to ultra-high molecular weight PE (UHMWPE) particles were examined in vitro in human primary osteocyte-like cultures, in vivo in the mouse calvarial osteolysis model, and in the acetabulum of patients undergoing revision total hip replacement (THR) surgery for PO. Osteocytes exposed to UHMWPE particles showed upregulated expression of catabolic markers, MMP-13, carbonic anhydrase 2 (CA2), cathepsin K (CTSK) and tartrate resistant acid phosphatase (TRAP), with no effect on cell viability, as assessed by Caspase 3 activity. Consistent with this catabolic activity causing perilacunar bone loss, histological analysis of calvarial sections from mice exposed to UHMWPE revealed a significant (p<0.001) increase in osteocyte lacunar area (Lac.Ar) compared to sham-operated animals. Furthermore, acetabular biopsies from patients with PO also showed significantly (p<0.001) increased osteocyte lacunar size in trabecular bone adjacent to PE particles, compared with osteocyte lacunar size in bone from primary THR patients. Together, these findings suggest a previously unrecognised action of UHMWPE wear particles on osteocytes, which directly results in a loss of osteocyte perilacunar bone. This action may exacerbate the indirect pro-osteoclastic action of UHMWPE-affected osteocytes, previously shown to contribute to aseptic loosening of orthopaedic implants.

STATEMENT OF SIGNIFICANCE

This study addresses the clinical problem of periprosthetic osteolysis, bone loss in response to polyethylene wear particles derived from materials used in orthopaedic implants. Periprosthetic osteolysis has been thought to be due largely to wear particles stimulating the activity of bone resorbing osteoclasts. However, in this study we demonstrate for the first time that polyethylene particles stimulate another type of bone loss, mediated by the direct activity of bone mineral embedded osteocytes, termed osteocytic osteolysis or osteocyte perilacunar remodelling. This study provides new mechanistic insight into wear-particle mediated bone loss and represents a new paradigm for the way in which bone cells, namely osteocytes, the key controlling cell type in bone, react to biomaterials.

Innate immunity sensors participating in pathophysiology of joint diseases: a brief overview

The innate immune system consists of functionally specialized "modules" that are activated in response to a particular set of stimuli via sensors located on the surface or inside the tissue cells. These cells screen tissues for a wide range of exogenous and endogenous danger/damage-induced signals with the aim to reject or tolerate them and maintain tissue integrity. In this line of thinking, inflammation evolved as an adaptive tool for restoring tissue homeostasis. A number of diseases are mediated by a maladaptation of the innate immune response, perpetuating chronic inflammation and tissue damage. Here, we review recent evidence on the cross talk between innate immune sensors and development of rheumatoid arthritis, osteoarthritis, and aseptic loosening of total joint replacements. In relation to the latter topic, there is a growing body of evidence that aseptic loosening and periprosthetic osteolysis results from long-term maladaptation of periprosthetic tissues to the presence of by-products continuously released from an artificial joint.

Innate immune reactions in septic and aseptic osteolysis around hip implants

According to the long-standing definition, septic and aseptic total joint replacement loosening are two distinct conditions with little in common. Septic joint replacement loosening is driven by bacterial infection whereas aseptic loosening is caused by biomaterial wear debris released from the bearing surfaces. However, recently it has been recognized that the mechanisms that drive macrophage activation in septic and aseptic total joint replacement loosening resemble each other. In particular, accumulating evidence indicates that in addition to mediating bacterial recognition and the subsequent inflammatory reaction, toll-like receptors (TLRs) and their ligands, pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPS), play a key role in wear debris-induced inflammation and macrophage activation. In addition, subclinical bacterial biofilms have been identified from some cases of seemingly aseptic implant loosening. Furthermore, metal ions released from some total joint replacements can activate TLR signaling similar to bacterial derived PAMPs. Likewise, metal ions can function as haptens activating the adaptive immune system similar to bacterial derived antigens. Thus, it appears that aseptic and septic joint replacement loosening share similar underlying pathomechanisms and that this strict dichotomy to sterile aseptic and bacterial-caused septic implant loosening is somewhat questionable. Indeed, rather than being two, well-defined clinical entities, peri-implant osteolysis is, in fact, a spectrum of conditions in which the specific clinical picture is determined by complex interactions of multiple local and systemic factors.

Innate Immunity and Biomaterials at the Nexus: Friends or Foes

Biomaterial implants are an established part of medical practice, encompassing a broad range of devices that widely differ in function and structural composition. However, one common property amongst biomaterials is the induction of the foreign body response: an acute sterile inflammatory reaction which overlaps with tissue vascularisation and remodelling and ultimately fibrotic encapsulation of the biomaterial to prevent further interaction with host tissue. Severity and clinical manifestation of the biomaterial-induced foreign body response are different for each biomaterial, with cases of incompatibility often associated with loss of function. However, unravelling the mechanisms that progress to the formation of the fibrotic capsule highlights the tightly intertwined nature of immunological responses to a seemingly noncanonical “antigen.” In this review, we detail the pathways associated with the foreign body response and describe possible mechanisms of immune involvement that can be targeted. We also discuss methods of modulating the immune response by altering the physiochemical surface properties of the biomaterial prior to implantation. Developments in these areas are reliant on reproducible and effective animal models and may allow a “combined” immunomodulatory approach of adapting surface properties of biomaterials, as well as treating key immune pathways to ultimately reduce the negative consequences of biomaterial implantation.

Interaction of Materials and Biology in Total Joint Replacement - Successes, Challenges and Future Directions

Total joint replacement (TJR) has revolutionized the treatment of end-stage arthritic disorders. This success is due, in large part, to a clear understanding of the important interaction between the artificial implant and the biology of the host. All surgical procedures in which implants are placed in the body evoke an initial inflammatory reaction, which generally subsides over several weeks. Thereafter, a series of homeostatic events occur leading to progressive integration of the implant within bone and the surrounding musculoskeletal tissues. The eventual outcome of the operation is dependent on the characteristics of the implant, the precision of the surgical technique and operative environment, and the biological milieu of the host. If these factors and events are not optimal, adverse events can occur such as the development of chronic inflammation, progressive bone loss due to increased production of degradation products from the implant (periprosthetic osteolysis), implant loosening or infection. These complications can lead to chronic pain and poor function of the joint reconstruction, and may necessitate revision surgery or removal of the prosthesis entirely. Recent advances in engineering, materials science, and the immunological aspects associated with orthopaedic implants have fostered intense research with the hope that joint replacements will last a lifetime, and facilitate pain-free, normal function.

Bone and the innate immune system

The immune system and bone are intimately linked with significant physical and functionally related interactions. The innate immune system functions as an immediate response system to initiate protections against local challenges such as pathogens and cellular damage. Bone is a very specific microenvironment, in which infectious attack is less common but repair and regeneration are ongoing and important functions. Thus, in the bone the primary goal of innate immune and bone interactions is to maintain tissue integrity. Innate immune signals are critical for removal of damaged and apoptotic cells and to stimulate normal tissue repair and regeneration. In this review we focus on the innate immune mechanisms that function to regulate bone homeostasis.