Background/Purpose: The destabilization of medial meniscus (DMM) model of instability-induced OA replicates disease-related tissue pathology (including cartilage erosion, osteophytosis) and pain-related joint dysfunction, making it a useful model to investigate OA pathogenesis. Synovial inflammation characterized histopathologically, although common in OA, is not a prominent feature in this model leading some to suggest lack of significant synovial involvement. The aim of the current study was to determine if molecular analysis of synovium post-DMM could identify changes in synovial activity not otherwise revealed by histopathology.

Methods: 10-12 week old male C56BL/6J (WT) mice were subjected to DMM surgery of the right knee. Six weeks (early OA) and nineteen weeks (advanced OA) after surgery, groups of 5 mice were sacrificed and joints evaluated by histopathology for synovial hyperplasia. Hyperplasia was scored on a scale of 0-3 based on lining cell layers in three anatomic locations: superior to the medial meniscus (S.M.M), inferior to the medial meniscus (I.M.M) and in the medial peri-patellar (M.P.P) region. For molecular analysis, anterior synovial tissues were harvested at 1, 2, 4, 8 and 16 weeks post-DMM, and tissues from 3-5 mice were pooled per sample, RNA was isolated, and mRNA quantified using QX200^TM Droplet Digital^TM PCR System (BioRad, Hercules, CA). Expression of CD68 (monocytes/ macrophages), CD3δ, (T lymphocytes), and specific chemokines (CCL19 and CCL21, upregulated in OA patients) were expressed relative to TATA box binding protein (TBP)

Results: Evaluation of synovial hyperplasia showed mild or no hyperplasia (score≤1) at 6 weeks (Mean±SEM at S.M.M: 1±0.32, I.M.M: 0.33±0.33, M.P.P: 0.8±0.2) and 19 weeks (S.M.M: 0.5±0.29, I.M.M: 0.5±0.29, M.P.P.: 0.4±0.25) post-DMM. Furthermore, synovial hyperplasia post-DMM was not increased significantly compared to un-operated or sham-operated controls. In contrast, CD68 mRNA expression significantly increased early and peaked at 4 weeks post DMM surgery (21.61±3.03, p<0.0001) as compared to unoperated controls (2.72±1.47). No significant variation in expression of CD3d or CCL19 was detected over the time course. However, analysis of CCL21 expression indicated significantly elevated and sustained expression starting 2 weeks post-DMM (17.82±1.55), which continued to increase up to 16 weeks (34.1±3.79) post-DMM.

Conclusion: The current study suggests that molecular changes indicative of synovial inflammation are detectable in the DMM model of OA, despite minimal changes in synovial hyperplasia. Increased CD68 expression suggests an early increase in macrophages, along with a progressive increase in chemokine (CCL21) expression up to 16 weeks. The significance of these expression patterns to disease development has yet to be determined. A more comprehensive evaluation of synovial activation and its relationship with disease features, will be the focus of future studies to better understand how molecular and cellular activation within the synovial microenvironment may promote disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/transcriptional-analysis-of-synovial-tissue-reveals-sustained-inflammatory-chemokine-expression-despite-minimal-histopathologic-change-in-the-destabilization-of-medial-meniscus-model-of-murine-knee-os/