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Thesis Format



Master of Science


Physiology and Pharmacology

Collaborative Specialization

Musculoskeletal Health Research


Appleton, Tom

2nd Supervisor

Heit, Bryan



Osteoarthritis (OA) is the most common form of arthritis, which affects hundreds of millions of people worldwide with no medical treatments that can stop joint damage. Tissues that line the interior of joints are essential for maintaining joint health, specifically the synovium. Previous research from the Appleton lab has shown that efferocytosis, the process for the clearance of dead and dying cells by macrophages, is impaired in OA patients. This project further explored the OA-specific mechanisms controlling efferocytosis in macrophages and investigated whether there was variation in different processes, gene expression, and pathways. We hypothesized that the macrophage receptors that are required for engaging dead and dying cells are decreased and that targeting these pathways can rescue the defect. To study the specific altered mechanisms, this study used macrophages differentiated in vitro from primary blood monocytes, which were treated with synovial fluid from patients with high levels of inflammation due to knee OA. By feeding labeled apoptotic cells to macrophages stimulated with synovial fluid and fixing the cells during the process of efferocytosis, we were able to identify the steps in the process that were impaired. We found from our studies that the engulfment stage of efferocytosis is impaired. Next, RNA sequencing was used to compare the healthy control versus the diseased sample to measure the differential gene expression of receptors and enrichment of physiologic pathways. We found that the defect is reversible with the treatment of IL-4. We also found that signs of cell stress, such as the unfolded protein response (UPR), were significantly enriched. Selective inhibitors were used to block the UPR pathway and there was a rescue effect. Overall, this study identified mechanisms contributing to impaired efferocytosis in OA and helped to inform treatments to improve efferocytosis and patient outcomes.

Summary for Lay Audience

Osteoarthritis (OA) is a long-term joint disease and has no effective medical treatment to stop or reverse joint damage. This is a severe disease which causes disability, loss of quality of life, and in some cases a shorter life span. Tissues that line the insides of joints (called synovium) are especially important for maintaining the health of the joint. We recently discovered that a critical function for joint tissue health – the clearance of dead and dying cells to prevent them from causing tissue damage – is impaired by OA. The mechanisms leading to the impairments in the function of the immune cells (macrophages) that clear dead and dying cells are not known. This project used the joint fluids from patients with high severity of knee OA to stimulate macrophages and test the mechanisms that control their ability to clear dead and dying cells. We tested candidate treatments to improve or rescue this impairment. We found that treating the macrophages with an anti-inflammatory molecule called interleukin-4 (IL-4), important for clearing cells out of tissues when they die to maintain tissue health. The IL-4 treatment rescued the engulfment defect. We also found that blocking a cell stress response pathway, the unfolded protein response (UPR), which affects the correct folding of proteins and can induce cell death, if this pathway is blocked the defect can also be resolved. These findings could then be tested in future studies to see if they could provide benefit to patients with knee OA.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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Available for download on Friday, February 28, 2025