The relationship between synovial inflammation, structural pathology, and pain in post-traumatic osteoarthritis: differential effect of stem cell and hyaluronan treatment

Background Synovitis is implicated in the severity and progression of pain and structural pathology of osteoarthritis (OA). Increases in inflammatory or immune cell subpopulations including macrophages and lymphocytes have been reported in OA synovium, but how the particular subpopulations influence symptomatic or structural OA disease progression is unclear. Two therapies, hyaluronan (HA) and mesenchymal stem cells (MSCs), have demonstrated efficacy in some clinical settings: HA acting as device to improve joint function and provide pain relief, while MSCs may have immunomodulatory and disease-modifying effects. We used these agents to investigate whether changes in pain sensitization or structural damage were linked to modulation of the synovial inflammatory response in post-traumatic OA. Methods Skeletally mature C57BL6 male mice underwent medial-meniscal destabilisation (DMM) surgery followed by intra-articular injection of saline, a hyaluronan hexadecylamide derivative (Hymovis), bone marrow-derived stem cells (MSCs), or MSC + Hymovis. We quantified the progression of OA-related cartilage, subchondral bone and synovial histopathology, and associated pain sensitization (tactile allodynia). Synovial lymphocytes, monocyte/macrophages and their subpopulations were quantified by fluorescent-activated cell sorting (FACS), and the expression of key inflammatory mediators and catabolic enzyme genes quantified by real-time polymerase chain reaction (PCR). Results MSC but not Hymovis significantly reduced late-stage (12-week post-DMM) cartilage proteoglycan loss and structural damage. Allodynia was initially reduced by both treatments but significantly better at 8 and 12 weeks by Hymovis. Chondroprotection by MSCs was not associated with specific changes in synovial inflammatory cell populations but rather regulation of post-injury synovial Adamts4, Adamts5, Mmp3, and Mmp9 expression. Reduced acute post-injury allodynia with all treatments coincided with decreased synovial macrophage and T cell numbers, while longer-term effect on pain sensitization with Hymovis was associated with increased M2c macrophages. Conclusions This therapeutic study in mice demonstrated a poor correlation between cartilage, bone or synovium (histo)pathology, and pain sensitization. Changes in the specific synovial inflammatory cell subpopulations may be associated with chronic OA pain sensitization, and a novel target for symptomatic treatment.