Impaired Efferocytosis by Synovial Macrophages in Patients With Knee Osteoarthritis

Abstract

Objective. Osteoarthritis (OA) exposes all joint tissues to physiologic stresses, increasing the need to clear apoptotic cells from tissues, including the synovium. We undertook this study to assess the burden of apoptotic cells in synovial tissue in patients with late-stage knee OA and to investigate whether OA impairs the macrophage-mediated clearance of apoptotic cells via efferocytosis.Methods. Synovial tissue was collected from individuals with healthy knees and patients with late-stage knee OA during arthroplasty. Synovial apoptotic cell burden was assessed by immunofluorescence for cleaved caspase 3. Efferocytosis of apoptotic Jurkat cells by CD14+ synovial tissue macrophages and peripheral blood-derived macrophages was quantified using immunofluorescence microscopy. Effects of OA on macrophage-mediated efferocytosis were modeled by stimulating blood-derived macrophages with synovial fluid collected from individuals with healthy knees and patients with early-or late-stage knee OA.Results. Patients with late-stage knee OA had more apoptotic synovial cells compared to healthy individuals. There was a marked reduction in the fraction of synovial tissue macrophages engaging in efferocytosis and the quantity of material efferocytosed by individual macrophages in OA patients. Blood-derived macrophages exposed to synovial fluid from patients with knee OA recapitulated the defective efferocytosis, with the greatest effect from patients with early-stage knee OA and higher disease activity (pain and inflammation).Conclusion. Apoptotic cells accumulate in the synovium of patients with late-stage knee OA. Our results suggest that OA impairs critical homeostatic functions of synovial macrophages, leading to accumulation of apoptotic cells.

Synovial macrophage diversity and activation of M-CSF signaling in post-traumatic osteoarthritis

“…The resident-like A cluster that emerged after injury expressed Mrc1 (CD206), historically associated with an anti-inflammatory and reparative phenotype, and which has been shown to mediate collagen turnover 58, 59 . In addition their potentially beneficial role in regulating extracellular matrix, the resident-like A population also expressed Gas6 , which is required for efficient efferocytosis of apoptotic cells by synovial macrophages 60 – an important aspect of inflammatory resolution that goes awry in OA 16 . It is noteworthy that conventional “M1” and “M2” markers, such as Il1b , Tnf , Arg1 and Il10 , revealed no appreciable separation across our clusters, placing further impetus to move beyond the simplistic M1-M2 spectrum of macrophages.…”

“…Macrophage ablation in mice by clodronate liposomes yielded generally favorable OA outcomes, including reduced osteophyte formation 13, 14 , however pharmacogenetic macrophage depletion using the MaFIA mouse resulted in systemic inflammation, exacerbated immune influx into the joint, with no attenuation of OA severity 15 . Indeed, synovial macrophages play critical physiological functions such as the timely clearance of apoptotic cells, which is impaired in OA 16 . In a model of inflammatory arthritis, Huang et al showed that resolution requires the suppression of pro-inflammatory, systemically-derived F4/80 hi MHC Class II + macrophages and that macrophages with a resident-like phenotype promote inflammatory resolution 17 .…”

Targeting STAT6-mediated synovial macrophage activation improves pain in experimental knee osteoarthritis

“…Synovial tissue was dissected from the entire knee as previously described [ 16 ], pooled from 3 or 4 animals per replicate, providing n=5 replicates per condition at 4 and 12 weeks after joint surgery. After rinsing in PBS, synovial cells were enzymatically dissociated as previously described [ 7 ]. CD11b+ cells (monocyte/macrophage) were sorted using magnetic-activated cell sorting separation columns (Miltenyi Biotec) and lysed with TRIzol (Fisher Scientific) for RNA isolation using the Direct-zol RNA MicroPrep Kit (Zymo Research).…”

Efferocytosis: Current status and future prospects in the treatment of autoimmune diseases

“…It is characterized by multi-articular, symmetrical, and invasive inflammation of the small joints of the hand and foot, often accompanied by involvement of extra-articular organs and positive serum rheumatoid factors, which can ultimately lead to joint deformity and loss of function [ 45 ]. Although impaired efferocytosis and elevated AC levels have been observed in the synovial tissue of patients with osteoarthritis [ 46 ], comprehensive evaluations of efferocytosis within RA synovial tissue are lacking. Inflammatory cytokines, including IL-1β, IL-6, IL-8, and TNF-α, have important roles in the pathogenesis of RA.…”