Bacterial Pathogen-associated Molecular Patterns Stimulate Biological Activity of Orthopaedic Wear Particles by Activating Cognate Toll-like Receptors

Abstract

Aseptic loosening of orthopaedic implants is induced by wear particles generated from the polymeric and metallic components of the implants. Substantial evidence suggests that activation of Toll-like receptors (TLRs) may contribute to the biological activity of the wear particles. Although pathogen-associated molecular patterns (PAMPs) produced by Gram-positive bacteria are likely to be more common in patients with aseptic loosening, prior studies have focused on LPS, a TLR4-specific PAMP produced by Gram-negative bacteria. Here we show that both TLR2 and TLR4 contribute to the biological activity of titanium particles with adherent bacterial debris. In addition, lipoteichoic acid, a PAMP produced by Gram-positive bacteria that activates TLR2, can, like LPS, adhere to the particles and increase their biological activity, and the increased biological activity requires the presence of the cognate TLR. Moreover, three lines of evidence support the conclusion that TLR activation requires bacterially derived PAMPs and that endogenously produced alarmins are not sufficient. First, neither TLR2 nor TLR4 contribute to the activity of "endotoxin-free" particles as would be expected if alarmins are sufficient to activate the TLRs. Second, noncognate TLRs do not contribute to the activity of particles with adherent LPS or lipoteichoic acid as would be expected if alarmins are sufficient to activate the TLRs. Third, polymyxin B, which inactivates LPS, blocks the activity of particles with adherent LPS. These results support the hypothesis that PAMPs produced by low levels of bacterial colonization may contribute to aseptic loosening of orthopaedic implants.Osteoimmunology is a newly emerging field that refers to bi-directional interactions between the skeletal system and the immune system (1). One example of regulation of the skeletal system by the immune system is inflammatory osteolysis, which is the local bone loss induced by inflammation in response to pathogens or nonpathogenic stimuli (1). Inflammatory osteolysis in response to either pathogens or nonpathogenic stimuli can contribute to loosening of orthopaedic implants and the resultant need to replace the implant. For example, implant infection is a devastating complication that often necessitates implant removal and long term intravenous treatment with antibiotics (2). In contrast, "aseptic loosening" results from inflammation induced by polymeric and metallic wear particles derived from the implant components in the absence of any clinical signs of infection (3, 4). The primary events involved in this process include: phagocytosis of the wear particles by macrophages; activation of the macrophages to produce pro-inflammatory cytokines; stimulation of bone resorptive cytokines, such as RANKL, in response to the proinflammatory cytokines; stimulation of osteoclast differentiation by the resorptive cytokines; and local osteolysis caused by the increased number of osteoclasts (3-5).Subclinical levels of bacteria may contribute to wear particleinduced inflammation, and...

Do Genetic Susceptibility, Toll-like Receptors, and Pathogen-associated Molecular Patterns Modulate the Effects of Wear?

“…Consistent with that view, responses to titanium wear particles that involve TLR2 or TLR4 have been shown to require adherence of cognate PAMPs to the wear particles [37]. Those results are strong, albeit indirect, evidence that the alarmins are not sufficient to activate those TLRs in the absence of PAMPs in the cell culture and murine models that were studied (Fig.…”

“…Moreover, it has recently been reported that human TLR4 can be activated by cobalt or nickel ions [88,110], both of which can be released by corrosion of cobaltchromium implants [39]. Functional studies have shown that deletion of TLR2 and/or TLR4 in murine systems substantially reduces the in vitro and in vivo responses to titanium [8,37], UHMWPE [40], cobalt-chromium [85], and hydroxyapatite [34] particles. Moreover, similar results have also been obtained for deletion of the myeloid differentiation primary response gene (MyD88) with both PMMA and cobalt-chromium particles [82,85].…”

Novel biological strategies for treatment of wear particle-induced periprosthetic osteolysis of orthopaedic implants for joint replacement

“…Furthermore, cobalt ions have been shown to directly bind to and activate TLR4 signalling [92][93][94]. Exo-or endogenous DAMPs can bind to wear particles surfaces and mediate particle-induced inflammation via TLR signalling [95][96][97][98]. As discussed above, TLR signalling, especially via TLR4, is one of the cues that can induce M1-like macrophage phenotype.…”

Aseptic Loosening of Total Hip Arthroplasty as a Result of Local Failure of Tissue Homeostasis

“…An important role in the transduction of hypoxic signal to bone-implant interface may be played by osteocytes via increased expression of HIF and osteopontin leading eventually to osteocyte-induced osteoclastic bone resorption (Gross et al 2005). Activation of PRRs by occurrence of PAMPs in periprosthetic tissues can distort local tissue homeostasis at any time postoperatively (Greenfield et al 2010). PRRs have a strong potential to trigger inflammation via several pathways including differentiation of more aggressive M1 and M17 macrophages, multinuclear foreign body giant cells, osteoclasts and granulomas (Anderson et al 2008).…”

“…on wear debris particles. Some researchers have demonstrated that i) PAMPs adhere to the particles; ii) adherent PAMPs increase significantly the biological response to the particles; iii) PAMPs do this by activating their PRRs (Greenfield et al 2010). In addition, particles deliberated from THA are coated immediately with serum proteins creating a protein-particle complex that may result in a damage/danger-associated molecular pattern picture irrespective of endotoxin (Sun et al 2003).…”