Inflammatory response in equine joints: studies on proinflammatory cytokines in diseased joints and chondrocyte cultures

Proinflammatory cytokines mediate inflammatory responses as well as regulate tissue metabolism. Thus, they may provide a link between inflammation and other pathologic findings seen in equine joint disease. The aims of this thesis were to gain a deeper understanding of the development of chondral pathology in equine osteoarthritis (OA) by obtaining increased knowledge of inflammatory processes in the joint, and to investigate proinflammatory cytokines as markers of joint pathology. Measurements of interleukin (IL)-6 and tumour necrosis factor (TNF) activities in synovial fluid from clinical cases of carpal joint disease revealed increased concentrations of bioactive IL-6 in joints with osteochondral fragments (OCF). Immunohistochemical staining for IL-6 and high mobility group box protein (HMGB)-1 in tissues from diseased carpal and fetlock joints identified both OCF and synovial membranes as cellular sources of IL-6 and extracellular/cytoplasmic HMBG-1. Histological assessment of synovial membrane biopsies from immunostained joints showed increased grade of synovitis in diseased compared to healthy joints. The effects of IL-6, IL-1β and HMGB-1 on chondrocyte metabolism was studied in vitro, and focused on gene expression analyses. A catabolic response to IL-1β was detected, whereas the response to IL-6 and HMGB-1 was varied but indicated promotion of cartilage formation. Analyses of the ultrastructural immunolocalisation of HMGB-1 in IL-1β and IL-6 treated pellets, showed a trend for decreased nuclear/cytoplasmic ratio, and increased extracellular matrix density of HMGB-1 after IL-6 treatment. Theses studies gained insight to presence of TNF, IL-6 and HMGB-1 in equine joints, and indicated an increased inflammatory response in joints with OCF. The difference in metabolic response of chondrocytes to IL-1β versus IL-6 or HMGB-1 in vitro, and differences in HMGB-1 translocation after IL-1β and IL-6 treatment suggest that the outcome of cartilage pathology in equine joint disease may relate to the individual contributions of cytokine activity and cytokine interactions.