Considerable evidence exists that aseptic loosening is initiated by wear particles that recruit macrophages and stimulate their production of pro-inflammatory cytokines. The cytokines primarily act indirectly by inducing production of RANKL, which stimulates osteoclast differentiation, osteolysis, and inflammatory bone loss. There is also considerable evidence that activation of macrophage Toll-like Receptors (TLRs) contributes to this cascade of events. It is however controversial whether bacterially-derived immunostimulatory molecules known as Pathogen-Associated Molecular Patterns (PAMPs) can contribute to aseptic loosening by stimulating their cognate TLRs on macrophages. Priming and subsequent activation of the NLRP3 inflammasome is essential for macrophage production of mature, active IL-1β in response to wear particles. We recently confirmed that wear particles can activate pre primed NLRP3 inflammasomes in the absence of PAMPs. Thus, activation of the NLRP3 inflammasome is the only macrophage-based event in the aseptic loosening cascade that we have found to date is independent of PAMPs. In contrast, priming of the NLRP3 inflammasome by wear particles requires PAMPs as well as their cognate TLRs. These results add to the growing body of evidence that bacterially-derived PAMPs can contribute to aseptic loosening.

Aseptic loosening is a major cause of revision surgeries and occurs when osteolysis is stimulated around the implant by pro-inflammatory cytokines including IL-1β. Production of active IL-1β in response to orthopedic wear particles depends on processing by the NLRP3 inflammasome which requires priming followed by activation. We found that pathogen associated molecular patterns (PAMPs) adherent to wear particles are necessary to prime the NLRP3 inflammasome. In contrast, in pre-primed macrophages, particles themselves are sufficient to activate the NLRP3 inflammasome and induce secretion of active IL-1β. Particles themselves also induce cell death, kinetically preceding the release of active IL-1β. Phagocytosis of particles is required to initiate both responses as the phagocytosis inhibitor cytochalasin blocks cell death and IL-1β release. Lysosome membrane destabilization is also critical as inhibition of lysosomal function with bafilomycin or chloroquine significantly abrogated the release of active IL-1β and cell death in response to wear particles. The pan-cathepsin inhibitors Ca-074-Me or K777 also inhibit cell death and IL-1β release indicating that cathepsin release from lysosomes is also a necessary step in the particle-induced response. Our results open the possibility of clinical intervention with lysosomal or cathepsin inhibitors to treat aseptic loosening as these drugs have better specificity and less in vivo toxicity than the phagocytosis inhibitors. Testing of these inhibitors in vivo in models of particle induced osteolysis is a key future direction.