Kyron McAllister

Soluble and particulate Co-Cr-Mo alloy implant metals activate the inflammasome danger signaling pathway in human macrophages: a novel mechanism for implant debris reactivity.

Immune reactivity to soluble and particulate implant debris remains the primary cause of aseptic inflammation and implant loosening. However, the intracellular mechanisms that trigger immune cells to sense and respond to exogenous nonbiological agents such as metal particles or metal ions released from orthopedic implants remain unknown. Recent studies in immunology have outlined the importance of the intracellular inflammasome complex of proteins in sensing danger/stress signals triggered by nonbiological agents in the cytosol of macrophages. We hypothesized that metal implant debris can activate the inflammasome pathway in macrophages that causes caspase‐1‐induced cleavage of intracellular pro‐IL‐1β into its mature form, resulting in IL‐1β secretion and induction of a broader proinflammatory response. We tested this hypothesis by examining whether soluble cobalt, chromium, molybdenum, and nickel ions and Co‐Cr‐Mo alloy particles induce inflammasome‐ mediated macrophage reactivity. Our results demonstrate that these agents stimulate IL‐1β secretion in human macrophages that is inflammasome mediated (i.e., NADPH‐, caspase‐1‐, Nalp3‐, and ASC‐dependent). Thus, metal ion‐ and particle‐induced activation of the inflammasome in human macrophages provides evidence of a novel pathway of implant debris‐induced inflammation, where contact with implant debris is sensed and transduced by macrophages into a proinflammatory response.

Soluble ions more than particulate cobalt-alloy implant debris induce monocyte costimulatory molecule expression and release of proinflammatory cytokines critical to metal-induced lymphocyte reactivity.

Aseptic osteolysis has been associated with excessive immune reactivity to particulate implant debris; however, innate and adaptive immune mechanisms that underlie implant debris reactivity remain incompletely understood. Although particulate debris has been implicated as the major type of implant debris mediating macrophage-induced osteolysis, the degree to which metal ions affect a proinflammatory response (if at all) remains unknown. We hypothesized that both soluble and particulate metal implant debris will induce proinflammatory responses in human monocytes resulting in cytokine production and elevated expression of T cell costimulatory molecules, facilitating adaptive immune responses. We tested this hypothesis by characterizing the response of a human monocyte cell line (THP-1), isolated primary human monocytes and PBMCs challenged with Co-Cr-Mo alloy particles and soluble cobalt, chromium, molybdenum, and nickel ions. Our results indicate that soluble cobalt, nickel, and molybdenum can induce monocyte up-regulation of T cell costimulatory molecules (CD80, CD86, ICAM-1) in human monocytes/macrophages. Furthermore, cobalt, molybdenum ions, and Co-Cr-Mo alloy particles similarly induce elevated secretion of IL-1beta, TNFalpha, and IL-6. Antibody blockade of CD80 and CD86, crucial secondary molecules for adaptive responses, abrogated lymphocyte reactivity to metal challenge in metal reactive subjects. Also the addition of IL-1 receptor antagonist (IL-1ra), (which indirectly blocks pro-IL-1beta and thus IL-1beta release), significantly reduced lymphocyte reactivity in metal-reactive subjects. Thus, both soluble and particulate metal implant debris induce monocyte/macrophage proinflammatory responses that are metal and individual specific. This suggests metal-induced up-regulation of costimulatory molecules and proinflammatory cytokine production is necessary to induce lymphocyte activation/proliferation to metal implant debris.

Orthopedic implant cobalt-alloy particles produce greater toxicity and inflammatory cytokines than titanium alloy and zirconium alloy-based particles in vitro, in human osteoblasts, fibroblasts, and macrophages†‡§¶

The performance of total joint arthroplasty (TJA) depends on the size/shape, material, and amounts of implant debris. Much remains unknown in terms of which types of debris are most reactive. We compared the responses of human periimplant cells, osteoblasts, fibroblasts, and macrophages, exposed to particles of different metal-based particles (i.e., cobalt-chromium-molybdenum (CoCrMo) alloy, titanium (Ti) alloy, zirconium (Zr) oxide, and Zr alloy. CoCrMo-alloy particles were by far the most toxic (p < 0.05) and decreased viability and proliferation of human osteoblasts, fibroblasts, and macrophages by >50% at a dose of only 50 particles per cell. All particle types induced the production of interleukin (IL)-6, tumor necrosis factor (TNF)-α, and IL-8 by osteoblasts, fibroblasts, and monocytes/macrophages. However, the greatest cytokine responses of macrophages were to CoCrMo alloy (TNF-α and IL-8) and Ti alloy (IL-1β). Likewise, the greatest responses of fibroblasts and osteoblasts were to CoCrMo alloy (IL-6 and TNF-α) (i.e., IL-6 300 pg/mL; 30-fold max, TNF-α 150 pg/mL; 15-fold max) versus controls. For macrophages, CoCrMo particles induced IL-8 (> 2000 pg/mL; approx 100-fold max) above controls and were also significantly elevated above levels produced by Zr-based particles. Submicron sized (0.2-0.9 μm) Zr-based particles (originally presumed to be more reactive) induced less toxicity and inflammatory responses when compared with larger (approx 1 μm) CoCrMo-alloy and Ti-alloy particles.

Increasing both CoCrMo‐alloy particle size and surface irregularity induces increased macrophage inflammasome activation in vitro potentially through lysosomal destabilization mechanisms

Recent investigations indicate that innate immune “danger‐signaling” pathways mediate metal implant debris induced‐inflammatory responses, for example, NALP3 inflammasome. How the physical characteristics of particles (size, shape, and chemical composition) affect this inflammatory reactivity remains controversial. We examined the role of Cobalt–Chromium–Molybdenum (CoCrMo) alloy particle shape and size on human macrophage phagocytosis, lysosomal destabilization, and inflammasome activation. Round/smooth versus irregularly shaped/rough CoCrMo‐alloy particles of ∼1 and 6–7 µm diameter were investigated for differential lysosomal damage and inflammasome activation in human monocytes/macrophages. While spherical/smooth 1 µm CoCrMo‐alloy particles did not measurably affect macrophage IL‐1β production, irregular 1 µm CoCrMo‐alloy particles induced significant IL‐1β increases over controls. Both round/smooth particles and irregular CoCrMo‐alloy particles that were 6–7 µm in size induced >10‐fold increases in IL‐1β production compared to similarly shaped smaller particles (p < 0.05). Larger irregular particles induced a greater degree of intracellular lysosomal damage and a >3‐fold increase in IL‐1β versus similarly sized round/smooth particles (at an equal dose, particles/cell). CoCrMo‐alloy particle‐size‐induced IL‐1β production was dependent on the lysosomal protease Cathepsin B, further supporting lysosomal destabilization as causative in inflammation. Phagocytosable larger/irregular shaped particles (6 µm) demonstrated the greatest lysosomal destabilization (observed immunofluorescently) and inflammatory reactivity when compared on an equal dose basis (particles/cell) to smaller/spherical 1 µm particles in vitro.

In vitro reactivity to implant metals demonstrates a person-dependent association with both T-cell and B-cell activation.

Hypersensitivity to metallic implants remains relatively unpredictable and poorly understood. We initially hypothesized that metal-induced lymphocyte proliferation responses to soluble metal challenge (ions) are mediated exclusively by early T-cell activation (not B-cells), typical of a delayed-type-hypersensitivity response. We tested this by comparing proliferation (6 days) of primary lymphocytes with early T-cell and B-cell activation (48 h) in three groups of subjects likely to demonstrate elevated metal reactivity: group 1 (n = 12) history of metal sensitivity with no implant; group 2a (n = 6) well performing metal-on-metal THRs, and group 2b (n = 20) subjects with poorly performing metal-on-polymer total joint arthroplasties (TJA). Group 1 showed 100% (12/12) metal reactivity (stimulation index > 2) to Ni. Groups 2a and 2b were 83% (5/6) and 75% (15/22) metal reactive (to Co, Cr, or Ni), respectively. Of the n = 32 metal-reactive subjects to Co, Cr, or Ni (SI > 2), n = 22/32 demonstrated >2-fold elevations in % of T-cell or B-cell activation (CD25+, CD69+) to metal challenge when compared with untreated control. 18/22 metal-activated subjects demonstrated an exclusively T-cell or B-cell activation response to metal challenge, where 6/18 demonstrated exclusively B-cell activation and 12/18 demonstrated a T-cell only response, as measured by surface activation markers CD25+ and CD69+. However, there was no direct correlation (R(2) < 0.1) between lymphocyte proliferation and % T-cell or B-cell activation (CD25+:CD69+). Proliferation assays (LTT) showed greater ability to detect metal reactivity than did subject-dependent results of flow-cytometry analysis of T-cell or B-cell activation. The high incidence of lymphocyte reactivity and activation indicate that more complex than initially hypothesized immune responses may contribute to the etiology of debris-induced osteolysis in metal-sensitive individuals.

Macrophage reactivity to different polymers demonstrates particle size- and material-specific reactivity: PEEK-OPTIMA® particles versus UHMWPE particles in the submicron, micron, and 10 micron size ranges†

Biologic reactivity to orthopedic implant debris is generally the main determinant of long-term clinical performance where released polymeric particles of Ultra-high molecular weight polyethylene (UHMWPE) remain the most prevalent debris generated from metal-on-polymer bearing total joint arthroplasties. Polymeric alternatives to UHMWPE such as polyetherether-ketone (PEEK) may have increased wear resistance but the bioreactivity of PEEK-OPTIMA particles on peri-implant inflammation remains largely uncharacterized. We evaluated human monocyte/macrophage responses (THP-1s and primary human) when challenged by PEEK-OPTIMA, UHMWPE, and X-UHMWPE particles of three particle sizes (0.7 um, 2 um, and 10 um) at a dose of 20 particles-per-cell at 24- and 48-h time points. Macrophage responses were measured using cytotoxicity assays, viability assays, proliferation assays and cytokine analysis (IL-1b, IL-6, IL-8, MCP-1, and TNF-α). In general, there were no significant differences between PEEK-OPTIMA, UHMWPE, and X-UHMWPE particles on macrophage viability or proliferation. However, macrophages demonstrated greater cytotoxicity responses to UHMWPE and X-UHMWPE than to PEEK-OPTIMA at 24 and 48 h, where 0.7 μm-UHMWPE particles produced the highest amount of cytotoxicity. Particles of X-UHMWPE more than PEEK-OPTIMA and UHMWPE induced IL-1β, IL-6, MCP-1, and TNF-α at 24 h, p < 0.05 (no significant differences at 48 h). On average, cytokine production was more adversely affected by larger 10 μm particles than by 0.7 and 2 μm sized particles. While limitations of in vitro analysis apply to this study, PEEK-OPTIMA particles were more biocompatible than UHMWPE particles, in that they induced less inflammatory cytokine responses and thus, in part, demonstrates that PEEK-OPTIMA implant debris does not represent an increased inflammatory risk over that of UHMWPE.

Asymptomatic prospective and retrospective cohorts with metal‐on‐metal hip arthroplasty indicate acquired lymphocyte reactivity varies with metal ion levels on a group basis

Some tissues from metal‐on‐metal (MoM) hip arthroplasty revisions have shown evidence of adaptive‐immune reactivity (i.e., excessive peri‐implant lymphocyte infiltration/activation). We hypothesized that, prior to symptoms, some people with MoM hip arthroplasty will develop quantifiable metal‐induced lymphocyte reactivity responses related to peripheral metal ion levels. We tested three cohorts (Group 1: n = 21 prospective longitudinal MoM hip arthroplasty; Group 2: n = 17 retrospective MoM hip arthroplasty; and Group 3: n = 20 controls without implants). We compared implant position, metal‐ion release, and immuno‐reactivity. MoM cohorts had elevated (p < 0.01) amounts of serum Co and Cr compared to controls as early as 3 months post‐op (Group 1:1.2 ppb Co, 1.5 ppb Cr; Group 2: 3.4 ppb Co, 5.4 ppb Cr; Group 3: 0.01 ppb Co, 0.1 ppb Cr). However, only after 1–4 years post‐op did 56% of Group 1 develop metal‐reactivity (vs. 5% pre‐op, metal‐LTT, SI > 2), compared with 76% of Group 2, and 15% of Group 3 controls (patch testing was a poor diagnostic indicator with only 1/21 Group 1 positive). Higher cup‐abduction angles (50° vs. 40°) in Group 1 were associated with higher serum Cr (p < 0.07). However, sub‐optimal cup‐anteversion angles (9° vs. 20°) had higher serum Co (p < 0.08). Serum Cr and Co were significantly elevated in reactive versus non‐reactive Group‐1 participants (p < 0.04). CD4+CD69+ T‐helper lymphocytes (but not CD8+) and IL‐1β, IL‐12, and IL‐6 cytokines were all significantly elevated in metal‐reactive versus non‐reactive Group 1 participants. Our results showed that lymphocyte reactivity to metals can develop within the first 1–4 years after MoM arthroplasty in asymptomatic patients and lags increases in metal ion levels. This increased metal reactivity was more prevalent in those individuals with extreme cup angles and higher amounts of circulating metal.

Cobalt Alloy Implant Debris Induces Inflammation and Bone Loss Primarily through Danger Signaling, Not TLR4 Activation: Implications for DAMP-ening Implant Related Inflammation.

Cobalt alloy debris has been implicated as causative in the early failure of some designs of current total joint implants. The ability of implant debris to cause excessive inflammation via danger signaling (NLRP3 inflammasome) vs. pathogen associated pattern recognition receptors (e.g. Toll-like receptors; TLRs) remains controversial. Recently, specific non-conserved histidines on human TLR4 have been shown activated by cobalt and nickel ions in solution. However, whether this TLR activation is directly or indirectly an effect of metals or secondary endogenous alarmins (danger-associated molecular patterns, DAMPs) elicited by danger signaling, remains unknown and contentious. Our study indicates that in both a human macrophage cell line (THP-1) and primary human macrophages, as well as an in vivo murine model of inflammatory osteolysis, that Cobalt-alloy particle induced NLRP3 inflammasome danger signaling inflammatory responses were highly dominant relative to TLR4 activation, as measured respectively by IL-1β or TNF-α, IL-6, IL-10, tissue histology and quantitative bone loss measurement. Despite the lack of metal binding histidines H456 and H458 in murine TLR4, murine calvaria challenge with Cobalt alloy particles induced significant macrophage driven in vivo inflammation and bone loss inflammatory osteolysis, whereas LPS calvaria challenge alone did not. Additionally, no significant increase (p<0.05) in inflammation and inflammatory bone loss by LPS co-challenge with Cobalt vs. Cobalt alone was evident, even at high levels of LPS (i.e. levels commiserate with hematogenous levels in fatal sepsis, >500pg/mL). Therefore, not only do the results of this investigation support Cobalt alloy danger signaling induced inflammation, but under normal homeostasis low levels of hematogenous PAMPs (<2pg/mL) from Gram-negative bacteria, seem to have negligible contribution to the danger signaling responses elicited by Cobalt alloy metal implant debris. This suggests the unique nature of Cobalt alloy particle bioreactivity is strong enough to illicit danger signaling that secondarily activate concomitant TLR activation, and may in part explain Cobalt particulate associated inflammatory and toxicity-like reactions of specific orthopedic implants.

Osteoclasts Lose Innate Inflammatory Reactivity to Metal and Polymer Implant Debris Compared to Monocytes/Macrophages

Long-term aseptic failures of joint replacements are generally attributed to implant debris-induced inflammation and osteolysis. This response is largely mediated by immune and bone cells (monocytes/macrophages and osteoclasts, respectively), that in the presence of implant debris (e.g. metal particles and ions), release pro-inflammatory cytokines such as IL-1β, TNF-α, and IL-6. The relative degree to which implant debris can illicit inflammatory response(s) from osteoclasts vs monocytes/macrophages is unknown, i.e. are osteoclasts a viable target for anti-inflammatory therapy for implant debris? We investigated relative monocyte versus osteoclast inflammatory responses in a side-by-side comparison using implant debris from the perspective of both danger signaling (IL-1β) and pathogenic recognition (TNF-α) reactivity (Challenge Agents: Cobalt-alloy, Titanium-alloy, and PMMA particles, 0.9-1.8um-dia ECD and Cobalt, and Nickel-ions 0.01-0.1mM, all with and without LPS priming). Human monocytes/macrophages reacted to implant debris with >100 fold greater production of cytokines compared to osteoclast-like cells. Particulate Co-alloy challenge induced >1000 pg/ml of IL-1β and TNF-α, in monocytes and <50pg/mL IL-1β and TNF-α in osteoclasts. Cobalt ions induced >3000pg/mL IL-1β and TNF-α in monocytes/macrophages and <50pg/mL IL-1β and TNF-α in osteoclasts. The paracrine effect of supernatants from debris-treated monocytes/macrophages was capable of inducing greater osteoclastogenesis (TRAP+, p<0.06) and inflammation than direct debris challenge on osteoclasts. Our results indicate that as monocytes/macrophages differentiate into osteoclasts, they largely lose their innate immune reactivity to implant debris and thus may not be as relevant a therapeutic target as monocytes/macrophages for mitigating debris-induced inflammation.

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.