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Aseptic loosening of hip replacements is driven by the macrophage reaction to wear particles. The extent of particleinduced macrophage activation is dependent on the state of macrophage polarization, which is dictated by the local cytokine microenvironment. The aim of the study was to characterize cytokine microenvironment surrounding failed, loose hip replacements with an emphasis on identification of cytokines that regulate macrophage polarization. Using qRT-PCR, the expression of interferon gamma (IFN-g), interleukin-4 (IL-4), granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-13, and IL-17A was low and similar to the expression in control synovial tissues of patients undergoing primary hip replacement. Using immunostaining, no definite source of IFN-g or IL-4 could be identified. IL-17A positive cells, identified as mast cells by double staining, were detected but their number was significantly reduced in interface tissues compared to the controls. Significant up-regulation of IL-10, M-CSF, IL-8, CCL2-4, CXCL9-10, CCL22, TRAP, cathepsin K, and down regulation of OPG was seen in the interface tissues, while expression of TNF-a, IL-1b, and CD206 were similar between the conditions. It is concluded that at the time of the revision surgery the peri-implant macrophage phenotype has both M1 and M2 characteristics and that the phenotype is regulated by other local and systemic factors than traditional macrophage polarizing cytokines. Keywords: joint replacement; wear debris; foreign body response; macrophage polarization; lymphocyte Aseptic loosening is one of the main long-term complications of total joint replacement surgery. Peri-implant osteolysis is mainly driven by macrophage-mediated inflammation to implant-derived biomaterial wear particles. 1,2 Wear particle activated macrophages secrete chemokines and pro-inflammatory cytokines that lead to further monocyte-macrophage recruitment, increased osteoclastogenesis, and suppression of osteoblast formation and function. Together these changes create a microenvironment, which favors bone resorption over bone formation, ultimately leading to periimplant osteolysis and implant loosening. Macrophages activated by wear particles are thus considered an essential component in the pathogenic cascade of aseptic loosening.Macrophages form a dynamic and adaptive population of cells that can assume various phenotypes as instructed by signals derived from the local microenvironment. The best-established example of this macrophage plasticity is the phenomenon of macrophage polarization. 3,4 Macrophages exposed to lipopolysaccharide, the T helper cell type 1 (Th1) signature cytokine interferon gamma (IFN-g), and/or granulocyte-macrophage colony-stimulating factor (GM-CSF) assume a pro-inflammatory phenotype known as classical macrophage activation or M1 polarization. These cells are effective in antigen presentation and killing of phagocytosed pathogens, and produce inflammatory cytokines and chemokines. In contrast, macrophages exposed to the Th2 signatur...

IntroductionPatients with rheumatoid arthritis (RA) have disturbances in the hypothalamic-pituitary-adrenal (HPA) axis. These are reflected in altered circadian rhythm of circulating serum cortisol, melatonin and IL-6 levels and in chronic fatigue. We hypothesized that the molecular machinery responsible for the circadian timekeeping is perturbed in RA. The aim of this study was to investigate the expression of circadian clock in RA.MethodsGene expression of thirteen clock genes was analyzed in the synovial membrane of RA and control osteoarthritis (OA) patients. BMAL1 protein was detected using immunohistochemistry. Cell autonomous clock oscillation was started in RA and OA synovial fibroblasts using serum shock. The effect of pro-inflammatory stimulus on clock gene expression in synovial fibroblasts was studied using IL-6 and TNF-α.ResultsGene expression analysis disclosed disconcerted circadian timekeeping and immunohistochemistry revealed strong cytoplasmic localization of BMAL1 in RA patients. Perturbed circadian timekeeping is at least in part inflammation independent and cell autonomous, because RA synovial fibroblasts display altered circadian expression of several clock components, and perturbed circadian production of IL-6 and IL-1β after clock resetting. However, inflammatory stimulus disturbs the rhythm in cultured fibroblasts. Throughout the experiments ARNTL2 and NPAS2 appeared to be the most affected clock genes in human immune-inflammatory conditions.ConclusionWe conclude that the molecular machinery controlling the circadian rhythm is disturbed in RA patients.