Complement system dysregulation in synovial fluid from patients with persistent inflammation following anterior cruciate ligament reconstruction surgery

Introduction: Patients with anterior cruciate ligament injury are at high risk of posttraumatic osteoarthritis and their response to reconstructive surgery and rehabilitation vary. Proteins identified in the orchestration of the acute inflammatory response may be predictive of patient outcomes. Objective: An unbiased, bottom-up proteomics approach was used to discover novel targets for therapeutics in relation to dysregulation in the orchestration of inflammatory pathways implicated in persistent joint inflammation subsequent to joint trauma. Methods: Synovial fluid was aspirated from patients at 1 week and 4 weeks after anterior cruciate ligament reconstruction (ACLR) and interleukin 6 (IL-6) concentrations were quantified by enzyme-linked immunosorbent assay. Patients were segregated into IL-6^low and IL-6^high groups based on IL-6 concentrations in synovial fluid at 4-weeks postoperation and proteins in synovial fluid were analyzed using qualitative, bottom-up proteomics. Abundance ratios were calculated for IL-6^high and IL-6^low groups as 4 weeks postoperation:1 week postoperation. Results: A total of 291 proteins were detected in synovial fluid, 34 of which were significantly (P <.05) differentially regulated between groups. Proteins associated with the classical and alternative complement cascade pathways were increased in the IL-6^high compared to IL-6^low group. Insulin-like growth factor-binding protein 6 (IGFBP-6) was increased by nearly 60-fold in the IL-6^low group. Conclusions: Patients segregated by IL-6 concentration in synovial fluid at 4 weeks post-ACLR demonstrated differential regulation of multiple pathways, providing opportunities to investigate novel targets, such as IGFBP-6, and to take advantage of therapeutics already approved for clinical use in other diseases that target inflammatory pathways, including the complement system.