Osteoarthritis (OA) is a debilitating degenerative joint disease affecting 27 million Americans over the age of 25. OA is characterized by total joint changes including the degradation of articular cartilage and inflammation of the synovium, a specialized lining that envelops the knee joint. Whereas OA is a disease of the entire joint organ, the contribution of the synovium in OA and to cartilage degeneration has been underappreciated. Synovial inflammation often precedes the development of cartilage damage and is observed in early and late stage OA. The onset of synovitis is driven by both elevated concentrations of pro-inflammatory cytokines and tissue debris in the joint space resulting from trauma, such as meniscal or anterior cruciate ligament tears. Accordingly, surgeons have observed cartilaginous debris embedded within the synovial membrane of OA patients presenting with severe capsular synovial hyperplasia and metaplasia. It has been hypothesized that the fibrotic shortening of the synovial capsule results in pain and stiffness often associated with OA and contributes to further joint destruction through the release of degradative enzymes. Current strategies to treat synovial inflammation and joint pain, such as intra-articular injections and synovectomy, have had limited and variable success.

To this end, cell culture and tissue engineering culture models provide a versatile platform to study the tissues and cells involved in OA. Our lab has typically employed mechanical overload or cytokine insult of chondrocytes and cartilage explants to study cartilage degradation. Alternatively, using cells derived from the pathologic joint provides the opportunity to study these within their de novo extracellular matrix (ECM). Increasingly more physiologic models of OA have cultured synovial explants with injured cartilage tissue. Within synovial explants, the inflammatory response of fibroblast-like synoviocytes (FLS) cannot be separated from the response of synovial macrophages. To isolate the role of FLS in the progression of OA, FLS can be exposed to chemical (e.g. cytokines) or physical (e.g. fluid shear) OA stimuli. Although often overlooked as an instigator of OA, cartilage wear particles have been reported to induce synovial inflammation and OA-like joint changes in various animal models. As opposed to non-biological (metal or plastic) wear particles, small (sub10μm) cartilage wear particles are comprised of ECM constituents that are degradable and may interact with cells beyond just being internalized. The work presented in this dissertation aims to combine knowledge from basic science and pre-clinical models of OA to develop a clinically relevant disease model using cells derived from clinical samples.

Previous studies have shown that synovial explants produce increased concentrations of degenerative enzymes in response to cytokines. To better understand the effects of these cytokines on the synovium, we looked at how cytokines regulate the cellular and ECM composition of the synovium in a physiologic co-culture system. In support of clinical observations, synovial explants were able to engulf large cartilage particles. To study the contribution of FLS only to synovial inflammation, we developed a high-density monolayer culture model of the synovium. Using this model allowed for the characterization of the interaction of FLS with cartilage wear particles. The resulting FLS proliferation and inflammation was determined to be caused by both phagocytosis and cell attachment to the particles. Using small latex particles and large collagen coated beads, we determined that this response was likely due to cell-attachment and activation of integrins. Cartilage particles also reduced the mechanosensitivity of FLS to fluid shear, implicating additional deleterious effects of cartilage particulates within the synovium and in the joint space.

Cells isolated from pathologic tissue provide another approach to studying OA. Using cells derived from an animal model of OA allowed us to make direct comparisons between synovial fluid and media concentrations of OA biomarkers. These tissues had different media concentrations of MMPs, chemokines and cytokines for up to two weeks of culture. This confirmed that disease cells can be used for the development of pathologic culture models.

As has been shown in the literature, the observed behavior of cells isolated from other species (bovine or canine) may not be a direct comparison to findings in human cells. For the final studies, we sought to translate our finding from a synovium disease model using juvenile bovine FLS and a chondrocyte disease model using canine chondrocytes to a clinically relevant cell source. Specifically, we compared the response of normal (healthy, non-diseased) and diseased (OA) FLS to different OA stimuli to model synovial inflammation in early and late stage OA. These results show that diseased cells are much more sensitive to inflammatory stimuli and that cytokines stimulate a larger inflammatory response than particles alone. Taken together, the sum of this work can provide a new research platform to study disease progression and identify new diagnostics and therapeutics that target joint inflammation and pain.