**4.1 Hierarchy of immune reactions to prosthetic wear particles** *(Gallo)*

Aseptic loosening and osteolysis can be understood to be the result of chronic high-grade inflammatory disease induced by exposure of local tissue to massive quantities of prosthetic particles. For this reason predominantly investigated is the role of the immune system in its pathophysiology. Currently, there is a large body of evidence supporting the concept of the immune system acting major influence on particle disease. In this section, we will focus on the role of the different parts of the immune system in the process of aseptic loosening and osteolysis.

Basically the immune system consists of two tightly associated segments: *the innate immune system and the adaptive immune system*. The former consists of a set of pattern-recognition receptors (PRRs) that evolved to recognize pathogen-associated molecular patterns (PAMPs), triggering rapid and robust response to invading pathogens following their recognition. The adaptive immune system evolved to produce a highly specific response to unique antigen presentation after innate instructions are conveyed to the second line of host-immunity associated in particular with T and B cells. A distinction should be made between at least the *inflammatory TH1 response* inducing and perpetuating inflammation via expression of IFN-γ, IL-2, TNF-alpha and activation of macrophages, NK cells, cytotoxic lymphocytes, and *anti-inflammatory TH2 and TH3 responses* associated characteristically with expression of IL-4, IL-5 and TGF-β and IL-10 cytokines, respectively (Matzinger 2007). From a functional point of view we can distinguish *cells/pathways/molecules that trigger the immune response to wear debris and those that regulate host response,* assuming the latter may play an important role in resolving inflammation and prevention of tissue damage.

The immune response is triggered after recognition of special surface motifs on invading pathogen (PAMPs) by PRRs. Intact pathogens can be composed of a number of PAMPs which induce activation of multiple PRRs simultaneously and, vice versa, different PRRs can recognize the same PAMP (Kumar et al. 2011). Currently, there is growing evidence that activation of PRRs is an important part of host adverse reaction against prosthetic particles (Lahdeoja et al. 2010). This could be explained by adherence of immunoreactive substances (lipopolysaccharides, lipoteichoic acid, fragments of bacteria, components of complement etc.) 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). Engagement of TLRs leads to activation of the NF-κβ signalling pathway with subsequent expression of pro-inflammatory cytokines, strongly boosting the immune response (Pearl et al. 2011). In the same way also other receptors (e.g. NOD-like receptor proteins) may participate in activation of the immune response to wear debris (St Pierre et al. 2010). Some of these are structurally involved in the cytosolic multi-protein complex known as the "inflammasome" which is considered a critical part of the inflammatory response. After appropriate stimulus, IL-1β and IL-18 convert from their inactive precursor molecules (pro-1β and pro-IL-18) into biologically active molecules that are then released from inflammasomes (Caicedo et al. 2009). From the clinical viewpoint, lipopolysaccharide has been detected in periprosthetic tissues without any other signs of infection in patients (Nalepka, Lee et al. 2006). In addition, some patients who underwent revision arthroplasty due to aseptic loosening had positive culture and/or PCR detected bacteria (Dempsey et al. 2007). In one recent study, it was even found that the extension of osteolysis correlated with proportion of positive sonication cultures (Sierra et al. 2011).

Participation of the adaptive immune system in host response to prosthetic wear debris is still a subject of controversy despite the fact that several reports have described lymphocytes in periprosthetic tissues, retrieved from patients with aseptically failed metal-on-metal (MoM) or non-MoM implants (Fujishiro et al. 2011; Ng et al. 2011). One recent study revealed increased serum concentration of cobalt and chromium in patients with MoM THA which positively correlated with increased proportion of HLA DR+ CD8+ T-cells in these patients (Hailer et al. 2011). In the same study, it was proposed that macrophages around THA could create haptenic metal ions in the context of self peptides (metal ion-altered self) as antigens in combination with MHC class II molecules, leading to T-cell priming. Weyand et al proposed antigen-recognition events when they characterised T cells in periprosthetic membranes (Weyand et al. 1998). They found identical T-cell receptor sequences suggesting identical antigen specificity and in addition, transcription of IL-2 and IFN-γ, indicating functional activity of at least some lymphocytes. Moreover, in this study, IFN-γ transcription correlated with extension of bone loss. Sensitization to various heavy metals after THA is described elsewhere in this chapter (*Part 4.5*).

### **4.2 Chemokines and cellular chemotaxis during the inflammatory response to wear particles from orthopaedic implants** *(Gibon, Goodman, Gallo)*

The biological reaction to polymers, ceramics and metallic wear particles is *a non-specific foreign body and chronic inflammatory response*. A type IV lymphocyte-driven hypersensitivity reaction involving a specific antigen has been found for a small percentage of cases with predominantly metal-on-metal bearing surfaces (*Part 4.5*). This section will focus on the inflammatory processes associated with wear particles from orthopaedic implants, emphasizing local and systemic cell signaling mediated by chemotactic cytokines called chemokines.

### **4.2.1 The inflammatory reaction to wear particles**

328 Recent Advances in Arthroplasty

Aseptic loosening and osteolysis can be understood to be the result of chronic high-grade inflammatory disease induced by exposure of local tissue to massive quantities of prosthetic particles. For this reason predominantly investigated is the role of the immune system in its pathophysiology. Currently, there is a large body of evidence supporting the concept of the immune system acting major influence on particle disease. In this section, we will focus on the role of the different parts of the immune system in the process of

Basically the immune system consists of two tightly associated segments: *the innate immune system and the adaptive immune system*. The former consists of a set of pattern-recognition receptors (PRRs) that evolved to recognize pathogen-associated molecular patterns (PAMPs), triggering rapid and robust response to invading pathogens following their recognition. The adaptive immune system evolved to produce a highly specific response to unique antigen presentation after innate instructions are conveyed to the second line of host-immunity associated in particular with T and B cells. A distinction should be made between at least the *inflammatory TH1 response* inducing and perpetuating inflammation via expression of IFN-γ, IL-2, TNF-alpha and activation of macrophages, NK cells, cytotoxic lymphocytes, and *anti-inflammatory TH2 and TH3 responses* associated characteristically with expression of IL-4, IL-5 and TGF-β and IL-10 cytokines, respectively (Matzinger 2007). From a functional point of view we can distinguish *cells/pathways/molecules that trigger the immune response to wear debris and those that regulate host response,* assuming the latter may play an

The immune response is triggered after recognition of special surface motifs on invading pathogen (PAMPs) by PRRs. Intact pathogens can be composed of a number of PAMPs which induce activation of multiple PRRs simultaneously and, vice versa, different PRRs can recognize the same PAMP (Kumar et al. 2011). Currently, there is growing evidence that activation of PRRs is an important part of host adverse reaction against prosthetic particles (Lahdeoja et al. 2010). This could be explained by adherence of immunoreactive substances (lipopolysaccharides, lipoteichoic acid, fragments of bacteria, components of complement etc.) 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). Engagement of TLRs leads to activation of the NF-κβ signalling pathway with subsequent expression of pro-inflammatory cytokines, strongly boosting the immune response (Pearl et al. 2011). In the same way also other receptors (e.g. NOD-like receptor proteins) may participate in activation of the immune response to wear debris (St Pierre et al. 2010). Some of these are structurally involved in the cytosolic multi-protein complex known as the "inflammasome" which is considered a critical part of the inflammatory response. After appropriate stimulus, IL-1β and IL-18 convert from their inactive precursor molecules (pro-1β and pro-IL-18) into biologically active molecules that are then released from inflammasomes (Caicedo et al. 2009). From the clinical viewpoint, lipopolysaccharide has been detected in periprosthetic tissues without any other signs of infection in patients (Nalepka, Lee et al. 2006). In addition, some patients who underwent revision arthroplasty due to aseptic loosening had positive culture and/or PCR detected

**4. Local host response to prosthetic wear debris** 

aseptic loosening and osteolysis.

**4.1 Hierarchy of immune reactions to prosthetic wear particles** *(Gallo)*

important role in resolving inflammation and prevention of tissue damage.

Resident macrophages are amongst the first cells involved in particle-associated inflammation and initiate cell recruitment via the release of chemokines. Macrophage activation may occur with or without phagocytosis, by cell membrane contact with particles (Goldstein et al. 1975). Several receptors in the outer membrane of macrophages (e.g. CD11b, CD14, Toll-like Receptors and others) are involved in the activation of macrophages after contact with particulate debris. These receptors act through transmembrane proteins and different intracellular pathways and result in the release of cytokines, chemokines and other substances that induce cell recruitment. One of the most important intracellular pathways involved in signal transduction is the *mitogen-activated protein kinase (MAP-Kinase) pathway*. The MAP-kinase pathway then activates transcription factors such as the Nuclear Factor Kappa B (NFB), which activates a cluster of genes for the production of pro-inflammatory cytokines, chemokines and related substances (Tuan et al. 2008). Besides macrophages, other resident cells are involved in the inflammatory process including fibroblasts, mesenchymal stem cells, osteoblasts, lymphocytes and others. Retrieval studies demonstrated that the periprosthetic cells produced high levels of pro-inflammatory cytokines and other factors including TNF, IL-1, IL-6, M-CSF, RANKL, and others (*Part 4.3*).

Aseptic Loosening of Total Hip Arthroplasty as a Result of Local Failure of Tissue Homeostasis 331

through the blood stream to the areas of particle generation. Recent murine studies involving continuous intra-osseous infusion of ultra high molecular weight polyethylene (UHMWPE) particles have demonstrated an increase in the systemic trafficking of reporter macrophages to the particle infusion site using non-invasive imaging (bioluminescence) and immunofluorescence microscopy (Ren et al. 2010). Furthermore, a decrease in bone mineral density has been observed within the UHMWPE particle-infused femora (Ren et al. 2011). This suggests that wear particles stimulate a systemic response, mediated by chemokines released locally. Recent unpublished studies in our laboratory have demonstrated that inhibition of the MCP-1-CCR2 chemokine-receptor axis can mitigate this response, including

Fig. 3. Biologic reaction induced by orthopaedic wear particles on host cells; note the complex interaction of different cell types and factors to the presence of wear particles. Chemokines play a central role in cell recruitment. This diagram does not include the type 4

*B ligand, TLR = toll-like receptor, FGF = fibroblast growth factor, OPG = osteoprotegerin, TGF*

*, PGE2 = prostaglandin E2* 

*stimulating factor, MCP-1 = monocyte chemoattractant protein-1, MIP-1 = macrophage inhibitory protein-1, ROS = reactive oxygen species, NO = nitric oxide, VEGF = vascular endothelial growth* 

*, GM-CSF = granulocyte macrophage colony-*

 *=* 

 *B, IFN = interferon, RANKL = receptor activator of nuclear factor* 

reversing in part, particle-induced bone loss.

immune reaction to metallic wear particles.

 *= tumor necrosis factor* 

*B = nuclear factor* 

*IL = interleukin, TNF*

*transforming growth factor* 

*factor, NF-*
