Synergistic induction of cyclooxygenase-II by bacterial lipopolysaccharide in combination with particles of medical device materials in a murine macrophage cell line J774A.1

Activation of Macrophages by Lipopolysaccharide for Assessing the Immunomodulatory Property of Biomaterials<sup/>

The design paradigm of biomaterials has been changed to ones with favorable immunomodulatory effects, indicating the importance of accurately evaluating the immunomodulatory properties of biomaterials. Among all the immune cells macrophages receive most attention, due to their plasticity and multiple roles in the materials and host interactions, and thereby become model immune cells for the evaluation of immunomodulatory properties of biomaterials in many studies. Lipopolysaccharides (LPS), a polysaccharide in the outer membrane of Gram-negative bacteria, elicit strong immune responses, which was often applied to activate macrophages, resulting in a proinflammatory M1 phenotype, and the release of proinflammatory cytokines, including tumor necrosis factor alpha (TNFα), interleukin (IL)-1, and IL-6. However, there is no consensus on how to apply macrophages and LPS to detect the immunomodulatory properties of biomaterials. The lack of scientific consideration of this issue has led to some inaccurate and insufficient conclusions on the immunomodulatory properties of biomaterials, and inconsistences between different research groups. In this study, we carried out a systemic study to investigate the stimulatory effects of LPS with different times, doses, and conditions on the activation of macrophages. An experimental pathway was proposed accordingly for the activation of macrophages using LPS for assessing the immunomodulatory property of biomaterials.

Lipoteichoic acid modulates inflammatory response in macrophages after phagocytosis of titanium particles through Toll-like receptor 2 cascade and inflammasomes

Toll-like receptor 2 (TLR2) and nucleotide-binding and oligomerization domain-like receptors with a pyrin domain 3 (NLRP3) inflammasomes have been presumed to participate in the pathogenesis of aseptic implant loosening. The aim of this study is to analyze the cellular localization of TLR2 and NLRP3 inflammasomes in the periprosthetic tissue from aseptically loose hip implants as well as the expression of these molecules in macrophages stimulated in vitro with titanium particles (Ti) coated with lipoteichoic acid (LTA). Using immunohistochemistry, immunoreactivity of TLR2 and NLRP3 inflammasomes was found in macrophages within the foreign body granulomatosis. Using RAW264.7 cells, stimulation with Ti increased the messenger RNA (mRNA) levels of TLR2 and TNF-α. Stimulation with LTA-coated Ti enhanced mRNA levels of NLRP3 and IL-1β, whereas reinforced secretion of IL-1β was not detected in spite of marked release of TNF-α. Finally, the same cells with silenced Irak2, an adaptor protein in the TLR2 cascade, suppressed this NLRP3 upregulation. This study suggests that TLR2 and NLRP3 inflammasomes are factors involved in cross-talk mediating the foreign body type response to wear particles. In addition, discrepant behavior in the release between TNF-α and IL-1β release may explain the variable pathomechanisms of aseptic implant loosening without acute inflammatory reactions.

Potential proinflammatory and osteogenic effects of dicalcium silicate particles in vitro

BACKGROUND

Due to their biocompatibility and bioactivity, dicalcium silicate (C2S) and hydroxyapatite (HA) are used as coating materials for prosthetic orthopedic and dental implants or as bone substitute materials to fill bone defects. However, prostheses and bone substitutes can release particles that trigger an immune response in the recipient. The immunological effects of C2S particles have not yet been studied.

OBJECTIVE

The aim of this study was to determine the cytotoxic effects of C2S particles on primary human monocytes, a human monocyte cell line (THP-1) and an osteoblast-like cell line (MG-63). The proinflammatory effects of C2S particles on THP-1 were also detected. Moreover, the osteogenic effects of C2S and HA on MG-63 cells were investigated.

METHODS

Characterization of C2S and HA was performed using scanning electron microscopy (SEM), energy dispersive analysis (EDS), X-ray diffraction (XRD), Brunner-Emmett-Teller (BET) measurements and laser diffraction. The cytotoxic effect of C2S on primary human monocytes as well as THP-1 and MG-63 cells was measured using Trypan blue assays, Cell Counting Kit-8 (CCK-8) assays and flow cytometry to detect apoptosis. THP-1 human monocytes with or without lipopolysaccharide (LPS) stimulation were exposed to C2S and HA for 6 and 24h. Thereafter, the mRNA expression and protein concentrations of MMP-2, MMP-9, TIMP-2, TIMP-1 and TNF-α were evaluated using real-time PCR and ELISA, respectively. RANKL and OPG mRNA expression levels in MG-63 cells were examined using real-time PCR.

RESULTS

No significant cytotoxicity was recorded when cells were directly cultured with C2S/HA particles. After THP-1 cells were cultured with C2S/HA for 24h, MMP-2, MMP-9 and TNF-α expression increased, whereas TIMP-2 and TIMP-1 expression decreased. Compared with HA, C2S slightly increased MMP-9 expression and slightly decreased TIMP-1 expression. The MMP: TIMP ratio increased in the C2S and HA groups; however, HA significantly increased the MMP-9: TIMP-1 ratio compared with C2S. Compared with HA, C2S caused less TNF-α production. C2S/HA did not modify the expression of proinflammatory mediators in LPS-stimulated cells. Furthermore, C2S/HA significantly increased OPG expression and slightly increased RANKL expression in MG-63 cells. C2S and HA decreased the RANKL: OPG ratio.

CONCLUSION

Our in vitro data suggest that C2S is relatively safe when directly cultured with cells. In addition, C2S may exert proinflammatory effects; however, compared with HA, C2S had fewer proinflammatory effects on THP-1. C2S and HA did not alter the LPS-induced production of proinflammatory mediators and had similar osteogenic effects on MG-63 cells.

Macrophage response to high number of titanium particles is cytotoxic and COX-2 mediated and it is not affected by the particle's endotoxin content or the cleaning treatment

Periprosthetic osteolysis is a progressive deterioration of bone around prostheses resulting primarily from the presence of wear debris. Particulate material, number, and their interactions with environmental factors play important roles in macrophage activation around implants. We have previously shown that macrophages cultured in the presence of high numbers of cleaned titanium (Ti) particles released significant amounts of PGE₂ that is potentially detrimental for bone. Cleaning of particles has become routine in most studies of macrophage/particle interactions as contaminating endotoxin elicits a macrophage cytokine response and since numerous studies have suggested that endotoxins may be present on implant materials. However, the strenuous cleaning procedure itself represents a possible source of other contaminants (such as material by-products) that may be relevant to the prostanoid response of macrophages. To analyze this hypothesis, the macrophage response to high numbers of cleaned Ti particles was compared to that of unclean particles and to particles that were subjected to a short version of the cleaning procedure. It was found that neither the high amount of endotoxin on the unclean particles nor the duration of the cleaning procedure had an effect on the release of PGE₂ and the decrease in cell viability in response to high number of Ti particles. Evidence of a possible relationship between these two variables is presented.

Toll-like receptors and their adaptors are regulated in macrophages after phagocytosis of lipopolysaccharide-coated titanium particles

Macrophages phagocytose metallic wear particles and produce mediators, which can induce cellular host response and aseptic implant loosening. Lipopolysaccharide (LPS) on the wear debris can stimulate macrophages via Toll-like receptor 4 (TLR4) and enhance the response. However, the precise functional role and interaction of TLRs and their adaptor molecules is still unclear. Rat bone marrow macrophages were stimulated with titanium particle (Ti) coated by LPS (Ti/LPS+) and LPS-free Ti (Ti/LPS-). mRNA levels of cytokines, TLRs and their adaptor molecules were measured using real time PCR. mRNA levels of TNF-α, IL-1β, and IL-6 increased in Ti/LPS+ than Ti/LPS-. In contrast, mRNA levels of TLR4, TLR5, and TLR9 decreased in Ti/LPS+ compared to Ti/LPS-. mRNA levels of MyD88, IRAK1, IRAK4 decreased gradually, and TRAF6 underwent an initial transient increase, followed by suppression in Ti/LPS+. However, mRNA levels of TLR2 and IRAK2 increased after phagocytosis of Ti/LPS+ than Ti/LPS-. The increased expressions of proinflammatory cytokines found in Ti/LPS+ indicated that their productions cytokines could be enhanced by phagocytosis of LPS-coated particles. Subsequent down-regulation of TLR4, TLR5, TLR9, MyD88, IRAK1, and IRAK4 suggests that self-protective mechanisms to regulate excessive host responses are activated in macrophages. Increase of TLR2 and IRAK2 and a transient increase of TRAF6 in Ti/LPS+ suggest that another possible pathway to modulate TLR-mediated cellular response to prolong inflammatory response in foreign body reaction of aseptic loosening. This down- and/or up-regulation of the potential TLR-mediated responses to LPS-coated particles reflects the proactive behavior of effector cells.

What other biologic and mechanical factors might contribute to osteolysis?

An overwhelming consensus exists that wear particles are the primary driving force in aseptic loosening of orthopaedic implants. Nonetheless, considerable evidence has emerged demonstrating that various other factors can modulate the biologic activity of orthopaedic wear particles. Two of the most studied modulating factors are bacterial endotoxins and implant motion.

The balance between endotoxin accumulation and clearance during particle-induced osteolysis in murine calvaria

Bacterial endotoxin may contribute to aseptic loosening of orthopedic implants even in the absence of clinical or microbiological evidence of infection. One potential source of endotoxin during aseptic loosening is systemically circulating endotoxin, derived from intestinal flora, minor infections, or dental procedures, that may bind to wear particles. The current study demonstrates that systemically derived endotoxin accumulates when 'endotoxin-free' titanium and polyethylene particles are implanted on murine calvaria. Time-course experiments and experiments using germ-free mice rule out the possibility that the observed endotoxin accumulation may be due to bacterial contamination. In contrast, endotoxin is cleared from titanium particles that originally carry high amounts of adherent endotoxin. The mechanism of endotoxin clearance is not dependent on induction of a respiratory burst. Taken together, these results indicate that a balance between endotoxin accumulation and endotoxin clearance controls the steady-state level of endotoxin surrounding orthopedic wear particles implanted on murine calvaria. This balance may regulate the rate of osteolysis in the murine calvaria model as well as in patients with aseptic loosening.

Lipopolysaccharide found in aseptic loosening of patients with inflammatory arthritis

Aseptic loosening of orthopaedic implants occurs in the absence of clinical signs of infection. Nevertheless, bacterial endotoxins derived from subclinical infections, systemic sources, or the implant manufacturing process may contribute to aseptic loosening. Also, the rate of implant infection is greater in patients with inflammatory arthritis than in patients with osteoarthritis. We hypothesized that lipopolysaccharide, the classic endotoxin derived from gram-negative bacteria, is more prevalent in periprosthetic tissue surrounding aseptically loose implants in patients with inflammatory arthritis than in patients with osteoarthritis. To test this, we used a modified Limulus amebocyte assay not affected by beta-glucan-like molecules in mammalian tissues. Lipopolysaccharide rarely was detected in periprosthetic tissue from patients with osteoarthritis and aseptic loosening (one of six patients). In contrast, lipopolysaccharide was detected despite the absence of any clinical signs of infection in peri-prosthetic tissue from all four patients with inflammatory arthritis (rheumatoid arthritis, juvenile rheumatoid arthritis, and systemic lupus erythematosus). Lipopolysaccharide also was detected in two patients with gram-negative infections, who were included as positive control subjects. Endotoxins derived from low-grade or systemic bacteremia may be important contributors to aseptic loosening particularly in patients with autoimmune conditions such as inflammatory arthritis.

Quantitative assessment of the response of osteoblast- and macrophage-like cells to particles of Ni-free Fe-base alloys

In the present study, the effect of mechanically alloyed particles of new FeAlCr alloys developed for potential applications as surgical implants has been tested on osteoblast- and macrophage-like cells and compared to particles of the Ti6Al4V alloy, for which there is a good clinical experience. After microstructural characterisation of the particles, cells were cultured with particles for 24-48 h using three different concentrations of particles, and the response of cells was quantified by assessment of viability, proliferation, and morphology. Mineralisation by osteoblasts was verified after 21 days. The amount of aluminium and chromium ions in the culture medium of macrophages was measured by graphite furnace atomic absorption and phagocytosis of particles assessed by light microscopy. Viability and proliferation of osteoblast- and macrophage-like cells were substantially unaffected by the presence of particles of the new alloys, which were phagocytosed according to their size. Aluminium and chromium ions were released in the culture medium, but no direct correlation with the cell behaviour was found. In vitro mineralisation was achieved by osteoblasts in due time. The new alloys are well tolerated in in vitro systems, and, due to their chemical and mechanical characteristics, they are under development for surgical implants.

Does endotoxin contribute to aseptic loosening of orthopedic implants?

Aseptic loosening of orthopedic implants caused by wear particles is a major clinical problem. This review examines the hypothesis that bacterial endotoxin contributes to aseptic loosening. Clinical findings support this hypothesis: bacterial biofilms exist on many implants from patients with aseptic loosening and antibiotics in bone cement reduce the rate of aseptic loosening. Three approaches were used to demonstrate that adherent endotoxin increases bioactivity of titanium particles. These experiments measured cytokine production and osteoclast differentiation in vitro and murine calvarial osteolysis in vivo. First, removal of >99.9% of the adherent endotoxin from titanium particles significantly ablates their biological activity. Second, adding lipopolysaccharide back to these "endotoxin-free" particles restores their biological activity. Third, cells or mice that are genetically hyporesponsive to endotoxin are significantly less responsive to titanium particles than are wild-type controls. Other investigators have confirmed and extended these results to include virtually all orthopedically relevant types of particles, including authentic titanium alloy particles retrieved from patients with loosening. Our recent studies suggest that adherent endotoxin on orthopedic implants may also inhibit initial osseointegration of the implants. Taken together, these studies suggest that bacterial endotoxin may have a significant role in induction of aseptic loosening.