Conditional Macrophage Depletion Increases Inflammation and Does Not Inhibit the Development of Osteoarthritis in Obese Macrophage Fas‐Induced Apoptosis–Transgenic Mice

Abstract

Objective Macrophages are believed to play a critical role in the inflammation associated with the development of osteoarthritis (OA) in obesity. The objective of the study was to investigate whether short-term, systemic depletion of macrophages would mitigate OA following injury in obese mice. Methods CSF-1R–GFP+ Macrophage Fas-Induced Apoptosis (MaFIA) transgenic mice that allow conditional depletion of macrophages were placed on a high-fat diet and underwent surgery to induce knee OA. A small molecule (AP20187) was administrated to deplete macrophages in MaFIA mice. The effect of macrophage depletion on acute joint inflammation, OA severity, and arthritic bone changes were evaluated using histology and microCT. Immunohistochemistry was used to identify various immune cells. Serum and synovial fluid cytokines were also measured. Results Macrophage-depleted mice had significantly fewer M1 and M2 macrophages in the surgical operated-joints relative to controls and exhibited decreased osteophyte formation immediately following depletion. Surprisingly, macrophage depletion did not attenuate OA with obesity; instead, it induced systemic inflammation and led to a massive infiltration of CD3+ T cells and particularly neutrophils, but not B cells, into the injured joints. Macrophage-depleted mice also demonstrated markedly increased pro-inflammatory cytokines including granulocyte-colony stimulating factor (G-CSF), IL-1β, IL-6, IL-8, and TNF-α in both serum and joint synovial fluid, although animals showed trends for decreased serum insulin and leptin levels after depletion. Conclusion Our findings indicate that macrophages are vital in modulating the homeostasis of immune cells in obesity and suggest that more targeted approaches of different macrophage subtypes may be necessary to mitigate inflammation and OA with obesity.

Intergenerational Transmission of Diet‐Induced Obesity, Metabolic Imbalance, and Osteoarthritis in Mice

“…Obesity‐induced systemic and local inflammation plays an important role in the progression of OA damage. The results of the present study and other previous studies have shown that immune cells colocalize within the knee joint during obesity‐induced OA . In addition, the systemic proinflammatory milieu has been shown to be important in the pathogenesis of OA, potentially through interactions with local injury‐induced inflammation .…”

Synovial macrophage M1 polarisation exacerbates experimental osteoarthritis partially through R-spondin-2

“…Furthermore, systemic depletion of macrophages reduces osteophyte formation in a collagen-induced arthritis model 40. However, in another recent study conditional macrophage depletion increased inflammation and did not inhibit the development of OA in obese mice 16. The role of macrophage polarisation and its regulatory mechanism in the pathogenesis and progression of OA remains unknown.…”

“…Earlier studies have shown that macrophages accumulate and become polarised (M1 or M2) in the synovium and articular cavity during OA development, implying a correlation between macrophages and OA 14 15. However, a recent study demonstrated that macrophage depletion (with significantly fewer macrophages of both M1 and M2) increased inflammation and did not attenuate the severity of OA in obese macrophage Fas-induced apoptosis-transgenic mice 16. These studies indicate that OA is a complex disease that encompasses many pathways leading to degenerative conditions that progress to a common final outcome of end-stage OA, and some of these pathways involve inflammation especially macrophages.…”

Inflamed synovial fluid induces a homeostatic response in bone marrow mononuclear cells in vitro: Implications for joint therapy

“…19,35,37,113 The flow cytometry markers used in this study (CD14, CD86, and CD206) were selected to replicate those commonly used in mouse and human studies, providing means of comparative analysis. 7,25,114 Inclusion of gene expression for a robust panel of consensus markers aids the interpretation of macrophage activation states in our study.…”