Background/Purpose: Intra-articular calcium hydroxyapatite (HA) crystals are present in the majority of osteoarthritic (OA) joints. They activate macrophages, synovial fibroblasts and articular chondrocytes, resulting in increased cell proliferation and the production of pro-inflammatory cytokines and matrix metalloproteases. This suggests a pathogenic role in OA by causing extracellular matrix degradation and subchondral bone remodeling. Total joint arthroplasty is performed when conservative therapies have proved unsuccessful in OA. However, particles generated from wear of prosthetic implants also induce pro-inflammatory cytokines, which drive osteoclastogenesis through up-regulation of receptor activator of nuclear factor kappa-B (RANK) ligand, an essential cytokine for the differentiation of osteoclasts. These bone-resorbing cells drive periprosthetic osteolysis through secretion of acid and the proteolytic enzyme cathepsin K and contribute to implant loosening. Both interleukin (IL)-6 and interferon (IFN)-γ exhibit anti-osteoclastogenic activity via activation of the JAK/STAT pathway. Titanium and bone cement wear particles can inhibit this effect, via activation of mitogen-activated protein kinases (MAPK) and induction of suppressor of cytokine signaling (SOCS) proteins. As synthetic HA is used as a biomimetic to coat orthopaedic implants, we sought to 1) determine whether HA crystals promote osteoclastogenesis, 2) determine whether HA crystals alter anti-osteoclastogenic signaling by IL-6 and IFN-γ and 3) elucidate the mechanism by which HA crystals may contribute to periprosthetic osteolysis.

Methods: Murine and human macrophages were stimulated with HA crystals for 3, 6 and 24 hours and expression of RANK ligand and cathepsin K was analysed by quantitative PCR. Human osteoclast precursors were stimulated with HA crystals and MAPK activation was detected by Western blot. Osteoclast precursors were treated with HA crystals for 1 hour prior to IL-6 or IFN-γ stimulation, and phosphorylation of signal transducer and activator of transcription (STAT) 1 and 3 was detected by Western blot. Up-regulation of (SOCS) 1 and 3 was analysed by q-PCR

Results: HA crystals (50μg/ml) up-regulate RANK ligand and cathepsin K in murine and human macrophages by 3 to 5-fold. Stimulation of human osteoclast precursors with HA induces the activation of p38, JNK and ERK MAPK. Pre-treatment of human osteoclast precursors with HA crystals inhibits both IFN-γ and IL-6-induced activation of STAT1 and STAT3 respectively (figs A and B). Furthermore, HA crystals up-regulate the expression of SOCS1 and SOCS3, an effect which is reduced by MAPK inhibition.

Conclusion: Based on these studies we propose that HA crystals could contribute 1) to periprosthetic osteolysis and 2) subchondral bone remodelling in OA through up-regulation of RANK ligand, and inhibition of the anti-osteoclastogenic activity of IL-6 and IFN-γ.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/calcium-hydroxyapatite-crystals-inhibit-interleukin-6-and-interferon-induced-anti-osteoclastogenic-signaling-in-human-osteoclast-precursors/