Interplay between cellular changes in the knee joint, circulating lipids and pain behaviours in a slowly progressing murine model of osteoarthritis.

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Synovial inflammation has known contributions to chronic osteoarthritis (OA) pain, but the potential role in transitions from early to late stages of OA pain is unclear. The slowly progressing surgical destabilization of the medial meniscus (DMM) murine OA model and sham control, was used in male C57BL/6J mice to investigate the interplay between knee inflammation, plasma pro- and anti-inflammatory oxylipins and pain responses during OA progression. Changes in joint histology, macrophage infiltration, chemokine receptor CX3CR1 expression, weight bearing asymmetry, and paw withdrawal thresholds were quantified 4, 8 and 16 weeks after surgery. Plasma levels of multiple bioactive lipid mediators were quantified using liquid chromatography with tandem mass-spectrometry (LC-MS/MS). Structural joint damage was evident at 8 weeks post-DMM surgery onwards. At 16 weeks post-DMM surgery, synovial scores, numbers of CD68 and CD206 positive macrophages and pain responses were significantly increased. Plasma levels of oxylipins were negatively correlated with joint damage and synovitis scores at 4 and 8 weeks post-DMM surgery. Higher circulating levels of the pro-resolving oxylipin pre-cursor 17-HDHA were associated with lower weight bearing asymmetry at week 16. The transition to chronic OA pathology and pain is likely influenced by both joint inflammation and plasma oxylipin mediators of inflammation and levels of pro-resolution molecules. Using a slow progressing surgical model of osteoarthritis we show how the changing balance between local and systemic inflammation may be of importance in the progression of pain behaviours during the transition to chronic osteoarthritis pain.